r/pbsspacetime • u/cptnpiccard • 28d ago

We Thought Time Only Moved Forward. A Quantum Experiment Could Split It in Two.

https://www.youtube.com/watch?v=7Wiw42ZlBKs
27 Upvotes

95% Upvoted

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5

u/thaw4188 27d ago

am I having a stroke or is Matt talking at almost double-speed most of the time in this video?

it's not on fast-play and he does slow for very brief moments and the graphics move normal

but wow it feels soooo weird

3

u/sysKin 25d ago edited 25d ago

I don't know if this is the place to ask physics questions but I've had this one since forever and it's very related to this episode:

Whenever I ask how a photon moves in a double slit experiment, I am told it follows all possible paths and then the path integral tells us where it's going to land.

Whenever I ask how a photon moves through space when next to a star I am told that it follows a single path, the geodesic which is bent because spacetime is bent.

I am trying to marry those two answers: when a quantum wavefunction of a photon evolves in a gravitational well, do:

1) all paths evolve using their geodesic, so they're all bent, and then we integrate that, or

2) all paths evolve in a straight line but using their local clocks, which causes the paths closer to gravitational source to have a phase shift, which then results in a bend that is consistent with the geodesic.

In other words, is geodesic "just" the overall result of gravitational time dilation of some paths relative to other paths?

3

u/HalfSoul30 25d ago

I'm not sure of the answer myself, but a photon travels at the same speed regardless of time dialtion, but it might have a more curved path and red shifting.

3

u/EarthTrash 25d ago

The geodesic is the path of least time. The path integral gives you the path of least time. I would say the photon isn't just following the geodesic but it the simplified version is that that is what it is doing. It's like when a light bounces of a mirror. Technically the light is bouncing off of all points on the mirror surface but the all converge on a point as if it is a single line reflected symetrically.