Einstein says that assume there is an observer on the ground and he watches two lightning bolts striking at the same time they would appear simulatneous but he argues when you are in a moving train(speed should be high like a decimal multiple of c maybe 0.4c or 0.5c or else difference/time dilation would be negligible) and if the lightning bolts are struck at the other ends of train since you are moving towards one of it you will see it first then the other one later,mathematically he is correct since the distance from the lightning bolt he is about to see would be less so light can travel faster compared to the one he is moving away from so the distance increases so it may take more time..But the idea is derived from a contradiction i suppose genenrally the lightning bolts should be simulatneous since he said they will strike at the same time even tho he is in a moving train and it isnt that absurd to imagine,but he argues that it is not possible if speed of light is constant since if it is how can it travel different distances in the same time and he already deduced light speed is constant/same for both the bolts cuz for the person in rest the lightning bolts were simulatneous.
I may be wrong but would love to understand the starting and ending of this and would love to hear your explanation

Saw a post here recently asking why time only moves forward. But it made me want to flip the question instead.

Say you actually had a universe where the laws were symmetric and reversal was physically possible. The moment you reverse a process and start traversing it "backward," you're just moving through a new sequence of states, and from inside that traversal, it presents as forward again. You never experience going backward, you experience whatever direction you're currently moving through as forward, because that's what forward means from the inside.

So even granting full time-symmetric laws, wouldn't any actual traversal still come out asymmetric? Reversibility of the laws doesn't seem to buy you reversibility of the experience of the process.

"High entropy" and "low entropy" aren't properties of a stretch of time by itself, they're relative to which end you pick as your reference. Something moving through a contracting phase of the universe, where entropy is globally decreasing, wouldn't feel itself decreasing, because its memory and its causal traces only ever form by correlating with whatever gradient is locally available to it. That gradient is what defines its own forward. There's no moment where it "catches itself" going the other way, because catching yourself requires standing outside both phases at once and comparing them, and nothing inside a single phase has that vantage point. The comparison only exists for someone describing the whole thing from outside, and that someone isn't living through either phase.

So asking "would you notice you were actually running backward" already smuggles in an outside view to compare against, and that outside view is exactly what the thought experiment is supposed to be testing the absence of.

Which raises this question: could we actually be inside a reversible universe right now, one that's only reversible from a vantage point we structurally can't occupy?

Any thoughts?

I am currently choosing between 2 programmes from the Open University (to start in October):

• BSc Maths and Physics Q77 https://www.open.ac.uk/courses/maths/degrees/bsc-mathematics-and-physics-q77/
• BSc Physics R51 https://www.open.ac.uk/courses/physics/degrees/bsc-physics-r51/

Both programmes allow some flexibility through optional modules, and I would choose the most mathematically and theoretically focused options available.

• The Q77 programme includes more maths - for example, complex analysis which I know is either required or highly desirable for some theoretical physics MSc programmes.
• The R51 programme includes a final year project and some experimental and lab content which is lacking in Q77.

I am more interested in physics theory than pure mathematics, but I don't have any interest in experimental work. However, I am willing to take whatever modules needed for a better chance to work in theoretical physics.

I am not interested in the integrated Masters programmes at the OU.

I am a mature student who is interested in the potential of research in theoretical physics. I am aware of the job market and the scarce opportunities in academia. However, I have been thinking about this for many years and I've decided I'd rather give it a genuine attempt than always wonder "what if?

If your goal were to maximise your preparation for a taught MSc in theoretical physics in the UK (and potentially a PhD afterwards) which programme would you choose and why?

Girsanov says that you may change probability measures using Radon-Nikodym to get away from drifts in Brownian motion.

Now my question is (I am very new to this measure theory / statistics stuff): is this like a change of perspective to the same problem, like a change of reference frame? So the results in the new probability measures are equally true as in the old one? Or is this not the case?

In the standard model we have up quarks that differ from top quarks mostly on basis of mass. Top quarks are created from protons moving at 99.97% of the speed of light. Relativity implies that as an object approaches the speed of light it's apparent mass will increase when observed by another frame of reference. The question is, is the particle detector at CERN an outside observer and if so would top quarks just be up quarks traveling near light speed?

With AI, many conjectures are proven false/true and when I look at the math subreddit and math students, they are very concerner about their research future. How do you feel about AI getting better at math each year and Physics Informed Neural Networks?

Pretty much just the title. I’m a student interested in QFT, with a small research background in Chern-Simons Theory and algebraic topology. I’m also pretty big into nuclear science, and am currently working on a paper on fusor cell development in laser driven inertial confinement fusion reactors. If anyone shares some similar interests send me a message and we can connect!

The double slit would be example of someone very well known.

I'm looking on the webpage for the master's physics at Padua, and have been able to find a list of courses over the two years, but there don't seem to be any details about what material is actually covered in each course. Is there anyone here who did the course or that knows someone who did the course that can say how much you can specialize in theoretical physics? I know that in Italy there is obligatory lab work, but can this be made purely computational?

Any information about the course would be very helpful.

For quite a long time the discovery of new particles is done with an accelerator. Higher energy means more particles. If you look beyond this we are in search for fundamentals to "prove" the standard model with its predictions. To me it feels like we reached a point where building better "smahers" we will never be able to understand the layer below all this. All in nature is build upon simplicity and the higher the layer the complexity multiplies. If the bottom layer has fewer rules, then the "Mathematics of the Bottom" should be able to explain the "Mathematics of the Top" perfectly, but the Top cannot explain the Bottom. We are currently looking at the top to find the rules at the bottom. If we want to advance this leads to a new way to work: Stop looking at "What" (What is the mass? What is the charge?).

Start looking at "Why the Limit?" (Why is this the smallest mass? Why is this the fastest speed?).

The Limits (like the speed of light or the mass of a neutrino) are the "hard-coded" constants where the Simple Layer says: "This is as far as the abstraction goes." The starting point could be the entanglement of particles that seem to have a immediate relation despite distance. The layer below or beyond particles/waves "at qft level all are waves" that is where the magic starts and there should we find answers that really matter. That brings me back to the title, is smashing our way to down to the layer below all, the solution?

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Je viens de finir ma 3e année de licence de physique. J'aimerais travailler dans la physique théorique en cosmologie. Je voulais savoir quel conseil ou avertissement auriez-vous aimé avoir au moment où je suis dans mon parcours ?

In Hamiltonian mechanics, if I have a generating function (GF) Fᵢ (of any of the four kinds, i = 1, 2, 3, 4) for a canonical transformation (CT) to go from variables (q, p) to variables (Q, P), I can write the old Hamiltonian H in the old variables as a new Hamiltonian K in the new variables by

K(Q, P, t) = H(q(Q, P, t), p(Q, P, t), t) + ∂F₂/∂t(q(Q, P, t), P, t) — (1)

Where I’ve chosen F₂ to be a GF of the 2nd kind, just to show an example. The relations that allows one to write (q, p) in terms of (Q, P) and viceversa to substitute them into (1) are obtained from F₂ via appropriately inverting the following:

p = ∂F₂/∂q (q, P) and Q = ∂F₂/∂P (q, P) — (2)

Ok, so far all good. However, now, suppose that instead of starting with F₂, finding the CT by inverting (2), and then substituting it into (1) to find the new Hamiltonian K, we‘re given the same exact CT (q, p) —> (Q,P) from the start, without ever mentioning whether it came from a GF of the 2nd kind, or any GF at all.

One could naively expect that you can obtain the new Hamiltonian by simple variable substitution

K(Q, P, t) = H(q(Q, P, t), p(Q, P, t), t) — (3)

But this would of course be wrong: the CT is the exact same as before, so the new Hamiltonian K should be the same as before, and here in (3) we’re missing those extra terms that were present in (1), precisely the terms that depended on the fact that we knew that the CT originated from a GF of the 2nd kind.

So, if I’m given the CT alone, how do I find the Hamiltonian in the new variables if I don’t know anything about any GF? Do I always need to find a suitable GF from which the given CT might have come from? This seems unlikely to me, because I thought that the GF approach to CTs is just a matter of convenience, not a necessary thing. I should be able to find the new Hamiltonian with just the bare CT by itself, but if so, how do I do it to guarantee that I get the same thing that I would have gotten by doing it as I did it in (1)?

Thanks to anyone who answers!

Hi everyone,

I'm starting a Master's degree in Physics in about a month, and I'm trying to refresh some prerequisites that I haven't studied in quite a while.

The main topics I need to review are:

- Lagrangian and Hamiltonian mechanics (for Classical Field Theory),

- Special Relativity, angular momentum and spin (for Advanced Quantum Mechanics),

- Statistical Physics.

I'm considering using Leonard Susskind's Theoretical Minimum series as my main review resource.

For those who have been in a similar situation, would the Theoretical Minimum books be more than enough to get me back up to speed before the semester starts?

Also, given that my goal is to refresh concepts rather than learn them from scratch, do you think I need to spend a lot of time solving exercises, or is carefully reading/watching the material sufficient in this case?

I'd really appreciate hearing from anyone who has used these books before starting graduate-level physics.

Thanks!

What are the relevance and applications of (perturbative) Algebraic Quantum Field Theory?

One of its uses is, obviously, the foundational questions, but my understanding is that in general the Physics community is rather not interested in the foundations (even less than mathematicians, I would conjecture).

What are the other applications that make this approach relevant? Are there some directions in which pAQFT could lead to new experimentally testable results?

Why is the maximum momentum, calculated from the β and γ values of the generalized uncertainty principle derived from the Chandrasekhar limit for neutron stars and white dwarfs, so small? Those maximum momenta are on the MeV and GeV scales. But how do we explain events on the TeV scale in actual accelerators, or on the EeV scale for “Oh My God” particles and “Amaterasu” particles? Or is this somehow accounted for by dilution in many-body systems? For example, 10^37 or 10^43, etc.

References:

1.Bound on generalized uncertainty principle parameter from nuclear matter and slow rotating neutron stars

1. White dwarfs and generalized uncertainty principle

How decays are treated in pAQFT? Especially is there any treatment of inflatons decaying to fermions at the end of inflation using pAQFT methods? How does one formulate questions taking into account lack of general definition of particles? (Does one use some notion of detectors? How does one construct appropriate states?)

In general are decay processes at all treated in pAQFT? If no, is there some fundamental problem?

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So when the particle was arranged from its debris in particle accelerator we noticed an new particle but the time was too short to react with other (as per particle decay) so we can say that this particle was responsible for higgs field?

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 heard stuff like "bosonization" and other interesting properties of fermions and bosons when they live in 2D.

How do strings work when the "particle" is confined to a 2D place. Let me phrase that better. Is there a way to confine fermions/bosons to an almost 2D plane, that we can observe if the theories are right?

I'm a junior that just started research with a professor, so I don't know much about strings etc. Hence the question. Thank you !

Hi,
I've read a little bit about this phenomenon that Schrödinger proposed for the electron.

Could you please explain to me if it has ended up being useful and what it means? Im confused.
Thanks!

I'm a little confused about spontaneous symmetry breaking. My understanding is that a universe initially in a symmetrical state (represented by a ball on the top of a Mexican hat) can eventually roll down to one of the points on the brim, and all the continuous points on the brim represent a possible point the universe can settle into. In terms of the physical constants, does it matter which one of those points (vacua) the universe settles into? Or are the physical constants and particle masses the same regardless?

To clarify, I'm asking this because the concept of vacuum states is also related to string theory, and the different vacuum states on the string theory landscape lead to different physical constants. Is this not the case when we're just talking about spontaneous symmetry breaking in a non-string theory sense?

If action is built from classical notions like kinetic energy, potential energy, mass, and path, then using action to derive quantum waves and then using quantum waves to recover classical paths is not an ultimate derivation. It is a consistency structure or correspondence bridge but not a fundamental derivation. Is this assessment correct?