The concept is simple. You have some system, say a spaceship, that produces waste heat as it functions. By some, unknown mechanism you take this heat and output it as a coherent laser, to keep you from having big radiators or from being spotted or for shooting at someone or whatever reason the story demands.

As I understand it, this is a complete abuse of the idea of "waste" heat at best, and completely violates the second law of thermodynamics at the worst. If you could get this waste heat into a coherent laser, you could presumably turn that into any other form of power, which feels very much like you're getting a perpetual motion machine, and at the very least it wasn't really waste heat- your equipment is just inefficient. Since a laser beam has a very low entropy for every unit of energy it outputs, is it just that the energy source of the ship would have to be even lower entropy per unit of energy? Am I misunderstanding the problem? Sorry if this is worded badly, I'm not sure how entropy applies to things like chemical reactions, nuclear reactions, or light, only that it does.

I was thinking about what happens when you place two mirrors in front of each other. Then I thought that a room with floor/ceiling/walls made of mirrors would be interesting, but a person would still be able to understand where the walls were due to the edges they would form. So I thought about making it a perfect sphere of mirrored surface.

My questions are: how would a human perceive this room (being inside the sphere)? How would his reflection even look? Wouldn't it reflect everywhere and get mixed with reflections-of-reflections-of-(....)?

I have read literature on how Oppenheimer taught in the university of California, CalTech and Berkely, along with giving numerous lectures in other institutions. How does he compare with the "great" professors of his and our time, such as Feynman?

I’m a second year in college and am currently a physics major. I love being in nature and am an avid backpacker and love to travel. I would love to be able to work in nature and was wondering if anyone else worked in a nature related field.

When I think about 2D beings living on the surface of a sphere, they could measure the curvature by measuring the internal angles of a large triangle - but so long as the sphere is relatively large compared to their size, they wouldn't see any special distortion in their 1D+depth view compared to if they'd be living in flat space. If the sphere would get relatively small compared to their size though, let's say so small that the surface area would be just a couple times larger than the area the beings occupy, they would for example see a distorted version of themselves in all directions wrapped around them (as looking in any direction would just show them the backside of their 2D head.

I'm trying to imagine how a very small (let's say 1000m³⁾ positively curved universe with three spatial dimensions that only contains me, a small lightsource and another person might look like?

What kind of distortions would I note in my direct vicinity, let's say when looking at my feet? What would I see in the distance?

While I agree that QFT has been tremendously successful for many parts of physics, it still has some major gaps. There isn’t an interaction picture so perturbation theory doesn’t “actually” make sense. It wildly overestimates the cosmological constant.

Do you think these issues are because gravity isn’t quantized or because of these issues gravity can not be quantized in terms of qft?

I have an inefficient cooling system. If wine varies by more than 1°F it could be ruined over time. Without physically testing liquids in the space can you calculate if I'm violating that threshold?

Wine bottle volume: 750mL

Air temperature range: 47°F - 54°F

Time between air cycles (time between each 47°F low point: 30min

Does the liquid in the bottle get too warm over these cycles?

I know the water ball will have greater terminal velocity but isn’t that exactly why the air one will reach it first?

I'm not good at physics, but if I remember correctly that temperature is come from energy that shaking or moving the atoms right? So how deos cold wind work? What are they do to energy or the movement of atoms?

(Apologies for my bad English)

I know a lot of people wonder if a nuclear weapon could ignite Earth's atmosphere but that's not what I'm asking here. I know that the density of the atmosphere is too low and thus the pressure and temperature of the atmosphere could not sustain a reaction. But what if a nuclear weapon was ignited on a gas giant, like Jupiter or Neptune? I know the answer is probably no but hypothetically, could a gas giant with absolutely perfect conditions for an atmospheric ignition exist?

The concept of action at a distance in physics involves an effect where the cause can be far away from the effect. To be more precise, it involves an action where there is no signal traveling through space or any sort of medium between cause and effect.

And yet, there are versions of quantum mechanics that posit some sort of action at a distance, such as Bohmian mechanics. Even the interpretations of quantum mechanics that don’t seem to posit this instead posit something equally unintuitive: correlations over large distances occurring without a cause (breaking the Reichenbach’s common cause principle).

In Newton’s time, action at a distance was heavily criticized since it seemed to indicate an occult-like/magical quality to the universe. Others told the criticizers that their intuitions are wrong and that the universe doesn’t need to obey their intuitions. Surprisingly, although perhaps not so surprisingly, they turned out to be correct after Einstein’s general relativity which posited that gravity does have a travel time and it propagates through space.

Is there something inherently philosophically untenable about action at a distance? If so, could this give us clues about how arguably incomplete theories like quantum mechanics might evolve in the future?

Recently I've found information stating that external particles can measure qubits (of kinds that have already been made). However, as I understand, it's impossible to empirically distinguish measurement from uncontrolled entanglement (otherwise the delayed erasure experiment would have a variant without the catch), and last time I checked it was unknown when measurement happens. The Wikipedia page on quantum measurement doesn't seem to give this information, either

Hi!

I'm not a physicist, just a layman with a moderate interest in physics and rusty maths skills (so any explanation for my question that steers away from maths would be good, but it's physics, so I understand if maths is unavoidable)

So I'll keep my query short: baed on my understanding, the "speed of light" is sort of a misnomer, in so much that light simply travels at the highest speed possible within our universe. Any massless particle travels at the same speed (i.e. gluons) along its spacetime trajectory because the speed limit is hardcoded into the fabric of spacetime.

So my question is this: would it be accurate to say that we are all traveling along our own trajectory through space at the speed of light, we just aren't moving through time at the speed of light? And if you were to magically, slowly siphon away my mass, my trajectory through space time would increase in speed until I had no mass, at which point I would reach the universal maximum?

Looking for a quick yes/no or any insights from folks with accelerator or fusion experience:

I’m thinking of taking a proton beam (~4 GeV) and firing it at a mercury or tungsten target to induce spallation. The idea is to then filter the resulting fast neutrons using a collimator, and direct them into a test chamber filled with a sample material (e.g., for neutron multiplication tests). I'd place He-3 detectors in the walls to measure the neutron output. I am doing this to find an appropriate neutron multiplier to replace beryllium.

Three quick questions:

1. Would this setup theoretically work?
2. How would I measure the number of neutrons entering the chamber without disturbing the beam too much?
3. Is this even remotely useful or "creative" as an experiment?

Any thoughts are appreciated. I'm still learning, so even brutal takes help.

Hi! I’ve been rewatching „The Flash” recently and Plack temperature was mentioned. I got interested and researched Planck temperature a bit. Everything I read said that the temperature is 1.416784 x10³² kelvin, but it’s hard to scale. Could some please help me understand how truly big of a temperature it really is? Thanks in advance!

A diving bell (open bottom vessel) lowered into the water will experience a certain amount of upforce.
When it is lowered or pushed down further, the air gets compressed more and the water level rises.
If there is no limit on the strenght of the vessel, lowering this vessel to 10000 feet will compress the air enormously. (and raise the water level inside)
But will the vessel experience the same amount of upforce, no matter the depth ? (the deeper the less volume of air)

A month ago (this post)[https://www.reddit.com/r/AskPhysics/s/6l8TUgB74m] was made asking whether two hydrogen atoms at two opposite edges of our observable universe exert a gravitational force on each other at all.

In short, the topmost answer was “yes” (“mass affects spacetime curvature which will either expand or contract which equals a force anyhow”); the second most upvoted answer was “no” (“the two hydrogen atoms are causally disconnected and gravitationally unbound”).

So I ask once and for all - which is it? Are both of these answers correct (up to two different interpretations of the question)? Is one of the commenters wrong? Is there some lack of consensus?

The underlying direction of this would be could our universe exist as part of a surface of an expanding 4D sphere, or other multidimensional structure?

Just please be polite in telling me how this doesn't make sense, thanks!

Sorry if this comes across as a silly question but I’ve been taking an interest in reading about black holes recently.

I've heard a lot about kittel and asthroft, mermin , which one should I get?, or any other book? Your suggestions would be appreciated 🙏

First year uni student here, I was fairly confused by this question on my as it seemed to have 2 correct answers. Is anybody able to clarify why the answer I chose is incorrect? Here’s the question:

If the velocity of an object is zero, does it mean that the acceleration is zero?

1. No, an example would be an object coming to a stop (my answer)

2. No, and an example would be an object starting from rest

(There were more options, but these were the only choices for no, which I think is the right answer)

I got this question wrong, and I assume the other ‘no’ answer was correct, anybody able to explain this?

So there’s this instructor that needs us to do a physics toy. But the twist is that the physics toy should be very interesting for it to be chosen by her son (he is aged 3-5) and if chosen, you get to have additional point.

If you have any ideas, and/or suggestions pls pls help ur girl out.

After thinking about it for two days, i really can't see what i'm doing wrong, so I ask for your help. Consider a situation where i want to send a satellite, positioned at a certain height from the surface of the Earth, to the Moon, such that the satellite makes half of an elliptical orbit. Assuming the mass and radius of the Moon to be zero, and knowing the initial velocity of the satellite, at which velocity does it reach the Moon. If you want the numbers: Initial velocity=1,082×10⁴ m/s Initial height=3,2×10⁵ m Mass of the Earth=5,97×10²⁴ kg Radius of the Earth=6,38×10⁶ m Distance Earth-Moon=3,84×10⁸ m My first assumption was to use the conservation of energy, but the result was different than the one given (1,89×10² m/s), so i looked at the solution and it wanted me to use the conservation of the angular momentum. And here's what i'm confused about: shouldn't they give you the same result? Why isn't energy conserved in such a situation? I already tried to assume that mayne he wanted me to consider the initial velocity as additional to the one necessary for the satellite to remain ina circular orbit at that height, but it simply diverges even more from the result so that cannot be it. Edit: Adding calculations

I'll use V0 for the initial velocity, R for the radius of the Earth, h for the height and d for the distance between the Moon and the Earth.

1/2mV0²-GMm/(R+h)=1/2mV²-GMm/d

Solve for V and it becomes

V=sqrt(V0²+2GM(1/d-1/(R+h)))

If you input the numbers in a calculator it comes out as about 530,1 m/s.

If i follow the solution given instead

mV0(R+h)=mVd

Solve for V

V=V0(R+h)/d

Inputting the numbers, it comes out as about 189 m/s.

Genuinely curious to hear what physicists think of new emerging evidence suggesting that dark energy may be “evolving” so to speak. Thoughts?

https://www.reuters.com/science/evidence-mounts-that-universes-dark-energy-is-changing-over-time-2025-03-19/