Amazing thought worth a look

Main Question or Discussion Point

according to the theory nothing is faster than light so if there were a rod very very long(eigth ligth min long) and then we were to pull it across with a velocity(applying either a force or impulse) then what will happen to the length of the rod?

will it increase becoz the other end of the rod will start moving only after eigth minutes..........

Last edited:

Related Special and General Relativity News on Phys.org
.. will it increase becoz the other end of the rod will start moving only after eigth minutes..........
Yes. Relativity requires nothing is infinitely rigid (unstretchable).

russ_watters
Mentor
Impulses travel through a rod at the speed of sound in the rod.

What if the rod were infinitely rigid?

What if the rod were infinitely rigid?
Relativity prohibits objects to be infinitely rigid (as already posted above).

Of course. Sorry.

yes nothing is rigid bt nothing is elastic to tht extent the rod is bound to break.........isnt it

that was what i had in mind. but then it doesn't necessarily have to. it depends on the distance the rod is moved and the velocity of the displacement.

Crazy question but I have to ask:

What if instead of a rod you had a very long (eight light minutes long) billiards table and lined up the balls touching each other in a straight line with guides and struck the first ball with the cue ball. How long would it be until the ball at the other end jumped off?

ur question is similar to mine and the answer to ur ques is the answer to mine

"NOTHING IS PERFECTLY RIGID" and thus only after eigth minutes will the ball at the other end move..........
crazy isnt it

DaveC426913
Gold Member
ur question is similar to mine and the answer to ur ques is the answer to mine

"NOTHING IS PERFECTLY RIGID" and thus only after eigth minutes will the ball at the other end move..........
crazy isnt it
It will take much, much longer than eight minutes, since the pulse will travel at the speed of the billiard balls, not the speed of light.

Nice thread title, BTW. You in bulk marketing?

It will take much, much longer than eight minutes, since the pulse will travel at the speed of the billiard balls, not the speed of light:
If the balls are touching they don't move. Mechanical pulses through "rigid" touching objects should travel at the same speed as mechanical pulses through a single "rigid" rod, although perhaps not at the speed of light. Mechanical pulses are not photons. I think mechanical pulse speed depends on the molecular structure of the "rigid" material. I wonder what the fastest mechanical pulse would be through? Perhaps a diamond? How fast would the pulse travel?

It will take much, much longer than eight minutes, since the pulse will travel at the speed of the billiard balls, not the speed of light.

Nice thread title, BTW. You in bulk marketing?

bulk marketing why do u think that??

bulk marketing why do u think that??
Because your post sounds like spam.

Because your post sounds like spam.
i dont get it dear and moreover i asked for an answer frm dave not u peter

DaveC426913
Gold Member
i dont get it dear and moreover i asked for an answer frm dave not u peter
Peter is my voice when I'm at other functions. His answer is bang on. (The only thing your thread title didn't do was promise I'll give my girlfriend a night she'll never forget.)

DaveC426913
Gold Member
If the balls are touching they don't move. Mechanical pulses through "rigid" touching objects should travel at the same speed as mechanical pulses through a single "rigid" rod, although perhaps not at the speed of light. Mechanical pulses are not photons. I think mechanical pulse speed depends on the molecular structure of the "rigid" material. I wonder what the fastest mechanical pulse would be through? Perhaps a diamond? How fast would the pulse travel?
(Actually, the billiard balls will move because they are not perfectly rigid. The cue ball will compress one side, the wave of compression will pass through the ball and press on the next ball. In doing so, the centre of mass of the billiard ball will have moved a very small fraction.)

Whether you bother to go into it in this detail or not, the point is that the shock wave will take much longer than 8 minutes - it will take as long as the speed of sound in the material that makes up the billiard balls, which will not be more than a few multiples of the speed of sound in air.

hey zarbanx, just ignore ppl like dave. your question was interesting all right (if it really did come out of curiosity). i don't think they comprehend how physics or science for that matter really is done - it's not about remembering formulae but being curious.

ZapperZ
Staff Emeritus
2018 Award
OK, everyone, please stay on topic!

Zz.

russ_watters
Mentor
DaveC426913
Gold Member
hey zarbanx, just ignore ppl like dave. your question was interesting all right (if it really did come out of curiosity). i don't think they comprehend how physics or science for that matter really is done - it's not about remembering formulae but being curious.
Should he ignore the helpful advice on the topic I've been giving since the beginning of the thread, or should he focus on the throw-away comment?

okkkkkkkkk everyone chill sry dave sry peter lets concentrate on the topic i will change the thread title

P.S--how do we change the thread title

DaveC426913
Gold Member
Well, were there any outstanding issues? It seemed pretty wrapped up.

A rod eight light minutes long is not prefectly rigid. SR forbids it. If one end is moved, the rod will transmit that movement through the length of the rod at no more than the speed of sound in whatever material the rod is constructed from. (The hardest substance known, diamond, will only transmit at 1/25000th of the speed of light.) But even if this rod is constructed of some incredibly advanced, super-hard material, the speed of transmission cannot reach the speed of light. Thus, moving one end of the rod will not cause movement in the other end in any less than eight minutes.

1 light year long staff, used to point with. (dont do this at home)

Wold a soft push travel in the same speed as a shocking hit? edit:(Thinking about the case with the billiardballs) perhaps it would just generate mutch longer wavelength but same speed? / edit

What if a man had a 1 light year long staff made of diomond and pointed to the left and then stright up - using 6 moths (and ignoring the fact that the staff would brake and the man had to be twice as strong as me). Would the other end of the staff travel 2x the speed of light? I guess the answer is no due to some space-time stuff. But how would it look like if observed form a far distance?

Last edited:
DaveC426913
Gold Member
Wold a soft push travel in the same speed as a shocking hit? edit:(Thinking about the case with the billiardballs) perhaps it would just generate mutch longer wavelength but same speed? / edit
Wavelength and frequency are reciprocals. A change in one results in an opposite change in the other.

If you pluck a guitar string softly does it play a lower note than of you pluck it strongly?

What if a man had a 1 light year long staff made of diomond and pointed to the left and then stright up - using 6 moths (and ignoring the fact that the staff would brake and the man had to be twice as strong as me). Would the other end of the staff travel 2x the speed of light? I guess the answer is no due to some space-time stuff. But how would it look like if observed form a far distance?
The rod would bend. Same reason.

Striking the end of the rod send a compression wave through it. Waving the rod sends a transverse wave through it, just like if it were a stiff jump rope.