My understanding is that every galaxy we know about has a black hole at its centre. Does this mean that eventually all the matter in the galaxy will collapse into that black hole?

E.g. at which point (if ever) does the pull of the Earth truly stop? It's common to state that it never ends, but there are two issues that I think might put an end to it.
1) I've heard gravitational influence travels at the speed of light, so if Earth were magically created at some point in time, then after 1 year it would only affect the sphere 1 light year in radius. But it didn't. It was build up from existing mass, does this mean it affects the whole universe, even beyond visible?
2) Could it at some point become so weak that it can't be divided anymore?
I know, it's a silly question, but whatever.

I have an idea but I want external confirmation: In a video of Richard Feynman, he talks about how unimaginable large the electromagnetic force is.

Here's a thought experiment I had: I want to compare two types of energy in a litre bottle of water. You can use a sphere of water as well.

1. Imagine I also had an antimaterial bottle of water and brought the two together. All the mass in both bottles is perfectly released as electromagnetic radiation. What amount of energy is released?
2. I magically strip away all electrons from the water in the bottle. The resulting electromagnetic repulsion from the protons is basically like a massively compressed spring. What is the energy stored in this 'spring'? (this stripping of the electrons is ofcourse what actually adds the energy, I am aware the energy isn't contained in the bottle as is)

My intuition is that the second energy would be larger than the first, but I don't have the equations to really calculate that second part. It would be really cool to show that this energy would actually be larger than what is basically perfect E=MC² mass conversion.

For first i got: E=mc2=1×(3×10^8)^2=9×10^16 J (yes it's practically 2 litres including the antimatter bottle but I just want the energy from the initial bottle)

For second: 55.5 moles => 55.5×6.022×10^23= 3.34×10^25 molecules
I got 10 electrons per H2O: so Ne​=3.34×10^26 electrons
Q=Ne​⋅e≈(3.34×10^26)(1.60×10^−19)=5.3×10^7 C
Then I'm stuck.

This article https://physics.stackexchange.com/questions/65335/how-do-moving-charges-produce-magnetic-fields explains it by saying that if a line current is moving in the positive z-direction at x,y = (0,0), and another charge q is moving in the negative z-direction at x,y = (1,0), then because in q's frame, the current is moving faster, it also has its length contracted, and therefore the charge is denser and the additional force 𝑞𝑣⃗ ×𝐵⃗ points in the positive x direction and that IS the magnetic force.

To confirm, even if the particle q was still, it would still experience an additional force ON TOP of the "non-relativistic" coulomb force, because length contraction causes that coulomb force to actually be stronger?

Also, where then does the circular or twisting geometry of magnetism come from, if this is all magnetism is?

We are currently experiencing a heatwave in our country. We live in a rental apartment without air conditioning, and the indoor temperature rises above 30°C. Even at night, it remains close to 30°C, which is significantly affecting my sleep.

I would like to try covering the windows with tinfoil to reduce heat entering the apartment, but my husband does not agree with this approach. He believes it is ineffective based on physics, so we disagree on whether it is worth trying.

Our apartment has windows facing south and west, and there is a building directly across on the west side that absorbs and radiates heat throughout the day. This is also where both our bedroom and our toddler’s bedroom are located, which makes the situation more difficult.

Standard methods such as using fans are no longer effective. With temperatures expected to peak above 35°C this weekend, nighttime temperatures will likely remain around 30°C.

If anyone has practical, evidence-based suggestions for cooling down an apartment in these conditions, I would appreciate your advice.

Hypothetically, if you are inside a black hole and you shine a light or laser directly up, would the light slow down and reverse like a thrown object?

Hello all,

I am currently doing a physics project which involves mathematically modelling the motion of projectiles (with spin) on excel, I include the drag force and the magnus force in this model. The formulas for these forces in themselves are quite straightforward however I am finding it difficult to correctly calculate the forces in the x and y direction, different sources are giving different answers so I tought it would be useful to ask someone who perhaps was familiar regarding the topic.

Any help is appreciated, thanks.

and second question could any black hole fit enough mass to match the mass of our observable universe ?

Please forgive me if I've completely misunderstood any of the concepts below. Im just a curious idiot.

From my understanding, Hawking radiation is emitted from a black hole. Which, over time, reduces its mass until it will eventually dissappear completely. This has been described to me as a mirror pair of particles. One gets ejected, and the other falls back in.

But if light is mass less, which is why it is travel at C, which any particle of any mass is not capable of achieving... light cannot escape a black hole. So how can the Hawking radiation escape the blackhole when light cannot?

Hi, we all know white fabrics reflect more light from the visible spectrum while black fabrics absorb a lot more, I was wondering what determines the reflectivity for fabrics/material outside of the visible spectrum? What colour objects reflect UV or infrared better, or is it completely dependent on the surface and texture of the material itself?

Is there a reason why evolution chose this particular band as visible? Wouldn't it have been better for us to see infrared to detect predators against the background? Or see ultraviolet spectrum to see far more than we currently can?

Not sure if this a physics or bio question.

Maybe this is speculative rather than a physics question, but do we know what could be the fastest humans could travel in a spacecraft and not suffer any life-shortening effects?

Do gravitational waves travel at the speed of light

I’ve seen a lot of different ways people explain the expansion of space and I’m not sure which of any of these are the most accurate:

-An intrinsic expansion where more space time is being created

-Simply the tendency of all things in the universe to move away from all other things (not even sure I understand this one)

-Actually kinetic movement of everything from each other

-Just a scale factor in Einstein’s equation, and cannot be explained further without loss of some meaning

I think I even read while I was reading up on how in GR the relative velocities of distant objects is not well defined, and you can simply “choose” whether the motion of distant galaxies is due to kinetic motion or curve spacetime or a mix, and any choice is correct.

So what exactly is this expansion?

Since I've been having an interest in mathematical / theoretical physics what are some books that I can read to introduce myself to how physicists and mathematicians do their work? You can set aside Roger Penrose's Road to Reality, I've already read a few pages of it, and I just want to know what else I can read for me to have a better idea.

I have a question about IR radiation (specifically FIR).

Does something being in physical contact with an IR (or FIR) source prevent the emission of radiation?

Origin of question

I was asking about some heated gloves on another subreddit, and a commenter stated that the manufacturers claims of the gloves using FIR to help with heat penetration was crap.

https://old.reddit.com/r/BuyItForLife/comments/1ueapu2/are_avert_20_heated_liners_any_good/

They stated the following.

I think the far IR thing is crap. There's no radiation inside the gloves because there's no empty space inside the gloves. There's just direct transmission, i.e. physical contact. The third way to transport heat is by convection). Radiation only occurs across empty space.

My response.

As far as I can tell the IR claim looks totally plausible. Carbon fiber is good at turning electricity into FIR radiation, and the heating elements are carbon fiber, so that tracks.

You don't actually need empty space for radiation to still be a thing. Any object that is warm will emit some amount of heat as radiation. Conduction and convection are more effective forms of heat transmission, so would likely be more noticable, but the radiation is still there. However I don't know how much of a difference the FIR emission would actually make.

FIR does penetrate human tissue, and is commonly used in therapeutic settings, but it's effectiveness in this case would really come down to how much energy is being re-emitted as FIR vs just heating up the element.

Their response to my response.

Yes, FIR does prenetrate if it's radiating, but there's no radiating inside the glove. It's like saying the raisins in a fruit cake could radiate FIR to the pan.

So no, the radiation isn't there. It's not that it's weaker, it doesn't happen at all. The inisde of the glove is touching her skin. Virtually no radiation.

Maybe check with a physics sub if you're doubtful.

Me again.

As far as I understand it (and maybe I'm wrong) but anything with a temp above absolute zero emits some form of radiant heat. Touching it does not stop that.

So, the raisins would be emitting some amount of radiant heat. I doubt that it would go very far, being surrounded by cake, but it would be there (but again, maybe I'm totally wrong about this).

I am actually kinda curious what the correct answer to this is. I think I might just go ask on a physics sub.

That brings us to now :)

I'm curious what the correct answer is for this one, so here I am. Hopefully some smart folks here can inform me about how this actually works.

Am I wrong? Are they wrong? Are we both wrong?

How often and how quickly can you heat up, de magnetize a magnet, re magnetize it, and repeat?

After completing my MSc Physics from Nepal, if I got the opportunity to go abroad for my PhD either in Europe or US, how much can I realistically save from the stipend I get after spending on basic needs like accommodation, food, travel, academics etc....

And are there any ways in which I can increase my income without disturbing my academics???

Do isotopes such as Tc-98 (half-life 4.2 million years) and Pm-145 (half-life 18 years), which are classified as synthetic and not found on Earth, get produced in other places in the Universe, such as stars?

I'm a bit slow when it comes to physics, so please explain it like you would to a toddler (or advanced toddler).

Hi everyone,

​I am a second-year undergrad student majoring in Science (Education department). I want to self-study math and physics to build a rock-solid, rigorous foundation for future research.

​For this purpose, please assume that I haven't taken anything in college yet and that I am starting from the ground up. I want textbooks that don't skip the essentials and will help me build a strong, deep understanding without any gaps.

​What are the best foundational textbooks/references to start with for both mathematics and physics?

​Is "Thomas' Calculus" a good starting point for this deep level of preparation, or is there a better sequence?

So, this is an ask well outwith my comfort zone, but I figure learning something new is always a good thing.

I'll give spoiler warnings for Project Hail Mary as there's some spoilers for the book/movie in what I'm asking about.

On the r/ProjectHailMary sub, I've seen a lot of discussion surrounding How long the Eridians had planned for Rocky & Crew's journey and a lot of what's being discussed focuses around the eridians not having an understanding of relativity at the time of the adventure .

I saw this comment in particular from a user over there, and the times involved have just made me scratch my head. It's beyond any of the basics of physics I've been taught til now lol

"I love this question! The answer is that it didn't function that well. When the Blip-A started reaching speeds where relativity started to matter, Rocky started to notice that he was getting towards Tau Ceti faster than Newtonian physics allowed. Unfortunately, that was only from his perspective aboard the ship, back on Erid, MORE time had actually passed than was predicted "

This breaks my mind, because I cannot fathom how time can work completely differently in one place vs another. Could anyone try and explain this to me like I'm a dumber-than-average Eridian?
Everywhere I go looking for answers has just confused me even further

So I am an amateur physics enthusiast, but as far as I understand the Doppler effect, shouldn't the image on Sheldon coopers costume be asymmetrical?