# What is the difference between gravitation and electromagnetism?

I am currently studying electrodynamics. And when looking at Maxwell's equations, I don't see any reason, why we cannot apply them to gravity.

We know that charges generate a force field that attracts opposite charges. From this and using special relativity we can deduce that there must be a magnetic field (see this nice video from minutephysics).

But masses also generate a force field that attracts other masses. Then by the same reasoning, moving masses must also generate some kind of field that interacts with other moving masses. And indeed, this happens. It's called Lense-Thirring effect.

So if these fields are so similar, why aren't there some kind of Maxwell's equations for gravity? What is the difference between electromagnetism and gravity?

EDIT: Thanks for the comments. So it basically boils down to the question why charge is invariant under Lorentz transformations while mass is not.

• Very similar question: physics.stackexchange.com/q/15366 – Kyle Kanos May 28 '14 at 16:51
• Related: physics.stackexchange.com/q/944/2451 and links therein. See also the notion of gravitomagnetism, cf. e.g. Wikipedia. – Qmechanic May 28 '14 at 16:56
• "Then by the same reasoning, moving masses must also generate some kind of field that interacts with other moving masses. And indeed, this happens. It's called Lense-Thirring effect." Correct me if I am wrong, but isn't this just a theoretical prediction? I do not know of any experiment sensitive enough to show presence of such small effect. – Ján Lalinský May 28 '14 at 17:31
• One Line Answer: Like charges repel each other. – Schrödinger's Cat May 28 '14 at 20:59

## 2 Answers

So if these fields are so similar, why aren't there some kind of Maxwell's equations for gravity?

There is a section in the book "Gravitation" where precisely this question is investigated in the sense that a gravitational four-potential, analogous to the electromagnetic four-potential is postulated and explored.

Since like gravitational charges attract rather than repel, there is a sign difference in the Lagrangian for the gravitational case versus the electromagnetic case.

The result is that the wave solutions transport negative energy rather than positive energy as is the case with electromagnetic waves.

The relevant section is on page 179 which, thanks to Google books, I include below:

The analog of Maxwell's equations in gravity are the Einstein field equations, $$R_{\mu \nu} - {1 \over 2}g_{\mu \nu}\,R + g_{\mu \nu} \Lambda = {8 \pi G \over c^4} T_{\mu \nu}$$