Deriving The Feynman Path Integral Part 2

Today I finish the derivation of the Feynman path integral for quantum mechanics.

Like my hat? Get it at:

Derivation Part 1:

Baker Cambell Hausdorff derivation:

BCH properties and Justifying Neglected Terms:

Info on probability measures:

An axiomatic approach to the euclidian path integral:

Other nice discussions on the measure:

[bakirhusremovic]

Me: *just finished freshmen year of high school*
Me:*watches video*
Me: Why is he writing in minecraft enchantment table?

[seandimmock]

Incredible video! You will not find physics like this so well explained any where else on the internet. Phenomenal. Keep doing what your doing, please.

[JoseTheWrangler]

Terrific. Have read a dozen or so derivations of the path integral, but this is by far the clearest and most step-by-step.

[danielcheung]

I don't even study physics but here i am

[blurb]

I barely understand this & I probably misunderstand everything I *think* I halfway-understand.

This is a great video, but also very humbling to me.

Can’t wait ‘till I get all the tools for this & suddenly understand “ohhhh, so THAT meant THAT”

[flyyufelix]

Thanks. Crystal clear in your derivation. Good job! I actually find it easier to understand a complex concept through vigorous math than through some sort of qualitative analogy. As physics get more advanced it becomes harder and harder to explain difficult concepts via simple analogy.

[Aaron-zihw]

i swear to god i love you, hope my gauge theory teacher were u

[SuperPBrady]

0:13 “but we’re all gonna be okay” first of all, don’t lie to me Andrew

[clth]

thank you for the great video! I think I spotted a small mistake at 31:40. I think there was an extra "N" being added to the numerator when you substitute delta-tau with i-delta-t. I guess you were going to substitute delta-tau with (i T / N) and then substitute in i-delta-t.

[jackhudson]

Andrew: "It's like a bajillion degrees in New Mexico"
Also Andrew: *wears a flannel*

[ratman]

Ahh good shit Andrew.
I would be so down for a video on measures or actually solving the integral for the harmonic oscilator or something.

[dangernuke]

An absolute marvel of a video duo! The path integral has ALWAYS been a mystery to me quantitatively. I had no issues conceptually, but this brought it all together. Many thanks! However, as much as I hate to say it, but also stoked that I found it, there is a mistake... Around the 32 minute mark, when you're substituting delta-t for delta-tau, it's a direct substitution. There is no N in the numerator of the Gaussian measure.

[varunvenkat]

Amazing video!!! Absolutely loved it Andrew! Great job! It would be amazing if you could do a part-3 of this talking about different measures and also how this leads us to the partition function, correlation functions etc.

[vladkovalchuk]

Hi Andrew,


I have a question regarding the factor raised to N/2 power, you say at 25:46 that it is making the integral converge, but that seems to be making it explode since you have a growth like N^N. Great work on the video!

[dikshantdulal]

Very good job Andrew! I started to read path integral on Altand's QFT textbook. Didn't make sense, so I came here. Now I can read it properly this time.

[joshuapasa]

I've been waiting for this for so long, 38 mins of Feynman. :)

[IonKattTMCE]

So what career can I do to avoid this completely

[michaelmyers]

Idk why I’m here. Just had my tonsil removed and this was somewhat entertaining 😄

[clem]

Thank you very much ! Great demonstration ; it makes me remember with happiness about my studies in physics !

[mihirsingh]

Damn man. Couldn't understand Path Integrals but this video saved me. This made my day, my week, my month. 🔥