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Author Topic: Can gravity exist without matter?  (Read 4656 times)

Offline thedoc

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Can gravity exist without matter?
« on: 11/08/2012 03:30:01 »
Jim asked the Naked Scientists:
   
I've been wondering about dark matter recently.

As I understand the issue, we know matter causes a gravitational attraction and we can observe the movement of galaxies and calculate the amount of matter required to cause the effects. The visible matter is insufficient to cause the observed effects so we can calculate the amount of matter that is not visible.

It seems to me that we're assuming that the spacetime is uniform and all deformations that we observe are due to some form of matter. Is there evidence for this? Is it possible that there are regions of space that just exhibit gravitational effects without requiring matter (either dark or normal).

Thanks so much,
Jim

What do you think?
« Last Edit: 11/08/2012 03:30:01 by _system »


 

Offline evan_au

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Re: Can gravity exist without matter?
« Reply #1 on: 11/08/2012 11:22:15 »
It is possible to have energy without having matter; but since energy and mass are interchangeable, that energy will exhibit gravity.

Light (or electromagnetic radiation in general) is a perfect example of this - it has no rest mass, but it does have energy, and so it interacts with gravitational fields, as recent gravitational lensing experiments show.

But light cannot be responsible for the properties attributed to dark matter, because light is not confined to a galaxy. It takes the intense gravitational field of a black hole to confine light.

So that leaves particles which travel slower than light. For these to interact with gravity, they must have a rest mass - and that makes them matter of some kind (even if we haven't discovered what kind, yet).
http://en.wikipedia.org/wiki/Dark_matter#History_of_the_search_for_its_composition
 

Offline lightarrow

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Re: Can gravity exist without matter?
« Reply #2 on: 13/08/2012 13:37:35 »
It is possible to have energy without having matter; but since energy and mass are interchangeable, that energy will exhibit gravity.

Light (or electromagnetic radiation in general) is a perfect example of this - it has no rest mass, but it does have energy, and so it interacts with gravitational fields, as recent gravitational lensing experiments show.

But light cannot be responsible for the properties attributed to dark matter, because light is not confined to a galaxy.
I'm not sure what you mean here. The light's energy density emitted from stars is greater inside the galaxy that outside and light does give some mass to the region of space where it is, just because the (electromagnetic) energy is there, even if light has no mass (very tricky concept, I know).
As far as I know the explanation is simply that this increase of mass given from light is too little to account for dark matter.
 

Offline Pmb

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Re: Can gravity exist without matter?
« Reply #3 on: 13/08/2012 15:04:57 »
Jim asked the Naked Scientists:
   
I've been wondering about dark matter recently.

As I understand the issue, we know matter causes a gravitational attraction and we can observe the movement of galaxies and calculate the amount of matter required to cause the effects. The visible matter is insufficient to cause the observed effects so we can calculate the amount of matter that is not visible.

It seems to me that we're assuming that the spacetime is uniform and all deformations that we observe are due to some form of matter. Is there evidence for this? Is it possible that there are regions of space that just exhibit gravitational effects without requiring matter (either dark or normal).

Thanks so much,
Jim

What do you think?

It there is no matter then the stress-energy-momentum tensor is zero, by definition. However if the cosmological constant is not zero then there will be a gravitational field.
 

lean bean

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Re: Can gravity exist without matter?
« Reply #4 on: 16/08/2012 12:21:43 »
 Can gravity exist without matter?
I think gravitational waves may be a candidate here, since they also have energy, and hence will exhibit gravity.
 

Offline @/antic

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Re: Can gravity exist without matter?
« Reply #5 on: 16/08/2012 20:06:11 »
But is matter and mass interchangable?
I don't think it necessarily is.
I thought that that is why the Higgs Boson was posited?

 

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Re: Can gravity exist without matter?
« Reply #5 on: 16/08/2012 20:06:11 »

 

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