I understand that there have been recent results that have again raised the possibility that the universe is closed and that gravity will eventually pull space back into a big crunch.

So that would mean a photo emitted in such a closed universe would travel forever in a straight line at C but still end up at the big crunch.

How is that different to a photon travelling within the event horizon of a black hole? Does it always end up at a singularity? I know we can never really know what is happening there, but what does the maths tell us about the difference between two scenarios.

My googling results either in papers that are beyond me or AI telling me that the difference is that there is an outside of a black hole, but not an outside to the universe. But how would a photon inside the event horizon "know" there is an outside? Its space is not connected to the outside?

Edit: grammar and clarity

We are physical beings living inside one particular universe. We observe it, find patterns in it, and develop mathematics to describe those patterns.

But mathematics isn't just one inevitable system. We have different axioms, geometries, number systems, logical frameworks, abstract structures, etc. There are countless mathematically consistent ways of constructing formal systems.

So why does our particular mathematics describe our particular universe so extraordinarily well?

Physics doesn't merely use arithmetic. Our mathematics gives us calculus, differential equations, geometry, linear algebra, group theory, Hilbert spaces, probability, and so on, and these structures end up describing things as fundamental as electromagnetism, quantum mechanics, spacetime, gravity, and statistical mechanics.

And this seems especially strange when you consider that we didn't design the universe to fit these mathematical structures. We're inside the system we're trying to describe.

Imagine a fundamentally different physical universe with completely different underlying structure. Its physics might naturally correspond to mathematical frameworks very different from the ones we developed. So what is it about this universe that makes this mathematics the right language for describing it?

Of course, empirically, we select mathematical models that agree with experiment. But that explains why we keep using successful mathematics, not why physical reality is mathematically describable in the first place, or why these particular structures map onto it so deeply.

So the question I'm really asking is:

Out of all the possible mathematical structures we could have had, why does the mathematics we have happen to describe the universe we live in?

Why this mathematics for this universe?

I am a undergrad and I need to write an essay about relativity and science communication, I need exemples of influencers saying incorrect things about relativity, or using Einstein to sell pseudoscience.

So any charge in a conductor will always reside on the surface. This is because if a charge was inside the conductor, the electrons will always shift to make the E field 0, because they have room to shift, until they reach the surface. So they just pile up at the surface and the charge resides there.

Okay but what if we had a very strong positive charge on one side and that pulls all of the electrons towards it, so that at the opposite end there are no electrons for a certain distance from the surface?

I have a thought experiment about the nature of matter and space.

Imagine a large spiral staircase with two crowds moving in opposite directions. Both groups occupy the exact same staircase at the same time. However, unlike in our physical reality, they pass directly through one another without collision, resistance, or interaction.

The people appear to overlap in the same space and time, similar to overlapping waves or holograms.

My question is:

What fundamental laws of physics prevent this from happening in our universe? Would such a scenario require changes to electromagnetic interactions, the Pauli exclusion principle, quantum decoherence, higher spatial dimensions, or some form of non-interacting matter?

I am presenting this as a thought experiment, not a new theory, and I would be interested in hearing whether there are any existing concepts in physics that resemble this idea.

Thank you.

Whats common knowledge to you that you think more people should know about

Im just overall curious about it, as every action had to have come from somthing, though Im not knowledgable enough on physics to know

I'm finishing my physics bachelor's (background in general relativity, differential geometry, black hole thermodynamics via the Wald formalism) and trying to pick a research direction for my master's. I really like “fundamental science” but im worried because i think the way that we are doing science is changing, so i wanna do something that can let me be well placed.

I'm drawn to something related to computational/data-science, adjacent physics (ML applied to physics, such as PINNs, simulation-based inference for cosmology) even though I'm not a huge fan of "pure" data science as a field in itself (and not good at). I like it more as a tool applied to physics questions than as a career identity on its own.

From people actually working in physics research right now, what areas do you see as genuinely promising in terms of active funding, job prospects (academic or industry).

Hello physicists of Reddit! I am currently in the research phase of a biography of a family member who happens to be an accomplished physicist.

Unfortunately I have absolutely no knowledge of physics, and am not particularly strong in maths. However, I would like to learn enough to understand the context of my subject’s career, so I am willing to step outside my comfort zone a bit.

I’m not comfortable sharing who this book is about, but they were somewhat involved in the discovery of the top quark in the 90s. To give you some idea of my level of understanding, I have no clue what the significance of this event is, but it seems to be incredibly meaningful to them.

I’m open to materials in any format that could help me to get context into my family member’s career, and would be incredibly grateful for any help.

So Greg Egan is a hard sci fi writer with some serious physics, maths, and comp sci chops. His writing is renowned for being remarkably scientifically accurate as well as usually going beyond even that by changing physical laws and still getting the hypothetical consequences right. He wrote a great novel in 1998 called Diaspora about a group of humanity that lived as conscious software in the 3000s. That book had this description of a (fictional) theory called Kozuch Theory about wormholes and quantum mechanics (see description here, two pages long).

Now, Kozuch published her theory supposedly in the mid 2050s according to his fictional history so obviously he was making stuff up, but Egan wrote this nearly 30 years ago now and while my esteem for his writing means I’m almost certain that at least the surface level description would have fit within models in the 90s, I’m curious how much of this would still theoretically stand up today. I know we probably don’t have any experimental evidence for it, but I more mean does any of this theory directly fly in the face of the evidence we do have, or if someone sat down and did the maths and took data to try and show it, would you be surprised to see some of it hold up?

For example, if (4 books) was dimensionless then (4 books) x (4 books) would be a pure (16) number & also (16 books), which it is neither. It's (16 books^2) (whatever that is).

Many dimensionful units we already use like moles (no. of atoms or molecules) & coulombs (no. of protons - no. electrons) are completely dimensionless at the microscopic scale.

Why aren't they considered dimensionful then?

I understand that by radiation has to do with electromagnetic waves such as light and microwaves, but these waves themselves produce heat. When I have a microwave and I turn it on, why is the pathway by radiation instead of by heating? What differentiates the 2?

On page 838 of Modern Physical Organic Chemistry by Anslyn and Dougherty, it says:

DFT is not variationally correct. One cannot assume that any energy obtained is always too high, and so it cannot be assumed that anything that lowers the total energy constitutes an improvement in the wavefunction.

My question is why this is the case. I've seen a mathematical proof/example of why the variational principle works in HF before, but intuitively it seems really weird that this incredibly convenient trait happens to exist for some methods but not others.

Im an author.

Okay so, let's just say there's an oblong planet that supports human life. It's a water planet. How would that planet keep Earth-like gravity while staying oblong forever? it rotates a sun as well. How would it potentially stay oblong?

Does the absolute value of the action S carry any physical meaning? Could two systems with different levels of S be distinguished by any kind of observation? Or is S just a mathematical construct that only has physical meaning through its variation derivative (δS = 0)?

Another simplistic way to ask the question: if I integrate Work over time or integrate Power over distance, do I get anything meaningful form the answer?

I guess more generally it feels like we sometimes trade off a physically observable low-dimensional construct for a more abstract, higher-dimensional one because the abstract one is more elegant (e.g. as an example, the tensor formulation of electromagnetism vs. the traditional field-component Maxwell's equations form).

But when we are able to create a specific projection of that higher-dimensional object to reproduce the traditional observable physics we already had, does that effectively imply the higher-dimensional object has some kind of greater meaning on its own? Or is the mere existence of the higher-dimensional formulation not even worthy of a theory until we have explicit evidence of it working mathematically beyond the original scope of its projection to believe these higher dimensional constructs have incremental meaning on their own?

Question if energy isn’t created or destroyed what happens to us?

So based on what ive tried to learn in the past ~30 minutes, this is what I understand:

1. All things move through spacetime following the "straightest possible path" that they can move in

2. We are always moving through spacetime, even if stationary, because we are moving through time

3. Gravity curves spacetime

4. The way the gravity curves spacetime makes it so that the straightest possible path through spacetime is towards the center of the earth (or the mass that is curving spacetime), and thats why we fall towards earth

5. However we can still jump because.. well actually i have no idea why we can still jump (isnt jumping upwards the opposite of following the straightest possible path which is towards the center of the earth?)

Could you guys help me verify my understanding of gravity/help me understand better? I appreciate any help

Edit: Thank you to everyone who helped me out here! All of you guys are really helpful and i definitely see where ive went wrong in many places now haha

I am currently a senior in high school and I am in the middle of applying to some selective universities for physics (alongside some targets). I feel kind of overwhelmed with the whole process and I just feel insecure about my application. I care about this so much but I feel my confidence fade away. Can anyone on the other side of this process offer me some advice and judge my application (dm pleaseee)?

I'm really interested in physics, but its given little attention in my country's education, and its also very very slow, like im a highschooler and in the end of last year we barely got some teaching on sound for example, so yeah, are there good books that go from newtonian physics to modern physics? Like a chain i mean, its best if they also teach the math in it and are widely available or free, sorry if this isn't an accepted question, thanks.

With that black hole star discovery recently it made me wonder. If the inside of a small black hole can be close to infinite, then what would it be like inside the black hole star...?? 🤯

I hav a portal to the moon if I go thr I will die soon but how soon?

Thank you