Search results

Fμν = ∂μAν- ∂νAμ ∂μFμν = ∂2μAν - ∂ν(∂μAμ) = ∂2μAν Why ∂ν(∂μAμ) and not ∂μ∂νAμ ? And why does ∂ν(∂μAμ) drop out? thank you

Why do I perceive "now" as a special moment compared to all the other moments in my life, past and present? Which physics equation, concept, theory accounts for one of the most human conscious experience, the perception that time flows, that the past becomes the present, that the present becomes...

For some strange reason I have not found this site before. Looks perfect! Thanks PAllen!

That looks very good and might be just what I wanted. I will check it out at my library on Monday! Thanks George!

Thanks so far for your answers. I am still interested in thoughts about the nature of time from physicists that go beyond the pure operational approach. Again: Does anybody know of papers, talks or chapter of books where trained physicists speculate about what time is (or might be) besides...

Repeating? You mean, repeating in time?

This I knew, too. But I wanted to know what a clock is. Can you explain it without time?

What is a clock? Can you define it without the concept of time?

There are quite some pop-sci books (by Greene, Smolin, Carroll and others) that deal with the "nature of time". Why does time appear to flow? Why is there a special moment, the "now"? Does simultaneity in SR imply a block universe? Why time-symmetric laws but a time-unsymmetric universe? Does...

Fantastic! Thanks Samalkhaiat! Kronecker Deltas from derivatives and indices swapping in the Levi-Civita. Got it. I must study a little tensor calculus..

Orodruin, I really appreciate your time and effort. I read carefully all your posts in this thread and the link you gave. But unfortunately, I still can not answer my initial question. Maybe I try somewhere else. Thank you!

L = aμ∂νaλ ∂L/∂aμ - ∂ν (∂L/∂(∂νaμ)) = ∂vaμ - ∂v ? I do not know what and how to differentiate in the second term. Also, I need to add two identical terms to get the factor two. But there is a minus sign.

I differentiate γεμνλaμ∂νaλ w.r.t. aμ and I get γεμνλ∂νaλ and γεμνλaμ∂νaλ w.r.t. ∂νaλ which gives γεμνλaμ. Thus γεμνλ∂νaλ - γεμνλaμ = 0 , which is not 2γεμνλ∂νaμ = 0.

I get the equation but without the 2 in front. I do not see how the 2 comes about. How to sum over the indices. I find the indices confusing. Hence my question.

Right. And a derivative in front of one a. Do I get one term from the RHS and one from the LHS of equation of motion and then I add them together?

The Lagrangian (Maxwell Chern-Simons in Zee QFT Nutshell, p.318) has as equation of motion: Where does the 2 in front come from? Thank you very much

In this Nima Arkani-Hamed paper on page 5 I found the sentence: These constraints are an artifact of using fields as auxiliary objects to describe the interactions of the more fundamental particles. In Schwartz's QFT book I also get away with the impression that the Poincaré irreps (i.e...

There does not happen to be some software for such kind of calculations? Also, is it easier to transform the triangle matrix back into the original form than the other way around?

That sounds good. But that way I will not get the top row given in the transformed matrix. Because the row simply does not appear anywhere in the original matrix, so I can not swap it to the top!

Thanks for the replies. But still not quite clear. As I understand multiplying is useless in this case. I can just add rows to get zeroes. To be concrete , what about the tenth line in the transformed matrix (0000 0000 0011 0011 0011 0011), which rows do I have to add to get this transformed...

I want to transform the first matrix below into the second one. The book ( Neutsch, "Coordinates") says this can be done by elementary transformation. I guess he means by some Gaussian elimination. But the entries of the matrix are from the finite field 2, so I can not multiply rows, that would...

In the Standard Model fermions interact via exchanges of massless (virtual) spin-1 particles. Fermions are turned into a boson. How is that different from the SUSY transformation that turns fermions into bosons?

So a light signal is sent off some space behind me. At the same time I start accelerating extremely quickly. Even though the light signal will always be faster than me it will never catch up with me. I have difficulties to understand that something that is always faster than you can still never...

Where does the 1 in the last line come from? Thank you!

Nice overall interview. But this must probably the dumbest thing she ever said: I have never heard of the fact that “physical models are commonly regarded as beautiful and in a sense minimal” and even if that was so I don’t know why it would matter. Yes, quite possibly it’s a pretty bad idea to...

Some last words before this thread disappears into oblivion. It is always amazing to see how much physics and deep insight someone can gain from dimensional analysis!

That makes sense. But when we compare Planck mass and Planck time with other mass and time scales. Like described here

Perhaps, one final question. What physical meaningful observation/ conclusion can we make about the Planck units? They are special units after all, ratios of three (dimensional) constants. The fact that the Planck mass is many orders higher than Planck time and length, would be one, as I read...

That's good stuff. What got me confused, among other things, was the Bronstein cube, that also appears in the opening chapter of Zee's "Einstein gravity", which makes you think that by increasing c, h or G gets you deeper in the domain of the respective theories.

I'm convinced now. If I choose Planck units for measuring, it becomes super-obvious that whenever I measure a different c,h,G my Planck units change as well. Thanks everybody!

If light would reach the moon in two minutes instead of one second, my units would change? How?

Not well-defined? Does light reach the moon in one second or in two minutes? I still fail to see why this is a not well-defined question.

I do not want to redefine anything. I want to measure the speed of light and I want to use the metres and seconds as commonly defined. Are you saying if the speed of light turns out be 792458 m / s or 458 m / s instead of 299792458 m / s we would not notice any changes?

Suppose the speed of light were only a tenth of what it is and Planck's constant were 100-fold bigger. Would not that mean that relativistic effects and quantum effects were bigger? Does not c, h, G tell us how relativistic, quantum-physically and spacetime-curved the universe is?

There are 25 or so dimensionless constants in the standard models, such as the masses of the fundamental particles (that can be divided by Planck mass or some other mass to become dimensionless). And there are the three dimensionful constants c, h, G (speed of light, Planck's constant, Newton...

In holographic entanglement entropy notes like here, they let alpha go to one in (2.41) and get (2.42). But (2.41) goes towards infinity, when doing that! Can someone explain how alpha --> 1 will make (2.41) into (2.42)? Thank you!

https://perimeterinstitute.ca/news/new-face-feynman-diagrams/deeper-dive-shell-and-shell that if you add two momentum vectors of two on-shell particles, you get an off-shell particle. Two questions: 1. Since on-shell solutions are solutions to the free equation of motion, should no adding two...

Below is a snipet from http://file:///C:/Users/Christian.Hollersen/Downloads/Britto_2011_2%20(1).pdf [Broken] of Britto. Similar explanation can be found in the QFT books of Zee, Schwarz or the Scattering Amplitude text of Huang. Or any other text that covers BCFW recursion. My dumb question...

The EM wave and the photon have two degrees of freedom. Their polarization directions and spin states, respectively. But they move in space, too. I mean light has the freedom to go in all directions in space. Like a macroscopic ball in 3-D space, which can go all three directions, if there are...

Hi Silviu, gauge symmetry is arguably the central tenet of particle physics and qft. But in many older textbooks its physical meaning is only badly explained. Check out Schwartz QFT textbook chapter 8 for a deep yet readable explanation. One of the best layment yet precise explanation of it can...

I found in Thompson "From Error-Correcting to Sphere Packing and Simple Groups" this on page 131 How do you compute m/n in a finite field? Take the equivalence class 5 given above. Why does 2/5 and 18/22 give 5? thanks

From Schwartz http://isites.harvard.edu/fs/docs/icb.topic521209.files/QFT-Schwartz.pdf p. 257 or his qft book p. 455 1. Why and how does the integral in (24.24) go imaginary, when M > 2m? Is it because the logarithms can not take negative real numbers, thus we have to switch to complex...

OR is it simply because the Compton wavelength is much smaller for protons than for electrons?

Thanks! And why is that so? Is there a simple physical argument behind this (curious) effect that photons scatter off particle with less mass with higher probability than they do with more massive particles? My guess is that in Compton scattering the virtual particle in the Feynman graph is...

I read many times that the early universe was opaque foremost because of the scattering of photons off free electrons (Thomson scattering). Why is the scattering off free protons not equally important? Btw, the same they say about stars. Photons within stars need a very long time to get out of...

How are those two geometries realeted? Conformal geometry is a metric geometry. Projective geometry is not. But the stereographic projection is related to the conformal geometry. Or does someone know a book/ notes where the individual geometries (affine, projective, euclidean, hyperbolic...

Can someone recommend a good chemistry overview/ review? Some pdf document perhaps, not more than 200 pages or so. Notes that do not assume you are complete beginner, but you have a science (physics) background. That gives brief summaries and the highlights of all the topics that are dealt with...

Here http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf, I find on page 31 in (2.1.5) I assume that it is childish calculus that connects both sides of this equation. But still, can someone help me why the integral can be rewritten like that? thanks!

Got it! Thanks DrDu and mfb!

But on what else could the partial derivative act on than on A?