Saturday, January 02, 2021

Is Time Real? What does this even mean?

[This is a transcript of the video embedded below.]

Time is money. It’s also running out. Unless possibly it’s on your side. Time flies. Time is up. We talk about time… all the time. But does anybody actually know what it is? It’s 3:30. That’s not what I mean. Then what do you mean? What does it mean? That’s what we will talk about today.

First things first, what is time? “Time is what keeps everything from happening at once,” as Ray Cummings put it. Funny, but not very useful. If you ask Wikipedia, time is what clocks measure. Which brings up the question, what is a clock. According to Wikipedia, a clock is what measures time. Huh. That seems a little circular.

Luckily, Albert Einstein gets us out of this conundrum. Yes, this guy again. According to Einstein, time is a dimension. This idea goes back originally to Minkowski, but it was Einstein who used it in his theories of special and general relativity to arrive at testable predictions that have since been confirmed countless times.

Time is a dimension, similar to the three dimensions of space, but with a very important difference that I’m sure you have noticed. We can stand still in space, but we cannot stand still in time. So time is not the same as space. But that time is a dimension means you can rotate into the time-direction, like you can rotate into a direction of space. In space, if you are moving in, say, the forward direction, you can turn forty-five degrees and then you’ll instead move into a direction that’s a mixture of forward and sideways.

You can do the same with a time and a space direction. And it’s not even all that difficult. The only thing you need to do is change your velocity. If you are standing still and then begin to walk, that does not only change your position in space, it also changes which direction you are going in space-time. You are now moving into a direction that is a combination of both time and space.

In physics, we call such a change of velocity a “boost” and the larger the change of velocity, the larger the angle you turn from time to space. Now, as you all know, the speed of light is an upper limit. This means you cannot turn from moving only through time and standing still in space to moving only in space and not in time. That does not work. Instead, there’s a maximal angle you can turn in space-time by speeding up. That maximal angle is by convention usually set to 45 degrees. But that’s really just convention. For the physics it matters only that it’s some angle smaller than ninety degrees.

The consequence of time being a dimension, as Einstein understood, is that time passes more slowly if you move, relative to the case when you were not moving. This is the “time dilation”.

How do we know this is correct? We can measure it. How do you measure a time-dimension? It turns out you can measure the time-dimension with – guess what – the things we normally call clocks. The relevant point here is that this definition is no longer circular. We defined time as a dimension in a specific theory. Clocks are what we call devices that measure this.

How do clocks work? A clock is anything that counts how often a system returns to the same, or at least very similar, configuration. For example, if the Earth orbits around the sun once, and returns to almost the same place, we call that a year. Or take a pendulum. If you count how often the pendulum is, say, at one of the turning points, that gives you a measure of time. The reason this works is that once you have a theory for space-time, you can calculate that the thing you called time is related to the recurrences of certain events in a regular way. Then you measure the recurrence of these events to tell the passage of time.

But then what do physicists mean if they say time is not real, as for example Lee Smolin has argued. As I have discussed in a series of earlier videos, we call something “real” in scientific terms if it is a necessary ingredient of a theory that correctly describes what we observe. Quarks, for example, are real, not because we can see them – we cannot – but because they are necessary to correctly describe what particle physicists measure at the Large Hadron Collider. Time, for the same reason, is real, because it’s a necessary ingredient for Einstein’s theory of General Relativity to correctly describe observations.

However, we know that General Relativity is not fundamentally the correct theory. By this I mean that this theory has shortcomings that have so-far not been resolved, notably singularities and the incompatibility with quantum theory. For this reason, most physicists, me included, think that General Relativity is only an approximation to a better theory, usually called “quantum gravity”. We don’t yet have a theory of quantum gravity, but there is no shortage of speculations about what its properties may be. And one of the properties that it may have is that it does not have time.

So, this is what physicists mean when they say time is not real. They mean that time may not be an ingredient of the to-be-found theory of quantum gravity or, if you are even more ambitious, a theory of everything. Time then exists only on an approximate “emergent” level.

Personally, I find it misleading to say that in this case, time is not real. It’s like claiming that because our theories for the constituents of matter don’t contain chairs, chairs are not real. That doesn’t make any sense. But leaving aside that it’s bad terminology, is it right that time might fundamentally not exist?

I have to admit it’s not entirely implausible. That’s because one of the major reasons why it’s difficult to combine quantum theory with general relativity is that… time is a dimension in general relativity. In Quantum Mechanics, on the other hand, time is not something you can measure. It is not “an observable,” as the physicists say. In fact, in quantum mechanics it is entirely unclear how to answer a seemingly simple question like “what is the probability for the arrival time of a laser signal”. Time is treated very differently in these two theories.

What might a theory look like in which time is not real? One possibility is that our space-time might be embedded into just space. But it has a boundary were time turns to space. Note how carefully I have avoided saying before it turns to space. Before arguably is a meaningless word if you have no direction of time. It would be more accurate to say what we usually call “the early universe” where we expect a “big bang” may actually have been “outside of space time” and there might have been only space, no time.

Another possibility that physicists have discussed is that deep down the universe and everything in it is a network. What we usually call space-time is merely an approximation to the network in cases when the network is particularly regular. There are actually quite a few approaches that use this idea, the most recent one being Stephen Wolfram’s Hypergraphs.

Finally, I should mention Julian Barbour who has argued that we don’t need time to begin with. We do need it in General Relativity, which is the currently accepted theory for the universe. But Barbour has developed a theory that he claims is at least as good as General Relativity, and does not need time. Instead, it is a theory only about the relations between configurations of matter in space, which contain an order that we normally associate with the passage of time, but really the order in space by itself is already sufficient. Barbour’s view is certainly unconventional and it may not lead anywhere, but then again, maybe he is onto something. He has just published a new book about his ideas.

Thanks for your time, see you next week.

You can join the chat on this topic today (Jan 2nd) at 6pm CET/noon Eastern Time here.

1. Indeed, time could be defined as an emergent quantity of matter structure due to causiation (radiation at speed c) deviating from straight effective line.

Not surprisingly that definition could be the same for gravitating/inertial mass/energy.

I think something like that fundamentality is described by the holographic emergent gravity. Via the information theory almost invariant mass/curvature has steady info and quantized items cannot be figured out. Only the changing distribution would emit measurable quanta.

1. Time is the mental delusion of motion. Just as light is the mental delusion of electromagnetic energy in a tiny range of frequency.

BTW, the author asserts that one can remain still in space, but not time. I would love to a single example of anything or anyone who is not in constant motion. That pure fantasy - it does not exist. All things are in constant motion. And, THAT is manifest in the human mind as time.

2. Hominid9:08 AM, January 04, 2021

"Time is the mental delusion of motion. "

You mean illusion rather than delusion. Again, refer to a dictionary.

" All things are in constant motion. And, THAT is manifest in the human mind as time."

So if I show you a video of a room in which everything is still and a "video" of a still of the room, then you can distinguish between them because "all things are in constant motion"?

I think you need to re-think that theory, petal...

3. Hmm. The information content of both above comments remain quite obscure.

4. tyy5:15 PM, January 06, 2021

Hominid claims that because objects are in constant absolute motion, we see the illusion of time. I claim he can't distinguish between an object that is relatively motionless in space and moving in time, and one that is relatively motionless in space and not moving in the time dimension. The former is moving in absolute terms though and so Hominid should be able to determine that it is also moving in time. But he can't.

Further, in terms of absolute motion, it's possible the universe is a sphere and the centre is moving in the time dimension but is motionless in space, I suppose. This would be something still in space but moving in time, which Hominid claims is impossible.

5. Your analysis is absurd. What you can "tell" is not reality - that's my very point. Absolute motion is a PROPERTY of the universe. Just because you can IMAGINE something doesn't mean it exists or can exist. You are a mystical thinker.

6. Hominid6:59 AM, January 08, 2021

"Absolute motion is a PROPERTY of the universe."
Now you just need to prove this assertion. You can start by proving the universe isn't an expanding sphere with an origin motionless in space. Off you go, petal.

" Just because you can IMAGINE something doesn't mean it exists or can exist. You are a mystical thinker."
Velocity vectors cannot cancel out. Interesting idea.

" All things are in constant motion. And, THAT is manifest in the human mind as time."

So if I show you a video of a room in which everything is still and a "video" of a still of the room, then you can distinguish between them because in the former "all things are in constant motion"?

Answer the question, all caps boy.

7. Motions are always relative. Photons don't move - they are lying in emitting and absorbing excitates. What really is absolute? The direction of interaction at local collapse is absolute for spatial structures and the non-orientation nature of quantum state is absolute for entangled correlations.

8. ...when properties of the universe concerned.

9. Eusa, motions (vectors) can only be observed (described) with reference to other motions - THAT is what "relative" means.

PROPERTIES are absolute. Motion is a PROPERTY of all things in the universe - there is nothing that is not moving - just as gravity is a PROPERTY of mass and wavelength is a PROPERTY of energy.

Your confusion exemplifies my point about language ambiguity. You don't understand what "relativity" means.

Are there no PROPERTIES of real things? Are there actually no properties and rules for science to discover? Are we scientists just masturbating?

To what is the universe relative? Fantasy multiverses?

10. I just told what can be relative and what can be absolute. Motion cannot be absolute by definition, it's 1-form in coordinates. Absolute and invariance are tangential - e.g. an acceleration can be seen as absolute in coordinates by gradient of 2-form...

What you think about background absolutism is metaphysics. There is space only due to interaction chains. The direction of lightlike propagation can be absolute right because it defines the coordinates you imagine in your head and so on via the fact that other observers see the same direction (0-form). With relative motion timelike objects can move in opposite direction depending on chosen coordinates.

When acceleration with interactive gradient, it's again possible to be absolute in direction. When you have form of odd rank, there is mixed state of transition without antipodal pole that is extra loose degree of freedom - just to consider in relation to outer entity.

(Sorry busy poor language).

2. Great job, as usual very interesting and, as usual, a lot of to learn. And great video editing too.
PS: I think you used a wrong image for Herman Minkowski (as you know he died at the age of 45 so it can be him).

1. You are right; I think the image shows his brother. Sorry about that.

3. Hi Sabine
Very interesting video. I am not a scientist so forgive me if my post is too simplistic! While time seems intuitively not to "exist" in the sense that the physical universe exists, the fact that it seems to pass at different rates suggests it may be a real phenomenon. For example if we were to travel to a distant black hole and then return to Earth, time would have passed more slowly for the traveller than for the people back on Earth. For time to pass at different rates suggests that it is a real phenomenon.

1. You are right. But we don't need to travel to a black hole; the effects are detectable on Earth. Your watch loses one or two picoseconds while you commute to work because of the so-called "twin paradox". You can actually get a few picoseconds' more work done with home office! And even more if you choose to work in the attic. ;-)

Sure, the effects are hard to measure. (Remember that it takes light about 1000 picoseconds to travel from your wristwatch to your eyes!) But they are real, and absolutely essential for deriving the "Coordinated Universal Time" from the atomic clocks in laboratories scattered over the globe. And for a functioning GPS.

2. I am not a scientist. But I do have a couple of science degrees and have spent the past forty-four years developing real-time systems. Before reading your comment, I would have said that time doesn't exist, merely events, some connected by causality. For practical purposes, that is the case in the kind of work I do. But your remark about the differences in the passage of time, due to Special Relativity (due to acceleration) or General Relativity (due to gravity), was quite enlightening. I knew this already, if for no other reason than systems like GPS have to take both into account. I know that the GPS-disciplined chip-scale cesium atomic clock that ticks away in my living room has to constantly adjust itself if only because it sits a mile above sea level in my home in the Denver Colorado area. But the fact that the way we measure time can actually detect these effects is important. So I've changed my mind: time does exist, even if we can't quite say what it is. I am fortunate enough to have seen the ytterbium optical atomic clock at the NIST labs in Boulder Colorado, a clock so accurate it can detect a change in altitude of inches. Time is in fact real. I'm now convinced.

3. Could you cite an example of an event that is NOT "connected by causality"? I ask because I know of none and have been inclined to conclude that a noncaused event is an impossibility.

4. The amazing thing is that different (good) clocks agree among themselves as well as they do. There must be some underlying structure (which we call time) which allows us to predict patterns of events ("next meeting at ...").

5. Hominid: I merely meant that there are events that occur outside of my light cone that cannot be causally connected to events that are within my light cone. As for events that are not connected causally to any other event, I'm not a physicist but AFAIK there are spontaneous events like radioactive decay, or perhaps the appearance of virtual particles.

4. The existence of time is something that has been of concern by some. In quantum mechanics, which is an ingredient for quantum gravity, something exists if it is the eigenvalue of a Hermitian operator. This does not occur for time; time cannot be such an operator.

While there is an operator for the Hamiltonian there is no such corresponding operator for time, or a well defined meaning to a commutator [H, T_{op}] . This in part stems from the classical picture, where there are Posson brackets {q, p} = 1 or in greater generality a Hamiltonian flow dz^a/dt = Î©^{ab}∂_bH, but no associated flow for a symplectic variable z_a = t .

We might propose a time operator T_{op}, which acts on Hilbert space vectors |t ⟩ . The time operator then acts on |t ⟩ for Ïˆ(t) = ⟨ t|Ïˆ ⟩ , which returns a time eigenvalue with a Born rule probability of a particle occurring at time t according to p[Ïˆ(t)] = |Ïˆ(t)|^{2} . Pauli proved that such a construction is not consistent with quantum mechanics. Consider a Hermitian time operator T_{op} such that [T_{op}, H] = ihbar . From this Hermitian operator a unitary operator exists U_Îµ = exp(-iÎµT_{op}) . This unitary operator is an energy development operator, and the energy scalar Îµ is in the set of reals. The state Ïˆ in the eigenbasis of a Hamiltonian HÏˆ = EÏˆ , with commutator

[U_Îµ, H] = sum_{n=0}^∞ (-iÎµ)^n/n![T_{op}^n, H] = - ÎµU_Îµ.

defines the composite operator HU_Îµ

HU_ÎµÏˆ = (U_ÎµH - [U_Îµ, H])Ïˆ = (E + Îµ)U_ÎµÏˆ.

The state U_ÎµÏˆ is an eigenstate of the Hamiltonian with eigenvalue E + Îµ . This Hamiltonian is not discrete or bounded below, since Îµ may assume a continuum of values on the reals. If the Hamiltonian is discrete and bounded below the U_Îµ maps these eigenvalues on the entire set of reals. This also implies energy eigenvalues are not bounded below. This is not physical and the time operator does not exist in an ordinary sense.

Does this absolutely preclude the time operator? Not entirely. We have anti-de Sitter spacetimes that have negative energy vacua and quantum states are potentially not bounded below. We then have the prospect that quantum states can endlessly cascade down a bottomless staircase and there is a horrid divergence. So what might prevent this? It would be string theory, where the quantum vacuum state is negative, but it is the final state. It is also a possible mechanism whereby cosmologies are endlessly generated out of the vacuum. The possible application of a time operator may be what generates cosmologies in eternal inflation.

5. The video/audio quality was noticeably better.

Lorentz invariance is approximate ... IMHO.

My gut says both the wave and the particle as well as some absolute frame of reference are all fundamental.

Is a non local hidden variable pilot wave theory compatible with a string theory landscape?

6. The professional editor does improve the visual presentation. The harmony between speaker (you) and graphics with animation is superb, outsourcing editing removes the burden from yourself. The number of scenes however has greatly increased and I find distracting from the narrative. Reducing the number of scenes would be less distracting. My two-cents.

7. Good to learn the provenance of, “Time is what keeps everything from happening at once,” In checking it out, I came across an interesting website: https://quoteinvestigator.com/2019/07/06/time/
There I found that John Archibald Wheeler had, in a footnote, attributed it to "graffiti in the men's room of the Pecan Street Cafe, Austin. For me, this was a strange coincidence because I first saw the quote written on the lavatory wall of a men’s bathroom at an interstate highway rest stop in central Texas. One doesn’t think of Texas as a place of roaming philosophers.

And your article reminds me of my experience as a framing carpenter. Given a pile of boards that are potential rafters, you must judge their suitability. So, you pick up the end of each one, sight down its length and then you can see its otherwise hidden properties of warp, twist, bow and crown. The mantra for rafters is, “Crown up!” because that creates the most resistance to potential snow load.

Anyway, when I learned about the probabilistic nature of quantum mechanics, I likened it to looking down the length of the time axis and thereby seeing all the variability at once. In a sense, that would be looking as if everything was happening at the same time and I wondered if there actually was such a point of view.

8. bee:

your last sentance says "Thanks for your time, see you next week.". are you making a joke here?

1. I am not joking when I say I'll have another video next weekend. Was I trying to be witty with my reference to time in a video about time? Yes, you figured that out correctly.

9. Does mind obey physical law?

1. Of course mind and everything else obeys physical laws.

2. Of course not. Mind is not real. The brain obeys physics; mind is delusion.

No one has been able to show a property of mind or measure mind or demonstrate a physical phenomenon that underlies a thought, emotion, perception. No bridge has been built linking physiology to psychology.

3. Hominid,
Perhaps you forget that your physics is a bounded region, kind of like an exclusive sports club where you must have a uniform to get on the playing field. That uniform is awarded only through act of measurement defined by an agreed upon uniform metric. There are real things in the universe that don’t pass the test. Meaning is one. There are many.

4. Sorry - I'm just not interested in obfuscatory blather of the sort you post. Perhaps someone else will take you seriously.

5. Point scored. But my limitations aside, determination of path is the salient observable. In your reply you typed 122 keystrokes to create text likely unique in the history of English language. Is that simply a product of your physics?

6. Hominid10:32 AM, January 04, 2021

"No one has been able to .. demonstrate a physical phenomenon that underlies a thought, emotion, perception. No bridge has been built linking physiology to psychology."

There are lots of correlations between subjective reports of mind experience and brain activity. That's why we think the mind is emergent from the brain.

7. Correlations are not causations. "We" don't "think" the mind is "emergent" from the brain, we KNOW it is beyond any doubt.

"We" may one day discover that mind can emerge from a high capacity computer with highly sophisticated programing (I'm doubtful), but until then, only brains are known to experience delusions.

8. Hominid7:03 AM, January 08, 2021

So now there is "a physical phenomenon behind a thought". Make your mind up.

We don't know it is beyond any doubt. See the post on weak v strong emergence. Also, neuroscience isn't a complete theory. Granted, it seems likely based on current knowledge.

Try to stick to the reality of the evidence, rather than making wild assertions in all caps. You are on the path to psychosis.

10. Unfortunately, General Relativity, same as all conventional theories I've heard of, does not model one particular feature of time as we experience it, namely the "present moment" being special. This is called "problem of Now", and it troubled Einstein himself.

Some physicists swipe the problem under the rug, saying that the present moment is an illusion, but for me this does not explain anything.

It looks like "geometrical" theories (including the network-based ones) are not compatible in principle with describing present moment...

1. There is no problem of now. This is complete nonsense. You get a "problem of now" if you consider systems that have no path-dependence. In this case they can have no memory, so of course you have no "now". I don't understand why people are still discussing this.

2. @No-Go
I completely agree with you.

Sabine seems to believe that
special relativity describes a "Now"
that "moves through time with a velocity of c":
http://backreaction.blogspot.com/2020/08/do-we-really-travel-through-time-with.html

@Sabine
Could you explain how physics explains that
when you read this "early 2021" is in some
obvious sense more
relevant to you than "early 1991"?

3. Yes, we move through time with a velocity of c. This has been recognized for decades. It's posted on many places on the internet. "Duckduckgo" it for yourself.

4. Time (motion) is CONTINUOUS. That's why there is no "now" - it is just a part of the continuum - NOT a discrete POINT. The beginning and end of "now" is undefinable.

5. I have a book edited by Hawking of Einstein's papers, A Stubbornly Persistent Illusion. Einstein was concerned about the perception of a "now" or moment in time. If time is with space a single manifold then it is odd that we perceive some passage of time through this.

6. Perception is a part of the illusions/delusions of mind. We perceive color where none exists in reality; we perceive tone where none exists in reality; we perceive pain where none exists in reality. But, for each there are fuzzy correlations with physical events that are useful to us as an evolutionarily emergent survival strategy. They are illusions that USUALLY correlate well enough with our physical surrounding that they enable us to interact with that surround in an adaptive way. They are useful, but fallible, illusions.

The same applies for "time" - it's an illusion that correlates roughly with motion (as generally defined) and it serves our survival. Many organisms - your dog, for instance - appears NOT to have much of a time illusion, if any at all.

Time concepts like velocity and acceleration describe motion and are useful to physicists and engineers for their predictive quotients.

7. “The distinction between the past, present and future is only a stubbornly persistent illusion.”
Albert Einstein

A photon travelling in a vacuum does not experience time. Regardless of the distance traveled, time, from the photon's point of view, has not changed. There are no old photons from the photon's reference frame.

8. Hominid12:01 PM, January 04, 2021

"Time (motion) is CONTINUOUS. "

It is not known that time is continuous so you don't need to write it in all caps. Time is represented mathematically as a continuous variable, but that's a model not necessarily reality.

9. Yup, a mystical thinker - everything and anything is possible. Try scientific thinking - it's proven far more gratifying and useful.

10. You're proffering assertions that are well-known to be false. Is that because you are unaware of scientifically established reality or have an ideological agenda?

11. Hominid7:21 AM, January 08, 2021

You assert absolutely that there is nowhere in the universe that is not at absolute rest spatially.

So the universe cannot be a sphere with motionless centre.

Amazing. Publish this evidence you have about the topology of space-time.

Remember, writing it in all caps doesn't count as evidence in Physics, sweetheart.

12. Hominid7:06 AM, January 08, 2021

"Yup, a mystical thinker"

So physically measured time is continuous, petal? I think the mathematical variable in the model is continuous and the physical reality not known to be so.

What time is it?
Epsilon past 1?

You have made the cardinal sin of confusing model with reality. I don't think our relationship can recover, my all caps love.

11. Time is information we don't have. Time is our ignorance.

Bonus points if you know who wrote that. Hint: he has commented here in the past.

1. I received this comment 3 times. I only approved it once. Please do not submit a comment multiple times. I usually approve comments within 12 hours.

2. "Time is information we don't have... who wrote that"

Someone who is unaware of the existence of clocks. Tell him it's half six.

12. testing a new way to post but not seeing the usual message. :-(

1. Sorry, here is the usual message:
When will you be publishing the paper with "evidence" of universal fine-tuning? Or is this Nobel Prize level work low priority?

2. Why do you not hold yourself to the same standard of evidence?

3. I have considerably higher standards of evidence than lunatics who randomly believe in universal fine-tuning, my darling.

13. To the newcomers: PLEASE READ THE COMMENT RULES. I do not approve links to personal webpages or any other website other than scientific journals and major newspages whose URL I immediately recognize. If I'd have to click on a link to find out what it is, I'll not approve it.

14. Sabine,

Alright, interesting and provocative post. Here’s what I’m seeing.

Time is a four-letter word.
We have found a need for it in our language and in keeping appointments.
Wiki says, “time is what a clock reads,” and I can live with that if we stipulate a multiplicity of clocks wherein on the back some say made in China and some say made in Nature.
Clock making is more of a cottage industry than a universal franchise.
Nature creates a multiplicity of clocks.
Nature creates clocks in many tempos through the physical dynamics of each enduring structure.
So, is time a tyrannical governance by one clock, or rather a democratic federation of many temporal domains with time emerging from enduring micro-tempos to enduring macro-tempos.
We are free to pick a tempo to set our clock.

“The music of earth sings through a thousand holes.
The music of man is made on flutes and instruments.
What makes the music of heaven?
Master Ki said:
Something is blowing on a thousand different holes.
Some power stands behind all this and makes the sounds die down.
What is this power?”
– Thomas Merton, The Way of Chuang Tzu

That said, what does quantum mechanics do to make sure everything doesn’t happen at the same time? Is time simply hidden in its formalism?

And, is positing the state of the universe at one particular time actually a valid proposition?

15. If you haven't already checked it out, Julian Barbour has an interesting timeless theory called "Shape Dynamics". It's like the Mach Principle taken to the extreme.

There's a tutorial here: https://arxiv.org/abs/1409.0105

1. @ Jason
That is an impressive piece of work. Its density seems to produce its own gravitational attraction. My appreciation though, is that of someone familiar with a craft of fitting things together and not one of real understanding. If it were a kitchen appliance, there would be a list of major selling points for me to read. Can you judge whether it is deterministic in the sense that it is used on Sabine’s blog, lack of free will etc.? Thank you.

16. Time is not real it's actually... imaginary...

1. Exactly. The other big problem we have is that language is a representation of the mind, not of reality. When scientists try to describe an event using our intuitive languages rather than math or a display, it creates enormous ambiguity.

Then there's the nagging problem of unit boundaries otherwise known as continuity. Boundaries apparent at the macro or even micro levels, often disappear at the ultramicro level. Where does one put the boundary between two different elemental atoms in a molecule when the two have overlapping electron clouds and are in fact sharing particles? Where is there an actual boundary between the surface of your skin and the air it's immersed in?

2. "Time is not real it's actually... imaginary..."
LOL Good joke. Compared to space in the metric it is an imaginary number.

3. Could you explain HOW his statement is a joke? Or, do you just make pronouncements?

4. On page https://en.wikipedia.org/wiki/Minkowski_space
"In his second relativity paper in 1905–06 Henri PoincarÃ© showed[5] how, by taking time to be an imaginary fourth spacetime coordinate ict, where c is the speed of light and i is the imaginary unit, Lorentz transformations can be visualized as ordinary rotations of the four dimensional Euclidean sphere

x 2 + y 2 + z 2 + ( i c t ) 2 = const . {\displaystyle x^{2}+y^{2}+z^{2}+(ict)^{2}={\text{const}}.} {\displaystyle x^{2}+y^{2}+z^{2}+(ict)^{2}={\text{const}}.}
So the space time coordinate can be written (x,y,z,it).
Usually, now-a-days, the time coordinate is usually written first ( it, x, y, z ), where i is the square root of -1. Also, it is usually abbreviated as ( -, +, +, + ). Also, ( +, -, -, - ). These theories are so complex that all of the math is coded and re-code to make the symbols and equation appear simpler. Wikipedia has many good articles and YouTube has many good videos that explain these concepts more clearly and in greater detail than I can here in a comment. I'm sorry that my comment was so terse that it left you frustrated. It is an occupational hazard for theoretical physics. I spend half of my life trying to find patient physicists who can explain their work. I like this channel because Dr Hossenfelder explains exceptionally well.

5. Hominid7:35 AM, January 05, 2021

"The other big problem we have is that language is a representation of the mind, not of reality. When scientists try to describe an event using our intuitive languages rather than math or a display, it creates enormous ambiguity."

Yes, it would be so much easier if Physics was just taught by showing equations with no natural language explanation. You are right.

"Then there's the nagging problem of unit boundaries otherwise known as continuity. "
There is no known continuity in nature.

"Where does one put the boundary between two different elemental atoms in a molecule when the two have overlapping electron clouds and are in fact sharing particles?"

There is no boundary when it is observed more closely. And?

" Where is there an actual boundary between the surface of your skin and the air it's immersed in?"

There is no boundary when it is observed more closely. And?

17. "However, we know that General Relativity is not fundamentally the correct theory."

could this apply to quantum mechanics ? especially since it contradict gr theory of time

1. What does "correct" mean? It's a value judgment - i.e., subjective. I have yet to see a theory that explains all observable events. Ergo, no theory is "correct" at some level.

2. Hominid7:36 AM, January 05, 2021

"What does "correct" mean? It's a value judgment - i.e., subjective."

Correct means fitting with the data, so it's not subjective, it's objective. Good try though.

3. Who decides how good the "fit" is? You? I'm not sure there is any scientific theory that "fits" completely with "the data," perhaps YOU are.

4. Hominid7:24 AM, January 08, 2021

Every physical measurement comes with an error bar, chuck.
How good the fit is?
Well, for example, the theory that the electron exhibits quantum behaviour, is a good enough fit to make your computer work.
Is the functioning of your computer a subjective question, petal?

18. Dr. Hossenfelder,

What a fascinating topic that I think is worthy of a critical discussion. I have been seriously thinking about this topic for the last couple of days and have basically come up with a lot more questions than answers.

Consider the following hypothetical situation; ET lands his craft in front of the United Nations. He steps out and says, I need a leader to meet me here is twenty-three and a half minutes so that I can pass along all of the information we have regarding our technology and medicine. If nobody contacts me I will leave in twenty-four minutes.

What we do not know is that ET’s world is a planet with half of the mass of our Earth. The Equivalence Principle tells us that clocks on ETs world will run faster than our clocks due to the lesser gravity. Of course there is the possible answer that now that ET is stationary and in our gravity his clock runs at our rate but that does not highlight my point; we cannot view time as a constant in our universe. The one thought that I have been kicking around about this more than others, is there a quantum concept of locality that needs to be considered here? A rhetorical question. There may not be any specific references in time in QM math, but there are still initial and final conditions with action in between and it could be argued that this points toward a “time” condition.

Dr. Hossenfelder, as always thank you for this thought provoking topic, and for the information about Julian Barbour’s book.

19. Lalplace's demon states that tome is a function of bandwidth, the more bandwidth you have the less time you need.
We need 50 hours to read War and peace with imperfect retention and comprehension, 5g enabled computer can download in 1 second with perfect retention and soon perfect comprehension.
The more bandwidth you have the less time you need or put another way the less you need time. For a powerful enough computer, yesterday, today and tomorrow will be the same.

1. Computers do not "comprehend."

As far as any evidence of which I'm aware, only biological nervous systems comprehend - and, then, with suboptimal accuracy.

Don't mistake SIMULATION for PROPERTY. It's a common fallacy among techies.

2. Hominid7:42 AM, January 05, 2021

"It's a common fallacy among techies."

It's not, though, is it. You're just making it up as you go along.

3. Laplace's demon states that an entity with perfect understanding of the laws of physics could plot the position of any body or matter, backwards or forwards and therefor time would be unnecessary, A Sufficiently powerful computer/god/alien will have no experience or need for time.
Einstein explained time as a ant would view a patch of grass, full of obstacles to overcome. Something with our intelligence in relation to an ant would simply step over the grass. In other words, when you see the universe from big bang to whatever as one big picture, why would you need time?

4. Evans - Your hypocrisy is on full display - YOU are the only one here "making it up as you go along."

5. Hominid7:24 AM, January 08, 2021

You have made at least 2 absolute assertions with no evidence provided:
There are no spatially motionless points in the universe.
Time is continuous.

Provide the evidence or withdraw the claims, otherwise you can join the universal fine-tuners and string theorists in the Physics sin bin.

20. Dear Bee,

In regards to your post there are several issues I’d like to bring up; my context will always be only what can be directly observed, in that respect only, I believe I am completely factual. Please point out anything you feel might have failed that effort. With respects to atomic clocks the information I’ll mention is from US National Institute of Standards and Technology and University Physics departments. While I read your blog post on the function of atomic clocks, I was already aware of the dynamics you wrote about.

Every clock you mentioned and atomic clocks measure an increment of the same fundamental physical phenomena, a change of position in space. You already alluded to that with the Earth’s orbit, rotation, and a pendulum clock so I’ll jump straight to the atomic clock. The physical dynamics that produce the clock’s output frequency are a microwave emitter in a feedback loop with a quartz oscillator, they are tuned to the fluorescence of the cesium atom (9,192,631,770 Hz). Regardless if we are talking about a specific frequency of light or a consistent vibration of a quartz oscillator, they are still increments of a change in position in space that define the clocks increments of time (output frequency). In this respect time is observable as a physical change of position in space, i.e., a photon, a quartz oscillation, the earth, a pendulum, etc.

Sabine said, “We can stand still in space, but we cannot stand still in time”. At best we can seemingly stand still relative to something else. We are still moving through space on the Earth, plate tectonics are changing our position imperceptivity slowly, and the more precisely you’d try to measure a distance be between two objects even temperature would show them to be in constant minute motion relative to each other. While distance can appear to be physically static, how would one observe or experience any distance without motion. Considering a distance’s physical meaning is relevant to motion and clocks measure increments of motion as time it offers a logical reason why they are inseparable and what time represents as the 4th dimensional coordinate.

I brought this up because there might be a foundational consideration here. I will not speculate more than saying a physical assessment of time seemingly can function in the dynamics of special relativity, which is compatible with quantum mechanics, and would appear to fail in GR as best I can determine because of the curvature of space-time. However, you did say GR may be imperfect.

1. You might be surprised to learn that when you perceive yourself to be sitting "still" in you barcalounger, you're actually hurtling through space at a velocity of tens-of-thousands of miles per hour. Besides the slow migration of your tectonic plate, you are (1) rotating around the Earth's axis, (2) orbiting the sun, (3) orbiting the black hole at the center of our galaxy, and (4) moving with the expansion of the universe. There are other forces moving you about as well, but pale in comparison with these velocities.

2. Another thing to consider is while there are many disagreements on what time is, almost everyone would agree time can measure change, for the past and for potential change we might predict in the future. Again, speaking only in the context of what causes change that we can directly observe, at the most fundamental level we can see it is mass, energy, particles, etc. changing position due to other physical interactions as we understand them. Considering that observable change is coincidently exactly what we can observe clocks to measure (a change of position in space). Is it really that much of a leap to consider time may be as simple as that? A measure of the phenomenon that is a change of position in space, exactly what can observe clocks to measure and what is at the foundation of all change we can directly observe? It doesn’t hurt this speculation that space (distance)-time is naturally inseparable for the phenomenon too.

3. Louis -- What is "change" if not movement?

4. Form, direction, complexion.

5. All involve and are a consequence of movement. Try again.

6. Quantity of movement is an incomplete measure of path. Path is a complex of energy and information.

7. Don,

Other than a singularity which the existence of is dubious at best, how does energy exist without movement? I not including potential energy however even matter that can be considered as potential energy will have some movement simply due to being at a temperature above absolute zero, let alone how can anything not be in relative motion to something in the Universe?

21. You might convince me that 'time' is 'real' in the sense of being an objective fact. But what about the 'present'? Isn't it a bit of an indoor pachyderm?

22. Time is not real. It is just a convenience for humans to compare motions like velocity (v) where V= S/t. (Space/time) However,momentum (p=mV) mass x velocity is real. where masses can be compared and its effect can be compared too. So motion (v) should really be defined by V= p/m.
Another concept that involves time is Frequency (f) where f=1/T where T is the period (t= time it takes to make one cycle) of a sound or electromagnetic wave. For example: c velocity of light= frequency (f) x Lamda wavelength(L): c=f x L where Frequency can be compared and wavelength can be compared and so real. Again if time or Period T = 1/f is an elusive concept that could be used for another system.

23. Time = Dark Energy (the expansion of our universe). Example: about 6 billion years ago equals the 1/n'th size (n depends on the expansion rate) of our universe compared to its current size. So, no, we are not able to stand still; neither in time nor in space. Nonetheless, time behaves/emerges as a dimension.

24. Time is treated very differently in these two theories

This sounds very interesting.

Besides that, I wish you an excellent year.

25. Time is money. Knowledge is power. But power is energy per time. Solving for money, we see that it is energy per knowledge. Thus, in the limit that knowledge goes to zero, the money diverges, regardless of the energy expended. :-)

1. Phillip Helbig11:26 AM, January 05, 2021

"Solving for money, we see that it is energy per knowledge."

So you are infinitely rich?

26. If time doesn't exist, and time is just an illusion, then the illusion of time exists, so whats the difference??? Maybe its time to trim Occam's whiskers.

1. "time is just an illusion" is more philosophy than physics. Time and space are among the the oldest physical concepts. Time is from the movement of the sun and the changes to biological systems (humans age). Space from assessment of taxes to farmland. Physicists and mathematicians don't like to say that previous theories and concepts are wrong. We prefer to present a bigger solution that includes the previous ideas as a special case. Just as quantum mechanics reduces back to classical mechanics when a certain limit is taken, new theories must include old theories. All of these relativistic and quantum theories are not really about time, but that the "Standard Model" has "ultraviolet divergences" leading current theories to discrete time and space ideas. Also, General Relativity is incompatible with the Standard Model. I am wondering if these two systems, relativity and quantum, can't be combined with the 100 year old mathematics used by mainstream physics. So I am looking at new mathematics that might combine them. Maybe String theory. But my favorite is Causal Fermion Systems ( in Wikipedia) which make no assumptions about spacetime. Spacetime emerges from a solution to a problem, or at least is independent of old spacetime ideas. I wonder if quantum entanglement also points to this. This new math and physics is the most complicated that I have ever seen. And people who know more than me dare not make specific statements. So I won't either. I hope that this helps you understand the problem of time a little better. Maybe watch Dr Hossenfeld's video again. She is a rare physicist who can explain well. I really enjoy commenting on these deep, fundamental problems.

2. Steve Bullfox12:55 PM, January 05, 2021

The concept is different in the 2 fundamental theories of Physics, and this needs to be resolved if theoretical progress is to be made.
See the blog post and Lawrence Crowell's comment.

27. You're using the term "exist" in an absurd way. One who doesn't understand the difference between reality and illusion is usually deemed psychotic or, perhaps, just ignorant of reality.

Science is the attempt to understand the universe that exists INDEPENDENTLY of the human mind - i.e., the cause-and-effect relationships that existed before the human mind arrived and will continue to exist after the human mind no longer exists.

1. Hominid2:29 PM, January 05, 2021

It's verging on psychotic for you to use the word in this context. ;)

Steve's aware of the possibility that time could be an illusion, and therefore we don't need to put him in a strait jacket just yet.
The word psychotic is usually reserved for a loss of contact with reality, rather than pondering subtle questions of emergence in Physics.

2. I said "psychotic OR ignorant of reality." It seems you need remedial work in reading comprehension.

3. And there you go again using "psychotic" incorrectly. Check the dictionary definition of psychotic.

4. A person who is unable to distinguish between reality and fantasy or illusion has a delusional mental disorder. Psychosis or psychotic behavior is an outburst of chaotic behavior, which may be violent. Delusional disorders are usually chronic long lasting mental illnesses, which a certain president might suffer from. Psychosis is usually transient.

5. “A person who is unable to distinguish between reality and fantasy or illusion has a delusional mental disorder.” Not always. Environments where masses of people are spoon fed a reality that is crafted to create a particular illusion do not have a collective mental disorder, they are simply being misled. A case in point was the riot in our nation’s capital yesterday. It was completely out of character for supporters of that “certain president”, but fully in character for his opponents, as witness the billions in property destruction incurred by them last year. Yesterday’s insanity is highly reminiscent of an event that occurred on 31 August 1939, during another time of mass illusion. While we may argue that time doesn’t exist it is inextricably woven into our language with sayings like: “Those who cannot remember the past are condemned to repeat it”.

6. David Schroeder8:45 PM, January 07, 2021

Trump's mob were sore losers trying to overturn an election result. There was no mass delusion. They *can* remember the past, and they were actively trying to reenact it.

" It was completely out of character for supporters of that “certain president”"
So who ran down the poor woman in Charlottesville - the Boy Scouts?

"but fully in character for his opponents"
You're not seriously suggesting this was a false flag op., I hope. Is that what the Aug 31, 1939 reference is? Are you suggesting Joe Biden is now going to invade Canada? You've lost me, chuck.

7. @Steven Evans:

Time will tell the truth in the course of investigating this horrible event. By and large the president’s supporters have been peaceful. You can’t rule out anything in the Capital assault. For example, why was a so-called member of “Trump’s mob” waving an upside down American flag? We saw that in Portland and elsewhere all summer, and they weren’t Trump supporters. A better analogy would have been the 27 February 1933 event rather than the August 1939 event. But what happened in Washington on Wednesday may very well have had elements in common with both of those historical tragedies. Time is needed to thoroughly vet this atrocious act, rather than a rush to judgment as happened in those earlier episodes of madness.

8. All you is insist on your version of things - which is derived of irrational thinking. Not surprisingly, you've revealed that there is an ideological agenda behind your obfuscatory posts.

I would encourage you to take that to a political site where you can blather away at your fellow psychotics. This site is intended for folks who want to rationally consider scientific questions.

9. @ David
Sorry lad, I find your comments more troubling than the incident itself. That at least was a logical consequence. You don’t need to look further for historical reference than Jan, 6, 11:00 AM and the much-amplified suggestion of “trial by combat.” A Tucker Carlson massage will not ease your discomfort, but hang in there, be well, really.

10. David Schroeder6:20 AM, January 08, 2021

The whole world saw Trump tell those present at the rally to march on and enter Congress to stop the confirmation of the vote. This is called a putsch.

You appear to be a fan of Alternative Facts.

11. Hominid:

There is nothing irrational about pointing out potential historical parallels to recent events. There were too many anomalies in both the election and eruption of violence on Capitol Hill to be passed off as my supposed, personal ideological agenda. Lots of people have pointed out suspicious things that should be investigated. For sure time is on the side of truth seekers, and Trump’s people will, no doubt, continue long past Biden’s inauguration, to examine possible fraud in multiple states. It may be that nothing definitive will be found, as with the 3 year Russiagate investigation. Well, since the consensus is that time doesn’t exist I guess no one will be building a time machine to peek into the future to see how this nightmare pandemic unfolds in the coming months and years.

12. Wow, I did not think this would make it here. Do you really think the "buffalo Viking man" was some sort of BLM or ANTIFA plant? There are mass delusions that occur, and this happened in Russia, Italy, Japan and Germany early last century. This sort of thing has been growing in the United States, and in other parts of the world. It is dangerous.

BTW as a sideline, Vikings did not wear helmets with horns. That motif came about with the stage productions of Wagner's Valkyrie.

On the issue of time as an illusion, it is possible that time is emergent from quantum physics or entanglements that make up space. As such it is a statistical result, and for large N = # states etc, it is a sort of emergent property or epiphenomenon. It could in one sense be an illusion, but it is not a delusion. Thinking of which it is time to get back to work.

13. Could you please take your political kerfuffles elsewhere. This is a place to discuss the reality of time, not BLM or ANTIFA, thanks.

14. Steven Evans is a political TROLL who has invaded your forum. He seeks only to provoke with illogical, ideological claims and ad hominem taunts, rather than seriously discuss scientific concepts.

15. Steven get pissed off by nonsense and has troubles staying polite, both of which are problems I am sympathetic to.

The major problem with my comment section is that it is overrun by people who confidently make statements about things they clearly know nothing about, such as you.

16. Ah - I get it now - you're a charlatan. Don't worry - I won't be visiting your site anymore.

BTW - YOU and Evans clearly know nothing about the brain and perception (psychophysics).

17. Louis,
Take what I say with two grains of table salt such as you might have around the house.
Hominid’s question to you was, “What is "change" if not movement?”
I think movement is an incomplete index of change. The movement to which our eye is so attracted is a measurable manifestation of energy and that measurement always reveals energy to be in some form. It is the form that changes while the quantity of energy does not. This might be better said by a physicist.

18. Don,

You said, “I think movement is an incomplete index of change”.

Being fully aware that what we can observe directly is not all there is to the physical functions of the Universe; the reason I discussed what we can observe clocks to measure, what almost everyone would agree time is used for, and how, at that root, directly observable evidence shows a clear and simple physical dynamic, it’s my starting point. It appears fundamentally and structurally sound under the scrutiny of directly observable evidence.

I read Sabine’s’ blog because of her penchant to prioritize testable predictions, not to assume (“the Universe doesn’t have to be anything”), and a basis for strong foundational axioms in a theory, hypothesis or scientific speculation, whenever possible. The question I’d like to ask you, to explore this more is; what do you think is an index of change that contains no movement?

19. Louis,
Sabine has expressed concern that the statued halls of physics are being invaded by an aroused mob of fur-clad nativists. She is understandably distressed by the chalkboard screech of imprecise language and erroneous assertion. Keep an eye out for those guys and be cautious whom you ask questions of.
My statement was, “I think movement is an incomplete index [indication] of change.” While this makes some sense in the context of the discussion, it is clearly not explicit as to the physics. I was wanting to point out that energy is not just about movement. One would have to include energy’s 'form’ for a more complete description and form can change while the quantity of energy does not. More broadly I guess we are talking about entropy. A given volume of air might evidence a lot of movement but contain less energy than a grain of rice at ‘rest.’ As to whether energy is always moving, though perhaps just in smaller orbit, that is another question.

28. Maybe I missed this in all the discussion about causal connections, the -- perceived, at least -- one-way flow of time, and the specialness of 'now', but ... suppose the flow flow of tim was reversed and we start living backwards, as it were. Suppose this was not a one-time event, but a common happenstance? So common, in fact, that the odds of either direction average out to 50:50.

My question is how would you know? Particularly since memories are being erased, not written?

1. "How to know time is running backward". The entropy of a closed system would decrease: a chicken generates a lot of entropy(disorder) to produce an egg. If a cooked or broken egg reformed including the embryo coming back to life without any outside intervention, then the entropy of the egg would "go negative". That would be very noticeable. Many people use thermodynamics for an arrow of time.

2. Who would observe this supposed 'very noticeable decrease in entropy?

29. Hi Sabine
Near the end of your video you say :
"...Barbour calls the Janus point, from which time flows in two directions, its currents driven ...". Is this correct? In Barbour's universe there is NO time flowing; there's only complexity growing in both +/- time directions, where time is treated as an (intrinsic) parameter only. This is similar to the Wheeler-deWitt equation with ADM foliation or 3+1 split.

1. rhkail:

"Near the end of your video you say :
"...Barbour calls the Janus point, from which time flows in two directions, its currents driven ..."

No, this is not what I say. I know that my English isn't the best, but if that's what you managed to hear, you may want to turn captions on.

30. Oh, I am so sorry, you are right! Of course, this erroneous text is not yours; it is published by goodreads.com, the website I visited as I got interested in Barbour's new book: The Janus Point.

1. That's much of a relief ;)

31. Answering my own question, I'm guessing that it is impossible even in principle to tell if the direction of time had been reversed, running the movie backwards as it were, because we ourselves are in the movie. Psychologically at least, we associate the accumulation of memory with running the movie in the usual way, that is, with the counter incrementing, and we say our most recent memories are the ones with the highest count. Since reversing the flow of time is exactly the same thing as decrementing the counter, we forget our most recent memories first. Reversing the flow of time means we can only read (and only destructively to boot), never write. Metaphorically at least, the universe is implemented as a stack data structure; the last addition will be the most recent push and the first pop will be the last push. And that certainly does distinguish the direction of time. But only to an outside observer ;-)

32. Hi Sabine,

I think my last comment was removed because of a link to arxiv, my bad, I thought that site would be appropro
iate.

I'm still interested in your opinion of Shape Dynamics and how it relates to Time. Thanks for all the awesome content you have.

33. Language is one of the tools we use to create a useful representation of reality for our minds to hang on to. Sure it creates ambiguity, but my view is that ambiguity is there by design. Ambiguity in language makes it possible to say counterintuitive things about time. It also makes it possible to agree to disagree so I am a big fan of ambiguity.

One of the things I have observed following Backreaction for a couple of years is that a few of the posters, as well as our host, have such a deep understanding of physics and mathematics that they are able to clearly explain some really complicated physics. With Sabine it is not even her native language. So while ambiguity has its place, language does not necessarily have to be ambiguous when it comes to physics, which would lead me to wonder if the mathematics used in physics actually is completely free of ambiguity.

Finally, my wife has a degree in clinical psychology so I asked her if I am psychotic. Her response was ambiguous.

34. The mention of Julian Barbour immediately rang a bell, as I knew I had received a book written by him as a Christmas present years ago. After a bit of searching I found it. It was an older book written in 1999 titled: "The End of Time". Looks like a rather interesting read.

35. Thinking that time is negligable might lead to a fatalistic leitmotiv like "Take therefore no thought for the morrow: for the morrow shall take thought for the things of itself". (Matthew 6:34)

36. As I divide my web browsing/participating time principally between Backreaction and threads at 23andme, a genetic site, I cannot help but notice that there are outstanding posters with deep and comprehensive knowledge in their respective fields in both places. Over at 23andme there is poster who is as knowledgeable in paleoanthropology as Lawrence Crowell is in fundamental physics here. It is a pleasure to read both authors comments, though I cannot honestly say I understand everything they say, as much of it is highly specialized. But I do try, and will continue to work on upgrading my knowledge.

One nugget of information that was finally cleared up in my mind in this post was Sabine’s mention of “boost”, as I had encountered this term before in reading papers on the hypothetical Alcubierre warp drive. I was perplexed by the use of that term in reading Sonny White’s Warp Drive 101 paper, but now it makes perfect sense. And particularly provocative was Sabine’s mention of the idea that our space-time universe might be embedded in a space-only background; an allusion to an additional dimension which we haven’t yet detected, but which has formed the basis of speculations in recent decades. When the ADD paper “The Hierarchy Problem and New Dimensions at a Millimeter” appeared in 1998 I was completely astonished, marveling at the fertile minds of physicists. Since then constraints have been placed on the size of any additional spatial dimensions through laboratory experiments, but I believe the search for them is still ongoing.

And one final thought. As a long time student of the UFO phenomena I’ve long been aware of close encounter reports where temporal effects were noted. Typically time measuring devices like wrist watches and dashboard clocks would both be offset by exactly the same amount after a close encounter with an aerial object of unknown origin. Taken at face value, the suggestion here is that a region within some tens of meters of these objects is permeated by a ‘time field’ altering the rate of temporal passage for everything within its range of influence, be it mechanical or chemical/biological. Indeed, there was a sheriff in Texas who had a cut on his arm, which remarkably healed up faster than normal following a close encounter, as if normal metabolic processes were speeded up. Now the Holy Grail of our terrestrial physics is quantum gravity. It might be that civilizations older than ours have passed that stage of unraveling nature’s secrets, exploiting that knowledge for technological application. We know that strong gravitational fields can alter the rate of temporal passage, but it takes humongous quantities of matter, concentrated in a very small region to significantly change clock rates or length scales. Just maybe, there is some aspect of a future theory of quantum gravity that would enable significant changes in those quantities with many orders of magnitude less material.

37. @Lawrence Crowell at 11:00 AM, Jan. 8, 2021: “it is possible that time is emergent from quantum physics or entanglements that make up space.” With my current rather modest comprehension of quantum physics that makes sense. In Julian Barbour’s theory time is dispensed with and replaced by the configurations of matter in space, as Sabine explained. But matter is ultimately quantum, so I guess every time a particle’s quantum state; energy, spin, etc. is updated that would correspond to the passage of time, or a ‘click’ in time, so to speak. The average of gazillions of ‘clicks’ would then correspond to the rate of temporal passage for a bulk piece of matter.

As I sort of alluded to in the previous comment (10:09 AM, January 9th, 2021) do you think it might be possible for a (hypothetical) Alcubierre Warp to run on just the local manipulation of time alone? Or space alone? Being aware that negative energy density of the vacuum is not permitted the thought occurred to me that if just one component of space-time were altered that it might be a way to circumvent that restriction. To elaborate on that a bit, the idea would be, in the case of time, to create a field behind the spacecraft in which time is speeded up, while an opposite field would be created forward of the craft slowing time down. But probably more sensible would be length scale expansion behind the craft with a corresponding length scale contraction forward of the craft.

38. James M. Chappell et al have a paper: Time As a Geometric Property of Space (2016) in which time is a dependent scalar dimension within a geometric algebra description of space. This generally fits with my own view that the direction of time is equivalent to the sign of the trivector (pseudoscalar; either + or - sign of eijk), although I am not clear why the paper stressed the virtue of a scalar dimension as a point property. The + or - property of eijk in geometric algebra may perhaps be related to dS or AdS? Time being the direction of torsion in a trivector volume of space may be connected to the virement between time and space within the constraints of time conforming to the sign of eijk. This seems to fit also with the entropy arrow of time.

39. Hi

I was wondering on your take on "retrocasual" theories, which try to explain quantum mechanics by backward in time dynamics.

1. There's no such thing as retrocausality, please read Section 7 of this paper.

2. A simple logic says that if you get results of superluminal propagation, none of the members of the observation is a cause but they can be an effect of common cause. With entanglement and virtual particles the cause is likely the global wave environment correspondence mechanism - just like free fall or Planckian rhythm (2-string-rope analogue reveals something perhaps)...

3. "retrocasuality": As most physics theories are time symmetric, many people wonder about time moving "back". And Feynman noticed that this could imply that a positron, antimatter electron, could be viewed as just an electron moving backward in time.
I tend at agree with Dr Hossenfelder here.
However, I prefer that theory not get too far ahead of experimental results. So that is one reason that I study Causal Fermion Systems as causality is built into the theory.
It's the same with faster than light travel: show me something going faster than light and I will talk about faster than light theories. Until then, it is just mathematical physics.

40. Time is the process the measuring an object's motion. The concept of time in the brain would not exist unless there was a way to measure the regular motion of an object. Time is a concept like "hot" which measures the relative amount of heat in an object.

1. Ok. Time can be an amount of motion but an uniform motion cannot be like an invariant proper time. If time is a process connected to measuring the motion of an object then it should be got out of non-uniform motions i.e. accelerations.

Do you think there is the equivalency of the proper time and the gravitating energy?

2. There is no invariant proper time. Time is only the measurement of any "periodic" motion and recording the amount of motion measured under the concept of "time" in the human brain.

41. PIO I agree with your view of time and motion. But I think that you seem to confuse heat energy with temperature. Perhaps it is just due to the conciseness of your comment.

1. The comparison of heat and time is only an analogy made to show that the human brain has invented a number of concepts/categories to record amounts of motion. The heat of an object is the amount of motion in the object. Time is the amount of periodic motion undertaken by an object.

2. BTW, space is the same sort of thing as time. It is the the classification scheme the brain has developed to record and relate amounts of distance. Time functions the same way for relative amounts of motion.

42. Sabine,

In so far as words suffice, here is a metaphysical conjecture on the mechanics of time’s origin:

Time is begotten, born at the beginning, emergent as essential attribute of the physical universe. Its heritage lies in a hidden domain, a lineage that can be explicitly inferred, but not directly observed. Time must shoulder its way into being, emerge to make a tiny interim for itself, compete to find space among other tempos and join the conjugate dynamic that creates time’s volume in the universe. Time must be declared by the endurance and resonant nature of physical structure. It is made manifest like the sounding of a bell, and there, it is neither the action of the striking nor the elastic constraint of the casting, but rather their conjugate dynamic that produces the enduring cycles and leaven time.

You can listen here to an audio analogue of time’s inception in the sounds produced by a 36” brass gong (in particular that at 7:50) or 13:00): https://www.youtube.com/watch?v=1vjzXRQc_zA

If you can imagine putting a black velvet drape in front of the visuals and only attend to the sound, you will hear an audio metaphor of the emergence of the physical universe from an implicit but hidden domain.

Once again, this is conjectural and perhaps an unwarranted assertion. Is there room for it in physics?

43. Don Foster,
If I knew what any of that meant, perhaps physics could accept it.
Giving it the benefit of the doubt, I believe metaphysics arose in humanity thousands of years before physics did. Again, that's just a believe of mine, so if a reader disagrees, I won't be offended.
People have a very limited comprehension of the universe. Even free thinking scientists fall into mental ruts. I think that is where I see physics today - in a rut. We have some beautiful theories, but then data, pieces of reality, have been measured that don't fit into our wonderful theories. Eventually, a younger person interprets a bit of metaphysics and a new theory develops. That's why I have always enjoyed working with students and have tried to encourage them instead of only instill just the "facts".
When I taught computer science I had a student that I couldn't decide if he was brilliant or insane. We don't have a place for them in our world and he had no job and was poor.

So keep thinking and posting.

1. @ Edward Lulfos

Thanks, your comment has me chuckling here on a snowy morning. I have heard that humor occurs when there is a difference between expectation and what happens next, but then, that would be the same for tragedy. It was useful that your comment was not dismissive or mean-spirited.

I try to use language with precision, but that often produces a density of the prose that is difficult to decipher and one may suspect a slight-of-hand. While I prefer to think that my observations are those of natural philosophy, it is difficult to tell if I venture into philosophy of the unnatural kind. I like to think my observations are grounded in reality and arise most clearly as reflections during a walk between the river and the garden, perhaps crossing a thoroughfare or two and passing by a playground. That and my being by long experience a tool user and seeing how device turns energy to particular effect. (I may have lost you at this point)

Letting go of the details of physics and looking at nature from a less exacting vantage point is easier for some than it may be for a physicist. Granted, that’s no guarantee of utility, but it does get one out of the box. And, looking from at nature from particularly rarified and abstracted point of view, one can observe the dynamics of the universe as resulting from the interplay of change and constraint. ‘Change and constraint,’ two plump and broad-bottomed words that are perhaps placeholders for something more exacting. Yet, such a general observation does not seem to do damage to physics proper. Is it useful?

Well consider the observation that the universe exhibits cyclical wavelike and iterative dynamics over spatiotemporal scales that vary by many, many orders of magnitude and manifest in a myriad of diverse physical mechanisms. Physics proper is exactingly descriptive of these dynamics on a case-by-case basis, but there does not seem to be a general rationale as to why cyclical dynamics should be such a ubiquitous motif. Given the dearth of straight lines in nature, perhaps really the only motif.

So, consider this primitive verbal equation:

Change + Constraint = Cycle

What does that mean if anything? I view it as an equation in which the property of each addend is preserved in the sum. A statement that a cyclical dynamic is the accommodation of change to constraint, the wheel to the axle, the energy to the orbit. To me, the ubiquitous presence of cyclical dynamics in the universe suggests that it is part of its basic fabric and arises on a metalevel, is ongoing.

Regarding time, at maximal constraint there is no change of tick to tock. At maximal change, there is no clock.

That’s my mental orbit. In theory, I am open to perturbation.

Regards,

44. I understand and agree with much of what you say.
1) "out of the box". Yes that's what I'm talking about. That what I see as needed in physics at this time. From biology, I take the guideline that "diversity can help stability". And you are contributing to diversity and so to progress in our understanding of the universe. Another example is that there is a man who is using astronomical data to postulate negative mass and repulsive gravity. People are ignoring him. I really see no way to model the data as he says. But I do encourage him as there might be somebody somewhere that might expand our understanding. It will probably be a young, malleable mind of a student.
2) Physics does seem to suffer from a very narrow perspective in this age. And I frequently consider "Physics proper is exactingly descriptive of these dynamics on a case-by-case basis, but there does not seem to be a general rationale as to why cyclical dynamics should be such a ubiquitous motif." I agree that this is where physics is.
3) If I was teaching beginning physics again I would use the metaphor "Change + Constraint = Cycle". That is a good high level understanding of waves.
4) I have been around Nobel prize winners and try to look for that quality of work in other scientists. For the last few years I have been following a group of mathematical physicists who seem to be doing breakthrough work. When I talk to grad students, they listen and wonder. When I talk to colleagues, they are silent. The only response I get is "That's not mainstream", or "I don't understand". One Nobel prize winner on LIGO would only say "That's not my area of expertise." I wonder if they are embarrassed that they don't understand it.

That's why I find your words so important. You present ideas that should be examined and so I try to listen. There are people in the world that need to see your work.

45. @ Edward Lulfos
I appreciate your comments and having the ball returned over the net.

Sonar engineers found that adding noise to a weak source can boost its signal over the threshold of detection. I am willing to do my part and it is nice to have a little room to speculate without doing the work of physics proper. For example, a link to a paper was posted in these comments:

“A Shape Dynamics Tutorial” – https://arxiv.org/abs/1409.0105

Here one can appreciate the amount of exacting work that goes into physics proper, more than that of building a house by far and that is a lot of work. Everything has to fit precisely together and one is working in a realm of high abstraction rather than tangible objects. You have to give a complete list of foundational references upon which you have built, put your name on it and present it for the kindly ministrations of your peers with the possibility of it being knocked down along with your reputation. Tough business, physics.

Here my appreciation is superficial, the mathematics is impenetrable. I am more at home within a less exacting general systems level of observation, necessarily looking at physics proper from some degree of remoteness. Still, one can find little comments in the paper that appear to confirm previous discussion:

“… SD does without spacetime: the existence of a pseudo-Riemannian4-dimensional manifold with Lorentzian signature is not assumed among the axioms of the theory.”

“…Temporal relationalism: the flow of time is solely due to physical changes.”

“I will show now how to realize Mach’s aphorism “time is an abstraction at which we arrive through the changes of things” in a dynamical theory.”
Time zones originated at the behest of railway companies because rail travel was faster and needed to be coordinated within a broader framework. Perhaps time itself is local, the creation of each physical structure with complex variety of tempos. In sum, more like the steady accretion and richness of a coral reef than product of a single clockwork.

46. Well yeah, time coordination was needed long before the railroads. Synchronized chronometers were necessary for determining accurate longitude for naval vessels far from Greenwich. Universal time was a missionary endeavor.

47. Why have people given up on unifying electrodynamics and GR? Or they pretend that they are unified. I find that strange.

1. They haven't "given up on it", they have realized that electromagnetism isn't a fundamental theory, but needs to be quantized, which makes it QED, which is (kind of) part of the standard model (it's somewhat more difficult than that), and the standard model has to be unified with GR.

48. No one has given up on unified field theories. There are several. The most popular being M-theory and loop quantum gravity. There is no experimental evidence to favor one over any of them. At this time, I'm following/favoring causal fermion systems. But don't look at it if you are afraid of mathematics. It has quantized more concepts than the other unifying ideas. causal-fermion-system.com

49. Don Foster, "adding noise to a weak source can boost its signal". I'll need to see a reference for that as it seems counter intuitive.
"Shape Dynamics": I'm having trouble in the abstract: "replacement of GR's relativity of simultaneity with a more tractable gauge symmetry, namely invariance under spatial conformal transformations." I'll look at it some more.

1. It's called stochastic resonance.

2. Ah, a non-linear effect. When students are only taught linear methods, many phenomena seem counter-intuitive.

50. @Sabine:
GRT and quantum theory are both very well proven but accepted to be wrong/imperfect where both have to be applied (black holes, urknall).
It seems to me, most physicists think or believe that he quantum theory is somehow more 'true' or nearer to reality than Einstein. Simply coz a real singularity or rather infinities are undesirable effects.
Q: Is this correct?
Reading this blog I receive the impression that U too tend this way. And that makes me wonder remembering your book with the main declaration, that the reality will not be what scientists want it to be.
Q: Where I'm wrong?
Thx!

1. First, physics is not right or wrong. All we have are models. Female fashion models are different than average females. A model is just an idea. If the data from experiment match the model, then we accept the model.
Second, I think that you can find infinities in any physics model. Quantum theory has infinities just about everywhere. Then some math is applied that removes the infinity. If that model matches experiment, then we use it. For example, I've never found an explanation for using a "symplectic vector space", except that it works.
Third, GRT only has infinities in conditions very far for which it was created. So that's quite good.
Fourth, GRT doesn't have forces! Mass moves according to space-time. In my opinion, then it doesn't make sense to talk about quantum gravity and the graviton, that's a minority opinion.
Fifth, I agree with Dr Hossenfelder, no one really knows what reality is. I think most scientists agree with that. We all just love playing with equations and lab equipment.