what topic in physics you never really or fully understood?

  1. I agree. When you are still thinking of light as a particle wiggling through space, it makes sense. When you get quantum and probability about it, it gets weird.

  2. Funnily enough, apparently there is a recent research paper that combines spin, circular polarization, and light polarity on a rather fundamental basis

  3. Stay away from Jones vectors - they're an overly-academic exercise that doesn't really do anything for thinking about propagating E&M waves from an antenna.

  4. Yeah, I remember my undergrad professor deriving it on the board using infinitesimal forces from tiny areas on a surface. And then when he scaled it up it turned out that an object that is really large in terms of surface area with a given mass spreads the individual fractional points out across a large area so each infinitesimal square contributes a small force times the friction coefficient. For the same mass (and surface material), a very small object has fewer points over the area integral but each of them contribute a higher force since the friction force is the normal force times the coefficient to friction. So it ends up with the same friction force due to a higher contribution from fewer infinitesimal squares.

  5. It is. The friction rabbit hole is deep. I did my dissertation on friction at the micro and nano length scales. They (for good reason) don't bother explaining the details at the large scales and just assign an experimentally determined number (that encompasses all the many complicated contributions such as surface area) since it works well for many cases of interest but in reality is more of a "rule of thumb" engineering quantity rather than a fundamental principle.

  6. I had the same thought too but I think it makes sense if you consider that friction is proportional to the force needed to lift an object a microscopic distance out of contact with a surface, so it can slide past the rough edges. This is a function of the object's weight, and not surface area.

  7. Because mass of an object is constant, if it lies on a smaller or larger area the pressure changes but the total normal force - the underlying consideration regarding friction - depends on the mass.

  8. correct me if im wrong but my understanding is that because no matter how large the surface area is, force applied to single point is inversely proportional to area. in a constant mass case, bigger the area less portion of weight pushes single point, smaller the area, more. like a pressure i guess. if you think that pressure is intermediate step in calculating friction coefficient everything makes sense, kinda?

  9. Tensors are remarkably simple but so poorly explained by every source I’ve seen that I’m convinced it’s a conspiracy or something.

  10. This video says magnetism in electric wires can be interpreted as electric fields in a relativistic frame. Not sure how it relates to bar magnets.

  11. I love magnetism, as a child magnets were true magic and enamoured me. Growing up I was often told learning the secrets behind something magical ruins it and sucks the magic dry.

  12. It’s so cool to wonder about it. It travelled so much distance just to hit a small detector in speck of dust floating out there in space just at correct time that we can observe! But yeah, it brings existential crisis along with excitement :)

  13. That's just because there's 10,000 bad pop sci explanations out there made by people who do not understand entropy themselves. It's not actually a hard concept.

  14. Suppose you have a coin and you want it to always come up heads when you flip it. You have two options: you can move weight from the heads face to the tails face so it lands on the bottom more often, or you can scratch a portrait onto the tails face. Moving weight makes a heads more energetically favourable; turning the tails into a heads makes a heads more entropically favourable.

  15. Well, that's easy. Levers. I think that I can fully grok leverage. Everything else is a work in progress.

  16. When I learned about Gamma Rays, I always wondered why visible light bounces off a mirror but Gamma Rays pass right through. I learned then that they interacted less with matter due to their higher energy. But then I wondered why higher energy would imply less interaction, then I learned that quantized electron energy states influenced the probability of interaction. I wondered why that was, and I learned that the energy levels of were based on the eigenvalues based on matrix formulation of electron energy operators. I wondered why that is but it seems that's as far as I can pull that thread

  17. https://physics.stackexchange.com/questions/8156/eigenvalues-of-an-operator-correspond-to-energy-states-in-quantum-mechanics-why

  18. I think the easier "back of the napkin" explanation is that waves always interact with stuff at the scale of their wavelength.

  19. I’m not a physicist by any stretch, I actually work in computer science but am quite passionate about physics. I remember reading in a book that you could essentially slap something so hard it catches fire due to the immense transfer of energy (please correct me if I’m wrong) and to this day thermodynamics has fascinated me

  20. Gyroscopes. Yes, I have had up to graduate level physics classes. As near as I can tell, a gyro is spinning so fast it can't decide which way to fall.

  21. I'm going to be a bit snarky and note that cutting edge physics is all about contending with physics that isn't fully understood. That's basically the job of a theoretical physicist.

  22. When you're done learning for classes you start to forget the things you don't need anymore and even more textbooks fill up. I think I peaked somewhere during exams for my masters. You also get dumber with age.

  23. Who the hell does? Just say something, something, SU(3) and hope that no one is paying too much attention to the words coming out of your mouth.

  24. Scary to think that 99% of mass in matter is QCD binding energy which isn't amenable to prediction by perturbative approaches due to asymptotic freedom so is basically empirical.

  25. Those band structure diagrams are such a terrible way of displaying band structures to students (although they are very useful once you understand what the image is showing you). They look so complicated because you're basically condensing 3 phase space coordinates into a single one by plotting a path through phase space instead of a full section of the phase space.

  26. If it helps, many features of such band structures can be anticipated by symmetries of the corresponding lattice, and of the translational symmetry shared by all periodic lattices. There is some rhyme and reason to that mess!

  27. Agreed. Angular momentum along one axis from a body rotating along a completely different axis is strange. I remember being so surprised that moment of inertia was a tensor. What does that even mean? Hoping someone here can give a nice wordy description of what all those off-diagonal terms actually represent.

  28. Noether's Theorem. You tell me that the electromagnetic force arises from phase symmetry of quantum fields. I understand what all those words mean in isolation, but I have no idea what they mean when you stick them together in that order.

  29. Had a physics professor tell my class if someone ever tells you that understand quantum mechanics they are lying. This was a 400 level physics class... on quantum mechanics... quantum mechanics was his area of research.

  30. This quote is the epitome of what is wrong in physics: authoritarianism and people repeating a quote just because an influencial figure said it 50 years ago.

  31. Can anyone recommend a book for quantum mechanics for a total beginner? (I have been trying to understand Schrodinger's wave equations but have a hard time grasping it)

  32. If a neutron and an anti-neutron collide will there be a typical matter/antimatter annihilation or will nothing happen because electrically neutral is electrically neutral, it doesn’t matter what the combination of quarks or antiquarks are.

  33. They will annihilate. Neutrons and anti-neutrons have opposite baryon number, and so there is no conservation law that prevents them delaying into photons. The cross-section is probably somewhat smaller than for proton-antiproton collisions though because they aren't charged

  34. Flux linkage. My E&M prof (an EE guy with a photonics background) insists that it has no definition and is just a product of our "engineering intuition".

  35. Time. no body knows what time is it right now. the time we see on our clocks is just made up time.

  36. I've never understood, at any level whatsoever, anything at all about string theory. Every single explanation I've ever heard or read just sounded like gobbledygook.

  37. The same reason it does the closer you are to gravity. More gravity = slower time. Forgive me as I won’t use the exact right terms but it’s simple gravitational time dilation. An analogy that helps me is that the more gravity the less energy is able to move. So it moves slower. Think of it like waking through air vs walking through a pool of water with weights on your feet. One has significantly more drag so takes longer. Time is just a measurement of change. Atomic clocks just measure 1 second as how long an atom takes to do a thing a specific number of times. 1 second will always be 1 second in our brain (assuming we aren’t high or tired or NDE) but that 1 second can take longer for our brain to process than if we were further away from gravity. GPS satellites show this and have time drift that has to be accounted for.

  38. Given a disk mounted to a shaft, in the manner that a car's axle is attached to a wheel, and you somehow were able to spin the shaft up to a speed where a single point along the circumference of the disk were traveling at 99.9999% the speed of light, what would prevent you from increasing the diameter of the disk to the point that a single point on it's circumference meets or exceeded the speed of light? What would actually be the limiting factor? Asking for a friend, who assumes material strengths and friction losses are non-factors.

  39. From the point of view of an outside observer a small length on the outside of the disk will be compressed tangentially from the Lorentz contraction so the circumference will become a bit smaller (which means the circumference divided by the diameter does not equal Pi anymore). Since the circumference is smaller the outer edge isn’t travelling as far meaning its speed isn’t as fast and it’ll never hit light speed.

  40. Potential energy. Would be glad if someone suggests some resource. Most of the working of the physical world seem to be on conversion of energy from one form to another. I have never been able to phrase/comprehend this conversion myself.

  41. Entropy, someone said to me (23M) that entropy, when you fully understand it, is terrific and if you are not terrified of it then you don't fully understand it.

  42. I couldn't get my head around the maths of Special and general relativity, and cosmology. This annoys me because I think it would be cool to understand where the impressive results you hear about in pop science come from.

  43. I have to admit that I don’t really understand temperature. The canonical definitions don’t help. Kinetic theory breaks down. In gas equations, I understand P and V but not T so much. It is part of boiling point. Why? It’s part of triple point. Why? My skin can sense it. But that seems like heat, but it’s not.

  44. Lattice Quantum ChromoDynamics... BS field I never understood and never want to understand.. fine they have a cool large Hadron Collider but F me!! Lattice QCD simulations make me want to put straws in my eyes and let my brain drain slowly..

  45. That a photon can change from a particle to a wave the moment you observe it. I'd like to know who was around the first 13.7 billion years to make that happen. Or did it start just as we came around to observe it. It doesn't make sense. Something else must be happening.

  46. Photons don't change from particle to waves. Photons are always photons. Particles and waves are just models that we use to describe how photons behave, but are each only applicable in certain scenarios

  47. What your intuition is telling you about force is really about momentum. A net force is a change in kinetic (or potential) energy. The bowling ball that’s dropped higher will apply a larger force when it hits your stomach but you don’t accelerate because the ground also applies a larger force upwards. The falling bowling ball has a constant force because it’s kinetic energy is changing.

  48. Why does interacting with a photon cause something to collapse into a single quantum state? Does it go back to being undetermined till the next photon bumps into it? Why?

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