Understanding the Relationship Between Gravity and Light

[saderlius]

Well, vacuum is not nothing, nor a non-thing.. vacuum is space, which in GR is a thing like another.

How does one know that the energy is derived from the fabric of a spatial dimension, and not just the oodles of ghosting neutrinos and other phenomena?

 

[xantox]

How does one know that the energy is derived from the fabric of a spatial dimension, and not just the oodles of ghosting neutrinos and other phenomena?

Recent observations indicating that the universe is undergoing an accelerated expansion tend to favor the hypothesis that the energy producing such expansion is associated with space itself (or perhaps with another field dubbed 'quintessence'). As if it was energy of matter (at least the ordinary one) or of radiation, then the energy density would decrease when matter (or radiation) would dilute into the expanding space.

 

[Chris Hillman]

On reading what was written...

How does one know that the energy is derived from the fabric of a spatial dimension

I don't think anyone said that; in fact, I don't know what "energy is derived from the fabric of a spatial dimension" would even mean.

 

[Chris Hillman]

Is there any "thing" which isn't tied into gravity somehow?

Don't forget that questions like this are always theory-dependent, so you should mention which theory you are asking about. The default in this context is probably gtr, but strictly speaking some of the stuff you have been talking about lies outside the domain of gtr. In particular, gtr can model the effects of hypothetical dark energy via the cosmological constant, but (regarding dark energy) it can't hope to answer Pauli's question: "who ordered that?!"

You might have to bubba-ize geodesics for me. What i gather is that it describes the curved path of a photon relative to time, space, or nothing.

A geodesic is simply a path on a (possibly curved) manifold which is analogous to a straight line in ordinary euclidean space. More precisely, any curve (even in a curved manifold) has a "path curvature" defined at each point along the curve. A geodesic is a curve such that this quantity vanishes all along the curve. In gtr, a timelike curve is interpreted as a world line of a particle (representing the kinematical history of the particle), a null curve can be interpreted (more or less) as the world line of laser pulse,
and the path curvature is interpreted as the magnitude of the acceleration vector. Here, a curve is said to spacelike, null, or timelike according to whether its tangent vectors are everywhere spacelike, null, or timelike. A vector is said to be spacelike, null, or timelike--- well, see Taylor & Wheeler, Spacetime Physics, first edition.

This seems to build on rather than replace euclidean systems.

It generalizes notions already familiar to you (if you studied elementary differential geometry) from euclidean spaces.

Does a straight line exist in reality? Perhaps Light doesn't travel in a straight line, but does it exist?

Oh dear, now it sounds like you are veering off into philosophy or even mysticism. I hope you can clarify the question so that it can be interpreted as an unambiguous question about physics.

 

[saderlius]

Whoa, saderlius, slow down!

Hullo Chris,
thanks for the time and effort it took to write such a large post. As you can see, I know just enough about physics to make me dangerous(in the annoying sense). Thanks for you patience, you've no doubt had to explain these things 1x10^38 times already. Assume that if i don't reply to one of your points, i agree or am silently intrigued.

See my response in the thread "photon dimensions" (and please see also https://www.physicsforums.com/showthread.php?t=5374 if you haven't already done so).

interesting thread- anantchowdhairy sounds allot like me... so instead of "photon", you call it "a test particle with a null geodesiac" or something like that. hopefully, the forum guidelines link wasn't because i was violating the "overt crackpottery" rule...yet. Without the esoterica, jargon, and math, i have a hard time understanding the gtr concept of light and gravity... and why plow through an entire textbook when i can just ask you? (rhetorical)

You can probably answer this one yourself. Imagine an isolated ball in deep space, far from any other objects. Imagine that you use some laser beams to heat it up (carefully balancing so that the momentum they impart cancels) and then turn off the laser beams and measure the mass of the heated ball. Bearing in mind that heat is manifested by increased molecular motion inside the ball and that you just transferred a large amount of energy to the ball from someplace else, would you expect the gravitational mass of the heated ball to be larger?

to answer your question: i suppose so, yes... however, since heat describes a pattern in something else, i don't quite see how heating a ball and measuring shows that heat energy exerts gravity. For all i know, some of the energy of the laserbeam was converted into matter upon impact, accounting for the increased gravitational mass, so it would still be heavier after it cooled. I suppose, however, if the mass decreased as the ball cooled, that it was indeed the heat which caused the increase...

You really need to slow down---now you seem to be confusing several distinct notions.

Granted. I'm trying to understand the connectivity and universal principals of things. I'll be more clear in my next posts.

Physics is indeed fascinating, but you are confusing a whole buncha ideas here, and it would take a long time, I think, to sort it all out. Maybe you should just file this question away and continue your study of physics--- eventually, it will all sort itself out.

good advice, but there might be a tad more milk in this thread. one particular concept has arisen which inspires me, so i'll ask you about it in the relevant post.
cheers,
sad

 

[saderlius]

I don't think anyone said that; in fact, I don't know what "energy is derived from the fabric of a spatial dimension" would even mean.

Hey Chris,
Well, Xantox understood what i meant(see post 32).
Even though i started a thread called "does light exert gravity", really i want to figure out the nature and source of gravity. Xantox stated that the energy of a vacuum exerts gravity. To me, that means that the spatial dimensions x/y/z exert energy, and that, in turn, exerts gravity. Now, i am curious as to what sort of energy is inherent to a vacuum, or whether its just called energy because it exerts gravity. To me, this would seem like the most primitive occurrence of gravity, and thus a good study for the source of gravity.
I would like to know if energy can be isolated from gravity, or vice versa. Is gravity treated as a type of energy? Energy pressure and energy density may very well determine the power of a gravitational field, but i can't yet see how they cause one object to be attracted to another to begin with.

you don't have to answer according to gtr- just whatever best explains it.
thanks,
sad

 

[pervect]

Hey Chris,
Well, Xantox understood what i meant(see post 32).
Even though i started a thread called "does light exert gravity", really i want to figure out the nature and source of gravity. Xantox stated that the energy of a vacuum exerts gravity. To me, that means that the spatial dimensions x/y/z exert energy, and that, in turn, exerts gravity.

The question of whether or not a vacuum has energy is very much up in the air.

Furthermore, cosmological observations suggest that "the vacuum" if it does anything at all, does not exert "gravity" but rather anti-gravity.

More formally, we say that cosmological observations show that the universal expansion is accelerating, and that we attribute this to "dark energy".

How do we reconcile this with the fact that the vacuum (may) have energy? Well, remember that pressure also influences gravity in GR, so that if the vacuum had negative pressure, it would exert anti-gravity.

I know this is a lot, so let me recap:

1) We don't have any direct observations of the energy of the vacuum.

2) We have some quantum models of the vacuum, but nothing that will directly predict the energy of the vacuum or the pressure of the vacuum. At least, we don't get any values for these parameters consistent with experiment from existing theoretical models.

3) We do have observations that show that the universal expansion is accelerating. One way of explaining this is to assume that empty space is not really empty, but has a sort of "anti-gravity", which is actually more correctly known by the name of "dark energy". This dark energy is usually associated with a positive energy of the vacuum, and a negative pressure of the vacuum. The reason that the vacuum should behave this way is not really known or predicted. It's just what makes the cosmological models work.

 

[AWolf]

One issue that I've not yet seen mentioned in this thread is the speed at which gravity propogates.

Unless I am mistaken, the speed of gravity has been measured at C.
If we can ignore the mythical graviton for a moment, then the effect which we associate with gravity must be caused by something which also has a velocity of C.

So, it seems, you can't have one without the other.

Is this simply because gravity is an attribute of energy ?

 

[Schrodinger's Dog]

I'm surprised this thread hasn't gone that way, that's testament in itself to the people contributing. I once asked this question and eventually ended up with the answer, light has no mass, but there's no reason it couldn't have an infinitessimal one, it's just unlikely if we'll ever measure it, but anyway, that's of course extremely contraversial. But if light had a mass of say 3.2372878^-60 then would it violate GR is my question? And feel free to call the crank police it's only a question

 

[AWolf]

This seemed to be something of an answer to my question, although wrapped up in a lengthy tirade against cranks and everything 'not mainstream'.

Chris, you raised the issue by saying that you can't have energy without having gravity. This can be interpreted as saying that gravity is synonomous with energy. Your words, not mine.

From some of the rest of the text there seems to be some measure of acceptance that gravity propogates at C, although clearly you are waiting on LISA to produce a figure before nailing your colours to the mast.

This was not a crank posting, but merely making an assumption based on your postings. Obviously one which you feel very uncomfortable about.

If shouting CRANK at the first sign of an awkward question is where science has gotten to, then we are the poorer for it.

 

[ZapperZ]

I'm surprised this thread hasn't gone that way, that's testament in itself to the people contributing. I once asked this question and eventually ended up with the answer, light has no mass, but there's no reason it couldn't have an infinitessimal one, it's just unlikely if we'll ever measure it, but anyway, that's of course extremely contraversial. But if light had a mass of say 3.2372878^-60 then would it violate GR is my question? And feel free to call the crank police it's only a question

But you also need to examine your question and in what way would you accept an answer!

For example, ask anyone on the street if, let's say, you have a vase that has been broken into a thousand pieces, that if you throw these pieces onto the floor, will it assemble itself back into the original vase?

You will get definite answer that no, it will not!

Yet, ask this in physics, and you will get an answer that the probability of it happening is extremely small, but it is definitely not zero.

When you ask for the "mass of a photon", do you expect an answer via standard, textbook physics (i.e. "NO, it has no mass"), or do you expect an answer at the forefront of cutting-edge physics research in which, by definition, we challenge many of the things we know of in physics today?

It appears that even when the simple textbook answer is given, inevitably the question is THEN made more complicated by bringing in all of these exotica that is part of research-front physics. That is why something that can be answered easily never made it through to its clear conclusion. When you ask for something based on work that is still on-going, don't expect a clear, definite answer because there isn't any! However, if you want standard, textbook answer, then THAT is clear, because any textbook will have such a thing.

Now, while the question on what is the speed of gravity isn't a crank question, insisting that there is a definite answer is! We expect that gravity travels at c, or at least, the information that allows us to detect gravity travels at c. However, any good physicist will qualify that by saying that this has not been verified YET! This must always be the first check in any claims in physics - has that been empirically verified? So if you are insisting that there is one definite answer to this question, now that is inviting crackpottery.

Zz.

 

[nakurusil]

One issue that I've not yet seen mentioned in this thread is the speed at which gravity propogates.

Unless I am mistaken, the speed of gravity has been measured at C.

Gravitational waves have been predicted to propagate at "c". No experimental confirmation is available yet (if you consider the controversy surrounding the Kopeikin experiments). LIGO and/or its enhancements is expected to measure the actual speed of propagation for gravitational waves.

 

[Schrodinger's Dog]

But you also need to examine your question and in what way would you accept an answer!

For example, ask anyone on the street if, let's say, you have a vase that has been broken into a thousand pieces, that if you throw these pieces onto the floor, will it assemble itself back into the original vase?

You will get definite answer that no, it will not!

Yet, ask this in physics, and you will get an answer that the probability of it happening is extremely small, but it is definitely not zero.

When you ask for the "mass of a photon", do you expect an answer via standard, textbook physics (i.e. "NO, it has no mass"), or do you expect an answer at the forefront of cutting-edge physics research in which, by definition, we challenge many of the things we know of in physics today?

It appears that even when the simple textbook answer is given, inevitably the question is THEN made more complicated by bringing in all of these exotica that is part of research-front physics. That is why something that can be answered easily never made it through to its clear conclusion. When you ask for something based on work that is still on-going, don't expect a clear, definite answer because there isn't any! However, if you want standard, textbook answer, then THAT is clear, because any textbook will have such a thing.

Absolutely not wanting to encourage crackpottery, I just wanted a second opinion, the answer then is depends how you ask the question, fair enough.

 

[saderlius]

The question of whether or not a vacuum has energy is very much up in the air.
Furthermore, cosmological observations suggest that "the vacuum" if it does anything at all, does not exert "gravity" but rather anti-gravity.More formally, we say that cosmological observations show that the universal expansion is accelerating, and that we attribute this to "dark energy". How do we reconcile this with the fact that the vacuum (may) have energy? Well, remember that pressure also influences gravity in GR, so that if the vacuum had negative pressure, it would exert anti-gravity.
3) We do have observations that show that the universal expansion is accelerating. One way of explaining this is to assume that empty space is not really empty, but has a sort of "anti-gravity", which is actually more correctly known by the name of "dark energy". This dark energy is usually associated with a positive energy of the vacuum, and a negative pressure of the vacuum. The reason that the vacuum should behave this way is not really known or predicted. It's just what makes the cosmological models work.

Anti-gravity would be (mutually?)repulsive instead of attractive. If i were to find myself in deep vacuum, the concentration of my energy would be drawn out of me, yes? Wouldn't that mean that the energy inside of me is attracted by the vacuum instead of repulsed? Where has my reasoning gone awry?

cheers,
sad

 

[pervect]

Anti-gravity would be (mutually?)repulsive instead of attractive. If i were to find myself in deep vacuum, the concentration of my energy would be drawn out of me, yes? Wouldn't that mean that the energy inside of me is attracted by the vacuum instead of repulsed? Where has my reasoning gone awry?

cheers,
sad

Consider a hollow sphere. The gravity inside the hollow sphere is zero, everywhere.

Thus, if you were in a uniform hollow sphere of anything (including normal matter and other stranger possibilities) you wouldn't feel a thing.

I really have no idea why you think "the concentration of my energy would be drawn out of me" (??).

The point is this: in a universe filled with a true vacuum (no stress-energy tensor), two objects a long distance apart will experience no relative acceleration.

In a universe containing only normal matter, gravity will make any two distant objects accelerate towards each other.

One must distinguish acceleration and motion - a universe containing only normal matter may expand, but the rate of expansion is always slowed down by gravity.

In a universe containing dark energy, the expansion of the universe is not slowed down by gravity, but it is sped up. This means that as objects get further and further apart, they will see the other object accelerate away from them faster and faster.

Note that all of the objects being discussed above are in free fall. They don't feel any "push" like one does when one is in a car that accelerates. They are in free fall, but their natural free-fall motion is to accelerate towards each other (in a universe containing only normal matter), or away from each other (in a universe containing dark energy) - or not to acclerate at all (in a universe containing no energy).

 

[xantox]

Anti-gravity would be (mutually?)repulsive instead of attractive. If i were to find myself in deep vacuum, the concentration of my energy would be drawn out of me, yes? Wouldn't that mean that the energy inside of me is attracted by the vacuum instead of repulsed? Where has my reasoning gone awry?

The vacuum is also inside of you, between your atoms and particles. If vacuum had enough negative pressure (in respect to his energy density), then the accelerated expansion would diverge in finite time, the vacuum ultimately ripping all particles apart.

 

[cosmicnomad]

I'm curious. Since light can be "bent" by gravity (i.e. gravitational lensing), does light have gravity, albiet a small gravity? Now since this gravity would be small, it would require a lot of photons to exhibit an effect. Wouldn't this effect be similar to the effect we see by the faster orbital patterns seen at the outside of galaxies? The massive amounts of photons generated by all of the photon generating bodies on the inside of the orbit would at some point become significant. I can't do the math, but the inverse square law of luminosity should allow one to calculate the actual number of photons from each object as volume, and utilizing E=mc2 would allow you to figure the equivalent mass. Would this not "explain" dark matter? It just seems to me that energy IS gravity but energy that is dependent on scale for it's actual effect. And temperature and density would be somehow directly proportional to this scale effect?

Sorry if I didn't get this completely coherent.

 

[cosmicnomad]

this i breaking my brain: If E=mc2, then if mass has gravity, shouldn't no mass at the speed of light also have gravity? The density and temperature would be lower? And if the density and temperature were higher, while the constant of the speed of light would not change, the energy would. As a result of the energy increase, the gravity would also increase due to the increase in mass? Forgive my basic understanding of physics.

 

[pervect]

This is a rather old thread...

The answer has been given before, though, in many other threads. The idea that "mass" causes gravity is a Newtonian idea. In GR, we say that energy and momentum and pressure cause gravity, or we say equivalently that the stress-energy tensor (which contains all of the above) causes gravity.

So light has energy, and momentum, and thus it has gravity. It doesn't need "mass". Even defining "mass" in the context of GR is rather surprisingly difficult, making it an advanced topic in the field.

 

[jtbell]

Would this not "explain" dark matter?

No. As far as I know, this idea doesn't predict results that are compatible with the observations that led to the idea of dark matter. But I'm not a cosmologist, so you'll have to ask in our Cosmology forum for details. Search there a bit, though. Dark matter is a common topic there.