Using Light as a absolute frame of reference?

[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 ?

 

[Orodruin]

The point is that whether the light is traveling 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.

 

[FrancisClinton]

The point is that whether the light is traveling parallel to the bottom or not is a frame dependent statement

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.

 

[Mister T]

If light travels in a straight line , why can't we use it as absolute frame of reference?
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 ?

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.

 

[Nugatory]

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.

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.

 

[Nugatory]

This thought experiment is done in the absence of external forces and gravity.

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

 

[FrancisClinton]

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

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 ?

 

[Orodruin]

am i right ?

No.

Since the light always travels the shortest distance between two points

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.

 

[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 ?

 

[Orodruin]

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.

 

[Orodruin]

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 traveling parallel to the floor in all frames.

 

[FrancisClinton]

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 traveling parallel to the floor in all frames.

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 ?

 

[PeroK]

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 ?

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.

 

[FrancisClinton]

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.

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 ?

 

[PeroK]

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 ?

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.

 

[Orodruin]

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.

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.)

 

[Nugatory]

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 ?

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.

 

[russ_watters]

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 ?

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..

 

[FrancisClinton]

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.

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 ?

 

[Nugatory]

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 ?

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.

 

[FrancisClinton]

Can you explain why it happens ?

 

[Nugatory]

Can you explain why it happens ?

Why acceleration is different, you mean?
If you google for "equivalence principle path of light" you'll find many good links (and some bad crackpot ones too - be careful).

 

[Orodruin]

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.

Lately this experiment has started producing different results for me than earlier. Gravity must be changing ...

 

[FrancisClinton]

Thanks guys for your discussion, my doubt has been cleared .

 

[phinds]

Lately this experiment has started producing different results for me than earlier. Gravity must be changing ...

I agree. My experience is that scales here on Earth seem to be showing things as getting heavier and heavier.

 

[FrancisClinton]

Why acceleration is different, you mean?
If you google for "equivalence principle path of light" you'll find many good links (and some bad crackpot ones too - be careful).

Yes i understood the equivalence principle , but how we know that Einstein theory should be the only theory of gravity? and what is the drawback in Newtoninan theory of gravity ?

 

[Orodruin]

Yes i understood the equivalence principle , but how we know that Einstein theory should be the only theory of gravity? and what is the drawback in Newtoninan theory of gravity ?

The ultimate drawback of any physics theory is not being able to describe observations. It can still be a good approximation in some limits, as Newtonian gravity is for weak gravitational fields and low velocities.

Of course we do not know that GR is exact or complete, in fact we know it is not. But it is the best we have at the moment. It describes several things Newtonian gravity does not, such as the perihelion shift of Mercury and gravitational time dilation.

 

[FrancisClinton]

So whether theory is correct or not ? depends on the observation . and one more doubt why can't we say that space time around a charge should also be curved , we can say space around the electron will be a curvature such that it will attract positive charge and repel negative charge , the curvature changes depending on the what is placed there .
can it be like this ?

 

[phinds]

So whether theory is correct or not ?

Not sure what you are saying here but if you meant to say "neither theory is correct?", then that is true but General Relativity is more correct/complete than Newtonian gravity because Newtonian gravity only applies on small scales and at low speeds and is wrong otherwise. General Relativity seems to apply everywhere except at the center of black holes, where it too is "wrong" in that it gives results that cannot represent physical reality. General relativity COULD be used on small scales, as in building a bridge or a sky-scraper for example, but one would be utterly insane to use it when in those limited cases Newtonian gravity gives results that are plenty accurate and it's so much easier to use.

What is really needed is a quantum theory of gravity that could extend General Relativity so that it DOES work at the center of black holes, then it would work everywhere that we know of.

 

[FrancisClinton]

Not sure what you are saying here but if you meant to say "neither theory is correct?", then that is true but General Relativity is more correct/complete than Newtonian gravity because Newtonian gravity only applies on small scales and at low speeds and is wrong otherwise. General Relativity seems to apply everywhere except at the center of black holes, where it too is "wrong" in that it gives results that cannot represent physical reality. General relativity COULD be used on small scales, as in building a bridge or a sky-scraper for example, but one would be utterly insane to use it when in those limited cases Newtonian gravity gives results that are plenty accurate and it's so much easier to use.

What is really needed is a quantum theory of gravity that could extend General Relativity so that it DOES work at the center of black holes, then it would work everywhere that we know of.

No, i just re confirmed what the previous comment said , for a theory to be accepted it must agree with observation .
My doubt is why can't we say that space time around a charge should also be curved , we can say space around the electron will be a curvature such that it will attract positive charge and repel negative charge , the curvature changes depending on the what is placed there .
can it be like this ?

 

[SlowThinker]

So whether theory is correct or not ? depends on the observation

Yes, there are many theories, logical, nice and simple, such as the existence of absolute time. Sadly, this theory does not agree with observation, so it is wrong. Physics works like that. Unlike math, it studies the behavior of the real universe.

and one more doubt why can't we say that space time around a charge should also be curved , we can say space around the electron will be a curvature such that it will attract positive charge and repel negative charge , the curvature changes depending on the what is placed there.

We use the idea of spacetime curvature in GR exactly because it does NOT depend on what is placed there. Otherwise it would make no sense. There were some attempts to view electricity as a 5th dimension but AFAIK not very successful.

 

[Nugatory]

h

Yes i understood the equivalence principle , but how we know that Einstein theory should be the only theory of gravity? and what is the drawback in Newtoninan theory of gravity ?

I said that you will find good explanations if you google for "equivalence principle path of light". This is not because the problem has anything to do with gravity or the difference between Einstein's and Newton's theories of gravity - it doesn't. It is because the first step in applying the equivalence principle is to work out what happens inside an accelerating room according to classical physics, and that is exactly the question you asked.

 

[FrancisClinton]

hI said that you will find good explanations if you google for "equivalence principle path of light". This is not because the problem has anything to do with gravity or the difference between Einstein's and Newton's theories of gravity - it doesn't. It is because the first step in applying the equivalence principle is to work out what happens inside an accelerating room according to classical physics, and that is exactly the question you asked.

I found an good explanation for the equivalence principle and i have understood it clearly.
According to General relativity , faster object curve less when compared to slower objects when moving around the heavy object , since space and time are single entity, time will run slow for the faster object so they will curve slower. Am i right ?

 

[PeterDonis]

I found an good explanation for the equivalence principle and i have understood it clearly.
According to General relativity , faster object curve less when compared to slower objects when moving around the heavy object , since space and time are single entity, time will run slow for the faster object so they will curve slower. Am i right ?

Whether this is right or wrong as a description of what GR says in general about objects moving near a massive body (it looks too vague to me to decide whether it's right or wrong), it has nothing to do with the equivalence principle. The EP asserts the local equivalence of an accelerating frame in flat spacetime with a frame at rest in a gravitational field in curved spacetime. What you are talking about--the "shape" of orbits of objects around a massive body--is not local.

 

[Mister T]

There were some attempts to view electricity as a 5th dimension but AFAIK not very successful.

Kaluza-Klein. Endorsed by Einstein. As I understand it, that theory isn't useful because it's classical. That is, it ignores quantum theory. Something that is acceptable in a theory of gravity but not in a theory of electromagnetism because the latter is so much stronger than the former. A nonquantum theory of electromagnetism can't account for how an atom behaves, for example.