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RickJ · Mar 4, 2009, 05:18 AM

Please refresh my memory. If I'm on a spaceship traveling at the speed of light - and shine a flashlight out the front window (forward), will I see the light going forward in front of my ship?

If so, then wouldn't someone watching from afar see the flashlight beam going faster than the speed of light?

I remember from school days that there's an "elementary" answer to this, but googling is no help with the odd terms :)

tickle · Mar 4, 2009, 06:17 AM

You are travelling at the speed of light, so if you shine a flashlight out the window, forward, you won't see the light going forward because the light will be behind you. I don't know about someone watching and seeing the beam going faster. How could that be if you are going at the speed of light, shining a flashlight beam that is supposed to be fast too, but can't be ? Okay. I had to reply to this post and am hopeful of seeing an answer from soneone else too !

Tick

RickJ · Mar 4, 2009, 06:23 AM

Are you sure?

I thought that relativity might mean that since I (in the ship) am "stationary", I would see the light going forward.

I DO know that If I'm standing a train that is moving at 100 mph and throw a ball forward at 10 mph then I see the ball going forward at 10 mph but an observer at a distance sees the ball moving at 110 mph.

That's the "tickler"... pun intended :)

I'm wondering if the speed of light scenario is different since it's the maximum speed of anything...

ebaines · Mar 4, 2009, 07:06 AM

Fist off, you can't travel at the speed of light. So let's talk about someone traveling at 99% the speed of light - what would that person see?

The person traveling at 99% the speed of light would have the same perception as if he is standing still and everything else is travling past him at 99% the speed of light. So he would see everything coming towards him from the front as very much blue-shifted, and everything traveling away from him towards the rear as red-shifted. If he shines a light out the front he would see that light as "normal" - the photons move away from him at the speed of light (relative to him) and is neither red-shifted nor blue-shifted. An outside observer who sees that light would see the photons coming towards him at the speed of light relative to him but very much blue shifted. No one ever meausres photons traveling at faster (or slower) than the speed of light.

RickJ · Mar 4, 2009, 07:56 AM

Haha, well neither a person nor a craft can travel at even 99% the speed of light :)

... so to clarify that I understand correctly:

If I'm traveling at 99% the speed of light and shine a light forward, then to me that light seems to go forward normally (that is, at the speed of light).

OK... that part I understand.

... but if the observer from afar is measuring how fast the flashlight beam is moving from my craft to some distant object, how fast would that observer see it (the light shined from my craft) as moving?

ebaines · Mar 4, 2009, 08:09 AM

Originally Posted by RickJ

...but if the observer from afar is measuring how fast the flashlight beam is moving from my craft to some distant object, how fast would that observer see it (the light shined from my craft) as moving?

Answer is at the speed of light, as that observer measures it, no matter what the relative velocity is between the observer and that object.

This is the fundamental insight that Leibnitz and Einstein quantified. If you think about "normal" relativity as we see it happening around us every day, velocities are additive. For example, if I travel on a train at 60 MPH and throw a ball forward at 60 MPH, to observers on the train the ball is going 60MPH whereas for observers on the ground the ball is going at 120MPH. Conversely, if I throw the ball towards the rear at 60MPH, the observer on the ground would see the ball as standing still. I think it was Galileo who first quantified the mathematics for these types of relative velocities.

But as velocities apporach the speed of light the rules change - velocities are no longer additive. The speed of photons is always a constant for all observers, regardless of the observer's relative motion to the source of the photons. Hence if I am on a ship moving at, say 2.95x 10^8 m/s (~99% the speed of light) and shine a light out the front I will measure the speed of those photons as moving away from me at 3 x10^8 m/s (the speed of light). Similary, an observer on the ground would see me coming at him at 2.95 x 10^8 m/s and the photons coming at him at 3 x 10^8 m/s, not (3+2.95) * 10^8m/s. So the velocity of the photons is not additive. The only difference that the observer on the ground sees is that the light will appear very much blue-shifted (higher frequency photons).

RickJ · Mar 4, 2009, 08:12 AM

Originally Posted by ebaines

But as velocities apporach the speed of light the rules change - velocities are no longer additive.

That is what I was missing. Thanks!

daeta · May 9, 2009, 09:11 AM

This question always gets confusing when you don't add the TIME element.

The person on the ship traveling 99% of light speed, shining the torch out has a different TIME than the observer on earth. In this case slower. So while LIGHT may appear at the same speed, its only that the observer on the SHIP has TIME slowed down due to the speed.

Always remember TIME and SPACE are connected:-)

RickJ · May 11, 2009, 03:52 AM

I just picked up Discover Magazine's special on Einstein. It's a wonderful issue!

ngasnier · May 13, 2009, 10:57 AM

Originally Posted by RickJ

Please refresh my memory. If I'm on a spaceship traveling at the speed of light - and shine a flashlight out the front window (forward), will I see the light going forward in front of my ship?

It depends on wheather you "know" that you are in a spaceship which is traveling as you say.

Since everything appears to be perfectly as normal to people on the spaceship (as long as they don't know about the fact that they are moving), I think that they would see the beam from the flashlight traveling at the speed of light.

dudi5717 · Sep 12, 2010, 02:17 AM

Originally Posted by ;

will I see the light going forward in front of my ship?

Hello RickJ

Exact answer is YES. When you travelling with the space ship with the speed of light c, you can see back and front views. But some deformations occours on sight.
You can find the first order information about your question at this site.
Alice In Physics
Look at the following two papers there :
Space deformation and Doppler Effect.
That pages are interactive so you can see how it occours and what happens.

dudi5717 · Apr 3, 2011, 12:23 PM

Speed of light is relative

I wrote two new papers on this subject. First paper shows how the light behaves. The second paper is about the reason of this behaviour.

Alice Law, Relativity Theory Part 1

Alice Law, Relativity Theory Part 2

Gnostradamus · Aug 5, 2012, 12:04 AM

But as velocities apporach the speed of light the rules change

That's technically incorrect. Velocities are never additive. But at slow speeds relative to light, such as those in normal day-to-day life, the difference is negligible and undetectable. As speeds approach that of light, the difference is no longer fractional. At the speed of light, it's not additive at all.

And it's possible to come up with such a scenario. Suppose a photon emits another photon. In a sense, from an observer's point of the view a photon's time would be constant (frozen), but the second photon would still travel at the speed of light, from the view of the first photon.