Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: stragen on 15/09/2005 18:10:20

Title: Light emitted *at* the Speed of Light
Post by: stragen on 15/09/2005 18:10:20
If you are in a vehicle travellling at *exactly* the speed of light, what would happen if you turned on the "headlights" and therefore projected light in front of you.

Nothing can travel faster than the speed of light so where does this light go? does it build up like a shockwave then disperse when you slow down?

Also, would it look different to someone travelling IN the vehicle and to a bystander on the "roadside" ?

This ones been bugging me for a while so a definitative answer would be great!
Title: Re: Light emitted *at* the Speed of Light
Post by: stragen on 15/09/2005 18:50:50
ok, so i read that thread and understand now that the light would look like its travelling away from you at light speed if you are in the vehicle, and everything would just look like its going the same speed to a bystander...

but where does the emitted light *go*. it looks like its moving away at the speed of light, but this cant be possible if you are already at that speed, otherise the light beam would have a "real" velocity of lightspeed+lightspeed

42....?
Title: Re: Light emitted *at* the Speed of Light
Post by: stragen on 15/09/2005 22:37:40
the bit i cant get my head around is the light you are emitting is already travelling at the speed of light, so as you emit more light it cant travel away from you as it is already going max speed possible.


42....?
Title: Re: Light emitted *at* the Speed of Light
Post by: gsmollin on 16/09/2005 18:36:55
quote:
Originally posted by stragen

If you are in a vehicle travellling at *exactly* the speed of light, what would happen if you turned on the "headlights" and therefore projected light in front of you.

Nothing can travel faster than the speed of light so where does this light go? does it build up like a shockwave then disperse when you slow down?

Also, would it look different to someone travelling IN the vehicle and to a bystander on the "roadside" ?

This ones been bugging me for a while so a definitative answer would be great!



The definitive answer is: You can't travel at the speed of light, so none of the rest of it can happen either.


"F = ma, E = mc^2, and you can't push a string."
Title: Re: Light emitted *at* the Speed of Light
Post by: Dr. Praetoria on 17/09/2005 17:31:52
The light would appear to "stand still" to you and being travelling at the speed of light to some bystander standing still.
Title: Re: Light emitted *at* the Speed of Light
Post by: Atomic-S on 18/09/2005 06:36:18
Well, you know, that if you travel at the speed of light, several other significant phenomena come into the picture. One of which is that your thickness in the direction of travel is now zero, due to the Lorentz contraction. Another is that your time now stands still, as far as the outside world is concerned, meaning that every attempt by you to emit light fails, because nothing can happen!
Title: Re: Light emitted *at* the Speed of Light
Post by: David Sparkman on 18/09/2005 18:13:20
Or in other words, you are thinking I do this and that should happen, but since time is standing still there is no "If-Then" because there is no passage of time.

Hint: keep your speed down, and enjoy life.

David
Title: Re: Light emitted *at* the Speed of Light
Post by: Solvay_1927 on 18/09/2005 23:23:43
Einstein's formula for the addition of velocities in Special Relativity is:
U = (V+W)/(1+VW/c^2)

For someone standing outside the vehicle, they see the light emitted at speed U, where V is the speed of the vehicle and W is the speed of what the vehicle is throwing out.  So V=c, W=c, and U=(c+c)/(1+c^2/c^2)=2c/2=c.  So the outside observer sees the light travelling away at the speed of light.

For someone inside / travelling with the vehicle, V=0, W=c, so U=(0+c)/(1+0)=c.  So they also see the light beam travelling away at the speed of light.

So what's the problem? :P
Title: Re: Light emitted *at* the Speed of Light
Post by: David Sparkman on 19/09/2005 00:55:32
The deciding question is: is there a perception of the passing of time when traveling at the speed of light. It is a boundry thing. If there is no passage of time, then we can't push a button and turn on the headllights as these are all functions that can only occur in time. It is a thought puzzle only similar to the little things A.E. came up with. It is totally impractical except to help us understand what these theories teach.

David
Title: Re: Light emitted *at* the Speed of Light
Post by: ukmicky on 19/09/2005 02:01:55
If they wanted to see what happened to time at near light speeds they could always make a nanotech clock and place it in a Particle accelerator [:D]



Sorry i appologise itís the only way I get to participate in these intellectual topics.[:)]


Michael
Title: Re: Light emitted *at* the Speed of Light
Post by: Solvay_1927 on 19/09/2005 02:35:47
Relativity says that if your vehicle is moving at constant velocity relative to something else, you have no way of telling if you're moving or standing still (other than by looking out of the window, of course - doh!)  You would experience the same laws of physics whether you're moving (uniformly) or stationary.  So the person sitting in the vehicle doesn't see time coming to a stop - they'll look at their watch an it will still be ticking at the normal rate.  They can reach out and flick the lightswitch just like they would normally.  They can sit on the bonnet (US: hood) of the car and hold a 1 metre stick out in front of the lights and confirm that when the headlights are switched on, the light takes 3.3 nanoseconds to reach the end of their stick (i.e. 1/3x10^8).

The person standing on the road, however, won't see this, as to them the car will appear to have shrunk in length (i.e. in the direction of travel) - but not in height or width - and if they could see the clock on the dashboard it would appear to be frozen.

Well, that's the theory anyway.  But certain practical considerations mean that it will always be theory - you could never demonstrate this in practice (e.g. the fact that the car would become infinitely massive if it could be accelerated up to the speed of light).

And you'd have to be a very good driver to keep the car on the road at those speeds anyway.

Maybe it would have to be driven by a German?
Title: Re: Light emitted *at* the Speed of Light
Post by: vanvinhhoang on 22/09/2005 10:21:52
do you think that the limitation of lightspeed close relate with the bound of Our universal?