Photograph via snooOG

For physicists and physics students. See the rules before posting, and the subreddit wiki for common questions. Basic homework questions are not allowed.

/r/Physics is for physicists, scientists, graduate and undergraduate physics students, and those with a passion for physics. Posts should be pertinent and generate discussion.

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Encouraged submissions

Open-ended discussions
  • Debates and discussions on all topics related to physics are welcome. Please make an effort to engage the community rather than simply stating your views.

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Encouraged in weekly threads

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Careers questions

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Homework problems

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  • The title of your submission should accurately reflect its contents. If in doubt, use the title of the original research.

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Weekly schedule

All threads are posted at 9am EDT (1pm UTC).

Day Post
Mon What are you working on?
Tue Physics Questions
Thu Careers/Education Questions
Fri Resource Recommendations


2,552,426 Subscribers


AI with Physics

With the development of artificial intelligence (AGI) and the use of deep learning/machine learning in research areas, what kind of things do you think will happen in the field of physics and quantum physics? Will there be things that will completely change humanity?

21:05 UTC



Need help with that question I'm self studying and need help with that

I chose D but answer is A

I chose D because of the net torque

20:55 UTC


Careers/Education Questions - Weekly Discussion Thread - April 04, 2024

This is a dedicated thread for you to seek and provide advice concerning education and careers in physics.

If you need to make an important decision regarding your future, or want to know what your options are, please feel welcome to post a comment below.

A few years ago we held a graduate student panel, where many recently accepted grad students answered questions about the application process. That thread is here, and has a lot of great information in it.

Helpful subreddits: /r/PhysicsStudents, /r/GradSchool, /r/AskAcademia, /r/Jobs, /r/CareerGuidance

13:00 UTC


Webinar on Novel Materials & Light Outcoupling Technologies for OLEDs & QD LEDs

Dr. Sudhir Kumar from the ETH Institute who will be presenting the following topic:

Overcoming intrinsic light outcoupling limit in perovskite quantum dots LEDs using low refractive index anisotropic nanocrystals

Date & Time: Tue, April 9, 10:00 AM - 11:00 AM (CET)

Joining Dr. Sudhir will be Dr. Balthsar Blülle from Fluxim who will be presenting a talk on Angular luminescence spectroscopy to explore novel emitter and light conversion materials.


12:32 UTC


What is your favourite equation in Physics and why?

08:13 UTC


Shower thought about water molecules

Original post with proper formatting

Text version with bad formatting:

Shower thought

When you are in the shower, watching the million billion molecules of water spray from the showerhead every nanosecond, take a moment to reflect on how, despite their innumerable multitude, each molecule has traveled a completely unique and remarkable journey for billions of years over and through the Earth to you.

Except, it's not true. None of the water molecules in your showerhead will last long enough to reach you.

At room temperature, pure liquid water is ionized at a ratio of about 2.8 parts per billion: for every billion water molecules, there are 2.8 H+ ions and 2.8 OH- ions. Lone protons are never found in liquid water, but rather are associated with water molecules to make hydronium H3O+, and frequently bigger associations (H5O2+ "Zundel ion", H9O4+, etc). The bonds in hydronium are very weak, not much stronger than hydrogen bonds, and have similarly short lifespan: hydronium typically lasts around a picosecond before one of the hydrogens jumps ship to sign on with a new water molecule. This is the Grotthuss process which causes protons to diffuse very rapidly in water:


Thus, for every H+ ion (by which we mean H3O+, etc, collectively) in a mass of water, there are about one trillion hydrogen swaps between nearby water molecules each second. This comes to around 400 swaps per water molecule: every water molecule in liquid water changes one of its hydrogen atoms on average 400 times per second.^[This figure is super approximate, and could easily be more than an order of magnitude off.]

If it takes, say, 140 ms for water in the showerhead to reach you, that is 100 times the half-life of a water molecule, and only 1 in $2^{100}$ water molecules (about 10000 liters) last that long.

(There is some nuance; liquid water forms large-scale networks or clusters of molecules connected by hydrogen bonds, which are constantly shifting and rearranging, but the timescale at which molecules turn and move is slightly longer than the timescale of the Grotthuss process. Thus when H+ moves from one water molecule to another, the most likely next place for it to go is back where it just came from, and the above calculation will modestly over-estimate the diffusive rate of H+.)

In the whole lifetime of the visible universe, no water molecule has ever persisted intact for a minute in liquid water at room temperature through chance alone.^[It is possible that there is some stabilizing process that I don't know about -- water is complicated -- but that would not be through chance.]

Perhaps, one might think, although water molecules are rapidly intermixed, some water molecules might reform from the same constituents as at some point previously; if a billion H+ are shuffled between a billion water molecules, on average one of them will end up where it started. This would be reasonable if water molecules had one hydrogen, but with two hydrogens this would require both to return to the same starting molecule at the same time. The probability of this falls rapidly as the number of molecules involved increases; and as more time passes, there are more opportunities for the body of water to be mixed with other bodies, and the original constituents of a water molecule to be separated forever.

Let us relax our expectations a bit and ask whether any arrangement of an oxygen atom and two hydrogen atoms has ever recurred. Out of $N$ water molecules there are $N^3$ possible such arrangements. By the birthday problem, you need to sample about $\sqrt{N^3} = N^{3/2}$ random arrangements to have even odds of having sampled the same arrangement twice. 400 times a second, $N$ new such arrangements are made.

Recall that deep in the mantle of the Earth, crystals are frequently found to have small pockets of water which may have been sealed for billions of years. If such an inclusion had a milligram of water, which is $10^{19}$ molecules, it would need $10^{9.5} / 400$ seconds, or 3 months, for some water molecule configuration to have occurred more than once.^[Or maybe not: "Recent research, however, has shown that water can sometimes be gained or lost from minerals at magmatic temperatures (>1000 C) in a matter of minutes. If this is true, then the fidelity of mantle xenoliths is questioned."] This remarkable coincidence will last for perhaps a millisecond (or, if a little unlucky, only a picosecond) before dissolving away.

02:23 UTC


What is the coolest physics-related facts you know?

I like physics but it remains a hobby for me, as I only took a few college courses in it and then switched to a different area in science. Yet it continues to fascinate me and I wonder if you guys know some cool physics-related facts that you'd be willing to share here.

23:57 UTC


Precession of the Planets

This is a bit of a historical question, in modern textbooks on gravitational theory and the theory of General relativity one of the key selling points is that Einstein's theory accounts for the famous missing 43 seconds per century that were required in the precession of the perihelion of Mercury.

To me one of the remarkable things about Einstein and co.'s calculation is not that it accounts for correct physics, but that the classical 8+ body problem calculations were good enough that the discrepancy could be calculated in the first place with a reasonable degree of accuracy.

Does anyone know how 19th century physicists like Newcomb and Seeliger approximated the N-body problem to calculate the expected period of mercury's precession? It seems extremely sensitive to the exact relative positions of the planets and the shape of their orbits, making it look very tricky to solve without computers and sophisticated numerical simulation.

13:45 UTC


Secondary cosmic ray occurrences, energies, wavelengths, and pulse shapes?

I'm interested in building a cosmic ray detector with a few level discriminators to be a conversation piece. I'm imagining something with LEDs that light up and hold for a second or two whenever it detects a > 100 MeV, > 1 GeV, > 10 GeV, or > 100 GeV ray.

There seems to be plenty of clear, easy to understand information on primary cosmic ray occurrence and flux, like this energy-vs-flux graph on Wikipedia. But since my detector will be at ground level I think I should be looking to detect secondary cosmic rays, and I'm having trouble finding anything that I can understand.

My general plan is to use a LYSO crystal and a SiPM in a dark box, and feed it to an amp (maybe a log amp?) and then to a set of ultra-fast comparators with latches, one per LED. Each comparator will compare the incoming signal with a level (set by a pot) and then latch. I'll have a delay timer on the latch output to reset the latch after some fixed time. Probably short for more common, low-energy rays and hold longer for less common, higher-energy rays. I'll roughly calibrate it against whatever data I can find for daily ground level cosmic ray flux at my location.

I'd like to understand:

  1. Is a LYSO crystal a good choice for scintillation of secondary cosmic rays? What wavelengths should I expect to be receiving?
  2. What do secondary cosmic rays look like in the time domain? What is the FWHM of a "single" cosmic ray? Does it even matter, or will the crystal absorb and remit whatever energy is delivered, minus inefficiency, regardless of pulse shape?
  3. LYSO crystals advertise decay times of about 50 ns. Does that mean that no matter how fast a secondary cosmic ray dumps energy into the crystal, the resulting scintillation pulse will have a FWHM somewhere between say 20 ns and 50 ns? Or does the shape of the scintillation pulse depend on the shape of the incoming ray?
  4. What energies should I expect from cosmic rays at ground level? What flux or rate of occurrences for different energy rays?
  5. Should the detector report the sensed energy of the secondary ray or the inferred energy of the primary ray that caused it? I guess this is a design question more than a physics question.

Any info or pointers is appreciated! If this isn't the right sub let me know.

12:44 UTC


Vacuum Tech: Seeking information on old thermocouple gauge technology.

I have an old Varian type 501 thermocouple gauge tube and I want to find gauge it's compatible with.

How can I know that a particular guage will function with this tube? I have not had much luck finding a manual/datasheet for the tube itself.

02:49 UTC


I feel like my research isn’t as interesting enough for a general audience. Can I make it interesting enough?

I have to present some of my research soon at a small thing for my school. I see some of the topics other people and the titles advertise themselves: spider research, sleep studies, building an engine, etc.

I’ve spent months modeling and coding so I can see how spin waves affect magnetic vortices. I think it’s kinda neat, but I’m worried that I’ll have a tough time connecting my enthusiasm with a more general audience.

Is this a typical feeling?

20:45 UTC


Curious coincidence between Dimensionality of Space and Spin State

I was studying introductory quantum mechanics. There I came across the classic two state quantum system, the spin state. Let directions of space (physical) is denoted by x,y,z.

Spin can be represented as a two dimensional Vector. Let's take spins along direction |z+> and |z-> as basis. Now other Spin states along each orthogonal direction of physical space can be represented as,

|x+> = 1/sqrt(2) |z+> + 1/sqrt(2) |z->

|x-> = 1/sqrt(2) |z+> - 1/sqrt(2) |z->

|y+> = 1/sqrt(2) |z+> + i/sqrt(2) |z->

|y-> = 1/sqrt(2) |z+> - i/sqrt(2) |z->

That's it! We can not represent any more pair of mutually orthogonal state vectors using complex number coefficients which give 1/2 inner product square with the previous states.

So if by chance, we happen to live in 4 dimensional space lets say, x,y,z,k ... how could the |k+> and |k-> states be represented?

Is it a coincidence that complex numbers allow us to represent spin states living in upto 3D space and we are actually living in 3D space????

19:20 UTC


If you've ever wondered what Bell's Theorem REALLY means, I explain it in this video. Hope it's helpful to someone!

16:18 UTC


Physics Questions - Weekly Discussion Thread - April 02, 2024

This thread is a dedicated thread for you to ask and answer questions about concepts in physics.

Homework problems or specific calculations may be removed by the moderators. We ask that you post these in /r/AskPhysics or /r/HomeworkHelp instead.

If you find your question isn't answered here, or cannot wait for the next thread, please also try /r/AskScience and /r/AskPhysics.

13:00 UTC


What are you working on? - Weekly Discussion Thread - April 01, 2024

Hello /r/Physics.

It's everyone's favorite day of the week, again. Time to share (or rant about) how your research/work/studying is going and what you're working on this week.

13:00 UTC


How to derive kramers law for bremsstrahlung as energy distribution per wavelength interval

Heyho physicists,
on the german wikipedia I found a formula to determine the energy distrbution of a bremsstrahlung-spectrum as energy per wavelength interval (https://de.wikipedia.org/wiki/Bremsstrahlung#cite_ref-7):


It's noted that this form is derived from a frequency-domain form of kramer (https://doi.org/10.1080/14786442308565244). But, I'm even not sure about which formula is the original one I should start with transforming. May someone can explain how to derive the energy distribution dE/dl from the orignal paper.

Kind regards

09:26 UTC


Circular reasoning: Solving the Hubble tension with a non-π value of π

08:50 UTC


April Fool's arXiv posts

It's that time of year again! Share your favourite April Fool's arXiv posts in the comments, I'll be collecting them here with the abstracts.

[Echoes from a long time ago: Chewbacca inflation] (https://arxiv.org/abs/2403.20143)

The cosmic microwave background (CMB) radiation offers a unique avenue for exploring the early Universe's dynamics and evolution. In this paper, we delve into the fascinating realm of slow-roll inflation, contextualizing the primordial acoustic perturbations as the resonant echoes akin to the iconic sound of Chewbacca from the Star Wars universe. By extrapolating polynomial potentials for these primordial sounds, we illuminate their role in shaping the inflationary landscape. Leveraging this framework, we calculate the scalar spectral index (n_s) and tensor-to-scalar ratio (r), providing insights into the underlying physics governing the inflationary epoch. Employing a rigorous chi-square (χ2) analysis, we meticulously scrutinize the Planck data combined with that offered by the BICEP/Keck collaboration to identify the Chewbacca sound profile that best aligns with observational constraints. Our findings not only shed light on the intricate interplay between sound and cosmology but also unveil intriguing parallels between the cosmic symphony of the early universe and beloved cultural icons.

06:35 UTC


Graduate Course Videos on Youtube

I have recently decided to go back to school to get my masters in Physics (I have a BS). I am planning on joining a program in January and I want to start familiarizing myself with the material so I can try to make the most of my time while in the program. Are there any really good courses on youtube that teach the graduate versions of mechanics, quantum, E&M and thermo?

Also, what textbooks do you recommend to get a head start?

00:36 UTC


Is there a way to generate reliable data of the viscosity term in a 2D Darcy's flow?

I am doing research on Physics-Informed ML, and I am testing an optimization algorithm to optimize a network with the purpose of learning the function that solves the 2D Darcy'flow PDE as shown in the figure below. I have found online a dataset from the "neuralops" library.

But the derivative calculation are not correct if we consider the diffusion coefficient as a constant, since the viscosity term seems to be dependent by the input values "x". Therefore, it is necessary to compute the derivatives of the diffusion coefficient term in another way. Is there a possible function that it can be used to obtain the diffusion coefficient value from the input "x"?

21:55 UTC


Journal and conferences rank

Hello all Are the conference called "APS march meeting" and the journals "physical review E", "physical review B", "physical review letters" considered goon in physics?

22:13 UTC


Physics Art: Open call for art/science exhibit exploring the captivating realms of fluid dynamics, aerodynamics, and flight

Traveling exhibitions for Gallery of Fluid Motion is thrilled to announce an open call for an upcoming art/science exhibit exploring the captivating realms of fluid dynamics, aerodynamics, and flight, inspired by the pioneering spirit of Leonardo Da Vinci. This particular exhibition aims to showcase the historical interplay between art and science, with Da Vinci serving as a guiding luminary whose multifaceted genius continues to inspire innovation and creativity.

Artists and scientists from diverse backgrounds and disciplines are invited to submit their works, whether new creations or existing pieces, that delve into the fascinating themes of fluid dynamics, aerodynamics, and flight. The exhibition will take place at The Leonardo Museum in Salt Lake City, Utah, during the Fall/Winter season of 2024.

We welcome submissions in any medium, size, or stage of production, including, but not limited to video, photography, painting, 3D printed models, sculpture, installation, mixed media, and beyond. Although not a requirement, artists, and scientists are encouraged to explore the intersections between art and science by drawing inspiration from Da Vinci's legacy while infusing their unique perspectives and interpretations.

Submission Deadline: April 1, 2024

Please spread the word to your students, colleagues, artists, etc. To submit your artwork for consideration, please complete this form.

We look forward to receiving your innovative and thought-provoking projects that celebrate the timeless connection between art, science, and the boundless wonders of flight.

Please find out more information on our Instagram Page.

Warm regards,

Azar Panah
Gallery of Fluid Motion Coordinator

18:53 UTC

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