# I Light in a moving train

1. Sep 10, 2016

### ewr

Say a person is on a train moving to the right. He then flashes a light upwards perpendicular to the motion of the train. Directly on top of the person there is a mirror. If the light travels in a straight line why does it hit the mirror and bounce back? Why does the light act in a similar way to, say a tennis ball being thrown upward in a moving train? Why doesn't the light travel to the back of the train hitting just to the left of the mirror?

If the train is moving to the right and the light is flashed upwards, why do the light also move in a horizontal direction along with the motion of the train?

I understand why a tennis ball seems to travel up and down when thrown upwards from the perspective of the person in the moving train. But I don't understand why this is the case for light.

Thank you for the help.

2. Sep 10, 2016

### Ibix

Same reason as the ball, in short. Conservation of momentum.

3. Sep 10, 2016

### Drakkith

Staff Emeritus
Why would it be any different?

4. Sep 10, 2016

### PeroK

The light on the train doesn't know that it is "moving to the right". As far as the person on the train, the torch, the light and the mirror are concerned, they are at rest and it's the person on the platform that is moving to the left.

Also, the train is on the Earth and the Earth is moving round the sun, so souldn't the light to go off in some entirely different direction depending on how fast the train is moving round the sun?

Someone yesterday asked a very similar question:

5. Sep 10, 2016

### jartsa

Let us consider a light bulb that is moving to the right, and a parabolic reflector moving with the light bulb, and a ray of light that is not moving with the reflector.

Now because the ray is not moving with the reflector, the reflector collides with the ray, and after the collision the ray is moving with the reflector.

And that is a reason why moving parabolic mirrors produce light beams that are tilted to the direction of the motion of the mirror.

(Imagine a upwards pointing parabolic mirror moving to the right, and a ray that points upwards in the frame where the mirror moves to the right)

Last edited: Sep 10, 2016
6. Sep 10, 2016

### PeroK

That, if anything, only confuses things.

7. Sep 10, 2016

### Staff: Mentor

That is a good thing to point out. You can also get a similar analysis by considering the beam produced by a spherically symmetric source and a pinhole.

8. Sep 11, 2016

### jartsa

Maybe I just happen like mechanical explanations.

Let's say an electron is bouncing up and down between two walls, or between a floor and a ceiling. Said electron radiates EM-waves equally to the left and to the right.

in a frame where the electron is moving to the right, it radiates more to the right than to the left. Why is that?

Let's see ... the direction of the acceleration is the same in both frames, but the shape and the orientation of the electric and magnetic fields of the moving elecron are different in the two frames, so that's the reason, probably.

9. Sep 12, 2016

### harrylin

Maybe it's useful to phrase what others already said in again other words. A tennis ball that is thrown straight upward relative to the train, is thrown at an angle relative to the tracks. And just the same, a light ray that is emitted straight upwards relative to the train, is emitted at an angle relative to the tracks.

10. Sep 12, 2016

### jbriggs444

The words "light ray" conjure up an image of a line with an unambiguous direction and may not be the best to use. "light pulse" is more helpful in my opinion.

11. Sep 13, 2016

### Raymond Potvin

If what you say was true, then we would see a star where it actually is, not where it was when it sent its light. Light is independent from the motion of a source, and if the train is moving the same way a star is moving, then it should not follow the motion of the train. Why would motion be different for stars than for trains?
For the same reason.

12. Sep 13, 2016

### Staff: Mentor

The speed of light is independent of the source, but the direction is not. The rules for both trains and stars are the same, both involve the same relativistic aberration. If you think that they are different then you are misunderstanding something.

13. Sep 13, 2016

### jbriggs444

Funny that you mention this. There is a second reason that we do not see stars where they actually are. https://en.wikipedia.org/wiki/Aberration_of_light
If you consider reception of light and transmission of light as inverse phenomena then this is exactly the same reason that a light is not always seen to shine in the direction it is pointed.

14. Sep 13, 2016

### Raymond Potvin

Motion is about two parameters: speed and direction. Considering that one is independent and the other is not seems contradictory. A ball depends from the motion of the source on both parameters, its speed and its direction, so why would light be independent only for speed? When a photon is sent from a star in the direction of the earth, the star is no more there when the photon strikes the earth because it has moved away from the direction the photon has towards the earth, so why would it be different for a train?

15. Sep 13, 2016

### Ibix

It isn't - that's what we keep telling you. The only special thing about light is that the magnitude of its velocity is unaffected by a frame change, while both are affected for things travelling slower than light.

16. Sep 13, 2016

### Raymond Potvin

Why only velocity would be unaffected? Is there any experiment that shows that the direction of light is affected by the direction of its source?

17. Sep 13, 2016

### Ibix

Speed is unaffected, to be precise. Why? It's a consequence of the geometry of spacetime. Or else it can be seen as a fundamental fact from which relativity can be deduced. We're pretty much running into the limits of our understanding here.

Direction of light depending on the relative speed of source and detector: https://en.m.wikipedia.org/wiki/Aberration_of_light

Edit: Also, see the FAQ on the experimental basis for special relativity, a sticky thread at the top of this forum.

18. Sep 13, 2016

### PeroK

You are misunderstanding the whole concept. Imagine a light source "at rest" and two observers moving with different velocities relative to the source. They cannot possibly measure the same velocity for the light, unless all three are moving in the same direction.

If the light comes out of the source in, say, the y direction, and one observers is moving along the x-axis, and the other is at rest at the origin, then they will measure different x components of the light's velocity.

This would be true for the motion of any object.

What is different about light is that it's speed is the same to both observers. That would not be true of a ball or a train.

In short, the speed of a light beam can be the same for all observers, but the velocity of the light cannot be the same for all observrs.

19. Sep 13, 2016

### Raymond Potvin

Here is the animation from that wiki page:

On the right part of the drawing, the earth is considered at rest while the star is moving, and the direction of the photon is not affected by its motion, but if a ball had been sent instead of a photon, it would not have reached the earth since its direction would have been affected by the motion of the star.

20. Sep 13, 2016

### PeroK

Those animations would apply exactly if, say, the star was a gun, the Earth the target and the light ray a projectile. It's pure geometry. If the projectile hits the Earth in one frame, it must hit in the other.

For example, in the animation on the left, why would a projectile inevitably miss the Earth?