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Comment by thailandstartup | Hacker News Reader

Something I wonder about gravity - While it is often mentioned that objects with a large gravitational force can 'bend' light (gravitational lens), can a sufficient amount of light 'pull' objects towards itself?

So I wonder if a star's gravity is mostly the mass of the star itself, but also, in small part, the huge amount of light that surrounds the star on all sides, being densest nearest the star itself.

There's some evidence that gravity may change a little during a solar eclipse http://en.wikipedia.org/wiki/Allais_effect and also the Pioneer anomaly, http://en.wikipedia.org/wiki/Pioneer_anomaly

I am most definitely not a physicist!

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> can a sufficient amount of light 'pull' objects towards itself?

Yup. As the article says, all forms of energy cause gravitational attraction. Even light! But, as you can see from E=m * c^2, it takes an insane amount of energy to make even a little mass-equivalent. C=3 * 10^8 m/s, so c^2 = 9 * 10^16 or 90000000000000000 m^2/s^2. So 1kg of mass at rest is equivalent to 9 * 10^16 joules of energy. It's about equal to a 4.5 megaton hydrogen bomb blast.

Mmmm. Okay, but a star puts out a lot of energy, over a long period of time. Is it enough that we calculate the gravitational effect of all those photons, rather than just the mass of star itself when calculating say the acceleration of the pioneer crafts?

http://www.wolframalpha.com/input/?i=3+cubic+au+times+682+ki...

so 2.5 * 10^14 kg inside the earth's orbit, or equivalent to 254 cubic kilometers of water - on an astronomical scale, insignificant. Still, bigger than I expected.

I'm not sure I understand your question correctly, but photons have no mass. So light can't pull objects towards itself.

In fact it'd to the opposite, which is why solar sails work. Radiation landing on you propels you away from the object emitting the radiation.

The Pioneer anomaly is very interesting. But we don't have sufficient data to determine what it is exactly. We should really send out more probes like that to find out.

I'd always understood gravity to be a mutual attraction between two masses. So the thing I have difficulty understanding is how light can only be attracted to mass, but itself cannot attract mass, no matter the quantity present.

I think I'm correct in saying that the light isn't attracted to the mass, but the mass bends spacetime around it, so the path of everything traveling through that medium is bent, including the paths of photons.

This effect causis gravitational lensing: http://en.wikipedia.org/wiki/Gravitational_lens

Hmm, thanks. I did a little more searching and reading about this with my new vocabulary (bending spacetime). It seems like photons also bend spacetime.

More info here. All of it contradictory :) http://www.physicsforums.com/showthread.php?t=287888

So I'm still wondering if it is proper to think of a star's gravitational field being caused not just by its matter, but also by all the photons it has pumped out over its lifetime.

It doesn't make any sense to define a gravity field by photons. They're massless, so they don't attract anything, and most of the photons a star has produced over its lifetime are speeding away across the universe billions of light years away from it.

Only things with mass can attract other objects with the gravitational force. Photons aren't in that category.

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