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# Sonic Boom, but at the speed of light.

Posted on 2004-04-30
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I was watching TV, ironically it was a comedy show, and a serious question was posed, what happens if you are travling at the speed of light with your head lights on. Now ofcourse they dident mean this seriously but I am curious.

If you travel past the speed of sound, we all know a sonic boom occours.

What happens if an object moves faster than the speed of light?
Physically what would the result be?

-Brian
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Question by:BrianGEFF719

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ID: 10957670
ohh yah, one more thing...

I am already aware that this completely contradicts Einstiens Theory of Relativity, because nothing of 'mass' can travel at the speed of light, but would would happen if we were to (hypothetically).

-Brian
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Mercantilum earned 50 total points
ID: 10957755
Interesting link - see especially the 2nd answer (bottom), where they really graph what would happen if one could travel faster than light [ and underlining the fact that it is not possible ].

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SteH earned 150 total points
ID: 10957785
To the second case. It would require new physics to allow objects to travel faster than speed of light. With Einsteins theory you can show that objects having a lower speed will always have alower speed. But it is true for the other part as well. If an object would happen to be faster it must remain faster. In principle you could allow a world where all objects would be faster than the speed of light.  But in the known world objects are always slower.

In the case where you travel at the speed of light time would stand still:

t' = (t - vx/c^2) / (sqrt [1 - v^2/c^2]) = ( t - vx / c^2) / 0 = oo.

And all other dimensions will reduce to 0.

L' = L * sqrt (1 - v^2/c^2) = L * 0 = 0;
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PointyEars earned 100 total points
ID: 10957904
Relativity says that you cannot cross the speed of light because it would require an infinite amount of energy, but it doesn't prevent an object from moving faster than light if it is created already travelling at that speed.  Several years ago some scientists postulated the existence of particles born already faster than light.  They called them tachyons.  You might have heard them mentioned in Startrek!

Anyhow, the sonic boom occurs when an object flies faster than the speed of sound in air.  The sound waves generated by the plane are more-or-less spherical when the plane is at rest, but when it moves faster than sound, they compose themselves to form a conical wave that trails the plane.  It is this shock single wave hitting the ground which is perceived to be like an explosion.  This is because also an explosion causes a single compression wave.

A spacecraft will irradiate heat and light (and what-have-you) spherically.  I expect that if it moved faster than light, these waves would also form a conical front of waves like the compression wave of a supersonic plane.  They would be trailing the spacecraft and move outward (like the water surface waves generated by a speedboat).  I imagine that an observer would see a flash of intense light (or some other electromagnetic radiotion, like X-rays or gamma) when hit by that wave.

There is an effect which takes place when a charged particle moves in a medium at a speed higher than the speed of light in the medium.  It is called Cherenkov effect and is possible because the speed of light in a medium is lower than that in void.  This effect generates a blue glow inside the medium, but this is something different from what you are talking about, because in space there would be no medium to perturbate.
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Sergio_Hdez earned 50 total points
ID: 10958026
Light always travel away from you (the emiter) at speed of light MEASURED from your point of view and from any other point of view as long as he/she/it is moving at a constant speed (no aceleration), it doesn't matter your speed: In the case of your point of view, just think that you are always stoped, if you measure speed in relation with your self!

As you don't notice any change in the speed of light (emited by you) regardless of your speed relative to a third person, if you should ever travel faster that light (always in relation with that third person, as speed have to be meassured against something) you would meassure a perfectly normal speed of the light traveling away from you.

The point here is how will this third person measure your speed and the speed of these light ray.... if the light travel from you straight ahead, then the light will pass you, so he/she/it will see that the light is traveling FASTER than you, that are in fact traveling faster than light (always from his/her/its point of view).... so this third person will see light traveling much facter than speed of light.... sounds imposible.

Just imagine you are traveling heading to him, at a speed higher than light, then you emit light, also heading to him: If this light travel at speed of light, he will reach you BEFORE the light, but the light is traveling away from you, so the light ray (the photons) are in front of you, not in your rear, so how can he recieve you BEFORE the fotons if they are ahead you?

There are many other "imposible" consecuencies of traveling faster than light, this is just a couple of them.
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grg99 earned 50 total points
ID: 10958985

There is such a phenomenon, it's called Cerenkov radiation.

If a particle traves thru some medium where the speed of light is lower than the speed of the particle (quite possible , as the speed of light is proportinal to the index of refraction), then the particle does form a "wake" and the wake is quite visible.

If you've ever seen a picture of the insides of a nuclear reactor, the blue glow is caused by beta particles (electrons) that are travelling fater than light can in the water.  They leave a bluish wake for some centimeters, until they lose enough energy to be going slower than light speed in water.

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PointyEars earned 100 total points
ID: 10959063
grg99, I already mentioned the Cherenkov effect.  In any case, the speed of light in a medium is inversely proportional to the refraction index, not directly proportional.
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klepa earned 50 total points
ID: 10963695
With Cherenkov effect the emitted eloctromagnetic radiation is blue because it uses water as a medium. Is that correct?
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Expert Comment

ID: 10963703
I guess what I mean would the color be differrent if it used a different insulator.
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Urhixidur earned 50 total points
ID: 10969606
Not really.  Cerenkov radiation (discovered by Mallet but studied by Cerenkov) has a peculiar spectrum which grows asymptotically as the wavelength of the light shortens --as one over wavelength squared, in fact (although I presume there must be a cut-off occurring at short wavelengths, since the particle responsible for the Cerenkov emission does not have infinite energy...).  It is always intrinsically blue.

See
http://www.cakes.mcmail.com/cerenkov/cerenkov.htm
and
http://dustbunny.physics.indiana.edu/~dzierba/LST1/data.html
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