Is energy faster than light?

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Scooter_xkcd
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Is energy faster than light?

Postby Scooter_xkcd » Thu Aug 18, 2011 6:29 am UTC

If you have a massively long non-compressible object, say a light-second long rod though space, and push it, the other end moves instantaneously with the rest of it.

However, if you have a photon head from one end to the other without any interference, obviously it would take a second.

How is this possible? Not even the forces of gravity and electromagnetism are faster than the speed of light, yet the force applied to the rod travels to all particles in the rod in zero time? If this is the case, then energy would be instantaneous too, and therefore the travel of heat (radiation?).

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Re: Is energy faster than light?

Postby gmalivuk » Thu Aug 18, 2011 6:44 am UTC

It's not possible. Absolute incompressibility violates special relativity, because impulses travel through a material at the speed of sound in that material, which is mediated by the electromagnetic force between atoms and therefore can't be faster than light.
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Technical Ben
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Re: Is energy faster than light?

Postby Technical Ben » Thu Aug 18, 2011 7:20 am UTC

Has this been asked 4 times in a month now? Perhaps some large sticky, banner or cheese grater for "long rod". :lol:
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Scooter_xkcd
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Re: Is energy faster than light?

Postby Scooter_xkcd » Thu Aug 18, 2011 3:10 pm UTC

Technical Ben wrote:Has this been asked 4 times in a month now? Perhaps some large sticky, banner or cheese grater for "long rod". :lol:


I saw one like this, but it asked something slightly different, such as how to make a photon traveling through the substance move faster than light.

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Re: Is energy faster than light?

Postby douglasm » Thu Aug 18, 2011 3:30 pm UTC

I've seen something like this discussed many times in this forum, though I can't give a good estimate of how often.

The misconception to be cleared up is always that no, the other end actually doesn't move immediately. The other end moves when the compression wave, which travels at the speed of sound in the rod, reaches it. A true non-compressible object would be one with an infinite speed of sound, which is impossible. Nothing can have a speed of sound greater than c without violating Relativity, and it takes something like a neutron star to even get close to that.

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Re: Is energy faster than light?

Postby Coffee » Fri Aug 19, 2011 12:28 am UTC

Good luck pushing a rod made of neutronium.
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Re: Is energy faster than light?

Postby KittenKaboodle » Fri Aug 19, 2011 1:46 am UTC

"Random thought on transmitting info @ C+ velocities" viewtopic.php?f=18&t=73157 still on the first page of "Science", almost precisly the same question, and got some decent answers, videlicet , movement travel at the speed of sound which is allways less than the speed of light in a vacuum and in ordinary materials, much much less. (there was some doubt about neutronium, but still, the speed of sound would not exceed c)
ShuRugal wrote:A perfectly straight rod of a material with low compressibility (such as high-speed steel or, to be silly, diamond) measuring 1 light-second in length is set up with an actuator on one end and a contact sensor on the other.

Set up parallel beside the actuator is a device which will, when the actuator pushes the rod, emit a photon towards a photocell beside the contact sensor.

The actuator pushes the rod, and simultaneously a photon is emitted. Does the photon arrive at the other end of the rod before, after, or concurrently with the movement of the other end of the rod?

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Re: Is energy faster than light?

Postby dumbzebra » Fri Aug 19, 2011 4:35 pm UTC

Oh Trollphysics...
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Radium
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Re: Is energy faster than light?

Postby Radium » Sat Aug 20, 2011 1:12 pm UTC

The whole rod doesn't travel at c. You get a compression wave which travels along the rod (think like a slinky). In addition, considering that energy is related to mass by E = mc^2, it doesn't make much sense that energy should be able to travel faster than light if matter can't either.


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