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### Author Topic: Can light exhibit mass in a nonlinear direction?  (Read 4575 times)

#### Robro

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##### Can light exhibit mass in a nonlinear direction?
« on: 04/03/2010 08:29:00 »
Does light exhibit any mass or force perpendicular to linear propagation during the bending of it's path?
« Last Edit: 05/03/2010 01:13:47 by Robro »

#### JP

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##### Re: Can light exhibit mass in a nonlinear direction?
« Reply #1 on: 04/03/2010 09:06:20 »
Light has no mass, so the answer is no.

However, linear and nonlinear directions don't mean anything?  Do you mean parallel and perpendicular to the direction of travel of the light?  Also, mass isn't a quantity that has a direction associated with it, so nothing can exhibit mass perpendicular to a direction.  It either has mass or it doesn't.

#### Robro

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##### Re: Can light exhibit mass in a nonlinear direction?
« Reply #2 on: 04/03/2010 09:18:05 »
I was wondering how the path of light is altered in a gravity field, does light have a property of gravitation proportional to the massive body creating the gravity?

#### JP

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##### Re: Can light exhibit mass in a nonlinear direction?
« Reply #3 on: 04/03/2010 09:23:46 »
Light takes the shortest path possible through a gravitational field.  Since the gravitational field is bending space-time, the shortest path is no longer a straight line, but a curve and so the light bends.  At each point along its path it still has no mass and it's still traveling at the speed of light.

#### LeeE

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##### Re: Can light exhibit mass in a nonlinear direction?
« Reply #4 on: 04/03/2010 15:37:08 »
Does light exhibit any mass perpendicular to linear propagation during the bending of it's path?

Like JP, I'm assuming you mean perpendicular to the direction of travel, but it's a good question and something that I've thought about too: it would seem to fit, or be implied in some interpretations.

It should be relatively easy to test though: set up two very close parallel lasers, one at a time, such that their beams travel a very great distance before reaching a target, then turn both of them on at the same time to see if the beams are affected by each other and converge.  The trouble is that I think that the distance you'd have to use before you detected any convergence would be so great as to be impractical (reflecting the beams a few million times between mirrors might work, if they don't end up absorbing too much of the energy).

#### Robro

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##### Re: Can light exhibit mass in a nonlinear direction?
« Reply #5 on: 04/03/2010 18:09:10 »
So, it could be said that light would only have relativistic mass as in a change in velocity. If a beam of light were shined through a thick piece of glass, the light would add to the overall mass of the glass during the time it takes for the light to pass through, since it takes up a bit of time for the light to penetrate the glass and so travels at slightly below 'C' ?
Then as the light leaves the glass, there would no longer be a medium and the relativistic property would return to zero as the light would then be back to it's unhindered linear propagation?
« Last Edit: 04/03/2010 18:21:23 by Robro »

#### LeeE

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##### Re: Can light exhibit mass in a nonlinear direction?
« Reply #6 on: 05/03/2010 00:10:18 »
So, it could be said that light would only have relativistic mass as in a change in velocity. If a beam of light were shined through a thick piece of glass, the light would add to the overall mass of the glass during the time it takes for the light to pass through, since it takes up a bit of time for the light to penetrate the glass and so travels at slightly below 'C' ?
Then as the light leaves the glass, there would no longer be a medium and the relativistic property would return to zero as the light would then be back to it's unhindered linear propagation?

Hmm... that seems to be completely unrelated to your original post: there's no directionality factor to just shining a light through a thick piece of glass.

#### Robro

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##### Re: Can light exhibit mass in a nonlinear direction?
« Reply #7 on: 05/03/2010 01:11:17 »
Yes, your right, I was digging around to see if light can exhibit any force, I mean with the glass, if shining light into glass adds mass to the glass, then the glass would have a bit more gravity associated with it and also why is this? Maybe I need a different approach. If the light going through the glass is accelerated in an anti-linear direction, does this create added gravitational force? So, with the light accelerated away from the original linear direction in the gravity Field, would it show any gravitational force of it's own? I think this question has been answered above as 'no' still zero mass. Then will the light in the gravity Field exhibit any variations in it's electric or magnetic Fields due to the asymmetry of the bent path? If so, can this be constituted as a force within itself?
« Last Edit: 05/03/2010 03:40:02 by Robro »

#### JP

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##### Can light exhibit mass in a nonlinear direction?
« Reply #8 on: 05/03/2010 03:40:37 »
Well, energy is the real cause of gravitational fields (or to be more precise, the stress-energy tensor in general relativity).  Light has no mass, but it does have energy so it should be able to bend space-time and therefore create a gravitational field.  Is that what you were asking about?

#### Robro

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##### Can light exhibit mass in a nonlinear direction?
« Reply #9 on: 05/03/2010 04:45:43 »
Well JP, in a way. It's difficult for me to understand the space/time concept. I know it states that space and time are curved, but gives no mechanism for it. I give my thoughts to other ideas and try to make sense of the universe through observation and explanation. The curved space/time concept doesn't give the thinker anything to think about when it comes to 'how' this curved space/time 'grabs' onto matter and all the associated fields. Relatively speaking, that area of the theory is blank. So, I search for keyholes that may unlock the 'why'. Yes I have my own ideas, but if a thread is to stick around very long it is wise not to post other theories. So I try to post keys in the form of questions about mainstream theories in areas that have either contradictions to observation or no answer at all. I guess I should have asked if light exhibits any force perpendicular or antilinear, to it's linear direction during the bending of it's path via a gravity field or other means.?
« Last Edit: 05/03/2010 04:59:11 by Robro »

#### JP

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##### Can light exhibit mass in a nonlinear direction?
« Reply #10 on: 05/03/2010 06:44:50 »
The curved space/time concept doesn't give the thinker anything to think about when it comes to 'how' this curved space/time 'grabs' onto matter and all the associated fields. Relatively speaking, that area of the theory is blank. So, I search for keyholes that may unlock the 'why'.

General relativity does explain this.  It says that gravity isn't actually 'grabbing' anything--things are just moving along geodesics (which are what straight lines become when you have to move in a curved space-time), where the curvature is given by gravity.

Of course, like all models, it's just a tool for predicting (very accurately in the case of general relativity) what happens in nature.  But of course it can't answer the question of why gravity is described so well by the curvature of space-time in the first place.

#### Farsight

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##### Can light exhibit mass in a nonlinear direction?
« Reply #11 on: 06/03/2010 14:52:31 »
Robro: see http://physicsworld.com/cws/article/news/41873 re light and momentum transfer when it travels through glass.

"For 100 years physicists have been struggling to reconcile two different formulations describing the momentum of light travelling through a transparent medium. One, put forward by German mathematician Hermann Minkowski in 1908, stipulates that light's momentum increases when it enters a medium, while the other, advanced a year later by the German physicist Max Abraham, instead says that the momentum of light decreases. Now, Stephen Barnett of the University of Strathclyde in the UK has concluded that both formulations are in fact correct, with the difference essentially boiling down to whether one considers the wave or particle nature of light."

Also check out the Compton effect for light exerting a force when its path is bent by an interaction with a free electron. This is a classical force, and the light loses energy. The hyperphysics page is quite good: http://hyperphysics.phy-astr.gsu.edu/Hbase/quantum/comptint.html

Light being bent by gravity involves a different type of "force", the force of gravity. If a photon skims a planet, we say its blueshifted then redshifted by equivalent amounts, with no overall change. Thus whilst its path has changed, it hasn't lost any energy.

Since energy causes gravity and light is energy, the photon does exert a gravitational force upon the planet. But it's very small, so you can't in practice measure it.

#### yor_on

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##### Can light exhibit mass in a nonlinear direction?
« Reply #12 on: 14/03/2010 23:25:28 »
Just to mess it up a little more :)

Assume that a 'straight' line is the one a photon takes, at all times. What does that make what we call 'light bending'? Light doesn't bend, gravity isn't a 'force' bending it. If it was a 'force' it would expend some 'energy' 'bending' our photon. If it did so it would be reasonable to expect that photon to lose some energy it seems to me? But as far as I know it doesn't.

But then we come to photons 'accelerating' down a gravity well?
They bend, right? And they also gain energy (blue shift) as observed by someone being at rest with the gravity well? So what would that make gravity? A 'force' bending a photon, as well as transferring an energy at it does so?

However you see it seems to me as you would have to retard that photon ever so slightly when changing its 'path' as we observes it? As that photon would like to continue that 'straight path' we normally define when we discuss a line. And to hard draw the analogy, if you want to argue that you're increasing its 'energy' by changing its path, then the ultimate would be to stop it, right? As you then would get the ultimate 'blue shift'? But that's not true as we know from photons leaving a VMO (very massive object). You actually will see them redshift, according to you being at rest versus the VMO (far observer)

But it should be testable.

You will need something to measure its energy and create a constant emitter of equivalent photons. Then you place invariant mass in its 'path'. You measure them at three points, one at the start A. The other point where gravitation 'blue shifts' it B, and the third at some point after that gravitational influence becomes 'unmeasurable' C.

A gives you the energy/momentum from the beginning for all of them assuming that you have a perfectly tuned emitter. B gives you the energy at impact under that gravitational influence. C gives you the impact when having passed it. Assuming that there is some 'energy transfer' to the photon and assuming that it actually travel :) we will now see if there is a difference between A, B and C.  Lets call A = 1 (energy/momentum)

So A = 1
At B it will blue shift ever so slightly as it 'falls' into our gravity well, right?  So B = 1.1 at impact.
But when measuring the photon at C, I will bet you that it will be 1 again, just as it was in the beginning :)

Now, if this was a normal 'energy', considering a perfect vacuum with no resistance. Shouldn't it keep this 1.1 as it actually gained a 'energy' free falling into our gravity well? You could argue that it loses that energy as it 'fights' its way up through that gravity well though, to keep on to C?

But assuming that gravity act with a 'force' on the photons path seems also to imply that it should lose 'energy' doing so, and then that should mean that gravity should change over time, shouldn't it? And also that both the photon and gravity should transform some work to 'work done' under the process it seems to me. Which then should be measurable, well, as I see it? And then the photon should have lost some energy at C.
==

But then again, considering a photon emitted and walking up a gravity well, it will to you being at rest with the well be red shifted, right? Which then implies it lost 'energy' walking up?
==

So then the question becomes, when does that photon redshift?

If we assume that the surface of that VMO emitting the photons contains the 'ultimate gravity', as it normally is as I understands it, then that redshift should be most pronounced there, shouldn't it? and then 'shrink' as it leaves the gravitational 'field'. Or should I assume that the redshift it will present on the surface is just the beginning of an 'energy' getting 'bled off' from that photon as it continues up the gravity well? And that the red shift then will increase with distance?

That's a real tricky one, it seems?
And if we discuss it in form of 'light quanta'?
==

As I think of it you can only 'steal' the energy at 'impact'. So thinking of a black hole, is there a possibility of light being so red shifted according to you at rest versus that BH that it will disappear?

It phreaks me out..

« Last Edit: 15/03/2010 00:15:04 by yor_on »

#### JP

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##### Can light exhibit mass in a nonlinear direction?
« Reply #13 on: 15/03/2010 09:35:18 »
I'm slightly confused by your post, but here's what I think happens.

First, regarding light bending in a gravitational field.  The concept here is geodesics.  Think of trying to draw a line between two points on a flat map of your town.  The shortest path between them is a straight line.  Now, let's say you want to take a path between two points on the earth's surface.  What's the shortest path you could take (without burrowing through the earth)?  It's a curved path.  The same thing happens when you bend space-time.  You can't talk about straight paths anymore since the stuff your moving through is curved, so light takes curved trajectories called geodesics.

At any rate, I don't think that has anything to do with your next point, about light losing or gaining energy from mass.  As we've discussed before in the forum, the energy of the gravitational system is what's conserved and it's hard to account for.   However, things seem to be OK if you look far from the object.  Before the light gets close to the mass, it's got some frequency.  It blueshifts as it moves towards the mass and redshifts as it moves away from the mass.  By symmetry, it should blueshift and redshift equally so that when it's done interacting with the mass, it has the same frequency as when it went in (it's just been delayed somewhat because it traveled along a curved path).

Regarding when redshifting occurs, I suspect it should be most pronounced when the curvature is strongest, i.e. close to the mass.

By the way, why are you insisting on talking about photons here?  You should be able to understand this by just talking about a beam of light and then once you have a firm grasp on what's going on with classical light, you can start worrying about photons.

#### yor_on

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##### Can light exhibit mass in a nonlinear direction?
« Reply #14 on: 15/03/2010 14:37:04 »
Ah, I really enjoy my photons JP :)

But looking at that beam of light reaching out from a VMO. Where does it red shift? Am I right in assuming that as the gravity is the largest at the surface you will get the redshift immediately? Assume that we have several detectors, at rest with the VMO, on its way from the 'beams' start-up, to that end of the gravity field where our observer sits above the VMO. Will the light have the same red shift at all its observations?

It should, shouldn't it? Even through the intergalactic wastes it will be redshifted, and as it reach earth it will still have that red shift. And if so, would I be wrong to state that it is red shifted due to the VMO, and that this light then actually have interacted with gravity in 'objective' manner? By traveling towards the oncoming light it will become more blue shifted, so in that manner it's all relative, but it still seems true that gravity/mass can give it a 'gold standard' from where all relations observed after will differ depending on that origin aka VMO:s mass? It's just me trying to see how it works.

#### yor_on

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##### Can light exhibit mass in a nonlinear direction?
« Reply #15 on: 15/03/2010 14:49:11 »
As for the first one, that's how I look at SpaceTime in fact. As an 'illusion' and our definitions of 'straight lines' being a limited truth :) The real straight line seems to me to be described by those photons. But you're right, I didn't make that clear.
« Last Edit: 15/03/2010 14:50:53 by yor_on »

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##### Can light exhibit mass in a nonlinear direction?
« Reply #15 on: 15/03/2010 14:49:11 »