# Action at distance

1. Jul 23, 2013

### Darko M

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?

2. Jul 23, 2013

### Staff: Mentor

From the FAQ at the top of this forum: https://www.physicsforums.com/showthread.php?t=536289 [Broken]

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.

Last edited by a moderator: May 6, 2017
3. Jul 23, 2013

### Darko M

It should be possible to use a rigid bar or even a cable or why not?

4. Jul 23, 2013

### phinds

Did you read the FAQ that was pointed to ???

5. Jul 23, 2013

### Darko M

I have read the FAQ. but not so convincing.

6. Jul 23, 2013

### WannabeNewton

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

7. Jul 23, 2013

### phinds

Well, you're on your own then. The rest of us are convinced.

8. Jul 23, 2013

### Staff: Mentor

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.

9. Jul 23, 2013

### Staff: Mentor

Why not, the logic in the FAQ is completely sound. What specifically do you disagree with? "Not convincing" is not a valid criticism.

10. Jul 23, 2013

### nitsuj

:rofl: But what if it's DOT 4 from Willy Wonka's brake fluid factory?

Last edited by a moderator: May 6, 2017
11. Jul 24, 2013

### harrylin

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.

12. Jul 24, 2013

### Darko M

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. Jul 24, 2013

### Darko M

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. Jul 24, 2013

### Staff: Mentor

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. Jul 24, 2013

### Staff: Mentor

Huh? The pythagorean theorem is about geometric distance and has no connotation about any material properties whatsoever.

16. Jul 24, 2013

### Darko M

Thank you Dalespam; answers well noted.