2

If something is moving past me at a relativistic speed, does its gravitational force on me increase as its speed increases? That is, not as if its speed is changing, but if multiple of the same (rest) mass go past me in the same path, is the gravitational attraction at the closest point the same for each, or is it higher the faster it's moving?

As some background, I was recently surprised to see that I should discard the idea of "relativistic mass", so I've been trying to work on that. Please forgive me mixing Newtonian physics with relativity, but I'm trying to get a high level understanding before going into the math too much. I understand now that instead of $F=m_{relativistic}a$ I should use $F=\frac{d}{dt}(\gamma m_0v)$, but what about $F=\frac{Gm_1m_2}{r^2}$? Is that still roughly as it is, or do I need to throw a factor of $\gamma$ in there, or something else entirely?

Yes, I know this is talking about acceleration and so it doesn't really fit into special relativity, and lots of other disclaimers, but I'm hoping for a simple answer to get a general understanding.

Neutronium
  • 21
  • 6
    Possible duplicate of Does relativistic mass have weight? – Kieran Moynihan Oct 19 '17 at 23:49
  • "Yes, I know this is talking about acceleration and so it doesn't really fit into special relativity" - acceleration fits into SR just fine as has been stated here and elsewhere many times. From the link: "It's a common misconception that special relativity cannot handle accelerating objects or accelerating reference frames. Sometimes it's claimed that general relativity is required for these situations, the reason being given that special relativity only applies to inertial frames. This is not true." – Alfred Centauri Oct 20 '17 at 02:52
  • I tend to object to closing this question. It's not a great question, but the OP is confused as to how he could possibly understand how a higher velocity could have the body's gravitational field get stronger. The answer is very clear in General Relativity, and totally un-related to Special Relativity which does not claim to deal with gravitation. Relativistic mass usage purity is not the issue, there is no science there, the science is in how GR accounts for velocity in creating gravitational fields, i.e., curvature. My answer tried to do that. And other answers could improve it. – Bob Bee Oct 21 '17 at 05:35

2 Answers2

1

To be more correct and more specific, the answer is yes because the source of a gravitational field, which in General Relativity (GR) is spacetime curvature, is the stress energy tensor $T^{\mu\nu}$. The 0i and i0 components of that tensor, $T^{0i}$ and $T^{i0}$ (which are equal because the tensor is symmetric in GR), are actually the $i^{th}$ component of the momentum density, $p^i$, for i = 1-3 the 3 spatial components, and 0 the timelike coordinate.

Since higher velocity is higher momentum there is a stronger source if it is larger. Of course it depends on the reference frame and how things are arranged geometrically.

See the stress energy tensor in Wikipedia at https://en.m.wikipedia.org/wiki/Stress–energy_tensor In GR, the Einstein tensor (including the Ricci curvature) is proportional to the stress energy tensor.

So, you can deal with the effect of higher velocities in a more correct way than dealing with relativistic mass. Velocity, as momentum, is part of the source for GR curvature.

Bob Bee
  • 13,996
0

Yes. Gravitational attraction in general relativity is based on an object's energy, and an object's energy is greater when its speed is greater.

Travis
  • 2,057