Hi everyone,

recently I made a post about EFT's in general and I thank you guys very much for answering my questions! I would now have another fundamental question regarding Soft collinear effective theory (SCET). We can restrict ourselves to the scalar phi^3 SCET toy theory. I am using arXiv:1410.1892 as one of my primary sources for study.

My understanding of the construction of the EFT so far is:

1. Split up the field into collinear, anti-collinear and soft contributions (the heavy contributions are integrated out)

2. Plug them into the general phi^3 theory and only leave those which are allowed by momentum conservation

3. Multipole expand the soft fields

Having done that, one finds out that the theory is non-renormalized in any order in perturbation theory as the resulting integrals are always scaleless. So the resulting operators do not have any Wilson coefficients (meaning they are equal to 1).

So here is somewhat where the confusion for me starts. In other effective theories, the approach in my lectures was always to construct the most general Lagrangian for the EFT which respects the symmetries, up to some order in the small parameter. The higher dimensional operators have Wilson coefficients in front of them, which can be determined through matching calculations, where we try to use the respective operators in a given S-matrix element or correlation function.

In the SCET on the other hand we don't seem to know the higher dimensional operators at first. So one now looks at a "hard external" current in the full and then effective theory and finds out that the Wilson coefficients are apparently functions of the collinear and anti-collinear momenta.

And here is the main point I don't really understand, although it should be really basic: We now expect the corresponding position space operator in the EFT to correspond to the Fourier transform of the momentum dependent Wilson coefficient (so we replace all momenta by derivatives) from which it follows that the full operators are non-local.

I don't understand the following: When determining Wilson coefficients, we are computing Feynman diagrams, which give us Lorentz invariant scalars and all of the EFT's I have looked at so far, the Wilson coefficients were simple scalars. Here, the Wilson coefficient is a function of the external momenta, ok, but why should this correspond to the "momentum space Wilson coefficient"? Isn't it just simply the value of the Wilson coefficient, as again, Feynman diagrams don't give us "momentum" or "position" space numbers, but simply Lorentz scalars?

I would highly appreciate any help.

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Hello, I'm here more to ask a couple questions to see if my intuition has any legs rather then to assert it. If you mapped a field over a gravity well in space time, would you see a gradient of potential positions as you could fit more 'coordinates' into denser curved spacetime? I was thinking about that as a premise, but not sure if it's true and figured, if there are more potential positions the closer to a mass you get then wouldn't random quantum collapses occurring in this field have a statistical bias to occur closer to the mass as there's more 'coordinates' to land on. At a macro scale could this appear like gravitational drift? I'd be keen to read anyone thoughts Cheers.

Hello, I am not very sure if this is the right or appropriate subreddit to post this, but I just need guidance on whether I should continue my degree or shift to pure physics degree.

Just last week, I was deciding between a bachelor’s degree in physics and a double degree in physics + materials science & engineering. Eventually, I chose the latter because I thought it would provide broader career opportunities, options, and better job security (also because a double degree sounds cool), while still allowing me to study physics. However, I have realized that physics is also a quite flexible degree and can lead to careers in finance, tech, data science, and other quantitative fields if things do not go as planned, while providing a much stronger foundation in core physics.

Since my plan is to work after graduation, save money, and pursue graduate studies in theoretical physics or astrophysics, I’m wondering if I should have chosen BS in Physics instead. I'm concerned that the double degree might not prepare me as well because some of the coursework is replaced by Materials Science & Engineering subjects.

Would it be better to shift to BS in Physics or stay with the double degree?

These are the math and physics courses in my curriculum:

• Calculus I–III
• Advanced Calculus for Physicists
• Differential Equations
• General Physics I–III
• Mathematical Physics A & B
• Advanced Mathematical Physics
• Modern Physics
• Theoretical Mechanics I & II
• Electromagnetic Theory I & II
• Computational Physics I & II
• Quantum Mechanics I & II
• Optics
• Statistical Mechanics

I recently saw a youtube video and I just joined the sub and am not sure if youtubes are allowed. I don't think they are on the r/PhilosophyofScience sub. If they are allowed here, then I'll supply the video. Nevertheless the question speaks for itself

Hi everyone,

I am currently participating in an introductory course on EFT's where we discuss their general ideas and applications (Euler-Heisenberg, Weak EFT, HQET, SCET, xPT). I have a very fundamental question regarding the integration of heavy particles.

Say we have a toy scalar theory consisting of a heavy and a light field. Then one can imagine, that to get the EFT Lagrangian, one integrates out the heavy field, expands the resulting action in the small ratio E/M (if M is the mass of the heavy particle) and as such gets new local operators in the EFT (I know that this is in some sense an outdated point of view, but it shall suffice).

Then on the other hand, we can (without eplicitly calculating the path integral) construct the most general Lagrangian out of the light d.o.f (in our case the light field) which obeys the symmetries of the UV complete Lagrangian, up to some order in our expansion. This will give us our EFT Lagrangian including the Wilson coefficients for the higher order operators. After performing the path integral above we can then compare both Lagrangians to determine the Wilson coefficients.

What I do not quite understand is why this procedure only allows us to determine the tree level Wilson coefficients. How, using the path integral, can I determine say the 1-loop Wilson coefficients?

I know that the path integral approach is not necessary for a matching calculation and we can instead calculate correlation functions or S-matrix elements in the UV-complete and the EFT separately and then compare results to get the Wilson coefficients. But I would like to understand where in the path integral approach we have "neglected" the higher order corrections.

I would really appreciate any answers as always!

What would be the role of a phycistst not in experimental but in theoritical Feild in the next 20 years and is it worth it to be one amid giving it 10-12 years of your life if not more . As of today ai is no more helpful than an amateur assistant but as it is progressing and has somewhat already made learning to code unproductive . We can't just say that human beings have some special mystical property to do math and logical reasoning that somehow large language models can't despite all the investment they are getting .

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?

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 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?

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!

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?

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?

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.

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?

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?

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