5
$\begingroup$

Pondering this question: Casimir effect and negative mass and, in particular, the response of John Rennie "as the mass of any bound system is slightly less than the mass of its parts" I thought: "This statement hold true even for gravitationally bounded system?"

If we have m1 and m2 so close that their gravitational interaction is not negligible, the system composed of m1 and m2 has a total weight less than m1 + m2 ?

If so, if we consider 2 galaxies, since each galaxy is composed by masses (M) and gravitational binding energy between those masses (E), it is possible that the gravitational attraction between those 2 galaxies is proportional to (M1 + E1/c^2) * (M2 + E2/c^2) ? (with E1 and E2 < 0)

Googling "Gravitational binding energy as alternative to dark matter" i have found this that should be relevant for the discussion: An explanation for dark matter and dark energy consistent with the standard model of particle physics and General Relativity

$\endgroup$
2
  • $\begingroup$ Think of a galaxy as a 3D black hole and a 2D disk. The interaction goes from isotropic to anisotropic the further out into the disk one is. The binding energy's dimensional reduction brings stronger interaction. $\endgroup$ Dec 18, 2021 at 16:58
  • $\begingroup$ It's just like the gravitomagnetism thing people keep asking about -- all these effects are way too small (generically, order $(v/c)^2 \sim 10^{-6}$) to work. $\endgroup$
    – knzhou
    Dec 18, 2021 at 18:42

2 Answers 2

6
$\begingroup$

It isn't an explanation because as an effect it is many orders of magnitude too small and it is in the wrong direction. Dark matter increases the gravitational influence of galaxies.

Approximately speaking, you could assume that the binding energy (which is negative) is at most equal to the gravitational potential energy. Dividing that by $c^2$ then gives a correction to the gravitational mass. So the size of the effect is roughly $$\frac{\Delta M}{M} \sim -\frac{GM}{Rc^2}$$ For a galaxy like the Milky Way, the mass, including dark matter is roughly $10^{12}M_\odot$ within a radius of 100 kpc. The fraction above is $-5\times 10^{-7}$.

$\endgroup$
2
  • $\begingroup$ Ok, thank you. What about the cited article? In particular, "Calculations have shown that field self-interaction increases the binding of matter inside massive systems, which may account for galaxy and cluster dynamics without invoking dark matter." $\endgroup$ Dec 18, 2021 at 17:31
  • $\begingroup$ It is somehow related to the positive energy of gravitons? $\endgroup$ Dec 18, 2021 at 17:53
3
$\begingroup$

That statement about "any bound" system is inaccurate. Some bound systems' mass can be greater than sum of masses of its parts, due to repulsive forces between the parts. For example, mass of nucleus of uranium 235 is greater than sum of masses of its decay products, because of the strong enough EM repulsive forces between protons.

Of course, for gravity, we don't expect such repulsive forces being involved. Gravity is known to be always attractive, so total energy of purely gravitationally bound system is lower than energy of parts in a disassembled state.

If we have m1 and m2 so close that their gravitational interaction is not negligible, the system composed of m1 and m2 has a total weight less than m1 + m2 ?

It's possible, although we don't have as direct experimental evidence for this as we have for nuclear and electromagnetic forces, and gravity has some very different properties from other interactions.

But the extrapolation does make some sense; it order for the system to decrease its gravitational potential energy, it has to get rid of it somehow. For example, via EM radiation. This should decrease its total mass.

Assuming this is true, this still does not help us explain rotation curves of galaxies better than dark matter, because assuming only Newtonian gravity, observations suggest more gravitational mass being present than sum of visible masses being observed, not less.

But hypothetically, if some weak long-range repulsive forces were present between stars/parts of galaxies, analogously to repulsive forces between protons in the uranium nucleus, bound state (galaxy) would have greater energy than expected with gravity forces alone.

Maybe this increase in energy and implied increase in gravitational mass and its gravity could be enough to explain the rotation curves, but I have not done any calculations on this.

$\endgroup$
3
  • $\begingroup$ @safesphere yes I agree, I've implicitly assumed we are talking about bound systems that have lost the binding energy via radiation or other way. $\endgroup$ Dec 19, 2021 at 0:15
  • $\begingroup$ Otherwise the system would not be "bound" in newtonian mechanics. With black holes it may be like you say, that binding energy won't get out. $\endgroup$ Dec 19, 2021 at 0:32
  • $\begingroup$ Hmm. Your statement “of course, for gravity,we don’t expect such repulsive be forces to be involved” immediately made me think “hey, but what about dark energy? That’s a seemingly repulsive force!” Just playing devils advocate. I almost think you must be baiting someone to retort. $\endgroup$ Dec 24, 2021 at 21:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.