As the title says. I'm a 28 year old physicist, working as the lead engineer for a large space and defense manufacturing company. Part of my job is to work directly with all engineering team heads, doing project engineering, project management, and systems engineering. AMA

Original post: https://www.reddit.com/r/AMA/s/ziHdUgtHrt

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

Are electrons stationary or do they move? Let's say you had 3 equal lengths of copper wire. A. Sits on a desk B. Used to complete a DC circuit C. Used to complete an AC circuit

Over time the electrons in piece A will always be the same electrons because it's not connected to anything. Is it fair to say that the electrons in B would flow in one direction like a river of electrons and C would dance back and forth at 50hrz/60hrz?

Researchers have used electrochemistry to increase the rates of nuclear fusion reactions in a desktop reactor. When a beam of deuterium was fired at the deuterium-filled palladium, they saw a 15% increase in fusion events. Link to the publication:

https://www.nature.com/articles/s41586-025-09042-7

August 2025

I’m thinking of applying for PhD programs in the US with a focus on string theory, and I’m trying to get a sense of how competitive it is.

From what I gather, string theory positions are pretty limited compared to other areas, and a lot of US departments now emphasize condensed matter, quantum information, or phenomenology. For string theory, the number of faculty taking students seems small, and you often have to match very closely with someone’s research.

For anyone who’s gone through this process:

• How tough is it to get admitted specifically for string theory?
• And what about funding issues?

I’d love to hear about recent experiences or advice.

Starting Mechanical Engineering in a month and a half and would like to rewise some material on mechanics. Used to use coursera, but now they changed their policies on free courses. Do you guys know some courses like org chem tutor for maths or smth?

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.

I’m choosing electives this semester:

• Thermodynamics & Statistical Physics (definitely taking this)
• FLUKA
• Cosmic Rays

The issue:

• Cosmic Rays is taught by a poor lecturer, so I’m considering skipping it.
• FLUKA is relevant for HEP, but I’m not sure if admissions committees or future supervisors will actually value it.
• Instead of Cosmic Rays, I’d like to take German (I’m already B1 and want to improve for studying in Germany).

My question:
Would taking Cosmic Rays and/or FLUKA give me a real advantage for grad school applications or research in HEP, or are they things I can learn later?

Hi all,

When I was in undergrad, we learned that you can do some averaging of the Schrödinger equation and get something sort of like F = MA (although closer to something like -<U> = m d/dt <v> where <> is an average over a large amount of particles).

Now that I’m studying fluid dynamics in my graduate studies, when we study two-phase systems (such as water and air) we often consider a surface tension coefficient to solve for both velocity fields using a jump boundary condition in stress in the normal and tangential directions of the air-water boundary.

I was talking with another graduate student about some philosophy of math stuff about when there is a “lower level description” that maps onto a “higher level description” ie kind of some emergence-like discussion. The Schrödinger equation mapping onto Newton’s second law seems like one such example, but I’m wondering if the same thing exists for surface tension using (I’m guessing?) molecular dynamics onto this description in Navier stokes problems. Seems like something I should just know, but I don’t :). I’m aware that the continuum hypothesis assumes some descriptive length scale used in NS is much greater than the mean free path of fluid particles, so I’m not sure how to go from one to the other.

Anyone have any idea about this? Thank you :)

The title. I have the impression that complex systems are controversial, as if it is really valid to apply physics tools and frameworks to understand emergent phenomena, not just physical in the traditional sense.

I'm referring to things like vote models based on the Ising model, modeling bird flocks and bacterium, the works.

I'm personally interested in the field, but sometimes I have the weird sensation that maybe it's bulls***t.

What do you think?

Edit: I should add that this is not an attack against this discipline and those who practice it. I myself do so, like for my thesis. This post was more like a 3:00 AM "what am I doing with my life" thought than "yo what's up with these bozos"

Hi everyone,

I’m looking for some advice on grad school pathways and would really appreciate some outside perspective.

*Background Information\*

I'm currently a 3rd year Honors Physics major at Bishop's University (in Quebec Canada). By the time I graduate I'll have about 12K CAD in government debt. Finally, I hold Canadian, US, and Uruguayan citizenship (so I have a bit of flexibility with where I can study). I'm originally from Uruguay.

*My Goals\*

My final goal academically is to be able to get a PhD in Physics, with a focus on Theoretical Cosmology or Theoretical Particle Physics. I’m debating whether to first do an MSc (in Canada or abroad) or try to go directly into a PhD program (like in the US).

Funding and cost of living are really important for me. I don’t want to take on huge debt, but I’m open to reasonable loans if it’s worth it for the long run. I also don't necessarily want to be tied down to one country/city for really long (3+ years) unless its really worth it, I want to be able to explore new countries. So ideally I'd want a scenerio where I can leave in 2-3 years if I don't like it but also stay for longer if i do.

*My Options\*

• MSc in Canada: My realistic options are "University of British Columbia" or "University of Victoria", I know there are other options with great physics programs but I don't think I can survive an extra 2 years in -20 weather.
  
• PhD in USA: I have a few options that interest me but I'm mostly worried about the current situation of funding for PhD student in the US. I've heard a few stories of people being rejected or simply defunded 2 years into their program. But I do know that out of all the options the US has the most prestigious programs.
  
• MSc in Europe: I've been checking for universities in Europe and I've been intrigued of living in Spain or Italy and I know they have some good programs. The main issue of doing an MSc in these countries is I'd probably have to take out a 20K CAD loan to do the 1 year degree, I don't know if its worth it.
  
• MSc in Latin America: In this case I would apply only to "National Autonomous University of Mexico", "University of Chile", "University of Buenos Aires", and "National University of Colombia". Since I'm latin american these are the countries that culturally fit the most with myself. What worries me hear is the lack of general funding and the prestigiousness. If i do my MSc here would I theen be able to my PhD in Europe.

*Summary\*

I have talked about this with my advisor and parents but I still want an outside opinion.

Given my situation (triple citizenship, moderate debt, physics BSc from Canada), would it be smarter to:

• Stay in Canada for an MSc then apply for funded PhDs abroad?
• Go straight to a funded PhD in the US?
• Take out a loan (~20k CAD) to do a n MSc in Spain or Italy and then aim for a PhD?
• Take out a smaller loan (if needed) to do an MSc in Latin America and then aim for a funded PhD abroad?

Thanks in advance!

Hello (I use automatic translation) How do we explain the reflection of photons in corpuscular theory? but also with what formula or means can we model it. If you have course or study documents on the subject, I am interested. Thank you have a nice day

I know that moving electrons generate a magnetic field, and that magnetic fields can generate power, but what I don't understand is something like a neodymium magnet. I cannot perceive any motion, and yet there is a magnetic field. How does that work? Is there, in the seemingly static structure of a magnet, electronic motion? If there isn't, what's going on there?

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.

I don't care how many of them there are or anything, just the idea that states of matter actually exist and take energy to change between.

I always thought matter was just a continuum and we demarcated a point where it was moving a little, a lot, and a whole lot, more or less, and that point (while fairly accurate) was more of a human construct than a state of nature.

But I was reading about how temperature doesn't actually increase in a object during a phase change until it has changed to the next state of matter.

Is there any debate on this or doubts about the existence of states of matter or is it more or less a case closed sort of deal?

Edit: Follow up, since people seem to be saying states of matter are actually thought to exist, how could any physicists not believe in god/higher power if the universe has such an exact ordering of things? It's almost too magical.

Obviously it could just be the way things are, but man is that weird. I always thought the states were more of a Pratchett-esque "fairy stories for children" type of deal.

(I am an atheist, but come on).

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)

Source : "ALSJ Apollo 14 Tape-01-1" on YouTube.

Timestamp: 10:05

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.

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