Mastodawn

ferret.imploading (xylia)Sep 3, 2024

since we live in 4dimentions

why do we say where and when?

shouldnt we just say where? and include the time in that? why do we distinguish time as seporate from the other 3 dimentions?

i know there are diforences(like we cant decided direction or speed through time) but why is it seporrate for there something is going to happen

why is it "time and place" and not just place?

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𐌀𐌌𐌙 Sep 3, 2024

@BlahajBlast interestingly everything in the universe moves at the same speed through four dimensions: c, the speed of light. It's defined as the magnitude of the four-vector that describes your motion. For an object that is not moving spatially, their velocity through the time dimension is c. If you do have spatial velocity, the speed of time slows down for you such that your total velocity in four dimensions remains c, when you add up the contribution from each dimension.

What's interesting about this is that massless particles follow the same rule, but they travel through the spatial dimensions at speed c, which implies a speed through the time dimension is 0. So it's valid to say that a photon doesn't experience time passing, no matter how far it travels.

What were we talking about again?

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ferret.imploading (xylia)

@k2tog yeah! i know its so cool! (i have been studying physics since i was like 7 and thos is one of my favourite areas of the subject please info dump more if you want)

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𐌀𐌌𐌙 Sep 4, 2024

@BlahajBlast omg yes photons are so cool! This gives me a chance to bring up one of my favorite quantum mechanics demonstrations, that's really only tangentially related, 'asking photons where they have been'. The experiment doesn't actually uncover anything new, observationally, but it makes the two state vector formalism of quantum mechanics seem really appealing as an intuitive explanation of why certain observations actually happen. Given two state vectors for a photon, one that describes forward progression through time and one backward, you can find that they only agree some of the time about where a photon can exist, and these agreement points comprise the photon's experiences, as observed when you collapse the wave function.

The experiment tries to give each photon a memory of which mirrors in the contraption it has bounced off of using vibration. When the photon hits the detector at the end, the frequency distribution can be observed and the peaks show where the photon has been. The experimenters then create a set up where the frequency distribution showed that, of the photons that hit the detector, most of them came from an arm of the set up that couldn't possibly reach the detector.

The reason this makes sense in the tsvf because a photon traveling backward in time from the detector can end up in the dead end arm of the experiment, even though forward traveling photons in the arm will never reach the detector, due to clever beam splitting placement.

Really interesting experiment: https://arxiv.org/abs/1304.7469

Asking photons where have they been

Quantum mechanics does not provide a clear answer to the question: What was the past of a photon which went through an interferometer? Various welcher weg measurements, delayed-choice which-path experiments and weak-measurements of photons in interferometers presented the past of a photon as a trajectory or a set of trajectories. We have carried out experimental weak measurements of the paths of photons going through a nested Mach-Zehnder interferometer which show a different picture: the past of a photon is not a set of continuous trajectories. The photons tell us that they have been in the parts of the interferometer which they could not have possibly reached! Our results lead to rejection of a "common sense" approach to the past of a quantum particle. On the other hand, they have a simple explanation within the framework of the two-state vector formalism of quantum theory.

arXiv.org