How do we know that gravity results from mass?

[mrspeedybob]

There are lots of other things that will almost always exist wherever mass exists in large enough quantities to be measured. Photons are one example. The quantity of electromagnetic interactions between particles in a given body of mass will depend on the number of protons and electrons the mass contains. How do we know that gravity results from the existence of the mass and not the existence of the photons?

 

[diazona]

Actually general relativity predicts that gravity results from both the mass and the photons, but the contribution of the photons is so small as to be negligible.

I believe that if all the gravity were caused by the photons and the mass itself had no effect, the large number of photons filling the universe would have a significant gravitational effect which could be observed, and which would invalidate general relativity. But general relativity works exceptionally well in every experimental test it has ever been given, so we have every reason to believe that photons' contribution to gravity is indeed negligible.

 

[mrspeedybob]

Actually general relativity predicts that gravity results from both the mass and the photons, but the contribution of the photons is so small as to be negligible.

I believe that if all the gravity were caused by the photons and the mass itself had no effect, the large number of photons filling the universe would have a significant gravitational effect which could be observed, and which would invalidate general relativity. But general relativity works exceptionally well in every experimental test it has ever been given, so we have every reason to believe that photons' contribution to gravity is indeed negligible.

Isn't there a large discrepancy between the amount of matter we see in the universe and the amount needed to account for the apparent gravitation? Isn't this the infamous "dark matter" mystery? Wouldn't it be ironic if "dark matter" turned out to me light?

 

[Pengwuino]

Isn't there a large discrepancy between the amount of matter we see in the universe and the amount needed to account for the apparent gravitation? Isn't this the infamous "dark matter" mystery? Wouldn't it be ironic if "dark matter" turned out to me light?

Ironic because it's not right. Based on our observations, everything we know of has been ruled out, especially photons.

 

[alxm]

Then different particles with different interactions, should be expected to have different gravity even if they had the same mass.

But, the Sun is over 70% hydrogen, so its mass is mostly (~85%) protons. The planets are mostly heavier elements, and have an almost 50-50% ratio of protons to neutrons. Yet the same laws of gravity describe the Sun and Earth as well as the Earth and Moon, even though they have a fundamentally different ratio of elementary particles to mass, and different ratio of electromagnetic or nuclear interactions to mass as well. If it were as you say, I'd expect there to be a measurable difference in interplanetary gravity compared to stellar-planetary gravity.

As it were, E=mc^2, so any interactions going on are already contributing to the mass and hence gravity.

 

[PhanthomJay]

How do we know that gravity results from the existence of the mass and not the existence of the photons?

Gravity is the curvature of spacetime in the presence of mass and energy. In that sense, since a photon has energy, it must contribute to that spacetime curvature, however small, or however large, and all particles/waves , whether with mass, or without mass, must follow that curvature.
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" Physical objects are not in space, but these objects are spatially extended. In this way the concept “empty space” loses its meaning. "

Albert Einstein, “Relativity ...”, Notes to the Fifteenth Edition

 

[AC130Nav]

Gravity is the curvature of spacetime in the presence of mass and energy. In that sense, since a photon has energy, it must contribute to that spacetime curvature, however small, or however large, and all particles/waves , whether with mass, or without mass, must follow that curvature.
___________________________________________________________________________
" Physical objects are not in space, but these objects are spatially extended. In this way the concept “empty space” loses its meaning. "

Albert Einstein, “Relativity ...”, Notes to the Fifteenth Edition

Has the contribution of binding-energy-becomes-mass been considered in the search for Dark Matter? The effect is cumulative: The gain to the solar system becomes mass in the calculations for the local star cluster becomes mass for the galaxy.

 

[PhanthomJay]

Has the contribution of binding-energy-becomes-mass been considered in the search for Dark Matter? The effect is cumulative: The gain to the solar system becomes mass in the calculations for the local star cluster becomes mass for the galaxy.

I'm not an expert on this, but I doubt it. Nuclear binding energy is just nuclear 'power' per E=mc^2, and is well understood. Dark matter is not understood. It could be postulated 'WIMP' particles that interact weakly with mass, or super-massive black holes, or , as theorized by the Brane Theorists, the gravitational effects from other universes that penetrate through the extra spatial dimensions of spacetime. I'm about to post a question on Dark Matter to our Cosmology experts in the Cosmology subforum.

 

[pallidin]

I don't think anyone really understands what MASS is.
We clearly know it's effects(gravity), but not it's cause. Higgs boson maybe? Who knows.
Perhaps in time the LHC can provide insight. Hopefully.