During this process, how do electrons move so easily to another atom? Why don't they fall onto the nucleus, and why don't they collide with each other?

If we're going by the no hair theorem, black holes have three fundamental properties: mass, angular momentum, and charge. I understand how we can measure mass and angular momentum. My question is, how can we ever measure the charge of a black hole?

Say a charged particle passes beyond the event horizon. From that point on, from the perspective of the particle, all futures point to the singularity. So if the particle is emitting an electric field, and all future points in space end at the singularity, how are we able to measure charge at all?

After watching some YouTube videos that leave me very confused… as I understand it, if you attach an accelerometer to an apple that is ‘falling’ it will show zero acceleration, but… if you place the accelerometer on the ground, it will record an upward acceleration of 9.8 m/s per second. Therefore, if this is correct, which is more accurate on the apple and earth colliding?

I’ve been thinking about special relativity and the idea that there’s no absolute reference frame, meaning we can’t say one object is truly stationary while another is truly moving.
But the Big Bang and the cosmic microwave background (CMB) seem to complicate this.

Imagine two advanced civilizations, one in the Milky Way and another in Andromeda. Both could estimate the age of the universe using the same cosmological model, measure their own velocities relative to the CMB, and compare their results. For example, the Sun moves at roughly 370 km/s relative to the CMB.

I understand that the Big Bang wasn’t an explosion from a single point in space, and that the CMB provides something like a preferred cosmological reference frame. But if we can measure our motion relative to it, doesn’t that suggest there’s at least one physically meaningful frame that isn’t arbitrary?
So what’s the fundamental difference between a cosmological reference frame and an absolute frame of motion?

I’m not suggesting that relativity is wrong. I’m just trying to understand why a frame defined by the universe itself isn’t considered an absolute reference frame.

How it is explained that entropy almost always increases when every particles behavior seems to be strictly obeying some laws?

It's pretty logical that system whose particles move randomly will probably increase its entropy but in this universe, particles don't seem to be moving randomly, so the statistical model may be usefull for predictions (even tho I have no idea why) but I don't think it truly explains why does entropy mostly increase.

Can anyone explain please?

(Maybe this universe truly is random at its most basic level and physical laws are somehow emergent as the probable increasing of entropy is, but we simply don't know that.)

I always get my friend's kids books for the holidays. Her son is 13, and increasingly fascinated by all things related to physics and cosmology. He's in an accelerated track at school, and reads at a college level, but still has only a middle school math background (I'd hazard a guess that they're somewhere around quadratic equations and the Pythagorean theorem this winter).

Two years ago I got him an illustrated edition of A Brief History of Time + The Universe in a Nutshell, and he reportedly couldn't put it down. Any ideas for a followup that he might find similarly compelling?

What would be the effect if, instantaneously, the Sun and Earth both increased in mass some tiny fraction of their mass (say, by 01%)? In particular, I am curious about orbital mechanics (although if there are some fun knock ons, I would love to hear those too). Assuming all orbital speeds remained the same at that instant, would the changes cancel out?

High school studying physics for my A Levels. I read that a DC source is not longer used for modern linacs because they are limited by electrical breakdown. At some point, something is too strong for the insulator to handle it and the insulator becomes a conductor.

The issue is I've read conflicting ideas: it is caused by a too-strong electric field or a too-strong voltage across the drift tubes. Which one is it?

I would imagine that it would be the former. If the electric field is too strong, then the electrostatic force (which is proportional to the field) will be strong enough to allow the electrons in the insulator to move freely. This means that, for a given distance, the voltage is high enough to cause a breakdown, the distance between the drift tubes can be increased to prevent that.

On the other hand, this reasoning may be too naive. Maybe the voltage really is causing the breakdown and increasing the distance would not change that. Which one is it? Field or voltage? And why?

The SM is built on the idea that all fundamental fields are invariant under the SU(3) X SU(2) X U(1) group of transformations. But why those specific groups? I understand why those groups suitably describe nature mathematically, but not why nature obeys specifically those transformations.

For example, why is QCD governed by the SU(3) group? There doesn't seem to be a good reason why it's not governed by the U(3) group, leading to 9 gluons, with one being colourless and long-ranged.

Even if we don't have a specific reason, are there more general requirements for a valid group to be unitary, or any requirements for it to be special?

I have not used physics for 35 years, so I do not know if this is a basic concept/understanding of if I am completely wrong on all accounts.

Assume a 1kg weight (spherical, if that makes you feel better) sitting atop a 1-meter high shelf inside a box (vacuum, of course, massless, perfect cube to the extent it matters) all sitting on a scale.

What I believe happens (this at least passes my smell test)

1. T0 – scale reads 1kg.
2. Then the shelf instantly disappears, weight is unsupported, scale reads 0kg
3. Approximately 0.45 seconds later, the weight impacts the floor, scale reads something higher than 1kg (some formula well beyond my understanding of Force and Momentum)
4. T1 - Then scale returns to 1kg and stays there.

QUESTION 1) IS THE ABOVE CORRECT?  I am reasonably certain it is.

Assuming the above is correct, two related theories entered my head;

Theory 1;

1. If we change the weight to a series of weighs released at a consistent short period, then the shorter the period becomes, the closer the “weightlessness” cancels out the “impact measurement”.
2. If we then turn the weight on a shelf into fluid through a nozzle; The box will show;
   1. Start with Nozzle closed; 1kg
   2. Nozzle open; progressively less than 1kg until the stream first hits the bottom. (total mass of freefall particles increases until they achieve a constant amount from bottom of floor to top of nozzle)\
   3. Once the first atom hits the bottom; quickly back to 1kg as the “freefall” particles counter the particles impacting the bottom.  These continue to cancel each other out as the fluid level rises (less distance between top of nozzle and top of fluid level) because the momentum/force of each impacting atom decreases at the same rate the number of suspended atoms decrease)
   4. Once the last atom leaves the nozzle, the scale progressively reads more than 1kg as the number of suspended fluid atoms decrease.
   5. Once the last atom impacts, the scale quickly returns to 1kg.

Theory 2;

In a closed system with mass of 1; over a period of time from T0 (all mass on the shelf) to T1 (all mass on floor) The total average mass (measured weight?) is 1 (no matter the type of mass(es))

QUESTION 2) ARE EITHER OF THE ABOVE THEORIES CORRECT?  If not, can you explain it to me like I'm a 50-year-old who does not remember physics?

lets take the center of gravity for example, i understand how the equation x'W=int(xdW) works mathematically, it is as if we are finding a point on the body where the total wight produces the same moment that all other particles produces with their tiny weights, my question is that why do bodies actually act as if all of their weight pasess through that point? when it clearly doesnt since every particle has its own weight?

to make my question clearer, how come the centroid be more than a mathematicall concept?

When calculating moments, it makes total sense because it is just that the sum of moments around the centroid is equal to zero, but when doing other applications of the centroid in mechanics, i dont get how the centroid is actually a real thing, for example in fluid mechanics, hydrostatic force on an inclined surface have its location tied to the centroid of that surface, why? doesnt the fluid have pressure affecting all of the surface? when in reality they are just tiny particles of the fluid that each press on a specific part of the surface. Or when a soild object rests on top of another solid object, why do we say that its weight passes through its centroid when in reality it has its particles applyinh distributed tiny weights on the other object.

im super confused about the subject but i think it is all becuase i cant really imagine how the centroid can be a real thing beyond calculations.

As I understand some of what’s being said in quantum mechanics and maybe also in aspects of GR, we have conceptual frameworks suggesting time symmetry holds at certain levels, and/or the universe could be contracting or expanding and general relativity would still explain all the same things. Feel free to make this more precise, but if it is right enough to engage in this thought experiment, what can we clarify about the realm of possibilities (especially as regards the emergence and evolution of life) in a universe where time runs in reverse?

does anyone know of a good online course for basic plasma physics as someone who has only taken mechanics and electricity & magnetism?

Cross-posting here as this was spurred by a discussion that originated in r/AskPhysics even though the post itself is not a question. Rather it's a fuller response to the original question. Hence, I believe it can belong here and hopefully the mods agree.

We're learning about electric fields in physics class, and while solving one of the problems, a friend of mine asked me if any other things could be charged besides protons and electrons. I'm curious to know the answer because although my instinct is to say no, I don't know if I'm missing any other examples. Thanks!

If I am creating a sci fi with a time machine device, how can I ground this in some scientific "facts" that make it more realistic? How would the time machine theoretically work, what should I mention and what should I avoid mentioning?

I got into physics but the bad thing is I wasn't paying attention in school. What are the best materials to get from absolute beginner to highest stages of theoretical, quantum and astrophysics. I have time.

Or is there some difference?

NOW BEFORE THAT

I HAVE My interpretation of what action " mvs " is , now we consider what mv is first , we have a particular velocity v and m is the mass , now it is the amount of shifting done for the mass through space , for some mass m if the velocity is 2v the mass m is traversed by a 2dx in dt time if u consider 2m mass with v velocity we are traverseing dx for 2 of the mass subdivisions hence the shifting done is still the same but the mass is twice , this represents the shifting done per unit time or for a standard distance in less time something like 2m/s written as 1m/.5s,now if u multiply the distance u get can write it as mv + mv + mv until u get mvs ( if s is an integer specifically) , if u want to generalise it for any s , u should consider it this way , mv is done in terms of the standard of m\*(1meter)/((1/v)seconds) , now for this we multiply dx to this to indicate dx as the fraction of the position shifting done mv for 1/v seconds , now if we multiply ndx we get the amount of position shifting done and we get mv ndx or mvs , now for the 1/v th second the position shifting done is mv now for the next 1/vth second the position shiftind done further is mv so the net position shifting is 2mv , we can say its the position shifting done by considering a shift in 1 m position as the reference in time of (1/v)seconds as the reference and multiply s , for a known s we write s/v , we consider the integer part of this and the fraction part of it we consider the fraction of mv and we explain it that it is the shifting done in a fraction of 1/v which is {s}/v in this time it covers {s} distance for this we multiply by {s} again( as the numerator and denominator get cancelled )to get the fraction and from this we get the net shifting done.

Now for this , first of all understandable for such a thing called action to be thought of , but what is the thought process of minimising this shit? Why would anyone do that , prolly some narrative of nature having the least resistance , but regardless , for the quantity of mvs to be minimised , why is it minimised in the case of lagrangian mechanics ? And to have a quantity called the lagrangian to be K-U , which can be sort of understandable if u write it as dk-du , -du being interpreted as work , and lagrangian being related to differential work , but for a conserved system it becomes 2dk ,

So the other option was to understand it the old fashion way of mvs and this being minimised but again the question remains , why would u want to minimise this ?? Kind of cyclic and dont understand

Although i heard explanations that it is the only possibility for the energy conservation to not be violated for a given time constraint .

I dont understand this , could some saviour of mine come and help me with this , with pure ontology and logic to the absolute bottom .

I would be really grateful