Understanding the Relationship Between Gravity and Light

[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:tongue:

 

[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:tongue:

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.