Action at Distance: Can Brake Speed Exceed C?

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Discussion Overview

The discussion revolves around the concept of action at a distance in the context of braking in a hypothetical long bus traveling at relativistic speeds. Participants explore whether the speed of the brake fluid could exceed the speed of light and the implications for passengers experiencing the braking action.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question the validity of the speed of light as an upper limit for all moving reference frames, particularly in the context of a long bus and its braking system.
  • Others argue that using a rigid bar or cable to transmit the braking force would not allow for faster-than-light communication, citing principles from relativity.
  • A participant mentions the concept of Born rigidity as a way to understand rigidity in relativistic contexts.
  • There is a discussion about the simultaneity of passengers being lurched forward during braking, with some suggesting it depends on the speed of sound in the bus's material.
  • One participant expresses skepticism about the FAQ's logic, while others assert its soundness and encourage a deeper understanding of special relativity.
  • Another participant raises a question about the implications of a collision with a stationary object and how it affects the timing of the passengers' reaction.
  • There is a mention of the Pythagorean theorem in relation to rigidity, which is challenged by another participant who states it pertains to geometric distance rather than material properties.

Areas of Agreement / Disagreement

Participants generally disagree on the implications of relativity regarding action at a distance and the nature of rigidity. Some find the FAQ's arguments convincing, while others remain unconvinced and continue to challenge the established principles.

Contextual Notes

Participants reference various principles of relativity, including the speed of light as a limit speed and the concept of simultaneity in different frames of reference. There are unresolved questions about the assumptions underlying these discussions, particularly regarding the nature of rigidity and the transmission of forces over large distances.

Darko M
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Hello fellows, it just feels good to be physics forum, But I'm still having a nightmare believing that the speed of light should be the upper limit for all moving reference frames.
Now, if we had a long bus whose length is past 3.0 ^8 m, say 3.0 ^12 m, (Oh just assuming) If the driver of this long cosmic bus were to suddenly apply his brakes and if assuming the passengers had no seat belts on, wouldn't the speed experienced by the brake fluid in its action (according to the Bernuli principle) be faster than c, if the distance between the brake pedal and the last brake pad/shoe is say 3.0 ^10 m ?
Also aren't all passengers on board (no matter distance from brake pedal) expected to be lurched forwards almost all simultaneously?
I mean, just to prove that action and reaction are equal and opposite in this scenario?
 
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From the FAQ at the top of this forum: https://www.physicsforums.com/showthread.php?t=536289

If you can't use a rigid bar between the brake pedal and the most distant brake shoe to generate a faster-than-light signal, hydraulic fluid won't work any better.
 
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It should be possible to use a rigid bar or even a cable or why not?
 
Darko M said:
It should be possible to use a rigid bar or even a cable or why not?

Did you read the FAQ that was pointed to ?
 
I have read the FAQ. but not so convincing.
 
Darko M said:
It should be possible to use a rigid bar or even a cable or why not?
Classical rigidity is not available in relativity (this is the kind of rigidity you are used to). What you can do is use Born rigidity: http://www.mathpages.com/home/kmath422/kmath422.htm
 
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Darko M said:
I have read the FAQ. but not so convincing.

Well, you're on your own then. The rest of us are convinced.
 
Darko M said:
It should be possible to use a rigid bar or even a cable or why not?

You might try googling around (and searching this forum) for "Born Rigidity", "Ehrenfest Paradox", "Bug-Rivet Paradox" to see more. But i will caution you that you're kinda starting starting in the middle by approaching relativity this way; it's generally better to start with the basic principles, nail them down, before you start retraining your intuition about how rigid bodies work at relative speeds near that of light.
 
Darko M said:
I have read the FAQ. but not so convincing.
Why not, the logic in the FAQ is completely sound. What specifically do you disagree with? "Not convincing" is not a valid criticism.
 
  • #10
Nugatory said:
From the FAQ at the top of this forum: https://www.physicsforums.com/showthread.php?t=536289

If you can't use a rigid bar between the brake pedal and the most distant brake shoe to generate a faster-than-light signal, hydraulic fluid won't work any better.

:smile: But what if it's DOT 4 from Willy Wonka's brake fluid factory?
 
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  • #11
Darko M said:
I have read the FAQ. but not so convincing.
Maybe you will find the derivation of SR more convincing?

Basically, if we assume, as has been rather well established, that the relativity principle holds (first postulate) and that the speed of light is independent of that of the source (reduced second postulate), then the speed of light is a limit speed.

You can find the derivation for SR's assumption that c is a universal constant in many places, for example here:

http://www.bartleby.com/173/a1.html
(click on the link to XI to see the sketch)

With GR things become more complex but c remains the limit speed locally.
 
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  • #12
I'm impressed by the references given thanks a lot. Well pardon my ignorance, but approached in a different perspective, assuming the bus were already in motion with a relatively low uniform acceleration, as such as would be quickly 'hushed' by a collision with a stationary object, will the farthest passengers be lurched forward 'almost simultaneously' or depending on the time factor of the distance(say now, 3.0 ^11) from the object at rest, relative to speed of sound in the bus' material? My not been convinced actually borders around an event as this.
 
  • #13
Also in in deriving the Lorentz transformation there is a semblance of the Pythagoras theorem which can connotes some considerable level of rigidity? I am still doing some reading on SR. But I must confess I've got a lot more reading to do.
 
  • #14
Darko M said:
assuming the bus were already in motion with a relatively low uniform acceleration, as such as would be quickly 'hushed' by a collision with a stationary object, will the farthest passengers be lurched forward 'almost simultaneously' or depending on the time factor of the distance(say now, 3.0 ^11) from the object at rest, relative to speed of sound in the bus' material?
The speed of sound in steel is about 6E3 m/s. So it would take 5E7 s for an impulse to travel 3E11 m in steel. The passengers won't lurch until that impulse arrives.
 
  • #15
Darko M said:
the Pythagoras theorem which can connotes some considerable level of rigidity?
Huh? The pythagorean theorem is about geometric distance and has no connotation about any material properties whatsoever.
 
  • #16
Thank you Dalespam; answers well noted.
 

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