If Einstein was a bat.

[Slaihne]

Hi there,

 

I posted this question in the Introductions forum and was told to post it here...

 

I’ve had this idea knocking around for a long time now, after skimming through a small hardback edition of Einstein’s Theory of Relativity around 20 years or so ago.

 

Basically, it boils down to my perception that Einstein performed a lot of thought experiments using clocks and trains. Thus he then put forward the idea that as velocity increases so does mass until it becomes impossible to accelerate faster than the speed of light because you would need infinite energy. (Excuse the massive over simplification).

 

My problem with this is that the experiments used the fact that observers viewed these clocks. And as human beings our method of viewing things naturally involves the speed of light.

 

If we were creatures with a different fastest sense, say, a hypothetical bat, who sees by sound alone, or some other highest order sense that perceived radiation that travelled at the speed of sound but could travel through a vacuum. Would C = the speed of sound?

 

rgds

 

David

[CraigD]

Welcome to hypography, Slaihne! :)

If we were creatures with a different fastest sense, say, a hypothetical bat, who sees by sound alone, or some other highest order sense that perceived radiation that travelled at the speed of sound but could travel through a vacuum. Would C = the speed of sound?

In short, no.

 

There are at least a couple of somewhat standard answers to this good question.

 

One is to note that, if applied to the speed of sound, the “constancy of c” postulate of special relativity is experimentally disproved. If you perform the equivalent of the Michelson–Morley experiment using sound – this would involve looking for a difference in interference pattern, which in sound are called “beats” when moving in different directions and speeds toward and away from a sound source – you’d find the expected differences. Performed in its usual way, using light, you won’t.

 

Put in terms of a simpler, less historical experiment, if you measure the time between a sound being made – say by a firecracker – in the middle of moving train of flatbed cars, and being heard on the forwardmost “upwind” and rearmost “downwind” car, you’ll find that the rearmost listener hears it first. If you did this with an electromagnetic signal, you’d find no such difference.

 

Another is to note that, to our best understanding of physical reality, EM radiation is more “fundamental” than sound and other means of signaling. What sound actually is is a moving, longitudinally compressed area of a column of some gas, liquid, solid, or even plasma medium. What’s doing the compressing, in any case, is a repulsive force between what makes up the medium – in all but bizarre cases, the charged parts of its atoms. The force between like charged particles is due to the boson that carries this force, which is the same as the one that carries light, the photon. So, in a real physical sense, even in absolute darkness at all EM frequencies, photons are involved in any sort of signaling. EM signals are, therefore, more fundamental, and special, than other kinds.

[Moontanman]

Slaihne, are you talking about a being that actually uses sound but cannot perceive light or are you talking about a hypothetical sense that uses something else that can travel through a vacuum and behaves like light but travels much slower? If the latter I'm not sure the analogy between light and sound would hold up exactly but none the less the speed of light is special and even if you found a way to detect and or transmit something faster than light like tachyons the speed of light would still be special and the laws of relativity would still hold sway. what I'm trying to say with out math skills i do not have is that the speed of light is special not because nothing can travel faster but due to inherent properties of space time.

[Slaihne]

Hi there,

 

Thanks for the reply and the links.

 

I can understand the fact that when you send a sound wave from a source, it is possible to move the detector in relation to that source and thus cause the Doppler Effect. Although this only works to a limit, if you travel away from the source at a velocity faster than the speed of sound you would theoretically hear the sequence of compressions, which make up the sound wave train, in reverse. I’m not sure whether this is similar to the ‘time dilation’ effects of theoretical light speed travel or just plain coincidence.

 

I’m not quite sure what would happen when you moved towards the source at greater and greater velocities, or indeed if you travelled away at many times the speed of sound. Basically, this hinges on the point when sound is no longer sound. I suppose it would depend on the range of frequencies my mythical bat could detect.

 

I had a read through the Michelson-Morley link you gave but, as far as I can tell, sound would behave identically to light in those experiments. To my understanding, this is due to the fact that sound’s ‘luminiferous aether’ is air and it moves along with the room the experiment is carried out in, thus the medium is at rest to the experimental apparatus and no matter which direction the sound beam travels and then arrives back at the detector it would have a constant degree of interference with itself, duplicating the results for light. I’m not suggesting that there is such a thing as luminiferous aether and it is held static within objects such as walls etc. That is something I never thought about.

 

I’ve thought about the rail cars example you gave. If the cars were totally enclosed and had direct access from the observers to the firecracker, more like a single long car rather than individual ones with doors. By my reckoning both the front and back cars would hear the firework sound simultaneously, if it is exactly in the middle of the train. This would be so because the air the sound is propagating in is at rest to both the observers and the firecracker.

 

If the cars are flatbed wagons then, yes, I must agree with you, the rearmost car would hear it first.

 

If this experiment were to be carried out at relativistic speeds would the firecracker be seen simultaneously? And would that mean that the light heading towards the front of the train actually travels faster than the speed of light? This is a similar puzzler to another question i posted today i’m afraid.:)

 

I understand what you mean by ‘more fundamental’ but still find it 'coincidental' that we can directly perceive the radiation that travels at this limiting speed, literally with our own eyes. If we were totally different in physical make up and could perceive some form of radiation that’s several million orders of magnitude faster than light would we then be setting that speed apart as some limiting factor. I know that last statement is a bit of a cop out since it answers its own question, hence the puzzling about sound and bats.

 

Hopefully I’ve not completely lost the plot and thanks again for your replies.

 

Rgds

 

David

[freeztar]

Hey David, kudos for the most provocative thread title I've seen in a while. :)

 

What Craig posted is most assuredly true.

 

Sound waves need a medium to propagate through whereas light does not. The speed of sound in an ideal vacuum is zero. The speed of light in an ideal vacuum is [math]c[/math]. This is part of what makes light so special.

 

As far as detecting other radiation that humans cannot physically detect, I would suggest the the electromagnetic spectrum for study. A bee can see flowers completely differently than humans, yet, through the use of UV and IR photography, humans can see the flower with a "bee-like" perception.

 

Another great "hypothetical bat" thought is the rumbling of the Earth. Infrasonic investigations allow us to artificially perceive what we cannot naturally perceive.

 

If there is some sort of radiation beyond EMR, we are ill-equipped to sense it and it will remain either mysterious or non-existant. :shrug:

[Janus]

Hi there,

 

Thanks for the reply and the links.

 

I can understand the fact that when you send a sound wave from a source, it is possible to move the detector in relation to that source and thus cause the Doppler Effect. Although this only works to a limit, if you travel away from the source at a velocity faster than the speed of sound you would theoretically hear the sequence of compressions, which make up the sound wave train, in reverse. I’m not sure whether this is similar to the ‘time dilation’ effects of theoretical light speed travel or just plain coincidence.

 

I’m not quite sure what would happen when you moved towards the source at greater and greater velocities, or indeed if you travelled away at many times the speed of sound. Basically, this hinges on the point when sound is no longer sound. I suppose it would depend on the range of frequencies my mythical bat could detect.

 

I had a read through the Michelson-Morley link you gave but, as far as I can tell, sound would behave identically to light in those experiments. To my understanding, this is due to the fact that sound’s ‘luminiferous aether’ is air and it moves along with the room the experiment is carried out in, thus the medium is at rest to the experimental apparatus and no matter which direction the sound beam travels and then arrives back at the detector it would have a constant degree of interference with itself, duplicating the results for light. I’m not suggesting that there is such a thing as luminiferous aether and it is held static within objects such as walls etc. That is something I never thought about.

 

I’ve thought about the rail cars example you gave. If the cars were totally enclosed and had direct access from the observers to the firecracker, more like a single long car rather than individual ones with doors. By my reckoning both the front and back cars would hear the firework sound simultaneously, if it is exactly in the middle of the train. This would be so because the air the sound is propagating in is at rest to both the observers and the firecracker.

 

If the cars are flatbed wagons then, yes, I must agree with you, the rearmost car would hear it first.

 

If this experiment were to be carried out at relativistic speeds would the firecracker be seen simultaneously? And would that mean that the light heading towards the front of the train actually travels faster than the speed of light? This is a similar puzzler to another question i posted today i’m afraid.:D

 

I understand what you mean by ‘more fundamental’ but still find it 'coincidental' that we can directly perceive the radiation that travels at this limiting speed, literally with our own eyes. If we were totally different in physical make up and could perceive some form of radiation that’s several million orders of magnitude faster than light would we then be setting that speed apart as some limiting factor. I know that last statement is a bit of a cop out since it answers its own question, hence the puzzling about sound and bats.

 

Hopefully I’ve not completely lost the plot and thanks again for your replies.

 

Rgds

 

David

 

Okay, here's the thing about light, its speed(in a vacuum) is invariant. That means that everyone always measures its speed relative to themselves as being the same. The speed of sound, however is not invariant, you can measure it as having different values depending on the circumstances.

 

For example, imagine two flatbed railway cars of equal length. One is traveling down the tracks and one is standing still an an adjacent track. When rear of the cars are directly across from each other, a sound wave passes the rear of the cars heading towards the front. Observers on each train measure how long it takes for the wave to travel the length of their train. The observer on the moving train will measure a longer time than the one on the stationary train. For him, it takes longer for the sound wave to travel from the back of his car to the front.

 

Now do the same experiment with the same cars, but with light instead of sound. Both observers will measure the same time for the light to travel from the back to the front of their respective cars.. Remember, they are measuring the same light.

 

Now, if you have a speed that is invariant, such as light's is, it becomes a logical necessity that it also is the natural speed limit of the universe. IOW, you can have only one invariant speed in the universe.

 

Do not get caught up in the fact that we "see" by light. This really doesn't have anything to do with Relativity. Given fine enough measurements, an intelligent being that only "saw" by sound, and had no knowledge of the existence of the electromagnetic spectrum, could, in theory, work out the rules of Relativity and come to the same conclusion as to the value of the ultimate speed of the universe.