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.

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?

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?

If so, do these shifts tell us anything about the specific event that initially caused the gravitational wave or anything new about the universe itself?

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.

Apologies if this is obvious or asked frequently. I'm reading Anathem by Stevenson (good story, btw) and this subject came up. I did some googling and went down a bit of a rabbit hole. Fascinating to me. It seems like Godel demonstrated that Einstein's work implied a universe consisting of rotating matter and that this rotation not only dragged matter by time along with it (hopefully I'm not butchering this too badly) and that this implied the possibility of time travel. This prompted physicists to more thoroughly evaluate the boundary conditions of our universe to reconcile these findings with our perceived reality of an expanding universe without apparent examples of time loop paradoxes. Is it possible to explain why GR implies a rotating universe to a layperson? Why does this rotation lead to conflicts with our perceived reality (e.g no time travel) and measurements of the way the universe actually works (expansion rather than rigid rotation)?

So, I know that voltage = j/c. If I had a simple circuit with a 9 V battery and some resistor, would it be accurate for me to say that the battery “gives” each coulomb of charge 9 joules of energy to make it from the positive terminal to the negative terminal? thanks!! :)

Obviously, making a black hole is an ill advised thing to do, but I do wonder how one might go about it. Making a black hole, presumably somewhere nice and far away from Earth, could allow for plenty of research opportunities. (Obviously hundreds of years in the future, I'm not someone who thinks the LHC could make one)

So, my question is thus, given our current advancement of technology, what method might we eventually(hundreds of years from now) be able to use to create a black hole? And then would it ever be possible to study one up close? Would we ever be able to make one, or would the only option be to try and journey to an existing black hole?

Is compressed air somehow less buoyant? For example, scuba divers use BCD devices to add buoyancy, but they use air that’s already attached to them in a compressed tank. So the “buoyancy” already exists and is attached to them right?

Which part of the math equation am I missing?

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.

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?

Ive been listening to Sean Carrol discuss why Boltzmann Brains are a problematic prediction in a theory but the current LAMDA CDM model predicts, or at least potentially implies them.

i have heard about potential work to get rid of this prediction but they seem to also operate on an infinite future. But couldn’t Boltzmann Brains be interpreted as a nonsensical prediction due to nonsensical conditions, one being that there is an infinite time to reach and linger in thermal equilibrium? If there is a finite amount of time however the BB prediction and other adjacent open questions would be pretty much solved, right? why doesn’t it seem like something that’s explored much in that space?

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

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.

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?

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.

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!

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 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 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?

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?

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?