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Author Topic: Why does two parallel lightbeams bend towards each other, its momentum?  (Read 3678 times)

Offline yor_on

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How is the mechanism thought to work?

Light is intrinsically timeless according to the theory of relativity, that should mean that there can be no action taken by light, it can only be acted upon.

Am I wrong there?
Why?

As I see it the answer to my question is the momentum created that is responsible, but I'm not sure how it is thought to work. That momentum should exist immediately the photon comes to be, but is the momentum a part of the photon, or is that something SpaceTime 'lays upon it'? "Einstein showed that, if Planck's law of black-body radiation is accepted, the energy quanta must also carry momentum p=h/λ, making them full-fledged particles."

Yep, but what is it? In a way, naively speaking, I could see momentum as relating to mass and velocity and so present me a oposite to inertia, possibly :)And for all things of invariant mass it makes sense, but when considering a massless photon it loses that 'common sense' If momentum is relative both light and mass it makes me wonder what creates it, the photon may be the instigator of it as it comes into existence but ??

It reminds me of that idea of gravity being just another type of massless particles, ah, as i understood it :) that somehow will collect around what we call motion. Is momentum something similar?


 

Offline JP

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I think the reason this paradox seems to arise is that special relativity doesn't actually tell us what a photon "experiences" in its own reference frame.  SR only describes the reference frames of objects with non-zero mass.   The reason for this is that the equations of SR were designed to describe points of view in which light always moves at the speed of light.  However, if you wanted to experience things as light does, you'd be riding along on the light, and it would be stationary with respect to you.  SR doesn't cover this case.
 

Offline yor_on

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Yes, you are perfectly correct in that it, like dividing with zero, is frowned upon.
But if we find it having a momentum I still want to understand what this momentum should be blamed on :) It's a little like this, I have this weird notion that nothing is what it seems to be. There's even a name for it, paranoia :)

And my suspicion is that light traveling is one h* of a weird idea ::)) that only a few tribes in this universe believes in, amongst them us:) But if that would be correct, how would we explain momentum?
==

Btw: JP apropos that, take a look at this one How does a massless photon have momentum 
And, could you introduce me to Bianca there? I like her, ahh, would mind be the proper word here? yep, that too :)
==

(Ahh, the last part was me trying to make a innocent joke, what can I say.. I love physics? If you read this Bianca I assure you it was most innocent, so you can stop throwing stuff at me now, please.)
« Last Edit: 08/09/2010 06:10:12 by yor_on »
 

Offline JP

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Hmm... one way of looking it is that momentum tells you about how something moves in space.  Energy tells you about how it moves in time.  The ratio of the two tells you about its velocity.  These are both measured from your (the observer's reference frame), so that you can measure them for light without running into trouble.

A photon always moves at light speed WRT the observer, so the ratio of energy to momentum has to always be constant, which is how you get p=E/c (so that E/p=c always).

==

Re: the answer in the link, I don't agree with it!  It's that whole "trying to apply the equations of special relativity to light" problem--when you try to plug in light-speed, you divide by zero, which is an indication that SR can't cover the case of "riding on a photon."  Dividing by zero doesn't mean you can pick whatever answer you want--it means there's a problem somewhere.
 

Offline yor_on

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I found it fun though :)
Don't know how serious it was..
But worthy of notice as (s)he both explained momentum, ahem.
And divided with zero to prove his point :)

But I agree on not agreeing, with him that is :)
If you need to divide with zero to get a answer you will get an infinity, of zero proportions :)
===

Or hers? As it might be?
« Last Edit: 08/09/2010 05:35:13 by yor_on »
 

Offline Vern

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If two parallel beams of light bend toward each other, there would need be communication between the most forward points of them. If such communication existed, it would necessarily be at faster than light speed, since the points are moving at the speed of light.

Present theories demand that the points converge. The mechanism to accomplish the convergence is not yet defined.

We can say that space time is bent. But what about other things present in the area for which space-time is not affected.

When I think in these terms, I wind up with the notion that space-time is only an experience enjoyed by material objects that exist within it.
 

Offline LeeE

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WTF!!!  :o

Has this experiment actually been done?  Has it actually been shown that two parallel light beams converge?

This is a prediction of a model of hypothetical n-order dimensional sub-atomic particles that I've been playing with.

Could you please post a link to the experiment that showed this phenomenon?
 

Offline yor_on

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Well, you put me on the spot there LeeE :)
I've seen it over the years, but for an actual experiment?

Hey, it bends towards the Sun, doesn't it!! And as we all know, that's where the real light comes from, well, mostly it does, doesn't it? For us at least, ahh, not me though as I live in the land of snow and ice, but, for someone? Somewhere?

Alas QED.

But yeah, you're right, as far as I know it's still a theory.

Farsight had what he saw as an example though, and looking I found this pdf. Tunable bipolar optical interactions between guided lightwaves.

But it seems subtly different?

"State-of-the-art advances in nanophotonics allow light to be highly concentrated in nanoscale waveguides or resonators with high refractive index contrast. In closely spaced devices the coupling between the guided light waves gives rise to an optical force known as the “optical binding force”. According to recent theoretical predictions, the polarity of this force is either attractive or repulsive depending on the relative phase between the interacting light waves."

That sounds somewhat different than gravitational effects to my ears, and to yours too, perhaps? :)
« Last Edit: 08/09/2010 22:40:29 by yor_on »
 

Offline JP

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They certainly can in matter, since the energy in a light beam can change the refractive index of matter, which would cause the beams to bend.

They might be able to in space... I'm just speculating here, but the tip of one light beam could start bending space due to its energy.  Some time later, this bend would have propagated out far enough to attract a latter part of the second light beam. 
 

Offline Vern

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Maybe we could think about it in terms of, what is it that makes the most forward points of light beams go forward.

If we agree that Maxwell's equations explain the functions, we have to accept that it is electromagnetic change, or the change in amplitude, of adjacent fields. Then we have to conclude that change in the amplitude of adjacent fields drive the beams forward.

Gravity changes the direction of the beams travel. Maybe gravity derives from the abundance of amplitude change of electric and magnetic fields present in any given area.

And so we would have electromagnetic gravity.  ::)
« Last Edit: 09/09/2010 15:45:06 by Vern »
 

Offline LeeE

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That sounds somewhat different than gravitational effects to my ears, and to yours too, perhaps? :)

Interesting, but yes, it does sound like something different.
 

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