# Thread: Faster than the speed of light?

1. If I had a rod 1 light year in length then pushed and pulled the rod back and forth... would this push and pull be felt on the far end of the rod before they would witness me pushing it visually?

2.

3. The transmission of the push via a compression wave in the material would reach the far end of the rod at the speed of sound in the rod's material. So the observers would see your effort long before they felt its effect.

4. Shadows are faster than light and proven so

5. Originally Posted by mauriecb
Shadows are faster than light and proven so
So what ?

6. So what? Thats the typical response from science majors when they can't explain how something is moving faster than light. School is only meant to teacj you how to research on your own. You have to learn beyond what you are taught.

7. Originally Posted by mauriecb
So what? Thats the typical response from science majors when they can't explain how something is moving faster than light. School is only meant to teacj you how to research on your own. You have to learn beyond what you are taught.
Sonny, I have been doing research on my own for decades.

Learning on your own and doing original significant research, is a degree requirement in my circles.

"So what ?" signifies that the fact that you can make a shadow, or a spot of light, or the intersection of a guillotine blade wth the base, move at speed greater than c is well known in special relativity and rather insignificant.

Go lecture someone else.

8. Originally Posted by treats
If I had a rod 1 light year in length then pushed and pulled the rod back and forth... would this push and pull be felt on the far end of the rod before they would witness me pushing it visually?
As Ophiolite already noted, there is no such thing as a perfectly rigid body. This means that the answer to your question is, no.

When you push on one end of the rod, it creates a moving compression wave down the rod itself... the particles closest to your hand compress, squeezing the particles next them, and when those compress it squeezes the particles next to them... and when those compress, they squeeze the particles next to them... and this progess continues all the way down the length of the rod until the end.

When you push it, it does not instantly move at the other end. It takes time for the compression wave to travel down the length of the rod (since there is no such thing as a perfectly rigid body), and that compression wave travels slower than the speed of light.

9. Originally Posted by inow
Originally Posted by treats
If I had a rod 1 light year in length then pushed and pulled the rod back and forth... would this push and pull be felt on the far end of the rod before they would witness me pushing it visually?
As Ophiolite already noted, there is no such thing as a perfectly rigid body. When you push on one end of the rod, it creates a moving compression wave down the rod itself... the particles closest to your hand compress, squeezing the particles next them, and when those compress it squeezes the particles next to them... This continues down the length of the rod until the end.

When you push it, it does not instantly move at the other end. It takes time for the compression wave to travel, and that compression wave travels slower than the speed of light.
Moreover, the rigid body idealization is consistent with Newtonian mechanics, but inconsistent with relativity.

The process that you are describing involves elasticity. Stress waves propagate along a material at speeds that are weakly dependent on amplitude. The limiting case of small amplitude corresponds to what we call the speed of sound.

10. Originally Posted by mauriecb
So what? Thats the typical response from science majors when they can't explain how something is moving faster than light. School is only meant to teacj you how to research on your own. You have to learn beyond what you are taught.
Yes, so what?

The fact that effects like the edge of a shadow can exceed light speed is meaningless as far as Relativity is concerned. They have no mass nor can they be used to transfer information. If you have a shadow edge passing between point A and point B, there is no method that someone at point A can use it to communicate with or affect point B.

The universe is full of such superluminal effects, none of which in any way violate Relativity.

11. I've deleted a number of posts from this thread as they had no bearing on the topic on hand and were quickly devolving into a flame war.

12. Originally Posted by Ophiolite
The transmission of the push via a compression wave in the material would reach the far end of the rod at the speed of sound in the rod's material. So the observers would see your effort long before they felt its effect.
haha i would like to witness this! that would be odd

13. Originally Posted by 432145
Originally Posted by Ophiolite
The transmission of the push via a compression wave in the material would reach the far end of the rod at the speed of sound in the rod's material. So the observers would see your effort long before they felt its effect.
haha i would like to witness this! that would be odd
I bet you already have. For example, you can see an approaching baseball long before you can feel it or it's effects.

14. Forgive me for not agreeing as I would have thought DrR would have. A shadow is nothing but the absence of light. You need to remove light from an area to create shadow. The removing light can only move at c. The shadow therefore cannot advance faster than c. Or am I missing something.
Shadow may be an absence of light but it can still convey information and as far as I recall, information ( being a form of energy, ie. entropy) is also limited to the speed of light. Correct me if in error.

15. Originally Posted by inow
Originally Posted by 432145
Originally Posted by Ophiolite
The transmission of the push via a compression wave in the material would reach the far end of the rod at the speed of sound in the rod's material. So the observers would see your effort long before they felt its effect.
haha i would like to witness this! that would be odd
I bet you already have. For example, you can see an approaching baseball long before you can feel it or it's effects.
hahaha yes indeed i have, but that is kinda what i expect
i dont expect to see someone pushing a rod which i hold and waiting till i feel anything

16. Originally Posted by 432145
hahaha yes indeed i have, but that is kinda what i expect
i dont expect to see someone pushing a rod which i hold and waiting till i feel anything
You could do it with a long block of jello (or anything else which would propagate a compression wave slowly).

17. Originally Posted by MigL
Forgive me for not agreeing as I would have thought DrR would have. A shadow is nothing but the absence of light. You need to remove light from an area to create shadow. The removing light can only move at c. The shadow therefore cannot advance faster than c. Or am I missing something.
Shadow may be an absence of light but it can still convey information and as far as I recall, information ( being a form of energy, ie. entropy) is also limited to the speed of light. Correct me if in error.
Nope

Neither the movement of a shadow nor a spot of light can transmit information superluminally.

The usual example is the movement of a spot of light on a distant screen , say from a laser, as the laser is swung. The apparent transverse movement of the spot (or shadow if you set it up differently) can easily exceed c -- it is just trigonometry. But the photons still travel at c, and no information is transmitted from one location to another faster than c. This does not violate relativity.

18. See Pulsar bursts move 'faster than light' at http://physicsworld.com/cws/article/news/41378 . It isn't really faster-than-light of course, it's something like a Mexican wave in a football stadium. You can time things so that the apparent propagation speed exceeds c, even though nothing is actually moving faster than c.

19. Originally Posted by Farsight
See Pulsar bursts move 'faster than light' at http://physicsworld.com/cws/article/news/41378. It isn't really faster-than-light of course, it's something like a Mexican wave in a football stadium. You can time things so that the apparent propagation speed exceeds c, even though nothing is actually moving faster than c.
Interesting article, but I think this is the proper link http://physicsworld.com/cws/article/news/41378

And here is the pre-print http://arxiv.org/PS_cache/arxiv/pdf/...909.2445v2.pdf

20. Darn, sorry about that. I inserted a space and the hyperlink now works.

All: there's no moving faster than light guys. It's science fiction I'm afraid.

21. Well, let's consider the equation:

If our velocity, v, was greater than the speed of light, c, than the above formula suggests we would have a momentum that couldn't be modelled in the real numbers, i.e., complex.

I suppose that extant physics only models motion in the Cartesian-coordinate system so it seems impossible even from a mathematical viewpoint (rather than simply a physics one, considering physics is based on the scientific method, whereas mathematics uses the axiomatic method).

22. The Lorentz factor doesn't really prove anything, Ellatha, it's essentially a restatement of Pythagoras' Theorem. But don't rule out complex numbers, they are applicable to electromagnetism. I don't know if you're know, but they often indicate rotation, see for example quaternions. Thus one might argue that in a circularly-polarized photon moving linearly at c, there is an additional "motion" as it were, so in a sense the local rate of motion exceeds c. However neither the photon nor anything else can travel from A to B faster than c. As to the real reason why, IMHO pair production has been rather overlooked.

23. Originally Posted by Farsight
The Lorentz factor doesn't really prove anything, Ellatha, it's essentially a restatement of Pythagoras' Theorem. But don't rule out complex numbers, they are applicable to electromagnetism. I don't know if you're know, but they often indicate rotation, see for example quaternions. Thus one might argue that in a circularly-polarized photon moving linearly at c, there is an additional "motion" as it were, so in a sense the local rate of motion exceeds c. However neither the photon nor anything else can travel from A to B faster than c. As to the real reason why, IMHO pair production has been rather overlooked.
I'm well aware of the applications of complex numbers to electrical engineering, but I don't think that velocity can be expressed as a complex number, which would be the case if one moved faster than the speed of light (as I explained by looking at the formula for momentum in general relativity). I would be more sure of your argument if you explained why the aforementioned reasoning is incorrect (rather than simply stating it is).

24. Originally Posted by mauriecb
Shadows are faster than light and proven so
How can that be!

Shadow is the region where there is no light,
So shadow also travels with the same speed of light isn't it?

25. I didn't mean to suggest your reasoning was incorrect, Ellatha. Just that it doesn't get to the bottom of why we can't move faster than light.

I think I know why. Take a look at pair production and electron properties. An electron (and a positron) can be created from a +1022keV photon interacting with a nucleus. The electron and positron have magnetic dipole moment and spin angular momentum. Now see the Einstein-de Haas effect which demonstrates that the spin angular momentum is "of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". That means the rotation isn't intrinsic after all, it's a real rotation. That means the electron has to be like the photon in the mirror-box. Minus the box. It can't move faster than light because at some fundamental level it's light going round. Sounds surprising perhaps, but check it against what Einstein was saying in Does the Inertia of a Body Depend upon its Energy-Content?. A radiating body loses energy and thus mass. Electron/positron annihilation is like "opening the box". The radiating bodies radiate away all their energy, losing all of their mass, and then they're not there any more.

26. Originally Posted by Farsight
I didn't mean to suggest your reasoning was incorrect, Ellatha. Just that it doesn't get to the bottom of why we can't move faster than light.

I think I know why. Take a look at pair production and electron properties. An electron (and a positron) can be created from a +1022keV photon interacting with a nucleus. The electron and positron have magnetic dipole moment and spin angular momentum. Now see the Einstein-de Haas effect which demonstrates that the spin angular momentum is "of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". That means the rotation isn't intrinsic after all, it's a real rotation. That means the electron has to be like the photon in the mirror-box. Minus the box. It can't move faster than light because at some fundamental level it's light going round. Sounds surprising perhaps, but check it against what Einstein was saying in Does the Inertia of a Body Depend upon its Energy-Content?. A radiating body loses energy and thus mass. Electron/positron annihilation is like "opening the box". The radiating bodies radiate away all their energy, losing all of their mass, and then they're not there any more.
Well if my reasoning isn't incorrect, than it must be correct (logic is binary). It may not be as comprehensive as many other reasons, but it still works. This is similar in a way to how one can use the law of cosines to prove the Pythagorean Theorem, even if it isn't as descriptive as proving it via construction of three squares. Mostly I agree with your post, however.

27. Thanks Ellatha. There's a lot more to all this but I can't talk about it because people have got papers being refereed.

28. Shadow is far from being absense of light. Shadow comes with it and disappears if a light is put off or turns another direction. A dark night is not absense of light but being in the shadow of the earth.

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