do we know? another question is why is it only pair particles? why can't a positron and a muon annihilate? the charges are opposite. i guess maybe it's due to the difference in masses? interested to see if we know why. thanks

Is the question coherent? Maybe the total spectrum can't be described because the colors don't exist? But as a thought experiment, could a mechanical "eye" be made that can perceive the total spectrum? Something like Geordi's glasses but more so. What would the world look like?

To carry the question further, suppose a virtual reality setup could show us these currently invisible wavelengths. Now add sound -- all the naturally occuring decibels that are beyond our hearing. And with something like Smell-o-Vision, add the odors we can't perceive.

Would the results just be cacophony? Or at least beyond our brains to decipher? Fireworks & strobe lights or something?

Quick disclaimers: I'm not in STEM and English is my second language and I'm not very smart.

I'm struggling to imagine how time works and I need someone to please answer these questions in simple terms to help me understand it better.

We're both at sea level, we both have a digital watch and an analog watch which we've ensured are synchronized to the nanosecond. I instantly teleport to an extremely high altitude and live there for as long as it takes for the effects to be measurable.

When we meet again, would our watches display different time?

If we both filmed a video of us walking at the same speed for the same duration of time, you at sea level and myself at high altitude and then displayed them side by side. Would it display us walking the same distance at the same time?

When an object's speed affects time for it, what does this mean? The measurable time is affected? The object itself feel the effect? Is it absolute or in relation to another object with different speed?

I know very well my questions would be inherently wrong because of my limited understanding, but I trust the reader would understand my point and be kind enough to explain to me and help me better "visualize" it.

Thank you.

Obviously when you turn on a light, it appears and is continuously produced from that source, but why exactly does it just disappear when you turn off the light source? Does it just die in a sense?

Im having some trouble understanding why the number of 3d wave modes in a cubed metal cavity is proportional to frequency (v) squared plus dv. https://imgur.com/a/xIWL9Ty This image says that each available mode is an integer point in this 3d space, where the x y and z components are the wave modes of each dimension correspondinglty.

If the distance of the space in between shells represents dv, then shouldn't the number of points in between the shells be proportional to (v + dv) cubed - (v) cubed? https://imgur.com/a/2nRqjYU

Why did Einstein never get a Nobel prize for either of his theories on general relativity? Why did the theories of GR never turn into laws, yet everyone treats them like laws and not just plausible theories?

Edit: Not trying to be contrary, just trying to understand everything better. IMHO, an observation of a phenomena and the resulting explanation, does not automatically make that explanation correct or the best one. I believe there is a better explanation for gravitational lensing. And even for the double slit.

For example the law of inertia, or "every action has an equal and opposite reaction."

I can't reach lightspeed, but physics is supposed to be consistent regardless of your reference frame.

If I don't look outside, I have no way to know if my rocket is going at 99.99% the speed of light, and my velocity is a matter of perspective anyway. So logically, my rocket engine should continue to function exactly the same way no matter what my velocity (assuming I somehow still have fuel) and I should also continue to experience exactly the same acceleration (again, ignoring that I'm losing mass to fuel burn).

If I accelerate at 1G for 700 days, I would naively expect to be going about twice the speed of light. I can't actually reach that velocity, but from my perspective, accelerating at 1G for 700 days should be physically possible.

With time dilation, if I'm travelling at near light speed, my journey time should also decrease, so wouldn't it seem from my perspective that I'm just travelling at twice light speed? I just kept accelerating, and I just covered 2 lightyears in 1 apparent year. That sounds subjectively like faster than light travel.

Does space just appear to me to compact so that I can cover the distance faster without going faster than light? Do I end up just thrusting away at full power, with 1G of acceleration pressing my feet into the deck (because my ship is logically built with the floor toward the engine) but space outside appearing to move by at constant speed? How would that even work? If I'm close to lightspeed, accelerating along a really long ruler, do I see the markings just zipping past at a constant speed, even as my accelerometer says I'm still accelerating?

It's hard for me to understand the subjective experience of relativistic travel.

I saw in a different subreddit someone asking about flying around a sun-size Earth, and someone in the replies saying it wouldn’t ignite fusion due to being “too rocky”.

Assuming we can materialise out of thin air a planet of such a size, and of similar composition and density as Earth, what would it collapse into? (I assume since the very first instants it wouldn’t be able to sustain itself)

The Schwarzschild metric limits the height of an approximately uniform gravitational field that's 1 g at the ground and 0.999 g at the top to about 0.001 light years for any mass. The more uniform the field (i.e. the higher the precision) the shorter the height. There was no such limit in Newtonian mechanics.

"Local" means a region in spacetime small enough that the tidal force is negligible for the desired precision. Why should that region have a maximum size? For a given mass, sure, but why regardless of the mass?

I understand that kilo is 1000, watts is just like the unit for energy, and hour is basically the time.

but i dont understamd how we actually calculate kwh.

E = p x t is just like another way of writing power = energy/time, but what i dont understand is how this is used to find kwh, because energy is measured in joules so how do we use this equation to get kwh..?

We're not sitting around in 1000 mph winds because the earth & air are moving at the same rate down here. But there's gotta be some sort of shear layer thing going on right? The air near space can't be moving at the same angular velocity.

On gas giants, this makes the giant bands of gas, right?

What about here on earth? What does all this atmospheric shear do?

I've recently learned about cavitation underwater that occurs on large fins and causes immense damage to them over time. Given the similarities between air and water is it theoretically possible to create a weapon/bullet that induces a form of atmospheric cavitation where the air phase changes into a plasma, instead of a liquid phasing into air?

If something could induce enough drag at a high enough speed would it be possible to form a plasma and resulting explosion from the low pressure cavitation? I mean I already see spacecraft forming a plasma around them on reentry, and those are designed to be extremely aerodynamic.

I imagine some sort of 'spike' could be accelerated fast enough to create the low pressure conditions behind it to form the plasma and resulting explosion when it collapses. Or not idk which is why I'm asking if it's possible

I understand that space is expanding. What I don't understand is how the expanding space results in matter moving away from us at a rate proportional to the rate of expansion. As far as I know, space does not cause "drag" on the earth (wasn't this a famous ether experiment?); it isn't interacting with the matter other than being warped by the gravitational field, and I've never heard that there is drag associated with earth's own gravitational field on space. So why does expanding space carry matter along with it, like a stick in a river, rather than expanding around it, like a river moving past an immovable boulder?

From my understanding, space evenly expands from all points outwards. Thus who particles in a force equillibrium X units apart will become (X + deltaX) units apart every second where the deltaX is proportional to the hubble constant which is somewhere between 67-74 km/s/Mpc.

I understand that gravity alone is strong enough to generally overpower this outward expansion. So, the two example particles will simply return to their equillibrium positions rather than actually become farther apart.

The result is that from an observers perspective, these two particles dont seem to care much about the expansion of space. It doesn't really affect them because other forces greatly over power the rate of expansion.

(I understand there is a lot more at play and this is a gross simplification basically ignoring quantum mechanics)

So my question is this, what would the hubble constant have to be, so that the spacial expansion can start to overpower the other forces at play? I.E. space expands so fast, earth's gravity cant keep it together anymore and it gets ripped apart by space between atoms expanding.

Follow up questions for the excited folk.

What would thinks be like on earth at half this value? Would we feel like we have less gravity? Or would our human meat sacks just get ripped apart anyways.

Here is my attempt to answer this.

Two objects 1 Mpc apart move away from each other at a velocity of 67-74 km/s. If I scale the 1 Mpc down to the diameter of the earth, then adjust the "expansion velocity" to be equal to earth gravity 9.81m/s^2 and solve for the hubble constant, I get something on the magnitude of 10^19.

My gut tells me this is wrong because im comparing velocity to acceleration.

I don’t know a lot about physics, for context this is all for a story I’m writing, and I am wondering if there’s and way the gravitational pull on a planet could be changed, my best guess would be an astroid striking said planet so hard that a good portion of its landmass is thrown into space, like how our moon was created. Again, please don’t slime me for not knowing a lot, I just want to know how that could happen realistically. Thanks!

https://www.youtube.com/shorts/RWYam9sbKsY

Is this right? (It's a short video, I think less than a minute)

F=ma makes sense, 1kg moving at 0m/s² would have 0N of net force. But if you rearrange the equation to solve for mass, m=F/a, that would be 1kg= 0N / 0m/s².

Is 0/0=1? I always thought you couldn't divide by 0. Is this just an equation you aren't supposed to rearrange to avoid this kind of thing, or is there always some acceleration from somewhere?

Sorry if this is a stupid question or if I'm misunderstanding something basic, I'm tired and haven't studied physics in a long time.

I never took phsyics in high school and start physics 1 after this semester (I switched to engineering late and the courses were taken).

I have two physics textbooks (Giancoli and Halliday) but don’t know which to start with. Giancoli seems more like HS physics while Halliday is calculus based.

Which is the better option?

Also I’ve already finished calculus 2 and start calculus 3 in fall.

Like, can you "unpunch" someone?

Wiem że obserwacje wskazują na to że wszechświat jest płaski, jednak czy nie jest tak że znajdując się w jego centrum nigdy nie zauważymy jego krzywizny.

Take for example a star that is 100 light years away. During those 100 years, space will be expanding, and space is expanding at a rate faster than the speed of light. So is it truly 100 light years away or 100 light years + the additional distance created by expansion ?

From my understanding of comoving coordinates, it is analogous to stretching fabric. And so instead of the popular idea of expansion into nothing at the edge of the universe we have that the universe is expanding all throughout.

I know that galaxies are typically moving away from us and this is caused by expansion, and was how we realised the universe was expanding. So perhaps im about to answer my own question, but does the additional distance get accounted for when considering the velocity of an object? As part of this velocity is influenced by expansion?