Scientists find upper limit for the speed of sound | Hacker News Reader

Scientists find upper limit for the speed of sound

phys.org

107 pointshhs60 comments

Very well-written progressive explanation of the experiment that I, a total layman, could understand the basics.

And it made me curious to know more about those mentioned constants that relate to different physical phenomena.

Edit: one doubt I have though, can it still be called an ”experiment” if it is “just” a calculation in a computer and not actually a measurement in the physical world?

>Edit: one doubt I have though, can it still be called an ”experiment” if it is “just” a calculation in a computer and not actually a measurement in the physical world?

The article doesn't use the word 'experiment' anywhere.

Yes, but I did. I was wondering if it would be correct to use it in this case.

Experiments are used in the scientific method to invalidate hypothesss. I'd say that yes, a simulation is an experiment, since it can do that.

Pure logic can also invalidate hypotheses sometimes, but pure logic isn't experimental.

Pure thought experiments are useful because they make predictions which can then be validated. It’s part of the basic foundations of science, otherwise you’re just curve fitting across all exponential data.

Sure but thought experiments are not actual experiments. In fact thought experiments are... exactly the same as simulations.

To be more clear a pure thought experiment doesn’t invalidate a theory on it’s own. It’s the conflict between the results and either past or future observations/experiments that invalidates the theory. Plenty of theories that gave what seemed like insane results happened to work out.

Absolutely agree. Fun fact;

> Gedankenexperiment, (German: “thought experiment”) term used by German-born physicist Albert Einstein to describe his unique approach of using conceptual rather than actual experiments in creating the theory of relativity.

It would be ridiculous to say that experiments can only be done physically. Thought experiments are responsible for developing one of the most famous theories known to man.

A simulation is only as good as the model it’s based on. So sure, it’s an experiment, but one that’s only valid within the context of the model. Validating the model itself would take a “real” experiment.

Most experiments are only valid within the context of a model. Calculating X-ray diffraction patterns only matter because of how we model orbitals and light diffraction for example.

What I’m fumbling to get at is that a physical experiment can tell you that your model is flawed in ways that simulation alone can’t.

If you don’t already understand the double slit experiment, you’re unlikely to simulate light in a way that reproduces its results.

Yes, each have their advantages, e.g. here the speed of sound couldnt have been calculated in these circumstances.

The simulation could've also revealed flaws and disproved the theory w/r to itself.

They can perform experiments to validate the more generic result: the relation between the speed of sound and the mass of the atoms. The specific result about solid hydrogen will still be an extrapolation though.

I don’t think strict definitions that allow putting things into well defined “experiment” and “not experiment” bins really matter. Being understood matters, and in science context is all important, but being understood just doesnt depend on strict definitions of common words like that, and when it does those words are explicitly defined instead of left for assumptions.

It is the kind of questions about scientific literacy you see quite a bit, loading terms with really consequential definitions and then asking at the edge cases... the answer usually lies in a combination of correcting usage that feels a little odd while also trying to break down the assumption of the question itself, in other words rejecting the premise.

It’s like the many arguments about the usage of the word “theory” especially when with the pejorative “just a theory”, the reality is scientists would never be confused by an odd usage of the word and themselves don’t place much importance on it.

“mu”

This paper is well written. I was able to extract the mathematics into the project I created to document the relations and steps of derivations [0].

[0] https://derivationmap.net/review_derivation/608598/?referrer...

Wow, this derivationmap site looks very interesting. I desperatly want something like this for graph rewriting on abstract syntax trees.

A fullscreen option for the d3 graph would be nice, this way it could leverage large monitors. Some interface for creating derivations directly from graph view would be amazing, but this is probably hard to get right.

If you already have a yacc/bison in your project’s CLI or compiler parser, you could use `bison —graph=` then feed that graph file into DOT/gv layout for outputting to PNG or SVG.

Note that there are two approaches to calculating v in this artcile, and this derivation covers only the more hand-wavy one. For example, they drop the √f factor, because "[f]or most strongly bonded solids, f varies in the range of 1 to 4".

Here's the arxiv link, for those who want to read the original article paywall-free: https://arxiv.org/abs/2004.04818.

To be clear, this isn't an unconditional speed limit. It appears to be something like "the maximum speed of sound in matter which consists of atoms" (maybe I've missed a few conditions). In particular, this speed limit is known to be violated inside of neutron stars (where the speed of sound is near 1/sqrt(3) times the speed of light, and may even exceed that).

The paper also mentions a proposed lower bound on viscosity. This is the KSS bound (https://arxiv.org/abs/hep-th/0405231), and various theories violating it have been constructed. The violations, of course, don't get mentioned nearly as much as the proposed (and all-but-debunked) bound.

The article is open access under Creative Commons: https://advances.sciencemag.org/content/6/41/eabc8662

Hah! I didn't even bother to check, I'm so used to just going for the arxiv version when not on campus. (Usually has better formatting too.)

Thanks for pointing that out.

> At those pressures, hydrogen becomes a fascinating metallic solid conducting electricity just like copper and is predicted to be a room temperature superconductor

I didn't get the room temperature superconductor part.

If you compress hydrogen to that pressure and keep it compressed in let’s say a really strong shell, then it will have superconductivity even at room temperature.

Quick question, can you really have room temperature at that pressure? I thought extreme pressures would always result in extreme temperatures.

> I thought extreme pressures would always result in extreme temperatures.

At least classically, it's the process of compressing that yields the extreme temperatures, not the state of having been compressed. If you maintain the pressure, the hot compressed material will transfer thermal energy to its surroundings until it has cooled back down to room temperature, and it does not require further work to be done on the compressed material to keep it compressed.

If you keep it compressed, it will eventually cool down, but stay compressed.

If it's a solid in that state, will it remain a solid if you remove the container?

Ow... That's disappointing.

I think it’s still an open question if it is metastable (like diamond>graphite), but it definitely wouldn’t be stable.

https://physics.stackexchange.com/questions/308290/are-there...

Depends. If you remove the container quickly, then it explodes. Dong it slowly, some of the material evaporates and thus cools the rest, so that you can end up with piece of ultracold solid. But it will most likely not have same properties as the original material.

That sentence is not great because copper is not a superconductor. https://www.engineeringenotes.com/metallurgy/metal-conductiv...

It's fascinating that the upper bound of the speed of sound is analogous to the speed of light in a vacuum, but it's highest when propagating through the thing that's the complete opposite of a vacuum.

Now I wonder what the speed of sound in, say, neutron-degenerate matter, would be.

I’m not sure, but I bet it is highly dependent on your frame of reference. :)

> the thing that's the complete opposite of vacuum

Solid matter is very nearly entirely empty space, and the vacuum state is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of existence.

Sooo disappointed that it wasn't the speed of light.

While not mind blowing, this result is somewhat surprising to me.

The result in itself as presented in the article derives from a somewhat easy to follow train of thoughts:

- sound is a longitudinal wave propagating through a medium;

- its speed of propagation depends on some properties of said medium;

- thus a physical uperbound on these properties imply an uperbound on the speed of sound.

The fact that an upper limit exists is in itself obvious (we already knew one: the speed of light in a vacuum). Properly deriving it is the interesting part.

The jump from one to two is where I'm lost, mostly due to ignorance. Sound propagating is just molecules smacking into each other, but it's not evident to me why the speed this can happen at would be limited other than by the speed of the molecules. I guess one molecule won't immediately accelerate when it's "hit" by another and this causes the slow down?

Isn't it the same as the speed of light? If sound is an abstract wave-like signal, then it can be transmitted equally well with radio waves or molecules in the air. The paper should probably mention that it's the upper limit of sound in a atomic based medium.

Light waves are electromagnetic. Light can travel without a medium because the photons are themselves moving forward. They have a frequency, but the frequency is caused by movement side to side, on a different axis than their heading.

Sound waves are physical. They are by definition in an atomic-based medium. Matter at the source of the sound is pushed, compressing it, which pushes the matter in front of it (uncompressing the original matter). If there's no matter to push, sound isn't a thing (thus nobody in space being able to hear you scream). It's not so much information being transmitted as matter being shoved. The frequency is amount of time between shoves, and that wave is in the direction of the motion (because the forward shoving is the frequency and the motion). Sound waves are not an abstract transmission of information.

It's certainly true that you can encode a sound as an information and send that information electromagnetically, but that transmission is not itself sound. Similarly, you could measure the speed of traffic in LA, email those times to New York, and then drive some cars in New York in a way that produces the same traffic speed, but you did not transmit the traffic at the speed of light, and this exercise would not be meaningful to a discussion on the maximum velocity of a car.

I think OP is talking about the fact that it is possible to represent and transmit sound via an electromagnetic medium and that therefore this limit is not a limit on sound information (which is nothing but generic information that can be transmitted at the speed of light) but rather a limit on sound as it occurs physically (in the way that you've described).

The key word there is “represent”. Radio isn’t sound. If I make a recording and mail it to you, the delivery time isn’t “the speed of sound through the mail”.

You can get into a whole thing about trees falling in forests and how a pressure wave only becomes sound when it’s interpreted as information, but that’s totally irrelevant to an experiment about pressure waves through a medium.

Here's an experiment. One cubic mile of space in a gravity free region is filled with tennis balls that freely float around and bounce each other from time to time. If we produce a wave in this medium by oscillating one of the walls, would we call this wave "sound"? If not, what makes atoms "better" than tennis balls? What if we replace atoms with electromagnetic balls exhibiting similar bouncing properties, would it be good enough to count waves in this medium "sound"? Getting back to the original experiment. Now, the tennis balls float not in vacuum, but in a gas, e.g. argon. In this case, both gas atoms and tennis balls would transmit waves induced by the oscillating wall. What makes one wave more "sound" than the other? What if we don't know for sure that the gas atoms are real atoms and not some atom substitutes? At what point does sound become not really sound? My point is that if we can substitute the medium carrying the waves, than we may as well remove the medium from the definition of sound.

I would call it sound in tennis balls. They're still using the same fundamental mechanism of pressure to transmit sound. But light isn't sound because the mechanism of propagation is fundamentally different, not just at a different scale or with a different medium. It's subject to different laws and behaves qualitatively differently. For example, it can be polarized.

I wonder about a neutron star though? Is that still subject to this theoretical speed of sound limit, or is it only atomic substances?

We can go even deeper and ask how we'd call sound-like gravity waves in spacetime? Some sort of oscillating system of heavy stars can produce a periodic gravity wave that we'd call sound probably.

No, we don't call that sound. Those are called Gravitational Wave. Admittedly this is a bit confusing vs Gravity Wave, but we appear to be stuck with that one in English now.

That's sound. Very lossy sound, presumably at a very low frequency, but sound nonetheless. There's a physical medium, and a signal is being passed through it via tennis ball pressure. If you had a sufficiently dense field of tennis balls, you could visually observe the wave moving across it as the field compressed (in the direction the signal is traveling) and then uncompressed, cyclically.

A good trick to telling the difference is thinking about the direction of the frequency. Frequency is a back and forth movement. If it's going back and forth in the direction the overall signal is moving (like a tennis ball going faster towards the destination and then more slowly or backwards, or like the wall at the source of the signal vibrating towards the other wall), that's a physical wave. If the back and forth movement is happening in an entirely different plane, that's like an electromagnetic wave.

So just to be clear, as it seems the other poster is quite confused, whether waves are "physical" or not is independent of whether their traverse waves or density waves. Sound is an example of density waves. The waves at the interface between the ocean and the air are gravity waves. Both are physical.

EM waves are physical as well, they just are in the EM field itself, rather than having a medium like matter.

You are way over complicating this in avoiding just googling the actual definition of sound in a physics context. It's acoustic waves in a medium. Acoustic waves are adiobatic compression/rarefaction waves.

All your examples are just sound. There's no difference if the medium is all gas, all tennis balls, or a mixture of both along with some very confused corgis.

The medium, and whatever objects it exists as, are not sound itself. The notional particles of sound waves are called phonons.

Propagation of transverse waves in the electromagnetic field is what we call light, radio, and other electromagnetic radiation. There's also constraints of symmetry for how the electric and magnetic portions of the field relate to each other. The notional particles of these waves are photons.

To address your last point, it would help to stop thinking of waves as platonic objects with their own independent existence as objects, and instead see them as patterns of activity/interaction within ongoing dynamic systems.

All of your examples are simply sound. There's no confusion in this. And yes the definition of sound still requires a medium.

> What if we replace atoms with electromagnetic balls exhibiting similar bouncing properties, would it be good enough to count waves in this medium "sound"?

What do you mean by "electromagnetic balls"?

> My point is that if we can substitute the medium carrying the waves, than we may as well remove the medium from the definition of sound.

Again, sound, simply by definition, is a compressive wave. Compressive waves can only happen in media that can be compressed, which rules out fundamental fields like the EM field or space-time. Atoms may not be entirely fundamental to sound, but matter is - you may be able to have sound waves in a neutron star for example.

I wonder if it's possible to talk about sound waves inside the radius of a black hole - that I'm not sure about.

Correct me if I am wrong but I believe sound should be differentiated from other waves as traveling through a physical medium of atoms as opposed to a fundimental field. Under this definition the following sentence quote makes sense.

> the speed of sound is dependent on two dimensionless fundamental constants: the fine structure constant and the proton-to-electron mass ratio.

> For example, [sound waves] move through solids much faster than they would through liquids or gases, which is why you're able to hear an approaching train much faster if you listen to the sound propagating in the rail track rather than through the air.

I am sceptical, I don't think this makes a big difference until the train approaches the speed of sound. What makes a difference, however, is that the energy is constrained into moving one-dimensionally, along the rail, so that it travels much further.

The speed of sound in steel is in the neighbourhood of 6000 m/s, whereas in air it's a mere 343 m/s.

Sound travels through steel ~17 times faster than through air at 1 atmosphere.

At 500 meters, the "kachunck" of a train wheel going over a bump will take ~1.4 seconds to reach you by air, but only ~0.08 seconds to reach you through the rail.

A pretty big difference.

The assumption behind my comment is: Most of the time, you only care about whether the train is coming. A 1-second advantage isn't really interesting. Of course, maybe that's the interesting part in that kind of situation? I doubt it, though.

At 5km, sound will take 15 seconds to cover the distance. That's negligible compared to the time the train will take, assuming a good old train going at ~40 km/h (7'30"). Using the quoted 6 km/s, that gives ~ 0.8 s inside the rail.

I admit I was initially expecting a smaller difference. But I still think that the 14 seconds advantage is negligible over just knowing that the train is arriving from a far away distance: to hear the train at all, wind needs to blow in the right direction, and the train needs not be too far away (~d^2 energy propagation, so ~log(d) in dB vs virtually no attenuation in a simplified model). I wish I had time to look up and compute the attenuation in both cases....

The fact that the sound will arrive much faster through the rail was not presented as having practical value in this scenario. The delay is merely an interesting fact of physics to observe, just like observing the delay from lightning to thunder can be interesting.

As for energies, I used a short example of 500 meters to make it more probable, but near ~150 meters you still have around half a second of difference, which is more than enough to notice.

> The fact that the sound will arrive much faster through the rail was not presented as having practical value in this scenario

Got it, thanks. I had somehow lost sight of the original context.

Next time you are in a swimming pool, listen with one ear in the water and one out. You’ll hear sounds that have come through the water before they come through the air. It works best with a big pool and something noisey at the other end.

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