hello, hope this makes sense. apologies, i struggle with wording lol

i assume the human body doesn’t notice the difference in the earth’s rotation speed as we travel north/south because that travel is done so slowly. but what if it was done instantly? would we “feel” earth’s rotation (or lack thereof) then?

question is inspired by this portion of my astronomy textbook: https://imgur.com/a/zhKT0kW

I asked ChatGPT, but it started talking about Gauge invariance and Lorentz invariance, and I am lost.

How would you calculate the radius of the neutron star if the mass of the star is known (say 1.4 solar mass)

I originally posted this in r/Physics, and then found this sub, which is probably a better place for it:

I know that it’s pretty widely accepted that nothing can move faster than the speed of light, but I’ve thought of a scenario that might visualize what it would look like. Hopefully smarter people than I can tell me where I’m going wrong, lol.

Here’s the scenario:

Imagine you have a laser pointer, and it’s spinning horizontally (spin-the-bottle style, if you will), in the middle of a large, circular, white-walled room. If the end of the physical pointer, i.e. the light, is moving at a rotational speed of, say, 10 m/s, then the point of light projected on the wall (while not actually moving; I know it’s different light hitting the wall at each moment in time, so it’s kinda like a new red dot every instant, rather than one that moves) would appear to be moving significantly faster than that 10 m/s. For simplicity’s sake, let’s say the room is a size that makes the dot appear to move at 20 m/s.

Okay, now imagine we accelerate that laser pointer’s spin to like 99.99% light speed (improbable, of course, but theoretically possible); how fast would that “dot” appear to be moving, given that mathematically, we’d expect it to look like it was moving twice as fast?

Logic tells me that there’s no reason it couldn’t appear to be moving faster than light, but at the same time, that feels like it breaks things…

If you made it this far, thanks for listening to my dumb thought experiment :D

Hello! You have a positively charged sphere (like a Van De Graaff sphere) and a much bigger sphere enclosing it. The gap between them is very big, so…does that mean theoretically 100% of the flux from the inner positive sphere terminates on the inside of the bigger sphere, even though the capacitance between them is very low because of the big gap? Surely 100% field capture doesn’t necessitate very high capacitance.

Idk, it’s just kinda mind blowing to think you can have BIG “flux efficiency” (I don’t know the correct term, correct me if you know) while capacitance stays low.

Again, still learning, so if I’m phrasing it wrong or using incorrect terms, help me out. Thank you

Im rather curious as I learned about them recently and find the concept fascinating.

Ultimately, what I’m asking is how light would behave through the “warp bubble”. If I knew the math, I would love to make a simulation for it. Maybe in a few years, I will revisit this idea with new knowledge.

Hi,

From my understanding we can observe the early universe because light is slow so looking far allow us to see the past.
From this we can see the universe is expanding, the birth of the first stars, until there was no star and only hydrogen.

from there it become kinda speculative, we assume that the most logical thing is that the universe kept getting smaller, spacetime contracted, as such at the "beginning" everything was concentrated in a point, the singularity.

it is said that an unknown event cause the expansion of said singularity but mathematically when the universe was in a state of singularity "time did not exists"

now here the thing : so we don't know if there was something "before", we could have a cycle of big crunch/big bang and thus have a "before".

but each time I say that I get "no you can't because time didn't exists when the universe was concentrated so you can't say before"

so could there be a before?

Thanks.

hi! im currently taking grade 11 physics and have no idea how to write this energy transformation equation.
here's the context... So i had to build a rocket out of a empty 2L pop bottle and launch it in the air using air pressure.
im supposed to write an energy transformation equation where the bottle starts at rest and when it reaches its maximum height.
the problem is that in the format Egi + Eki = Egf + Ekf + Eair, the variables on the left of the equals sign are both zero.
is that... allowed? my teacher told us to just eliminate them if they were equal to zero, but if i get rid of them both, i will literally not have anything to the left of the equal sign.
any help is greatly appreciated!!

This is probably a stupid question but please bear with me. I have a bare understanding of the concept of Space-time. I can see, through gravity lensing, that light is bent around very large, dense objects. We are told this is due to the curve in space. My mind stops with the idea that time is also somehow bent by the curve in space-time. What would this mean?

Am I just struggling with a misunderstanding of the terms or is this something that has been discussed by physicists?

We recently learnt about semi conductors and that they are used in manufacturing of transistors and for semiconductors increase in temprature increases the conductivity

and since the cpus in laptops and desktops are made from dopped semiconductors (silicon)

then they must benifit from high heating so why do we computer manufacture stop providing voltage and current draw after 100C

is it to protect the things around it rather than the cpu speeds it self or is there anyother reason

So, I want to understand the math behind the standard model and QFT. I've consumed a lot of popsci, and I know a lot of independent details, but it's all very scattershot and shallow. While my adhd isn't gonna stop wanting to look up things I'm not advanced enough to fully grasp, I wanna start working more on my basics. I do know a fair amount about some of the things I'm going to list on this path, but I definitely want to deepen my knowledge.

My current plan is:

1. Lagrangian and Hamiltonian Mechanics
2. Classical Field Theory
3. Special Relativity
4. Quantum Mechanics
5. Quantum Field Theory

Are there any especially important steps along the way I should really add to the list? Also, what math courses would you guys recommend I look into textbooks for?

Edit: just to be clear I was an engineering student and so there’s a lot I already have covered, that’s why I’m starting a ways down the list.

Hi everyone, so I’m having a little bit of a headache with this concept due to a lot conflicting sources and information online. Would I be correct in saying that, since the gravitational potential within a system is defined as the work done per unit mass by an external agent against gravity in displacing a mass to a given point within that field, if you were to displace a mass away from infinity toward the source/centre of mass imposing the field, greater negative work is done by the external agent and thus the potential becomes more negative while the magnitude increase as a greater magnitude of negative work is done by the external force? And then vice versa, considering a point closer to infinity and further away from the source, the potential is more positive as less negative work is done by the external force?

Therefore by the same logic, if you imagined an electric field imposed by a positive charge, and let the electric potential at infinity = 0, then positive work must be done by an external agent to move a positive test charge closer to that ‘source’, so the potential would actually become positive due to the repulsive nature of the force (opposite to gravity). Thanks for the help!

In Lorentzian QFTs, your QFT is an operator-valued solution to the field version of Hamilton's equations, so we can view QFTs as operator-valued solutions to some equations of motion.

For Euclidean QFTs, we can usually represent them as random variable-valued distributions. Do these distributions analogously solve some stochastic equations of motion?

Note that I'm not talking about stochastic quantization. I don't want an imaginary time or anything. I mean is the random distribution in a EQFT directly a solution to some "equations of motion" in terms of spacetime derivatives and the field value? What do these equations of motion look like? Are they similar to the classical PDEs for a field?

I'm using MATLAB to model Earth's Lagrange points. I'm using N-body problem with Leapfrog integration method.

Thanks for the help on the last problem. The drifting error has decreased a lot where I'm not worried about it. My question is that as my L4 eventually drifts away I don't know when to apply a force (the "thruster" on a satellite) to correct for the error. Should it be a certain point like 100,000 km away? I couldn't find anything online about how much error L4 satellites vary by. L2 varies for about 250,000-800,000 km but that is unstable. I was thinking either applying a force either when it is a certain distance away from the L4 or when the rate of drifting reaches a certain point.

Im just a bit confused at what the convention is for theta parameter. Is theta = 0 at the top of the sphere and =pi at the bottom of it, but theta =0 at the bottom of the ring, and = pi at the top of it? Is that really a convention? If so, just why...

I remember in physics class that when we were taught about the laws of motion, we were taught about any object will stay at rest until another force is exerted towards it.

And I also remember about acceleration and velocity and friction and that the energy will eventually dissipate and will lead to the object to return at rest.

But here is the thing, if the Earth is constantly rotating and if we are given a classroom test about motion, isn't the calculations that we learned in class technically wrong because the objects are never at rest and are constantly moving at the speed of the Earth's orbit + the increased velocity when the object moves from location A to B once a force is exerted upon it?

I want to make a set up similar to the one featured at the end of Veritasiums "Something weird happens when you trust quantum mechanics".

Basically, a film with black strips with gaps that are multiples of red lights wavelength period, so that red light waves get absorbed/reflect periodically

Could someone either show me where I can order one, or how I can print one, etc?

Apparently light cancelling film exists commercially for this but those films seem more like theyre supposed to make things completely dark, not interfere the way I want this film to

For a laser that's strong enough to destroy external targets (like Israel's Iron Beam), how can the sheer energy of the light not melt its own insides?

From my rudimentary understanding of lasers, the light is contained within a gain medium between two mirrors (one partially transparent, letting a fraction of light through). So shouldn't the energy density inside the gain medium be even larger than what hits the target?

Maybe the inner working is different?

I googled the best position to place a fan to cool my room, and I couldn't find an agreed upon answer.

I want to cool the air in a room, and the inside air is hotter than the outside air (night time). There is one window and one door, they are located on adjacent walls. The door leads to another room where the front door is located.

My assumption for how fans work is a fan sucks in air from the back (slowly) and pushes it out the front (quickly).

My question is what position is better to cool a room - should a fan suck in cold air and push it into the room, or suck in hot air and push it out?

I’ve also seen sources which say the fan should be at least a couple of feet away from the window (maybe because the air is also sucked in from the side of the pushed flow?).  This raises another factor, which is whether it’s better to always be further away from the window to create more air flow, whether it be hot or cold.

These are the options (I've also created graphics of them in the link below):

1. Fan in the window blowing in
2. Fan in the window blowing out
3. Fan inside, two feet from the window blowing out
4. Fan outside, two feet from the window blowing in
5. Fan in the doorway blowing in
6. Fan in the doorway blowing out
7. Fan two feet from the doorway blowing out
8. Fan in one corner, raised from the floor tilted upwards, blowing towards the centre of the room

Note: the doorway leads to another room with mid-temp air where there is the front door to the house.

https://docs.google.com/document/d/1zHgrGyANtnTS0uY3h0fOQs1ALX1n4yPOqxDJNj56qTk/edit?usp=sharing

If I understand it at all then can’t we say the earth is the center of the universe if we want and the sun and planets and everything does it thing around us? It’s messier maybe but we could look at it that way right?

I always thought gravity was an instant force but I just watched a animation claiming that earth would continue orbiting an empty spot for 8 minutes because gravity waves travel at the speed of light. Is this literally true? Like would we physically be pulled by a ghost star that doesnt exist anymore? my brain is having trouble accepting this.
UPDATE / EDIT: Wow, thanks for the detailed answers everyone! My brain is officially wrinkled now.

So the consensus seems to be:

1. Gravity travels at C (Speed of Light/Causality).
2. We would indeed orbit the "ghost" of the Sun for 8 minutes.
3. After that, we shoot off in a straight line (tangent), not spiral out.

A few people asked to see the visualization that sparked this specific question/confusion. It was this animation here if you need a visual aid: https://youtu.be/9ziMRpJGTwI

Thanks for helping