apt45

Any idea about this question math.stackexchange.com/questions/3814630/… ?

@BalarkaSen ok thanks!

Hi all! Is U(N) a subgroup of the compact version of the symplectic group, USp(N)?

sure, I will eventually consider asking on SE.

Is my question unclear?

Does this blueshift occur? Is there a time after which I cannot describe the dynamics of the wave with my EFT?

For example, assume that you are sending a wave with high enough frequency $\omega \lesssim M$, where $M$ is the particle that you have integrated out. In flat space, at $\omega \sim M$, the EFT breaks. In the contracting universe, after a finite period the wavelength decreases (thus the frequency/energy increases) and could reach the value $\omega = M$.

Imagine an EFT evaluated on a gravitational background which is contracting (for example, FRW with a cosmological constant). In this universe lengths contract, so waves would experience a blueshift as the time flow. Does this mean that the EFT approximation breaks down after a finite period from the emission time?

In flat space, EFTs can be written as a derivative expansion of the low-energy fields, and this expansion breaks when the energy of particles in the EFT is roughly the mass of the heaviest particle that has been integrated out.

I don't understand something regarding Effective Field Theories in expanding/contracting universe. I

So let's put aside the tachyons. Let's talk about the diagram I posted

We don't even know if tachyon do exist 😂

A and C cannot communicate instantaneously, so how do you know that causality is violated?

Immagine the message from A to C is so powerful to kill the observer in C. Localilly, this is perfectly fine to me. The observer C does not know that his/her future counterpart will kill him/her. Who can establish that the signal came from the future?

And not some other particle? What is the special property of tachyon you are using?

Why tachyons?

i wonder whether there is a local definition of causality.

For example, gamma rays detected with some delay after black hole merging could also have emitted before the merging, as far as we know from the Earth

In the diagram I showed you, the two observers in A and C cannot communicate instantaneously, therefore they cannot agree on who sent the signal

Indeed, that's the point

You could classify the radio signal of informations about future events as a fake news. I would like something more rigorous

How can the observer C know that the message comes from the future and not from some other observer in its past?

Why is this configuration inconsistent with causality?

This is the diagram of an observer sitting in the space point A sending a superluminal message.

I don’t like papers/books written in Word :P

That’s also true

I am sorry, but I think that the authors should not decide what the reader should know or not

I don’t see any way writing “it is well known” could be useful

So, if anyone states a result and then write that it is well known, then it means that the read could have followed the discussion in the text without referring explicitly to this result

@ACuriousMind the point is that You wouldn’t spend time and words to write something that is a triviality. We don’t write that a ball is a sphere, unless you are talking about rugby balls :))

@Charlie yes, that’s the same for me. I am trying to ban from my drafts/papers words like straightforward, easy, simple, well known etc...

@Charlie ah sure, I am referring to two separate sentences, sorry :)

Where is the typo? Ahaha

@Charlie the fact they assume I know what they are referencing.

Am I the only one feeling uncomfortable when I read sentences as “the well known result, as it is well known...” in physics notes/papers?

yeah, sure

otherwise, it does not make sense, right?

yeah ok, but you are always assuming that your integral on the real p-axis converges

The integrand grows exponentially for negative imaginary part of p

I am not sure about the integral in the lower half plane

@Slereah very nice slides! Thank you

the integral on the two red curves is the same?

but these are all closed contours

@ACuriousMind Is the "contour trick" unique? I mean, there could be any other choice of the integration path that makes the integral convergent but with a different result?

@ACuriousMind Thanks. The fact the numerical integral was oscillating lead me to think that the contour trick was a prescription

I am also happy to pay for a service, but I don't want a person to proof-write my draft. I was wondering whether there is a service AI-based, for example, which allows you to do it on your own. I am already using Grammarly, but it is not enough.

@Cerberus kind of, yes. But I don't want to pay someone else to do it, because I want to learn

Hi all! My English vocabulary lacks rare words and constructs. I want to improve it. For example, suppose I want to substitute the sentence "We follow the same spirit of the general treatment discussed in Ref.1" with an esthetically better sentence. Is there an online service that suggests similar sentences from which I can get inspiration?