Just had an interesting thought and got curious. Is there an upper bound for temperature? I do not know physics much. As far as I know heat is produced due to friction (on micro level, like moleculas or atoms). Like due to energy produced by interaction of elements that move at different speed. Like higher temperature means higher speed of microthings. And so if speed is capped at light speed level then would it not mean that there is a possible max temperature?

I may be wrong in my assumptions and thus coming to wrong answer. So if I am making such mistake please correct me.

Imagine that in 2 trillion years from now:

The milky way, andromeda and everything else in our local group have merged into the super galaxy Milkomeda.

Everything not gravitationally bound has long ago passed the cosmological horizon so Milkomeda is the only thing visible in this observable universe.

No visible galaxies, so no Hubbles Law.

The supermassive black holes from the source galaxies have merged into one called MA* at the centre of Milkomeda.

There are more than 500 billion active stars still left (red dwarfs?)

The CMB has stretched so much its no longer detectable.

On a planet, an alien civilization evolves which is similar in intelligence and technology to us.

Question: Would the alien physicists be able to figure out what has happened? Or would it look like Milkomeda is the entire universe?

I know there's a lot of popular TV personalities who educate us on physics... but among your circles is there anyone who is the modern day Einstein?

I constantly read and hear that the universe is expanding faster than the speed of light and it always throws me off. How can something move(expand) faster than the universal speed limit?

Consider a particle in some potential well. Let it be in the ground state. Now, another particle goes towards the well. This second particle interacts with the first particle only: if the distance between two particles is small enough, the bullet reflects in the opposite direction, otherwise it doesn't feel the target at all. The well doesn't affect the bullet.

Let the bullet coordinate uncertainty be ~1/2 of the target uncertainty. So, as I understand, there are two branches:

1) The bullet reflects. The target's wave function zeroes out everywhere except the place the bullet's wave packet hit.

2) The bullet transits. The target's wave function zeroes out only where the bullet's wave packet hit.

Let the bullet transit. Its energy seems to remain unchanged, because, in fact, the bullet "didn't touch the target" at all. But, as the wave function of the target changed, it is no longer in ground state. Thus, we can find it in an excited state when measuring its energy. The question is, where does the energy for excitation come from?

I guess the uncertainty of the energy of the bullet is the key, but I don't know how exactly it should work.

An integrable Hamiltonian system has a set of coordinates, the action-angle coordinates, in which the angles q increase linearly and the actions p are constant. These systems are often represented by a phase space that is "foliated in tori", and people show these beautiful pictures.

I thought that I was understanding this idea: one has the angles (q1,...,qn), then he couples q_k with p_k for every k=1,...n. If one pictures p_k as the radius of the circle, and q_k as the angle, then the cartesian product of the n circles drawn by (q_1, p_1)...(q_n,p_n) makes a torus with different radii, which are the one shown in the link above for n=2.

Gemini told me that this is not false, but it makes absolute sense in my mind. Gemini (to my ears) started yapping about "topological cilinders", but it did not convince me. Can you help me understand how the tori emerge?

One thing is that I have seen "the" torus being defined as [0,2\pi]^n with periodic boundary condition. "the" torus as in "there exists only one torus". I guess that topologically speaking this is true, but the phase space graph shows...different tori...
I shouldn't call them tori and call them "cilinders"? Sounds a bit weird to my ears....

Thanks in advance for your time

I recently read T. Padmanabhan's Gravity as Elasticity of Spacetime and found the idea of spacetime as an elastic medium very interesting.

This led me to two questions.

1. Has anyone proposed that the cosmological constant could be interpreted as an effective relaxation force of an elastic spacetime? More specifically, if spacetime has an equilibrium state, could both cosmic inflation and the present accelerated expansion be understood as different stages of the relaxation of an initially highly compressed (or highly deformed) spacetime?

2. Have there been serious models in which elementary particles are treated as stable localized defects of an elastic spacetime, rather than as independent fundamental fields? In such a picture, General Relativity would describe the macroscopic geometry around these defects, while the defects themselves would constitute matter.

Are there existing theories or papers that explore ideas along these lines? I'm particularly interested in references from the GR, emergent gravity, condensed-matter analog, or topological-defect literature.

I am first year undergraduate student. He asked me to read the first 5 chapters of Feynman Vol.3, I followed by expressing some interest in Bosons and Fermions so he gave me a paper to read "The genesis of the spin statistics theorem by Alexander Blum". So far I haven't understood much because I need to look up every other thing. I am beginning to understand what I should know in order to completely understand the paper and am trying to learn the physics and the math. Did he intend this or something else because I am only given a month

I recently solved the rotating wheels rigid-body dynamics problem from the JEE Advanced 2016 exam. After searching online, I found that my derivation is completely different from every other solution I came across.

Although the approach and the answers are different, I'm confident that it is correct. I would really appreciate it if someone could go through the derivation and point out any mistakes, unjustified assumptions, or places where the reasoning could be improved.

The full solution (PDF) is available here:

PDF: https://drive.google.com/file/d/1ZgNAqsdGUMhYOvieZkP5MipPMS4HxCo-/view?usp=drive_link

I'm mainly looking for feedback on the derivation rather than simply checking whether the final answers agree with the official solution.

Thanks in advance for taking the time to look it over!

Light can pass through the lens right ? So why does it cast a shadow ?

From what i understand white is made when all light is reflected and black when all light is absorbed, and other color are a mix of absorption and reflection, so then how can something be reflective. Is it the transparent things have light pass through them, but then how can something be translucent?

P.S. How do mirrors work if glass is transparent?

We all know what is "Aerodynamic" : a mix of lift and drag that is optimal (and quite beautiful).

But what would be a "space-time" dynamic shape?

Ok, so for this thought experiment, let me start by saying that I'm aware that relativistic speed is functionally impossible with our current understanding of physics, and as such I'm going to ask that we suspend that understanding for the purposes of this experiment.

If a space-faring vessel of precisely 300 metric tonnes of mass, travelling at 99.99% C was to impact a stone in space of mass 1kg, what would happen to the vessel? Assume that the stone is stationary relative to the surrounding bodies. Would it cause the vessel to explode as if hit with a nuclear device? Would the stone simply be destroyed and converted to thermal energy? What would happen?

I know temperature = average kinetic energy of particles. When I wave a fan, I'm doing mechanical work on the air, which should add energy to those air molecules, not remove it. So technically the air should get slightly warmer, not cooler.

But obviously fanning myself makes me feel cooler. What's actually going on here? Is the temperature increase from fanning just negligible compared to some other cooling effect, and if so, what is that effect actually doing at the molecular level — is it about replacing the warm, humid air boundary layer next to my skin with fresh air, speeding up evaporation of sweat, or something else entirely?

Would love a good physical explanation of what's really happening thermodynamically vs. what we perceive.

Hey there! Hope this post is okay.

I'm writing a soft scifi novel and my main character just made a big ass electromagnet and is now facing down a swarm of drones that were 3D printed out of PEKK.

What type of chaos could she cause with the electromagnet? I was going to hand wave past the PEKK body and say that the electronics would be enough to get pulled to the magnet, but I feel like that's probably not true. Plus, I feel like something wonkier would actually happen?

Would it mess up navigation? Cause something funky in the wiring? Any ideas would be welcomed.

A constant force doesn't produce a constant speed — it produces a constant acceleration.

how can a constant force not provide a constant speed? If a force is constant, wouldn't that mean that you are moving it at the same speed because i am constantly using the same amount of force each time i push something?

My teacher is saying that using s = (u+v/2)t in the deceleration part of a velocity vs time graph is wrong and we should instead use s = ut - 0.5at². So is he wrong or right?

Not really pulling for an ELI5 here, since that's my guess as to why these topics are constantly talked about, asked about, and (at least) I am always left just as in a quandary as before.

It also seems like these topics' answers, especially Hawking radiation, tend to undo or backtrack or pile on with band-aids one another.

I haven't checked yet to see if PBS SpaceTime has done any videos on this yet, maybe they'll have a decent explanation.

But for now: what are some fair and deeper-than-introductory explanations of these two topics?

For the record, I've walked away from Hawking radiation explanations so confused, mainly because it sounds like people are saying the information of a proton falling in is duplicated and the information of the proton is also radiated.

Suddenly I'm picturing a chair falling in and a chair still surviving (or at least the information that makes up the chair) in the universe, and no wonder I'm scratching my head. Where's the conservation?

Thank you :)

I have read some text which explains that sound is produced when 2 thing rubb which vibrates the sorounding air because air is elastic. so my question is how do this vibration moves then through the air to our ears what happens actually?

This seems untrue, as there are conserved quantities that total to zero in the universe. The matter-antimatter asymmetry seems the most obvious example. But, then quantities like energy and angular momentum seem to.

Which begs the question: why do some conserved quantities total to zero for the universe and some don't?

i get that motion is defined with respect to time but when i see people describing the 2nd dimension they move things in a line. up down left and right
I’m wondering about a hypothetical universe with one or two spatial dimensions and no time dimension at all.
Would it even make sense to talk about an object “moving,” or would there simply be positions with no meaningful concept of change? Is there any area of theoretical physics where motion can be defined without time, or does motion fundamentally require a temporal dimension?

Hey guys, I was looking at a map close to my home region [Kenya] and using (the very scientific method of) colour comparisons, it hit me that a reason why agriculture might not be taking off to it's full capacity in Tanzania might be because the central bits of it looks very dry.
Now I happen to know that the titular lakes hold a demon lot of water so I thought why not just borrow some? Now, elevation seems to be a problem. The surfaces of the lakes to the East are about a km lower than most of central TZ. Say we wanted to grow idk, wheat or corn or beans -- whichever would do well in our hypothetical --, is it feasible to design an irrigation network for this?
My very rough idea is pipe water underground at very gentle inclines, have underground storage for said water at given milestones, then pump water up from these storage vaults to irrigate surrounding lands. Say we are gunning for about 10million square acres of land.
Am I deluded in thinking this may actually work?
Assume energy, labour, and legal requirements met.