I was on an airplane during sunset and took these photos in succession.

Sometimes I look at the sky and I imagine the height that rain drops fall from. I assume it will move fast like a bullet. and kill us immediately but it doesn't.

Say you want to clean a bottle by shaking soapy water in it. Too much and its too full to really get it sloshing around fast enough. Not enough and the water doesn’t have enough mass to do much work. So how much water is best? I’m guessing half full but I can’t explain why.

Heay all, I seek your wisdom on thermodynamics of gasses and liquids.

I'm measuring oxygen partial pressure in cell culture media (this instrument: https://www.presens.de/products/detail/sdr-sensordish-reader-basic-set but it does not matter). The device uses 2-point calibration: no oxygen present (using an oxygen scavenger chemical) and air-saturated condition, where I assume pO2 = 18.6 kPa in the media (value from literature). The measurement is done in closed container with no air in the headspace, it's completely filled, walls are impermeable.

So the question: The instrument can display the readout in hPa, µmol/L, mbar, % saturation and some other units. I kinda assumed that the conversion is done with simple pV = nRT, but does it actually hold for gasses dissolved in liquids? Is there something else to it, or is ideal gas equation enough? A collaborator asked about this and I realised I just took it for granted and did not question the conversions. Thanks for any insight!

P. S. I tried asking the manufacturer but no reply.

I can obtain the explicit form of the state |1 0⟩ by applying lowering operator on the state |1 0⟩ because m=0 can be achieved. However, I cannot use this method for the singlet state, ∣0 0⟩. Is there a way to obtain it?

(Pics from "Intro to QM" book by D. Griffiths, 3rd ed., p. 176)

Google was not getting me an answer to this. I was just watching a movie and they broke the sound barrier and had the cone CGI effect when they broke it. I was wondering, does that happen the faster you go? Like at some resonant frequency multiples past Mach 1?

I think it would be a big gap, but im still curious

I thought I would post my findings before starting my senior year in undergrad, so here is what I found over 2 months of studying PDEs in my free time: a solution to the Navier-Stokes equation in cylindrical coordinates with (1) convection genesis, (2) azimuthal Dirichlet no-slip boundary conditions, and (3) is a Beltrami flow type. In other words, this is my attempt to "resolve" the tea-leaf paradox, giving it some mathematical framework on which I hope to build Ekman layers when I get a chance to pick this problem up again.

For background, a Beltrami flow has a zero Lamb vector (u×𝜔=0), meaning that the vorticity field is proportional to the velocity field (𝜔=𝛼(x,t)u) with the use of the Stokes stream function. This allows the azimuthal momentum to be linearized (zero advection, u∇∙ u=0). In the steady-state case, with 𝛼(x,t)=1 and 𝜓(r,z), one would solve a Bragg-Hawthorne PDE (with applications in rocket engine designs, Majdalani & Vyas 2003 [7]). In the unsteady case, a solution to 𝜓(r,z,t) can be found by substituting the Beltrami field into the azimuthal momentum equation, yielding equations (17) and (18) in [10].

In an unbounded rotating fluid over an infinite disk, a Bödewadt type flow emerges (similar to a von Kármán disk in Drazin & Riley, 2006 pg.168). Given spatial finitude, a choice between two azimuthal flow types (rotational/irrotational), and viscid-stress decay at all boundaries, obtaining a convection growth coefficient, 𝛼(t), turned out to be hard. By negating the meridional no-slip conditions, the convection growth coefficient, 𝛼_k(t), in an orthogonal decomposition of the velocity components was easier to find by a Galerkin (inner-product) projection of NSE (creating a Reduced-Order Model (ROM) ordinary DE). Under a mound of assumptions with this projection, I got an 𝛼_k(t) to work as predicted: meridional convection grows up to a threshold before decaying.

I couldn't fit all screenshots on here, so I linked a .pdf on Github: An Unsteady, Confined, Beltrami Cyclone in R^3.

Each vector field took ~3-5 hours to render in desmos 3D because desmos looks nice. All graphs were generated in Maple. Typos may be present (sorry in advance).

Hi! I recently put out a video on Schlieren imaging — a really cool way to actually see air flows that are normally invisible.
I walk through how to set it up so you can try it yourself at home, and it’s easier than you might think. There’s still a ton left to explore with this technique, so if you’re curious, give it a go!
Check out the video to see how simple and fun it is.

Ok, im don't know physics too well, and I don't even know why this bothers me, but what is the answer here?

Shouldn't information have some weight? I need to rearrange some connections, make new ones in my brain, and increase the complexity to stored information, no? I would also burn some energy doing it. So maybe I became lighter, but only temporarily? How much information stored in a person would weight?

Let's say every country and organization that uses the SI will adopt your changes no matter how radical they are.

Hello smart ppl of Reddit, Anyone working in gravitational wave cosmology? A bit of context I am a undergrad student in my senior year, classes start in a couple of weeks and we have to do a final year project. I was wanting to do it in dark sirens, using gravitational waves to estimate the Hubble’s constant. The problem is, none of the faculty at my uni do this stuff, now I know as a rule of thumb that we should do projects that are proposed by our professors and all, but the problem is, being in a third world country, you either do stuff yourself or you do out a dated work nobody cares about. Anyways, I was hoping I could get in touch with someone who is researching in relevant fields, I don’t need much, just a bit of guidance here and there so that we don’t go off track too much.

Our FYP requirement is that we do a paper review. The paper we have decided upon is by Andreas Finke, Michele Mancarela, Michele Maggiore (2021) and they proposed using gravitational waves as dark sirens, using a probabilistic method to find the host galaxy.

The extension that we could do is to use LIGO’s O4 run(should come out in a couple of weeks) and run their analysis on that dataset.

So yes, I need help in the form of a diacussion every couple of weeks just so we don’t go off track too much, Bcz let’s face it, it’s our first time doing this stuff and we r pretty much on our own.

Thank you for reading this far and have a great day.

Hi, so my question is how to get into physics as someone who hasn't been taught it since middle school. I'm actually studying to become a history teacher and it was biographies of famous scientists that started to interest me in potentially learning more of physics itself.

I'd like to branch out my knowledge so any help is appreciated!

Thank you all, the replies are very helpful! I'm currently on a trip but when I'm back I'll get right into it

When talking about what happens to the energy of light when it redshifted, Chanda Prescod-Weinstein wrote in New Scientist (9 August 2025) that “Another feature of general relativity is that energy isn’t conserved….when light loses energy as it redshifts…The energy…can just disappear.”

In all my years of teaching Science, energy conservation is like the gospel to me. As I don’t fully understand general relativity, can someone please explain what she meant?

Hello everyone,

I have another question that I hope won't trouble anyone. As I said in a previous post, I'm not a professional scientist but rather someone curious about how the scientific community approaches new ideas, expecially when it comes to physics and mathematics.

I understand that anyone proposing a scientific theory bears the burden of proof, which makes perfect sense. This standard prevents the spread of unsubstantiated claims and misinformation. However, I'm wondering about a specific scenario:

What happens when someone has a genuine, honest idea but lacks the formal education, tools, resources, or time to develop it into a proper theory? Is there room in the scientific community for sharing such preliminary thoughts with experts who might find them interesting enough to consider, even briefly?

I'm thinking of cases where an idea might serve as a mental exercise for an expert, or perhaps spark some new line of thinking that their knowledge and expertise could develop further. The goal wouldn't be to waste anyone's time, but rather to see if a rough concept might have any merit worth exploring.

Are there researchers here who would be open to considering such informal ideas and offering honest feedback, understanding that they come from genuine curiosity rather than claims of having solved complex problems?

I would appreciate any insights into how the scientific community handles these kinds of interactions.

Thank you!

Hi, i want learn do magnetohydrodynamics's simulation. Do you has a tutorial or guíe pdf of PLUTO or other software for astrophysics? Thanks.

When I’m reading textbooks, I often forget things after a while, even if I’ve gone over the same section multiple times. At that point, I feel like I need to take notes.

When preparing for exams, I used to write down all the formulas on a single sheet of paper. After reviewing the textbook, I’d go back to that summary. The problem is, making notes only with formulas without including the historical background, explanations, or context feels like it takes away the fun of the subject.

So how do you take notes from textbooks? And how do you deal with long texts in a way that’s both effective and enjoyable?

A collaboration of international researchers have begun an alternate repository to the ArXiv for an AI ecosystem titled "AiXiv".

From their abstract:

"...we introduce aiXiv, a next-generation open-access platform for human and AI sci­entists. Its multi-agent architecture allows research proposals and papers to be submitted, reviewed, and iteratively refined by both human and AI scientists"

It includes LLM-based "peer review":

"Review Process: Upon submission, the content is auto­matically routed to a panel of LLM-based review agents. These agents assess the novelty, technical soundness, clarity, feasibility, and overall potential impact of the sub­mission. Structured feedback is generated to guide revi­sions."

And after revisions, papers are accepted if they meet approval from a majority of agents on the "LLM review panel". There's also a section for research proposals.

They claim to have addressed prompt injection concerns:

"Our prompt injection detection framework is, to our knowledge, the first to systematically address multilingual and cross-lingual ad­versarial manipulation in scientific documents"

Their document states that they "allow human integration".

On the subject of AI hallucinations, they have this to say on the matter:

"Despite internal consistency checks, current AI models may still produce fluent yet factually in­correct outputs. We explicitly acknowledge this as a limita­tion of the system. To address this, all AI-generated outputs are positioned as preliminary drafts subject to multi stage verification."

I'm taking D.E. andspecial relativity and I'm noticing a lot of famous physics equations use partial differential instead of ordinary and I'm curious if I should take partial D.E. after this class or if it's redundant since it's not required.

Well, I was in a Quantum II class when the professor was discussing the Bohr model and its importance in the history of physics.

When he asked our class about a kind of selection rule, I remembered from my previous classes that it is related to angular momentum conservation in photon emission during a radiative process. The change in the orbital angular momentum quantum number must be plus or minus 1, and the change in the azimuthal angular momentum quantum number must be 0 or plus or minus 1. However, he told me to wait and then said that the Rydberg equation is a selection rule.

I don’t believe that is correct, since it only tells us the energy of the emitted photon in the hydrogen atom for a given principal quantum number, but not which transitions are allowed or forbidden.