EM, Feynman, and Gravity

[Enola Straight]

Feynman stated that a particle travelling backward in time behaves precicely the same as its anti-particle moving forward in time.

A fellow poster on this board refutes this with a thought experiment:

Take two electrons and hold them close to each other...and let them go. They fly apart due to mutual repulsion of like charges.

If time goes backwards, you have two positrons attracting each other...in violation of the laws governing electromagnetic interaction.




However, this IS how Gravity works, since we only observe mass having a "positive charge".
(The theoretical Exotic Matter...with the quality of negative mass...has not been observed)

Does this inverse-temporal symmetry between G and EM tell us something mathematically deep regarding Grand Unification?


Something not found in Kaluza-Klein or M-Theory?

[Count Chocula]

Nonsense. There is no evidence whatsoever that the concept we label "time" has properties that can be explained by the behavior of charged particles. "Time" in a positive direction seems to be as fundamental to our perceptions as gravity, and equally immune to manipulation. From what I recall of Feynman's lectures, the ones on antiparticles and causality were largely thought experiments. Brilliantly constructed, with mathematics to back them, but no experimental verification.

Besides, we have no theory or proof that "time" affects EM interactions at all. For all we know, "forward" "time" and "backwards" "time" are different, non-interacting dimensions entirely. We're stuck in our own frame of reference.

[von Neufeld]

Feynman stated that a particle travelling backward in time behaves precicely the same as its anti-particle moving forward in time.

A fellow poster on this board refutes this with a thought experiment:

Take two electrons and hold them close to each other...and let them go. They fly apart due to mutual repulsion of like charges.

If time goes backwards, you have two positrons attracting each other...in violation of the laws governing electromagnetic interaction.

No, no, no. If you reverse time you have two positrons moving towards each others and getting caught and held close to each others.

[Dooey Jo]

Exactly. That's a pretty flawed argument, because if they don't behave as positrons, you'd have two electrons "getting attracted" to each other instead, which by the argument's own logic should also be impossible. Fortunately, that's not how time-reversal works. Indeed, the particles must repel each other, because in normal time, they accelerate away from each other, and so when you reverse time they must decelerate when they get closer; which is exactly what would happen if two like charges were to move towards each other.

[Enola Straight]

Parallel discussion on SDMB
http://boards.straightdope.com/sdmb/sho ... p?t=525386

[Simon_Jester]

However, this IS how Gravity works, since we only observe mass having a "positive charge".
(The theoretical Exotic Matter...with the quality of negative mass...has not been observed)

Does this inverse-temporal symmetry between G and EM tell us something mathematically deep regarding Grand Unification?

General relativity does not become electromagnetism under time reversal. It doesn't follow identical mathematical formalism, for starters.

As an example, take some general relativistic phenomenon like a rotating black hole as described by the Kerr metric, and apply time reversal. You don't get any kind of animal that can be covered by Maxwell's laws.

[Steel]

Ok that thought experiment that 'refutes' time antisymmetry is wrong. Well the experiment isnt wrong but your interpretation very much is. Under the time reversal analogy you get 2 positrons being thrown towards each other, repelling as they approach and slowing down until they are grabbed, precisely as you would expect.

Choocula's post deserves special mention for being an exceptional rape of physics.

[Count Chocula]

Choocula's post deserves special mention for being an exceptional rape of physics.

And again we discover why whiskey and science are poor bedfellows. However, I still don't see how we will ever be able to detect, or even set up a tangible experiment, to measure physical objects travelling backwards in time. Or if such a thing as backwards time travel even can exist in the real world.

[Enola Straight]

I didn't mean to imply that reversing time flow toggles between GR and EM.

I simply believed that my flawed observation of EM interactions (opposite charges attract) seemed to behave more like Gravity (like attracting like) depending on the direction of time flow...and that indicated some deep mathematical relationship.

[Simon_Jester]

Yes; it's called the "inverse square law." Which is a fundamental principle of macroscopic E&M, and a handy oversimplification of macroscopic GR.

Remember, gravity doesn't just have "like attracting like." It has "everything attracting everything." There's no evidence of anything having negative mass, or even that such a thing can exist. We can postulate it, but that doesn't make it real.

So there's no reason to assume that 'mass' is a precise analogue to 'charge.'

[Wyrm]

I didn't mean to imply that reversing time flow toggles between GR and EM.

I simply believed that my flawed observation of EM interactions (opposite charges attract) seemed to behave more like Gravity (like attracting like) depending on the direction of time flow...and that indicated some deep mathematical relationship.

It goes deeper than that: they behave exactly like themselves under time-reversal. It just happens that they are both governed by inverse-square laws (mostly), so of course they behave like each other.

[Surlethe]

Choocula's post deserves special mention for being an exceptional rape of physics.

And again we discover why whiskey and science are poor bedfellows. However, I still don't see how we will ever be able to detect, or even set up a tangible experiment, to measure physical objects travelling backwards in time. Or if such a thing as backwards time travel even can exist in the real world.

The point of describing antimatter as matter traveling back through time is not to generate some physical experiment that can reveal deep things about the nature of time, it's to tap into a convenient mathematical heuristic to better help describe matter and antimatter. Anyway, the idea that you have to be able to directly observe any particular assumption of a physical model is silly; we confirm or refute heuristics by indirect observation all the time. Not being well-versed in arcane quantum field theories, all I can say is: if QED incorporates this assumption, then I'm inclined to believe it even if we can't directly observe it simply because the predictions QED generates are so mind-blowingly accurate (as in, "measuring the distance from New York City to Chicago to within 10 mm" accurate).

[Kuroneko]

It's not an assumption. What's an assumption is that the Hamiltonian is hermitian, which is used to guarantee that the energy spectrum is real. It is a mathematical theorem that any such local Lorentz-invariant quantum field theory is going to be symmetric under simultaneous inversion of charge (matter↔antimatter), parity (spatial), and time reversal. This, time reversal is always equivalent to a simultaneous charge and parity inversion. However, electrons don't distinguish between left and right, so T-reversal is equivalent to C-inversion in QED.