I don't want to live on this planet anymore.

[The Mad Monk]

It's a thought that's been bouncing around my head since reading a Scientific American article on how Locality isn't really a thing. I think.

[The Mad Monk]

Found it.

In Brief

In everyday life, distance and location are mundane absolutes. Yet physics now suggests that at the most fundamental level, the universe is nonlocal—there is no such thing as place or distance.
Initially Isaac Newton's conception of gravity seemed to imply the phenomenon of nonlocality because the attractive force between masses appeared to act magically across expanses.
Albert Einstein's general relativity instead ascribed gravity to the curvature of spacetime. Yet it introduced a deeper sense of nonlocality by showing that spacetime positions have no intrinsic meaning.

[Lorizael]

I wouldn't say that spacetime positions have no meaning now. We just have to reevaluate what they do mean. In relativity, simultaneity is not a definite thing, but causality still exists. In special relativity, space can be contracted and time dilated, but the distance between any two spacetime points is an invariant "spacetime interval." The spaectime interval is the same for all frames of reference, looks a little like the Pythagorean Theorem, and takes into account the speed of light. In general relativity, spacetime can be curved arbitrarily and you can't necessarily say that we live in a Euclidean universe (we seem to on a universal scale, but not on local scales near big sources of gravity), but the distance between any two points can be calculated if you know the metric tensor.

[Jon Miller]

The principle of locality still very much exists in physics.

In fact, one of the proposed features of quantum gravity is non-locality. This non-locality could (in some proposed theories, none of which currently work) come from applying quantum principles to space and time.

General Relativity is very much a local theory.

Quantum mechanics may be non-local (that is what the EPR paradox first stated and Einstein's problem with quantum mechanics), depending on the interpretation. In the Copenhagen interpretation it is realism that is lost and not locality.

JM

[The Mad Monk]

That's why I like you guys. You think about this stuff so I don't have to.

I might add more when my eye stops killing me.

[Lorizael]

Quantum mechanics may be non-local (that is what the EPR paradox first stated and Einstein's problem with quantum mechanics), depending on the interpretation. In the Copenhagen interpretation it is realism that is lost and not locality.

This is something I wish more pop sci would try to get across. Violations of Bell's inequalities (if you accept the assumptions of Bell's theorem) imply that you can have reality or locality, but not both. Science articles always seem to phrase this in terms of Einstein's spooky action at a distance and say that it's locality we're giving up. I think people sometimes come away from that and say that QM must be garbage if it means (a) Einstein was wrong and (b) FTL without communication is somehow a thing. But if the alternative is to give up objective reality, then perhaps non-locality can be tolerated.

My understanding is that quantum foundations people have a range of more subtle stances, though, with some of them involving giving up on traditional causality, or giving up on objective reality as describing single things, or MWI approaches where what's real is the wavefunction, etc.

[The Mad Monk]

What gets me is this: nothing is ever in contact with anything. Not really, anyway. Atoms interact through various shades of electromagnetism, nuclei are held together with the strong nuclear force, and the weak nuclear force does something with quarks and so on. There's space between everything, no matter how far down you go, and the properties you get depends on how they all dance. Gravity isn't the exception, it's the goddam rule! Just at larger distances than the others; once you adjust for scale, it all looks the same. The forces all "jump" bewteen particles, with nothing to "carry" them. They just are. Do you see what I'm saying? Is this nonsense?

[Lorizael]

Well, the model of quantum field theory says that's not how we understand what's going. In fact, the three forces that aren't gravity are transmitted by particles known as bosons. So the force carrier for electromagnetism is the photon, for the strong force the gluon, and for the weak force W and Z bosons. These virtual particles are flying around (at the speed of light or less), colliding, exchanging momentum and the like, and the effect we see is of particles acting on each other at distance via force fields. So in that respect, force interactions are all local.

Gravity, too, is local as understood in general relativity. Masses aren't instantly affecting each other at great distance; instead, masses are simply following spacetime, which just happens to be curved by the presence of other masses. But this process isn't instantaneous. Gravity waves (disturbances in spacetime), for example, move at the speed of light.

[The Mad Monk]

So when a bat hits a baseball (which is really the electron clouds of their respective atoms interacting and repelling each other), photons are carrying that repulsive force? Oh God I need sleep.

[Lorizael]

Yes, although my understanding is that we're incapable of analytically (or even numerically) calculating the force of a baseball on a bat via the framework of virtual photon interactions. But we know what much simpler interactions look like in terms of electrodynamics, and we can analyze the simple stuff via Maxwell's equations (classical) or QED (quantum), so there's every reason to believe that could be, in principle, scaled up.

[Jon Miller]

Lorizael said it but didn't emphasize it. Interactions involve the exchange of virtual particles.

Also, there are other symmetries/etc that do not have fundamental force carriers but still cause things like repulsion. One of those is that for Fermions only one particle can be in a given quantum state (Pauli Exclusion). This is actually the prime consideration for things such as neutron stars... but there is no hypothetical exclus-ion.

JM

[C0ckney]

What gets me is this: nothing is ever in contact with anything. Not really, anyway.

that could be a novel argument in court: 'you see, your honour, according to quantum physics my fist never really came into contact with the plaintiff’s face, so how can anyone claim that i assaulted him?'

[OneFootInTheGrave]

to bring this thread back on topic

[Elok]

In other news, Lori picked one hell of a field to enter:

http://www.theatlantic.com/science/archive/2016/01/gender-discrimination-astronomy/422817/

[Aeson]

A field about "heavenly bodies" and with names like "Uranus" ... it's basically entrapment