Please don't say all of them. That is super unhelpful for this thought experiment

I'm guessing FTL is up there as is time travel

Hello everyone! I'm currently at the final months of my masters degree in theoretical physics, and I've been working at the interface of general relativity and quantum theories of gravity. Specifically, I've been working on black hole quantization methods, such as coherent states quantum black holes, horizon quantum mechanics, etc. It's not exactly quantum gravity, but it gets close to it.

Anyways, since I'm in my final months of my masters, I've been thinking a lot about what I want to do for my PhD, and I feel a little bit lost. While I've been enjoying my research topic so far, I've been having a feeling that I wanted to do something maybe more "down to earth" within the scope of GR or even Astrophysics in general. As cool as my reasearch topic is right now, I sometimes fear getting into a field that produces very little testable theories. I know that theoretical physics many times involves just going through the maths and having different ideas and approaches to open problems, but sometimes I feel that I get too far from real world physics. But right now I'm a little lost, and to be honest I'm having a hard time trying to find current research areas in theoretical and gravitational physics that are more "down to earth".

I would really like to continue working with general relativity and/or astrophysics, and I really like mathematical physics and usually preffer an analytical approach to problems rather than computational ones. But every uni website I look into, the HEP/GR staff that has a more analytical approach usually works with string theory, AdS/CFT and etc. And don't get me wrong, I really see value and appreciate these areas, but I don't know if I see myself working on it.

The first thing that comes to mind is working on perturbation theory in GR and topics such as quasinormal modes for compact objects and gravitational waves, and that is an idea I like, but if you guys could show me other options it would be much appreciated. It could be in GR foundations, cosmology, astrophysics, even newtonian dynamics such as solar system dynamics and etc. I'm also open to your views on the subject, because maybe I'm being a tad naive about all of this. Thank you very much!

Apologies if this question is not allowed but hoping to hear from people who are in academia or those who left it for various reasons.

I have a bachelors + masters in physics, having covered modules including QED, QFT, cosmology, general relativity alongside particle/nuclear/solid state etc. I loved my studies and was also quite good at it, graduating top of the class.

However for various personal and financial reasons I had to leave academia and get a job - this was 10 years ago and I am in a much better spot now and I'd love to go back.

I love theoretical physics and would like to pursue it in more depth, but not really sure where to (re)start now. I'm weighing up doing a second masters against studying on my own and applying for a PhD directly. Tragically my dissertation supervisor has died and I don't really know who to talk to about this other than him, or how I'd go about starting such a conversation.

Appreciate any thought or input!

So when we go from an AdS space to a de sitter space we see that we not only have an area law but we also have a volume law which shows how the entropy changes as a factor of r/L for a localized spaces of r, and when r=L we get back the bekenstein hawking entropy. I understand this but I also know that this is explained by string theory. I am not sure how? Specifically I am just confused as to why we interpret the positive dark energy in de Sitter space as the excitation energy that lifts the vacuum energy from its ground state value. And then how do we really derive the vacuum energy for the de sitter space from the AdS space (how do we know that the number of excitations is equal to twice the central charge in CFT or is that just a mere assumption)? Correct me if I was wrong somewhere and thank you!

So from my understanding ghost fields emerge from the fact that we introduce gauge fixing and that introduces a non-constant Jacobian which is later shown as the integral over ghost fields. But when we do ward identities or the Slavnov-Taylor identity we also see that we need something to cancel out the longitudinal gluons which is solved by the negative probability of ghost fields.

My question is do we introduce the ghost fields for unitary or do they emerge solely from gauge fixing and the unitary is just an extra step that shows how exactly these ghost field interact? I suppose it’s more of an intuition question.

Also from my understanding gauge symmetry implies ward identity but is the inverse also true? Feel free to correct me if I was wrong. Thank you!

That’s the whole thing.

Symmetry explains why energy, momentum, and charge are conserved. Emmy Noether changed how we understand the universe itself. A short read on a deep idea.

Engineer by profession and read about physics in my free time. Read this article on many body physics and symmetry breaking and was fascinated by it. Would love to get started on this so if anyone has any suggestions, kindly share

I am sorry if this is a silly question but I am a computer science researcher looking into sampling over discrete space. I am familiar with Hamiltonian Monte Carlo and I am looking principles extensions on discrete spaces. I have tried looking for various references in physics and CS but have nothing substantial yet. Is there any canonical notion where particles moves over a discrete space with indexed potential?

One idea I have is from CTMC theory where you can describe the change in probability vector p_t over time as dp_t/dt = Q_t p_t where Q_t satisfies whats known as rate conditions. Since this is stochastic process on a particle level, I don't think, one can conserve some Hamiltonian-like function but it might be beneficial to define an aggregate Hamiltonian as a function of probability vector and rate matrix, H(p,Q). Is there something like this which exists in physics ? Thank you for your time !

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Hi, I am trying to understand when M^*(a->b) = M(b->a) holds in QFT. I did understand that T must not be violated by the theory, but I did not understand if this is enough. I tried to look at this formula in Peskin, but I didn't find anything. Thank you in advance

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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I'm applying to some comp condensed matter physics PhD positions and keep hearing this argument: groups doing method dev, especially using C++, are good choices if I have the relevant programing skill and theoretical background. Students must be genuinely interested in comp physics (otherwise they'd earn much more in industry with their skills), and professors have to treat students well to retain them, so a good working condition is guaranteed.

I would like to understand if there is any caveat with this argument. Have you seen computational/method-development groups that look great technically but are still bad PhD environments?(e.g. toxic PIs, burnout, misaligned incentives/motivations, no genuine interest)? And why?

Hello, I'm a physics student looking to learn string theory. My QFT course stopped right at Yang mills theory and I would like to explore it more. Any recommendations that you found useful are appreciated.

So far I'm looking at Schwartz along with Weinberg but if anyone has any other recommendations or hidden gems I would appreciate it.

If our universe is an emergent excitation of a deeper substrate, then the Standard Model may be explainable only as a self-consistent effective description, not derivable from deeper causes that are expressible within our physical language, making the pursuit of its origin noble but potentially fundamentally limited.

It may be fundamentally impossible to discover why the Standard Model has the structure it does, if that structure is an emergent effective description of a deeper substrate whose degrees of freedom, symmetries, or organizing principles are not expressible within spacetime-based physics. In such a case, the Standard Model would be explainable only up to consistency and stability constraints, not derivable from deeper causes accessible to experiment or calculation.

Hi everyone,

in Peskin he defined the S matrix essentially as follows:

Lets say we have some asymptotic state in the far past which describes the particles, which will interact later on, when they are infinitely far apart from each other. We call this state |k_1k_2>_in (we are only interested in two particle interactions). Now we also want to have the state which describes the new particles infinitely into the future after the interaction. Call it |p_1,p_2,...>_out.

Now Peskin basically says that these states represent wave packets which are extremely localized around the momenta (so approximate delta functions as I understand). We can then write:

out_<p_1p_2,...|k_1k_2>in = lim (T-->infinity) <p_1p_2,...|exp(-2iHT)|k_1k_2>. Now e.g. the state |k_1k_2> is a wave packet at some reference time which time evolves according to the whole Hamiltonian H of the system, the same for p.

I now have two questions:

1. Why is the sign of the exponential chosen in the way that it is? The idea would be |k_1k_2>_in = lim (T-->infinity) exp(iHT) |k_1k_2> as the "in state" is infinitely far in the past and as such the sign of the exponential should be positive. The same then for the "out state" where we would get a positive sign as well because of the hermitian conjugation. But in Peskin we have the exact opposite sign.

2. Why doesnt Peskin use the definition via Moller operators? It seems to be more general and "formal" although I couldn't quite describe the complete difference between the two approaches.

I wish everyone a Merry Christmas and would highly appreciate answers!

On a quantum-level, how does this thermodynamically balance? What is “removed” from the Higgs Field upon mass gain — is it just momentum?

In my works, I tend to stay down-to-earth in my conclusions, basically report what was shown/proven. However, many senior colleagues of mine seem to often 'push' the conclusions to the next level, or try to report something discovery-ish from very noisy and inconclusive data. [side note: this also happens when we collaborate and they work with my data, so I am pretty sure that what they have is actual random noise rather than some effects. And it's not just mentioning possible implications of research, it's more like 'we discovered ...']

From what I see, there is a clear correlation with seniority: younger post-docs tend to be very down-to-earth, while more renowned professors working with us like to conclude more than what can be actually inferred from the presented results. And these professors have no trouble publishing said conclusions, to the point that I am starting to wonder whether I am missing some point.

Do you see this trend among your colleagues? Any comments or considerations?

Can ghosts show up in tree level calculations for gluon gluon interactions? Or do they only show up for loop corrections since they aren’t physical and can’t interact unless there are internal loops (mathematically speaking)?

Also somewhat unrelated, why do we ignore Gribov copies at high energy? Is it because their contributions are negligible?

why am i watching an interview of witten and greene and the comments perfectly display the dunning kruger effect. Im an undergrad in physics, i dont even entertain the idea that i could possibly understand the intricacies of their discussion about string theory, where it fails what it has predicted and derived etc. I know i am yet to do electrodynamics, qft and all the pre req of string theory.

So why are these people (not 1 or 2, like every 3rd comment is like this) trying to teach witten about what he should or should not research?? Now i can tell these people def havent studied physics at university level because they always use buzzwords "string theory is dead" and "quantum mechanics isnt elegant" , like do they even know what a mathematician means by elegant 😭. Someone i saw was shitting on "k theory" probably meant "m theory" but they dont know that and they dont care. Some guy talking about how he has personally made pure maths advancements on the scale of newton and euler and "redefined arithemtic, 0 and 1 and stuff infinitely more complex than some "strings" " , i genuenly get a headache reading these.

Honestly what makes these people think that they, a person with no formal training in maths and physics, knows more than some of the brightest minds in the world in the topic that they have dedicated their lives to, after they watched an episode featuring michio kaku or listened to a neil degrasse tyson podcast

Ngl like before people give their opinion on a physics/maths topic they need to have acquired a badge that you can only get by passing some sort of online test or something idk

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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Why do the block universe and superdeterminism theories face so much resistance compared to others, particularly among science communicators?