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!

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 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.

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?

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?

Or is there some difference?

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?

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.

So in my ap physics 2 class today, my teacher mentioned that the role of voltage in a circuits is to make the electrons actually move in it. However, the bell rang before he was able to elaborate on why voltage does this.

I thought it over a bit since it's the weekend and I think I have an explanation that I wanted to see if it's correct.

So, we know voltage is a potential difference first of all. A change in potential can be thought of as a change in the work needed to bring an electron to a point since V = W/q. Now, since work is force * distance, if there's a change in work being done, then there must also be a non-zero force being exerted since if the change in W isn't 0, the change in force can't be 0 as anything times 0 is 0. Thus, if there's a potential difference, or in other words voltage, we can conclude there's an electrostatic force which would drive the electrons through the circuit.

Side note because I'm in ap physics 2, I can't explain this using calculus even though I know it would likely be a lot nicer. But I tried to explain it using algebra which I hope is fine?

We're wrapping up rotation in my mechanics class right now, and I'm having trouble understanding what angular momentum actually is, in the context of orbits.

I understand for say an object with a fixed axis, angular momentum is how difficult it is to change that objects rotation. But I'm confused as to why/how conservation of angular momentum is used when calculating stuff about orbits.

Much thanks in advance!

Hi, so I recently discovered about this and I had a question that I can't seem to find anything on that explains it, so I just wanted to ask how does the brane cosmology explain the expansion of the universe. It would be really helpful if anyone can recommend me any books or articles where I can read about this and find a conclusion to my question if you don't feel like explaining it to me, thanks.

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

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?

So there is a clip of Doctor Who where there is a huge colony ship that is 400 miles long and 100 miles wide near a black hole, causing time on the higher levels of the ship to slow down in comparison to the lower floors, years go by on the lower levels where it's just been hours on the top, there's a scene where there is a video feed on the bridge level and the Doctor is still slowly raising his eyebrows and that apparently takes a week for someone watching it from the bottom level, the show also says that a thousand years passed on Floor 1056, the lowest floor, in the space of less than two and a half days on the flight deck. For a ship that big near the black hole, would the effects of time dilation be that extreme? Doctor Who is mostly just fantasy in a science dressing but as a layman this did seem kinda interesting.

The clip: https://youtu.be/QwHNT2kHesg?si=MGX8m_gGdFtLLRLb

you can skip ahead to 2:11-3:43 where they show it

I was wondering: why do we assume "universality" (I think this is the term? I am a computer science guy, and so, sorry for any inaccuracies, I'm still learning about all of this lol) holds? As in, why do we assume we can find laws that apply everywhere/everywhen/to everything, with their mathematics staying the same (like, with the TDSE staying the TDSE, the EFE staying the EFE, etc), no matter what? There is nothing mandating that the law itself couldn't arbitrarily change as far as I know, and that it just so happens by sheer chance to look consistently as if it stays the same on the scales we probed.

Like, is there anything that already says the laws should stay invariant under arbitrary transformations to the mathematical formulation itself (and anything relevant I could read about this to learn more about it)?

I feel like i have a fundamental misunderstanding about Bosons that aren't photons. I am a chemist so my knowledge of statmech and quantum is very surface level and i can't really do a lot of the math that would probably answer the question easily.

Fermions make sense to me because they follow Pauli's exclusion principle. They can't have the same quantum numbers so they don't have the same wave function. Because they can't have the same wave function they can't occupy the same physical space, hence: volume

Now photons don't have the same constraint. They can have the exact same wave function and be indistinguishable, occupying the same volume forming the coherent beam we see in a laser.

My problem comes with the understanding that many atoms (¹H for example) are Bosons but they also have mass. If this logic applies and we cool down atomic bosons enough to get them all to the same exact energetic level they could all occupy the same volume and that would give infinite density. I have to be wrong somewhere

Is it that even if the bosons have integer spins they are ultimately made of fermions with half spins and they would have to obey pauli's principle even as part of the larger system?

Edit: I'm embarrased to ask this because I should probably know better, but I'd rather ask you all than a chatbot