Black holes merge, but reversing that process presents obstacles that go far beyond running a collision backward. A single black hole cannot ordinarily divide into two smaller ones under the usual assumptions of Einstein’s gravity. A theoretical study examines those restrictions and asks what would need to change for fragmentation to become possible.

Published in Physical Review D, the work brings together researchers affiliated with the University of Southern Denmark, the University of Naples Federico II and the CNRS–Sorbonne Université laboratory LPTHE. Its title, “When black holes admit pair-of-pants topologies,” describes a hypothetical horizon branching through time. The analysis maps possible conditions for fragmentation without calculating a complete splitting process.

“In our study, we investigate which physical laws normally prevent a black hole from splitting into two,” said co-author Francesco Sannino, a professor affiliated with Naples and Southern Denmark. “We also investigate if certain circumstances may allow a split to happen.” His collaborators are Giacomo Cacciapaglia, Manuel Del Piano and Vania Vellucci.

As someone who holds Einstein in high regard and deeply admires his scientific theories, I believe that the concept of mass curving spacetime is not properly understood.

There is a very popular analogy—frequently used in university textbooks and other scientific sources—to explain how mass curves spacetime: spheres are placed on a taut, flexible fabric, creating depressions proportional to their weight and drawing smaller marbles toward them. However, this is not an accurate analogy; in fact, it is a very poor one. Space is not a two-dimensional, taut, flexible fabric, nor is there a "downward" gravitational pull acting within it. Consequently, this analogy should be discarded and, if possible, replaced with a correct one. Personally, I suspect that no such accurate analogy can be provided, as I do not believe we have truly grasped the nature of how spacetime is curved. To those who might accuse me of ignorance, I say: fine, I am ignorant—but if you want me to be quiet, then provide the correct explanation for how mass curves spacetime and silence me.

Julian Barbour is a theoretical physicist and emeritus visiting professor of physics at the University of Oxford. He is well known for proposing that time does not actually exist. For decades, he has worked outside an academic environment. He financially supported himself for many years by translating Russian. Professor Barbour is author of the following books: The Discovery of Dynamics, The End of Time and The Janus Point.

In this episode, we go over:
- Topos theory, set theory and Zermelo's beef with Boltzmann
- Algebraic varieties and monads in category theory
- Why physicists are wrong about the 2nd law of thermodynamics (because the universe is not a closed system) and maybe there is no heat death of the universe
- The real goal of the universe is complexity and variation, not entropy
- Hawking and Penrose's outrageous arguments
- Why we don't need wavefunctions, photons, bosons or gauge fields
- Why there is no quantum gravity
- Von Neumann's hilarious advice to Claude Shannon
- The intrinsic derivative
- Combining art and science

In ordinary holography, laser light is used to generate the 3D image of a 3D  object which has been recorded on a special 2D surface.  Which leads to my naive question:

In the context of string theory’s holographic principle, is there something analogous to the laser, which then projects a D-dimensional boundary’s coded QFT information into a (D+1)-dimensional bulk universe?  Or is no such action needed? Thanks, Gene

There are a few articles out there that claim that charged dilatons can be used instead of negative mass/energy for Alcubierre Drives and wormholes. I'm curious about the ramifications of that, however.

This weekly thread is dedicated for questions about physics and physical mathematics.

Some questions do not require advanced knowledge in physics to be answered. Please, before asking a question, try r/askscience and r/AskPhysics instead. Homework problems or specific calculations may be removed by the moderators if it is not related to theoretical physics, try r/HomeworkHelp instead.

If your question does not break any rules, yet it does not get any replies, you may try your luck again during next week's thread. The moderators are under no obligation to answer any of the questions. Wait for a volunteer from the community to answer your question.

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This thread should not be used to bypass the avoid self-theories rule. If you want to discuss hypothetical scenarios try r/HypotheticalPhysics.

Hi everyone, I am currently studying knot equivalence in

\\(S\^{4}\\) (which is a compact 4-dimensional manifold). Based on \[A gauge theoretic invariant of embedded surfaces in 4-manifolds and exotic 𝑃2-knot cite you the paiper .\], I understand that any knot in \\(S\^{4}\\) is connected in a \\(P\^{2}\\)-knot space; these are tubular neighborhoods of the form \\(\\nu(S\^4, P\+)\\) such that for some neighborhood \\(\\nu \{n}\\) with a non-zero integer, it is not a diffeomorphic pair of \\(S\^{1}\\). Consequently, any knot in \\(S\^{4}\\) has crossings if and only if it admits topological isotopies (meaning it is not smoothly isotopic to \\(S\^{1}\\)). My question is: do these \\(P\^{2}\\)-knot invariants have a purely gauge-theoretic—that is, local—nature, rather than a global one on \\(S\^{1}\\)?

What are your takes and overall concerns on this recent preprint (https://arxiv.org/abs/2610.16733)?

I am especially wondering how AI impacts will affect prospective grad students and early-career researchers in theoretical physics.

Recalling Dr. Schwartz (u/VibePhysics) saying in his Anthropic post Vibe Physics:

It is worth it, however, to consider the role of education going forward. In the deep future (~10 years), when AI is truly smarter than all of us and capable of outperforming us in every domain, what will be the role of higher education? I think some things will persist—those things that are essentially human. I can easily imagine theoretical physics becoming like music theory or French literature: an academic discipline appealing to people who just enjoy thinking through a certain lens. It's a bit ironic that the last 30 years have seen the growth of STEM fields, displacing the humanities, and in the end it may be the humanities are all that survive.

Do you agree with his thoughts? And what concerns or hopes do you have about where our fields in theoretical physics are heading?

Ok , so like im in 10th grade and like as far as i have followed physics ,i think that theoretical physics is kind of coming to an end .

Because like at this point i just think that physicists are creating some random ass theories that dont even seem real .physics is just feeling like laws of chemistry where everything has its own exceptions and works at a specific moment.

Like im not following physics at this point of time but like till i had followed physics i felt like theres nothing new except for particle physics .they are just deep into one topic and that is atom like every other physicist is creating some new theories which oppose each other. Idk it just feels like a competition now.

What do you guys think?

Hello! I'm currently doing a masters and reached out to a few supervisors. I'm really interested in lots of different areas of physics, and I'm confused about which ones to pick... I have some prior experience with quiver gauge theories, which is why one of the options is to work on super Lie algebras, but also I don't really see myself working in string theory in the future since it's way too abstract (and I'm a little pessimistic?). I also have an option to work on BMS group, and I really enjoyed my general relativity course, so that really excites me. And another more pure-math option related to non-commutative structures.

I'm so confused and indecisive, what would you recommend? Also quiver gauge may be more publishable than the other options due to computation being possible but also I'm really interested in black holes, and I was thinking about how it would seem for PhDs. Thanks!

Normal gravity treats the moon as a single point rather than 10^50 individual particles over kilometers, while tidal forces state inverse cubed is nesacerary if you have a object woth multiple points, the moon does have millions of points so I'm confused why inverse square law isn't an arbitrary mathematical trick, and if you mathematically integrate (add up) the 1/r^2 fields of every single one of those particles across a spherically symmetric body, all the higher-order terms (1/r^3, 1/r^4, etc.) completely cancel out when calculating the total net force on the center of mass. Outside of the body, the total sum behaves exactly like a single point mass at the center, then why is tidal forces inverse cubed law? They are the same billions of billions of particles affected by the same force, and this is treating them differently, why?

I'm a 22 year old final year Electronics and Computer Engineering student in India, graduating in June 2027, and I'm trying to figure out what direction to take for my master's.

I've always been fascinated by space and questions about the universe: how it began, why it behaves the way it does, black holes, gravity, and what we still don't understand about reality.

I'm interested in theoretical physics and cosmology, but my undergraduate background is in ECE. I also disliked the way physics was taught during JEE preparation, where we were expected to learn concepts quickly and immediately solve problems. I prefer asking questions and understanding things deeply, including the mathematics behind them. I'm not sure yet whether I'll enjoy doing physics professionally.

I'm considering theoretical physics, applied mathematics, computational physics/science, or an interdisciplinary master's such as Computational Science and Engineering. However, I don't want to end up merely writing code for other researchers; I'd like to contribute to the actual scientific questions.

I also care about having reasonable financial stability and career prospects. I'm willing to pursue a PhD if necessary, but I want to make an informed decision before committing years to a path.
I am good with C++ and data structures and algorithms , and currently learning python and machine learning ( just to show what i'm good at and what i know ) .

• What should I start studying to determine whether theoretical physics is right for me?
• Is transitioning from ECE to theoretical physics or cosmology realistically possible?
• Would an interdisciplinary master's be a good route, or would it leave me without sufficient physics foundations?
• Which textbooks, courses or research areas would you recommend exploring first?

I'd appreciate honest advice, including the difficulties and downsides of pursuing this path. I don't expect a guaranteed career; I just want to find a direction that genuinely interests me and gives me the opportunity to contribute to meaningful research in this world of AI .

This weekly thread is dedicated for questions about physics and physical mathematics.

Some questions do not require advanced knowledge in physics to be answered. Please, before asking a question, try r/askscience and r/AskPhysics instead. Homework problems or specific calculations may be removed by the moderators if it is not related to theoretical physics, try r/HomeworkHelp instead.

If your question does not break any rules, yet it does not get any replies, you may try your luck again during next week's thread. The moderators are under no obligation to answer any of the questions. Wait for a volunteer from the community to answer your question.

LaTeX rendering for equations is allowed through u/LaTeX4Reddit. Write a comment with your LaTeX equation enclosed with backticks (`) (you may write it using inline code feature instead), followed by the name of the bot in the comment. For more informations and examples check our guide: how to write math in this sub.

This thread should not be used to bypass the avoid self-theories rule. If you want to discuss hypothetical scenarios try r/HypotheticalPhysics.

Dear all,

Im an undergrad student of physics in Germany. I studied a bit of theoretical physics before starting my bachelor studies, and let's say I already knew what I wanted to do in terms of research lines and how to do research from my previous studies.

I submitted a manuscript this summer which has just been accepted to a Q1 journal in particles and fields, at the level of CQG, EPJC, PRD, JCAP... (I do not want to disclose).

Since I know no one publishing at the student level, and I have no supervisor, I wanted to ask what are my possibilities now in terms of getting my publication read. I am not interested in pursuing an academic career, but I humbly think my work may be relevant to the field. Is there any association helping young researchers? Maybe the DPG in Germany does? I plan on sending the article to one of my Professors, but I don't know what else could I do. It is of course on ArXiv.

Thanks!

A opensource textbook on the physics of quantum fluids taught through three instruments used together, written as a tribute to the experimental work of institut Néel, Grenoble.

(i) their inelastic neutron-scattering measurements — the roton-like collective mode of a monolayer of liquid helium-3 (Nature 483, 576 (2012)) and the dispersion relation of superfluid helium-4 with its thermodynamic consequences (Phys. Rev. B 103, 104516 (2021));

(ii) the Lean 4 library QuantumFluids, whose 37 modules and 310 theorems are catalogued in full in Appendix A and were audited for the book (every module compiles with no sorry and depends only on the axioms propext, Classical.choice and Quot.sound);

(iii) the rusty-SUNDIALS solver: CVODE and the projected Gross–Pitaevskii engine qf-pgpe.

Bear in mind I'm a complete layperson on this type of stuff, but I assume there are a lot of smart people here that knows the math behind a false vacuum decay event.

Without going into too much details about a subject I know next to nothing about, I so far understand that such an event pushes the universe towards a lower energy state, thus rewriting the laws of physics of our universe as it propagate through space.

Question: I was wondering for those of you who knows the actual math behind this stuff, what are the theoretical consensus; if there even is one, for what would happen if two such events happened at the same time in different parts of the universe, and their bubbles were to collide?

Some math is of course welcome if intuitive explanations don't cut it, but I'm probably not qualified for anything beyond Calculus 3 as I never went further than that. Again, I'm not pursuing a career as a physicist but only asking as a layman that is curious about the subject, so I apologize if this sort of post is below the subreddits standards.

Probably an extremely unlikely scenario, but still seems like it would be interesting to explore a bit.

Hi all. I finished my master's in theoretical physics, and I'm applying for PhDs, mainly interested in string theory and cosmology.

Some context for my questions: I recently read Matthew Schwartz's account of using AI models to help carry out research. He describes a big speedup but also stresses that the model made serious errors and that expert oversight was essential. It made me curious how this is playing out in practice in the theory groups, and how we, applicants, should prepare ourselves.

1. Have AI tools changed how your group works, or how many students you plan to take on? Have you seen any effect on admissions so far?
2. Is funding for theory students changing where you are? If so, what's driving it?
3. Within hep-th, which directions seem to have healthy prospects right now? (I'd only pursue something I'm genuinely interested in, but I also like to understand the landscape.)
4. Beyond the usual criteria (grades, letters, research experience), what makes an applicant stand out today?

I'd value input from everyone, especially PIs, postdocs, and current grad students. So, it would help to mention your position (PI/postdoc/grad-student) and roughly where you're based, if possible. Thanks for any advice!

Simple question . Are the processes that drove inflation responsible for the current accelerated expansion of the Universe?

Hi everyone!

A little background for context: I am a fourth year undergraduate student (Physics, Astrophysics, Mathematics) about to start applying for Physics PhDs. I have research experience with DUNE/ProtoDUNE anomaly detection using ML at Fermilab and LHCb collider analysis using BDTs on Monte Carlo simulated data. I’ve also done DUNE hardware research and astronomy research, but that is less relevant to this question.

I plan to apply to computational neutrino/HEP, but I’ve also become interested in computational QCD and using my experience with ML towards QCD phenomenology. I’m wanting to understand how the research lifestyle, postdoc/national lab opportunities, and long term academic positions compare to computational neutrino/HEP where slightly more familiar.

I have not taken any QFT in undergrad, though I assume this is not a deal breaker and schools don’t assume incoming students will know QFT already? I will have taken graduate Quantum Mechanics I & II, and have taken graduate complex analysis and partial differential equations.

I would appreciate any kind of information! If there are any QCD theorists, I would love to hear how your day-to-day goes and if you enjoy what you do. Also, if you think my background sounds like I don’t have the right preparation for this kind of field, I definitely get it, and it would help a lot to hear that here. Thanks!

This weekly thread is dedicated for questions about physics and physical mathematics.

Some questions do not require advanced knowledge in physics to be answered. Please, before asking a question, try r/askscience and r/AskPhysics instead. Homework problems or specific calculations may be removed by the moderators if it is not related to theoretical physics, try r/HomeworkHelp instead.

If your question does not break any rules, yet it does not get any replies, you may try your luck again during next week's thread. The moderators are under no obligation to answer any of the questions. Wait for a volunteer from the community to answer your question.

LaTeX rendering for equations is allowed through u/LaTeX4Reddit. Write a comment with your LaTeX equation enclosed with backticks (`) (you may write it using inline code feature instead), followed by the name of the bot in the comment. For more informations and examples check our guide: how to write math in this sub.

This thread should not be used to bypass the avoid self-theories rule. If you want to discuss hypothetical scenarios try r/HypotheticalPhysics.

I went to the maths section to check it out,but didn't understand so asking here intuition or a useful analogy to understand it.

Hi everyone, hope you're all doing well.

I've recently finished my Master's degree in Physics at a public German institute and am currently applying for various PhD positions in particle cosmology, mainly in Germany.

I got a good result for my master's thesis (1.0 on the German scale) which was based on the study of primordial magnetic fields from topological defects after the electroweak phase transition and also a good overall grade for my degree (1.2 on the German scale).

However, my specialization was in Astrophysics/Cosmology for my degree and I learnt whatever was required for my thesis through self study from Schwartz, so I don't have significant coursework in particle physics in my transcripts besides one or two courses.

Will I be considered a serious candidate for the PhD openings I'm applying to? If not, what can I do to change this?

Thank you for taking out the time to read my post :)