## Thursday, February 20, 2020

### The 10 Most Important Physics Effects

Today I have a count-down of the 10 most important effects in physics that you should all know about.

10. The Doppler Effect

The Doppler effect is the change in frequency of a wave when the source moves relative to the receiver. If the source is approaching, the wavelength appears shorter and the frequency higher. If the source is moving away, the wavelength appears longer and the frequency lower.

The most common example of the Doppler effect is that of an approaching ambulance, where the pitch of the signal is higher when it moves towards you than when it moves away from you.

But the Doppler effect does not only happen for sound waves; it also happens to light which is why it’s enormously important in astrophysics. For light, the frequency is the color, so the color of an approaching object is shifted to the blue and that of an object moving away from you is shifted to the red. Because of this, we can for example calculate our velocity relative to the cosmic microwave background.

The Doppler effect is named after the Austrian physicist Christian Doppler and has nothing to do with the German word DoppelgĂ¤nger.

9. The Butterfly Effect

Even a tiny change, like the flap of a butterfly’s wings, can making a big difference for the weather next Sunday. This is the butterfly effect as you have probably heard of it. But Edward Lorenz actually meant something much more radical when he spoke of the butterfly effect. He meant that for some non-linear systems you can only make predictions for a limited amount of time, even if you can measure the tiniest perturbations to arbitrary accuracy. I explained this in more detail in my earlier video.

8. The Meissner-Ochsenfeld Effect

The Meissner-Ochsenfeld effect is the impossibility of making a magnetic field enter a superconductor. It was discovered by Walther Meissner and his postdoc Robert Ochsenfeld in 1933. Thanks to this effect, if you try to place a superconductor on a magnet, it will hover above the magnet because the magnetic field lines cannot enter the superconductor. I assure you that this has absolutely nothing to do with Yogic flying.

7. The Aharonov–Bohm Effect

Okay, I admit this is not a particularly well-known effect, but it should be. The Aharonov-Bohm effect says that the wave-function of a charged particle in an electromagnetic field obtains a phase shift from the potential of the background field.

I know this sounds abstract, but the relevant point is that it’s the potential that causes the phase, not the field. In electrodynamics, the potential itself is normally not observable. But this phase shift in the Aharonov-Bohm Effect can and has been observed in interference patterns. And this tells us that the potential is not merely a mathematical tool. Before the Aharonov–Bohm effect one could reasonably question the physical reality of the potential because it was not observable.

6. The Tennis Racket Effect

If you throw any three-dimensional object with a spin, then the spin around the shortest and longest axes will be stable, but that around the intermediate third axis not. The typical example for the spinning object this is a tennis racket, hence the name. It’s also known as the intermediate axis theorem or the Dzhanibekov effect. You see a beautiful illustration of the instability in this little clip from the International Space Station.

5. The Hall Effect

If you bring a conducting plate into a magnetic field, then the magnetic field will affect the motion of the electrons in the plate. In particular, If the plate is orthogonal to the magnetic field lines, you can measure a voltage flowing between opposing ends of the plate, and this voltage can be measured to determine the strength of the magnetic field. This effect is named after Edwin Hall.

If the plate is very thin, the temperature very low, and the magnetic field very strong, you can also observe that the conductivity makes discrete jumps, which is known as the quantum Hall effect.

4. The Hawking Effect

Stephen Hawking showed in the early 1970s that black holes emit thermal radiation with a temperature inverse to the black hole’s mass. This Hawking effect is a consequence of the relativity of the particle number. An observer falling into a black hole would not measure any particles and think the black hole is surrounded by vacuum. But an observer far away from the black hole would think the horizon is surrounded by particles. This can happen because in general relativity, what we mean by a particle depend on the motion of an observer like the passage of time.

A closely related effect is the Unruh effect named after Bill Unruh, which says that an accelerated observer in flat space will measure a thermal distribution of particles with a temperature that depends on the acceleration. Again that can happen because the accelerated observer’s particles are not the same as the particles of an observer at rest.

3. The Photoelectric Effect

When light falls on a plate of metal, it can kick out electrons from their orbits around atomic nuclei. This is called the “photoelectric effect”. The surprising thing about this is that the frequency of the light needs to be above a certain threshold. Just what the threshold is depends on the material, but if the frequency is below the threshold, it does not matter how intense the light is, it will not kick out electrons.

The photoelectric effect was explained in 1905 by Albert Einstein who correctly concluded that it means the light must be made of quanta whose energy is proportional to the frequency of the light.

2. The Casimir Effect

Everybody knows that two metal plates will attract each other if one plate is positively charged and the other one negatively charged. But did you know the plates also attract each other if they are uncharged? Yes, they do!

This is the Casimir effect, named after Hendrik Casimir. It is created by quantum fluctuations that create a pressure even in vacuum. This pressure is lower between the plates than outside of them, so that the two plates are pushed towards each other. However, the force from the Casimir effect is very weak and can be measured only at very short distances.

1. The Tunnel Effect

Definitely my most favorite effect. Quantum effects allow a particle that is trapped in a potential to escape. This would not be possible without quantum effects because the particle just does not have enough energy to escape. However, in quantum mechanics the wave-function of the particle can leak out of the potential and this means that there is a small, but nonzero, probability that a quantum particle can do the seemingly impossible.

1. Isn't it true that in the Aharanov-Bohm effect the phase shift depends on an integral of a closed path and thus on the magnetic flux enclosed by the closed path? If so, the phase shift depends on the magnetic flux, i.e. an observable quantity, and not on the vector potential.

1. The integral over a field is a potential. And the whole point is that it's observable.

2. Whitewater:

The point here is that the experiment can be arranged so that the charged particle can only be present in the region where there is no magnetic (and no electric) field.

And, yet, despite the fact that there are no E and B fields where the particle can be, the motion of the particle is affected by the non-zero vector potential, which depends on the magnetic field in the region where the particle cannot go.

Superficially, it seems like action at a distance: it seems that the particle is being affected by the magnetic field in a region from which the particle is excluded. That is why the A-B effect was quite a surprise to physicists.

Speaking for myself, I still have trouble believing it is true, even though I understand the theoretical argument (which is almost trivial) and even though it has been confirmed experimentally.

Dave

3. Aharonov-Bohm paper is beautiful (1959, Physical review):
It is important to emphasize the "multiply-connected region." (page 490). Another paper begins: "the Aharonov-Bohm effect is simple and topological..." (Aharonov, et.al., Interplay of A-B and Berry Phases for Quantum Cloud of Charge, 1995). A beautiful paper by Waechter gives a path-integral derivation followed by a "topological perspective." Read: "for the Aharonov-Bohm effect to occur, we need a configuration space which is not simply connected." (2018 ETH Seminar).

4. I think you are missing a mathematical subtility: the fact that the electromagnetic field is null does not mean it does not exist.

It is the same enormity as saying that since the derivative of a function is null this function has no derivative.

See? Potential = function, field = derivative

5. The AB-effect can be explained in a fully gauge invariant and local way, but this requires a quantum mechanical description of the fields. When using classical potentials one will necessarily miss entanglement involving the fields:

https://arxiv.org/abs/1906.03440
https://arxiv.org/abs/1110.6169

2. Cool stuff ... One could do an honest count of how many effects one has known before and then post an answer here.

Me, for starters, I knew 7/10 for sure, and 1 I kind of knew, but did not know the name.

Nice collection anyhow!

3. I might expand the list to 12 with the inclusion of the Lense-Thirring effect and quantum entanglement. The Lense-Thirring effect is frame dragging of space by a large rotating mass. Quantum entanglement manifests itself in a range of phenomena from NMR to quantum optics.

1. Those are probably as real as The Hawking Effect.

2. Greg,

Your comments are very ill-informed and my patience with you is running low.

3. Quantum entanglement since the Aspect experiment has been found since the 1980s Aspect experiment. Frame dragging was measured with the Gravity-B probe and recently found in the orbit between a neutron star and white dwarf.

4. "Those are probably as real as The Hawking Effect."

Indeed; all are real.

4. Curious: electromagnetism-related effects dominate, and gravity is a bit player. The strong and weak nuclear forces are nowhere to be found (well, not explicitly).

Perhaps there's an observer bias, a kind of scale effect? At the scale of atomic nuclei, how many of these ten are notable?

2. Curious indeed.

3. There are plenty of nuclear effects (with my favorite being Moessbauer effect) but they are somewhat less general than the effects listed here.

5. Hello Dr. B. One typo. In effect 3, photoelectric effect, you wrote Einstein's bday as 1979.

1. Elver,

Yes, thanks, it's been pointed out to me by several people. There is no way to edit videos once they're published, so nothing I can do about it. Sorry about the blunder.

6. There is no experimental evidence for the existence of Hawking radiation. To claim that it is an important physical effect is disingenuous to say the least.

1. Norman,

2. To be fair, superfluids are simply an example of a system that is formally similar. Kinda like some general relativistic effects can be analogous to electromagnetic effects, with similar effects.

Nobody so far has measured Hawking radiation. I think Unruh radiation also has not yet been measured?

3. Cyberax,

What you say is not correct. The case of superfluids is not "formally similar" it is formally identical. It is the exact same effect. Hawking radiation has been measured in superfluids. Please check the reference I provided above.

4. We don't have any equations which combine gravity and quantum mechanics which have any experimental evidence to confirm them.

The geometry of space time in a superfluid is flat, the geometry around a black hole is not.

I cannot for the life of me understand how you can claim that these situations are "formally identical".

5. Norman,

"We don't have any equations which combine gravity and quantum mechanics"

How do you think Hawking did his calculation if he didn't have equations, huh?

"I cannot for the life of me understand how you can claim that these situations are "formally identical"."

The equations are identical.

6. Here is a view: "... physicists are impressed, but they caution that the results are not clear-cut. And some doubt whether laboratory analogues can reveal much about real black holes. “This experiment, if all statements hold, is really amazing,” says Weinfurtner, a theoretical and experimental physicist at the University of Nottingham, “It doesn’t prove that Hawking radiation exists around astrophysical black holes.” (Aug. 2016, Nature). Read Barcelo, Liberati, Visser: "analogy is not identity, and we are in no way claiming that the analogue models we consider are completely equivalent to general relativity — merely that the analogue model (in order to be interesting) should capture and accurately reflect a sufficient number of important features of general relativity..." (2011, Analogue Gravity).

7. You truncated my quote. I said that there is no evidence that any equation which combines gravity and quantum mechanics is correct. You may have some equation which purports to do that, but until you have experimental evidence all you have is speculation.

It may be true that you are using the identical equations in both circumstances but you have no experimental evidence that the application of that equation is correct.

Until we actually see Hawing radiation it will be no more than a prediction.

8. Gary,

No one claims that the observation of Hawking radiation in superfluids tells us that black holes also emit Hawking radiation. What I said is simply that Hawking radiation has been measured in superfluids.

9. Norman,

"It may be true that you are using the identical equations in both circumstances but you have no experimental evidence that the application of that equation is correct."

That is incorrect, as I already said. We have experimental evidence that the equations corretly describe superfluids, which demonstrates that the Hawking effect exists.

"You truncated my quote. I said that there is no evidence that any equation which combines gravity and quantum mechanics is correct."

Your statement is incorrect whether truncated or not. We do full well have evidence that equations combining quantum mechanics with gravity are correct.

I have the impression you do not know what you are even talking about. As I have said many times, not anything that has something to do with "quantum" and something with "gravity" has also something to do with "quantum gravity". Hawking radiation is not a quantum gravitational effect.

10. Sabine reminds me: "No one claims that the observation of Hawking radiation in superfluids tells us that black holes also emit Hawking radiation. What I said is simply that Hawking radiation has been measured in superfluids." However, does not Steinhauer, et al. make that claim ? As they say: "We find that the correlation spectrum of Hawking radiation agrees well with a thermal spectrum, and its temperature is given by the surface gravity, confirming the predictions of Hawking’s theory. The Hawking radiation observed is in the regime of linear dispersion, in analogy with a real black hole, and the radiation inside the black hole is composed of negative-energy partner modes only, as predicted." (2019, Nature). My previous post was merely intended to accentuate the all-important word "...analogy..."

11. An experiment proposes to rotate a tiny object with the polarization of a laser beam may detect a variant of this. An object in extreme rotation has its endpoints on an accelerated frame. This then results in a form of vacuum friction that is similar to Unruh radiation. This means on that frame the vacuum appears with a thermal bath of photons. From our inertial frame perspective the energy in the rotation of this object is lost. The object heats up and transfers the energy of angular momentum, and angular momentum, outwards as thermal photons. So there is a sort of friction of the vacuum that heats the spinning body up.

https://www.newscientist.com/article/mg20927994-100-vacuum-has-friction-after-all/

12. Gary,

The paragraph you quote says correctly that they have observed Hawking radiation in superfluids, that this confirms the prediction of Hawking's calculation, and that this situation is in analogy to black holes.

13. I was confused, and the source of my confusion was your description of the Hawking effect. As a mathematician I assumed that this had the status of a definition and so was surprised that you conflated two distinct effects under one name.

Thus what I should have said is that there are no observations which show black holes radiating. There is no indirect evidence for black holes radiating such as the indirect evidence for the existence of gravitational waves provided by the Hulse-Taylor binary pulsar.

I understand that you are claiming that an observation in a superfluid is evidence for the Hawking effect in black holes because it is an analogy.

My analogy is this: In the year 1880 an eminent natural philosopher blogged about the most important effects in their field of study. This list included the Michelson effect, which allows you to measure your absolute velocity by measuring the displacement of interference patterns. When asked about evidence for this effect this eminent natural philosopher replied that there was ample evidence as the effect had been demonstrated in gravity waves on the surface of water, in sound waves in air and in compression waves in a superfluid. Since all three of these situations obey exactly the same equation as case with light we can safely conclude that the effect is real and once the apparatus has been refined to be sensitive enough we will know our absolute velocity.

I feel that this is an important point becuase like you I feel that parts of physics have lost their way. Where we differ is in where that is. I believe that the point of failure was when theoretical results were accepted as true without experimental evidence. Howking radiation is the most egregious example of this failure.

7. Sabine,

concerning the Casimir Effect, I remember there were doubts that the vacuum energy is necessary to explain it e.g. https://link.aps.org/doi/10.1103/PhysRevD.72.021301:

>>Casimir forces can be calculated without reference to the vacuum and, like any other dynamical effect in QED, vanish as Î± → 0. The vacuum-to-vacuum graphs …do not enter the calculation of the Casimir force, which instead only involves graphs with external lines.<<

1. Stefan,

I am thoroughly unimpressed. One can calculate the effect, measure it, and that's the scientitif part. Then one can try to put words to the math. There will be more than one way to do the calculation and the words will always remain ambiguous. I therefore do not see what one learns from quibbling about them.

This of course is not specific to the Casimir effect. There is also more than one way to calculate and interpret the Hawking effect and many other quantum effects, depending on what technique or interpretation you use. What do we learn from that? Well, personally I'd say it means that structural realism makes not sense, but I'll leave that to philosophers to debate.

2. Sabine,

why do you object to the other explanation of the Casimir effect?

Literature says that the Casimir effect can also be explained as a van der Waals force. This avoids the use of vacuum energy. This is better because we know about vacuum energy that there is the discrepancy between QM theory and measurement of after all 120 orders of magnitude.

The van der Waals force is also in so far a better explanation as it is more classical physics and as such fits into a reductionistic system in contrast to Quantum Mechanics which do not.

3. antooneo,

I do not object to it. You misunderstood my comment.

4. When ships are in harbor precautions have to be taken to prevent the ship from banging into the harbor front. The reason is the waves between the ship and a seawall of a dock have a discrete 1/2 set of wavelengths. One the other side waves interacting with the ship have a continuum of wavelengths. Consequently the ship experiences a force towards the dock. That is one reason you see buffers on the dock to keep the ship from impacting the dock hard. This is a water wave analogue of what happens in the Casimir effect with the ZPE vacuum.

5. Lawrence Crowell ,that reminds me that a herd of whales has never caused a storm or ever, nor in theory; so, I don't know where butterflies get their great energies organized to create cyclones, making fun of all the Automation engineers

6. The so called Butterfly effect is just a heuristic statement that two systems nearly identical with a small separation in their initial conditions will separate. The two systems will at some time, the reciprocal of the Lyapunov exponent term, end up in states that have no apparent similarity. The "flapping butterfly" is a description of the tiny initial separation in initial conditions.

7. Thank you very much for your response, I agree with you, Sabine made a post that explains very well exactly what you say; but for me, chaos is usually temporary; the tendency of almost everything is to the balance; notice that a cyclone is precisely to restore thermal equilibrium; beyond a certain temporal chaos, systems tend to balance, they can have parts that evolve distinctly that break the balance; but in the end a new organization is established with a new balance. Chaos stands out precisely because it happens in a broad system in equilibrium; I think that space is a system in balance, the stars are also entities in balance. I am not saying that behind this there is nothing subjective; It is only a very general opinion.

8. ChaoChaos does not mean violence. The solar system has chaotic dynamics. The breakdown of predictability occurs on a very long time scale of 10s to 100s of millions of years. I did work on some of this, and did a numerical fast Fourier transform of this behavior and it has a remarkably 1/f signature. This 1/f noise spectrum occurs with the quantum states of electrons in condensed matter. This in the case of solid state physics has some possible connection to the Hossenfelder-Palmer idea of chaos in QM.

Nature does not quite go to equilibrium for systems that are open. Open systems with a small amount of energy flux exhibit quite complex and dynamical behavior. Planetary science has revealed some of this. The distant planets Uranus and Neptune are very cold due to a much reduced solar irradiance. They receive about 1/100 times the solar irradiance on Earth, about 1200watts/m^2. Yet interestingly these planets have incredible winds in their atmospheres and are far from being cold dead worlds. Even Pluto, what we thought was as dead as the ancient god it is named for, shows signs of considerable dynamics.
s

9. Lawrence Crowell
Thank you for your response and your patience, I realize that the chaos used in physics has a different meaning than the literal chaos, yes, chaos is not violence; Nor does balance mean immutable; in the case of the stars I was referring to the fact of, for example; all the particles that make up the Earth can have infinite configuration and correlations with each other; but the shape of the Earth and its distribution of density will not change almost; of course any particle within it can receive an energy that causes it to leave Earth; but below that energy the system is almost self-contained, it does not work as a set of particles with independent movement. In the case of an entangled electron-positron pair, I do not believe that the symmetric evolution between them is broken, unless the symmetry of the path between them is broken; the fact that in a Bell experiment the state of both particles can be correlated tells me that there is no arbitrary or chaotic evolution; if that correlation is broken later, I suppose the system does not become chaotic; It continues to evolve under the same rules, only that it is out of sync, I suppose. Remember, it is only the opinion of a neophyte,Be patient, hahaha, thank so much.

8. I read somewhere that the red shift of the CMB and distant stars is not really akin to the Doppler effect because it is not the stars that are moving but the space between us and the distant stars is growing and therefore the wavelength of the light is "streched". Please enlighten us. Thanks.

1. InConstruction,

That's correct. What I was referring is not the cosmological redshift of the CMB, but the distortion of the CMB due to our motion relative to it.

2. The cosmological redshift is not a Doppler effect in almost all senses (for the exception, see Bunn & Hogg). This was cleared up decades ago by Harrison, though many popular accounts still get it wrong. (Some professionals also get it wrong, but it doesn't matter that much since recession velocities, like the acceleration of the universe, are not directly observed and, while they can be calculated, there is little use for this.)

A topic about which there was genuine confusion was that of cosmological horizons, which was cleared up by the late, great Wolfgang Rindler in a seminal paper. One still occasionally encounters people confused on that topic.

The most recent example of cleared-up confusion in cosmology involves the traditional flatness problem; see the paper by Holman.

Bunn & Hogg: "The kinematic origin of the cosmological redshift"

Harrison: "The Redshift-Distance and Velocity-Distance Laws"

Rindler: "Visual horizons in world models"

Holman: "How Problematic is the Near-Euclidean Spatial Geometry of the Large-Scale Universe?"

All of these were published in leading journals in the field. I've linked to arXiv if a freely accessible version is not available at the official source or at ADS.

3. @Phillip Helbig:

I for 1, think that, the idea "CMB photons are MW (instead of gamma?) because their wavelengths got stretched on the way by expansion of Universe" is wrong!

I think, those photons are actually still the same (gamma?) photons, but we detect them as MW because of Relativity (imagine CMB sphere (all around) as a wall, that is moving away from us at relativistic speed)!

What I am really wondering is, if this view assumed to be correct, would it change Hubble constant calculation result using CMB, or not?

(Currently, a huge problem in astrophysics is the mismatch between 2 calculated values of Hubble constant, using Distance Ladder & using CMB!)

4. I for 1, think that, the idea "CMB photons are MW (instead of gamma?) because their wavelengths got stretched on the way by expansion of Universe" is wrong!

I think, those photons are actually still the same (gamma?) photons, but we detect them as MW because of Relativity (imagine CMB sphere (all around) as a wall, that is moving away from us at relativistic speed)!

It depends on the definition. Usually, the energy (or, equivalently, frequency or wavelength) of a photon determines what it is called (e.g. ultraviolet, gamma, radio). Sometimes the process does (e.g. gamma: nuclear transition; X-ray: deep atomic transitions, visual: normal electronic transitions, infrared: thermal etc). Of course, this way there can be overlaps in frequency between different sources. The photons are certainly cosmologically redshifted (not really a conventional Doppler shift).

What I am really wondering is, if this view assumed to be correct, would it change Hubble constant calculation result using CMB, or not?

Not.

(Currently, a huge problem in astrophysics is the mismatch between 2 calculated values of Hubble constant, using Distance Ladder & using CMB!)

5. I was going to write a detailed comment on this matter of "expanding space," but Phillip Helbig saved me the trouble with his link to the Bunn and Hogg paper.

I urge everyone to read their paper: it would have saved me a lot of confusion four or five decades ago if it had been available then; instead, I had to (painfully) figure it all out for myself.

One subject I wish Bunn and Hogg had expanded upon is the "Milne universe": anyone who wishes to pontificate on General Relativity needs first to check to see if what he or she is saying is true in the Milne universe.

The Milne universe is really just (part of) flat Minskowski space: it has space-like cross-sections that are just flat Euclidean geometry and a universal time coordinate. Indeed, Milne universe is only the part of Minkowski space lying inside the future light cone of the origin, so each space-like cross-section is just the inside of a finite ball with Euclidean geometry. Space is finite at any point in time.

And yet... if you use "co-moving" coordinates, including so-called "cosmic time," then each space-like section is an infinite non-Euclidean (hyperbolic) space.

So... is space flat or curved in the Milne universe? It is, as Einstein would have said, "relative."

Anyone who does not understand this does not really understand General Relativity, and, in particular, how the choice of coordinate systems can make one universe present itself in very different ways.

6. The paper by Bunn and Hogg should be required reading. Read: "Any statement in which “now” is used to refer to the present cosmic time at the location of a distant object is not about anything observable, because it refers to events far outside our light cone" and they conclude their paper with this:
"The common belief that the cosmological redshift can only be explained in terms of the stretching of space is based
on conflating the properties of a specific coordinate system with properties of space itself. This confusion is precisely
the opposite of the correct frame of mind in which to understand relativity."

9. Most of the posted are conveniently measurable. Which is to say they can be demonstrated in many well equipped university laboratories.
The Aharonov–Bohm Effect: it seems that my education has been sadly neglected. Of the posted effects The Casimir Effect has always been the most mysterious to me. It took a real reordering of my thinking to accept.
Thank you for the presentation.

10. I'm puzzled though how the word "effect" becomes attached to the names of some phenomena and not (perhaps) to others.

"This is a list of names for observable phenomena that contain the word effect":
https://en.wikipedia.org/wiki/List_of_effects

1. In a similar vein, I was wondering what sort of phenomena would qualify for your list. Does it have to be something that we currently refer to by the name "the * effect" ? In that case really important observations such the "laws" of electricity and magnetism (Coulomb, Ampere, Faraday, Ohm ...), the Michelson-Morley experiment and many more are not eligible because we don't call them "effects". Is that right?

Anyway, I have another candidate: the Edison effect. Actually, that name is rather dated. Most people nowadays call it "thermionic emission", and so it may not be eligible either. It's the phenomenon on which vacuum tubes are based. These lists always contain a significant dose of personal preference - that's what makes them fun. I enjoyed your list, but in my own world, the Edison effect rates above the tennis racket effect.

2. Philip Thrift: you are more than welcome to do a video about ALL those effects. Thank you.

3. I suppose from my particular philosophy-of-science (neopragmatism) view there are no "effect"s. (So the list would be 0.)

11. "A closely related effect is the Unruh effect named after Bill Unruh, which says that an accelerated observer in flat space will measure a thermal distribution of particles with a temperature that depends on the acceleration. Again that can happen because the accelerated observer’s particles are not the same as the particles of an observer at rest."

Another way of describing it: shake a thermometer in a vacuum and the measured temperature will increase. Nomen est omen: "Unruh" in German means a state of unrest, particularly appropriate here.

Although not in the Nomen est omen category, there are some papers by Daniel Holz and Bob Wald. "Holz" and "Wald" mean "wood" and "forest" in German. :-)

12. I have known about the Aharonov-Bohm effect for years, in the sense of having heard about it, but not really knowing what it was. So, last night I printed out a paper titled "A macroscopic test of the Aharonov-Bohm effect" by Adam Caprez, Brett Barwick, and Herman Batelaan, with the intent to read it while propped up in bed. As I expected, to truly understand the effect, comprehending the math is essential. And that requires coaxing the brain cells out of their natural laziness mode. Fortunately, Youtube has a smorgasbord of videos on the effect, which I just discovered this morning. Such visual-audio presentations I find easier to follow than dry words and equations on a sheet of paper.

13. Regards tunneling, years ago Condon and Gurney wrote a beautiful paper: "Quantum Mechanics and Radioactive Disintegration" (1929, Physical Review) where read "in quantum mechanics most statements of certainty are replaced by statements of probability" and "Now we throw the whole responsibility onto the laws of quantum mechanics, recognizing that the behaviour of particles everywhere is equally governed by probability." Razavy makes a claim: "we can describe the tunneling phenomena either partially or completely in terms of classical dynamics provided that we replace the simple system by a complicated interacting system." (page 139, 2003, Quantum Theory of Tunneling).

14. Funny thing, I thought some Youtube videos on the Aharonov-Bohm (AB) effect would be more transparent in explaining this phenomena than a text only presentation. Turns out the Wiki article on the effect was the easiest to follow, for someone at my level of knowledge. Even before I read the Wiki article, or comments on this post, I wondered if this effect was an illustration of quantum mechanical non-locality, such as embodied in quantum entanglement. That, in fact, was the argument advanced by Vaidman, in the Wiki article. In some sense that almost seems easier to comprehend than invoking the vector potential of the confined magnetic field in the solenoid version of the AB experiment. To be honest I don’t really understand what is meant by “potential” in this context. It’s weird that something is ‘there’, outside the solenoid, but can’t be measured like electric or magnetic field lines. Anyway, I started to read the Wiki article on the “Electromagnetic Four Potential”. Maybe that will help clarify things.

1. David Schroeder wrote:
> Even before I read the Wiki article, or comments on this post, I wondered if this effect was an illustration of quantum mechanical non-locality, such as embodied in quantum entanglement. That, in fact, was the argument advanced by Vaidman, in the Wiki article. In some sense that almost seems easier to comprehend than invoking the vector potential of the confined magnetic field in the solenoid version of the AB experiment.

As I said earlier, the A-B effect seems to violate locality, but, of course, it cannot change the electrons' path in a way that violates the no-signalling theorem. I.e., you cannot simply turn on the solenoid and get an instantaneous shift of the interference pattern, faster than light.

Somehow, there must be a not-faster-than-light way of explaining the A-B effect, though I have never seen it clearly done (it's not a high priority for us theorists, because, after all, the standard analysis is correct).

Dave also wrote:
> To be honest I don’t really understand what is meant by “potential” in this context.

Ut started out as a convenient mathematical technique for analyzing magnetic fields. It is a theorem that, because div B = 0, there must be another vector field A such that B = curl A.

In solving problems, it is sometimes useful to work with A rather than B.

And then it turns out that "momentum" in a certain sense in classical mechanics is not just mv but rather mv + qA. The "certain sense" has to do with the Lagrangian and Hamiltonian formulations of classical mechanics, and all that happens to be relevant to quantum mechanics.

And so in quantum theory, when you should have p^2/(2m), you actually need to subtract off the qA term to get (p-qA)^2/(2m). Then you remember that p in quantum mechanics is actually the gradient operator, and you start to see how qA interacts with the derivative.

A long chain of reasoning, each step of which was known to be necessary before the discovery of the A-B effect.

Any student of quantum mechanics is supposed to internalize everything I just mentioned, and then the A-B argument becomes pretty obvious.

By the way, the "modern" approach is to take the connection between qA and the derivative as the primary idea, not the long chain going back to classical mechanics. In that case, you have the basic idea of a "gauge theory," a so-called "abelian" gauge theory, which, to simplify, just means your A field is a number, not a matrix: non-abelian gauge theories, such as for QCD, have vector potentials represented by matrices.

So, if you truly get clear on everything about the A-B effect, you have grasped a lot about modern physics!

All the best,

Dave

2. @PhysicistDave,

Thank you for lengthy reply to my second post. I confess I got lost in the details, but the deep significance of gauge theories as they relate to the modern Standard Model is something I’m determined to understand through and through. I believe I understand the basic gist of it. For example, Maxwell’s formulation of the EM field was the first gauge theory discovered (going from memory from the Wiki article). In that article, which I was reading this morning, some quantity is conserved. For the EM field I assume it would be the speed of light. In Maxwell’s equations the interplay of the E and M fields constrains the speed of light to c (vacuum permittivity and permeability fixing the actual magnitude of c). This process is what I always thought was meant by “gauging” a field. Anyway I’ll try to make sense out of what you wrote with the various connections. Thank you again for this help. I’ve definitely got an uphill climb in knowledge absorbtion but am determined, as any mountain climber, to reach the summit, or at least high enough to see the physics landscape clearly.

I wrote the first version of this at a coffee shop on my notebook computer, but hopefully it didn’t get posted there.

3. Dave Schroeder wrote to me:
> I confess I got lost in the details...

Of course. I just wanted to give you hints as to what to look into if you want to know more and also to indicate that the historical development is rather roundabout.

Dave also wrote:
> I believe I understand the basic gist of it. For example, Maxwell’s formulation of the EM field was the first gauge theory discovered (going from memory from the Wiki article). In that article, which I was reading this morning, some quantity is conserved. For the EM field I assume it would be the speed of light. In Maxwell’s equations the interplay of the E and M fields constrains the speed of light to c (vacuum permittivity and permeability fixing the actual magnitude of c). This process is what I always thought was meant by “gauging” a field.

No, when you are looking at the E and B fields, you are not really clearly seeing the gauge-field aspects (there is a sense in which the E and B fields represent the curvature of the gauge fields). It is the vector potential (along with the scalar potential) that embodies the "gauging."

The fact that you get the speed of light out of the E and B fields is now viewed as a rather trivial matter (of course, it was not trivial historically!), since everyone knows the speed of light is just 1.

The term "gauge theory" goes back to an idea from Hermann Weyl early in the twentieth century in which the unit of measure (hence the "gauge") varied from point to point in such a way that after you went on a trip and came back home, you might be a different size! As goofy as this sounds (and of course it cannot work), this is the origin of the idea.

Weyl and others later modified the idea to refer not to linear measurements but to the complex phase of the quantum wave function, so that if you went on some path and returned to your starting point via a different path, you could suffer a net change in complex phase. It was realized that this is what happens with electromagnetism in quantum mechanics.

And from this comes the A-B effect.

If instead of just a change in complex phase, we have some internal symmetry that can be represented by matrices, we get the non-abelian gauge theories such as QCD. "Non-abelian" just means "non-commutative" and refers to the fact that matrix multiplication does not commute.

All of this would have seemed quite bizarre to Maxwell, of course, but then Maxwell did not know quantum mechanics. Nowadays, this rather radical reformulation of Maxwell's equations is viewed as the "right" way of thinking about not just electromagnetism but also the strong and weak forces.

Dave

4. If I may kindly interject, a nice overview of the physics and history of gauge invariance can be found in the AAPT Resource Letter, Gauge Invariance, by Cheng and Li (1988). Jackson and Okun later wrote a beautiful article (2001) "Historical Roots of Gauge Invariance." (arXiv:hep-ph/0012061). Both of these resources are pedagogical and include copious references.

5. @PhysicistDave,

I was very surprised, and delighted, to see your post this morning waking up in the snow blanketed, frozen wonderland of southwest New Hampshire, figuring you wouldn’t read it in California till late morning EST, with the 3 hour time difference. Off topic, I know, but California is an awesome place. I visited the state many times in the 80’s and 90’s with friends, who eventually moved out there. When they bought a home in San Francisco’s Marina district we pedaled our bikes from Broderick St., over the Golden Gate, and up Route 1 to a spectacular viewing area. On 3 separate rides I pedaled from their home to the summit of Mt. Tamalpais and back. They now live in Illinois, nowhere as scenic as California.

My very first exposure to the gauge concept was probably a 1980’s physics popularization book that described it in terms of the gauge measuring blocks used in machine shops, as you alluded to. Oddly, I woke up this morning with that gauge block connection in mind. That immediately made me realize I was missing something in my incorrect interpretation of the concept that I posted yesterday. But on reading your post it wasn’t necessary to dig up that book, as now I’m being steered in the right direction. Still this is going to take some thinking to understand it to my satisfaction. Sorry for being somewhat incoherent, but I’m under time pressure (going to a Connecticut casino with twin brother), and need to get some winks, to make up for too much caffeine before hitting the rack last night. I’m printing out the Gauge Theory page on Wiki, right now, and will read it on the second leg of our journey, after I arrive at my brother and his wife’s house, when he starts driving.

6. @PhysicistDave

Your synopsis of the twists and turns in the history of gauge theory was very helpful to me, clearing certain things up. There’s quite a bit here to digest, and I’ll be studying it for a while before I get back to you. Thanks again. Plus, I just noticed the link Gary Alan provided above that sounds like it might also help in bridging the gap between a layperson's understanding and a more formal appreciation of this extremely important topic.

7. The Wikipedia article "introduction to gauge theory" (the nontechnical account) is replete with misconceptions. Cheng and Li emphasize that "gauging" involves changing from a global symmetry to a local symmetry. They emphasize that the form of quantum electrodynamics is completely fixed by gauge invariance and renormalization. Gerard 't Hooft's twenty-page Scientific American article (1980): Gauge Theories of the Forces Between Elementary Particles, attributes to Weyl the idea of "machinists blocks." Robert Crease writes: "One of the biggest unsolved challenges of Yang–Mills theory is simply finding ways for outsiders to get a hint, at least, of the stunning achievement it represents." O' Raifeartaigh: "the emergence of gauge theory has been a gradual process, a slow evolution rather than a revolution." (Introduction, Dawning of Gauge theory).

8. Gary Alan, thanks for alerting us about the issues with the Wiki article "introduction to gauge theory". As I read Gerard 't Hooft's 1980 Scientific American article on gauge theories (online version), I knew I had read it before, long ago, as I recognized the diagrams in the article. Searching my collection of Scientific Americans, dating back to 1978, I couldn't find that June, 1980 issue. But the online version is just fine.

15. Also, an excellent non-specialist review of these misconceptions: https://arxiv.org/abs/astro-ph/0310808

1. This comment of mine was supposed to be a response to the comments of InConstruction and Philip Helbig. I dont know why it appeared at the end.

16. Dr. Hossenfelder,

Forgive me for not letting the discussion on Hawking Radiation go, it would appear that it is getting a bit trying. But it seems to me that there might be a loose end. In reading your link the words "irretrievably lost" are used to describe the entangled particle inside the black hole. How do we know that this particle and its information, communicated via entanglement, is truly lost? I ask this because of the entanglement the "information exchange" should be instantaneous. But we do not know how this instantaneous information exchange occurs so how can we conclude that the information is in fact lost. We do know that under standard speed of light info exchange the info would not be able to escape. But with entanglement speed of light info exchange does not occur. Thank you for you time Dr. Hossenfelder, I really enjoy your writing.

1. Steve,

"But we do not know how this instantaneous information exchange occurs so how can we conclude that the information is in fact lost"

There is no instantaneous information exchange in quantum mechanics.

Having said that, we do not of course know whether information is indeed lost in a black hole. What we do know is that if general relativity remained valid (no quantum corrections) it would be lost.

17. A nice thing about Sabine's list of "effects" is how it compels one to continue researching. In case it has been forgotten, many years ago (1968), Lamb and Scully published a fascinating paper which concluded thus: "The introduction of the photon concept is neither logically implied by nor necessary for the explanation of the photoelectric effect." (The Photoelectric Effect Without Photons).

18. Regarding whether the Hawking effect has been observed experimentally... The superfluid and other acoustic black or white hole analogue experiments certainly confirm experimentally that the math behind the Hawking effect is correct. Not that there was much doubt about that, but still nice.

That said, we don't know whether the semiclassical approximation that is used to derive thermal radiation from horizons is a good one. It might work, or other effects might become important. We won't know until and unless we are able to measure it in situ.

One example where I can see the Unruh effect might be breaking down is that to create an accelerating detector one has to apply force to it, which, in turn, changes the spacetime geometry, potentially enough to swamp the very weak Planck-level thermal bath.

19. To this collection of physical effects one should add
- the Quantum Zeno Effect (QZE)
- the astrophysical Sunyaev-Zel'dovic Effect (SZE), the boosting of CMB photons in clusters of galaxies
to round-up the dozen.

20. In your video when explaining the AB effect you should not use the same symbol(small phi) for both the phase shift and the magnetic flux in your formula; so correctly label the magnetic flux Phi(B)to avoid confusion arising for people which are less acquainted with the AB effect.

1. Dang, you are right. I hadn't paid any attention to this. (It's such a pain to get equations and special symbols into a video editing program.)

21. Photo-electric effect and "recording" at the quantum level, is there a connection? I am only wondering aloud.

22. Fun. Thanks. I know the tennis racket effect, does it have any use in physics besides just being fun? For additions I'll add the Hanburry-Brown effect. Interference of 'independent' photons. (or other things)

1. In a previous blog (Part 2 by Tim) I noted that I, personally at any rate as there were no further comments, saw a connection between the tennis racket effect and Tim's use of the Lorenz 'butterfly' diagram as they both had bistable patterns. This could, in a maybe-calculable way, lead to two entangled particles occupying the butterfly wing positions so that the wing positions of the two particles allowed pseudo-randomly opposed but coordinated measurements by Alice and Bob. This could be relevant to a Bell Test simulation. I wouldn't support it though until I saw a calculated simulation. And isn't a bistable state rather a special state for a Bell Test? Are particles spinning about intermediate axes? :)

Austin Fearnley

23. A curious fact about the AB-effect : it has been considered as a possible explanation for "scalar waves", a subject pioneered by Tesla but considered by many to be pseudo-science. One entry point to start making your own opinion about this stuff can be found on the CIA's web site (!) where also connections with supposed cold-war era secret Soviet weapons are discussed : https://www.cia.gov/library/readingroom/docs/CIA-RDP96-00788R001900680014-4.pdf

24. The CIA's report is a bit old (1984) but I have just stumbled on more recent stuff along these lines in the book "scalar wave driven energy applications" (Springer, 2019).
I have no idea whether this is serious science, but if someone has a (serious) opinion, do let us know! The author (Bahman Zohuri) seems to have a serious-sounding position in a U.S. university (associate research professor at the University of New Mexico).

1. A prominent Tesla replicator has given our open source group a number of metal samples that have been exposed to tesla waves. We are currently characterizing the effects of this exposure using detailed microscopic and SEM examination. The surface of the metal seems to be impacted in amazing ways by polariton petal condensation that has changed the structure of the metal consistent with the topology of these condensates. Videos of the results of this inspection and some opinions on causation are available on the internet.

25. Dr. Hossenfelder,
This maybe outside of the scope of this post but since this post started me looking into the Fulling-Davies-Unruh Effect I thought I might as well ask my question. My two biggest take aways from reading about the Fulling-Davies-Unruh Effect is virtual particles and accelerated reference frame. Considering this with Sergei bring up the the semi-classical approximation of the thermal effects a question came to me; Could there be a synergistic effect between Fulling-Davies-Unruh Effect and Hawking effect? The Fulling-Davies- Unruh effect requires an accelerated reference frame, does this acceleration include change of direction or is it limited to linear? Any curved motion by a black hole with respect to its motion around the center of a galaxy represents a change of direction and a classical accelerated reference frame. If this is good enough acceleration for the Fulling-Davies-Unruh effect this could represent an increase in virtual particles that could then transition into Black Hole Hawking Effect. I understand that the thermal aspect is very, very minimal, but the increase in particles from the Fulling-Davies-Unruh Effect could work with the Hawking Effect. Thank You!

26. (off- topic) sorry "the love lasted so little", readers and Sabine. due to a minor stroke I am no longer able to type, sign, my name, tie my shoes, hold a soup spoon.

thank you putting up with me both here and on you Tube.

I will miss you.

1. Ivan,

Jesus, sorry to hear. I hope someone is taking care of you. We will miss you too!

2. Sorry about you stroke. I have a bit of a problem myself with what are called giant cell tumors. that is what they are called, no big medical term. These are transformed cells, though fortunately not malignant, of tendon sheaths in the hand This has begun to seriously limit my dexterity. Some anti-tumor drugs are having some limited positive effect.

27. Ivan,
My wife and I are both cancer survivors from more than 10 years ago. That was a bleak time for us, and no doubt things seem bleak now to you. But judging by your post you seem like someone with a very strong mind. I suspect and hope you will be back soon. Best wishes for a speedy recovery.
Steve

28. I think an important physics idea that doesn't get enough attention is the the running parameter "effect". That is, quantities like mass and charge of a particle change with your resolution scale and how it does depends on the theory (specifically, the renormalization group).

Renormalization is often just explained as a mathematical trick to get rid of infinities, but it actually comes from deep physical insights. For example, you don't measure the "bare" charge of a particle because you have a cloud of virtual particles surrounding it screening the charge. How much of the cloud you see will affect what charge you measure.

The above maybe is a bit of an oversimplification, but re-normalization is extremely important to modern theoretical physics.

29. Sie haben den sehr wichtigen "Pauli-Effekt" nicht erwĂ¤hnt.

-drl

1. I think that the question for the Pauli Principle is a good one. Because it is a typical point in quantum mechanics. I have asked repeatedly in discussions whether we meanwhile have an idea why it is valid. The usual answer is that this question is not justified because this is a fundamental rule in physics where we cannot ask for a cause. – I see here the typical denial of reductionism; quantum mechanics declares something as a “principle”, and that’s it.

2. antooneo,

I believe you misunderstood this comment. The Pauli-effect is not the Pauli-exclusion principle.

And btw, it was #11 on my original list from 12 years ago.

3. Sorry, I have really mixed both. And I never heart about the "Pauli-Effekt" before . That sounds to me a bit like Murphy's law. And 12 years ago I did not have knowledge about your blog.

But anyway, the Pauli principle is in my view also a somewhat open point in present physics; open as not explained.

30. "judging by your post you seem like someone with a very strong mind."

Thanks, Steve and Sabine.

"fiercely lefty" kind of loses meaning when lose fine motor control on left hand!

on the other it is a new experience to me to try to write with the pen upside down and take a soup spoon to my mouth without being to register both that pen is upside down, and spoon shell is down!

I just woke up like this 3 weeks ago. That' what I didn't need: another Ivan to handle.(:-)

Coming attractions: Fiercely Righty Ivan...

31. Sorry to hear about your stroke. There's research into stem cell therapy for people who have suffered a stroke, as described at the link below. My brother tore cartilage in his knee joint from a jump off a boat bow 3 feet above the dock, mistiming the swells. His knee completely healed after new cartilage grew in the damaged area.

https://www.cryo-cell.com/the-benefits-of-banking/cord-blood-cord-tissue-research/stroke-motor-function-treatment-stem-cells

1. I forgot to mention that my brother had his stem cell therapy for his knee performed at a clinic in Colorado.

32. Thank you, this is the type of post I enjoy most because it educated me a little more about things I didn't know.

33. The photo-electric effect used to be my favorite; but I think these days, it's the Ahranov-Bohm effect. Thanks for giving it the thumbs-up.

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