1. Dec 25, 2017

Hello There,
I have basic doubt about light propagation .So I'm hoping to get some help here.
The problem is ...Let us assume two frames, 'MOVING FRAME'[one moving with a certain velocity(constant)] and 'REST FRAME'[one wrt to which the first frame is moving]. If now moving frame emits a laser beam ...I want to know how the path of that laser will look like in both the frames mentioned above.

2. Dec 25, 2017

### Orodruin

Staff Emeritus
It is better for understanding to talk about a light pulse rather than a continuous beam. A light pulse will move at speed c in a straight line in both frames.

Also note that your setup has nothing that actually identifies either frame as ”at rest”. The concept ”at rest” is relative and there is no way of identifying ”at rest” without a reference. Also, there is no such thing as a ”frame emitting a light pulse”. A frame is a way of assigning space and time coordinates to events, not a physical entity.

3. Dec 25, 2017

### Staff: Mentor

In both frames the speed will be c. The direction may differ.

4. Dec 25, 2017

### Ibix

You can always rotate your axes so the light is travelling in the x-y plane. Write down the position at t=0 and t=T, then apply the Lorentz transforms to work out what happens as described in another frame.

5. Dec 25, 2017

### vanhees71

Light is described by the electromagnetic field. Thus it's much easier and conceptually better to check how the plane-wave modes of the electromagnetic field transform under Lorentz boosts. You'll learn everything from this exercise relevant for the question, namely the Doppler effect for light and aberration.

6. Dec 26, 2017

### pervect

Staff Emeritus
I'm not sure how to answer a "doubt", but it sounds more like you have a question.

Let's take a very simple case. We have an inertial frame S, and in that frame, for light moving in the x direction, we can say that x = ct.

Now we imaging another inertial frame S', which is moving relative to frame S with some velocity v along the x-axis. Then in frame S', we can write x' = c t'.

Things get more complicated if S' is moving in a direction other than along the x-axis, but it's easiest to talk about the simplest possible case first.

Last edited: Dec 26, 2017
7. Dec 26, 2017

I'm sorry for not explaining my doubt clearly... I was in a kind of a hurry when I posted this.Well now, I have attached the situation which was in my mind then. The problem is what my intuition says in the image seems to be opposite to the real scenario, but I don't know WHY.
I hope now I'm clear in what my 'doubt' or 'Question' Is.

8. Dec 26, 2017

### Staff: Mentor

I suppose you've seen the bromide about the British and the Americans being divided by a common language? Indian English has its own characteristics which seem strange to the rest of us.

9. Dec 26, 2017

### Orodruin

Staff Emeritus
To be fair, this particular characteristic is actually not unique to Indian English. I have observed it quite often in southern Europeans as well.

10. Dec 26, 2017

I'm happy that you got that I'm indian, but what about the answer to my 'doubt'.

11. Dec 26, 2017

### Orodruin

Staff Emeritus
Your situation is not very well defined because you have not stated in which frame the pulse is emitted straight down. If the pulse is emitted straight down in the ground frame, then indeed it will go somewhat in the "back" direction in the ship's frame. If it is emitted straight down in the ship's frame, it will go somewhat in the forward direction in the ground frame. This is called aberration of light.

12. Dec 26, 2017

This is actually what i'm not understanding .Why it should go in forward direction,it is not obvious to me as is to some of my peers.

13. Dec 26, 2017

### Orodruin

Staff Emeritus
Because, assuming that it is emitted straight down in the ship's frame, it will always be directly below the ship in the ship's frame. Since the relative velocity between the ship and the ground is orthogonal to this displacement, it always needs to be directly below the ship in the ground frame as well, which means that it needs to have a velocity component $v$ in the direction of the ship's motion in the ground frame.

This is not particular to light. The same would happen if you would drop a ball from the ship. The ball will then have a velocity in the ship's direction in the ground frame.

14. Dec 26, 2017

With ball, I'm clear with this view.But why should light behave like a ball?

15. Dec 26, 2017

### Orodruin

Staff Emeritus
Did you at all read what I wrote before the ball?

16. Dec 26, 2017

Yes...I understood your argument. You are saying that since light needs to be below ship in ship's frame so should it be in the ground frame by which you are giving rise to that velocity in the positive direction.Am I right.
But can one in ground frame tell why should that pulse have that forward velocity like the way we can tell in case of the ball?
In case of the ball, it is obvious since every particle of that ball has that forward velocity because of motion along the ship, but the situation with light seems different than that which i'm not understanding.

17. Dec 26, 2017

### Orodruin

Staff Emeritus
It is no different at all. You could also emit the light so that it goes straight down in the ground frame, but then it will not go straight down in the ship's frame. It is a question of how you emit the light. You cannot have it going straight down in both frames.

18. Dec 26, 2017

### vanhees71

Again my advice: Write down your problem in proper mathematical language, which is a wave description. You can simplify the task by thinking about scalar waves first. For a massless field (as the electromagnetic field) a particular solution of the wave equation are plane-wave packets,
$$\phi(t,\vec{x})=\phi_0 (k^0 c t-\vec{k} \cdot \vec{x}-c t)=\phi_0(x_{\mu} k^{\mu}),$$
where
$$k^0=|\vec{k}|.$$
Now take a Lorentz boost (e.g., in $x$ direction) and see what results for that plane-wave packet! Then all your puzzles should be resolved, and you get the relativistic Dopplereffect and aberration formula for light.

19. Dec 26, 2017

### Ibix

If you analyse the emitter in the moving frame you'll also see that it should emit at an angle.

For example, a laser is basically a light clock with a lasing medium between the mirrors. In the rest frame it's light pulses bouncing "straight up and down" that go back and forth between the mirrors, repeatedly passing through the lasing medium and building up stimulated radiation. In the moving frame, the lasing medium and mirrors would move through "straight up and down" light and the amplification effect wouldn't happen. It's the "diagonal" bouncing light that repeatedly passes through the medium and produces the laser. So the beam comes out diagonally.

20. Dec 26, 2017