Comment by Xcelerate | Hacker News Reader

Comment by Xcelerate | Hacker News Reader

I think this article is missing the biggest thing that makes gravity different: it arises from and affects everything with energy, which includes everything that we know exists.

See this graph of the known standard model interactions: https://upload.wikimedia.org/wikipedia/commons/4/4c/Elementa...

Gravity isn’t shown because every type of particle in that graph interacts with every other type of particle via gravity (i.e., the stress-energy tensor).

A lot of people may not realize that a perfectly mirrored box containing photons would have “weight”. Similarly, a spinning top weighs more than the same top that isn’t spinning, a compressed spring weighs more than the same spring in an uncompressed state, and the earth+moon system weighs less as a unit than if you pulled them apart and added together their individual weights.

In fact, most of the “weight” of an atom isn’t due to rest mass at all but rather the kinetic energy of the motion of its constituent particles.

For electron/positron collisions, it isn’t so much “annihilation” as it is a change of form. After the collision, total momentum, angular momentum, and energy all remain the same. Even though there are no longer two particles that have the property called “rest mass”, the effect on the gravitational field remains exactly the same the moment before and after the collision.

How come the charts that show the mass distribution of the universe don't include energy at all? This one [0] says that "visible matter" is 5%, but "energy" isn't even present. From my light research the only answer I found was "because it's too small to include", but this seems to disagree with your comment. What am I missing?

[0] https://svs.gsfc.nasa.gov/12307

It is very small and almost perfectly homogeneous, so there is basically no gravitational attraction.

I think that gravity isn't shown simply because it isn't part of the standard model. IANAP, but standard model fields can only interact with other quantum fields and we do not yet have a quantum field version of GR.

This is one of those things that laymen often misunderstand, actually. (I don’t blame you, it’s often not well explained.) General relativity is easy to make into a QFT. There’s an action and you can easily derive amplitudes from it. And it works well, as long as you don’t ask questions of what happens at arbitrarily high energies. That’s what “nonrenormalizable” actually means: it only answers questions up to some energy scale, after which it stops being predictive. But believe it or not, physics has been here before, so we’re not totally clueless what to do! Fermi’s theory of beta decay [0] had the exact same problem. And the moral there was that there was something new that had been missed in Fermi’s theory that was only important at high enough energy. The theory was correctly predicting its own limitations. In fact we figured out what was missing. Fermi didn’t know about the W boson, and at the energy where W boson exchange becomes important, that’s where the old theory breaks down!

But Fermi’s theory works well up to that scale. It’s like how Einstein’s relativity didn’t mean that everything in Newtonian physics was totally wrong, but just that you need it if you want to think about really fast moving objects. Newton’s theory is still an outstandingly good approximation.

So if history is any guide, we don’t need to change the low energy theory of GR (at least not much), that works just fine. We just have to find out what’s the high energy phenomenon that we’re missing. That’s stuff that comes into play with black holes and very high energy collisions. Anyway we have some guesses here, too. In string theory, that’s the stringyness of the strings, their 2D nature, that fixes the problems automatically. (This was not by design, it was completely unexpected! You start playing with dimensions and it just falls out.) In LQG it’s about the fuzziness or discretization of space (which sounds appealing, although it’s really tough in practice to make this work).

See also [1].

[0] https://en.m.wikipedia.org/wiki/Fermi%27s_interaction

[1] https://en.m.wikipedia.org/wiki/Effective_field_theory

> I think this article is missing the biggest thing that makes gravity different: it arises from and affects everything with energy, which includes everything that we know exists.

To be fair, one of the physicists (Daniel Harlow) does say

> gravity is the only force that is felt by all kinds of matter

I didn't realize it was kinetic energy, I thought most of the weight of an atom was due to the strong nuclear force

According to this article [0], you're right:

> This property of the strong nuclear force is known as asymptotic freedom, and the particles that mediate this force are known as gluons. Somehow, the energy binding the proton together, the other 99.8% of the proton's mass, comes from these gluons.

[0] https://www.forbes.com/sites/startswithabang/2016/08/03/wher...

Yep, that’s correct. I misspoke (should have said “not due to rest mass”). You can think of this by analogy with the Coulomb Hamiltonian—it is the sum of a kinetic term and an electromagnetic potential term. With the nucleus, it’s similar except the potential term corresponds to the strong force and is even much higher than the kinetic term as you say.

Does gravity affect an electric or magnetic field?

amazing to think about, thank you

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