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Author Topic: Can we prove light is affected by gravity?  (Read 9616 times)

Offline jeffreyH

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Can we prove light is affected by gravity?
« on: 19/01/2014 02:56:44 »
I read the post about the photon and atomic clocks down a mine. Someone noted that to a distant observer the mirrors of the photon clock would appear further apart. What if the distant observer sent a message about how the distance of the mirrors differed from his perspective as a percentage and gave instructions for the distance of the mirrors to be adjusted by that amount? Would that give us any useful information from the perspective of the respective observers?
« Last Edit: 27/01/2014 09:19:12 by chris »


 

Offline Bored chemist

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Re: Can we prove light is effected by gravity?
« Reply #1 on: 19/01/2014 13:22:14 »
"Can we prove light is effected by gravity?"
Yes.
http://en.wikipedia.org/wiki/Tests_of_general_relativity#Gravitational_redshift_of_light

"What if the distant observer sent a message about how the distance of the mirrors differed from his perspective as a percentage and gave instructions for the distance of the mirrors to be adjusted by that amount?"
Then the clock would run at the wrong rate from the POV of the person near it.
 

Offline jeffreyH

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Re: Can we prove light is effected by gravity?
« Reply #2 on: 19/01/2014 16:28:20 »
"Can we prove light is effected by gravity?"
Yes.
http://en.wikipedia.org/wiki/Tests_of_general_relativity#Gravitational_redshift_of_light

"What if the distant observer sent a message about how the distance of the mirrors differed from his perspective as a percentage and gave instructions for the distance of the mirrors to be adjusted by that amount?"
Then the clock would run at the wrong rate from the POV of the person near it.

However, since the distance seen by the remote observer now agrees with the same distance in his frame this should show him how light has been effected. He can determine this by requesting from the observer underground what time the light now takes. The adjustment to length contraction can now show that light has been effected.
« Last Edit: 19/01/2014 16:31:10 by jeffreyH »
 

Offline Bored chemist

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Re: Can we prove light is effected by gravity?
« Reply #3 on: 19/01/2014 18:54:45 »
Do you mean "affected" rather than "effected"?
Anyway,
if you change a clock, it isn't a clock any more.
 

Offline RD

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Re: Can we prove light is effected by gravity?
« Reply #4 on: 19/01/2014 18:59:49 »
Q. Can we prove light is effected by gravity?

A. Yes, have a look at Einsten's ring, (stop sniggering at the back ) ...


https://en.wikipedia.org/wiki/Einstein_ring

https://en.wikipedia.org/wiki/Gravitational_lens
 

Offline jeffreyH

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Re: Can we prove light is effected by gravity?
« Reply #5 on: 19/01/2014 20:06:30 »
On a slightly different note. Is it thought that gravity interacts with itself?
 

Offline jeffreyH

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Re: Can we prove light is effected by gravity?
« Reply #6 on: 19/01/2014 20:09:46 »
Do you mean "affected" rather than "effected"?

I was wondering.

A. How long it would take anyone to notice and comment and
B. Whether others would start to use it this way.

This gives an indication of how ideas and habits propogate.
 

Offline syhprum

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Re: Can we prove light is effected by gravity?
« Reply #7 on: 19/01/2014 21:54:11 »
The first demonstration of the deflection of light by gravity was given by Sir Arthur Eddington in 1919 when he measured the apparent position of a star when its light passed close to the Sun during a Solar eclipse.
 

Offline yor_on

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Re: Can we prove light is effected by gravity?
« Reply #8 on: 20/01/2014 10:42:15 »
No gravity does not 'interact' with itself, as far as I know. Best is probably to think of it as a 'field' if you want to think of it, or as the way SpaceTime has to behave in the presence of mass, 'energy', motion (uniform accelerations) and a arrow. it's the way things has to move, called a geodesic. It doesn't matter if it is light or matter, they all take the 'straightest path' as defined by the geodesic. Any other path will expend energy.
 

Offline yor_on

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Re: Can we prove light is effected by gravity?
« Reply #9 on: 24/01/2014 09:35:23 »
What is remarkable with an idea of a geodesic is that it presumes no resistance to its path. That makes for Earths uniform motion as described by relativity. You can set up a experiment in where you introduce several bodies, all acting and being acted on by gravity. Then start to 'throw stones' in between them. As soon as those stones stopped accelerating they all must find geodesics in where there is no resistance, or 'friction' by gravity.

If you now think of gravity as a field of varying 'strength' acting on, and being acted on by, matter. How is that possible? That I can throw those stones in any direction, and that they all must find a geodesic in where they do not lose any energy?

That was the original definition, that Einstein preferred. Then later, the idea of gravitational waves was introduced, with Einstein finding himself of two minds on that one. In that one 'SpaceTime' can transport energy, created by gravity acting and being acted on by matter. The simplest example of that one is binary stars, rotating around each other, drawn to each other in a spiraling pattern, letting of 'waves' of distorted SpaceTime to 'propagate' inside itself.

But then again, geodesics does not disappear inside a gravitational wave, they just distort. Which makes the idea of a 'field strength' of gravity really difficult to understand as I can saturate that space with my thrown stones, all of them finding a path of no friction and resistance.
 

Offline yor_on

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Re: Can we prove light is effected by gravity?
« Reply #10 on: 24/01/2014 09:43:23 »
If you translate this to a Higgs field, then that field acts on a few types of accelerating particles, unless they are in a 'uniform motion', in which case there is no 'interaction'. That idea guarantees geodesics to exist everywhere, although it also seem to define an absolute 'global frame of reference'. Because in such a definition a uniform motion must find itself at rest, doesn't matter from which observer, or at what velocity/mass. So it is a observer dependent absolute frame of reference, but it will be impossible to define it as a 'globally' definable field, contained 'inside' a SpaceTime.
 

Offline jeffreyH

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Re: Can we prove light is effected by gravity?
« Reply #11 on: 24/01/2014 14:37:23 »
If you translate this to a Higgs field, then that field acts on a few types of accelerating particles, unless they are in a 'uniform motion', in which case there is no 'interaction'. That idea guarantees geodesics to exist everywhere, although it also seem to define an absolute 'global frame of reference'. Because in such a definition a uniform motion must find itself at rest, doesn't matter from which observer, or at what velocity/mass. So it is a observer dependent absolute frame of reference, but it will be impossible to define it as a 'globally' definable field, contained 'inside' a SpaceTime.

I have just finished reading "Gravity" by Brian Clegg. This discusses some of the issues facing Einstein whilst developing general relativity. I don't think many people understand all the issues. Einstein did think that gravity could interact with itself and that there was gravitational feedback. He also included a vibrational element to gravitation. It is interesting that some of the equations, according to "Gravity", have not been solved in the almost 100 intervening years. While anyone can discuss relativity, not even Einstein understood it fully himself. We have to bear this in mind when saying something doesn't happen in such and such a way without having an in-depth understanding of general relativity.
 

Offline JP

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Re: Can we prove light is effected by gravity?
« Reply #12 on: 24/01/2014 15:07:51 »
I have just finished reading "Gravity" by Brian Clegg. This discusses some of the issues facing Einstein whilst developing general relativity. I don't think many people understand all the issues. Einstein did think that gravity could interact with itself and that there was gravitational feedback. He also included a vibrational element to gravitation. It is interesting that some of the equations, according to "Gravity", have not been solved in the almost 100 intervening years. While anyone can discuss relativity, not even Einstein understood it fully himself. We have to bear this in mind when saying something doesn't happen in such and such a way without having an in-depth understanding of general relativity.

Very good points.  I try to avoid weighing in on physics topics where I don't have a thorough understanding of the material, and general relativity is one of those areas.  Unfortunately, some users do weigh in with opinions without making it clear that they're not experts, and you can get a lot of misinformation in these threads about how gravity works.  More unfortunately, on this forum, we don't have a general relativity expert, so I would be wary about trusting answers you get on this forum without double checking them.

You make another good point about Einstein.  Oddly, many threads on GR try to reference what Einstein believed.  He was unarguably one of the greatest figures in the history of physics, but we've advanced quite a bit since he proposed his theory and found that he wasn't always correct and have found better ways to explain some of his ideas. 

Finally, regarding your question above about gravitational interaction with itself, as I understand it (not being an expert), Einstein's field equations (that describe how momentum and energy shape the gravitational field) are non-linear.  This means that because a gravitational field can have momentum and energy, it can be a source of further gravitational field.  This is what makes the equations so hard to solve, and in many cases solutions have to be found numerically.  This is distinct from the other famous set of equations governing classical fields: Maxwell's equations, which describe electromagnetism.  These equations are linear because the electromagnetic field does not interact with itself. 
 

Offline jeffreyH

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Re: Can we prove light is effected by gravity?
« Reply #13 on: 24/01/2014 18:48:23 »
I have just finished reading "Gravity" by Brian Clegg. This discusses some of the issues facing Einstein whilst developing general relativity. I don't think many people understand all the issues. Einstein did think that gravity could interact with itself and that there was gravitational feedback. He also included a vibrational element to gravitation. It is interesting that some of the equations, according to "Gravity", have not been solved in the almost 100 intervening years. While anyone can discuss relativity, not even Einstein understood it fully himself. We have to bear this in mind when saying something doesn't happen in such and such a way without having an in-depth understanding of general relativity.

Very good points.  I try to avoid weighing in on physics topics where I don't have a thorough understanding of the material, and general relativity is one of those areas.  Unfortunately, some users do weigh in with opinions without making it clear that they're not experts, and you can get a lot of misinformation in these threads about how gravity works.  More unfortunately, on this forum, we don't have a general relativity expert, so I would be wary about trusting answers you get on this forum without double checking them.

You make another good point about Einstein.  Oddly, many threads on GR try to reference what Einstein believed.  He was unarguably one of the greatest figures in the history of physics, but we've advanced quite a bit since he proposed his theory and found that he wasn't always correct and have found better ways to explain some of his ideas. 

Finally, regarding your question above about gravitational interaction with itself, as I understand it (not being an expert), Einstein's field equations (that describe how momentum and energy shape the gravitational field) are non-linear.  This means that because a gravitational field can have momentum and energy, it can be a source of further gravitational field.  This is what makes the equations so hard to solve, and in many cases solutions have to be found numerically.  This is distinct from the other famous set of equations governing classical fields: Maxwell's equations, which describe electromagnetism.  These equations are linear because the electromagnetic field does not interact with itself.

I am about to do some work using Minkowski spacetime diagrams that may actually point out something novel for the curvature induced by gravity. It surprised the hell out of me when It came to me. I will start a new thread in new theories when I have it done.
 

Offline yor_on

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Re: Can we prove light is effected by gravity?
« Reply #14 on: 24/01/2014 21:55:37 »
Would you have references to that Jeffrey, preferable with citations? Would make interesting reading if it is correct. If you relate it to gravitational waves 'interacting' with a SpaceTime, then that's slightly different, and something of a headache to Einstein, as I've read it.
 

Offline yor_on

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Re: Can we prove light is effected by gravity?
« Reply #15 on: 24/01/2014 22:01:45 »
It also depends on what you mean with a 'interaction'. If you take a perfectly spherical shell, then we would expect no gravity in the middle. That one you either can describe as the 'force' of gravity taking itself out, or as there is no general direction for it, more than the observers own. To make a experiment of this you need a observer, defining it. That observer will have a gravitational influence, even if it is a 'test particle', assuming it to have mass. That direction is inwards, to a center.
 

Offline yor_on

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Re: Can we prove light is effected by gravity?
« Reply #16 on: 24/01/2014 22:58:25 »
Now, if one define gravity as interacting, assuming a test particle to have a mass, then that test particle interact with all other gravity reaching it, and as gravity's reach is defined as unlimited, you now truly have yourself a non linear system acting and being acted upon in all possible directions. How can there then be a unlimited amount of geodesics without resistance, or 'friction' inside that sphere? Either it is correct to assume that there is a undefinable amount of geodesic 'paths' possible inside the sphere, or you have a limited amount available for throwing that stone, with some paths 'forbidden'?
=

What  I mean by throwing a stone, is that no matter where you throw it from, and no matter what direction, vector etc you choose, that stone must find a geodesic, as soon as it stopped accelerating. There is no 'forbidden path' to it, that I can see? Although, you can naturally stop its path by placing obstacles in its way, or change that geodesic by introducing a new mass, it will still be no resistance to it. If you define a resistance/friction to a geodesic, then it can't be a geodesic.
=

The logic of applying a friction to 'gravity interacting with gravity' must then be that uniform motion can't exist. That as all heavenly bodies are 'losing energy' interacting with each other, SpaceTime is a dynamic proposition in where you have relative motion changing gravitational influences. That one you then can translate to 'decelerating' which now have changed relativity into a SpaceTime without uniform motion. One where we only will find accelerations, but it won't solve the Higgs field as you still have different masses, as Earth relative the moons, although both now defined as 'accelerating', instead of having its formerly 'uniform motion'.
=

'accelerating' as both decelerations and accelerations express the same. Inertia under a arrow, or 'gravity'. One more thing, I'm not totally sure of this one, but to me any such definition seems to define a absolute state of rest, versus absolute motion. Meaning that 'relative motion' then must be incorrect as we now will see all matter 'slow down' relative each other, until they all find that absolute frame of reference defining a zero velocity/speed.

all in all, it would not be relativity.
« Last Edit: 25/01/2014 00:17:49 by yor_on »
 

Offline jeffreyH

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Re: Can we prove light is effected by gravity?
« Reply #17 on: 25/01/2014 06:23:54 »
It also depends on what you mean with a 'interaction'. If you take a perfectly spherical shell, then we would expect no gravity in the middle. That one you either can describe as the 'force' of gravity taking itself out, or as there is no general direction for it, more than the observers own. To make a experiment of this you need a observer, defining it. That observer will have a gravitational influence, even if it is a 'test particle', assuming it to have mass. That direction is inwards, to a center.

I haven't had time to read your last post but this one can't be right. If gravity cancelled out at the centre of a mass there would be no gravitational collapse. The surface of a sphere is not pressing inward, because gravity travels outwards, it is being pulled in by the attraction of the internal mass. What we think of as cancelling out should really be thought of as a point pressure/stress relationship.

If we were in a cavity there is no mass at the centre to exert gravitational force. We are still at the centre of gravity but all the mass exerting the force now surrounds us. This mass is exerting force in all directions but it is still force. Every part of the inner surface of the cavity is pulling us towards it. That i not the same as cancelling out. Try tying four ropes around your waist and having four friend all pull on them. It will hurt and would would feel a crushing sensation at your waist. This is how I see the cavity problem. Except, if I am right, you would be stretched out, rather than crushed.
« Last Edit: 25/01/2014 06:29:26 by jeffreyH »
 

Offline Shervaan Singh

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Re: Can we prove light is effected by gravity?
« Reply #18 on: 27/01/2014 07:32:53 »
I read the post about the photon and atomic clocks down a mine. Someone noted that to a distant observer the mirrors of the photon clock would appear further apart. What if the distant observer sent a message about how the distance of the mirrors differed from his perspective as a percentage and gave instructions for the distance of the mirrors to be adjusted by that amount? Would that give us any useful information from the perspective of the respective observers?
 

Offline yor_on

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Re: Can we prove light is affected by gravity?
« Reply #19 on: 27/01/2014 12:48:21 »
It's not Einstein, but it's correct.
https://en.wikipedia.org/wiki/Shell_theorem

If you want to argue it as forces acting, then you also need to make that experiment proving those forces 'tugging' at the observer from all directions. The observer 'weightlessly resting', no discernible 'force' acting on him, in the center of that perfectly spherical hollow shell (in a flat space).

And gravity's direction is inwards.
 

Offline yor_on

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Re: Can we prove light is affected by gravity?
« Reply #20 on: 27/01/2014 12:54:19 »
Look up how the Higgs particle(field) is defined Jeffrey. Because that is a definition of 'force', as well as of a 'interaction'. And you need both if you want it your way.
 

Offline lightarrow

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Re: Can we prove light is affected by gravity?
« Reply #21 on: 27/01/2014 13:08:37 »
It also depends on what you mean with a 'interaction'. If you take a perfectly spherical shell, then we would expect no gravity in the middle. That one you either can describe as the 'force' of gravity taking itself out, or as there is no general direction for it, more than the observers own. To make a experiment of this you need a observer, defining it. That observer will have a gravitational influence, even if it is a 'test particle', assuming it to have mass. That direction is inwards, to a center.
I haven't had time to read your last post but this one can't be right. If gravity cancelled out at the centre of a mass there would be no gravitational collapse.
Don't understand this.
Quote
The surface of a sphere is not pressing inward, because gravity travels outwards, it is being pulled in by the attraction of the internal mass.
Don't understand even this. A point mass on the surface cannot be pulled inward by the other masses on the surface?
 

Offline yor_on

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Re: Can we prove light is affected by gravity?
« Reply #22 on: 27/01/2014 17:08:16 »
Are you thinking of gravity as 'sucking' Jeffrey? Some sort of decreased 'pressure' inside that shell, sucking at our 'point mass' equally from the shell, from all directions?

Why are you able to be 'at rest' on Earth? Shouldn't matter if you fill in that shell, should it? If the 'filling it up with mass' only add to the 'force' of gravity directed outwards?

Gravity is SpaceTime, it defines geodesics. The geodesics define the geometry we find. And a point-mass as Lightarrow described it, (better than using a 'test particle':) has its direction toward a center, as far as I know? Not directed outwards. What you see when you use two point masses in a flat space, is those two redefining space around them depending on their mass. But you don't need to make them interact to do it. Describing it as a 'dynamically updated' field is a very good idea, if it wouldn't be for observer dependencies. Using observer dependencies you need to define it as a locally defined, dynamically updated field at 'c', to make sense to me.
=

But it all comes back to what one would mean by 'interacting' here. If you use it as describing 'forces' acting on each other then that's one thing. If you use it as describing the way a field is described by the mass energy, motion etc, locally? then that is a interaction too, as the mass (etc) defines it, but it's not as simple as 'forces' acting upon each other, as I see it.

I don't think of it as forces, although it definitely seems able to be expressed that way.
==

The point is  that if you use one point mass, the direction must be inwards. Use two, or more, point masses they can be described as 'interacting' at 'c', for lack of a better expression? Or maybe we just should call it 'redefine', the SpaceTime you locally observe. 'Buckle it up' and introduce a locally definable 'field'. But you can also transform away a 'gravity' by a 'free fall'. And the way we define a repeatable experiment is from that it must be equally true under equal circumstances. Adding to that we also assume that our universe obeys the same laws of physics, everywhere you can report back from. So this 'transformation of gravity', depending on your frame of reference, 'free falling' or not, is a repeatable experiment that you can do anywhere.

Gravity is frame dependent, which makes it observer dependent.
« Last Edit: 27/01/2014 17:52:35 by yor_on »
 

Offline jeffreyH

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Re: Can we prove light is affected by gravity?
« Reply #23 on: 27/01/2014 20:01:29 »
OK then here is the argument. Gravitational field strength is proportional to both time dilation and length contraction. The stronger the field then the more intense both these effects are. It follows that if we say the gravitational field is cancelled at the centre of a mass then so must the effects of time dilation and length contraction be cancelled. No field (cancelled out) no effect on spacetime. It cannot be argued any other way. If a force is cancelled out then it CANNOT produce an effect.
 

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Re: Can we prove light is affected by gravity?
« Reply #24 on: 27/01/2014 22:30:54 »
Another thing to consider. We, personally, are length contracted in the earth's gravitational field. If we examine our structural makeup under a microscope and compare this to say a rock face the atomic structure will appear to be of the same scale. So therefore the earth will length contract itself as well as objects on the surface. This is part of the gravitational feedback mechanism described by Einstein. If the gravity cancels out at the centre then as we travel from the earth's surface to the core the intensity of the effects must diminish proportionately.
 

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Re: Can we prove light is affected by gravity?
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