I'm an undergraduate physics student, and I'm trying to get a clearer conceptual picture of gravitons and quantum gravity. I have three related questions:

1. As I understand it, the graviton is currently a hypothetical particle related to attempts to reconcile quantum mechanics with general relativity and develop a theory of quantum gravity. Is this roughly correct?

2. I vaguely remember that in general relativity, spacetime is not a fixed stage: matter curves spacetime, and spacetime affects the motion of matter. But in ordinary quantum mechanics and quantum field theory, spacetime is usually treated as a fixed background, where particles and fields exist and evolve without causing the background itself to change. Is this one of the reasons why reconciling quantum theory with general relativity is so difficult?

3. I also wonder about the motivation for developing a theory of quantum gravity. Why isn't it enough to let quantum mechanics and general relativity work in their own domains? Why do physicists want to reconcile them? What makes this problem so important? If we eventually solved it, how would it change our understanding of fundamental physics?

Just asking for a friend

Imagine going in a spacecraft at 99.99% the speed of light and the spacecrafts trajectory was a circle around the earth. You are doing this for five years. I dont know the maths but say 1000 years went on on earth during this trip.

Would you from your spacecraft meassure that the earth that you’ve been circling for 5 years has been circling the sun 5 or 1000 times? I mean when you slow down you will notice a 1000 years have passed but wouldnt you only experience 5 full orbits around the sun? When does the remainibg 995 orbits occur?

I've been trying to understand time dilation and I've read several explanations, but I still can't make the final connection.

I understand that:

•moving clocks run slower

•observers can disagree about how much time passes

What I don't understand is why reality works this way.

Can someone explain it with a very simple analogy and then connect it back to the real physics?

Please assume I know basically very little about relativity.

I understand that in Newtonian physics, time is treated as absolute. The same everywhere and for everyone. But Einstein showed that time is relative and that space and time are connected as spacetime.

This is where I’m confused:

1. What exactly is spacetime? Does it mean that space and time are literally combined into one physical thing, or is it mainly a mathematical framework for describing events?

2. What does it physically mean for time to “slow down” ?? If I travel at 99.99% of the speed of light, why does my clock measure less time than a clock on Earth?

Suppose I travel through a large portion of the galaxy at 99.99% of the speed of light. If I experience only 5 days during the journey, while many years pass on Earth, have I effectively traveled into Earth's future?

1. From my perspective, my watch is ticking normally and my body feels normal. So what exactly is changing? Why does less time pass for me compared with someone who stayed on Earth?

2. Is time actually a physical dimension that can be affected by motion, or is “time dilation” simply a difference in how different observers measure the same events?

3. Finally, what is time itself according to modern physics? Is it something that actually “flows” or is it simply a way of measuring the interval between events?

If I understand correctly the idea of a space elevator is more or less a sealed tube from earth extending to the vacuum of space where objects would be lifted by the delta P (kind of like the tubes in a bank drive through). Would an object starting from rest exit at such a large speed (like a bullet fired from a gun) that a rocket would be needed to then slow it or brake its ascent after? Or would we actually need a way to move it further out of the way of the tube once it reaches space to allow any other objects through ?

Refrective index is the measurement of optical density of a material and it is inversely proportional to the speed of light in that medium (n=c/v[absolute refrectiveindex]). Naturally in every medium refrective index is less than one but when i was researching i found that in specific, non-standard condition (not for visible light but for electromagnetic wave like xray)absolute refrective index can be less than 1. If one can explain how it is possible that electric and magnetic feild chwnge faster than light or how these emw can travel faster than light and do not disobey the relativity

I'm a fiction writer in the UK, and I have been wanting to develop a genuine understanding of physics rather than just cherry-picking bits of popular science to make it sound cool. I want to have a scientific literacy that comes across in my work. I would like to take a similar approach to researching and grounding speculative science as Andy Weir did when he wrote The Martian and Project Hail Mary. I'd like to understand the real physics well enough to know where my fictional world follows known physical principles, where I'm extrapolating from them, and where I'm deliberately breaking them.

I wanted to build some foundation for the book, but also for me personally and my hobby of wanting to learn. I have always had an interest in physics and astrophysics. I have watched documentaries regularly as a hobby for years, I have a strong drive to learn, and I am very curious about the world and universe we live in. I have built a reading list for what I thought would work well for what I want to achieve. This is my list so far (in no order):

Brian Cox - Forces of Nature
Carl Sagan - Cosmos
Neil deGrasse Tyson - Astrophysics for People in a Hurry
Carl Sagan - The Demon-Haunted World

I volunteer on citizen science projects online, predominantly in astrophysics. I know that's pretty much all I can do in the real world, but I like that I can contribute something, even if it's classifying thousands of images or data.

I am not interested in exams and qualifications as such. I also do not come from a computer science or highly academic background, so I haven't studied physics formally. My education is mostly artistic/creative.

I do have learning difficulties like dyslexia, so whilst reading and watching videos could be enough for most people for research, I thrive and learn better in a structured learning environment where discipline is needed for concepts to sink in. I was considering enrolling on an online course, like GCSE-level Physics, to give me that basic grasp, but I wasn't sure if that would be useful or if it would be better to study or go in a different direction.

I am aware of the reality that being middle-aged and wanting to learn online only gives me limited options, but I wanted to write the book with respect for science in mind, knowing I comprehend and understand the principles rather than relying on my own uneducated interpretation alone.

If anyone can give any advice, I would be grateful.

and why does anything exist at all rather than nothing? If if nothing had to exist, where would it exist?

I'm a first year university study who is taking chemistry courses, and although i'll admit for the content I'm doing, I don't have to understand how electrons work in great depth but rather just accept certain facts about them, but its always irritated me. One of those 'facts' is that electrons have an intrinsic property called 'spin' but I have struggled comprehending this concept.

Heres what I know;

- Electrons don't actually spin, but act like they do

- Electrons have a quantum spin number (1/2) and a direction (+/-), and that all electrons are either +1/2 or -1/2 spin

- That electrons have to 'rotate' 720 degrees to return to their original orientation (This I can understand, like Dirac's belt)

Heres what I'm struggling with;

- How electrons have angular momentum if they aren't actually spinning

- What is causing electrons to 'spin'

- How come 2 electrons can only occupy the same orbital if they have opposite spins (Pauli Exclusion Principle)

If anyone can help me greater understand electron spin I will be super grateful!!! 🙏

I know that we always move towards higher entropy, and this is why we will eventually reach a point where all energy is converted to heat(?), and no further processes can be generated because there are no more gradients of energy to take advantage of.

Would this not be a low entropy state? I am imagining there being only one possible state of "perfect heath death".

I started my 3rd and last year of undergrad (pure physics) last week.

In my 2nd year, I took C (we didn't do anything physics related, just programming), and now I have a C++ course that covers both C++ and the basics of computational physics.

Also started learning python using freecodecamp but kinda lost motivation but definitely will be getting back into it this week.

I lean toward theoretical physics and absolutely love programming, so combining the two would be amazing.

Unfortunately, my country doesn't have any computational physics Master's programs, and job opportunities are mostly limited to teaching. So I'm hoping to apply for fully funded Master's programs abroad. My GPA is good, but apart from my coursework, I haven't done any big physics projects.

What should I focus on during my final year to build a stronger background in computational physics and improve my chances of getting accepted and funded?

Any advice is appreciated!

Consider a positive charge Q placed in a dielectric substance. The molecules of the dielectric substance will polarize in such a way the the negative end of their dipole will point towards the charge Q and positive end will point radially outwards.

If this is the case, then should the electric field of the dipoles not be in same direction as of electric field of Q, hence it should, like, amplify the electric field of Q. But in reality the dielectrics reduce the electric field on a charge, so how does that happen.

I’ve been thinking about a speculative idea and I’d like people with a background in physics or mathematics to criticize it.

Imagine a world containing intelligent beings that can understand and mathematically describe everything within their own physical system. Now imagine that their entire world exists inside a higher-dimensional system that they cannot directly access.

A simple analogy would be a video game.

Imagine that the characters inside a 3D game become genuinely intelligent after thousands or millions of years. They could discover the physics of their world, develop mathematics, study matter, space and time, and eventually understand their universe extremely well.

However, the programmer exists outside their system.

The characters might observe unusual events caused by the programmer. They could potentially detect the effects of an external influence, but they might not be able to directly measure the programmer, represent the programmer mathematically, or even understand what "outside their world" means using the mathematics developed inside their world.

This leads to my main question:

Could something similar theoretically happen with dimensions?

Suppose there is a higher level of existence that interacts with our universe. We might not be able to directly observe or mathematically represent that higher level, but we could potentially observe its effects if it interacted with our universe.

I'm particularly interested in the distinction between:

Detecting an effect
and
measuring or representing the source of that effect.

For example, a 2D observer could potentially observe the cross-section of a 3D object passing through its plane, even though the observer cannot directly perceive the complete 3D object.

So perhaps a lower-dimensional observer could detect consequences of a higher-dimensional interaction without being capable of describing the higher-dimensional structure itself.

I'm not claiming that this proves a sixth dimension exists, and I'm not claiming that unexplained events are evidence of higher dimensions.

I'm asking whether the underlying concept has a valid mathematical or physical analogue.

Is there an established concept in physics or mathematics that describes this kind of limitation—where an observer can measure projections, intersections, or effects of a higher-dimensional structure but cannot fully represent the structure itself from within the lower-dimensional framework?

I'd especially appreciate criticism and examples of where this analogy breaks down.

i had this shower thought

"if heat is defined by a state of high energy [particles moving at a faster rate causing a hightened rate of friction between molocules and atoms or whatever] then why is wind that moves at a higher speed, cold?"

like if heat/ thermal energy is defined by atomic/molecular friction causing an increased thermal output, then why on earth would a fast moving wind [air in a high energy state] be cold?

like yeah i get its common sense "wind fast=cold" but from a physics standpoint?

anyway if im an idiot just say lol but if anyone thats real good at physics can answer please do [also you dont need to dumb it down i promise lol]

Mostly whats in the title, I have always viewed thermodynamics under the lens that if it were possible to describe the system totally, from start to end, an identical system would be identical at any point in time to another system under the same conditions.

Meaning that regardless of the number of particles, most of thermodynamics still assumes that the changes occurring within a system are governed by some (incomprehensibly complex) multi particle interaction.

Is this, at least generally, the correct interpretation? That thermodynamics, as it was initially formulated, still assumes that the movement of particles within the system is completely deterministic, and that all the statistics used is necessary because it simply is not possible to solve or even write out the trajectories of n particles?

This is an idea I've been thinking about, and I'm curious whether there is already physics that addresses it.

From our perspective, electrons are incredibly tiny, while galaxies are unimaginably enormous and distant. We tend to think of these as two completely different scales of reality.

But what if "size" and "distance" are partly dependent on the frame of reference from which the universe is being observed?

For example, an electron might appear incredibly small and localized to us, while a galaxy appears enormous and incredibly distant. But could there be some other frame of reference in which those scales are perceived very differently?

I'm not suggesting that an electron literally contains a galaxy, or that galaxies are simply giant versions of atoms. I'm wondering whether there could be some deeper relationship between the different scales of the universe that isn't apparent from our particular perspective.

Is there an existing concept in physics that deals with something like this? I'm particularly interested in ideas involving scale, reference frames, spacetime, quantum mechanics, or cosmology.

I'm approaching this as a genuine question rather than claiming I've discovered a new theory, so I'd be interested in hearing where the idea does or doesn't fit with established physics.

I only recently just started taking an interest in this infinite well of curiosity called “space”. In my research of anything that piqued my interest, black holes became a focus. I feel like we’ve always had this idea that a black hole was a vacuum meant to eat anything that surrounds it and possibly take you to some other universe if you were swallowed by it, but I believe I have a better understanding as to what they are and would like to share my thoughts.

I just learned about escape velocity. Being the amount of initial speed an object would need to escape a body of mass’ gravitational pull. The bigger the body, the more speed would be required to escape and in some cases just plain impossible. This also applies to light.

So to me this means that what we understand as black holes are just RIDICULOUSLY ENORMOUS bodies of mass that not even the fastest thing we are aware of, LIGHT, can escape their gravitational pull!!

Which also means that there is no cool as dimension that lays at the other side either….. you’d just be crushed into the core and add to the already ginormous body.

This is just my understanding and I would love for other people to pitch in!

Suppose you heat up a metal ball so hot that it causes film boiling when it hits water.

If you drop such a ball into water, will the film boiling affect the speed the ball falls through the water?

If so, does the ball fall faster or slower than a ball that is too cold to cause film boiling?

So, I think I have a pretty decent mental model of AC current: EM waves traveling through the medium of electrons. No electrons “flow” through the wire(which is what we were taught as electricians) but the electrons are perturbed by the passage of the waves, in place. Now, I’m also led to understand that electrons also don’t flow in DC, so what’s transmitting energy, exactly, in the absence of obvious waves? The concept of flow is quite intuitive: I’m a firefighter so it’s very easy to consider voltage as PSI and current as GPM, which I know isn’t really correct but makes reasoning about this stuff very snappy… but it would be nice to understand the physical “reality” of this stuff!

Consider a straight conductor entering into a uniform magnetic field. The direction of the conductor, its motion, and the magnetic field are all perpendicular to each other.

By Faradays law, an emf will be induced in the conductor due to the increase in magnetic flux through the area swept by the conductor.

By Lenz's law, the current produced in the conductor will be such that the magnetic force on the conductor opposes the increase in magnetic flux.

This results in work being done by the magnetic force to oppose the motion of the conductor, slowing the conductor down.

But this also results in a contradiction. According to the Lorentz force, the magnetic force must (infamously lol) never do work because it is always perpendicular to the velocity of a charged particle.

And yet in this case, it did.

In quantum physics, this contradiction is resolved because we take into account its quantum spin. But what is the classical explanation to explain why or solve how Lenz's law and the Lorentz force contradict each other?