## Saturday, March 07, 2020

### Is Gravity a Force?

I was sick last week and lost like 10 pounds in 3 days, which brings up the question, what is weight?

Weight is actually the force that acts on your body due to the pull of gravity. Now, the gravitational force depends on the mass of the object that is generating the force, in this case, planet Earth. So you can lose weight by simply moving to the moon. Technically, therefore, I should have said I lost mass, not weight.

Why do we normally not make this distinction? That’s because in practice it doesn’t matter. Mass just a number – a “scalar” – as physicists say, but weight, since it is a force, has a direction. So if you wanted to be very annoying, I mean very accurate, then whenever you’d refer to weight you’d have to say which direction you are talking about. The weight in East direction? The weight in North direction? Why doesn’t anyone ever mention this?

We don’t usually mention this because we all agree that we mean the force pulling down, and since we all know what we are talking about, we treat weight as if it was a scalar, omitting the direction. Moreover, the gravitational attraction downwards is pretty much the same everywhere on our planet, which means it is unnecessary to distinguishing between weight and mass in everyday life. Technically, it’s correct: mass and weight are not the same thing. Practically, the difference doesn’t matter.

But wait. Didn’t Einstein say that gravity is not a force to begin with? Ah, yes, there’s that.

Einstein’s theory of general relativity tells us that the effect we call gravity is different from normal forces. In General Relativity, space and time are not flat, like a sheet of paper, but curved, like the often-named rubber sheet. This curvature is caused by all types of mass and energy, and the motion of mass and energy is in return affected by the curvature. This gives you a self-consistent, closed, set of equations know as Einstein’s Field Equations. In Einstein’s theory, then, there is no force acting on masses. The masses are just navigating the curved space-time. We cannot see the curvature directly. We only see its effects. And those effects are what we call gravity.

Now, Einstein’s theory of General Relativity rests on the equivalence principle. The equivalence principle says that locally the effects of gravity are the same as the effects of acceleration in flat space. “Locally” here roughly means “nearby”. And acceleration in flat space is described by Einstein’s theory of Special Relativity. So, with the equivalence principle, you can generalize Special Relativity to General Relativity. Special Relativity is the special case in which space-time is flat, and there is no gravity.

The equivalence principle was well illustrated by Einstein himself. He said, let us consider you are in an elevator that is being pulled up at constant acceleration. There is one force acting on you, which is the floor pushing up. Now, Einstein says, gravity has the very same effect without something pulling up the elevator. And again, there is only one force acting on you, which is the floor pushing up.

If there was nothing pulling the elevator (so, if there was no acceleration) you would feel no force at all. In General Relativity, this corresponds to freely falling in a gravitational field. That’s the key point of Einstein’s insight: If you freely fall, there is no force acting on you. And in that, Einstein and Newton differ. Newton would say, if you jump off a roof, the force of gravity is pulling you down. Einstein says, nope, if you jump off a roof, you take away the force that was pushing you up.

Again, however, the distinction between the two cases is rather technical and one we do not have to bother with in daily life. That is because in daily life we do not need to use the full blown apparatus of General Relativity. Newton’s theory works just fine, for all practical purposes, unless possibly, you plan to visit a black hole.

1. Aren't all forces subject to an interpretation that makes them not forces (un-forces?) in the same manner as Einstein's claim that gravity is not a force? Every force is sliding down a slope in some appropriate choice of space.

1. No, they are not. You can measure acceleration. It's not relative, it's an absolute. If you are not moving on a geodesic (ie, if you are not freely falling), there is a force acting on you. You cannot interpret this away.

2. PaulTopping,

As Sabine said, in general relativity free falling is due geodesic form of the spacetime. Also, gravity is not like normal fictitious force depending only on another's acceleration. Its magnitude is due to your speed in relation to gravitating center too. The "force" limits double force when speed limits c.

Now, you can study if attractive force between opposite electric charges could be modelled as falling the same way...

At least the electric push force between objects seem to be real signal interaction at speed c.

3. Paul,

The force of gravity causes all objects to undergo the same acceleration (at the same point in spacetime, of course): that was the point of Galileo's famous experiments (whether or not he really did it off the Leaning Tower of Pisa!).

That is true for no other force. For example, an electric field will accelerate positive and negative charges in opposite directions.

Therefore, with gravity you can go to a frame of reference in which all the objects show no acceleration at all.

Of course, if you move a finite distance away, the gravitational acceleration may differ in magnitude or direction, and so some acceleration will still be apparent: those residual forces are the so-called "tidal forces." And, one way of viewing General Relativity is that it is the theory of tidal forces: tidal forces are real, while gravitational acceleration is relative.

For a detailed and readable (albeit eccentric) presentation of this viewpoint, see Wheeler's A Journey Into Gravity and Spacetime.

4. Eusa wrote:
>Also, gravity is not like normal fictitious force depending only on another's acceleration. Its magnitude is due to your speed in relation to gravitating center too. The "force" limits double force when speed limits c.
...
>At least the electric push force between objects seem to be real signal interaction at speed c.

What you wrote is confused: gravitational forces also do not propagate faster than light and also cannot accelerate something to faster than light.

And this thing about "limits double force"? No sense.

You are also confused about different forces' dependence on velocity.

5. Let's calculate in the frame of a gravitating body.

When slow particle moves past the effective part is curvature of time. As far as relativistic fast moving particle considered both time and space curvatures are effective - for light it's twice magnitude of bending towards a gravitating body in relation to what happens with slow timelike particle.

If gravitational falling would be dependent on signals at speed c from a gravity source, there would be tangential acceleration in direction of trajectory and no stable orbits.

6. The force of gravity causes all objects to undergo the same acceleration (at the same point in spacetime, of course)

The exception being photons.

Photons can experience angular acceleration, but not linear acceleration.

7. Euasa wrote:
>When slow particle moves past the effective part is curvature of time. As far as relativistic fast moving particle considered both time and space curvatures are effective - for light it's twice magnitude of bending towards a gravitating body in relation to what happens with slow timelike particle.

Okay, that clarifies what you meant by your doubling comment: yes, you are correct. As you imply, for slow-moving particles, the effect of spatial curvature is negligible (not quite zero, of course), and, as you approach the speed of light it becomes equal to the effect of varying rates of time. (Everyone: I know that this depends on the coordinates chosen -- Eusa and I are choosing the natural coordinates used to describe deflection of light by the sun.)

Eusa also wrote:
>If gravitational falling would be dependent on signals at speed c from a gravity source, there would be tangential acceleration in direction of trajectory and no stable orbits.

No, you are wrong on this: the force felt by the earth is from where the sun was eight minutes ago; of course, in the sun’s reference frame, this happens to be pretty much the same place where the sun is now!

I understand that you are saying this will tend to get wrecked in another reference frame, but the laws for transforming fields basically take care of this. I’ve been digging into the history of this recently (i.e., the early attempts to wed gravity to relativity), and some famous physicists early in the twentieth century did make the same mistaken assumption you are making.

A similar thing happens (and is well-known) in electromagnetism: an electron moving with constant velocity has an E field that points to where the charge is now rather than where it was, even though EM fields are only transmitted at the speed of light. Change the electron's motion right now, and the E field will still point to where the electron would have been had it continued in inertial motion, until of course the speed-of-light delay allows us to see the change in motion.

This is in fact the simplest way to see what causes EM radiation. I believe that Purcell's undergrad text, for example, discusses all this.

A similar effect happens for gravity.

Strange, I know, but that is what the math says.

I'm actually writing up a monograph on relativistic EM, so I have gone through the effect in EM in great detail.

Dave

8. Okay Dave,

Just take similar massive stars orbiting common center of mass distribution. With couple of them you have symmetric system again in the frame of the mass center point.

Now if attractive signal was delayed by speed c the force vector direction is other than diameter direction through origin... Or what is your model?

9. http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

If you want to postulate attractive signal flow from massive body, you cannot get right result via affecting directly on other body - result is a retardation.

Instead, you need to see that the "signal flow" is indeed equivalent to curved spacetime. When considering gravitons affecting only on spacetime curvature the mass distribution changes are only meaningful. At last in constant gravity you need no gravitons, only curved spacetime.

It's said usually that gravitons are always attractive (ao in Wikipedia) but when interpreted as gravitational waves you cannot define them neither attractive nor repulsive - they only change the curvature of spacetime.

10. Eusa,

>If gravitational falling would be dependent on signals at speed c from a gravity source, there would be tangential acceleration in direction of trajectory and no stable orbits.

The very short answer is that, as I assume you realized, if you were right and the speed of gravitation were infinite, this could be used to send signals faster than light and all of relativity would then be proven wrong. As you might expect, physicists thought of this issue long ago and analyzed it carefully, and, in fact, the issue you raise is well understood and does not show that gravitational influences travel faster than light. You'll be happy to know that Laplace hit upon this issue as early as 1805 (but of course could not give the correct answer, not knowing relativity).

Of course that leads us to the short answer: why then doesn't your paradox show that a finite speed of gravitation would lead to non-central forces and therefore bizarre orbital behavior? Briefly, the answer is that, even in special relativity, forces do not transform in a trivial manner between different frames of reference. If you have a radial force pointing directly towards the source – as in Coulomb's law or Newton's law of gravitation -- and then you transform that force into another frame of reference moving at velocity v, then you will find that the component of force parallel to the direction of v and the component perpendicular to v will transform in different ways so that, in the new frame of reference, the force will point to the position that the source would be in now, assuming the source continued moving with constant velocity. This is a straightforward calculation in relativity if you understand how to use four-vectors, Lorentz transformations, etc.

Of course, as you know, the force in General Relativity is not Newton's law: even for weak gravitational fields, there is the additional effect that doubles the deflection of light by the sun: this effect need not point directly towards the source. How does this fit in? Well, that additional effect is due to the curvature of space, essentially to the fact that a 2-D cross-section over a small change of radius (between r and r+dr) looks like the frustrum of a cone in terms of its (2-D) geometry. And you can make a cone by taking a wedge of a disk and connecting opposite edges: when you think you travel 360 degrees around the apex of the cone, in a sense you are only traveling a fraction of 360 degrees corresponding to the fraction of the circle represented by the wedge. In effect, this reduces the so-called “centrifugal force” in such a way that the test particle seems to curve inward. In a way, this is not a real force but more like the Coriolis effect or “centrifugal force.”

You find those explanation too vague and unconvincing? For longer explanations, I will have to refer you to outside sources. John Baez's site has a nice readable explanation by Steve Carlip. Carlip has a much more detailed, rather technical explanation on the arXiv.

You don't buy my or Carlip's explanation?

Then go through the detailed derivation of behavior in a Schwarzschild geometry in Chapter 25 of Misner, Thorne, and Wheeler, and then tackle J. Kim's “Gravitational Field of a Moving Point Particle.” And then you can move on to the issue of why there is no dipole radiation but only quadrupole from gravitational sources, the non-linearities in gravitational radiation, and a lot more.

Dave

11. This comment has been removed by the author.

12. Dave,

Thanks for response.

I haven't argued that any signals can affect at infinite speed. I said "At last in constant gravity you need no gravitons, only curved spacetime."

I repeat: the "signal flow" is indeed equivalent to curved spacetime and when considering gravitons affecting only on spacetime curvature the mass distribution changes are only meaningful.

There is no contradiction to Carlip's explanation when I say that curved spacetime need not any signalling - only changes need.

In fact, I linked to Baez's page of Carlip's explanation to show how complicated it is if you demand wannabe signals for static curvature. Easier way to think is that there is a flow of potential from surroundings into particles - not a flow of signals from particles to space.

Also, it's easier to consider gravitational waves being created by curvature changes of spacetime not by particles itself. It's a common semilocal effect, not net sum of local individuals.

Just follow the principles of general relativity and observations.

2. Einstein describes gravity as not a force, but as an effect being caused by the curvature of space and time. But in his 1972 book on Gravitation, Weinberg expresses doubt about this. In particular, on page 147: " ... the geometric interpretation of the theory of gravitation has dwindled to a mere analogy ...". Isn't it possible that gravitation IS a force, mediated by the graviton? And that spacetime is just the unstructured nothingness between events? The point is, we simply don't know whether gravity is a) a force - as described in Weinberg's book, b) some effect of physically curved spacetime, or c) something else (entropy, etc.)

1. Gravitation is an interaction, and it is (under certain assumptions) mediated by the graviton. Whether it is a force or not is a different question altogether. Weinstein in his book famously refused to use the geometric interpretation of Einstein's theory, but that is entirely irrelevant. It is an interpretation -- I think it's a useful interpretation -- but whether you use it or not plays no role for the correctness of the theory. Whether or not you use the geometric interpretation, fact is that a freely falling observer, in Einstein's theory, is not accelerated, hence there is no force acting on him or her.

2. ---fact is that a freely falling observer, in Einstein's theory, is not accelerated---

If a stone falls from roof down to the street, it is accelerating. One can measure that acceleration. Maybe it is not an example of free fall in Einstein's sense?

3. Andrey,

Newton would say the stone is accelerated, by insisting that acceleration be measured relative to the surface of the earth. But there is nothing intrinsic to the stone that would allow you to measure that (leaving aside friction, needless to say). If you put the stone in a black box and try to measure acceleration while the stone falls, there is nothing. That was Einstein's insight.

4. Only way you can manage to model the whole gravity by gravitons is that gravitons affect on space form and only on space form, not on matter particles.

5. Suppose somehow someday it is experimentally determined that spacetime is not physically structured. That is - it's not quantized; it's not physically curved; it's just the unstructured nothingness between spacetime points. Would you still consider "curved spacetime" a valid or useful "interpretation" of gravitation?

6. Bob,

We know as a matter of fact that curved space is a useful interpretation of gravitation. Ten-thousands of physicists have used it for almost a century. Even if it turns out tomorrow that it is only an approximation to an underlying theory, it will remain to be useful.

7. Well, a lot has happened since 1972. There has been a lot of work done on actually describing gauge theories, which describe all the non-gravitational interactions, in the language of differential geometry. In fact, Weinberg renounced his earlier position according to Paul Ginsparg :

"back to big steve w., when he wrote the gravitation book he was presumably just trying to get his own personal handle on it all by replacing any geometrical intuition with mechanial manipulation of tensor indices. but by the early 80's he had effectively renounced this viewpoint in his work on kaluza-klein theories (i was there, and discussed all the harmonic analysis with him, so this isn't conjecture...), one can look up his research papers from that period to see the change in viewpoint."

8. "Weinstein in his book famously refused to use the geometric interpretation of Einstein's theory, but that is entirely irrelevant."

I'm sure that Weinstein has never used any other theory of gravity either, though he experienced it after a fashion in his own downfall. :-)

Back to Steven Weinberg: Yes, he was not a fan of the geometric approach to gravity for a while, but later revised this view.

9. Maybe I'm more stubborn than Weinberg. Until someone experimentally determines that spacetime is physically structured, I will continue to think of curved spacetime as a mathematical analogy rather than physical reality.

10. Bob,

Space is a mathematical concept which has been spreaded among conscious life helping survive.

Tightly speaking only structures based on interactions can be thought as "physical reality".

Sort of simplyfied explanation: the free falling is recharged interactivity which discharges when lack of interactions. With this kind of postulates we can make try-outs in detailed physics.

3. The geodesic equation

D^2r/ds^2 = 0 = d^2r/ds^2 + Î“^r_{00}(dt/ds)^2,

has for Î“^r_{00} = ½∂_rg_{00, g_{00} = 1 - 2GM/rc^2 and dt/ds ≈ 1 (or c) this reduce to d^2r/dt^2 + GM/r^2 = 0. This is basically Newton’s second law with gravitation. General relativity and Newtonian mechanics converge for weak gravity and velocities v << c.

Newton’s second law is a strange physical principle. F = ma is the earliest fundamental equation we all learn. The force F is a dynamical quantity in that it has a measurable property and a direction in space. The mass m is a kinematic quantity that is scalar and the acceleration is a geometric quantity. It is as if dynamics = kinematics times geometry. Much of physics shares this property.

General relativity has an interpretation as a force over an extended region. The tidal acceleration of an extended body is a force. For point-like particles there is no tidal force across then, but for two or more particles interacting by some gauge force tidal acceleration or force enters the picture. In a QED setting the gauge potential is covariant in spacetime and so virtual particles can be red or blue shifted in the curved spacetime.

I trust your illness is better. I was taking a nap late this morning and my Labrador retriever got on the bed and I woke up with her there. In hugging her I found a lump on her side, which now has me really worried. This dog is a real sweetheart and I hate to think this might be cancer. Nature is not always kind.

1. "This is basically Newton’s second law with gravitation.

Not unless you assume that inertial mass equals gravitational mass, hence the more commonly known version of the equivalence principle that the two are equal.

2. Yeah, I sort of glossed over that. There are a number of EPs. I think most physicists consider inertial and gravitational mass to be equal and equivalent.

3. Yes, but then, we can follow the steps of E. Verlinde and, based on the Holographic Principle, study gravity and inertia as entropic forces, which may open up new perspectives in understanding what gravity

4. Hi Sabine, I hope you don’t mind me copying and pasting a slightly paraphrased version of something I posted in one of your earlier threads, but it seems relevant to this thread.

When it comes to the question of what gravity is all about, is it possible that it has something to do with the superpositioning and entanglement of our quantum underpinning?

For instance, a planet’s gravitational status is based upon its overall mass which, logically (from the quantum perspective), is the sum-total of all of the waveforms of a planet’s contents and features - all blending together (theoretically) into one superpositioned wave.

And when a random asteroid, for example, crashes to a planet’s surface, the asteroid’s wavefunction...

(which up to that moment was basically autonomous in the vacuum of space)

...is now subject to becoming entangled (cohered?) with the planet’s greater wavefunction.

In other words, upon contact with a planet, the asteroid’s wavefunction seamlessly intertwines itself (becomes one) with the planet’s overall wavefunction, thus becoming superpositionally enmeshed (at the “non-local” level of reality) with the planet’s phenomenal structures.

In which case, the occurrence of what we refer to as being the asteroid’s newly acquired “weight” is something that is proportional to the degree of the entanglement of its own unique waveform constituents with those of the rest of the planet.

And the point is that because the asteroid has a greater array of quantum attributes than that of a sparrow’s feather, for example, it is thus “heavier” than the feather due to a greater complexity of its entanglement with the “whole.”

And all that means is that as we attempt to move or lift the asteroid (or a bowling ball, or a freight train), we are, in essence, “tugging” on a vastly greater web of superpositionally entangled waves than those that comprise the feather...

...hence we therefore encounter a greater resistance to our effort.

Furthermore (and with the help of a rocket), if we were to send the asteroid back into space, it would simply be a situation of detangling (decohering?) its wavefunction from the greater wavefunction of the planet...

(with the degree of detangling still having something to do with distance, as per Newton’s law)

...thus restoring the autonomy of its wavefunction (and its prior weightlessness) in the vacuum.

Now I realize that what I am proposing is highly speculative, however...

...is it possible that the greater the volume and complexity of the entangled morass of quantum waves that comprise a planet’s overall wavefunction is what determines the strength of that which we call a planet’s gravity?
_______

1. Keith D. Gill wrote:
>And all that means is that as we attempt to move or lift the asteroid (or a bowling ball, or a freight train), we are, in essence, “tugging” on a vastly greater web of superpositionally entangled waves than those that comprise the feather...

>...hence we therefore encounter a greater resistance to our effort.

Ummm... Keith, young fella.

As I am pretty sure you know, things that are very, very far apart from each other (electrons or photons) can be quantum entangled with each other.

And there is no evidence, either experimental or theoretical, that such entanglement somehow involves tugging or "tugging": putting quotation marks around a word does not change nonsense into sense.

Your suggestions here are "not even wrong," in Pauli's famous phrase.

You've commented here enough that I think you do know how science works: mere pseudo-poetic word salad, that might well get published in Social Text just is not helpful in science.

There is nothing in quantum mechanics that connects at all with your suggestions.

Physicists have suggested, going back at least to Sakharov in the '60s, that somehow gravity is the residual effect of other forces. No one has really made that work in a convincing way.

If you want to try something like that, you need to learn a lot more physics than you now know.

If you are ever willing to do that, let people here know, and they can suggest some books you should spend the next decade studying.

Of course, that would requite real work........

2. Physicist Dave wrote:

Ummm... Keith, young fella.

As I am pretty sure you know, things that are very, very far apart from each other (electrons or photons) can be quantum entangled with each other.

And there is no evidence, either experimental or theoretical, that such entanglement somehow involves tugging or "tugging": putting quotation marks around a word does not change nonsense into sense.

Your suggestions here are "not even wrong," in Pauli's famous phrase.
----------------
Ah yes, thank you Mr. Condescension for making sure that everyone remains firmly contained within the tight little box of standard thinking.

(Btw Dave, young fella, I’m in my 70s.)

What? - you don’t like the word “tugging”?

Are you saying that when you reach down and pick up a bowling ball (or whatever) you don’t experience a force tugging (pulling) against your effort? (I’m sure you know what I mean, so just pick a different word that fits the circumstance – a word that you can live with.)

Furthermore, did you not read the part where I stated that I realized that what I was offering is “highly speculative”?

You clearly are not a fan of Einstein’s suggestion that...

“Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution.” - (From Wikiversity)

Einstein once imagined what it would be like to travel at the speed of light. I, on the other hand, merely try to imagine what it would be like to be fully-immersed in the quantum world prior to the collapse of the wavefunction.

I once pointed out to you the following:

“You seem to harbor a simmering hostility to any sort of philosophical speculations regarding the implications of what science is uncovering.”

To which you replied:

“You think so? Maybe if you give a particular example, I can answer more clearly.”

Well, your condescending response to my little speculative idea is a “particular example” of what I was referring to.

And btw, there is no experimental evidence that proves the existence of a multiverse, yet that doesn’t stop physicists from offering it up as a possibility.

*(Continued in next post)*

3. _______

*(Continued from prior post)*

Physicist Dave wrote:

You've commented here enough that I think you do know how science works: mere pseudo-poetic word salad, that might well get published in Social Text just is not helpful in science.

There is nothing in quantum mechanics that connects at all with your suggestions.
----------------
Again with your attack on imaginative approaches to things that are not yet fully understood by science. Do you also yell at the neighborhood kids to stay off your lawn?

I once asked you to describe the alleged superpositioned status of an electron as it transitions (in accordance with SchrÃ¶dinger’s equation) in the space between the slitted wall and that of the measuring screen in the double slit experiment?

To which you responded with:

“Have I mentioned that I like to reply, "I don't know"?”

...And...

“Humans need to acquire the courage to say, "I don't know."”

Now even though I highly doubt that you actually “like” to reply - “I don’t know,” that was nonetheless a refreshing display of humility.

However, in light of your recent comments, are you now implying that the idea of “superposition” is completely understood, and that everything that is taking place within the quantum realm is now fully visualizable and scientifically explicable? – including gravity?

In other words, are you now saying that you, Physicist Dave, so fully understand the workings of gravity and the quantum underpinning of the universe, that you absolutely know (beyond any shadow of a doubt) that what I have speculatively suggested is indeed pure and utter nonsense?

Physicist Dave wrote:

Physicists have suggested, going back at least to Sakharov in the '60s, that somehow gravity is the residual effect of other forces. No one has really made that work in a convincing way.

If you want to try something like that, you need to learn a lot more physics than you now know.

If you are ever willing to do that, let people here know, and they can suggest some books you should spend the next decade studying.

Of course, that would requite real work........
----------------
Yes, that would indeed require real work, and I highly respect anyone who has endured the process, especially someone like Sabine (and even you, Dave).

However, do you know what else requires real work? – Reaching a level of consciousness to where you are able to realize that all humans are basically sleep-walking through life.

I’m talking about a level of consciousness that allows one to see that humanity’s piddling efforts to understand the ultimate truth of reality is (as mentioned in an prior conversation) the metaphorical equivalent of amoebas in a petri dish trying to figure out the human level of reality.

Yeah, yeah, I know, more of my inane and useless offerings of “pseudo-poetic word salad.”

Nevertheless, seeing how your highly restricted diet of the meat and potatoes of physics has only been feeding your left brain and has caused your right brain (your imagination) to wither, then perhaps a little salad might do you some good.

Maybe you’ll even experience an Einsteinian moment of imaginative inspiration, instead of voluntarily limiting yourself to the mundane expectations of a “shut up and calculate” frame of mind.

4. Keith D. Gill:

When non-scientists suggest some "scientific" idea that is obvious nonsense, and a scientist bothers to point out this fact, the non-scientists tend to get very annoyed.

I have experienced this more times than I can count in the last forty years, and anyone who follows this blog knows the same thing happens to Sabine.

Most scientists have learned from such experiences to simply not respond at all to avoid the hassle.

I, alas, seem unable to learn that lesson!

The reason that science has progressed so rapidly during the last five centuries and has given us the only non-obvious, relatively reliable insights into the nature of reality that humans possess is that science is quite ruthless about shooting down ideas that do not agree with observation or experiment or that, even worse, cannot in principle be tested via observation or experiment.

I know that you do not like this. Most people do not like this. Most people really do not like science: how many college students voluntarily choose a hard-core science course -- not "physics for poets" -- as an elective if they are not required to?

On the other hand, lots of people (including scientists) choose real history or music or literature or philosophy courses even if they are not required to.

No, most people do not like science.

You wrote:
>Furthermore, did you not read the part where I stated that I realized that what I was offering is “highly speculative”?

Of course, I did. And your speculations are utter nonsense. And, as a scientist, I said so.

That is how science works. And you just do not like how science works.

Keith also wrote:
>What? - you don’t like the word “tugging”?

No, what I do not like is its misuse.

I realize that what happened is that physicists' use of the word “entanglement” brought to your mind some image or metaphor – perhaps of entangled threads or strings? – that made you think that entangled quantum systems are somehow “tugging” on each other.

But you're wrong.

There are lots of ways of seeing you are wrong. For example, entanglement effects are not limited by the speed of light (that is the whole point of the idea and what makes it interesting and intriguing). On the other hand, gravitational influences are limited by the speed of light.

You were misled by nomenclature: if instead of “entanglement,” we had used the more precise and revealing phrase of “super-luminal anomalies of non-classical probability,” I doubt you would have offered your speculations at all.

Now, if you were really interested in science, you would politely thank me for shooting down your nonsense because you would have learned something.

Alas, I have learned from experience that you won't.

Keith also wrote:
>I’m talking about a level of consciousness that allows one to see that humanity’s piddling efforts to understand the ultimate truth of reality is (as mentioned in an prior conversation) the metaphorical equivalent of amoebas in a petri dish trying to figure out the human level of reality.

You have too much contempt for the human race.

In the four-and-a-half billion years in which this planet has existed, we are the first beings who have transcended a mere struggle for survival and come to understand a great deal about our own origins, the origins of this planet, and, indeed, the universe itself.

The “level of consciousness” you defend is the inferior level of consciousness that prevailed among human beings before the scientific revolution.

Real courage, true enlightenment, an objectively higher level of consciousness consists of a willingness to face reality as it is, to ruthlessly reject ideas that make humans beings feel comfortable but that do not actually correspond to the universe as it really is.

I know we are in a transitional period where many people manage to cling to the sorts of perspectives you are peddling.

But, in the end, science provides the only non-obvious, relatively reliable knowledge of reality that human beings possess.

5. PhysicistDave wrote:

Keith also wrote:
>What? - you don’t like the word “tugging”?

No, what I do not like is its misuse.

I realize that what happened is that physicists' use of the word “entanglement” brought to your mind some image or metaphor – perhaps of entangled threads or strings? – that made you think that entangled quantum systems are somehow “tugging” on each other.

But you're wrong.
----------------
I fully admit that I could be completely wrong in all areas of my speculations about life and reality. Are you willing to admit the same?

And just to clarify, I never said that quantum systems are “tugging on each other.” No, I said that as we lift an object such as a bowling ball, we can feel something tugging against (pulling back on/resisting) our effort.

For the life of me, I cannot fathom why you are making such a big stink over the word “tugging.”

You are misapprehending what I meant, so please, just get over it already.

PhysicistDave wrote:

There are lots of ways of seeing you are wrong. For example, entanglement effects are not limited by the speed of light (that is the whole point of the idea and what makes it interesting and intriguing). On the other hand, gravitational influences are limited by the speed of light.
----------------
I am well acquainted with the idea of how the connection between entangled quantum particles is alleged to be superluminal (indeed – instantaneous) as opposed to the effects of gravity being limited to light speed. However, none of that, as far as I can tell, has any relevance to the point I was making.

Not that I expect you to accept any of this, but the speculative gist of my original post is based on the idea that if it is indeed possible that there is an instantaneous (literal and physical) connection between particles that were once in contact with each other...

(and according to the implications of the Big Bang Theory, all quantum phenomena were once in contact with each other in the initial crunch)

...then it stands to reason that through this real and instantaneous interconnection, anything we do here on earth (lift a bowling ball, for example) might - (THAT’S “MIGHT”) - have some extremely subtle (yet instant) affect on literally everything else throughout the universe (at the quantum level).

In which case, when we do feel the weight of a bowling ball when lifting it, my slack-jawed (non-scientist) THEORY suggests that we are in essence feeling the resistance from pulling against, if not the entire universe,...

...then at least from the more immediate (less subtle) connections between the quantum constituents that underpin the bowling ball and that of the quantum constituents that underpin the rest of the phenomena that comprise our little planet.

I mean, “something” that is basically inexplicable to science (right now we call it “gravity”) is magnetically adhering us to the surface of this spinning orb. And if the existence of gravity is not purely derived from the algorithmic processes churning away in the informational underpinning of the universe, then what else could be its cause?

To some extent, even Sabine, with her belief in super-determinism, must in some way accept the possible existence of a real and tangible (physical) connection between all quantum phenomena throughout the universe. Otherwise, how else could there exist an almost omniscient coordination between everything to the point where super-determinism could even be possible?

Again, I openly admit that I could be utterly wrong about all of this, so you don’t need to remind me.

(P.S., I love science. And no, I do not have contempt for the human race. I only have contempt for any and all scientific theories that try to assert the primacy of matter over the primacy of life and mind.)

6. Keith wrote to me:
>I fully admit that I could be completely wrong in all areas of my speculations about life and reality. Are you willing to admit the same?

No, because I actually know lots of things, and you do not.

And the problem is that you only admit that you "could" be wrong; you cannot bring yourself to admit that you are wrong.

Keith also wrote:
>In which case, when we do feel the weight of a bowling ball when lifting it, my slack-jawed (non-scientist) THEORY suggests that we are in essence feeling the resistance from pulling against, if not the entire universe,...

>...then at least from the more immediate (less subtle) connections between the quantum constituents that underpin the bowling ball and that of the quantum constituents that underpin the rest of the phenomena that comprise our little planet.

Yes, I understand what you were saying, and, yes, it is indeed ridiculous for reasons I pointed out above.

Keith also wrote:
> I love science.

No, you love saying you love science. But when an actual scientist tries to teach you some science, you do not love it at all.

Keith also wrote:
> I only have contempt for any and all scientific theories that try to assert the primacy of matter over the primacy of life and mind.

I certainly have not asserted "the primacy of matter over the primacy of life and mind." Nor do any of the theories I have been talking about do that.

In fact, I have said many, many times that I do not think anyone (yet) understands the nature of consciousness, and I do not see how physics as it now exists can explain consciousness.

No, the problem is that for some bizarre reason you seem to think that well-established scientific theories somehow "assert the primacy of matter over the primacy of life and mind" when they do nothing of the sort. And you then repeatedly and aggressively try to assert the primacy of mind over matter in ridiculous ways, and when anyone tries to point out that you have no idea what you are talking about, you get belligerent.

You hate science. Science is not about just oohing ans aahing over pictures from the Hubble Deep Field. Science is about aggressively trying to disprove hypotheses that do not agree with the evidence.

And you do not like it when your extremely flaky hypotheses are shown to be wrong. You hate science.

7. Are you guys seventy or seven?

8. I wish I could downvote Greg's comment.

5. The equivalence principal has always bothered me. Given two bodies in free space. If Able is accelerated by a gravitational field he will not notice it. Betty on the other hand if she is accelerated she will feel the force on her body when a rocket engine fires.
I have always suspected that quantum physics was looking in the wrong place for the graviton. Inertial mass seems to be tied to the existence of virtual particles and tied to the geometry of space.
One accepts that Quantum Physics and Generally Relativity calculations work when appropriately applied. One also accepts that an architects design for a house will get it built. Neither one is perfect but it seems to get done.

1. Lockley wrote:
>The equivalence principal has always bothered me. Given two bodies in free space. If Able is accelerated by a gravitational field he will not notice it. Betty on the other hand if she is accelerated she will feel the force on her body when a rocket engine fires.

You misunderstand: Einstein's point is that free fall in a gravitational field is not really acceleration: it is the natural unaccelerated (AKA "inertial") motion. When you are just standing there on the ground, the earth is in fact causing you to be accelerated upwards compared to your natural state of motion (i.e., free fall).

I know that is counter-intuitive, but it seems to work very, very well in terms of quantitative predictions.

What is surprising is that inertial motion here in the USA and inertial motion, say, in Australia are not inertial relative to each other. The definitions of inertial motion in different places conflict. Putting that into detailed mathematics just is the Einstein field equations.

It is mathematically consistent, it works nicely empirically, so life is good.

Nature does not care that humans find it counter-intuitive.

2. The equivalence principle can be seen in Newtonian physics, or Newton’s second law with gravitation. Einstein’s genius was in illustrating what was in everyone’s hands, but they did not know it.

If you have ever read the fanciful novel by Jules Verne, From Earth to the Moon you can see some of this. He had his crew fly to the moon on a giant cannon shell, where one obvious problem is the detonation and rapid acceleration of the shell would compress these people into pulp. Ignoring that the shell had two floors. One floor was at the base of the shell for the journey to the moon, and then after orbiting the moon the top end of the shell had a floor. There is clearly a disconnect here with physics. The thinking was the crew would stand on the base floor and the top opposite facing floors for the two parts of the journey. Of course, even within a Newtonian perspective the acceleration of gravitation on the shell and its inhabitants and contents are equal. This means there is no relative acceleration between them, which means the crew would experience weightlessness.

The crew of the international space station and any orbiting craft is falling with their craft or station and moving so fast they keep missing the Earth. The Earth curves away from their fall so they maintain an orbit. Orbits in the cislunar or interplanetary regions of space are much the same. So, weightlessness can be seen within Newtonian mechanics. Einstein realized this and said this means there is an equivalency between free falling frames and a frame in flat spacetime extremely far from any gravitating body. There is an accelerated version as well; the experience of a force in an accelerated frame is equivalent to being held fixed in a gravitation field by some force such as the bulk resistance of material comprising a planet. There are several varieties of this EP, and they generally focus on the equivalency of inertial and gravitational mass.

Einstein in framing physics according to this EP paved the way for looking at physics according to geodesic flows rather than according to forces or even potential energy. General relativity in some ways removes the dichotomy between kinetic and potential energy that is standard in Newtonian mechanics.

6. Why is there no weight in the direction of the past or the future? Why do forces work in spatial dimensions but not in the temporal one?

1. They do, and since force is the (proper) time derivative of momentum, and the temporal component of momentum is energy, the temporal component of force is... power!

>Why is there no weight in the direction of the past or the future? Why do forces work in spatial dimensions but not in the temporal one?

Properly speaking, forces are "four-vectors" and do indeed have a time-like component. However, it turns out that how that works is related to your velocity: tell my your velocity and the spacelike components of the four-vector force acting on you, and I can calculate the time-like component of force. So, you do not need to inform me of the value.

There are various other ways of viewing what is happening: in any case, the answer is, "Sure, space and time are intertwined in relativity, and if something like force shows up in space it probably has a time-like aspect."

But, in the case of force, for various reasons, you can often, for practical purposes, ignore that time-like component.

To understand all this in real detail, you'll have to learn the mathematical details of Special Relativity: I recommend Taylor and Wheeler's Spacetime Physics, now available online.

7. This seems like a good post to bring up the recent discussion on whether antimatter can potentially "fall up" (something about CERN ALPHA antihydrogen trapping).

To quote Wired:

> Standard physics theory actually does not predict how antihydrogen would behave in Earth’s gravity, and some researchers speculate that it might even fall upward.

"Standard physics theory,"namely, General Relativity, actually *does* predict that, even if antihydrogen had negative mass, it would still fall down, because it follows geodesics, and there is no geodesic that presents as apparent upward acceleration on the Earth surface. Any confirmed deviation from that would make us rethink classical General Relativity, no quantum gravity required.

1. Sergei,

That's correct, it is extremely hard (ie, implausible) to change GR and the Standard Model so that anti-matter falls up. I wrote about this here.

2. Yes, I remember that post, the pink flying pig and all! Very nicely explained the situation, too. Here I simply wanted to emphasize that the detectors that measure how antihydrogen behaves in Earth's gravity would not be able to tell between positive and negative mass. And non-equivalence of gravitational and inertial masses is not even a possibility in classical General Relativity, there are only massive and massless objects. I have no idea what kind of a change to non-perturbative GR would be required to accommodate such a non-equivalence.

The Standard Model can just shrug and say, "well, hydrogen has negative mass, it's just a parameter in the Hamiltonian, let's see what the consequences are," because it doesn't care about the gravitational mass at all. There is no equation in GR where you can do anything similar without also flipping the sign of the gravitational mass", because there is only one mass.

Maybe I am belaboring it too much, but it seems that the HEP crowd does not appreciate how vain is the hope for antihydrogen falling up. It's even less likely than it moving faster than light, because the latter can at least be accommodated, in principle, by creating an exotic equation of state. It would break the initial value problem, but at least it can be written down. No such luxury is possible for the former.

3. Sergei,

One problem not mentioned very often is, if antimatter has negative gravitational mass, what about particles that are their own antiparticles, such as photons and the Ï€0?

The obvious answer would be that they must have zero gravitational mass (the only way they can have the negative of their own gravitational mass). But, we know that the path of photons is bent by the sun, so they must not have zero passive gravitational mass.

And, if their passive gravitational mass is non-zero but their active gravitational mass is zero, then they will not pull on the sun when the sun pulls on them, and momentum conservation will be violated.

(Of course, we know that both photons and pions have positive inertial mass from numerous particle experiments.)

N.B.: I am using "mass" in the old-fashioned sense of "relativistic mass" which is what is actually relevant for gravity (AKA "energy").

To be sure, if there were particles with negative inertial mass-energy, as well as having both kinds of negative gravitational mass-energy, life would be more interesting: they would still fall down, but they would repel normal matter.

But there has never been any sign of such particles, and, indeed, they probably would lead to the vacuum being unstable.

Note that I am not assuming that gravity is tensorial rather than vectorial bur merely that the sun does indeed attract photons (known observationally) and that momentum is conserved.

It is very hard to propose in detail how antiparticles could behave in a negative way gravitationally that is not disproven by observations that already exist.

As far as I know, no one has really done that in detail: e.g., answering my question here about photons, deflection of photons by the sun, conservation of momentum, etc. If they cannot answer such questions, they do not really have a theory, merely words.

Just saying, "Hey, maybe antiparticles behave negative gravitationally" does not cut it. That needs to be fleshed out, and I do not think it can be.

4. Dave, photons have zero rest mass. There is no separate gravitational mass in general relativity. Your argument about bending therefore non-zero mass stems from your poor understanding of general relativity and is a common misconception. All your other questions betray similar lack of understanding the basics and are not novel questions in any way, they are addressed in most GR textbooks.

You do have one interesting point, however. Mesons that are their own antiparticles, cannot have just positive or just negative mass if somehow antihydrogen is found to have negative mass. But it's not out of the question that mass is not an eigenstate of these mesons, and so they might be in a superposition of a positive and negative mass, and possibly oscillating between the two, the way neutrinos do. This is extremely far-fetched, however, but still more likely than antihydrogen falling up.

5. There is a distinction between mass and matter. Still, mass is positive-definite.

A recent experiment with anti-hydrogen showed it behaved just like normal hydrogen (sorry, I don't remember the details ...)

6. When I first heard about the notion that anti-matter might fall upward, as a consequence of it having negative energy density, my first thought was that couldn't be right since the annihilation of a particle with its anti-particle produces a pair of high energy photons, both having positive energy. Another idea involving negative energy density - a propellantless propulsion system with a ball of negative matter chasing an equal sized ball of positive matter was first broached by Sir Hermann Bondi in the 1950’s. With the system having zero effective mass and zero total energy it can accelerate indefinitely across the universe. What’s interesting about this setup is that, generically, it’s almost the same as Alcubierre’s hypothetical warp drive with a region of expanding spacetime (negative energy density) behind the spaceship and a region of contracting spacetime (positive energy density) forward of the ship. The difference is the positive mass energy of the ship has to be pushed forward by the warp, and thus requires energy.
Am glossing over a lot of detail here, as I’m eager to get out on my bike to catch some positive-energy solar photons, adding to those happily absorbed in 110 miles of pedaling in 8 days.

7. Sergei wrote to me:
>Dave, photons have zero rest mass. There is no separate gravitational mass in general relativity. Your argument about bending therefore non-zero mass stems from your poor understanding of general relativity and is a common misconception. All your other questions betray similar lack of understanding the basics and are not novel questions in any way, they are addressed in most GR textbooks.

Sergie, young fellow, I specifically indicated that I was, as I said, "using 'mass' in the old-fashioned sense of 'relativistic mass' which is what is actually relevant for gravity (AKA 'energy')" Photons do of course have energy, and, in the old days, we would have said their "relativistic mass" was E/c^2.

In fact, gravity couples not to the rest mass but rather to the stress-energy tensor, which does indeed have non-zero values for photons, which is why photons do indeed have their paths altered by gravity.

And, photons do indeed serve as a source for gravity for the same reason. This is known to everyone (except people like you are ignorant of GR) and is a well-known fact in all of the standard textbooks. It is, for example absolutely central to the cosmology of the early universe.

Which you would know if you had any acquaintance whatsoever with the standard textbooks.

If you cannot understand that, you should not be talking about science in public. You embarrass yourself.

I took GR from the Nobel laureate Kip Thorne. I took QM and intro to elementary-particle physics from the Nobel laureate Dick Feynman. And I took QFT from the Nobel laureate Steve Weinberg.

And, you think I do not know what is in the standard textbooks! Sergie, I studied the subjects under the people who wrote the standard textbooks!

So, you have just proven beyond any shadow of a doubt that you are not acquainted with those standard textbooks.

What game are you playing, Sergie?

8. Sergei that was quite a turn ðŸ˜‚

8. But then there's the pesky little problem of reconciling quantum theory's graviton/boson approach to gravity with GR's curvature of space interpretation of gravity...

1. I don't know what you think the problem is. Speaking of gravitons makes only sense if you quantize perturbations around a background. The background might well be curved. How you get from the perturbative to the non-perturbative case is an unsolved problem, but that doesn't mean the perturbative limit itself is problematic.

9. bee,

is the graviton/QFT and string theory description of gravity able to reproduce all aspects of GR including equivalence principle and time dilation?

if GR picture is entirely correct as curved spacetime, would this falsify string and graviton picture

1. Yes, if GR is correct to arbitrarily small scales or there is no graviton, or the perturbative limit is incorrect, then string theory would be wrong. Which is why I remain perplexed that no one is even trying to make predictions for the quite possibly soon be testable weak-field limit.

2. does graviton/ perturbative QFT make any predictions that differ from GR that could be tested?

how does graviton/ perturbative QFT/ string M theory address the problem of time in QG?

10. "I was sick last week and lost like 10 pounds in 3 days"

are you feeling better? could it be the coronavirus?

1. Yes, thanks, I am better now. No, it's not coronavirus. I have no flu symptoms at all, no fever, no cough, not even a runny nose.

2. that's good to hear.

11. I 4 1 think that, spacetime is a superfluid (emergent property), created by collective behavior of virtual particles of quantum vacuum & spacetime curvature (aka gravitational field), is actually just the pressure distribution of the spacetime superfluid!

(If so then one may ask, why objects w/ mass have higher pressure around them?

IMHO, it must be because, all real particles have a (denser) cloud of virtual particles around them (according to Quantum Theory)!)

2. Spacetime superfluid is not my own idea:

https://en.wikipedia.org/wiki/Fluid_solution
https://en.wikipedia.org/wiki/Superfluid_vacuum_theory

Even Einstein himself seems to took the idea quite seriously:

https://physicstoday.scitation.org/do/10.1063/PT.6.4.20190717a/full/

3. Someone, somewhere may care what Einstein thought, but no one here cares what you think about what Einstein thought.

12. Wow. Super-interesting idea. For me, gravity is caused by very social, mass loving pink fluffy unicorns: they gather around mass, you see, the greater the mass, the more unicorns. And since they are very social, whenever they see other pink fluffy unicorns, they want to get to them, pulling and pushing the mass they are attached to with them. This is what we call "gravity". Only when the distance gets too great, the unicorns don't bother: this is why the universe is falling apart. I only have to work out some kinks with the maths so everyone can see how good my theory is: so should you.

13. @Keith D. Gill and PhysicistDave: The idea that entanglement is involved is spot on. In fact, the equivalence principle should be expressed according to that. The accelerated frame in relativity is in a Rindler wedge bounded by past and future horizons, a split horizon, where the future horizon obscures any signal to the future of it. This horizon occurs at a distance d = c^2/g in the direction away from the direction of acceleration. There is then Unruh radiation with temperature proportional to 1/g due to the production of radiation. This temperature increases 1K for every 10^{19}m/s^2, so it is not a huge effect so astronauts are not toasted by firing their rocket thrusters. This radiation, which is a transformation of a pure vacuum in an inertial frame to vacuum plus a statistical distribution of bosons, is decoherent. Hence if on that accelerated frame there is an entangled system this radiation will define a thermal bath or environment that will remove the quantum phase of this entanglement.

A statement of the equivalence principle then should be seen as any development of a system that maintains the entanglement of that system. When this is a reference to spacetime as a geometric system this then defines a geodesic as a path where the entanglement of a local portion of a system is maintained. This means if I have an EPR pair with one on a geodesic in curved spacetime, then the EPR pair maintains this entanglement on this or any other geodesic. Spacetime as a large N-entanglement or condensate of states could be similarly referenced, where the EP would have any entangled system maintain a quantum phase according to unitarity. I think in this setting the equivalence principle is then equivalent to or in some sort of duality with unitary principle.

In this setting I think it is possible to work with matters of the firewall. The firewall is a gadget imposed to prevent in effect a W entanglement comprised of bipartite entanglements from evolving into a GHZ tripartite entanglement. These two entanglements are separated by a 3-tangle topological obstruction. This obstruction is a manifestation of unitarity. Black holes emit Hawking radiation by absorbing a virtual particle in an EPR pair with another particle that escapes to infinity. In this way the virtual particle becomes “real” with the absorption of mass from the black hole. The problem occurs when the black hole has about half of its remaining mass. When this happens, a Hawking radiation boson is now not only entangled with the black hole but previously emitted Hawking radiation. This means a bipartite entanglement with the old Hawking radiation is converted into a tripartite entanglement. This is prohibited by this 3-tangle obstruction and is sometimes referred to as the monogamy principle. The firewall is a fix or hack that says the equivalence principle is violated at some point, so the horizon of the black hole becomes a sort of singularity that demolishes all which reach it.

However, if the EP and unitarity are equivalent to each other or in a duality, something similar to Bohr’s complementarity principle between quantum and classical observables, then this firewall is a manifestation of an incomplete analysis of this problem. Hawking radiation theory treats spacetime separately, and the response of spacetime to the emission of Hawking radiation is the so called backreaction. This is a semi-classical reasoning that is approximate.

1. Well, Lawrence, yes, I know that "entanglement" is involved in Unruh radiation, the firewall, and all that.

But, Keith's explicit suggestion was that a (supposed) greater level of entanglement is what causes a bigger mass to exert a greater gravitational pull.

Nothing you said addressed that, and if you think there is something to Keith's idea, by all means write it up as a real paper, and of course, give Kieth credit. Indeed, why not co-write the paper with Kieth and submit it to PRL?

Okay, I'm being cruel: we both know that what Keith wrote is nonsense. Of course we know -- everyone knows! -- that a full quantum theory of gravity will include entanglement simply because QM necessarily includes entanglement.

But, you will note that Keith was specific enough to be clearly saying something else.

2. I didn't say he was all right. A larger mass does have more quantum states in possible entanglements. However, that is as you say not relevant. The only thing that is relevant is that entanglement of a system is not changed by spacetime interaction. There is nothing in particular that makes bulk mass some measure of entanglement entropy. However, things might get a bit subtle with the Higgs field and how the condensate of Goldstone bosons has its quantum numbers taken up by the W and Z bosons and with fermions through Yukawa coupling.

3. Lawrence Crowell wrote to me:
>I didn't say he was all right. A larger mass does have more quantum states in possible entanglements. However, that is as you say not relevant.

Yeah, the only thing I am really trying to get across to Keith is that it is a good thing in science to shoot down bad hypotheses so that the not-so-bad hypotheses survive to fight another day.

I actually think this is the biggest problem in science communication. If it turns out that the challenge to the supernova estimates for the cosmic acceleration parameter stand up to criticism, that is good. If criticism shows decisively that the accepted analysis is correct, that is also good.

Either way, everyone wins.

I think this is very hard for most non-scientists to grasp. At least in the US, there are social norms that say that you do not point out how ridiculous someone's religious beliefs are (transubstantiation, the Book of Mormon, Young Earth Creationism, etc.) even if they are obviously bonkers.

And lots of non-scientists want to import that norm into science: no matter how absurd their "hypotheses" are, they think we scientists are bad people if we point out the absurdity.

They truly do not understand that they are advocating the death of science.

And, I think it is important that somehow we try publicly to explain that.

4. If you want to save science you should be battling against Celebrity Physicists Who Write Sensational Books With Outrageous Claims Just To Make Money and See Themselves On TV, not some random dude in one small corner of the blogosphere.

5. "social norms that say that you do not point out how ridiculous someone's religious beliefs are (transubstantiation, the Book of Mormon, Young Earth Creationism, etc.) "

Start with the most common crazy beliefs - existence of gods, creation, virgin birth, resurrection. Let's not pretend that these are not any less insane.

"even if they are obviously bonkers."
All religions are obviously bonkers.

The problem is that there are physicists with these crazy beliefs who are deliberately trying to corrupt the field. Doctors who promote anti-vaxxing are rightly struck off the medical register. Similar actions should be taken against physicists who use their credentials to try to promote "spiritualism" or primitive superstitions.

6. Greg Feild wrote to me:
>If you want to save science you should be battling against Celebrity Physicists Who Write Sensational Books With Outrageous Claims Just To Make Money and See Themselves On TV, not some random dude in one small corner of the blogosphere.

And, how I am I supposed to "battle" against these evild scientists? Burn their books in public?

Sorry, I don't burn books.

Guys like our friend Keith probably do get their goofy ideas from people of the sort you criticize. I think that engaging people like Keith or you or antooneo with a bit of "tough love" one-on-one may be the best I can do given that I have no intention of burning books!

7. Steven Evans wrote to me:
>Start with the most common crazy beliefs - existence of gods, creation, virgin birth, resurrection. Let's not pretend that these are not any less insane.

Well, yeah.

But, you know, lots of people -- ranging from Richard Dawkins to Biblical scholar Bart Ehrman -- have pointed this out. And it seems to me they know more about this than I do.

What I do know about is physics, and some related fields in math and engineering. I'm not shy about saying that I think existing religions are all preposterous, but perhaps my efforts are best spent talking about disciplines on which I am indeed expert.

Also, I think you and I may have an actual difference of opinion on some of this. I think that Young Earth Creationism or the Virgin Birth are just as certainly false as the flat-earth theory.

But I am not as critical of all sorts of theism, pan-psychism, philosophical idealism, etc. For example, I think theism is probably not true (God seems to hide himself awfully well), but I do not think it is certainly and trivially false.

I just do not know the ultimate nature of reality, the full role of consciousness in the universe, etc. I am pretty sure no one else does either.

Dave

8. "But I am not as critical of all sorts of theism, pan-psychism, philosophical idealism, etc. For example, I think theism is probably not true (God seems to hide himself awfully well), but I do not think it is certainly and trivially false."

Panpsychism, as discussed in a previous post, claims to be an ontology of matter as well as an explanation of consciousness. But it provides not one single fact about matter, or anything else, and makes no steps to explaining consciousness. It is an utterly meaningless and useless theory. It tells us literally nothing.

Theism is certainly and trivially false because humans only consider that the universe might have been willed into existence by a conscious being because we are conscious beings that will things. It would be like elephants thinking that the universe had been blown out of the trunk of a great elephant-like being. It's not just certainly and trivially false, it's a ludicrous idea.

"I just do not know the ultimate nature of reality, "
Quarks, so far.

"the full role of consciousness in the universe"
Consciousness is a function that evolved in brains.

9. Dave asked: What can I do (besides attack Keith) ?

- Write a book.
- Write a Monograph.
- Write a blog.
- Write the Department of Energy.
- Write the American Physical Society.
- Write the New York Times.
- Write the Celebrity Physicist.
- Write the publishers. Boycott.

- Write a talk and go on the lecture circuit and to conferences...

10. Physicist Dave4:49 AM, March 12, 2020

"And, how I am I supposed to "battle" against these evild scientists? Burn their books in public? "

Physics Today does book reviews and claims to have rigorous standards, so it would make sense if they pointed out egregious errors in popular physics books. But in the case of "A Fortunate Universe", as an example plucked out at random, they got one religious lunatic to review a book written by another religious lunatic. People like the editor of Physics Today need to try and do their jobs properly. How could a review by one lunatic of the ravings of another lunatic make it into the pages of a supposedly serious physics publication? Why don't we appoint psychopaths to act as judges in murder trials?

11. Greg Feild suggested that I:
- Write a book.
- Write a Monograph.
- Write a blog.
etc.

Greg, I (sincerely) appreciate your faith in my ability to move public opinion!

However, while I have a number of opinions that I think are (mostly) correct, I am not so confident of my ability to judge what and how the public is thinking so that I can address the public in a convincing manner. The problem is that writing books, giving lectures, etc. is rather one-way communication.

Whereas, going back and forth with you, Steve, and others here gives direct feedback one-on-one. For better or worse, I have a keener sense of where you and Steve are coming from due to our direct interaction than if you were merely a reader of a book I write.

Furthermore there is a cost-benefit trade-off: writing a book or monograph is just step one -- you still have to find a publisher. And writing a book is hard work. Conversely, writing the APS is easy, but not likely to have any result.

As I have said earlier, I am in fact working on two monographs: one on relativistic electromagnetism that shows simpler calculational techniques to get radiation/acceleration fields and the other a book that shows simpler calculational techniques to get the Schwarzschild solution, the standard cosmological solutions, etc.

I think it is important for physicists to take the solid results that we have and figure out how to calculate those results as simply as possible. Oliver Heaviside may not have discovered Maxwell's equations, but, if we had to work with the equations in Maxwell's form, instead of the enormously clearer form devised by Heaviside, it would be very hard for undergrads to grasp them at all.

So, it seems to me that the task of taking complex ideas and putting then into a (mathematically correct) form that can be understood by undergrads and, if possible, even by freshmen or bright high-school students is much more important than is generally realized.

Few of us physicists can be Maxwell, but we should at least do what we can to emulate Oliver Heaviside.

Anyway, neither I nor Sabine can do everything. I am actually stunned at the success she has had: I hope you have read her book, which does a great job of talking about serious issues while still giving human pictures of the guys she interviewed (Sabine gives a very authentic sense of the two interviewees I myself knew: Joe Polchinski and Steve Weinberg).

We each do what we can. And, sometimes, we may find the best we can do is engage with our fellow citizens one on one.

Dave

>People like the editor of Physics Today need to try and do their jobs properly. How could a review by one lunatic of the ravings of another lunatic make it into the pages of a supposedly serious physics publication?

Well... if you have been following Sabine's (and also Peter Woit's) blog long enough, you know that there are problems in natural science.

The good news is there always have been. The bad news is that I suspect it is worse today.

You can see the fundamental problem if you compare pure science to the business of running a restaurant. If I run a restaurant that churns out food that routinely makes patrons sick, even if I bribe the county health department to let me stay open, I will go bankrupt since people will not spend their own money to eat rotten food.

But there is no direct consumer of scientific research who takes money out of his own pocket to pay the scientist and who is also in a good position to judge if the product produced by the scientist is rotten.

Most research in pure science is financed by the government and, as you know, if you think some scientist who is funded ultimately through taxes is doing lousy work, you do not get to withhold your tax payments in protest!

But even if science were funded through voluntary charitable contributions, most philanthropists (Warren Buffett, Tom Steyer?) do not know enough about science to judge cutting-edge research for themselves. (The obvious exception is Jim Simons, but hard-core pure math or physics people who are incredibly rich are a bit rate!)

So, we trust scientists to judge each other, with limited external constraints.

And that can lead to very unfortunate perverse incentives, which Peter and Sabine have done a good job of documenting. (For an early prediction of the problems we face today, see Leo Szilard's 1948 satirical short story, "The Mark Gable Foundation.")

>Why don't we appoint psychopaths to act as judges in murder trials?

Don't we? I don't know if you have noticed but our system of justice, for somewhat similar reasons, does sometimes produce rather perverse results! The good news is that usually the issues before courts are comprehensible to ordinary people, and most jurors do seem to take their roles seriously.

Anyway, the point is that the incentive structure today in pure science is a bit like, say, the defense industry. And the institutional structure does not give us scientists who are critical of the system much power to do anything about it.

Sabine actually is having an impact, but I suspect that this will not result in her being asked to head the physics department at Harvard or Stanford!

Dave

13. PhysicistDave 4:19 AM, March 13, 2020

Dave, do you agree that theism is clearly just a primitive anthropomorphic myth about the origin of the universe? The Bible states that God made man in His own image, but clearly the reverse is true. How could such a ludicrous idea not be certainly and trivially false? We all understand that Zeus, Heracles, Harry Potter, Gandalf, etc. are fictional characters. How could any rational person not know 100% that God, Allah, etc., are also fictional characters?

14. Hello Dave

I appreciate your polite and measured response!

The Irony is delicious.

Greg

15. At 5 AM, March 12, 2020, PhysicistDave said: "I just do not know the ultimate nature of reality, the full role of consciousness in the universe, etc. I am pretty sure no one else does either."

Hopefully this isn't off topic, but I think it is relevant to what Dave said. At just after 4 AM, yesterday morning, I woke up from a vivid dream to use the bathroom. In the dream I was in what seemed like a bakery department, with a woman dressed in white overalls talking about cheese Danish pastries, as she worked on a roll-a-round rack, with multiple shelves, filled with pastries, and about 6 feet tall. She then gave me the number 9828, which I assumed was a code for that particular pastry. The number was so specific, and clear in the dream, that I jotted it down immediately on awakening, thinking perhaps it's a clue to some lottery game.

I went on line to check if New Hampshire has any 4 digit games, and they did, it's called "Pick 4". I drove to the nearest store, arriving 8 minutes before the 1st of two Pick 4 drawings that day. Unfortunately, I had never played this game before and filled out the ticket incorrectly, so missed the 1st of the two drawings. The clerk consoled me saying that the evening drawing was many hours away. Finally I had my ticket in the exact order of the dream, but wished I had chosen the any order option. It cost a dollar.

This morning I checked the number for last night's drawing. Interestingly, it was 6892. I matched 3 of the 4 numbers with the dream numbers, but in a different order. Of course this could have been mere coincidence, but it does make you wonder if consciousness, whatever its ultimate basis, is not entirely tethered to the present, but can somehow probe into the future (and past).

14. Yesterday one of my kids showed me an interesting science video about the twin paradox in special relativity. To remind you, one of the twins goes on a journey to the nearest star at relativistic speed. When returning to earth, he realizes that he is now several years younger than his earth-bound sibling. The “paradox” is that one could argue that from the point of view of the traveling twin, it is the earth bound twin who is moving away at relativistic speed. So from this point of view, the traveling twin should be the older one at the end of the trip. The solution to the paradox is that this symmetry of the twins’ experiences does not in fact hold. As argued in the video, one needs two inertial frames to describe the movement of the traveling twin (one for the trip from earth to the star, and another one for the return journey). But the other twin remains all the time in the same inertial frame.

When I saw the video, I thought that the twin paradox provides a kind of bridge from special relativity to GR. One can indeed argue that the age difference is due to the acceleration experienced by the traveling twin when he turns back to return to earth. Now imagine that GR has not been invented yet, but you want to build it on the equivalence principle. Since acceleration has an influence on clocks (as we saw in the twin paradox) one could deduce that the same must be true for gravitation. So one could deduce time dilation in GR only from special relativity and the equivalence principle. Is this analysis correct, and for those who know their history, has it played any role in the actual construction of GR by Einstein et al. ?
Another question about the twin paradox: the analysis in the video focuses only on what happens when the traveling twin turns back (change of reference frame). But after all the twin also accelerates when leaving earth, and decelerates when returning to earth. This is completely neglected in the video, so perhaps the actual age difference will be bigger (twice as big?) than in their analysis.
Thank you for your patience, and I hope that you have not experienced too much time dilation while reading this comment.

15. A plane falls and is torn apart; Where does the force that destroys it come from?.

1. From the impact. There is no force acting on the plane as long as it falls. Assuming there is no air friction, which, alas, is kind of bad news for planes in general.

16. This has come up a couple of times in the comments: the difference between inertial and gravitational mass. And yes, so equivalence principles.

Question for those who really follow this in depth (no, Wikipedia is only mildly helpful): to what extent is the distinction between inertial and gravitational mass open to experimental testing (even in principle)? Where such experimental tests are possible (in principle), how closely has it been shown that the two are the same?

1. I know that a couple of decades ago the inertial mass and gravitational mass of a body were found to be equivalent to within 13 orders of magnitude. There are several forms of the EP, weak which involves differences in composition, strong which has to do with position and momentum in spacetime and the Einsteinian form which says any experiment performed in a local inertial frame will yield the same result. The last is in some ways more general. It it includes the weak EP for material differences and the strong EP for change of frame.

These are taught in introductory GR courses. Except for those who perform measurements these tend to get forgotten a bit. I think most simply regard M_i = M_g without much fuss. Also most have few problems with the idea that all physics is the same in all local inertial frames.

There is of course an important caveat. The local inertial frame has be be really small. If it has much extent geodesic flow in one region may deviate slightly from another region. This is due to the geodesic separation or more commonly thought of as tidal acceleration.

2. Try Page 13, 14 and 15 of "Gravitation" by Misner Thorne and Weeler. It is in the free section of the book in Googles Play Books.

3. Thanks Lawrence Crowell.

I think I've answered my own question ... I had forgotten Living Reviews in Relativity. The most recent which seems to cover this topic is Will's "The Confrontation between General Relativity and Experiment" (2014), specifically "Tests of the Foundations of Gravitation Theory".

No doubt there are more recent reported results (and if anyone wants to read crackpot ideas on this topic, be sure to check out viXra ;-)), but I expect that there are no (or very few) new major experiments planned (or completed) that were not mentioned by Will.

4. This question never seemed properly motivated to me. What are the grounds for wondering if inertial mass and gravitational mass are equivalent?
What would the difference be and how would it be manifest? Yet another field permeating all space and time?

It is an observation/postulate/axiom that has worked well so far!

It may be a faux question asked just to make people look thoughtful.

17. Jean, I will comment on YT as soon as can write a bit better, as I have already mentioned the subject before.

In brief answer to your questions: only empirical evidence is enough, and it is invisible to normal people. I am far from normal, and qualify to comment it but not here... Not without risking a few beatings.

1. Thanks Ivan from Union.

I hope you are recovering, and look forward to you returning to your usual posting/commenting mode.

18. There is this different, apparently seldomly mentioned, equivalent formulation of GR (at least at the level of the equations of motion) in which there is no curvature, but only torsion - the socalled Teleparallel Equivalent of GR (TEGR). What Sabine, if I may ask, I am curious, is your attitude towards that theory? Is it something that you and your collaborators use, at least occasionally? Oppinions differ in the literature and online as to which one of the formulations is the superior one.

19. Grant the equivalence of inertial and gravitational mass. Do a thought experiment: an accelerated reference frame vs a frame in a gravitational well. Drop test masses in each and carefully observe their paths in each frame. In a strong enough gravitational field, the paths of the dropped test masses converge toward the center of the field as they fall. In the accelerated frame they do not. There is no "center of field". There is no field. So you can differentiate between the two frames by experiment. But equivalence is LOCAL. How local must it be? Suppose a uniform 100,000 g acceleration vs a field with a 100,000 g gravitational acceleration. Local then must be VERY local. Consider accelerations equivalent to the surface of a neutron star. Very, very, local, so that one can't discriminate between the uniform acceleration vs the gravitational field. Or take an extreme case of proximity to a singularity and the equivalent acceleration. At some point locality becomes meaningless-- can we have infinitesimal "locality"? Is there something wrong with this picture? The point being, if we can discriminate, in principle, if not in practice, between a gravitational field and an accelerated frame, the two are not equivalent, even though the measured masses are the same. Or does that matter?

1. The infinitesimal limit doesn't exist, so why worry about it.

2. The local inertial frame in an extreme gravity field can be large. A super-massive black hole can have a local inertial frame many kilometers in extent before geodesic separation or tidal forces are apparent. A freely falling observer can pass the horizon without noticing anything. Conversely, if there is a way of holding an observer close to the horizon again this will conform to the equivalency of an accelerated frame and a frame held fixed in a gravitational field. As an exercise try to take the Schwarzschild metric

ds^2 = (1 - 2m/r)dt^2 - (1 - 2m/r)^{-1}dr^2 - r^2(dÎ¸^2 + sin^2Î¸dÏ†^2)

with the radius r = 2m + â„“, for â„“ << 2m to derive the Rindler wedge for an accelerated frame. It is possible to show there is an upper limit to acceleration a = 10^{52}m/s^2 called the Planck acceleration. An accelerated frame held above the horizon of a black hole by â„“ = sqrt{GÄ§/c^3} requires this acceleration. This would be the same for a supermassive black hole and for a small black hole. For the supermassive black hole an accelerated frame of some extent will work, but for a much smaller black hole the scale of the frame is reduced. The important factor is not so much the magnitude of the gravitational field, but rather its divergence.

3. Correct, the infinitesimal limit doesn't exist. So, if under sufficiently extreme but possible circumstances the locality required for the equvalence principle to hold becomes physically meaningless, would the principle not break down?

4. This seems an unnecessarily laboured point. The equivalence of gravity and acceleration is along any line from the centre of mass out to infinity.

The simplest thought experiment shows this. In a sealed room with a perfectly flat floor I can easily determine if the force holding me down is acceleration due to a rocket motor or is gravity by measuring if the downward force is constant across the whole floor or if the force deviates inwards as I move towards the edges of the room.

So yes the locality is infinitely small but why would that invalidate the principle?

5. Infinitely small locality or locality of zero dimensional coordinates makes no physical or mathematical sense. Express locality with a system of differential equations in zero dimensions? That doesn't work.

6. Rick Lubbock wrote:
>The point being, if we can discriminate, in principle, if not in practice, between a gravitational field and an accelerated frame, the two are not equivalent, even though the measured masses are the same. Or does that matter?

Fifty years ago, when I first started learning about GR, I had the same thought: I was convinced I had caught the great Einstein in an error.

It is actually more interesting than that. What the GR field equations are actually all about is the deviations from a homogeneous acceleration that you mentioned-- the so-called "tidal forces."

So, while neither you nor I had in fact found an error in GR, we both did in fact figure out the really important point about the Einstein field equations.

For a book-length discussion that elaborates on this, try reading John Wheeler's A Journey into Gravity and Spacetime. The book is easy reading, albeit eccentric (Wheeler was well-known as an eccentric genius).

It's actually possible to run with this approach and derive the Schearzschild solution, the basic equations of cosmology, etc., using only first-year calculus: I'm working on a book showing how to do this.

Funny how one never knows whether worrying about an apparently piddling issue is in fact really focusing in on the essence of what is happening.

By the way, this is not how Einstein himself arrived at the GR field equations: his own approach was extremely tortuous, plagued with one error after another. Pais' book on Einstein, Subtle Is the Lord: The Science and the Life of Albert Einstein, has a lengthy discussion of Einstein's route to the right equations.

John Norton's online Einstein for Everyone is also interesting.

Dave

7. I want to thank you! I've ordered the Wheeler book and the link to Einstein for Everyone is appreciated. A very enlightening exposition, and the technical index is very helpful.

20. Any thoughts out there about the effect population density of "whatever" has on total chaos (energy) in a closed system. My research suggests there is a 2% increase in chaos for every 1% increase in population. And, to no surprise, studies in criminality show a close correlation to this theory. - So, what about observations in physics?

21. > Newton’s theory works just fine, for all practical purposes, unless possibly, you plan to visit a black hole.
Or unless you wanna use GPS :-)

22. If Kaluza Klein theory and similar extensions to higher dimensions are correct then none of the standard model forces are actually forces.

23. Ten pounds! A bit unsual ... to use pounds!

24. I have not found the time to read the tomes devoted to Einstein's theory of relativity, but I have the gut feeling that it does not fully expkain the changes which I am going through. I am perturbed as I am being pulled in many directions by forces I do not understand. I'll just fall and pick myself up again until my mortal coil loses the energy to react.

25. "Weight is actually the force that acts on your body due to the pull of gravity"
If these is the case, why then when an object is free falling is weight is zero but his gravity is not?

26. "...weight is zero but his mass is not?"
because the force you experience due to acceleration (i.e. the force you feel pressing you into your seat when a plane accelerates on the runway, provided, of course, you sit on that plane) equals the force of gravity. When the two balance each other out (the "free" in "free falling") you do not notice either: you feel weightless.

27. What is a force? F = ma. A force is a phenomenon that causes a mass to accelerate. In relation to an inertial frame of reference. It doesn't matter what the mechanism of that change in velocity is - the tension on a rope, the push from my shoulder, or the curvature of space. So, gravity is a force.

Or am I missing something?

28. "Or am I missing something?"

What an inertial frame is.

29. As one travels on the surface of the Earth there is no force tangent to the surface. Yet our motion, whether by vehicle or walking, is constrained to be on that surface and the path is not a straight line. The only force on you is local gravity, which is perpendicular to the surface.

It is said that gravitation is not a force, and this is because it really is a theory of paths or geodesic flows in curved space or spacetime. The path or geodesic flow is determined by an extremal principle, similar to the principle of least action in classical physics. Just as we must move around on the curved surface of the Earth gravitation is really about motion in the curvature of spacetime.

Gravitation does have a force aspect to it. An extended body will have different geodesic flows and this results in a tension or force within that body. This is the basis for tidal forces. Maybe we could say that gravitation results in a differential acceleration between a distribution of particles or a force within a body, where these are on geodesic flow.

Gravitation is then not considered a force as such. If you are falling in a container then you are falling with it and apparently weightless. An orbiting spaceship is falling but moving fast enough to keep missing the Earth as it curves away. A force you experience from gravitation is really the stuff in Earth pushing up to resist geodesic flow. After all, it is not the fall that kills you but rather the sudden stop at the end.

1. Just pointing out that you could also favour the tensor field view and ditch the geometric one. It's still general relativity, but more in line with the other fundamental forces.

30. Rick Lubbock writes: "Do a thought experiment: an accelerated reference frame vs a frame in a gravitational well. Drop test masses in each and carefully observe their paths in each frame. In a strong enough gravitational field, the paths of the dropped test masses converge toward the center of the field as they fall. In the accelerated frame they do not. "

Precisely. If each test mass is following a geodesic and the geodesics converge or diverge, then the underlying spacetime must be curved. That is the difference between acceleration in a flat spacetime and free fall in a gravitational field.

31. An inertial frame of reference is one that is at rest or moving at a constant velocity.

32. I'll mention why I brought up the issues I did: if there is any realistic physical scenario in which gravitational divergence forces GR locality into the quantum scale, that might be interesting to examine. But now I'm not so sure that even a small black hole would do that. Very disappointing.

33. Dave,
You are correct about the social norms around strange religious beliefs in the US. But its the general lack of critical thinking, lack of deep scientific knowledge (plus ego) that results in strange scientific theories, occasionally, even at Backreaction. Its not an agenda, its just poor thinking generating poor science.

In the US its also a general distrust of intellectualism and science. I think people take the benefits of science and technology for granted while ranting against the negative unintended consequences.

Science leaks into politics. Which is why I think there is such difficulty with climate science in America. Right from the start conservatives saw that effective action against global warming would require government intervention and regulation. They fought against that by denying the science.

I am not so much against science popularizers who simplify or maybe even muddy the science, or bring in God to get their message across. I firmly believe you have to start by talking to people where they are, not where you want them to be,

If an individual then wants to pursue some further interest in subjects such as global warming or cosmological foundations they can find more rigorous treatments easily now days.

Backreaction is one of the resources where people can get a better look at deeper science. All the back and forth leads to deeper understanding and frankly, is tremendously entertaining. I know that it has changed my thinking on several foundational subjects.

Its not all that hard to tell who knows what they are talking about and who doesn't. If a poster can explain their idea with out unnecessary qualifiers, or without a lot of math that doesn't go anywhere (but mainly if I can understand it) then they probably know what they are talking about.

That's enough I'm going now to read up on conformation bias.

1. Steve Bullfox wrote to me:
>In the US its also a general distrust of intellectualism and science.

I'm not sure that is the explanation: after all, most intellectuals throughout history were merely apologists for the ruling elite ("throne and altar" and all that) and deserved to be distrusted.

I grew up in a very conservative area, and I can assure you that most conservatives think they love science, just like our friend Keith. And they do, except of course when science challenges their cherished beliefs. Same as for leftists who rail against GMOs. Not much difference that I see.

Steve also wrote:
>I am not so much against science popularizers who simplify or maybe even muddy the science, or bring in God to get their message across. I firmly believe you have to start by talking to people where they are, not where you want them to be,

Well.... I think that empirically that approach fails. I have had members of the clergy tell me that of course they do not believe what they preach from the pulpit every Sunday, but, you know, you have to meet people where they are. I mean, the clergy (often even fundamentalist clergy) learn in seminary that the Bible is not literally true, but the people in the congregation are not "ready" for that.

Well, when are they actually going to simply start telling their congregation the truth? How are the members of the congregations supposed to figure it out if the experts who have the responsibility of informing them decide instead to treat them like children? (Not that I think even children should be lied to.)

Steve also wrote:
> If a poster can explain their idea with out unnecessary qualifiers, or without a lot of math that doesn't go anywhere (but mainly if I can understand it) then they probably know what they are talking about.

A bad filter, I fear. Lots of things in physics can only be explained in math or with lots of qualifiers.

I'd say the opposite of your approach is closer to the truth.

34. We seem to take GR for granted but a serious look at Lorentz Invariant gravity type theories might not be a waste of time. Might actually be easier to quantize, for one thing. On the one hand this LIGT looks a little cranky, but it does address some problems in GR. LIGT makes gravity a force again, and it takes its place among other forces-- no longer geometric. I guess I'll look into it-- something I've just run across although its been around a long time.

35. Dr. Hossenfelder,

If there was ever anything that showed you need to keep writing over videos, this post is definitely it. You took what I am going to presume is a common illness….(I hope that you have gotten better) and turned it into a lengthy discussion on gravity and relativity. Well done!

I must a different view point here, after I refer to a topic in a book I read not too long ago. The author was discussing how the subjective term and/or quality of “beauty” was injecting itself into current physics. The author rightly made the point that beauty is subjective and is not a valuable criterion for assessing a theory. I agree with the author’s position on this, but I also believe that very rarely are there absolutes in anything and this is one of them. To me, beauty is simplicity. And, in this case simplicity is known as an inertial reference frame, which in this instance is the earth itself and the laws of classical mechanics.

The discussion regarding curved space time and the like is very interesting and I have read the posts, but I am at a loss to understand how it applies to Dr. H losing 10 lbs. If Dr. H’s intention was for this to take the direction that it did, it was very successful. However, to me the “beauty” was in the simplicity of looking at what we have in front of us every day.

And, my answer to your question, is yes it, especially in the circumstance you have presented!

As always, nice work Dr. H.!!

36. I find these typical elevator-reasoning's outdated and sometimes refer to them as classic-analog-Newtonian reasoning. Therefore they bump with a progressive quantum-digital-approach and or make the transition towards a quantum theory of gravity more difficult.

37. Revisiting Taylor and Wheeler, they write: "force can be said to be a form of interaction" and "instead of talking about forces between particles, we can talk about their changes of momentum...to discuss both momentum and forces in relativity is to complicate the story" (page 102). Albert Einstein wrote: "Therefore, I have avoided introducing the force concept." (1935, Elementary Derivation of the Equivalence of Mass and Energy). Finally, John L. Synge writes "I have long since abandoned the attempt to attach relativistically viable meanings to the words 'force' and 'acceleration'." (page 152, Talking About Relativity). Zee is rather interesting, writing: "Einstein's insight was that the most commonly experienced force of all--the gravitational force--may be an example of 'fictitious force'." (page 279, Einstein Gravity in a Nutshell, 2013). Note, Zee's use of the word "may" in his sentence. James Anderson, long ago, reminded us that "...accelerative effects are not the same as geometrical effects." (page 337, Principles of Relativity Physics). He also reminded us that "the criteria of beauty and simplicity are subjective." (page 340, Principles of Relativity Physics).

38. Hi Sabine

Re this question - there is a bit in Frank Wilczek's book where he talks about 2 different ways of understanding GR, as curved spacetime vs as a metric field, and says he goes for the latter

(quote here some way down the page: https://www.goodreads.com/author/quotes/460100.Frank_Wilczek?page=4)

My question is - is this distinction relevant to this question? ie if you interpret GR as curved spacetime then you'd say it is not a force, but if it's a metric field then you'd say it is?

(more generally, I find this distinction a bit odd... my *very* amateurish understanding of GR is that the metric tensor is just a description of the geometric character of the space, and if gravity is present the space is curved which means the metric differs from place to place, so how precisely are the descriptions different?)

39. Hi Sabina, I would like to think that your belief in superdeterminism can somehow accept the possible existence of a real and tangible (physical) connection between all quantum phenomena in the universe and express your opinion about entropic gravity based on the Holographic Principle and .. ... There is a real engineering project that surprisingly demonstrates the connection with the Holographic Principle and quantum mechanics. Of course, I could be wrong, but we are talking about a device with hard parameters and not about ...theory.

40. (
"I would like to think that your belief in superdeterminism can somehow accept the possible existence of"...

Not a "belief", a modeling methodology as far as I understand it, and you can't "take it everywhere" anymore than you enter a car race with a tricycle.

As far as modeling is concerned, superdeterminism is an honest alternate, alas too little known as of the present.

"a real and tangible (physical) connection between all quantum phenomena in the universe":

My take: Real. Physical. Not "material". Only empty space connects all things. Only empty space is at all borders and betweens.

)

1. “Only empty space connects all things. Only empty space is on all borders and between them. ” - These are all spells. As soon as we take the side of superdeterminism, we automatically assume that the Holographic Principle works with its law of conservation of information. And space in the Holographic Principle is an illusion, only the surface with information is real. We have come to digital physics and there is an urgent need to separate reality from illusion.

2. ""space in the Holographic Principle is an illusion":

If Verlinde sustained that, Verlinde erred.

""only the surface with information is real":

If Verlinde sustained that, he erred. The surface is matter/material, not "space". There is no such thing as "unreal space", all space is physical.

I have been for months trying to not say publicly something about his theory, to the point of using wrong words intentionally in my comments on the Holography post, because I am simply out of an audience that would understand it. Believe me, there is a mind-blowing twist to his theory and am unwilling to part with it.

3. "... all space is physical." Let's conduct a thought experiment and answer the question: what is real and what is the illusion in an optical hologram: - 2D surface with information or 3D image of an apple that cannot be distinguished from the original?

4. Ok... I confess. I WAS with Verlinde on that day on Times Square. I was carrying the soap box.

As soon as we got close enough to the Naked Cowboy audience, he gave me the signal, I dropped the box, he immediately jumped on it.

He raised his finger in the air and screamed at the top of his lungs:

"MY THEORY PROVES SPACE IS AN ILLUSION".

Who said you can't get a standing ovation from two heroin addicts in an audience? We did!

5. With those who ignore the question not interested. Good luck and health to you.

41. Late to the party, but it took time to find this again. Here are remarks by Rai Weiss on March 16, 2016 at UCSB, in response to a question about why there was slow progress in analyzing gravitational wave theory. Weiss was teaching a course on GR in the 60's and his students wanted to know more about Joseph Weber's work on resonant bars:

"People didn't understand the theory. Many of the people who were objecting when that came up were people who were working on bars. Why could I not teach in that course the Weber thing. It was a perfectly legitimate thing, but by that time I had tutored myself into thinking that there are no forces left in gravity. This is the way I looked at it, it was all geometry. I was a young person and I looked at that and I bought it hook line and sinker. And so what happened is that the way you would analyze a bar was quite complicated to me. I didn't feel I understood it. Because the way you do that is you don't use the h, you use the curvature tensor and covert that into a tidal force. And you use space as being Euclidean. And so you don't write h, you use a new field, you do it more like Weinberg's book. You don't use geometry, you use a new field, a tensor field, and believe it as it stands. And I didn't want to do that, I didn't understand how the interaction between a gravitational wave and a bar works.

There's another step that you ought to know in the education of all of us. When all of this took place in the 60's, most physics departments did not have a course in GR, it was all in mathematics. I taught the first course in GR at MIT since 1930, in 1967. There was no basis, people had not learned any GR. GR became something to teach people again when GR began to do experiments, and began to have results in astronomy and cosmology.

And so that's the transition that took place. A lot of people were ignorant of what was in the theory. And that was the problem. What they did was they mixed up the two approaches. They mixed up the geometric approach with the tensor force approach. They came to the conclusion that if you do this (bars) you're not going to see anything.

And that (opinion) was held by 80% of the people who were working on bars. The thing that was always difficult, when you learn GR for the first time yourself, you find out that the coordinates don't mean anything, ... and that's the thing people didn't really realize. There was nothing wrong with the theory, there was misunderstanding of the theory.

The first time you read GR you read it in some popular science book and you just don't get it. Nobody sat down, except Joe Weber himself, and ... I guarantee you all the people who were working, Dave Douglas, Tony Tyson, even Heinz Villenes, probably never bothered to look in a GR book like Peter Bergman's or Landau and Lifschitz, who had it very straight. They never bothered with it, it's too complicated, the hell with all that tensor crap (laughter)."

42. Please note that the fundamental laws of gravity physics are not postulated by E Verlinde, but are mathematically derived from simple equations of thermodynamics on a holographic screen. Gravity is an entropic force. https://en.wikipedia.org/wiki/Entropic_gravity

1. "The theory has been controversial within the physics community but has sparked research and experiments to test its validity." So I guess we'll see IF he is right.

2. In a new type of mechanical gyroscope whose rotor produces coherent oscillations, we theoretically obtain entropic forces acting at a distance. And for several years there we have indirect experimental data.

43. Gravity IS "a force". ‘ See what Unruh says: .. "A more accurate way of summarizing the lessons of General Relativity is
that gravity does not cause time to run differently in different places (e.g., faster far from the earth than near it). Gravity is the unequable flow of time from place to place. It is not that there are two separate phenomena, namely gravity and time and that the one, gravity, affects the other. Rather the theory states that the phenomena we usually ascribe to gravity are actually caused by time’s flowing unequably from place to place... “ arXiv:gr-qc/9312027v2 17 Dec 1993

So, the vacuum is Time, and causality is a difference in the rate of Time. What do you call a higher probability of existence in one direction, if not a force?

1. Dear Marcel-Marie,

Curved time just defines the frame in space where no potentials occur i.e. free fall. When a force is defined as interaction by signalling, gravity is no force.

44. Eusa, the concept of space, like a ruler, is the idea we get from seeing or thinking of a bunch of points as being all at the same moment. Light still take some time to go from one end of the ruler to the other; no choice here, i.e., they are not at the same moment. There is no such thing as space. So, we invented space-time in order to keep doing physics. So, all there is out there is the vacuum/time process. To understand how things work, we have to let go of illusions even if they are of practical use. Physics studies how we experience the universe, and this experience is not the universe.

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