I found this in a physics lab closet, but I can’t figure out what experiment/lesson/phenomenon it could be used for. What do you think this could be? The front wheels rotate together, the back wheels are fixed, and there isn’t anything on the other side except another blue arrow.

Does anyone know how to measure the 'critical-ness' of a dynamical system in which it's configured to allow 2nd order phase transitions? For context, I'm an undergrad ML researcher that is doing research on Reservoir Computing. I'm looking into ways to modulate dynamical systems and treat them as artificial neural networks by maintaining their critical state (I dub this process as 'Homeostasis', no one really has a term for this officially yet I think). The dynamical system's architecture is an n-dimensional hyperlattice of nodes that evolves over time (input or no input).

The most apt comparison I can give for the system are Ising models. Ising models are modeled within a grid, with spin up or spin down states. The global state of Ising models evolves over time, and produces patterns.

The most viable candidate metrics I have are Shannon Entropy and Kolmogorov-Sinai Entropy for measuring the 'temperature' of these systems, and I'm doing measures on the correlation length, structure factor, finite size scaling, fractal dimensions of subgrid states, and dynamical scaling.

I haven't found a way to tie this all together as one coherent metric though.

I'm interested to know if anyone knows papers or can point me to any interesting resources on other viable metrics for measuring the criticality of these systems.

For my research, over the last year I've been developing my own code for conducting TISE and TDSE simulations. Specifically, I'm interested in analyzing both the energy and special distributions of electrons ejected while atoms are interacting with strong, ultrashort laser pulses.

The simulation is done broadly in three steps:

1. Solve the TISE to get the bound states/initial state.
2. Propagate initial state using Crank-Nicholson Scheme to arrive at final state.
3. Project the final wavefunction onto energy eigenstates to find population of ejected electrons.

If anyone is interested, I do plan on posting a link to the code, but there are a few more things l'd like to implement before I share it with others.

For now, however I'd just like to showcase some of the results I've gotten.

Laser Parameters:

Polarization: Linear in Z direction

Frequency: 0.085 au

Duration: 20 Optical Cycles

Intensity: 2E13 W/cm^2

Angle Integrated Spectrum

Angle Resolved Spectrum

In the first image you can see peaks in the energy spectrum. This corresponds to above-threshold ionization where the electrons absorb more photons that necessary to ionize, resulting in peaks separated by the photon energy to appear in the spectrum.

In the second image you can see in space where the photoelectrons are being ejected. In principle the angular distribution is sampled over a sphere, however for visualization this angular distribution is sampled over the XZ plane.

Laser Parameters:

Polarization: Circular in the XY plane

Frequency: 0.114 au

Duration: 2 Optical Cycles

Intensity: 2E14 W/cm^2

Angle Integrated Spectrum

Angle Resolved Spectrum

In this case the entire distribution looks different. This is mostly due to the duration of the pulse being much shorter. We see that the energy distribution no longer contains ATI peaks but has one distinct peak. The angular distribution is sampled over the XY plane, and we see that instead of there being multiple peaks and photoelectrons being emitted in many directions equally, we see that most photoelectrons are emitted to the "bottom right" direction.

Laser Parameters:

Polarization: Linear in Z direction

Frequency: 0.057 au

Duration: 6 Optical Cycles

Intensity: 3.51E14 W/cm^2

High Harmonic Generation

Finally, this is the high harmonic spectra produced. From classical physics we know that when a charge accelerates/oscillates it should emit radiation. By computing the expectation value of the dipole acceleration matrix at each time step and taking a fourier transform you can gain insight into the energy spectrum of radiation emitted.

I'm not sure if it is even possible but just had a thought and wasn't sure where else to ask.

Context: I have just purchased a 50cc classic moped and will take it to the mountainous region for work commuting next year. However I do not live in a mountainous region out of season so have nowhere to test its capabilities going uphill. So I need a way to test its ability to climb hills in a completely flat region.

Would adding extra weight onto the back of the bike be representative of a drag force similar to going uphill?

If yes then is there a way that I could for example work out that a 13% incline would be equivalent of carrying an extra 70kg on the flat for example to then able to test it that way. (Random numbers for example and also just random thoughts).

Let me know if I've lost the plot or whether it could be worked out that way.

If I haven't explained it correctly then please excuse me

Thanks in advance

Hey everyone!

I'm a third year undergraduate Engineering Physics + Math student, and recently I've been reading through reviews and textbooks on Topological Phases of Matter + other topology/geometry related condensed matter theory (CMT) topics, and I'm trying to join a research group studying Topological Matter.

I spoke with a postdoc in the group doing very cool stuff, and he essentially told me that if I could find something specific I wanted to work on, he'd be down to advise me.

He suggested three main subfields to look at, based on their popularity and potential, and told me to try and come up with a possible project, even if I may not currently have all the math + physics background necessary

(For context: I've done graduate differential geometry and algebraic topology but I'm just learning the basics of K-theory and I need to learn much more category theory. I have some QFT and SFT background, but I'm still learning and am not proficient)

The topics he suggested were:

1. Classification of Fermionic SPT phases
2. Categorical, non-invertible, higher symmetries in relation to Topological Phases
3. Quantum Information, Open Quantum Systems (this one's kinda vague, but still)

I'm trying to get an idea as to what the main questions are that researchers are concerned with in 2024, and which topics seem to be the most important.

Questions/Requests:

1. Does any one have any advice on how to narrow down my search without having to read (or at least, skim through) a crapload of papers?

2. If there are any standard resources/references used by the community to keep track of open questions and direct progress, I'd love to hear about it!

3. If anyone on this sub familiar with the current research situation could provide an overview of the major subfields in Topological Quantum Matter, and important questions in the subfields being investigated in 2024, that would be greatly appreciated!

I'd also just love to chat in general and learn about peoples' own research interests. Anyway, thanks!

I’m fascinated by Schrödinger’s cat! It really makes you think about reality. What’s yours?

The last time an atom was discovered was in 2009. (Tennessine)

When can we expect atom n°119 to be synthetized/created ?

I thought some may find this interesting. This is a space-time diagram, very similar to the diagrams in Davis and Lineweaver's 2003 paper (link in sidebar), to show various properties of the LCDM cosmological model.

The first toggle switches between proper, co-moving and conformal coordinates and the 2nd toggle adds inflation in (by hand).

https://www.desmos.com/calculator/dnopt98v8z

Though this is an approximation of the LCDM model, it is a pretty good approximation. The inflationary portion however is only to illustrate a few gross features of inflation, so shouldn't be taken too seriously.

Unfortunately it does take time for Desmos to do some of the calculations, so it can take some time (20+ seconds) for rendering in proper and co-moving coordinates.

Let me know if you have any questions or comments.

As the title suggests, i want to ask experience peopled about a very confusing time that I'm going through

Before anyone says I should have taken engineering if I was interested in working industry, I would have if I had any remote attraction towards engineering.

Being from a third world country that makes millions of engineering graduates yearly, the admission and process of becoming a engineer isn't something I find myself going through. Neither do I have any specific interest in any of those fields, only thing i know is that my interests lie within physics and natural sciences in general. I love understanding aspects of life and how they work from molecular to sub-atomic level.

Which brings me to my main question, being from a country with weak currency, I just want to pursue a phd abroad and then jump into industry if possible to gain enough financial stability (personal reasons, i choose physics because it had 1/3 of fees in comparison to any engineering still i had to take student loan to study.)

I love research, the institute i current study in is a research focused institute too although everything comes with it's own bad sides(only way to gain industrial experience is through internship.)

I know how tough it is to study through your phd, but i know i can work as long as it's physics that I'm doing. I don't mind academia i just don't like the competitive aspect of it. I don't think i want to go through such phases again. Industry gives me a chance to change trajectory of my family and myself.

Sorry if alot of things feel unnecessary I'm really just typing this with barely any sleep last night.

Any advice would be helpful.

I can't find an answer on Google. I see there's a lot of info about pilots not hearing the sonic boom, but my question is about after that.

Once they're past that point, do they hear anything? I mean the rumble of the engine, the nose cone tearing through the air, their own voice... or does it become a silent affair with no sound able to catch up with them?

Relatively the same question about light from a flashlight at the speed of light. Does it just go black relative to the carrier?

I'm currently a student attending high school. I think that physics is a really interesting subject and want to dive deeper into physics. what are some books, videos or websites you would recommend visiting to get extra information?

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.

I don't know if this is the right place, but I feel "disappointment" with the lack of technological progress on gamma ray beams. Even the production of electron-positron pairs by colliding two real photons ("Breit Wheeler pair production") is almost impossible to access experimentally, and it seems like it ought to be simple. The reverse process - two photon annihilation - is the main thing that happens when positrons enter matter. Certainly when it comes to higher energy things like meson photoproduction and the like, it seems no one is even trying. The entire scientific community seems like it just wants to "cheat" by doing all the experiments with virtual photons - by shooting high energy charged particles at the target in order to simulate photons. Where are the proposals to actually generate a 1 GeV photon beam and use it to test these things?

It seems a little strange, given that when high energy electrons are incident on matter, they actually lose most of their energy by emitting the photons. At higher energies, and higher atomic numbers for the target - bremsstrahlung losses actually exceeds ionization by inelastic scattering from atoms. So you can't claim that it's harder to generate a photon beam than an electron beam at such high energies!

Clearly, there are severe struggles with phase-space densities and the like - but why is so little attention going toward resolving it? I've read a few particle physics papers about photonuclear reactions and pair production - and most of them simply take for granted that "We don't do that, we don't even talk about it, and we don't even ask if we could do so in a future experiment".

Given that many physics teams are exploring very exotic and hard-to-reach tests such as dark matter detection and such, it seems strange that this relatively mundane topic just never even gets talked about.

Small background for context; I've over the past few months tried to work through easier parts of the math around general relativity and build a better internal visualization and intuition about the related phenomena. I've a few university math courses behind me, but not currently studying and haven't studied physics in the context of higher education. So my understanding is definitely lackluster in many ways.

I've tried to get a better understanding of gravitational waves. One thing that kind of eludes me is why do gravitational waves carry away energy from the system? If I've understood correctly, angular momentum of the orbiting bodies is transferred away as gravitational waves, which correspondingly leads to a loss in orbital velocity, and as such in lieu of other sources of drag and sources of energy for increasing orbital velocity, will lead to eventual collision between the orbiting bodies.

But why does this happen? I have some kind of an inkling that it would be the natural consequence of the fact that the propagation of gravitation too is bound by the speed of light. Is it related to the gravitational strain an the orbiting object being a function of time and thus uneven and a source of its own changes in its gravitational field?

Visualizing this in the terms of waves on e.g. surface of water seems rather impossible, and I don't think such waves would lead to similar effects. Googling this I also happened on the "sticky bead argument", which seems to look at the concept from the perspective of measuring the energy of a gravitational wave on the receiving end. I though find it harder to understand why orbital velocity is lost at the source of the gravitational waves.

I want rigorous theory, proofs, concrete practice problems!

So far I found:

Leonard Susskind from Stanford classical mechanics https://youtu.be/ApUFtLCrU90?si=mujz3-XJ-tTWAp2P

MIT classical mechanics Chakrabarty https://youtube.com/playlist?list=PLUl4u3cNGP61qDex7XslwNJ-xxxEFzMNV&si=Rw2CYChSDpM5m7Ss

Balakrishnan from IIT madras classical mechanics https://youtu.be/Q6Gw08pwhws?si=0mMWTK3zL3gKMJMt

MIT Walter Levin classical mechanics https://youtube.com/playlist?list=PLyQSN7X0ro203puVhQsmCj9qhlFQ-As8e&si=PPrgz1tfXmFbhwpL

Hello,

I am trying to find any good resources on learning effective field theories for classical systems.

I can’t find any resources, most are just for quantum field theories or quantum mechanics related.

[reposted from AskPhysics since it was not a question]

I received my PhD in Cosmology and Modified Gravity research and I am here to answer some common questions about Modified Gravity. I want to give a fair overview of the evidence for and against both positions for the benifit of everyone interested in this topic. I am also going to make my case for Modified Gravity. As I am not am active Dark Matter researcher, please inform me if I am in any way michareterizing the Dark Matter model or strawmaning it. I would apreciate the same respect given to me as an active Modified Gravity researcher.

The Missing Mass Problem [skip if you know it]:

Einstein’s field Equations of General Relativity (GR) are the presently best tested theory of gravity which work within our solar system but appear to fail at the scale of galaxies. If you plug the mass of an object into the GR equations you get out what type of gravitational field (space-time bending) the mass produces and from this you can find the orbits of smaller objects around that mass within its gravitational field. We have a good, simulation tested, understanding of how stars work and how their brightness/color/composition can be used to infer their mass. Take the mass of the sun, put it into the equations of GR, and you get out a prediction for the orbits of the planets which matches what we observe (within our observational precision). We can meassure the vissible mass of a galaxy from its brightness (its stars) + any amount of gas/dust content that it has. If we plug the vissible mass of a galaxy into the GR equations we find a major missmatch between the predicted rotational velocity of a galaxy and its observed rotational velocity, for nearly every studied galaxy. Galaxies are rotatating faster than is predicted which implies they have more gravity than is predicted. The 2 major proposals for explaining this "extra gravity" is comming from are "Dark Matter" (DM) and Modified Gravity (MG).

Dark Matter (DM):

A simple way to explain the extra gravity is to propose that galaxies contain aditional mass which we can't see, Daark Matter (DM), though the name "invisible matter" would probably have been better. Whatever DM is it has mass and produces gravity but has so far evaded our attempts to detect it by any direct methods. This implies that it interacts with the other fundmental forces either extreamly weakly or not at all. We have ruled out that it could be any type of known matter, meaning that it must be some new particle we have never detected yet. The only information we have gained about DM comes from what can be infered from its gravity (the extra gravity which we are atributing to this DM stuff). DM does not clump up in the same way that vissible matter clumps up to form galactic disks and stars, if it did then we would observe the gravitational effects of small dense clumps of DM but so far no such clumps have been observed. The present model of DM is that it exists in large gas/fluid clouds (called DM Halos) which suround nearly every galaxy, with the vissible galactic disk sitting at the center of a much larger, much more massive, and much rounder DM Halo. The gravity produced by the spread out DM Halo is signficant in effecting the rotation of the entire galaxy but has no signficant effect on the smaller scales when it comes to planets orbiting stars.

Modified Gravity (MG):

The Modified Gravity proposal is to replace the Einstien field equations of General Relativity with a different set of equations such that we get a different prediction for the strength of the gravitatioanl field produced by an object with mass. Since we already know that GR works at teh scale of the solar system, the MG equations must match the predictions of GR close to a massive object but then diverge at further distances. The strength of gravity (as a function of the distance from a massive object generating the gravity) in GR dies off to zero as you move away from the massive object, while in MG the strength of gravity decreases at first but then either platues or starts increasing again at further distances. This stronger gravity at further out distances is what is meant to explain the "extra gravity" which causes faster than expected rotation rates of galaxies, and it does so without the need to add in Dark Matter.

Modified Gravity (MG), Questions and Answers:

Q: Does MG break relativity?

A: No, not necesarily. MG is not one theory it is a collection of many theories which only share in common the idea of explaining the "extra gravity" problem by changing the equation for gravity rather than adding in Dark Matter. An individual Modified Gravity theory may or may not choose to stick with or violate the princples of relativity. For an MG theory within the framework of relativity, gravity is still the bending of space-time, all that has changed is that the "slope" of the bending of space-time is different (at large distances) than what it is in the Einstien Equations.

Q: I thought MOND was discredited because it can't predict gravitational lensing, cluster dynamics, or the CMB?

A: "MOND" is just one particular modified gravity theory. It was the first to gain prominance and is often falsely conflated with the entire diverse research area of modfied gravity theoreis. It acurately models a vast number of galactic rotation velocity profiles, and thats it. Most researchers consider MOND to just be a first order proof of concept and not something that is meant to be a stand alone theory. Active work on modified gravity research concerncs full relatitivistic field theories, MOND is just a simple aceleration equation. The goal of modified gravity research is to find a relativistic field theory of gravity which reproduces MOND in the apropriate limit and also explains everything else. MOND is to modified gravity what Newton's Law of Gravity is to Einstien's General Relativity. Saying that modfied gravity is discredited because MOND does not explain gravitaitonal lensing is like saying that GR is discredited because Newtonian Gravity doesn't explain gravitational lensing.

The selling poitns of the DM model:

By looking at the rotation rate of a galaxy we can fill in the amount of DM that would have to be present to expalin its rotatioanl velocity profile. This is not a prediction from the DM model, this fitting the model to the observations. What needs to be done to scientifically validate the DM model is to take the amount of DM we need to fill in in order to explain a galaxy's rotation rate and then use that to make some other indpendent prediction which can be verified. Lets look at a few other ways to infer how much extra gravity there is around a galaxy (which in the DM model tranlates to how much DM it has)

1.) Gravity distorts the path of beams of light, known as gravitational lensing, so we can look at how much lensing there is around a galaxy or galaxy cluster.

2.) The velocities of galaxies within a cluster are influenced by their gravitational pulls on each other.

3.) The Cosmic Microwave Background (CMB) is the earliest and most distant light we can see, it is the thermal radiation (remnant heat glow) of the plasma which once filled the universe everywhere before cooling down. We see this light today because light takes time to travel through space and we see that it contains a distinctive splotchy patern of hot/cold spots. The size of these spots has to do with the dynamics of matter, radiation, and gravity in the early universe. The amount of DM present in the early universe would play a large role in this procses.

These 3 examples represent indpendent observations which also depend on the "extra gravity" and thus by implication the DM quantity in galaxies or across the universe as a whole. It is claimed that the DM model agrees with all of these indpendent observations. However, I have also heard complaints about the model having too many free parameters so it is not clear to me on which of these points it can be said that the model is making acurate predictions vs just being fit to the observations. I am not a DM model expert myself and this is a general overview.

There is one other topic which comes up often in support of DM's existence which I feel should be adressed seperately.

4.) In high velocity collisions of galaxy clusters (like the Bullet Cluster) the bulk of the vissible mass rapidly decelerates in the collsion but the DM is predicted to pass through the collision mostly unafected and is therefore thrown out in front of the vissible matter. This appears to be what we observe from the gravitational lensing around high velocity galactic cluster collisions which seems to sugest a large invisible source of mass was ejected from the collsioin out ahead of the vissible matter.

The selling point of MG (and the crisis for DM):

The modified gravity equations (MOND and its relativistic generalizations) work for galactic rotations. Take the vissible matter of a galaxy and plug it into the modified gravity equations and you get out the observed rotation velocity profile of that galaxy, for nearly every galaxy from the smallest to the largest across many orders of magnittude of vissible mass. There are some outliers but they represent less then 1% of all galaxies studeied. It is very important to understand the difference between MG and DM on the crucial topic of galactic rotation velocity profiles. The DM model does not predict galactic rotation rates, it uses galactic rotation rates to model the amount of DM in and around the galaxy and then that can be possibly used to make testable predictions, with every galaxy having a unieque DM Halo. MG predicts the glactic rotation rate. MG gives a universal aceleration law for gravitational orbits, it takes in the vissible matter distribution of a galaxy as its indpendent variable and makes the testable prediction of what the galactic rotation rate should be, and its overwhelmingly acurate down to fine details.

The succses of MG theoreis at predicting galactic rotation rates is a crisis for the DM model. Why? Because in the DM model the bulk of the gravity in and around a galaxy is coming from its DM, not its vissible matter. The DM model doesnt make any strait forward prediction about there being any relationship between the vissible matter distribution of a galaxy and its rotation rate. The existence of any corelation between the vissible matter distribution and the rotation rate of a galaxy implies for the DM model that nearly every galaxy has a tight corelation between the mass density distribution of the DM Halo around the mass density distribution of the vissible matter in the galacitc disk. This is major problem because we already know from both observations and simulations that galactic formation is a chaotic procses which involves compelx dynamics of mergers and collisions between galaxies and dwarf galaxies of a wide range of sizes and masses, with some amount of mass being ejected from the galaxies in the collisions.

The DM model has an incredibly severe apparent paradox on its hands: on the one hand DM is supposed to be able to seperate out from vissible matter during galactic/galactic-cluster collsions because of how differently the DM and vissible matter interact with the other forces (hence the explanation of the gravitational lensing around the Bullet Cluster) yet on the other hand somehow nearly all galaxies (having expericied all manner of chaotic histories in their past) retain this very precise highly corelated density distribution between their vissible matter and their Dark Matter. I would love to know how the DM models resolves this aparent paradox.

The supposed issues with Modfied Gravity:

Most of the supposed issues with Modfied Gravity theories are the claim that MOND does not adress our observations of galactic cluster dynamics, collisions, gravitational lensing, and the CMB. This is a nonsensical argument against Modified Gravity and all it reveals is just how behind the times most physicists are with the current progress in Modified Gravity research. There are multiple *fully developed relativistic feild theories of modified gravity* and progress is ongiong to calculate what predictions these theoreis make. Here are some results:

1,) A modified gravity theory called SVT Gravity succeeded at moddeling the CMB temperature spectrum.

2.) Another modified gravity theory called Conformal Gravity succeeded at predicting the type 1A supernova brightness-redshift curve.

What is significant about these cases is that these theories already accurately predict galactic rotation curves, without dark matter, by emulating MOND in the non-relativistic limit.

A final speculative remark on the Bullet Cluster:

The Bullet Cluster and other similar galactic cluster collisions appear to present the greatest issue for any Modified Gravity theory because it is assumed that gravitational lensing necesarily implies a local source of mass. However, we already know of a counter-example: gravitational waves. When massive objects acelerate they radiate gravitational waves which propogate out away form the massive objects and which will lense the paths of light beams which cut across the gravitational waves. Furthermore, gravitational disturbances are poorly understood outside the linearized limit. As a quick bit of background on this: electromagnetism is a linear theory meaning that two overlapping electromagnetic fields add together in a simple way (vector addition). But gravity is not linear, two gravitiaonl fields which overlap in space do not simply add up, the resulting total field is not simply just the sum of the two, its much more complicated. This is already true in GR but it becomes even more severe in many MG theoreis in which there are aditional fields or higher order derivatives in the gravitiaonal equations. In my opinion, it is extreamly poor reasoning to assert that gravitational leansing necesarily implies a local source of mass, especially if we are considering modfied equations of gravity which may contain behaviors not seen in GR.

Here are the two candidate MG field theories I mentioned:

SVT Gravity: https://en.wikipedia.org/wiki/Scalar%E2%80%93tensor%E2%80%93vector_gravity

Conformal Gravity: https://en.wikipedia.org/wiki/Conformal_gravity

The refernces will take you to papers on the subject on the archive, you can cross check their publications in journals like physics physics review D. Both of these are relativistic metric tensor field theoreis of gravity with Lagrangians based on the Reimann tensor, just like GR.

What website do you visit to watch for new, post doc positions? In the U.S. is there something to follow other than APS Physics Jobs? My friend is in condensed matter

How are ultra-low temperatures measured? 0.5K or 0.1K for example?

I did NOT understand laser self-mixing physics at all. So I took the "explain it until you understand it" approach, and ended up making some animations and putting them in a video! Check it out for yourself if you're into that sort of thing.
https://www.youtube.com/watch?v=NL2_vVnqUSw

I'm an undergraduated student and during the 2+ years of college I never felt... creativity. I have ADHD and I am a VERY creative person during all my life, I am always creating something, I am always painting, making my own music, i LOVE doing things manually, I love feeling that something that I do is special to me because I AM CREATING THAT.

I never felt like this in Physics. I love physics but I always feels like i'm doing another person job, I feel like i'm never doing something BECAUSE of me and FOR me, but because and for someone else. I'm working with optics and it's the closest i've ever felt of DOING something but it's still weird for me and it's demotivating me a lot. I wish i could explain exactly how i feel.

I try to explain to my friends but the answers are always BUT IT IS CREATIVE, YOU ARE TRYING TO FIND SOLUTIONS FOR A PROBLEM. Yes, I know, but I don't feel like THAT type of creativity is what I'm looking for.

So how do you feel creative in physics, or research in general? Do you ever felt like this? I wish to know others perspectives about this !

i’m in my second year of my undergraduate for physics (minor in astronomy) and i’m looking at internships right now… there are next to none for physics degrees but significantly more for engineering degrees! i’m not even sure what engineers totally do but should i try to switch to be able to get more jobs? also i heard you can get an engineer job with. physics degree but is it hard that way?

Hi everyone, do you know if an app or an online website, newspaper, whatever the way, that brings last news about science ? I know some literature like Nature but it's a bit expensive for me.

Thank,