# Fashlights, Spaceships, and General Relativity

Ok, here is a physics type problem I have been wondering about, but can’t figure out on my own.

Suppose I am traveling in a spaceship approaching the speed of light
What happens if I shine a flashlight both ahead of me (in the direction that the spaceship is traveling) and behind me?

How fast does the light appear to be moving

1. From my perspective
2. From the perspective of someone independent of my spaceship

For example, if the light shining forward appears to be moving at its normal speed, then shouldn’t it appear to be moving 186,000 miles per second plus however fast my spaceship is traveling, and thus violate the universal speed limit that is the speed of light?

Light can never go faster than the speed of light. It can never go slower than the speed of light. When you shine the flashlight ahead it goes SoL, and when you shine it behind you it goes the same.

In Newtonian physics, speeds will add up. If you go 100 MPH and shoot a gun forward it will travel at its normal speed plus 100 MPH. But at speeds approaching the SoL that breaks down. Light always travels the same speed, and things start to appear funny to outside observers.

If you do a search of this site you’ll find this discussed in numerous ways. I’m not very well versed on this subject, others will come along soon.

> How fast does the light appear to be moving
> 1. From my perspective

Answer: The light appears to travel in both the forward and
backwards directions at “c”, the speed of light.

> 2. From the perspective of someone independent of my
> spaceship

Answer: The light appears to travel in both the forward and
backwards directions at “c”, the speed of light.

> For example, if the light shining forward appears to be moving
> at its normal speed, then shouldn’t it appear to be moving
> 186,000 miles per second plus however fast my spaceship is
> traveling, and thus violate the universal speed limit that is the
> speed of light?

Answer: No. It doesn’t violate the universal speed limit.

The problem is that in everyday life we are used to the idea that
velocities add in a very simple way: A man walking at 5 mph
forward in an aircraft going 500 mph is moving at 505 mph
relative to the ground. This was originally described by Galileo,
and the method for examining the difference between “frames of
reference” such as the airplane, the ground and the man in the
above example is called a “Galilean transformation”.

However, the universe doesn’t work this way: velocities add
according to the “Lorentz” transformation, which has some
startling consequences when you examine what happens
between frames of reference that have relative speeds near c.

Recommended reading: “Special Relativity” by A. P. French,
Publisher: W.W. Norton & Company; ISBN: 0393097935

JohnnyS

As mentioned light is always measured as travelling at light speed no matter who is measuring it. However, I think you are looking for how that could work on your near light speed spaceship.

On your spaceship you shoot a photon of light at a target. The photon does not gain momentum from your ship (a photon has no rest mass). As the photon is flying towards your target the target is moving away from the photon. You would think that you would then measure a lower speed for the speed of light but that doesn’t happen. Instead, your clock slows down and distance shortens such that when you stop your stopwatch when the photon arrives and do a quick calculation you will see exactly the same speed of light as anyone else in the universe does. If you extend this you will see that actually moving at the speed of light causes distances to shrink to zero and time to stop…if you went that fast you would essentially be in all places in the universe at once at all times in the universe. Among other problems this would require infinite energy to achieve so don’t hold your breath for it.

I forgot to mention what all of this looks like to you on the spaceship. It all looks perfectly normal to you. You wouldn’t feel or see anything different than you do right now as if you wren’t moving at all (even though you actually are moving quite a bit at this very moment).

I’m afraid I have to disagree with you on that last statement Whack-a-Mole. Things would look quite a bit different to the passengers of the spaceship due to the Doppler Effect.
Cite: http://www.arachnoid.com/sky/redshift.html

Photons have no rest mass, but they do have momentum. And from the perspective of someone watching your ship fly by, the forward moving photon will have more momentum than one going in the other direction. The velocity of both will be c, but in relativity, momentum is not velocity times rest mass.

If the traveller looked out a window then other objects might be Doppler shifted. As far as life on the sapceship goes everything seems normal. Everything loks the same. Your clock ticks forward just the same as it had on earth as far as you are concerned and so on.

In fact, there is no experiment our traveller could perform that would even tell him he’s moving or standing still. If he looks out a window it is just as reasonable for him to assume that everything else is moving and he is standing still. Imagine our traveller was born on that ship and no one told him any differently he would think he was stationary.

Welcome to the boards, JohnnyS. With clear well-reasoned answers like that, you’ll be a great addition. Thanks.

Ah, I misunderstood your previous statement. I was of course thinking of what the rest of the universe would look like to our traveler. Sorry.

a couple of little movies to give you a visual idea of whats what. Does a fairly good explanation too…

http://www.fourmilab.ch/cship/cship.html

What they all said. Another way to visualize it is with a garden hose instead of a flashlight.

Picture yourself spraying the hose while moving forward on a track. No matter how fast you go, the water isn’t going to come out of the nozzle any faster.

Same if you point it behind you.

The problem with the garden hose is that, although the speeds look the same to you while running, the fellow sitting in the bleachers will see the forward stream as going faster. This is not true of light: Any observer will measure any photon, from any source, as moving at exactly c. Unfortunately, I don’t think that there is any familiar analogy for this.

A question about the OP, by the way: You say that the ship is “approaching the speed of light”… Do you mean that the ship is at some high, but constant, speed, or do you mean that it’s fast and getting faster? The results will actually end up being a bit more complicated if you’re accelerating.

No, it’s not the same at all. If you shoot water out of a hose, speed of the truck is added to the speed of the water as observed by someone standing on the road. If the truck is going faster than the spray, the water shooting out the back is going forward as observed by a by-stander. This is in contrast to a beam of light. The speed of light is the same whether measured by a moving truck (or space ship) or by a (realtively) stationary observer.