# Is this situation possible?

1. Apr 6, 2013

### ash64449

Is this situation possible??!!!

Hello friends,

I have seen many videos that explain time dilation like this. you imagine a two mirrors which are apart by 186000 miles. a light goes and hits the mirror and reflected and hits the other mirror and goes on. This is used to explain a tick of the clock,if it hits the mirror,it is considered as one tick. Think that these mirrors are two points in y-axis. let is move in the direction of x-axis. Well,it the context of who is moving in IRF, Those two ticks travel straight and they consider as two ticks of clock. for an observer, that "light" needs to go in slant direction,so it needs to travel a longer direction,so time dilation to IRF that is moving can be understood in reference to the observer. But my question is: whether the second situation possible. I mean Light moving in slant direction? If you think that it is possible, then you will have to say what caused light to move in slant direction??!!

2. Apr 6, 2013

### Staff: Mentor

Yes, this is the way that Einstein used to compute the equations of time dilation. the observer in the moving frame of reference sees the light bounce off each mirror. Similarly the stationary observer sees the same event of the light bouncing off the mirrors. Both observers witness the same event although the stationary one see light travel the slanted path and also a longer path. The moving observer determines its one second but the stationary observer says no its much longer.

Why? Because the light has an x component to its velocity due to the moving of the light source and mirrors in the x direction. The caveat here is that light travels the slanted path at the speed of light (no faster) because that is the limiting speed but now it has to travel a longer path and take longer doing it.

3. Apr 6, 2013

### ash64449

what do you mean by x component to its velocity? Do you mean relativistic aberration?

4. Apr 6, 2013

### ash64449

and i am not including any source of light too. Just a photon of light is bouncing off. Mirrors are that polished that photon bounce off without getting absorbed.

5. Apr 6, 2013

### ghwellsjr

You're probably envisioning the diagonal line that we draw on a piece of paper (or computer screen) to be like what we would see if a laser beam were aimed in that direction showing a continual line of light. That's the wrong way to think about it. If we were using a laser, it would not be aimed along the diagonal but rather it would be aimed straight across. Then we don't leave the laser on all the time but just flip it on and off very quickly. Now the burst of laser light is traveling straight across but it traces out a diagonal line as it travels between the mirrors.

It's no different than when I recently told you to remember how a treadmill draws a graph of your progress and as you change speed, the slope of the diagonal line changes so that your path can have curves in it. But you're not taking a curving path, you can only go straight in one direction on a treadmill.

6. Apr 6, 2013

### ash64449

this is not the situation i meant. I imagined just a photon of light which hit the mirrors without being absorbed,i didn't imagine a laser and flip it on and off,just a photon light that bounces off.this happens when mirrors were at rest. Now think that the mirrors are moving in x-axis with respect to observer. What happens to Photon? Will it come together with the mirrors? If so why? that is the actual question.

Anyway i understood what you meant.Laser is aimed straight across along the diagonal with respect to observer. But this is not the case i meant.I meant the above what i said. Anyway what you said is important..

7. Apr 6, 2013

### Fredrik

Staff Emeritus
What second situation? You seem to be describing only one scenario, the one that's illustrated in this video. The only thing that happens is that the ship fires the laser "straight down" and it bounces back "straight up". To an observer that's moving with respect to the ships, this obviously looks like the path is two legs of a triangle. The only "cause" is that when the laser was fired, it was aimed in the direction that according to the observer on the ship is perpendicular to the ship.

8. Apr 6, 2013

### ash64449

I meant this one fredrik:
"this is not the situation i meant. I imagined just a photon of light which hit the mirrors without being absorbed,i didn't imagine a laser and flip it on and off,just a photon light that bounces off.this happens when mirrors were at rest. Now think that the mirrors are moving in x-axis with respect to observer. What happens to Photon? Will it come together with the mirrors? If so why? that is the actual question"

9. Apr 6, 2013

### ash64449

The reason why light traveled in the diagonal line in reference frame of observer because,Laser was shot across,this was explained by gh..

10. Apr 6, 2013

### Fredrik

Staff Emeritus
I don't know why you think anything would be different if we just don't mention the laser and just look at the light. Are you talking about the situation where light is bouncing between two mirrors, and then we move the mirrors away from that location. Then the light would obviously miss the mirrors, just as you would miss the chair if I pull it away as you try to sit down. If you're talking about what light bouncing between two "moving" mirrors looks like to an observer who's "not moving", then all you need to do is to look at video, because that's what it describes. Keep in mind that in the inertial coordinate system that's comoving with the asteroid, the rocket is moving, but in the inertial coordinate system that's comoving with the rocket, the asteroid is moving. Both coordinate systems are equally valid. It's impossible to say that one of them is "really" not moving.

What he described is essentially the scenario shown in the video I linked to in post #7. He said that the laser isn't aimed diagonally.

11. Apr 6, 2013

### 1977ub

In plain old Galilean relativity, pre-Einstein, a man on a ship moving past the dock drops a box. For him, it is seen to fall straight down toward the deck. For an observer on the dock, it is seen to stay in line with the man, and thus from the dock, he would have to trace a diagonal to follow the box's fall. This is where the diagonal line comes from, completely before relativity or constant c comes into the picture. With Einstein, the *speed* of a line followed by the photon is seen as c by both observers (one of which is straight toward the mirror, other observer sees diagonal toward the moving mirror), which is different from the classical prediction.

12. Apr 6, 2013

### ghwellsjr

Since you imagined just a single photon, then just think about one of the photons in the laser beam. It's path is different from the beam, isn't it? The beam goes straight across but each individual photon goes on a diagonal.

13. Apr 7, 2013

### Fredrik

Staff Emeritus
Now I'm confused about what scenario you're talking about. I thought ash was talking about a scenario that's equivalent to the one in the video I linked to in #7. In this type of situation, the path of the beam is the same as the path of the photons.

14. Apr 7, 2013

### Staff: Mentor

Maxwells equations if you are speaking classically, or QED if you need a quantum description.

15. Apr 7, 2013

### ghwellsjr

In your video, although there is a laser, there is not a beam of light, it is just a pulse of light. In the rest frame of the two spaceships, the pulse goes straight down and reflects straight up. If the laser were turned on continually, the laser beam would trace out the same path as the individual photons so the familiar experience that we have of a laser beam showing a thin line of light would look the same as the path of an individual photon.

In the rest frame of the observer on the asteroid, the video draws two diagonal lines that look like laser beams of light aimed at a slant. I was merely pointing out that even if the actual laser in the video were turned on continually the beam would still be going straight up and down between the two ships even in the rest frame of the asteroid but the individual photons would be traveling the diagonal path shown by diagonal lines in the video.

In other words, we can't always associate the familiar experience of seeing a beam of laser light with the path of a photon drawn on a diagram or video.