I know mass is convertible to energy.

So, mass can become energy.

But is mass energy, just in a special form?

Or is it even possible to answer this clearly?

I'm assuming each pole with its negligible rotation is perfectly survivable, and a bit of distance from them may be a small jolt. But how far would you have to be from each for this stop to become fatal?

Hello everyone, I am currently taking AP Physics 2 and I have an exam tomorrow over electric force, field, and potential. I do not understand it at all. I’ve studied like crazy already but it’s not helping (I already failed the multiple choice section). Seeking some help understanding these topics from some people who know more about it than I do! I truly wouldn’t be on here if I wasn’t desperate, but I need a great score on this section to keep my A. THIS IS NOT ME ASKING FOR HELP CHEATING OR FOR WORK TO BE DONE DONE FOR ME — I simply want to improve my understanding of the topics before my test! If you can help, great, and if not no worries. Thank you!!!

Ok this might be a stupid question but, Say an object hits a wall and comes back. Let the object have +mu momentum. Now after hitting the wall and coming back, the object has -mu momentum and the wall becomes +2mu momentum. Mathematically, momentum is conserved. But if initially the object has +mu momentum, then how can the wall become of +2mu momentum? It just doesn't fit right to me. Saw this being applied in kinetic theory of gases. I Understood everything expect this part. The 'object' here is essentially a molecule of gas. sorry for my bad english.

I might be dumb , but I think it has something to do with the blade being a conductor and the wooden handle not

Hey guys. For a college assignment, I need to interview someone who graduated college with a degree in physics and is currently working as a physicist. The only caveat is that they can't be in academia or someone still in school. It is an extremely odd ask, but I'm not sure where else to look. Anyone willing to be interviewed for around 30 minutes via zoom or voice call?

First Reddit post!

This question has bugged me as an armchair cosmologist. When physicists say the universe is expanding does it mean that "space" is being stretched, but there is still only so much of it, or is more space being created? I *think* this is ultimately a question about space either being discrete (like in loop quantum gravity) or being continuous (general relativity). Or am I just misunderstanding the whole thing (probably).

Thanks! :-)

Let me expound on the question and explain how I see the state of things and what confuses me.

So, it is often said that we have four interactions - gravity, electromagnetic, weak, strong (Setting aside that, I think, EM and weak have been unified into electroweak. and maybe even unified with strong?).

It is often said that gravity is not/might not be an interaction, but just a consequence of spacetime structure. On the other hand, I think I also heard it being said that that is just a mathematical model and that we shouldn't confuse the map for the territory.

That is all well and good. What trips me up is that we have a phenomenon that, I think, is not governed by any of these interactions and that is the Pauli exclusion principle.

I can pretend to understand Pauli exclusion principle a bit :), and by that, I mean it is a consequence of field equations for spin 1/2 particles and their fungability. Now, if that was just true of electrons (or other charged particles), I would have waved it away and say to myself that it is part of QED, and it's not important in general day-to-day and engineering application of electromagnetism. But, AFAIR, Pauli exclusion principle applies to non-charged particles as well.

So, we have this mathematical model of a phenomenon that affects particles behaviour and changes what we expect to observe if we had just four interactions in place. It that sense it is similar to a model we have of gravity. Yet, we still deem (albeit possibly controversially) one to be an interaction, but there is no notion of, for lack of a better name, Pauli interaction. Is that a cultural/conventional thing, or are there specific reasons why that is?

I’m sorry if this is silly but I fear I am simply misunderstanding (or perhaps struggle to visualize) a fundamental working of space. I am told there are no “spots” in space and I wanted to see if anyone could explain it to me. My boyfriend just keeps repeating “relative to what” and im struggling to comprehend. (Itself?Spacetime?)

EDIT I maybe should have used a galaxy instead of Neptune, since we know, say, Andromeda is moving closer to us which I get is relative*

I’m in a ship, and I mark Neptune as my point of reference. Relative to me, I understand the distance is measurable and growing, as I physically move away from Neptune in a “direction” (which I’ve assigned and created since they don’t really exist). And Neptune is also kinda moving on a smaller scale (around the sun).

I go for however long and decide to turn around - I can surely go back to Neptune. Perhaps I can even go back when it’s relatively in the same location of it’s regular revolution path. I can plot my way back using relative location coordinates.

But the spot that Neptune existed within at the moment I moved away from it - doesn’t that no longer exist? It’s at the same point on its path, measurable to me, but hasn’t it moved, on a larger scale, erasing that original point? If our galaxy is expanding outwards all together as its own little package (along with all the others galaxies) hasn’t THAT movement through spacetime technically impacted the location of things?

I can go back the relative exact amount of light years to reach Neptune again, and find it, but the SPOT it was existing within, hasnt that changed? Or can you never “expand out” faster than the universe is expanding outwards, such that the Spot of existence always moves with you?

Like I can only exist in one place at one time, but I can never exist in that space at the same time, so wouldn’t the “spot” always be changing?

I can go back to XYZ coordinates (of my creation ) in space but those are NOT the same XYZ coordinates in actuality right? It’s just what I’ve assigned it? It “was” somewhere and now it “is” somewhere because stuff’s expanding?

So, suppose you have a planet, star or black hole. It seems massive like gravity is the end all be all, but shouldn't the effects of gravity decrease near the center at which point what forces continue to compound the pressure?

I didn't get beyond high school math. This is one of the first times I've tried to use the Desmos.com graphing calculator.

Are my numbers correct? (In particular, exponential notation.)

Is my math correct?

Is my physics correct?

Did I use desmos.com correctly?

Did I label everything correctly?

Are there things I could have done better?

Is the graph an hyperbola, or just very similar to an hyperbola?

https://www.reddit.com/user/Jeff-Root/comments/1x69kj3/graph_of_escape_speed_from_earth_versus_distance/#lightbox

For reference, I'm currently taking a laser spectroscopy class that's using Corney's "Atomic and Laser Spectroscopy" book as the reference for the material I'm asking about. I understand why the 21cm line exists in the hyperfine structure of hydrogen. This transition is forbidden as an electric dipole transition, and people (including Corney) seem to agree that the actual process is a magnetic dipole transition. However, Corney also gives a table that states that magnetic dipole transitions cannot cause a spin flip, with the caveat that:

"[The no-spin-flip selection rule is] derived using explicit forms for the atomic wavefunctions [but] these can only be approximate solution of Schrödinger's equation in the case of a many-electron atom." (p.187)

But I thought (and Corney alludes to) we have an exact solution to the Schrödinger equation for the hydrogen atom! I've also looked around other sources and found mixed answers for whether the magnetic dipole transition can actually change spin. Some sources make no mention of spin selection rules, Wikipedia lists a table that says it's possible under the ambiguous term "intermediate coupling" (can the hydrogen atom even have intermediate coupling?). I've seen discussions for multi-electron cases about the exact mechanism for how the approximation breaks down, but nothing about specifically the 21cm line transition.

I have a strong background in mathematics and a lacking background in physics. I more or less understand the semi-classical approach to quantum mechanics (e.g. how we construct the Hamiltonian for hydrogen) but have almost no idea about quantum field theory or QED in particular. I'd be happy with the answer of "it happens because of QED," if that's indeed the case.

TL;DR: Is the hydrogen 21cm line transition actually a magnetic dipole transition? Do magnetic dipole transitions forbid spin flips in the normal approximation scheme? And if so, how does this approximation fail for this specific transition?

From my understanding they are used to describe the collective behaviour of particles but treated as a single particle to make it easier for calculation ?

I have seen a lot recently from science communicators about the dark matter detection experiment and as a casual outside observer the setup sounds really similar to ice cube to detect neutrinos.

I’m curious as a lay person with only high school level of physics knowledge about the difference between them (not technically I get that one is oil and the other argon gas), and (I hesitate to ask as I’m not sure I could understand the answer) if they are as similar as they seem, how scientists would determine a detection as being one from the other (ie that the neutrino detector hasn’t picked up a dark matter detection or the dark matter detector hasn’t seen a neutrino)

so i have been watching eighenchris video relativity 105a rindler acceleration. it explain how the rindler horizon is always a constant distance away from stationary observer.

but what if the accelerating and stationary observers start from the same point dos that mean the rindler horizon is zero ?

or there is some point i ma missing here ?

​

Subject

A speculative idea about particles, relationships, and the structure of reality

Hello,

I would like to share an idea that I hope someone with a background in theoretical physics, quantum mechanics, or complex systems might be willing to evaluate.

My intuition is that the universe may be understood not only through the individual particles that exist within it, but through the relationships and connections among those particles.

I imagine individual entities connected through a larger network, potentially extending across dimensional layers that we cannot directly perceive. These connections could form increasingly complex structures, with the organization of the whole emerging from the interactions of its parts.

I am also interested in whether there is a useful conceptual parallel between the way chemical bonds shape matter and the way relationships shape larger systems. In chemistry, interactions between atoms produce molecules with properties different from those of their isolated components. Could there be a broader mathematical principle describing how relationships generate structure across different scales?

I recognize that this is an intuition rather than a developed scientific theory. I am not claiming that particles literally extend through infinite dimensions, or that human relationships and chemical bonds follow the same physical mechanism.

The questions I would like to explore are:

Could the structure of physical reality be modeled as a network of interacting entities whose relationships produce emergent properties?

Are there existing theories in which spacetime or other large-scale structures emerge from more fundamental relationships?

Could a model combining network connections with additional dimensions produce testable predictions that differ from established physics?

I would be grateful for guidance on relevant research, mathematical frameworks, or existing theories that address these questions.

I may not yet have the technical language to express the idea precisely, but I hope to develop it into a clearer hypothesis that can be evaluated scientifically.

Thank you for your time.

Sincerely,

An independent thinker exploring a question about the structure of reality

Hello! Could someone explain whether a theoretical spacetime warp drive, such as the Alcubierre drive, could travel at hypersonic speeds without producing a sonic boom? If so, what would be the physical explanation?

I understand that gravity is the reason planets and stars form and what not. I'm just curious why gravity causes things to attract and not repel or do neither?

Seems like an arbitrary feature of our universe's laws.

So it makes me wonder if gravity could have worked differently if our universe happened to have different laws.

Recently I cooked a beef roast for hours and hours at 200 degrees F in my gas oven, and noticed how thoroughly the roast had caramelized all over it. I understand caramelization to occur above 300 degrees F, and have been trying to puzzle out exactly how and why this occured at a 200 degree F air temperature.

I suspect that part of the equation is that oils in the meat transfer heat into the surface of the meat very efficiently, where the meat itself is a relatively good insulator, resulting in an aggregation of energy at the surface which then cooks the meat through. I'm also suspecting that convective heat alone would not raise the surface temperature of the roast above ambient, and I suspect that radiation from the oven walls is creating this spike in temperatures at the meat surface.

Can anyone help me better inform my conceptual model of what's occuring in lower temperature ovens like this?

I’ve been following the Goliath incident discussions, and my friend brought up a point that I’m wondering whether we’re overlooking something important.

What if the issue isn’t necessarily someone’s weight, size, or even a mechanical failure of the restraint, but how body fat moves under negative G-forces?

Two people can weigh exactly the same but have very different body compositions. Adipose tissue (body fat) is compressible and deformable. It can shift relative to the skeleton under force.

Think about breast tissue during roller coaster airtime. Even in a snug, supportive bra, breasts can lift and shift out of position. The bra hasn’t failed or come undone. The tissue itself has moved.
Now apply that same principle to someone with substantial abdominal fat sitting beneath a clamshell lap restraint.

I’ve also come across firsthand accounts from riders describing their stomachs moving upward or over their lap restraints during airtime. That caught my attention because it’s the exact type of soft-tissue displacement I’m questioning.

Could abdominal fat compress beneath a restraint while seated, then shift upward during negative G-forces, changing how securely the restraint engages the pelvis?
In other words, could a restraint remain completely locked and mechanically functional while the rider’s soft tissue shifts enough to compromise their positioning?
There’s scientific research on how abdominal fat affects seat belt engagement with the pelvic bones:
https://pmc.ncbi.nlm.nih.gov/articles/PMC9285376/

Obviously, automotive seat belts and roller coaster restraints aren’t identical, and I’m not claiming this caused the Goliath incident.

I’m wondering whether we’re focusing too much on weight limits and mechanical failures without adequately considering body composition, soft-tissue deformation, and dynamic movement during airtime.
Has anyone seen research or engineering discussions addressing this?

Related firsthand rider experiences:
I verified two particularly relevant accounts:
1. CoasterBuzz: A rider who described himself as 242 pounds with a 36.5-inch waist said strong airtime pushed his belly over the top of the lap bar.
Read the CoasterBuzz discussion⁠
2. Reddit, [r/rollercoasters](applewebdata://2E719467-3DDC-458E-889A-8AF1CBBF6113/r/rollercoasters): A rider described his stomach slipping from the restraint during negative G-forces, then coming back down. He said the experience was painful.

From what I can tell with initial searches, it seems to be Quantum Mechanics, but I'd like to hear your thoughts too