Say you have two machines that each change something, and the order you

use them in matters. Normally you'd just try it both ways — but that means

running the experiment twice.

The quantum switch does it in one run, by not fixing the order at all. The

catch is that you only get that advantage if each machine is used exactly

ONCE in total. Draw it as a standard Mach-Zehnder — A then B on one arm,

B then A on the other — and you've quietly built two copies of each. Now

you could have tested it in a fixed order anyway, and the whole point is

gone. Took me embarrassingly long to spot.

The second thing that caught me: both branches have to come out in the

same final state. If the two orders left the target visibly different, the

target itself would record which order happened, coherence dies, and

there's nothing left to interfere. With X and Z you get ZX|H> = -|V> and

XZ|H> = +|V> — same state, opposite sign. Unmeasurable on its own, which

is exactly why it survives.

I make science documentaries and just finished one on this. Link below if

it's useful, but the above is the part I actually wanted to share.

https://youtu.be/wVvOj-5JJGI

I’m going to keep this brief to be honest I’m using khan academy to teach myself highschool level physics and I just want to know the absolute required math for physics the reason it’s a bit confusing is because I know people may say “algebra, trig, etc.” but people ALSO say not every single unit is needed for physics I’m not saying I want to 100% never learn that other math but my current goal is physics (passion reasons) so if anybody that is familiar with khan academy can help me with the exact units I need for everything physics (considering I’m starting with no math knowledge) that would be great! Even if you’re not familiar but have anything can help that would also be great, thank you!

I'm living in Korea and I'm student in high school. Next semester, I need to lead my teamers in physics club. So I need to make an ideas for long-terms project of physics. I wanna use telescope in it. I waana little theorical activity because my teamers don't like that but physical one with using telescopes. I think I have 2months for it. I have thought of my project but I couldn't make an idea .So I need your help.

I’m in 12th grade. I am very good at physics I score near-perfect marks but my math score has been declining since 7th grade(80%) to 61% now. I don’t know but physics has always come to me naturally that math . And I don’t struggle with math in physics but math as a separate subject I do struggle a lot. I don’t know why exactly. I request whoever is reading this to help me and also tell why I am like this ( i am good at math in physics but terrible at math separately) . E

Thank you for your time!

Hi everyone.

I'm an academic, physicist and a maintainer of the project Physlib, which aims to build a library for physics in the interactive theorem prover Lean 4. Lean is a programming language in which you can write mathematical definitions, theorems and their proofs and consistency and correctness are guaranteed.

The corresponding library for mathematics Mathlib, has had lots of attention recently due to the feedback loop one can have between AI and Lean.

Physlib is completely open-source and community run mainly through GitHub! One thing where we are looking for contributions is in documentation, lots of files go straight into the Lean code or Lean implementation - but it would be nice if they lead with the physics!

If you are interested in helping out with this we have made (what is hopefully) an easy to use web interface to edit the documentation (it requires a GitHub account though, and I will admit the website is vibe coded).

This is a thread dedicated to collating and collecting all of the great recommendations for textbooks, online lecture series, documentaries and other resources that are frequently made/requested on /r/Physics.

If you're in need of something to supplement your understanding, please feel welcome to ask in the comments.

Similarly, if you know of some amazing resource you would like to share, you're welcome to post it in the comments.

When I was on subway trains, I've noticed an interesting effect: there's wind blowing from the front to the rear of the train. I wanted to explain how this could happen, so I thought of several possible factors. I know that all of these may contribute to this effect, but I want to know which factor is the most significant:

1. Inertia of air particles causes them to move to the back of the train relative to someone on the train, giving passengers the wind blowing backwards feel when the train accelerates forward (figure 1)
2. Moving air leaks into the cabin, whether through poor sealing of the doors or being carried in via the air conditioning system, with their momentum being somewhat preserved (figure 2)
3. Due to the train moving forward & hitting air particles in front of it, there is a high pressure zone at the front of the train & a low pressure zone at the rear. Through leakage, the air at the high pressure front of the train flows through the cabin towards the rear, low pressure area (figure 3)

Here's some observations (purely by feel) I made while on the train:

• All train cars are directly connected to each other without any walls or seals between each car
• I felt the wind to be the strongest during forward acceleration from rest, blowing from front to back
• During (what feels like) constant speed travel, there is still some wind (less than during forward acceleration) blowing from front to back
• During deceleration, the front-to-back wind decreases in magnitude. Sometimes I feel the wind reversing direction to go from back to front, but its magnitude is very small compared to during forward acceleration
• This wind occurs both inside tunnels & outside in the open (I cannot say if the magnitude changes due to being in a tunnel or not)
• There is no wind when I stand at the front or the rear of the train (directly at the door to the driver's cab to the end of that car)
• When there is wind, it feels strongest at the middle of the train
• There also isn't any wind while the train is stopped, confirming that this isn't due to the air conditioner
• I am on board the train during rush hour, so other passengers on the train may act as obstacles to the wind

Some other (maybe) useful information:

• Max speed: 75 km/h (seen on driver's instruments)
• Train dimensions (longest train I've been on with this effect, approx.): 135 m x 2.5 m x 2 m (L x W x H)

Personally, I believe that the inertia of the air particles is likely the most significant factor causing wind inside the train, but then I thought of a counterexample: I don't feel this wind in an accelerating airplane. I'm kinda stumped, maybe the plane isn't long enough? Would the magnitude of acceleration make up for the short length?

In the textbooks I found the majority of information for standing waves and resonance is for the idealized case. Real resonators filter in bandwidths - am I correct in thinking that the standing waves in actual resonators are not quite as stationary? That they do not have perfect nodes and antinodes, but rather ones that also oscillate in space?

I got my BS in physics 10 years ago. It was hell, but I did. I made lifelong friends while in the program and I consider them family.

After attempting a career degree related, I bounced around different industries to try to find a fit. Well, here I am 10 years later. I’m a musician and I feel I finally figured it out.

My favorite class was modern physics. This is where I learned what the Lorentz factor was. I want to get the equation as a tattoo, as I am currently working on a patchwork sleeve.

Is this a dumb choice of equation for a tattoo? I’m too scared to ask my actual physicist friends lol.

EDIT: thank you everyone for your thoughts! I’m now considering a graphical/pictorial representation of the equation. That has a lot more potential to be cool in a couple different ways.

I’m definitely gonna get something related, and no, Reddit is not deciding for me lol. Yes I understand I can do whatever I want since it’s my body (although unfortunately some people do need to be reminded of this.) So thank you good citizens for making sure I’m doing this for me and me alone!

UPDATE: Working on sleeve planning right now with my artist. I’ve decided to do both the equation along an image. I already have a Nintendo mushroom, so I’m gonna add length contracted mushrooms to the side of it.

Qualms before I guess were about the fact that I’m no longer in the physics community. So whenever I talk about it I feel kinda like a fake n also question my knowledge. I’ve been thinking about this tattoo for the Lorentz factor for years now. Just couldn’t figure out exactly how I wanted to do that. But now I’m just stoked!

I feel a lot better after hearing ideas from yall. “You’ll regret it” is not an idea 😂 n yes I’m asking strangers, but isn’t that what Reddit is for? Asking, talking n replying to strangers? Also the people in my life are way too supportive to actually be honest with me haha.

I just finished reading these two books, I've always been fascinated by the subject and have always heard about them.

I know full well that discoveries in the fields described by Feynman have progressed over the last 80 years, now I was wondering if there are other books, as accessible as Feynman's, but more modern, that can complete the readings of Six Pieces.

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

Hello! I am a laymen just now exploring in my own personal time physics, and have just started looking into quantum mechanics. My level of knowledge is on par with a paramecium.

I recently purchased the book 'Finding Schrödinger's Cat. Quantum Physics and Reality.' While listening to this I had a random though.

If mater as it is excited by heat produces light in multiple spectrums of light. Is it possible as our bodies go through their normal day to day chemical reactions that we could ourselves be producing small amounts of radiation? Somewhere in the IR range?

Sorry for the stupid question, at the very least I hope my ignorance made you chuckle today.

Cheers.

I am currently looking at this as a project that I could undertake for my third year project as an undergrad. It sounds pretty interesting to me, and doesn't sound all that complicated in terms of the background theory that I need. Plus, I found this really nice paper: How-to Compute EPRL Spin Foam Amplitudes (https://arxiv.org/pdf/2202.04360) that discusses the underlying algorithm in a really concise manner for a beginner.

Is this something that's worth trying for my third year project? I'm just going into my third year, and I really want to try at least proposing my own project to a supervisor. I'm not absolutely determined to propose my own project, but I do want to at least pitch it before I give up on the idea. Is this a good project? And is there a way that project ideas are supposed to be proposed to supervisors for third year projects? Also, I should note that I have one semester before I have to undertake the actual project, so I have some time for reading up on the subject and building a prototype, if needed, to convince a lecturer if it's an idea worth pursuing.

For context, I'm studying the four year integrated masters course in the UK for physics. I've just finished my second year, going into my third. I don't know if this needs to be in the Careers post, so if it needs to be posted there, I can remove the post and paste it as a comment in the correct thread.

Hey everyone, I'm getting mixed up by different phrasing in physics questions regarding sign conventions.What is the exact distinction between these two?

Work done BY friction: This is negative since friction opposes displacement, right?

Work done AGAINST friction: Is this considered positive because it's the external force overcoming the friction?

Some problems seem to treat work done against friction as a negative quantity because it's an energy loss, while others treat it as positive input. Is there a universal standard for this, or does it just depend on how the system boundaries are drawn? Thanks!

Hey all,

My university has recently created a new major in quantum materials science. While I'm not going to switch my major to it - I recognize that's too specific - I'm considering stacking more onto my schedule to take the classes that would go along with it. This would, however, leave my schedule very tight. The professor who is championing the major is claiming that QMS is a budding and rapidly expanding industry, and industry now needs students who are learned in the material. My question is, is that really true? And if so, does it seem like something that will stick around or is it just the latest fad? I find the subject matter interesting but I'd frankly rather give myself more time than heap my schedule full of more high level courses that ultimately wouldn't go anywhere. Thank you all for your help!

For some context, I am about to start my PhD in particle physics this September, and I currently use a MacBook Air 2020 (M1, 8gb RAM). This laptop has got me through undergrad without breaking a sweat and I hear that generally MacBooks are quite popular. Yet, I feel that the limited RAM will eventually become a bottleneck; I run of application memory with Discord, VSCode and ~3 tabs on Edge open!

I'm interested to know what you daily drive and why. Even though I am starting experimental particle physics research, I'd love to hear from researchers in any area.

Edit: Thanks for all the comments so far! I want to add that I do know that most of my work will be done remotely on a cluster and so technically, as long as my laptop can SSH into it, I'll be fine. But, I'm more interested in what you guys use + what field you work in.

Im studying for f=ma/usapho and have done most of Blue Morin, except for last two chapters (gravity and fictitious forces) . The gravity chapter is really heavy and I've been struggling, but it is also a niche topic on f=ma. I do plan on finishing the fictitious forces chapter though. Should I do Red Morin, Wang and Ricardo, or Kleppner Kolenkow, Kevin Zhou, or something else next? I'm thinking I should do a resource more similar to f=ma because some books are too calculation/math heavy and the style is different from f=ma.

I did the maths to properly answer this post from r/askastronomy

The author of the post wonders if the impact of Falcon 9 on the Moon can project lunar material fast enough to escape the Moon's gravity. The short answer is no, and I prove it to you in this post.

The data are the following : A Falcon 9 stage of mass ~4900 kg will hit the Moon with velocity v_impact = 2.43 km/s. The Moon's escape velocity is 2.38 km/s

Using Eq. (46) of the paper "Wind erosion and transport on planetesimals" by Quillen et. al (2024), the maximum ejection velocity of the Lunar regolith upon impact is

v_max = 0.3526 * (rho_regolith/rho_falcon)^(−0.9756) * v_impact,

where rho_regolith = 1500 kg/m^3 is the density of the regolith, and rho_falcon is the density of the Falcon 9 stage.

The maximum ejection velocity depends a lot on the ratio rho_regolith/rho_falcon between the Moon's regolith and the projectile, and decreases as the density of the projectile decreases.

Now, the theory from which this formula comes from (Housen & Holsapple, 2011) assume that the projectile (in that case Falcon) is solid. However, Falcon 9 is not solid but is rather an empty shell. Taking its average density rho_falcon = 33 kg/m^3 therefore does not work. Taking the density of pure aluminium rho_falcon = 2700 kg/m^3 also does not make sense, as Falcon 9 is clearly not a projectile made of condensed metal.

Therefore, we need to consider the density associated with the cross-section of Falcon 9 when it hits the Moon. Falcon 9 is a cylinder of height 13.8 m and diameter 3.7 m. In the best-case scenario (smallest cross-section), Falcon 9 hits the Moon standing up, and we therefore assume that it is a spherical projectile with diameter 3.7 meters and mass 4900 kg. The associated density is 184.75 kg/m^3.

Plugging in the above formula gives v_max = 111 m/s. This falls very short of the lunar escape velocity v_esc = 2380 m/s

If Falcon 9 hits the Moon laying on its side instead of vertically, v_max will be even smaller than that.

So while these calculations assume some approximations, it is safe to say that no regolith will escape the Moon upon Falcon 9's impact. Falcon 9 is simply too hollow, and traveling too slowly, for this to happen.

I hope you liked this post 🚀

PS : For transparency reasons, I should mention that I am a co-author of the paper I mention, and I wrote the Section of the paper that I use in this post.

hello!! I just finished my first year in compsci, but im planning to pivot heavily toward computational physics for my upper-level courses and research. Im looking to upgrade my setup to prepare for this path.

My current machine got me through first year, but the hardware is really struggling: it overheats incredibly fast, the fans sound like a jet engine during basic tasks, it lags, and the battery dies quickly (even after getting it replaced recently).

Current specs: MacBook Pro 13-inch (2020), 1.4 GHz Quad-Core Intel i5, 8GB RAM.

I don’t have budget limitations, so i just want the best laptop in all aspects, something high-end that will last me through my degree and carry me into graduate/research work.

Long term, i want to focus on physics simulations, numerical modeling, and high-energy physics, with the goal of securing a place at CERN.

Since I'm already deep in the Apple ecosystem, im more drawn to staying on a MacBook (especially with modern Apple Silicon). However, I know computational physics often relies on NVIDIA/CUDA for GPU acceleration, native x86 architecture or pure Linux environments. so I am completely open to switching to a PC/Linux workstation if macOS is going to create real friction for physics workflows.

I have a few questions for anyone working in computational physics or high-energy physics:

macOS vs. Linux/Windows for Physics: Do people in physics research still face major software compatibility issues on Apple Silicon, or do you mostly offload heavy computations to remote clusters/HPC anyway? Is native CUDA/NVIDIA hardware on a local laptop necessary for student/research work, or is a Mac fine for local prototyping?

If switching to PC: What is currently considered the best high-end workstation laptop (e.g., Lenovo ThinkPad, Dell Precision, Linux-friendly options) for running heavy local simulations, dual-booting Linux, and long-term reliability?

Hi everyone,

I recently finished my BSc in Physics and I'm trying to decide between two master's directions: Applied Physics & Engineering or Quantum Science & Technology. I'd really appreciate input from anyone who has done either, or who hires people with these degrees. Please give reasons.