Sasha

At this point Labor is making a game out of opening new extraction projects after the net zero deadline...

ODing on ibuprofen is terrifying

Hate to say that video has been on my mind pretty much constantly these last 6 or so months...

Me

It's been one of three things for me:

• Like having a cool friend you see a lot. Good memories, but never a relationship that lasted.
• Someone working out how to twist my own mind against me, controlling me for their own gain and never actually understanding that a relationship isn't a transaction. (and hopefully that one isn't stalking me on lemmy again, otherwise I or one of my friends will get harassed and I'll be filling out an intervention order)
• The most beautiful thing that I didn't know was possible, I thought I'd been happy before but when you meet the right person you really do just click and life becomes worth living. Never felt so good about myself as I did then, just hurt all the more to lose it so suddenly. I've written a lot about others I've only met briefly, songs about people who'd never think of me that way, but when it's true love I just can't. I don't think anything I can say could really capture that. There just isn't enough poetry in the world to describe how magical it is to look up at a pair of beautiful brown eyes swimming in a field of stars and hearing them say "I love you" for the first time.

I've only ever done QFT in curved spacetime, but I don't see any reason why you couldn't do EM, it'll be a vaguely similar process. I never actually dealt with any scenarios where the curvature was that extreme, and QFT in a curved background is kinda bizarre and doesn't always require one to consider the specific trajectories, though you definitely can especially if you're doing some quantum teleportation stuff. In my area it's simpler to ignore QED and to just consider a massless scalar field, this gives you plenty of information about what photons do without worrying about polarisations and electrons.

It's been a long time since I did any reading on the geometric optics approximation (in the context of GR this is the formal name for light travelling on null geodesics), but for the most part it's not something you have to consider, even outside of black holes the curvature tends to be pretty tame (that's why you can comfortably fall into one in sci-fi), so unfortunately I don't know of any phenomena (in GR) where it's important. QFT in curved spacetime generally requires you to stay away from large curvatures, otherwise you start entering into the territory of quantum gravity for which there is no accepted theory.

Outside of GR, it breaks down quite regularly, including I believe, for the classic double slit experiment.

Edit: Another really cool fact about black holes is that even when you've got really large wavelengths, it often doesn't matter because they get blue shifted to smaller wavelengths once you get close to be horizon.

On that first point, calculating spacetime metrics is such a horrible task most of the time that I avoided it at all costs. When I was working with novel spacetimes I was literally just writing down metrics and calculating certain features of the mass distribution from that.

For example I wrote down this way to have a solid disk of rotating spacetime by modifying the Alcubierre warp drive metric, and you can then calculate the radial mass distribution. I did that calculation to show that such a spacetime requires negative mass to exist.

Yeah, once you add in a second mass to a Schwarzschild spacetime you'll have a new spacetime that can't be written as a "sum" of two Schwarzschild spacetimes, depending on the specifics there could be ways to simplify it but I doubt by much.

If GR was linear, then yeah the sum of two solutions would be another solution just like it is in electromagnetism.

I'm actually not 100% certain how you'd treat a shell, but I don't think it'll necessarily follow the same geodesic as a point like test particle. You'll have tidal forces to deal with and my intuition tells me that will give a different result, though it could be a negligible difference depending on the scenario.

Most of my work in just GR was looking at null geodesics so I don't really have the experience to answer that question conclusively. All that said, from what I recall it's at least a fair approximation when the gravitational field is approximately uniform, like at some large distance from a star. The corrections to the precession of Mercury's orbit were calculated with Mercury treated as a point like particle iirc.

Close to a black hole, almost definitely not. That's a very curved spacetime and things are going to get difficult, even light can stop following null geodesics because the curvature can be too big compared to the wavelength.

Edit: One small point, the Schwarzschild solution only applies on the exterior of the spherical mass, internally it's going to be given by the interior Schwarzschild metric.

Yeah it would fair point, I'll be honest I haven't touched Newtonian gravity in a long time now so I'd forgotten that was a thing. You'd still need to do a finite element calculation for the feather though.

There's a similar phenomenon in general relativity, but it doesn't apply when you've got multiple sources because it's non-linear.

Possibly?

A bowling ball is more dense than a feather (I assume) and that's probably going to matter more than just the size. Things get messy when you start considering the actual mass distributions, and honestly the easiest way to do any calculations like that is to just break each object up into tiny point like masses that are all rigidly connected, and then calculate all the forces between all of those points on a computer.

I full expect it just won't matter as much as the difference in masses.

I actually thought the answer might be never, but a quick back of the envelope calculation suggests you can do this by dropping a ~1kg bowling ball from a height of 10^-11^m. (Above the surface of the earth ofc)

This is an extremely rough calculation, I'm basically just looking at how big a bunch of numbers are and pushing all that through some approximate formulae. I could easily be off by a few orders of magnitude and frankly I didn't take care to check I was even doing any of it correctly.

10^-11^m seems wrong, and it probably is. But that's still 1,000,000,000,000,000,000,000,000 times further than the earth moves in this situation. Which hey, fun What If style fact for you: that's about the same ratio of 1kg to the mass of the Earth at ~10^24^kg.

That makes perfect sense because the approximations I made are linear in mass, so the distance ratio should be given by the mass ratio.

This is not correct, the force on the objects is the same sure, but the accelerations aren't so you can't calculate them both in one go like this.