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.

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

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.

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.

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.

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.

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.

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?

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.

Itâs 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 :)

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.

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.

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

Iâm 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 Iâm wrong) and to this day thermodynamics has fascinated me

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.

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.

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.

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.

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!

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.

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.

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.

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.

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)

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 doesnât matter what the combination of quarks or antiquarks are.

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

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".

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.

The same reason it does the closer you are to gravity. More gravity = slower time. Forgive me as I wonât use the exact right terms but itâs 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 arenât 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.

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.

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 isnât travelling as far meaning its speed isnât as fast and itâll never hit light speed.

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.

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.

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.

I have to admit that I donât really understand temperature. The canonical definitions donât 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? Itâs part of triple point. Why? My skin can sense it. But that seems like heat, but itâs not.

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..

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.

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

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 thatâs dropped higher will apply a larger force when it hits your stomach but you donât accelerate because the ground also applies a larger force upwards. The falling bowling ball has a constant force because itâs kinetic energy is changing.

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?

Light polarity.

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.

polarity doesn't seem so hard until you get to circular polarity

Polarization?

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

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.

I actually never heard of it đ

Why the hell surface area isn't a consideration of friction.

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.

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.

I know this and yet there must be more to it. Why do race cars have such wide tires?

https://physics.stackexchange.com/a/260997

It is! Itâs baked into the friction coefficient

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.

Increase in area causing friction is exactly offset by pressure since more area is less pressure.

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.

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?

I can help you with this

more of a math thing, but I'm reading a GR book and I'm really struggling with tensors

Tensors are remarkably simple but so poorly explained by every source Iâve seen that Iâm convinced itâs a conspiracy or something.

https://youtube.com/playlist?list=PLJHszsWbB6hrkmmq57lX8BV-o-YIOFsiG

An Introduction to Tensors and Group Theory for Physicists helped me a lot. Only need to read the first 2 chapters on tensors.

I made it through at PhD and never got particularly comfortable with tensors.

Have you watched MITs course of General Relativity, on YouTube?

Not a clue if this will help but I love this channel. Never done any maths work on GR

Magnetism.

Did my dissertation on magnetism. Can fully agree with you here.

I always think to a Richard Feynman âquoteâ that I am about to vaguely approximate.

ferromagnetism is two questions away from the boundary of the unknown.

Relevant SMBC

Fucking magnets, how do they work?

Magnet make metal go close to go boom

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.

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.

Yes

Yes

Yes

The Hamiltonian always baked my noodle.

Definitely my favourite musical

More than the Lagrangian?

How a photon can have travelled and not hit anything in 13 billion years until it hits my detector.

Itâs 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 :)

Space is very empty

Even more beautiful when you consider that 'my detector' could be your eye.

Entropy

You just need to be more organized.

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.

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.

For real

Why donât you ask what topic in physics you REALLY or FULLY understood?

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

I can tell you what will happen if you push a block with mass m down a frictionless ramp with force F.

Do we really understand anything fully, other than a harmonic oscillator?

Astrophysics made the maths make sense for some reason

Just about all of it.

Really?even simple laws?

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

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

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

gestures at universe

Thermodynamics

Iâm convinced that itâs comprehensible, but no one knows how to properly teach it.

the first thing my physics teacher told my class about it was that weâll never fully understand it

I was able to grasp nuclear, atomic, solid state, QM I and II but thermo kicked my butt

Iâm 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 Iâm wrong) and to this day thermodynamics has fascinated me

Thermo was my favourite module in uni I loved it

Yes

Thermodynamics: You canât win. You canât even break even

The fact that an electron's "mass is not distributed identically to its charge".

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.

Redshift was goldđ

Thanks for reminding me that I understand ring laser gyroscopes but not mechanical ones.

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.

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.

QCD

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.

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.

I did my dissertation on magnetism and can firmly conclude I understand less about it now

That's true of almost every topic I've ever studied

Solid state physics.

It's anything but boring! That mess has the same physics internally as particle physics

It's pretty simple, just ignore everything above or below the band gap! Problem solved, no need to thank me

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.

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!

Iâve never truly understood why the effective mass in a lattice is generally less than the mass of a free electron

Advanced angular momentum of rigid bodies with tensors and other stuff. It was too much for my brain

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.

Statistical relativistic quantum mechanics

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.

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.

I believe this is a direct quote from feynman

Isn't this a quote from Richard Feynman

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.

this is a famous quote that is just super wrong and should not be repeated as much as it is

Intuitively understanding the basics of QM isnât that hard. Understanding how to solve anything but the simplest problems howeverâŠ

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)

Relativity and quantum mechanics

They are pretty complicated

I do not understand what the fuck a tensor is.

Motherfucking angular momentum. Fuck that shit.

Momentum space

your name is phonon DOS and you don't understand k-space... there has to be a joke in there somewhere

QM....brute forced the math in undergrad.

undergrad QM: the study of a single orbit of a single atom by a single non-spinning electron for 5 months

Gravity

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 doesnât matter what the combination of quarks or antiquarks are.

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

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".

Rotation in general. Frisbees blow my mind.

Universe is made up of 23% Dark matter and 4% visible matter. Yet I cant find any Dark Matter around my house.

Have you checked in the refrigerator?

It's a combination of interstellar space being huge and dark matter density being so low

All of physics, iâm not the sharpest tool in the shed. I find all of it interesting but understand none of it.

Time

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

Quantum entanglement. The first time someone told me about it, I legit thought it was BS.

Physics

11 dimensional string theory. It hurts my brain

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.

Lets be realistic, no one actually knows what the fuck is the string theory

The first part

Physics. I donât understand physics.

Renormalization, shit feels patched and stupid

Continuum mechanics

Time delineation, why does time slow down closer to a black hole?

The same reason it does the closer you are to gravity. More gravity = slower time. Forgive me as I wonât use the exact right terms but itâs 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 arenât 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.

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.

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 isnât travelling as far meaning its speed isnât as fast and itâll never hit light speed.

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.

Superposition SchrĂ¶dingerâs âïž

Why light moves. WHY, not how fast or how. Simply why does it move. What causes it o leave itâs source.

How can light slow down in a medium and then accelerate once it exits into a vacuum again?

LSZ reduction formula

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.

Renormalisation.

I went and read about it,now I don't know my name

Math, just math.

The universe expanding everywhere at the same time.

Being able to see farther back in time with more powerful telescopes/equipment.

Many particle diagrams

Quantum mechanics. Then again no one does.

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.

The mass in relativity is scarier than anything

Conventional current

Quantumâboth undergrad and grad đ”âđ«

I'd say the Aramov-Bohm effect and the EPR paradox/Bell's inequality.

Any of it tbh

Equations related to magnetic fields creating current around wires. Wut?

Kinematics

Magnetism. Might as well be magic.

Eigen values of deformation and or rotation moment, woul be it for me.

Heat of fusion of ice, didn't get it, now, after geology, yeah, water is a curious phenomenon

Diagonalizing the Hamiltonian.

Nash embedding is black magic

Quantum physics

Gravity, and more specifically time dialation's relationship to gravity.

Fully understood?

Light polarization.

Truly and fully? All of them.

Friction haha. How is that shit not dependent on surface area?!

I have to admit that I donât really understand temperature. The canonical definitions donât 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? Itâs part of triple point. Why? My skin can sense it. But that seems like heat, but itâs not.

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..

all of it.

I thought I understood thermodynamics until I took an advanced class in grad school.

Graduate level Statistical mechanics was hard.

Everything with circuits for me.

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.

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

Gravity and dark energy (Fred)

Entropy. It was just some formulas that you ran and got a number from.

Optics

I never understood how light works. If someone could try and explain I'd love that

Probably spinor fields and Lie groups.

Physics

All of it

Huygens Principle.

Not so much didnât understand, but didnât like the answer.

Friction

working of transistor, how a small current control other big current. and current flow in general, specially after varitasium video.

string theory. I need not explain why.

Einstein

Escape Velocity

What makes matter move along lines in Spacetime?

Anything that has something to do with quantum physics should be an objective answer.

Quantum gravity!

Why force is considered the same (regardless of speed) for an object of a certain mass and acceleration.

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 thatâs dropped higher will apply a larger force when it hits your stomach but you donât accelerate because the ground also applies a larger force upwards. The falling bowling ball has a constant force because itâs kinetic energy is changing.

While I have books on gauge theory, I have never been able to get an intuition for it.

this question

Physicsđ”âđ«đ„Č

Thermo.... Never really understood how things down there... Also Electronics.

String Theory

physics

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?

Thermodynamics. I got the top grade in that class but feel like I understood nothing.

I've never understood how time could "speed up" or "slow down" or how it could be bound to physical space.

All of it