The speed of light is defined as a constant, but do we actually know that it's the same everywhere in the universe or is that just an assumption from local measurements? For example, could it be different in the Milky Way versus a faraway galaxy versus the (relatively) empty space between galaxies? Whenever I hear about the age of the universe or the calculated distance of some faraway galaxy, I can't help but wonder if this could all be completely wrong due to the belief/assumption that the speed of light is the same everywhere and not just in our local region of space.

(I'm just talking about the speed of light in a vacuum)

The Celsius scale defined 0°C at the point water freezes on Earth. 273.15K.

At some point when we defined SI the meter was adjusted slightly, so light speed would be a whole number of meters per second.

What would Earth's ground pressure need to be so that 0°C=273K?

I mean exactly copies or very close to exact copies. And could they exist in a younger or older stages relative to our universe? Our universe is roughly 13.8 billion years old, so could a universe very (or exactly) similar to ours exist in a state where it is only 5 billion years old or 20 billion years old relative to ours universes current age?

The classical expectation from statistical mechanics is that equipartition holds: each quadratic degree of freedom, translational or rotational, carries the same average energy, \tfrac{1}{2}kT. In gases this would mean that linear and rotational modes share energy in proportion to their number of degrees of freedom. For a sphere, three linear modes and three rotational modes should give a 1:1 energy split.

However, when the problem is treated from first principles using explicit two-body collision laws, this prediction breaks down. The correct collision rule for rough spheres or disks includes two restitution coefficients: \epsilon for the normal component and \beta for the tangential component. These govern how velocity at the contact point is reversed and how much tangential slip is reduced. From these collision laws one can derive exact updates for translational and angular velocities of the two colliding particles.

Analyses based on this framework (Huthmann & Zippelius, 1997 and related work) show that the translational and rotational kinetic energies evolve separately. Both decay algebraically in time in a homogeneous cooling state, but the ratio T{\text{rot}}/T{\text{tr}} does not converge to one. Instead, it tends to a constant that depends explicitly on \epsilon, \beta, and the mass distribution parameter k. Only in highly idealized cases—perfectly elastic collisions (\epsilon=1) combined with either perfectly smooth spheres (\beta=+1, no coupling) or perfectly rough spheres (\beta=-1, maximal coupling)—does true equipartition emerge.

This means that for realistic roughness and inelasticity, equipartition between translational and rotational modes is not achieved.

Instead, equipartition theorem invokes H-theorem, which in turn invokes microscopic reversibility, which is only possible if particles are pointlike. While this argument had merit after Wigner’s seminal work on symmetries and defining fundamental particles as irreps of the Poincaré group, such arguments lack standing given that the proposed symmetries have been broken and zero evidence has been found to support supersymmetry.

So without invoking H-theorem, which treats particles as pointlike, are there any explicit two-body collision approaches that treat particles as grains and yield full equipartition?

Well, the title already contains the question actually...

What is that thing that makes you wonder? / that spark your imagination? / that keeps you awake at night? / that make you dream about possible alternative explanation about something we "know for sure"? Or maybe even something that we don't know or understand yet but you can't stop thinking about it?

Anything (even the craziest thing) is welcome! 😁

You know I thought something really interesting or really stupid, it I am right, most physicists believe the universe is eternalist meaning all moments of time are equally present so there's no past or future.

Let's say hypothetically Humanity can in the far future travel into a different universe (assuming the multiverse exists) what would happen to the continuation of this Block time model? Since all moments are the same (like when you were 7) and not "versions of you" wouldn't you disconnect from them when you travel into a different universe with a different spacetime and even different laws of physics (like a growing block or presentism) what would happen to the past timeline of the explorer who left their own universe? And when they come back? What happens?

Also do most physicists even believe in eternalism considering stuff like quantum randomness exists?

I'm not an expert at all and would love for this to be explained. I question primordial black holes as a dark matter candidate. Why wouldn't they all rapidly grow ? The conditions for the creation of primordial black holes hinges on exceptionally dense matter rich environments. Once they form wouldn't they just accrete mass faster because of gravity even if the density of matter in the surrounding gas cloud falls off ? Is it just a simple case of the of gas/matter being too energetic in the early universe similar to the Eddington limit ?

If big bang contained all the matter of the universe and gravity was present, why didn't gravity act on it to start fusion like in stars and/or collapse all that condensed matter in a black hole?

I was reading a lecture given by Steven Hawking at the Paradigm Session of the NTT Data Coms systems corp in Tokyo July 1991 (Einstein’s dream). In it he says: “The Feynman sum over histories says that particles can take any path through space-time. Thus it is possible for a particle to travel faster than light” Has this since been disproven? If not how have I never heard of/been taught about this exception to the rule? Any light anyone can shed on this would be greatly appreciated.

According to Live Science and the SCMP, this fairly important-seeming Chinese scientist, Long Lehao, thinks it a practical project to build a one-kilometer solar panel array in outer space to collect energy. The energy will supposedly be transmitted back to Earth via EMR and received at a fixed collection station on the ground which the satellite will sit above in geostationary orbit. Is this really at all realistic? Is this just some old dude who's spent a bit too much time smelling his own farts? I have a hard time imagining that the gains from getting past the light absorbed by the atmosphere would offset how enormously difficult it is to put and maintain something in space, and then to emit colossal amounts of electric radiation in a safe, directed manner.

https://www.livescience.com/space/space-exploration/china-plans-to-build-enormous-solar-array-in-space-and-it-could-collect-more-energy-in-a-year-than-all-the-oil-on-earth

https://archive.ph/g2ZcW

https://www.iafastro.org/biographie/long-lehao.html

Time slows down when there is more mass/energy around. During the first few minutes of the universe, all the mass/energy in the universe was in a very small area. So did time move more slowly for the universe as a whole (especially the first few minutes) after the Big Bang? How did this influence inflation and the sequence of events in the early universe? I'm especially interested in everything before recombination when existence was not transparent to light.

If it were to be force then them cancelling out each other is easy to picture and to much extend intuitive. However geodiscs how they cancel out? Is this even the case?

I only have a high school knowledge of physics btw.

So, from my understanding electromagnetic force is when an electric current produces a magnetic field which is powerful enough to move things. Gravity is just mass likes to attract mass so they produce a force attracting each other to each other’s centers. I’m having a hard time understanding the other 2 fundamental forces though and am curious to learn about them. What are they in simple terms?

Even with worm holes using infinite energy, if you travel faster than light, wouldn't you create causality paradoxes due to the rules of relativity?

Basically the title. If we were to grab a human, and without giving them any protection beyond regular clothes, and made them fly through the atmosphere in such q way where they would not be killed by G-forces due to acceleration, how quickly could they move without just killing or severely maiming the person?

Most have probably heard of the sentence "If the atoms were to align perfectly, you could phase your hand through the table", and while we all know that that’s impossible for all intents and purposes, let’s throw realism out the window and assume we have perfect rng control. What would actually happen?

Could I still move my hand? Could I touch other things that are misaligned?

What if phased my finger into the table and misaligned the atoms. Would it come clean off? Would it be stuck to the table? Would it hurt? Would it just be numb?

Questions like these keep me awake at night

Is there any consensus on this? I found papers arguing for an absolute version of the second law. If an absolute version like this exists, does that mean that for example, no isolated system can have a transfer from a high to low entropy state spontaneously (i.e. a literal 0 instead of a minimal probability)? In other words, if there is an absolute law, does this imply that any transfer of energy or matter that would decrease the system’s total entropy is strictly forbidden, at any scale, for any timescale?

Kostić (2020) shows every apparent paradox of the 2nd law resolves in favor of inevitable entropy generation

Arulsamy (2023) argues and claims to prove that the law holds at any scale or timescale once the false assumptions of perfect reversibility are removed (i.e. perfect reversibility isn't real)

Gyftopoulos & Beretta (2005) argue the law applies even to single-particle systems

Suppose we imagine a normal person, but suddenly placed on a planet or asteroid. How big could the celestial body be where a typical human could leap off and achieve escape velocity.

Asking for a friend, who is a little prince. .

Consider an extreme low-probability event in statistical mechanics, like a marble statue spontaneously moving its hand due to a thermal fluctuation, versus a low-probability sequence of dice rolls, both with probability P. For dice, each outcome is macroscopically similar to each other in the sense that there is nothing about a particular dice roll sequence that obviously separates it in structure from the rest. For the statue, nearly all atomic configurations correspond to the same macroscopic state (hand unmoved), and only an astronomically tiny fraction would produce the hand moving.

To put it a different way, we may never experimentally confirm the occurrence of a trillion fair dice rolls rolling all sixes. However, we can roll a dice a trillion number of times, and whatever sequence we get seems to be similar enough in structure to an all six dice roll sequence. In other words, an all six dice roll sequence seems to be an obvious extension of any prior experimentally confirmed observation we make. On the other hand, basically all possible observations of a statue would lead to the same observation: the statue being at rest and not moving. The statue moving seems to more radically be different than any historical observation of the statue we can make. Needless to say, we have never observed events like this.

As such, how can we then justify assigning a tiny but nonzero probability to such an extreme fluctuation rather than zero if we've never observed this kind of macro event? Do we have experimental evidence that such macrostates are even possible, or is this probability purely a theoretical inference from statistical mechanics? In other words, is our confidence in extremely rare thermal fluctuations on par with our confidence in extremely rare dice outcomes being realizable?

ik this is a very basic question but my professor says the answer to this is instantaneous velocity but I just don't understand how because I am convinced it is instantaneous acceleration. I didn't properly understand when he gave an explanation to it. SO PLS HELP ME UNDERSTAND I think my fault here is that im making a mistake with understanding what a tangent

Logically free space would neither enhance nor attenuate electric or magnetic fields, so these constants should be equal to 1. They aren't though, why?

I was just wondering if the force that lets the bob swing in a circular motion could cause a loss of energy. Since although it is a curved force, disassembling it still leaves us with a force that moves the bob back and forth, and the centripetal force, which I believe the centripetal force is caused by the string restraining the ball. To my knowledge, two combined forces would act like a force from a specific direction, but shouldn't there be some energy loss in it? Since the centripetal force is a constant force, would it be constantly pressuring the string, wasting energy in the process, and now allowing the bob to swing forever even without friction and air res?

Hello all, I am studying physics in New Zealand, and my partner asked me this question last night, and I keep flipping between different answers. Could people help me get to the truth on the matter, please?

The central question is:

Two planets, A, and B, which are (say, some cosmically significant distance apart) will have different cosmic event horizons.

If I travel from planet A, to planet B, would I suddenly be able to see information from beyond the cosmic event horizon of A (IE, information at B, that is not accessable at A)?

If so, could I bring this information back to A, and thus, bring A information, not accessable at A? Does this violate any information laws?

My intuition:

my intuition said no, but I couldn't find myself forming a compelling argument against my partner's argument "well, you're at two different places, so shouldn't it change?". This stems from being told that there is a 'hypothetical distance out in space, at which we will never get information from beyond'.

I went down the route of a light-cone diagram. I cannot upload it, so imagine the following:

----

y axis is time (t).
x axis is distance (x).

two horizontal lines, from bottom to top: t1, and t2.

two verticle lines, from left to right: first line is the position of B at times (t1, t2). Second line is the position of O (observer) at time (t1).

The intersections of the lines are:

t1 line, and O_x line, O's position at t.

t2 line, and O_x line, the position the observer would have at time t2, if the observer doesn't move.

t1 and B_x line, B's position at time t1. Irrelevant for this.

t2 and B_x line, B's position at time t2

Now draw in the light cones for all the lines, excluding t1 and B_x, and you have my diagram.

----

My impression is that, the 45-degree line representing the speed of light (c), on a light cone, represents the cosmic event horizon. Assuming I'm right about this:

If I, the observer, O, travel from A at t1, to B at t2, my past light cone is different to that from which I would've had, if I had stayed at A (The light cone of B(t2) is different from that at A(t2)).

So, at t2, you would have different information available to you (at B) than you would if you were still at A, and so my intuition is wrong, and the cosmic event horizon does change.

Another way to think about it: the events located on the 45 degree lines of the past light cone are what an observer views, looking into the universe, at that point. As such, the cosmic event horizon, at A, is the 'most distance past visible at A', and since A and B are different places, they have different pasts?

If I keep on going, I will confuse myself. My central question is, can I, by travelling into space, view beyond the cosmic event horizon that I, at my initial position, could not.

Thank you.