# Macroscopic particles?

## Main Question or Discussion Point

Hi,
just a thought but if two elementary particles are travelling parallel to one another at the speed of light does particle a observe the gravitational wave from particle b and vice versa as they would in Newtonian gravitation?
i realise that gravity at this scale is negligible.

Best regards,
Mark

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bcrowell
Staff Emeritus
Gold Member
Hi,
just a thought but if two elementary particles are travelling parallel to one another at the speed of light does particle a observe the gravitational wave from particle b and vice versa as they would in Newtonian gravitation?
These have to be particles with zero rest mass, of course.

I'm not sure I completely understand what you had in mind, since the title you chose for the thread doesn't seem to relate to the question you asked.

I'm also not clear on why you say "gravitational wave" rather than "gravitational force." Newtonian gravitation doesn't have gravitational waves, and even in GR, I wouldn't expect there to be a gravitational wave in the situation you describe (in the sense of gravitational radiation that takes energy from a source and carries it away to infinity).

I doubt that the answer to the question depends on which specific type of zero-rest-mass particle you talk about, so let's assume we're talking about light.

I've heard that light rays traveling in the same direction experience zero gravitational attraction, which is different from what you would expect naively based on mass-energy equivalence. I haven't seen this statement given by an authoritative source, and I haven't seen an argument or calculation offered as to why it would be so. Light waves that are not traveling in parallel directions certainly will interact gravitationally. There is a whole subfield of GR concerned with colliding gravitational and electromagnetic waves.

I have to ask as bcrowell did... why the title?

sorry it was a bit of a mind dump. I’m currently occupied with simulating through software a well know physics experiment for my dissertation.
I have implemented Newtonian gravitation in the simulation; it just crossed my mind that the simulation may be wrong as particles travelling parallel and in the same direction as one another will have no gravitational attraction to one another (after just reading and reminding myself about GR). I was picturing the classic model the dent in the fabric around the particles and as the gravitational waves travel out at the same rate as the particles the waves never interact ahead of the particles.
As for why I implemented Newtonian gravitation the simple answer is it’s easier to do.
also I was under the impression that GR was only going to yield different answers if I was going to simulate massive bodies of mass with considerable distance between them

Ahhh, I see where you're coming from. As it happens however, Numerical GR simulates test particles too, even if they tend to be concerned with the 2-body problem (massive, distance, as you say). If light is moving (as trains in opposite directions) in the manner you describe, and it DID interact, that would lead to some funky optical effects I think.

I don't see a problem with what you're doing assuming normal m=0 test particles.

EDIT: To clarifiy, if you DO expect mutual attraction between the light, that would seem to be in error.

If light is moving (as trains in opposite directions) in the manner you describe, and it DID interact, that would lead to some funky optical effects I think.
that was my thoughts exactly!
EDIT: To clarifiy, if you DO expect mutual attraction between the light, that would seem to be in error.
im calculating gravity currently between all objects in the system using the clasical formula.
am is this in error?