Quantum Field Theory 3c - Photons III

[@ARBB1]

I just discovered this channel, and I need to say: Brilliant. The explanations being clear is already enough in a topic like this, but the fact you go through the effort of making it aesthetically pleasing, even including animated graphics, is a display of great care and respect for education. Massive props to you not just using a regular whiteboard as well.

[@viascience]

Thank you for the kind words.

[@streeturchin1683]

Great job! Congrats! It's really clear and the explanations take different ways than many standard books or classicial courses (repeating the same all the time..)

[@kalbismus]

1000 fold speed learning than from books. the first time i get a feeling for this material, in just a good hour, over "endless" hours looking into books

[@_BhagavadGita]

Thank you so much. You are such an excellent teacher.

[@viascience]

You are welcome.

[@mausamkhetani8661]

Dude this is gold , brilliant job

[@viascience]

Thanks.

[@Richardj410]

Don't know how I missed this one. Thank you again and again.

[@viascience]

You're most welcome.

[@awsomo594]

Just Wondering when can we expect another episode of the General Relativity series? Really interested to see how (◻)^2 gμν =0, since you only went over the weak field approx. Is there a difference and how do you solve it? Great series btw :D

[@gtarkanyi_dr]

we've learned as the "expectation value"

[@viascience]

Yes, in quantum mechanics the expected value of the measurement of an operator is typically called the operator's "expectation value." I should probably have used that more common terminology.

[@boffo25]

Nice episode. What was the intuition for $\hat{a}^{-1}|\Psi>=c|\Psi>$ ?

[@viascience]

We will see in the next installment, by working backwards, it's required if the expected value of a measurement of the field is to match the classical field value.

[@boffo25]

@Stijn Boshoven Thank You

[@TenzinLundrup]

Question about the state |psi>. It threw me when you introduced it at 2:17 because I wanted a wavefunction of the dynamical degrees of freedom q_k (and p_k?). I want a wavefunction psi_k(q_k) such that |psi_k(q_k)|^2 gives me the probability density of mode k having amplitude q_k. Why don't we introduce such a wavefunction? It seems as if you don't care about the field amplitude but only care about how many quanta it has. I have some kind of mental block I need to resolve. It is unclear to me what the "observable" is for an EM field? Isn't it the amplitude of the field?

[@monikalala3810]

Thank you for your great work!

[@viascience]

You are welcome.

[@streeturchin1683]

Dear sir, i have a question: How to put correctly c,h, m (normal units) with these results? What is the real expression of the final deviation (including c,h,m)? Thanks a lot in advance for your help..!

[@viascience]

I would recommended referring to a formal text after watching these videos. My goal is to try to simplify the notation and presentation as much as possible. That means I am not always rigorous with things like constants and units. The following is a great free text resource: www.damtp.cam.ac.uk/user/tong/qft.html

[@streeturchin1683]

@@viascience Oh Thanks a lot! That's great!

[@tessacortes9226]

Thanks!

[@viascience]

Welcome!

[@2tehnik]

is it just me or do the subtitles go crazy at the end (quickly going through text he isn't saying)?

[@viascience]

Good catch. I think it is fixed. Not sure where that extra text at the end came from.

[@yigal_s]

4:57 it's not because of the incoherence of the photons. The result is te same even for a one photon.

[@benjaminkaufmann2482]

The series is really good! But in this episode I see a mistake. The description of incoherent light is incorrect. Incoherent does not mean that the average field is zero.

[@viascience]

For incoherent light the expected value of the field is zero. This is true in both the quantum description and the classical description. If the field is cos(omega*t+phi) with phi random, then the expected value = 0 at any time t.

[@benjaminkaufmann2482]

@@viascience I reffered to 5:27 where it's said: "At a given time in place the total field being the sum of a large number of random contributions have an expected value of zero. That's the nature of incoherent light." This is not true. At 5:53 it's said about coherent light: "At a given time in place the total field ideally will be the sum of n identical contributions and have a defined expected value." Yes, but this is also true for incoherent light! In your comment you write "the expected value = 0 at any time t". Now you mix up the average in time and the average at a certain point in space. The average in time is zero for coherent and incoherent light, but the average of n coherent contributions at a certain point in space is not necessarily zero in classical physics - neither for coherent nor for incoherent light. BUT in quantum mechanics, for the single photon field, the expectation value of the field at a certain point in space is always zero! This is completely different from the classical field theory.

[@benjaminkaufmann2482]

@@viascience I reffered to 5:27 where it's said: "At a given time in place the total field being the sum of a large number of random contributions have an expected value of zero. That's the nature of incoherent light." This is not true. At 5:53 it's said about coherent light: "At a given time in place the total field ideally will be the sum of n identical contributions and have a defined expected value." Yes, but this is also true for incoherent light! In your comment you write "the expected value = 0 at any time t". Now you mix up the average in time and the average at a certain point in space. The average in time is zero for coherent and incoherent light, but the average of n coherent contributions at a certain point in space is not necessarily zero in classical physics - neither for coherent nor for incoherent light. BUT in quantum mechanics, for the single photon field, the expectation value of the field at a certain point in space is always zero! This is completely different from the classical field theory.

[@hafizajiaziz8773]

First comment ☝