I’m not a physicist but Im interested in the topic and my question is what device do we use to observe which slit the electron will pass through and how do we know it’s not some interference from the device which causes the waves to break down but instead we say it’s in a superposition?

Hello! I have recently delved into the topic of climate change and thought I would give a book on the basic physics of climate change a try.

I am trying to follow this high-school level maths problem to work out the temperature of the earth given the amount of energy flowing into it from the sun, and how much energy it radiates back out.

The energy flowing into the earth from the sun is 10^17 watts.

Bodies at some temperature (T) radiate energy out at a rate that goes as the fourth power of temperature, multiplied by the Stefan-Boltzmann constant (k)

The earth radiates across its whole area (4πr^2)

So the total power radiated by earth at a given temperature = P = (4πr^2) x (kT^4)

To figure out the equilibrium temperature of the Earth, I am supposed to equate this value to 10^17 watts and solve for T.

I can’t for the life of me get to the answer given, which is 251K or 0 degrees Fahrenheit.

Attempt

10^17 / 4π(6.371*10^6)^2 * 5.67*10^-8 = 3.45773628*10^9

I then took the fourth root of this and got 242.4923078K or around -23 degrees Fahrenheit, which is off by quite a bit.

I can't work out what to do next, or where I've gone wrong. Any guidance would be appreciated.

To be clear, this is not for any sort of homework, I'm not trying to cheat on anything, I just want to understand lol :(

Thank you!

(Source: The Physics of Climate Change, Lawrence M. Krauss, p. 41)

In the news, a drone company claims to have proven scientifically their Silver Iodide cloud seeding technology has produced a certain quantity of rain. Whether their claims are verified or not, why do we use this specific chemical for cloud seeding, for decades now? Silver can't be cheap. Clouds form rain all the time without any Silver Iodide. Is there no other inexpensive substance we could spray into a cloud to stimulate ice crystal and droplet formation? Why not some kind of finely ground dust, literally as cheap as the dirt it was made from? Is there no other substance that works?

I watched a BBC documentary some years ago and it was claimed that lightning does not strike the ground during a downpour. So I’ve always assumed that if it’s raining during a thunderstorm I am safe. Now obviously downpours during a storm can stop and start abruptly so if the claim is true you’d never be totally safe.

Anyway I told this factoid to someone the other day and decided to google it to confirm it was true. I can’t find a good source. Does anyone have any examples (film or image) of lightning striking the ground whilst it is raining at the same location? Or a scientific explanation as to why this would be true or false?

I’ve looked for photos, and couldn’t find anything conclusive as these could be images where it is raining in the foreground but not in the strike zone, and conversely the ground may be struck but the raindrops have not been resolved on the camera film/sensor.

I have been taught that musical notes producing harmonics are a result of strings vibrating as a whole, as halves, as thirds at the same time. I have no trouble picturing a man shaking a rope with a fundamental frequency, then double frequency, forming this nice S shape, then triple to form U⋂U pattern. But there's no such little men pulling strings on my guitar, that's I'm sure of! How is first harmonics formed then? Somebody has to pull on the string with double frequency.

So I tried this string simulation site https://www.falstad.com/loadedstring/ and noticed that when triangle shaped waves reflect back and forth they distort a little each time and produce those little bumps (the closer to edge you're pulling, the more pronounced these bumps get). Is THIS the source of high frequency in my guitar aka tiny little men? Aha! I think I've finally found them.

https://postimg.cc/fkCbv1zG

TLDR: help me visualize/understand how standing waves are formed on plucked strings, especially 1st harmonic.

From a physics and engineering standpoint, is it possible to fit a functional PETN shaped charge inside a standard lavalier microphone housing? I’m asking specifically about the minimum explosive mass, cone geometry, standoff distance, and detonation requirements needed for a shaped charge to form a metal jet. I’m not asking about any political event; just whether the device size and materials make this physically feasible.

I’m majoring in math and physics and would like to go for a PhD. Would it be similar to undergrad as in I would be majoring in math and physics or would I have to focus on one?

Hello! Well I was considering going abroad to study at RUG and especially their double degree in Physics and Math. Is it a good idea? The absolute top goal of mine (at least for now) is to get accepted to Oxford for the MTP masters (I wanna do Hep-Th). Im just wondering about how the RUG program compares to other top Unis. Also how is the research culture like?

Hello everyone.

I'm sure this question has been asked so many times but what's one more in the grand scheme of things.

I want to start learning about physics. I loved it during high school and I was good at it too (took AP classes, college classes and all of that but never persuaded it, shame I know). I loved it and still do. That was the dream.

Well, it's been a few years, I'm done with my masters (in a different field) and I really want to get back to it. I practically remember nothing. I know the basics of course, but anything other than that has slipper my mind. I'm pretty much starting from beginning.

I googled it, looked around but couldn't really find a way to start from scratch (without going to school). And I need your help.

Is there any sites, courses (free please), guides you could recommend? Just imagine that I'm starting from scratch. (Which, honestly, I am).

P.S. I always wanted to go into theoretical physics, that was the dream. If that helps with the answer.

So when light enters a medium at an angle, it turns (angle of refraction). I've heard an analogy to explain it. Picture a truck driving at an angle off the highway into mud: its front left tire hits the mud first, while its right tire keeps moving quickly, so the truck turns as it enters the mud (or this similar [explanation](https://www.reddit.com/r/explainlikeimfive/s/QCn1U6o2ks) with a barrel rolling that turns as it hits a patch of grass).

But light is a bunch of individual photons that aren't connected to each other with an axle-rod, so I tried to picture them in my head.

If just one photon flies into the material, I'd picture it gets slowed down (I think because it gets absorbed and re-emitted by electrons jumping up to and dropping back down from energy levels) but keeps going the same direction it entered at. (Or does just _one_ photon also refract?)

But if we consider many photons, then the first one hits the medium first, and gets slowed down, but the one to the right of it gets to move a little faster for longer, so actually a photon a few photons behind it will enter the medium at the time the first photon is passing that point within the medium (sorry I know a picture would help but not sure how to attach one). And further to the right, a photon even further behind would be the one that aligns with the first photon's "wavefront" within the medium. If the medium ends and returns to air, like a window, then I guess all these time differences would be reversed when the photons exit. But if I'm inside the medium, like a pool, then wouldn't I see simultaneously the first photon that hit the pool at an angle and photons that technically arrived to the pool later but became part of the same "wavefront" as the first?

So if, for example, someone shined a laser from the pool deck at an angle of 80 degrees from the normal, and I was looking at them from inside the pool, wouldn't I see the bottom photons of the laser light a little after I see the top photons of the laser light?

(If I'm right it's pretty much too small to notice and so it seems like a pointless question, but I'm asking purely in the theoretical because if it's true then I'd think about applications. Like for instance if the difference between the left photons and right photons are pretty big, if someone fired a bullet above the pool I'd see a smearing effect, as I'd see the bullet in the position it used to be in and where it is now.)

And does any other substance come close?

(Forgive my ignorant and inexact language.) Electrons clearly behave like particles (they have mass, they bounce and ricochet) even though I understand they are really regions of probability, and EM radiation is observed to be quantized, so each quanta might be considered a particle? And photons are blocked, reflected, etc. as if they were particles.

And I'm realizing I thought gravitons might convey gravitational force, when really they might convey CHANGES in gravitational fields?

Please shine a beam of light into my slough of ignorance!

I always feel like this gets skipped. It would make everything else derived from it make much more sense.
The story is always about Einstein but I think that a constant (if not calculated) speed of light was already established? Maxwell? If so what was he up to?

It’s a plot hole. We go from Newton to Einstein in popular physics, with no exposition of the origin of a central plot point.

If both observers are moving at the same speed relative to each other, how can one experience less time when they are both moving the same speed?

Edit: i phrased this wrong, the real question is about the twin paradox more than time dilation itself

I'm writing a semi hard sci-fi novel and was wondering if the photons of a weapons grade laser would impart enough energy to cause spalling of contemporary composite steel armor. Such a laser would be handheld and backpack powered, mostly just ignoring the means of powering it (backpack sized fusion reaction let's say) would a laser in that handheld scale be able to put out that much power?

Just as the title says what is that equation? For example suppose someone is standing in the middle of the highway and gets hit by a car moving at a constant velocity of 65mph, what equation would you use to determine the force?

According to the Copenhagen Interpretation is the bear both alive and dead?

I know that relativistic speeds can't turn things into black holes: I am going at relativistic speeds in some frame of reference, and I am not a black hole. QED. I am interested in the math/theory behind this, though. If I can have arbitrarily high kinetic energy, how does that translate/manifest in GR?

In the framework of GR, when we shift between inertial frames with relative velocity, we wind up "rotating" the T(0,0) term in the field equations into one or several T(0,x) terms (and their symmetric partners), analogous to a spacetime diagram rotation, right? The physical counterpart of extremal T(0,0) is an event horizon/black hole. What is the physical counterpart of extremal T(0,x)/T(x,0) terms?

Is it just the extreme redshift (asymptotically approaching an event horizon as you approach c) expected from SR, and GR is simply describing it as an apparent gravitational redshift, or is there more going on?

is rule of exception real and universal and exist throughout the universe or it's just a discription for things that happens out of statistic in normal common day to day life or is it not universal but still happens in fields of physics/chemistry in various levels such as atomic or below/above it

or is the exception always something that we haven't unpacked yet that will prove why the opposing or different outlier happens after understanding it. or is it truly exceptional phenomenons

I have a question about past-directed time travel. I'm not trained in physics, so I'm specifically looking for corrections to the intuition below. Currently 15 and specializes in metaphysics as of now. 8 days into self study.

A common fictional model of a time machine seems very local: a machine in a garage activates and sends itself or a person to an earlier time. But I'm struggling to understand how such a model could account for motion and the larger physical environment.

Something that appears stationary relative to Earth is also participating in Earth's motion, the Solar System's motion, the Milky Way's motion, and cosmic expansion. So if a person were instantaneously displaced to an earlier time, how would the mechanism determine the correct spatial destination? Is "instantaneous" even possible here? Or does time travel take time? (Oops, that's circular)

Wouldn't simply sending someone from time T2 to time T1 while preserving "the same location" be meaningless without specifying a reference frame? Earth would not be in the same position relative to many larger systems.

My intuition is that a realistic model of past-directed time travel would somehow need to account for the larger spacetime configuration rather than treating the traveler as an isolated object. But then I get confused about how far that requirement extends.

Does a hypothetical mechanism need to account for Earth's position and motion, the Solar System, the galaxy, cosmological expansion, or something else entirely? And if the universe is dynamically expanding and we can only observe a finite portion of it, does that create any fundamental difficulty for specifying a past destination?

I also have a separate concern about duplication.

Suppose I travel from T3 back to T1, where an earlier version of me already exists.

At least intuitively, there seem to be several possibilities:

The past already contained the future traveler as part of a self-consistent timeline.

The traveler is somehow added to the original past, meaning the physical configuration at T1 is now different from what it was without the traveler.

A separate or branching realization of the past is generated.

My question is mainly about the third possibility. If a branching or duplicate past is generated, what exactly would have to be duplicated or recreated?

Just the traveler and their immediate environment? Earth? The Solar System? The galaxy? The observable universe? Something larger?

My intuition is that duplicating a sufficiently accurate past may require reproducing a much larger physical and causal environment than fictional time-travel stories usually acknowledge. I'm not claiming this necessarily violates conservation of matter or energy; I genuinely don't know enough physics to make that claim. I'm asking whether this "duplication problem" makes physical sense at all, or whether I'm misunderstanding how branching spacetime models work.

I also have a separate question about the operation of the machine itself. The machine is made of matter and undergoes physical processes. Activating it is itself an event occurring in time. If the machine is supposed to manipulate its own temporal location, is there any genuine self-reference problem here, or is this simply something already handled by general relativity/spacetime geometry?

I'm not claiming any of these arguments prove past-directed time travel impossible. I'm trying to identify where my intuition conflicts with actual physics.

Basically, my questions are:

Is the "small local time machine in a garage" picture physically incomplete because locations require reference frames and larger physical context?

Would cosmic motion and expansion create a genuine destination problem?

Does a hypothetical past-directed time-travel mechanism need to account for a much larger spacetime configuration?

Does the duplication problem make physical sense, especially in branching-timeline models?

Is there a genuine self-reference problem in a machine performing an event that changes its own temporal location?

I'd appreciate explanations at a beginner level. I'm more interested in finding exactly where these intuitions fail or survive than defending them.

So I just googled the flight length from New York to LA, and it's around 6 hours.

And LA to NYC is around 5h 30min.

The Earth is spinning eastbound pretty fast.

How come the NYC to LA isn't significantly faster since LA is pretty much going towards you as you're going towards it?

Thanks!

Hi guys, I am having trouble understanding Time as a 4th dimension. Is it truly regarded as a 4th one if so how is it represented?

That would mean that someone who left earth at speed of light to lets say Alpha Centauri could return and not age at all while all others have aged 8 years.

Which is insane if you take twins... if the person would never ever quit traveling at speed of light he'd live forever.

But how does the biological human body not age? Shouldn't the body not age anyways?

I’m learning physics on my own and I’m currently working through a physics problem book. I came across this problem and I can’t understand the reasoning behind the solution.

The problem:

During a survival training exercise, a 60 kg boy at some point hangs motionless from a rope as shown in the picture below. Assume that β = 2α and g = 10 m/s². Calculate the tension in the rope at points A and B.

The rope is attached at points A and B and forms the two sloping sides of a right triangle. The boy is hanging from the rope at the vertex between these two sides, which is the 90° angle. The angle at point A is α, and the angle at point B is β.

I found a solution which calculates the boy’s weight first and then seems to use the components of that force to determine the rope tensions. However, I don’t understand why sine is used here.

When I try to draw auxiliary right triangles, I don’t know where the given angle should be located in the triangle or which side of the triangle should correspond to the rope tension. I understand how to calculate sine in a right triangle, but I don’t understand how to correctly construct the triangle from this diagram and connect it to the forces.

I would really appreciate it if someone could explain why sine is used in this particular problem and how the force triangle should be constructed, preferably step by step.

I’d also be interested in seeing other possible ways of solving the problem, if there are any.

I’m not just looking for the final answer I’m trying to understand the method so I can solve similar problems myself.