B Am I attracted to a hydrogen atom 1 billion light years away?

[jeff einstein]

Just a quick question, are we attracted by a hydrogen atom billions of light years away by gravity or is my gravity completely masked by earth's gravity and so I am not attracted by the hydrogen atom? I am talking on a very very very small and incomprehensible scale.

 

[Ibix]

There is no such thing as "gravitational masking". Our models predict a gravitational attraction between you and the atom, although it's got upward of 50 zeroes after the decimal point.

Whether the models are correct on this kind of long distance low mass extreme has not been tested, though.

 

[DaveC426913]

Massive bodies do not "mask" gravity.

Yes, you and a hydrogen atom millions of light years away are gravitationally interacting.

It may seem like a small amount and really far away, but consider: there's a galaxy cluster called the Virgo supercluster 65 million light years away that we are gravitationally bound to. It is mostly hydrogen, as we are here.

If every hydrogen atom in the Virgo supercluster weren't interacting with every hydrogen atom here, then we wouldn't be gravitationally bound.

Nature does not care how hard it is for humans to measure very small masses and very large distances.

 

[Dale]

are we attracted by a hydrogen atom

We don’t judge here either way

 

[topsquark]

We don’t judge here either way

-Dan

 

[sophiecentaur]

Yes, you and a hydrogen atom millions of light years away are gravitationally interacting.

This could be tidied up a bit. The gravitational effect of that hydrogen atom on you will be in the direction of where that atom was, millions of years ago because that is how long it will have taken for the gravitational effect to reach you.
A mechanical analogy would be pulling one end of a slinky. The other end would only move after the slinky wave had travelled from one end o the other. Gravitational effects propagate at the speed of light. If that hydrogen atom nucleus had fused with some other nuclei last year, we would not know about it for millions of years.
Warning: the numbers in the scenario of this thread are pretty meaningless to real life. Best to discuss things like merging black holes at distances of a couple of thousand light years. We can actually detect that sort of thing (but the delay is still there!)

 

[Ibix]

The gravitational effect of that hydrogen atom on you will be in the direction of where that atom was, millions of years ago because that is how long it will have taken for the gravitational effect to reach you.

This isn't correct, by the way. The solar system would be unstable within centuries if it were.

There are gravitational effects analogous to the magnetic field in electromagnetism that mean that your acceleration due to the gravity of an isolated body moving inertially points to where it is now, not at the delayed position you see it. For bodies under acceleration (for example an atom in a galaxy) in a complicated spacetime it will not be that simple. The acceleration will not be towards either its past or current position.

 

[jeff einstein]

ok i am satisfied with all these answers

 

[sophiecentaur]

This isn't correct, by the way.

Fine. Your reasoning about planetary orbits seems irrefutable but the isolated electron could be annihilated at some point. It wouldn't exist and its mass would have changed to massless energy. Can what you say deal with that?
I know GR has surprises everywhere.

 

[Ibix]

It wouldn't exist and its mass would have changed to massless energy.

Changes to gravitational fields do propagate at the speed if light in GR. So if you just leave the hydrogen atom alone and isolated, its field will always point towards it (or, to be a bit more precise, will be more or less a Schwarzschild spacetime centered on it). If you kick the atom somehow then the change in the field propagates outward, and far from the atom you still have the unchanged field while nearby you have a field centered on the new trajectory with a transition zone somewhere.

This is made more complex than electromagnetism because in EM you can have a charge with its Coulomb field and shoot it with an uncharged bullet and get a nice clean transition form one Coulomb field far away to another nearby. But there's no such thing as a gravitationally "uncharged" bullet. Everything is a source of gravity, including massless entities because energy is s source of gravity. So you never get the same simple and clean result - but the analogy is there.

You asked about annihilation. I'm genuinely not quite sure. The atom would have to collide with an anti-atom, so you'd have to feel their combined field. I suspect that's not measurably different from a Schwarzschild field long before they collide. When they do collide and convert to photons there would be a change in the field propagating outwards at light speed, but I don't know exactly what the field after the change would look like - and nor does anyone else because you'd need a quantum theory of gravity.

Strictly, that last applies to everything in this thread. We don't know what the gravitational field of a single atom looks like because we can't test it and we don't have a quantum theory to make solid predictions.