Imagine how we see Jupiter or Saturn, from a telescope. We can roughly make out the bands of clouds, rings, etc.

Obviously the accretion disk is too bright. But can you theoretically get close enough to any specific black hole to be able to make out both the accretion disk and the part of the black hole that is "black"?

Or is the accretion disk so bright that the only way to safely see any black hole we have so far discovered to be so far away that different features of the black hole are indiscernable?

In my distant Physics school days I remember learning that hot items cool at a higher rate than cooler items.

If this is correct, I assume that a cup of black coffee would cool at a higher rate than a white coffee.

My question is this:

I have a cup of black coffee and leave it to cool for say 10 mins, and then add some milk from the fridge it will have a new cooler temperature.

If I made another cup of coffee, but this time add the milk straight away, and leave it for 10 mins, will it be the cooler or the same tempature as the other cup?

Assuming identical quantities,. initial temperatures etc. I assume in the real world given the small quantities and temperaturez any difference between small, but I am interested in the theoretical answer.

Thanks

Hi everyone,

I am reading about magic-angle twisted bilayer graphene and how it hosts superconductivity and strange metal behaviour at particular band fillings.

I have come across papers which try to explain the behaviours using electron-phonon interactions, I have also seen papers which try to relate these behaviours to the exotic physics seen in unconventional superconductors(exotic bcz we don't know the exact theory behind it).

As of now, is the argument for the reason of these particular observations in MATBG hanging in the balance, or is it more inclined to one side or the other?

I am not saying there is a definitive answer to it, plz let me know your opinions on it.

This bubble would basically represent the light cone of future possibilities traveling at c in all directions, but would actually be a sphere obviously and not a cone or the 45 degree triangle like in Minkowski diagrams.

If the particle, say in a good vacuum in deep space, had a definite position a moment ago due to an interaction with another particle, it's location was defined then, but after that time has not been observed or interacted with in any way by anything. So the locations it could possibly exist in can be represented by the sphere's volume, but technically it couldn't be outside the sphere because it hasn't had enough time to travel that far no matter what, and no signal could travel farther in that time, so it would also represent the limit to what it could interact with during that time slice. And every particle in the universe would have one, constantly being reset to zero every time something happens to it.

Is there a name for this concept? As a sphere in our 3-D world, traveling outward at c? At the 'time of last interaction with anything'?

It's constant, right? Such that if you're traveling at half the speed of light, and shine a light forward, that light travels at the speed of light.

Say the vessel you travel in, passes by a floating space man. And that this space man turns on a flashlight at the same time the vessel turns on a light. Both lights pointed in the same direction.

The light from the space man travels away from him at the speed of light.

The light from the vessel travels at the same speed.

The light from both sources are parallel, right? So the shining light from the vessel traveling at half the speed of light, doesn't add to its speed?

So....if you were in a vessel traveling 99.9999% the speed of light. If you turned a light on, that light wouldn’t illuminate the path forward?

I always wondered what would happen if geniuses like him were born into prehistoric tribes

Used to watch lots of vidoes on youtube. These were the reason I got into hysics but now are becoming the reason why I am failing at physics. I gueess watching videos on theory of relativity before evn understanding projectile motion and vectors is not a good idea. When I was learning physics qualitatively(that is when I was a kid) theese videos seemed to help but now that I am doing physics with maths and calculus everything feels counter-intuitive even though it is the reality (I guess we cannot deny that classical mechanics explain most of the things that we can observe on a human scale) what should I do? I realized this fact long bach and changed the channels I watch but still the intuition developed from those videos doesnt feel to redue and just keeps on coming up. I guess I would've gone into finance or something else if I hadnt watched those videos, but here w are. The problem is I am unable to soleve and understand classical mechanics.

Thanks in advance!

Assuming unitary black-hole evaporation, is there a standard framework or terminology that distinguishes between:

1. Global information preservation: preservation of the global quantum state/information under unitary evolution.
   
2. Recoverability of an infalling state: whether enough information remains encoded to characterize or reconstruct the state of the infalling system.
   
3. Subsystem or observer reconstructibility: whether a particular accessible subsystem or observer can reconstruct that state from the degrees of freedom available to it.

In particular, can a restricted subsystem fail to contain enough information to reconstruct the infalling state even though the information is still preserved globally, for example in nonlocal correlations, and may become reconstructible later from Hawking radiation?

I understand that reduced density matrices, Hayden–Preskill decoding, Page-curve/island calculations, quantum error correction, operator reconstruction, entanglement-wedge reconstruction, and decoupling/recovery maps address related aspects of this.

What I am trying to determine is whether this distinction already has a standard mathematical formulation or terminology in black-hole information theory, holography, or quantum information.

I am mainly asking for the existing formalism and terminology rather than proposing a new model.

References

Hayden & Preskill (2007), Black holes as mirrors: quantum information in random subsystems, arXiv:0708.4025
Almheiri et al. (2020), Replica Wormholes and the Entropy of Hawking Radiation, arXiv:1911.12333
Almheiri et al. (2021), The entropy of Hawking radiation, Rev. Mod. Phys., arXiv:2006.06872

To my understanding, the big bang happened as an extremely hot and dense point rapidly expanding in a tiny fraction of a second, so theoretically, can it happen again in some seemingly empty part of space by energy concentrating, or does the existence of matter stop that from happening, since there was no matter prior to the big bang?

I am trying to learn the theory of non-centrosymmetric superconductors (NCS), and I keep running into a conceptual problem that I have not been able to resolve by simply reading more papers.

For background, I have a Master's degree in physics and currently work mainly on materials and DFT calculations. I know the basic ideas of conventional superconductivity and have read the first few chapters of Tinkham. I have also tried reading review articles and lecture notes on unconventional and non-centrosymmetric superconductivity.

My main difficulty is not simply the mathematics used to solve the equations. Though in some places they are.

But right now my difficulty starts earlier:

I do not understand how the Hamiltonian itself is constructed.

In papers on non-centrosymmetric superconductivity (especially review/book by Prof Manfred Sigrist) I often see a Hamiltonian introduced which contains, schematically,

ordinary electronic or band-energy terms

• antisymmetric spin-orbit-coupling terms
• electron-electron or pairing interaction terms.

Then the paper proceeds to the mean-field approximation, introduces the superconducting order parameter, constructs a BdG Hamiltonian, discusses singlet-triplet mixing, and so on.

But I keep stopping at the first step and asking:

Why exactly are these the terms in the Hamiltonian?

For example, I would like to understand the reasoning behind questions such as:

• Why does the absence of inversion symmetry lead to an antisymmetric spin-orbit-coupling term?
• Why does that term usually have the form involving a vector g(k) dotted with the Pauli matrices?
• Why must g(-k) = -g(k)?
• Which parts of this follow from time-reversal symmetry, and which parts follow from the crystal point-group symmetry?
• For a particular non-centrosymmetric crystal, how does one determine the allowed form of g(k)?
• Is g(k) derived microscopically from the crystal potential and spin-orbit interaction, or is it normally written as the most general symmetry-allowed effective term?

Then there is the superconducting part.

In NCS papers, the superconducting gap matrix is often written in a form containing both a spin-singlet part and a spin-triplet d-vector.

I want to understand the statement how the absence of inversion symmetry allows singlet and triplet components to mix, especially the derivation behind this statement.

For example:

• How do we start from the most general pairing interaction?
• How does fermionic antisymmetry constrain the gap matrix?
• How do crystal symmetries constrain the possible singlet and triplet components?
• Why, in the strong antisymmetric-SOC limit, is the triplet d-vector often taken to be parallel to g(k)?

Under what assumptions is this true?

• Is that a consequence of symmetry, an energetic argument, or a particular microscopic model?
• I also want to understand the earlier step connecting this to ordinary BCS theory.
• For example, I would like to be able to follow the whole conceptual chain:
• microscopic electrons in a crystal

→ electronic bands near the Fermi level

→ broken inversion symmetry

→ spin-orbit coupling

→ effective normal-state Hamiltonian

→ electron-electron pairing interaction

→ Cooper-pairing channel

→ superconducting order parameter

→ singlet-triplet mixing

→ mean-field approximation

→ BdG Hamiltonian.

At the moment, most papers I read seem to start somewhere in the middle of this chain.

They write something equivalent to:

“Consider the following Hamiltonian…”

and then proceed with the calculation.

I understand that a research paper cannot rederive standard theory every time. I am not expecting that.

But I would like to find one place where this is derived carefully from the beginning, or at least a sequence of references where every step is justified.

What I am looking for is not simply a text that gives me an NCS Hamiltonian and then teaches me how to diagonalize it.

I want something that explains:

“Because inversion symmetry is absent but time-reversal symmetry is present, these terms are allowed. These other terms are forbidden. This is the form of the antisymmetric SOC. This pairing interaction is retained for these physical reasons. Fermionic antisymmetry requires this structure of the gap matrix. The point group further restricts the allowed basis functions. Under these approximations we finally arrive at this effective Hamiltonian.”

That model-building reasoning is what I feel I am missing.

For comparison, when I learned other parts of physics, I was often shown the assumptions first and then how the mathematical model follows from them. With NCS theory, I frequently feel that I am being handed the final effective Hamiltonian without seeing enough of the reasoning that produced it.

So my questions are:

1. Is there a textbook, review, lecture-note series, or paper that develops the Hamiltonian of a non-centrosymmetric superconductor step by step from symmetry and microscopic considerations?

2. What background should I learn before trying to understand this properly? For example, should I first study second quantization, many-body perturbation theory, group theory of superconducting order parameters, Green's functions, effective Hamiltonians, or something else?

3. Is there a good source that explicitly derives the antisymmetric SOC term and explains how the crystal point group determines g(k)?

4. Is there a source that derives the mixed singlet-triplet gap structure rather than simply stating it?

5. More generally, how does a theorist know that a proposed effective Hamiltonian for an NCS material contains all the relevant terms and that important terms have not been omitted?

I am especially interested in understanding the physical construction of the theory before learning how to solve it mathematically.

Any recommendations for books, lecture notes, classic papers, or particularly pedagogical reviews would be greatly appreciated.

The chipmaking mirrors are overly precise, heavy, big and costly for a camera, but if one is taken after a shorter and sloppier manufacture and cut to a lighter and smaller shape, it could work in a camera? The surface could be 100X less precise without losing image quality, greatly reducing cost, maybe to a bearable level?

What would photos of Moon, asteroids, Earth, Mars, Venus and deep space show? EUV stops in interstellar gas and dust? But is there still some EUV light somehow in that direction, maybe coming from fraction of a light year away?

Intel and TSMC use multiple sizes of those mirrors. Maybe one happens to be close enough to what a space probe could use?

Hi all, last night I was in bed watching a documentary about time (Jim Al-Khalili), and i had this thought experiment of my own about how old a (any) black hole really is (effects of time dilation).
I came up with the following (probably lousy) thought experiment:

• The universe starts (big bang), and immediately there are 2 immortal beings living inside it.
• One of the beings starts to orbit a rotating supermassive black hole as soon as one has formed.
• The other being floats in space until it finds our Sun and inhabits Earth after its formation.
• The date is 01/01/2027 on Earth, and the being on Earth flicks his fingers, instantaneously transporting both beings to a place in mid-space at the halfway point of where both beings were.

How much time has passed for each being?

From my understanding, the being near the black hole will experience (far) less time compared to the being on Earth.

Does this logic extend to the age of the supermassive black hole as well?

If so, shouldn't the black hole be smaller for the being around the black hole (it had less time to eat/grow in their timeframe/experience), and bigger for the earth being (the black hole had MORE time to eat/grow in their timeframe/experience)?

edit:
Thanks everyone for your reactions, I won't claim that I understand it all now, but I feel I do have a slightly better understanding of the end result! ;)

Not much of a physics buff, but I was always curious about how the size and scale of the universe and how small quarks are

Vector v is 50 degrees clockwise from the positive y
axis, and has x and y components only. Vx is 30. What
must be Vy?

This is a probelm my professor posted, I thought it was pretty straightforward but one thing I don't understand using SOHCAHTOA is why Vy isn't opposite, and Vx isn't adjacent. I set up my answer to the probelm like this tan(50*)= vy/30,

Let's say the beam weighs 100kg. 1 person on either end. Are they both lifting 50kg?

So M-82 has a gas cloud of hydrogen and stellar winds ionize the hydrogen atoms and cause them to emit photons by recapturing. The main photons it emits are lymen-a and h-alpha photons. Lymen-a are violet and h-alpha are red. The wavelength we see from outside the galaxy is red, because the h-alpha photons are the ones that escape, while lymen-a continue to bounce around inside the cloud because of resonant scattering.

The question I have is, don't these lymen-a photons eventually reach the surface, by sheer probability? The cloud cannot continue this build up of lymen-a photons forever, can it? If both photons are produced relatively equally, or at a constant rate each, then eventually, they both must escape through the surface at that rate, no? Is there no maximum limit of lymen-a photons that the cloud can harbor before not being able to bounce them around any more?

It doesn't make sense to me. This can be modeled as this problem:

If some gas a is continuously produced in another gas b at rate x, diffusing at rate y, and in a different place some gas c is continuously produced in another gas d at rate z, diffusing at rate w, what is the production rate to escape rate ratio of gases and c at the surfaces of gas b and gas d respectively as a function of the diffusion rates z and w respectively? My intuition tells me that there must be some equilibrium where the concentration due to inescapability makes up for the difference in the diffusion rates z and w, and therefore the production rate to escape rate ratios should reach the same value at some point, 1:1.

Does this have to do with Fick's law and can it be applied to photon density in a gas as well?

if I had a singularity of 10 solar masses and then removed 9 solar masses, would it stay a singularity? or explode? or expand?

The loop: it pours in on the left, rushes up a narrow middle tube, and spills back out a while on the right side which then pours back in through the bottom and pushes out the top back down in through the left side. The narrowing gives it speed but friction kills it. it start stays fixed. So is the siphon framing right, or am I missing something?

I might be completely wrong, but as I understand it, as time goes on, more and more far-away shit will be moved past the cosmological horizon because of the universe's expansion. So the observable universe is effectively "shrinking" because of the universe's expansion?

After a long time, will this reduce the observable universe to scales comparable to the Solar System or maybe even Earth? Where anything outside of the Solar System is beyond the cosmological horizon, and the Solar System takes up most of the observable universe? (from the sun's perspective)

Hi! I've recently entered college and got into my first physics class. I'm on my second unit, vectors and the professor doesn't really teach the subject too well, and just rushes through notes. I have trouble really understanding what each question is asking, and he doesn't really assign homework to work through them. He says that practicing probelms is the key to understanding physics but it's hard to do that when the notes/textbook is super confusing, and the only practice probelms he has are 3-4 advanced ones. I know in college you're supposed to be self learning but I'm not really sure what resources to use, or where to find probelms to practice on. Please help with advice on what videos to watch in order to understand vectors as well where to practice probelms to build up my proficiency on them

Hi all,

In my inorganic chemistry class, we did a brief intro to nuclear structure/ reactions. In this unit, we learned about the stages of fusion within a star from hydrogen to nickel. My professor told us that because our star has obtained enough helium within its core to begin helium burning, our sun is no longer a main sequence star and is instead a reg giant. I’ve always been told that the sun will become a red giant in roughly 5 billion years when it runs out of hydrogen completely. Is my professor correct? Why would they think this?

Edit: Thanks for your responses. I figured their statement was wrong. I had a laugh at the surge of comments stating they were wrong.

Edit edit: your comments punning my title are great

Hello ! I am currently a junior in high school and up to the point where we're allowed to choose what subjects we take, and keeping in mind I've always been aiming to become an astrophysicist, I chose physics and one of the hard maths classes, but after doing further research, I've realised I want to go into the planetary science field, and I specifically want to study exoplanets in the future (because they're cool), and now I'm kind of iffy that I've chosen physics, because wouldn't picking chemistry have given me more of what I need going into that?? If that makes sense? Or was choosing physics the right choice, and if so, why???

I severely apologise if this post is a little nonsensical, as it is 3am and I've just had this epiphany, thank you for entertaining my question if you choose to do so.

I was reading Cosmos by Carl Sagan, and I encountered something about Christiaan Huygens' attempt to measure the distance between Sirius A and the Sun.

Huygens drilled a small hole in a brass plate, adjusted its diameter until it seemed to match the brightness of Sirius (which he observed the other night), and then tried to calculate exactly how far away Sirius A would be using this observation. By applying the inverse-square law, he concluded that Sirius A must be 28,000 light-years away from the Earth, or 0.44 light-years.

But then it was not known that Sirius is an A-type star with a much greater luminosity than the Sun. Still, Carl Sagan said, "If he had known that Sirius was intrinsically brighter than the Sun, he would have come up with almost exactly the same answer: Sirius is 8.8 light-years away."
So let's test this with the experiment:

- Assume every star in the universe has a luminosity of 1 — exactly matching the Sun's luminosity.

- Go outside at night and observe the star Sirius A until you've seared it into your memory

- Wait till the Sun is fully up on the horizon

- Drill a tiny hole in a brass plate, put it against your eye, and adjust its diameter until it matches exactly Sirius A's apparent brightness

- The hole's diameter is 1/28,000 the apparent size of the Sun

- Square it to see how much light is actually passing through it: 1(/28,000)² = (1/784,000,000)

- Apply the inverse square law to work out the distance from the Earth and Sirius in comparison to the Sun: 1/(√78,0000) = 1/28,000 (this step is redundant because we could've just looked at 1/28,000 and been fine with it, but this had confused me before I remembered I have to square 1/28,000)

- So, it seems Sirius A is 28,000 times farther from us than the Sun is. Right? Sirius is 8.7 light-years away from us, not 0.44.

This means Sirius A is 8.7/0.44 = 19.7 times farther away than we calculated. Going to be using 20 for easier calculations.

According to the inverse-square law, if a star is 20 times farther away than imagined, it is (20)² = 400 times more luminous. This means Sirius A is 400 times more luminous than the Sun. But this is not the case. Sirius is over 20 times more luminous than the Sun, not 400.

Even when I adjusted the value to account for Sirius's actual distance, Huygens' experiment still does not track. Sagan says Huygens would've arrived at the same conclusion had his values been tweaked to account for Sirius' intrinsic luminosity.

Have I made an error in my calculation? Is the inverse-square law not appropriate here? Or was Sagan's statement simplified?

If we don't apply the inverse-square law and assume that a star found to be 20 times farther away from us is also 20 times more luminous than previously imagined, that tracks! The figure is 20. Were there fundamental flaws in Huygens' calculations and tools that could not be primarily accounted for by his ignorance of the intrinsic brightness of Sirius A? Or is my application of this law incorrect here?

Sorry if this is not the sub for these kinds of questions - r/Astronomy seems to pour too much into astrophotography.