Other Special Speeds? (than light)

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The discussion centers on the implications of the speed of light (c) as the universal speed limit in physics. Participants assert that while disturbances in the electromagnetic and gravitational fields propagate at the speed of light, no invariant speed greater than c exists. They explore the concept of special speeds in various fields, including critical velocity in fluid mechanics, and emphasize that any speed other than c is not invariant under Lorentz transformations. The conversation also touches on the nature of entangled particles and the speed of thought, concluding that while tachyons are theorized to exceed c, their existence remains speculative.

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  • Understanding of special relativity and Lorentz transformations
  • Familiarity with electromagnetic and gravitational fields
  • Knowledge of critical velocity in fluid mechanics
  • Basic concepts of quantum entanglement
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It seems to me that the speed of light and its constancy have very important implications in physics as we know it. Isn't light a disturbance in the electromagnetic field? If so, shouldn't there be special speeds related to the other fields, (namely the Gravitational field?)
 
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Disturbances in the gravitational field also propagate at the speed of light.
 
There can only be one invariant speed. The invariance of c results in it being the "speed limit" for the Universe. Speeds greater than c don't exist, so you can't have any invariant speed greater than it, and if you had an invariant speed less than c, then it would be the speed limit of the universe.
 
"Speeds greater than c don't exist"

Uh, how does that sit with the data transfer rate between entangled photons ? IIRC, ~10^6 c over ~kilometre separation, though an upper limit was not established due to limits on the timing.
 
It seems to me that JonnyB42 deserves better answers than the kneejerking so far, for he has asked a profound question.

Yes special speeds abound in Physics.
A simple example would be critical velocity in fluid mechanics, but I'm sure many could present better examples.

Of course the exact nature and implications of each special speed will depend upon its field of application so will affect more phenomena than others.
 
Jonnyb42 said:
It seems to me that the speed of light and its constancy have very important implications in physics as we know it. Isn't light a disturbance in the electromagnetic field? If so, shouldn't there be special speeds related to the other fields, (namely the Gravitational field?)

If you want to get more profound, try http://arxiv.org/abs/physics/0302045" . You will see that symmetry and reciprocity leads to a single "invariant" speed, which happens to be a maximum speed.

The real question is why does anything travel slower?
 
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Jonnyb42 said:
It seems to me that the speed of light and its constancy have very important implications in physics as we know it. Isn't light a disturbance in the electromagnetic field? If so, shouldn't there be special speeds related to the other fields, (namely the Gravitational field?)

Sure- the speed of sound is another example: the speed at which elastic disturbances propagate. Shock waves are what happens when that limit is exceeded.
 
JDługosz said:
If you want to get more profound, try http://arxiv.org/abs/physics/0302045" . You will see that symmetry and reciprocity leads to a single "invariant" speed, which happens to be a maximum speed.

The real question is why does anything travel slower?

Hey, thanks. This is just what I've been looking for. I posted a similar question last year. Good things come to those who wait!

https://www.physicsforums.com/showthread.php?t=323238
 
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  • #10
Nik_2213 said:
"Speeds greater than c don't exist"

Uh, how does that sit with the data transfer rate between entangled photons ? IIRC, ~10^6 c over ~kilometre separation, though an upper limit was not established due to limits on the timing.

It's likely instantaneous. It has little to do with speed and travel, and more to do with the fact that the assumption that for two things to influence each other, they must be adjacent.

Essentially it can be looked at as evidence for the nonfundamentality of the concepts of space, rather than speed.
 
  • #11
I heard that the only thing faster than light was human thought, but surely this can't be true? Or maybe it "takes time" to think of something. I don't know how quick that is...
 
  • #12
FeDeX_LaTeX said:
I heard that the only thing faster than light was human thought, but surely this can't be true? Or maybe it "takes time" to think of something. I don't know how quick that is...
I heard that the only thing faster than light is bad news. Someone actually built a spaceship that was powered by bad news, but they weren't welcome anywhere.

Seriously though, the actual thought doesn't have a position, so you can't associate a speed with it. The place where the thought occurs is just your brain, and it's not moving very fast, certainly not faster than c.

Edit: Some of you might be interested in my thread here.
 
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  • #13
Studiot said:
It seems to me that JonnyB42 deserves better answers than the kneejerking so far, for he has asked a profound question.

Yes special speeds abound in Physics.
A simple example would be critical velocity in fluid mechanics, but I'm sure many could present better examples.

Of course the exact nature and implications of each special speed will depend upon its field of application so will affect more phenomena than others.

It seems we can respond to a question on several different levels.

Of global significance, I've included zero speed in the set; {0,c}. I'm hindsight, I'm not sure there's any justification for it, Is there?

Any candidate speed, other than c itself, is not invariant under a Lorentz transform.
 
  • #14
Any candidate speed, other than c itself, is not invariant under a Lorentz transform.

Why so?

Surely any limit that depends upon the ratios of the velocities of two particles in the same system will be invariant?
 
  • #15
Studiot said:
Why so?

Surely any limit that depends upon the ratios of the velocities of two particles in the same system will be invariant?

I'm not sure what you mean. Say we had some force that was mediated by a particle having a velocity u. Under a Lorentz boost v,
u' = (u-v)/(1-uv/c2).​
I'm sure there are characteristic velocities in particular media, if that's what you mean.

Under the more general question "Are there any special speeds other than 0 and c?", I can't think of any. If we posit additional, rolled-up dimensions, you could imagine a difference characteristic speed, b not equal to c, for these dimensions, but b/c may just be an immeasurable scale factor.

We might consider the magnitude of the 4-velocity to widen the search.
 
  • #16
Fredrik said:
Seriously though, the actual thought doesn't have a position, so you can't associate a speed with it. The place where the thought occurs is just your brain, and it's not moving very fast, certainly not faster than c.

Off-topic, but the speed of thought is in fact quite measurable.

Broadly speaking, the brain takes about a fifth of a second to act in habitual fashion and around half a second to react in attentive fashion. The distances that neural signals have to travel explain why one is quicker than the other.

And indeed the very question that got psychology started (Helmholtz, Wundt, Donders, etc) was about the speed of various mental processes, following measurements of nerve conduction speeds in frog's legs.
 
  • #17
I'm sure there are characteristic velocities in particular media, if that's what you mean.

Imagine a substantial disk of elastic material.

Place stations, A and B, on opposite edges of the disk and let the stations interchange elastic waves through the material of the disk.

Now imagine an observer, O, to be moving at a velocity v, a substantial portion of c, along the axis of the disk.
What velocity would this observer make for the speed of the elastic waves between A and B?

What would this observer now make of the critical velocity of a third station, D, moving from A to B in relation to the elastic waves?
 
  • #18
Phrak said:
Under the more general question "Are there any special speeds other than 0 and c?", I can't think of any.

Zero is not special. Only c is special, being invariant for all observers. Zero, like 10mph, is relative to your reference frame.

--John
 
  • #19
JDługosz said:
Zero is not special. Only c is special, being invariant for all observers. Zero, like 10mph, is relative to your reference frame.

--John
There are quantites associated bodies such as mass, energy, length, and time interval that are minimal or maximal when the relative velocity is zero.
 
  • #20
Saying there are no speeds greater than c may not be accurate.

Tachyons are a hypothetical subatomic particle that travels faster than the speed of light.
 
  • #21
gutti said:
Saying there are no speeds greater than c may not be accurate.

Tachyons are a hypothetical subatomic particle that travels faster than the speed of light.

It's not that simple. See http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html" in the Usenet Physics FAQ.

In other words, we can no longer "localize" our tachyon in any bounded region in the first place, so it becomes impossible to decide whether or not there is "unit propagation velocity" in the precise sense of part 1)

That is, you can't really say that they are "traveling" in a meaningful classical way, at all! You can solve SR formulas with superluminal velocities and not have the math break if you allow mass to be imaginary, but when you follow it through you get other weird effects that are hard to reconcile with our every-day concepts of space and time.

--John
 
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  • #22
Phrak said:
There are quantites associated bodies such as mass, energy, length, and time interval that are minimal or maximal when the relative velocity is zero.

I see. "special" in some sense, but not invariant.
 

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