# Light gravity

[SOLVED] Light gravity

Bonjour,

Just an interrogation this morning.

If, locally, I parallelize two laser beams separated by some distance "d" but pointing in the same direction. Would these beams converge and mix (after some distance), would they swirl forever, or would they ...?

Hummm!

Last edited by a moderator:

Gold Member
Dearly Missed

Originally posted by Imagine
Bonjour,

Just an interrogation this morning.

If, locally, I parallelize two laser beams separated by some distance "d" but pointing in the same direction. Would these beams converge and mix (after some distance), would they swirl forever, or would they ...?

Hummm!

may we make some ideal assumptions? the beam from a real laser would not be perfectly collimated and would spread out---more like a cone than a column----but perhaps we can assume that these two beams have classical perfection?

And we may assume you shine them into space so empty that they can travel undisturbed without limit? then one could imagine them having enough time to interact gravitationally and----if they were close enough together initially---weaving a kind of braid: crossing paths again and again.

by "classical" is often meant un-"quantical"---not quantum-mechanical. Quantum mechanics imposes limits on geometrical perfection and would probably forbid this waltzing interaction of lightbeams. Some other PF poster may wish to discuss your morning interrogation with different assumptions.

BTW if you carelessly make the distance d too large then even though the two beams start out parallel the expansion of space will (overcome their gravitational attraction for each other) and eventually pull them away from each other! The initial distance must be small enough so that they remain gravitationally bound.

Last edited:
re Marcus-
"overcome their gravitational attraction for each other"

I know that gravity causes light to bend but where is the proof that this is due to 'attraction' rather than a variation in field density?

Originally posted by elas
re Marcus-
"overcome their gravitational attraction for each other"

I know that gravity causes light to bend but where is the proof that this is due to 'attraction' rather than a variation in field density?

Note he's talking in the classical sense.

Gold Member
Dearly Missed

Originally posted by Imagine
Bonjour,

Just an interrogation this morning.

If, locally, I parallelize two laser beams separated by some distance "d" but pointing in the same direction. Would these beams converge and mix (after some distance), would they swirl forever, or would they ...?

Hummm!

MessieursDames (Imagine, elas, neutroncount) my impression was that Imagine posed the question "lightly" and confess that my answer was in a similar vein

in reality I cannot suppose one could ever detect the gravitational interaction of beams from two real lasers,
so that how one answers this question will depend very much on how one idealizes
the situation. I will be happy to withdraw or correct my account
if anyone wishes to reply with more "gravity"
and provide a serious quantum mechanical and general relativistic analysis.

Gold Member
Originally posted by Imagine
Bonjour,

Just an interrogation this morning.

If, locally, I parallelize two laser beams separated by some distance "d" but pointing in the same direction. Would these beams converge and mix (after some distance), would they swirl forever, or would they ...?

Hummm!
if they are parallel how would they converge they will never meet.
am i missing something here?

Originally posted by loop quantum gravity
if they are parallel how would they converge they will never meet.
am i missing something here?

I believe that you are. If I understand Imagine's question correctly, he is asking if the two beams will exert gravitational attraction on one another.

BTW, the more I hear about it, the more convinced I become that light has no gravitational influence.

Gold Member
Dearly Missed

Originally posted by LURCH
I believe that you are. If I understand Imagine's question correctly, he is asking if the two beams will exert gravitational attraction on one another.

BTW, the more I hear about it, the more convinced I become that light has no gravitational influence.

I've nothing but admiration and respect for skepticism and, in particular, the steadfast refusal to believe experts
but some of the pleasure of such a firm denial is missed if
one does not appreciate the expert view, and so here is
the conventional wisdom:

cosmologists with online cosmo tutorials (George Smoot, Charles Lineweaver, Ned Wright, Eric Linder) will tell you that if the universe were suddenly made empty except for light, and there was enough light, it would begin to collapse to a Big Crunch

and they can tell you how much joules of light per cubic kilometer would be needed.

I don't recall how much exactly, maybe I can edit this later and put in a figure, but certainly ONE joule per cubic kilometer should be enough to make otherwise empty space collapse.

(this does not involve any "dark energy" or "dark matter" or anything exotic, just empty space with plain old light in it, and even if there were howevermuch dark energy there as well, you could put enough light into overwhelm the effect of dark energy and cause gravitational collapse anyway)

this is what they say. the nice thing is that no one (unless you are taking a midterm exam) is obliged to accept it. So you can
declare light to be a complete no-show when it comes to gravity and who knows maybe you are right and not they!

Interrogation's Precisions:

Yes, beams would not spread out, would be in empty space to travel enough to interact, and would be "near" enough to "keep" interacting. (Imagine: Similar to my sparse neurons)

Source of my interrogation:

If we launch, at the same time and speed, two electrons to follow initial parallel path (similar to laser beam setup), what would be their behavior? Note: Since they repulse each other, after a certain time, they will have different speed components and therefore magnetic force... Right? In what the magnetic force would change their paths? Swirling?

IMHO, light beams would attract instead of repulse but could they follow the similar behavior?

P.S.: Nice morning

Originally posted by LURCH
I believe that you are. If I understand Imagine's question correctly, he is asking if the two beams will exert gravitational attraction on one another.

BTW, the more I hear about it, the more convinced I become that light has no gravitational influence.

Bonjour LURCH,

Due to conservation laws and based on my modest opinion and knowledge, I invite you to think about momentum, electric charge, spin and energy instead of light, mass, and/or whatsoever. You could find some interesting similarities.

Originally posted by loop quantum gravity
if they are parallel how would they converge they will never meet.
am i missing something here?

Bonjour lqg,

Photons don't have sexe but, IMHO, they are attracted each other. My "good morning" interrogation was concerning the existence of some derivative force.

IMHO, upon Maxell's equation, magnetic field is derivative of electric field. Does exist some similitude when we are talking about gravitational field?

Gold Member

Originally posted by marcus
I've nothing but admiration and respect for skepticism and, in particular, the steadfast refusal to believe experts
but some of the pleasure of such a firm denial is missed if
one does not appreciate the expert view, and so here is
the conventional wisdom:

cosmologists with online cosmo tutorials (George Smoot, Charles Lineweaver, Ned Wright, Eric Linder) will tell you that if the universe were suddenly made empty except for light, and there was enough light, it would begin to collapse to a Big Crunch

and they can tell you how much joules of light per cubic kilometer would be needed.

I don't recall how much exactly, maybe I can edit this later and put in a figure, but certainly ONE joule per cubic kilometer should be enough to make otherwise empty space collapse.

(this does not involve any "dark energy" or "dark matter" or anything exotic, just empty space with plain old light in it, and even if there were howevermuch dark energy there as well, you could put enough light into overwhelm the effect of dark energy and cause gravitational collapse anyway)

this is what they say. the nice thing is that no one (unless you are taking a midterm exam) is obliged to accept it. So you can
declare light to be a complete no-show when it comes to gravity and who knows maybe you are right and not they!

Considering the universe collapsing on itself and two parallel laser beams are two very different cases.

The answer is no they will not converge and will remain parallel, it is only systems of non-parallel photons that exert a gravitational force.

Originally posted by jcsd
The answer is no they will not converge and will remain parallel, it is only systems of non-parallel photons that exert a gravitational force.

Do you have an explanation?

According to Enc. Brit. there is at present no generally accepted explanation for the transmission of light between galaxies. As for gravity surely everyone should by now be aware that relativity does not explain recent observations by the Hubble telescope.
With the issues in so great a turmoil any opinion made as if it were a statement of fact cannot be justified. I believe the answer lies in a vacuum theory, but others prefer anti-gravity, strings and so on. Perhaps we should be debating the merits or otherwise of each proposal, using two light beams as the event to be explained.

Gold Member
Dearly Missed

Originally posted by Imagine

quote:
--------------------------------------------------------------------------------
Originally posted by jcsd
The answer is no they will not converge and will remain parallel, it is only systems of non-parallel photons that exert a gravitational force.
--------------------------------------------------------------------------------

Do you have an explanation?

Imagine, I too am eagerly awaiting the explanation!
When I picture two long thin beams of light starting out parallel,
it is hard to see how the photons in one beam would not
be influenced by the gravity of the photons in the other beam

Last edited:
Staff Emeritus
Gold Member

Originally posted by marcus
Imagine, I too am eagerly awaiting the explanation!
When I picture two long thin beams of light starting out parallel,
it is hard to see how the photons in one beam would not
be influenced by the gravity of the photons in the other beam

Thank you Marcus! I have been trying to get an answer to all of this for some time.

Marcus, as I understand you here, we can argue for gravity solely on the basis of the energy of the photons?

Gold Member
All I can say for the moment is that at ther moment I can't reemebr exactly how, but I'll get you an answer soon (IIRC it is a consequence of considering photons in GR).

Gold Member
Dearly Missed
Originally posted by jcsd
All I can say for the moment is that at ther moment I can't rememebr exactly how, but I'll get you an answer soon (IIRC it is a consequence of considering photons in GR).

Bravo jcsd, I can conceive of it's depending on how you picture the lightbeams.

Two individual photons traveling side by side in parallel might have a different outcome from two thin extended beams (each containing many photons) starting out parallel.

I hope you do find out a GR result of this type. It would come as an agreeable surprise.

Also jcsd, I may say thank you for apparently corroborating what I said about a universe filled with nothing but light collapsing by its own gravity. It is a standard case modeled in cosmology. Light does attract light---generically speaking. What you are saying, if I understand you, is that it is a special case when two photons (or possibly beams) are traveling in parallel formation.

------------

Ivan S, thanks for the kind comment on my minor contribution. The main idea was contained in Imagine's "morning interrogation". It will be interesting to see how the discussion turns out. All I can say is that the otherwise-empty light-filled universe is a textbook example and that the energy density (joules per cubic kilometer) is, as always, the parameter that decides future evolution. If the energy density (merely that of light in this case) is greater than the "critical" density, which is calculated from the current expansion rate H0[/sup], then the U will eventually collapse. This is only an idealized model but it shows that cosmologists think light exerts a gravitational effect as do other types of energy.

But this example has the light so-to-speak "in general position"---randomly directed and distributed. So we just have to wait and see if jcsd digs up anything about light that is parallel.

Gold Member
Dearly Missed

Originally posted by Ivan Seeking
...I have been trying to get an answer to all of this for some time.

Marcus, as I understand you here, we can argue for gravity solely on the basis of the energy of the photons?

one little footnote
the prevailing model of gravity is of course the 1916 einstein equation and the righthand side of that equation is an expression for the energy-density and Pressure (!) of some
region or locale

usually the effect of pressure is neglible (in the examples we consider, we are just partly-evolved fish and do not think of everything all the time) so we almost always neglect to mention the pressure

but you said "gravity solely on the basis of the energy of the photons" and that is not quite right, it is gravity solely on the basis of the energy density and PRESSURE of the photons.

But for crying out loud light has almost no pressure at all, so let's neglect it!

But in another thread of discussion one should talk about the fact that the SQUEEZING inside the core of a star actually contibutes to the gravlish attractiveness of the star. this is so strange as to be beautiful... the righthand side is called the "stress-energy" tensor and it is not merely an energy-density it also contains pressure terms

so if you put some energy in a bag it will exert gravitational attraction, and how much attraction depends on how much energy you put----but it ALSO depends (which I find delightfully unintuitive!) on the pressure inside the bag.

you should look at the Friedmann equation which is a simplified
form of the 1916 GR equation and has a righthand side which is essentially (density + 3 times pressure)

(rho + 3p)

Gold Member
Yep, Marcus the phtons of the universe defintely contribute to it's mass and the gravitational attraction, and yes it is related to the pressure exerted by light (the classic analogy is to consider a sealed box, containing photons, on a set of weighing scales).This wouldn't hold true for two parallel beams of phtons as they are acting in the same direction tho'(I'll try and get the correct equatins soon)>

wimms
zero impulss difference between parallel photons?

Ring
From sci.physics

"No, the situation is more subtle than the options you have presented.
What happens with two parallel beams of light is: their energy causes
them to attract each other gravitationally. But their _momentum_ (which also gravitates, since it's part of their stress-energy) causes them to_repel_ gravitationally, and this effect happens to exactly cancel out their attraction. This is worked out in an approximation (though it holds exactly) in Lightman et al."

Ring
Of course the above post only applies to parallel beams of light. Anti-parallel beams of light do attract gravitationally.

Gold Member
Dearly Missed
Originally posted by Ring
Of course the above post only applies to parallel beams of light. Anti-parallel beams of light do attract gravitationally.

Bravo!
Do you have a web reference to some sci.physics
conversation where a person who does not own
the parallel case?

Two parallel traveling photons do not interact gravitationally, since the deformation of space-time (for one single photon) has the form of a delta.

Last edited by a moderator:
Ring
Originally posted by marcus
Bravo!
Do you have a web reference to some sci.physics
conversation where a person who does not own
the parallel case?

No, all I could find were qualitative discussions of the subject. But if you look at the relativistically correct equation for mass:

m2 = E2 - p2 (c = 1)

You can see that a system of anti-parallel beams has a net momentum of zero so m = E. On the other hand a system of parallel beams of light has a momentum of p = E and therefore m = 0.

This isn’t Lightman or GR but it’s pretty convincing.

Shirley
Um, photons have no mass, do they?

Forgive my ignorance, as this is all very new to me, but the definition of mass involves an object at rest, and photons are never at rest. Right? If we accept that, then must they also not have any gravitational attraction? I have the impression that objects with mass warp space, but objects without mass cannot...

John[?]

Gold Member
They dopn't have invariant mass, but they do have a gravitational field.

ewoodlief

Originally posted by LURCH
I believe that you are. If I understand Imagine's question correctly, he is asking if the two beams will exert gravitational attraction on one another.

BTW, the more I hear about it, the more convinced I become that light has no gravitational influence.

The short answer is that light itself does not emit gravitational waves because it does not occupy any space-time. You will see why this question is difficult to answer: the scenario simply cannot occur under any existing physical framework.

First, for clarification: Gravity does not bend light. Gravity bends space and light travels in a straight line through the curved space.

The problem with the original inquiry is that the universe is bounded by the material contained within. For only two light beams to exist in space, and nothing else, would mean that this universe is of zero dimensions. Two reasons: first, light is pure energy and massless, and therefore does not take up any space; and second, gravity, as the facilitator between the transfer and conversion of energy--and subsequently the observed change in systems (also known as time), would be nonexistent in a universe consisting of only light, hence the zero dimensions (a system in which change cannot be observed.)

On a mathematical note two parallel lines, no matter the mutual distance, intersect at infinity. Oddly enough, this perspective can apply to the original question and even hint to the answer. Assuming it was possible to create a universe of only two parallel light beams (where there is a distance greater than zero between the beams and time is constant everywhere), in order to increase the accuracy of our measurement, we need to increase our rate of travel. As our rate of travel increases, the observed distance between the two light beams decreases so that as we reach the speed of light the light beams would appear to intersect. Of course, our observation does not always reflect what is actually occurring. Our observations would be perturbed by our own gravity field, which gets stronger and stronger as we move faster and faster. If we were to somehow "shield" the oncoming space of our gravitational effects, either by ceasing to emit any waves ourselves or by making space itself immune to its effects, the light beams would be observed to run parallel forever.

It's quite clear that this question cannot be fully answered by the known laws of physics, quantum or otherwise.