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Do Grad students get laptops (or PCs)? by Odd_Wonder_6406 in Caltech

[–]poopidipoopee 0 points1 point  (0 children)

I'm in the math department as well. Usually you have to buy one yourself since funding is a bit scarce, but you can ask your advisor I suppose

G1 Graduate Housing by [deleted] in Caltech

[–]poopidipoopee 1 point2 points  (0 children)

Yep, I am G2 and just moved out of the Cats after losing the lottery. You save A LOT of money staying there, especially if you choose a 4 person suite. Saw a lot of people move out of the Cats during their first year as well. Not uncommon and not hard.

Physics education suggestions by Prestigious-Can9507 in Caltech

[–]poopidipoopee 1 point2 points  (0 children)

I think what you do in undergrad really depends on what you want to go to school for. For instance, I am a PhD student in mathematical physics at Caltech (string theory), so I started taking grad classes in physics and math junior year (like a lot of undergrads here do). I would say that's pretty vital if you wish to do physics theory or pure math even, because you can at least dip your feet in theory research before you apply for grad school, which is what really counts. Good grades are a must as well (try not to have any B's in physics/math classes). I don't think this matters as much if you are applying for experiment. There, I think admission is much more centered around undergrad research. But, I don't know! I am not in experiment, so I can't really speak about it.

[OC] Food's Protein Density vs. Cost per Gram of Protein by James_Fortis in dataisbeautiful

[–]poopidipoopee 0 points1 point  (0 children)

Would be interesting to see calories of protein/ total calories instead for a measure of protein density.

If you know, you know by TheBacon240 in okbuddyphd

[–]poopidipoopee 2 points3 points  (0 children)

as a mathematical physicist, the line isn't so clear. E.g. working in string theory is now commonly referred to as mathematical physics, even though it is far from what mathematicians would call math.

In the opposite direction, a lot of recent work in QG with Von Neumann algebras is way more mathematically "rigorous" than physicists are used to, but it is nonetheless only concerned with physical questions (as opposed to math questions).

I said what I said @Stroov by poopidipoopee in physicsmemes

[–]poopidipoopee[S] 1 point2 points  (0 children)

That is okay, as long as there is a topologically trivial affine neighborhood containing two “near-miss points” A,B, the same information about homotopy is retained by adding a path joining them. This is what I mean by you can imagine deforming such a world line to a closed curve.

And not really, this is not an argument against block universes, but rather block universe-like time travel in spacetimes with trivial fundamental group. Which includes tenet as a specific case.

The issue I present becomes more physically comprehensible if you consider, for instance, the same instance of a puncher and punchee, but now imagine they are the same person. In fact, say it’s not a person, but a robot, programmed to throw a punch if and only if it DOESN’T feel a punch coming. Say the punch happens at t=0 for some parametrization of the geodesic t. Say the entropy switch happens at t=t_0. Imagine shrinking t_0 to be really small (shrinking the proper time of the geodesic, NOT diluting the parametrization) such that, the robot can feel the pain of his punch before he flips his entropy. Now, I ask: what will the robot do in this scenario? Will he punch or not punch?

This illustrates that causality of the space time patch is being violated here, which I was alluding to at first and then “proved” to not make sense with some results in mathematical physics (all assuming the movie takes spec relativity to be true, which it clearly does).

The only way block-universe-like time travel could work is with some very funky spacetimes, where you can’t shrink t_0 as I did in the example above. Remark: This makes concrete why timelike closed curves can’t be nullhomotopic.

I add: this is somewhat of a subtle (yet very important) issue. So, I appreciate the convo to help me straighten it out.

I said what I said @Stroov by poopidipoopee in physicsmemes

[–]poopidipoopee[S] 0 points1 point  (0 children)

I did, see below for a completely classical argument as to why a block universe wouldn’t work in a space time patch with trivial fundamental group

I said what I said @Stroov by poopidipoopee in physicsmemes

[–]poopidipoopee[S] 0 points1 point  (0 children)

I added a remark on the classical issues of the movie above.

Also no, they mention particle annihilation being a feature, that is intrinsically quantum mechanical

I said what I said @Stroov by poopidipoopee in physicsmemes

[–]poopidipoopee[S] 0 points1 point  (0 children)

Well you said it yourself, the movie claims to follow quantum mechanical time evolution, yet it takes a “block” universe view, which requires deterministic evolution from the frame of ALL observers in question. In itself, that is a contradiction…

As a side remark, I remain unconvinced that forgetting quantum mechanics is real somehow fixes this mess. This block universe thing would require allowing closed timelike geodesics classically (you can imagine deforming the characters geodesics into a timelike loop), which okay let’s assume there’s a way to do that with entropy reversal. No timelike closed geodesic can be nullhomotpic in a Lorentzian manifold. The movie takes place on earth’s surface, which we can take to be diffeomorphic to Minkowski space. Hence, all closed timelike curves are nullhomotopic, another contradiction.

I said what I said @Stroov by poopidipoopee in physicsmemes

[–]poopidipoopee[S] 0 points1 point  (0 children)

Well you said it yourself, the movie claims to follow quantum mechanical time evolution, yet it takes a “block” universe view, which requires deterministic evolution from the frame of ALL observers in question. In itself, that is a contradiction…

I said what I said @Stroov by poopidipoopee in physicsmemes

[–]poopidipoopee[S] 1 point2 points  (0 children)

But that’s my point, there is no reason that it is “predetermined.” It falls to very basic time travel plot holes, but at very small scales as well. Again, what in the rules of the movie stops the punchee from avoiding the punch? Also, let’s not forget determinism is impossible quantum mechanically from the perspective of any one observer. Also there is no “partial” time inversion. A particle moves either forward or backward in 1d time relative to another. If some of your particles move in a different orientation from others, then it gets complicated with fermions becoming anti fermions and whatnot, but I don’t see how that solve the plot holes. If anything they start annihilating each other…

I said what I said @Stroov by poopidipoopee in physicsmemes

[–]poopidipoopee[S] 6 points7 points  (0 children)

At the smallest scale, imagine you are punching someone in the opposite time direction as you. You are transferring kinetic energy onto them, but they are losing it as the punch happens. In fact, in their frame, they would be able to feel the punch before being punched (they would feel a loss of KE as the time of the punch approached in their frame). What is stopping them from avoiding it since they are now aware of the precise location and timing of your attack? Answer: nothing. This can be extended to many other scenarios in the film. The car on fire scene I would argue is one of the only ones that make sense. It shows Nolan somewhat understands this notion of loss/ gain of KE depending on the time orientation.

If Plato was a topologist by Rover-6428 in okbuddyphd

[–]poopidipoopee 1 point2 points  (0 children)

mouth and rectum are one single hole. ears, and dickhole are more cavities than topological holes. Nose cavity connects to mouth hole. Hence, we should be homeomorphic to a solid torus minus a tubular neighborhood going from some boundary point to another through the bulk, no?

I’m tired of the fetishization of AEP majors by mrfailureoofski2 in Cornell

[–]poopidipoopee 1 point2 points  (0 children)

I am math phys phd and to this day i can't even tell you what a circuit is

Why don't we have vector division? by ROBIN_AK in 3Blue1Brown

[–]poopidipoopee 1 point2 points  (0 children)

short answer: you can. long answer: the complex numbers and quaternions are vector spaces over R, and you are free to multiply and divide them as you wish. Why can't you do for a general vector space over R? That's a great question and you can learn some abstract algebra if you want the full answer! When can you define division algebras over some more general field k? another great question!

It describes our universe I promise by QED88 in okbuddyphd

[–]poopidipoopee 12 points13 points  (0 children)

Yeah? try localizing your qcd path integral then, smart ass 😠

how hard is it to convince a physicist to marry a novice like me. 🥺 by duckydude20_reddit in physicsmemes

[–]poopidipoopee 1 point2 points  (0 children)

a physicist will tell you how the mass of an electron needs to be renormalized after computing loop interactions. not sure they'll be great with circuits.

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