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Author Topic: Would a torch light go backward in space?  (Read 2707 times)

thedoc

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Would a torch light go backward in space?
« on: 12/12/2014 01:30:02 »
Shee Hong  asked the Naked Scientists:

If I turn on the light of a torch light in space, will it go backwards? What about a laser pointer?

What do you think?
« Last Edit: 12/12/2014 01:30:02 by _system »

jeffreyH

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Re: Would a torch light go backward in space?
« Reply #1 on: 12/12/2014 02:01:25 »
Only very near to a black hole.  [:0]

evan_au

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Re: Would a torch light go backward in space?
« Reply #2 on: 12/12/2014 09:34:41 »
One of the principles of the theory of relativity is that if you see a light beam passing you in a vacuum, it will always seem to be traveling at the same speed, which physicists write as "c". This speed c is about 300,000 km/second, or about 7 times around the world in 1 second.
• If you shine a torch or a laser pointer away from you in space, it will be traveling away from you at "c".
• If you shine a torch or a laser pointer towards you in space, it will be traveling towards you at "c".
• If you are in space, and shine a  torch or a laser pointer towards a mirror, you will see it coming back at you at "c".

In other materials (like air or water), the speed of light is lower than c.
In some exotic materials, it is possible to prime the material with a laser pulse, and slow down light by an extreme amount - some have claimed speeds as low as a walking pace. So I guess if you ran past such a piece of exotic optical material, you could overtake the light, and in that sense it would be going backwards compared to you.

However, you can't see it going backwards, because as soon as it escapes back into a vacuum, it resumes traveling at c (or in the case of air, light travels at 99.97% of c).

chiralSPO

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Re: Would a torch light go backward in space?
« Reply #3 on: 12/12/2014 18:23:33 »
Light is more well behaved and predictable in space than in our atmosphere.

In addition to the exotic materials mentioned by evan_au, there are also exotic  materials (metamaterials) that have an apparent negative index of refraction at particular frequencies. According to Snell's law of refraction, this implies that light would have a velocity greater than c in these materials. It is good to remember that Snell's law is just an approximation though, and that a slight correction to the formula shows that no super-luminal speeds are required for negative refraction.

JohnDuffield

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Re: Would a torch light go backward in space?
« Reply #4 on: 12/12/2014 22:54:43 »
Shee Hong  asked the Naked Scientists:

If I turn on the light of a torch light in space, will it go backwards? What about a laser pointer?
Yes. It will go backwards, because for every action there's a reaction. But don't expect it to shoot backwards like a squid. You probably wouldn't notice anything unless you sat and watched it for a year or something. Only its battery would go flat before that. So in practice, maybe you wouldn't see it going backwards at all.

Maybe somebody else can run some numbers.
« Last Edit: 14/12/2014 12:50:32 by JohnDuffield »

yor_on

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Re: Would a torch light go backward in space?
« Reply #5 on: 13/12/2014 20:34:11 »
'Backwards' is a tricky question?

Do you mean in time, as for some thought up observer of that light?

yor_on

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Re: Would a torch light go backward in space?
« Reply #6 on: 13/12/2014 20:38:19 »
If you meant that one, no, that would destroy the universe in where you exist´. It would destroy causality, and that is what joins us.

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Re: Would a torch light go backward in space?
« Reply #6 on: 13/12/2014 20:38:19 »