# Using Light as a absolute frame of reference?

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1. Oct 16, 2015

### FrancisClinton

If light travels in a straight line , why can't we use it as absolute frame of reference?
This thought experiment is done in the absence of external forces and gravity.
One person standing inside a box ,sends a photon parallel to the bottom of the box from one end to another end , whether this photon will hit the point at the other side of the box which is at the same height as the point from which the photon is sent or it will hit another point ? ,if this is the case , then by calculating the distanced between the two points and dividing the time it took the photon to hit the other end can we able to calculate the absolute speed of the box ? i know i am wrong some where, but can you help me out ?

2. Oct 16, 2015

### Orodruin

Staff Emeritus
The point is that whether the light is travelling parallel to the bottom or not is a frame dependent statement. This is nothing particular for relativity, this effect is there already in classical mechanics. So, no, you cannot use this to determine absolute motion. Even in Newtonian mechanics there is no such thing.

3. Oct 16, 2015

### FrancisClinton

Why we can't send a light beam parallel to the bottom surface , and i am not saying light should travel in parallel direction? just sending it from the parallel position.

4. Oct 16, 2015

### Mister T

That calculation will give you the speed of the photon. And since experiments have shown that you'll get the same speed regardless of the motion of the box, you can use it to establish the absolute speed of light. But you cannot use it to determine an absolute speed for the box.

5. Oct 16, 2015

### Staff: Mentor

If you are at rest relative to the room (that is, you are the guy in the room setting up the light) you can arrange things so that the light beam travels parallel to the floor and hits a spot on the wall at the same height - setting a laser pointer down on a table with a level top is all it takes.

However, anyone moving relative to the room or (it's the same thing) at rest while the room is moving relative to them will observe that the light beam travels on a path that is not parallel to the floor but ends up hitting the same spot on the wall. The wall moves just enough while the light is in flight to bring the same spot on the wall into position to be hit by the light even though its path is not parallel to the floor. Thus, the light always lands at the same spot so we don't learn anything from that.

6. Oct 16, 2015

### Staff: Mentor

One question: you are trying to detect whether the room is moving in the vertical direction, not the horizontal direction, right?

7. Oct 17, 2015

### FrancisClinton

Yes i want to detect the vertical motion of the box (motion which is perpendicular to the one in which the light beam was sent ) , I don't want the light beam to travel parallel to the bottom surface , until it hit the other side . Instead i want to release it from a position which is parallel to the bottom surface.
Since the light always travels the shortest distance between two points , if the box hasn't moved in the space , light beam would have hit the exact point on the opposite side from which it was sent , if it doesn't hit that point , so that point is moving relative to the light beam . am i right ?

8. Oct 17, 2015

### Orodruin

Staff Emeritus
No.
Your argument is circular, it will travel the shortest path between the points it is going between. This foes not imply that it must go parallel to the bottom.

I suggest you look for links on aberration and the non-existence of absolute motion in classical mechanics. As I said earlier, the non-existence of absolute motion is not particular to relativity. It is therefore more helpful to start looking at it in classical mechanics rather than trying to understand it using contrived arguments in a theory you are not familiar with.

9. Oct 17, 2015

### FrancisClinton

Now, in the elevator moving in space, if a light beam is thrown from one wall of the elevator in a horizontal direction towards another wall, the beam will not reach exactly opposite point on the other wall because the elevator would have moved up in the time the light beam would travel the width of elevator. Light beam will hit on the opposite wall at a slightly lower height than the height from which the light beam was emitted. Thus the path of light will be curved.
Did you agree with this one ?

10. Oct 17, 2015

### Orodruin

Staff Emeritus
Again, it depends on in which frame the light travels parallel to the floor. Until you understand this point you will not get any further. You really need to take the advice of understanding this in the setting of Newtonian mechanics first. Until you have done so, it is pointless to try to go any further.

11. Oct 17, 2015

### Orodruin

Staff Emeritus
And the path of the light is not curved in any of the reference frames unless one of them is accelerating. It is just not travelling parallel to the floor in all frames.

12. Oct 17, 2015

### FrancisClinton

I never said light beam should travel parallel to the floor , please read my comments above once again , i said light beam should be released from a parallel position to the floor.
Yes in accelerated frame light path will look curved from the perspectine of person in the space, yes i accept that but in constant velocity motion light path would be slanting line from the perspective of the person in the space , did you accept this ?

13. Oct 17, 2015

### PeroK

Let me try to explain this a different way, although you could learn a lot from nugatory's posts.

Imagine you have a tube across the room parallel to the floor and something fitted to the side of the room that fires light through the tube. You set things up so that the beam stays in the tube.

So, you would conclude you are not moving. Everything is always parallel to the floor.

But, to an observer to whom the room is moving vertically, the reason for the parallel motion is clear: The laser or light source is also moving!

So, you detach the light source from the room. Now the light does not stay in the tube, unless the light source is at rest with respect to the room. And there's the nub of your problem: The parallel test you have devised only works if the source and the room are relatively at rest.

14. Oct 17, 2015

### FrancisClinton

Yes you are right ,in my test both source and room are at rest relative to each other meaning they are attached together , but both are moving in vertical direction for a person in space .
So here is my question where will the photon hit the other side of the wall , i know how the motion of the photon will be different for the persons inside the lift and outside . Only thing i want to know where will the photon hit on the opposite side.
Answer should be whether this photon would have hit the point at the other side of the box which is at the same height as the point from which the photon is sent or it would have hit another point which is above or below that point ?

15. Oct 17, 2015

### PeroK

If the source is aligned to fire its beam parallel to the floor, then it will hit the opposite wall at the same height.

If the source is detached from the room, but stays in the parallel alignment, then the beam will only be parallel if the room and source are at rest.

If the room is accelerating, then the beam will be curved to the observer in the room.

If the source is accelerating, then the beam will be a straight line and parallel only if the source was relativity at rest when the beam was emitted.

So, you cannot distinguished motion of the source from motion of the room (at constant velocity). But you can distinguish between acceleration of the room and acceleration the source.

16. Oct 17, 2015

### Orodruin

Staff Emeritus
This makes no sense whatsoever. You do not release light. If you aim the light parallel to the floor it will travel parallel to the floor in the system where you aimed it parallel to the floor. Again this is a system dependent statement and you simply cannot use it to find a notion of absolute velocity, just as little as you can find an absolute velocity in classical mechanics by throwing a ball out of a train. (What you would find that way might be the relative velocity with respect to the surrounding air.)

17. Oct 17, 2015

### Staff: Mentor

Same height above the floor. You may find this problem easier to understand if you think in terms of an outside observer moving relative to the room instead of the room moving relative to an outside observer.

18. Oct 17, 2015

### Staff: Mentor

Let me try a more direct approach to answering:

If you tried this experiment with a gun and bullet, firing it from a fixed location toward a target on the opposite side of the ship, do you agree that this experiment would produce the same results regardless of the state of motion of the ship? (Non-accelerating)

That's the principle of Relativity and it has been known for hundreds of years. What Einstein did with Special Relativity was realize it also applies to light. So you see, you are viewing the issue exactly backwards. Prior to SR, people believed light traveled along fixed paths in a non-moving medium. Einstein discarded that idea and indroduced the idea that, like everything else, the motion was observer dependent....with one caveat: that it always traveled at C..

19. Oct 17, 2015

### FrancisClinton

Thanks guys for the discussion , i agree with all of you , but one doubt when the lift and source both accelerates ,whether the light will hit the same point or it will hit above or below that point ?

20. Oct 17, 2015

### Staff: Mentor

Acceleration is a completely different situation than constant speed. The light will follow a curved path relative to an observer inside the room and will land at a lower spot on the far wall.

The observer in the room could use this effect to detect the acceleration, but there are easier ways - standing on a spring scale and noticing that he's not in free fall, for example.