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https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
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2020-04-12T15:32:41Z
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x6YWpe9xHMg-t0.0
|
x6YWpe9xHMg
|
Hello everyone and welcome to the biggest ideas in the universe. This is one of our Q&A sessions.
| 0
| 6.24
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
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x6YWpe9xHMg-t6.24
|
x6YWpe9xHMg
|
I'm your host Sean Carroll and today we're going to be doing the Q&A associated with idea number three,
| 6.24
| 12.04
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t12.040000000000001
|
x6YWpe9xHMg
|
force, energy, and action. So as I go through these Q&A videos, I am moving away more and more
| 12.04
| 19.56
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t19.56
|
x6YWpe9xHMg
|
from the idea of specifically going down question by question and more and more toward a theory of
| 19.56
| 24.72
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t24.72
|
x6YWpe9xHMg
|
using the questions to inspire me to elaborate on things that maybe I glossed over or simply forgot
| 24.72
| 32.2
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https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t32.2
|
x6YWpe9xHMg
|
to talk about when I did the original video. This is the downside of having a very casual,
| 32.2
| 36.76
|
https://youtu.be/x6YWpe9xHMg
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The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t36.76
|
x6YWpe9xHMg
|
non-polished, non-rehearsed approach is that sometimes there are things I wanted to say and
| 36.76
| 41.4
|
https://youtu.be/x6YWpe9xHMg
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The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t41.4
|
x6YWpe9xHMg
|
I just forgot to say them. Also in this Q&A I'm going to use the opportunity a few people have asked
| 41.4
| 46.56
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https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t46.56
|
x6YWpe9xHMg
|
how I actually make the videos. So I will be at the end of this Q&A talking a little bit about the
| 46.56
| 52.48
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t52.48
|
x6YWpe9xHMg
|
setup and the technology that goes in to actually making the videos. If you want to stick around for
| 52.48
| 56.44
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t56.44
|
x6YWpe9xHMg
|
that, I thought I was sticking at the end because many of you couldn't care less. So clean up. In
| 56.44
| 61.92
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t61.919999999999995
|
x6YWpe9xHMg
|
other words, what are the things that I didn't talk about in the actual video that I want to talk
| 61.92
| 66.96
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t66.96
|
x6YWpe9xHMg
|
about? The most important thing is nothing very profound that I have to say, but remember we were
| 66.96
| 72.4
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t72.4
|
x6YWpe9xHMg
|
talking about the Newtonian versus Laplacian paradigms, right? So here is a particle at some point X.
| 72.4
| 79.8
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t79.8
|
x6YWpe9xHMg
|
This is X0 equals 0. That was another question that was healthfully answered. 0 is just a way of
| 79.8
| 91.24
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t91.24
|
x6YWpe9xHMg
|
saying 0 and T0. Then at some other time T1, the particle is located at X1. In the least action
| 91.24
| 102.84
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t102.84
|
x6YWpe9xHMg
|
way of doing things, we say of all the paths you could traverse between X0 and X1 and between time
| 102.84
| 109.48
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t109.48
|
x6YWpe9xHMg
|
T0 and T1, what is the one that minimizes this quantity called the action? And all I wanted to do
| 109.48
| 116.28
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t116.28
|
x6YWpe9xHMg
|
was to sort of highlight the question, how does it know? This is the big issue with the miraculous
| 116.28
| 125.88
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t125.88
|
x6YWpe9xHMg
|
feeling that we have about the principle of least action. So we're saying that I don't tell you the
| 125.88
| 133.16
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t133.16
|
x6YWpe9xHMg
|
initial velocity of the particle. I don't tell you the final velocity. I just tell you where it starts
| 133.16
| 137.4
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t137.4
|
x6YWpe9xHMg
|
and when it ends. I tell you when it starts and when it ends. And somehow the procedure for figuring
| 137.4
| 145.08
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t145.07999999999998
|
x6YWpe9xHMg
|
out all of the different ways of getting from here to there is that had I gone on another path
| 145.08
| 152.12
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t152.12
|
x6YWpe9xHMg
|
that is not the actual physical one. The action, this quantity that we defined, action is an integral,
| 152.12
| 163.16
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t163.16
|
x6YWpe9xHMg
|
if that's okay, the sum over all the different moments of time of kinetic minus potential energies.
| 163.16
| 169.08
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t173.0
|
x6YWpe9xHMg
|
Over time, okay. So, and then you minimize the action, you get the least action to get the actual
| 173
| 179.32
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t179.32
|
x6YWpe9xHMg
|
path. It seems like precognition is going on here, right? It seems like the particle leaves at point
| 179.32
| 187.88
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t187.88
|
x6YWpe9xHMg
|
X0 at time T0 and it immediately has a velocity, right? Like there's some velocity that the particle
| 187.88
| 194.28
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t194.28
|
x6YWpe9xHMg
|
actually has right at that moment and following that velocity in the Newtonian paradigm, we would
| 194.28
| 199.96
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t199.95999999999998
|
x6YWpe9xHMg
|
say at every moment we chug forward using f equals m a and we get to the final point. But in this
| 199.96
| 205.24
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t205.24
|
x6YWpe9xHMg
|
picture, we say, well, it could have taken other paths. It could have gone on other velocities and
| 205.24
| 210.2
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t210.20000000000002
|
x6YWpe9xHMg
|
done other things. But then the this integrated quantity called the action would have been higher.
| 210.2
| 216.36
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t216.36
|
x6YWpe9xHMg
|
So it didn't take those. It took the one with the least action, even though the action is this
| 216.36
| 220.92
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t220.92000000000002
|
x6YWpe9xHMg
|
non local quantity, this quantity that can only be evaluated by already knowing what the path is,
| 220.92
| 227.48
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t227.48
|
x6YWpe9xHMg
|
right? How did it know? What to do? So I don't want to, there's no answer to this in a simple way.
| 227.48
| 235.16
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t235.16
|
x6YWpe9xHMg
|
It's just to point out that the question is badly phrased. You know, it didn't know. The particle
| 235.16
| 241.08
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t241.07999999999998
|
x6YWpe9xHMg
|
didn't guess. The particle doesn't think about every single path ahead of time and then choose
| 241.08
| 247.24
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t247.23999999999998
|
x6YWpe9xHMg
|
to go on the path of least action. There is a way the particle moves and that way can be specified
| 247.24
| 254.36
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t254.36
|
x6YWpe9xHMg
|
by saying of all the paths, it is the one with the least action. So one way of thinking about this
| 254.36
| 259.56
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t259.56
|
x6YWpe9xHMg
|
is, you know, I said in words that minimizing the action gives us the same path as the Newtonian
| 259.56
| 268.04
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t268.04
|
x6YWpe9xHMg
|
way of doing things starting with f equals m a, but it's more specific than that. So let me talk
| 268.04
| 273
|
https://youtu.be/x6YWpe9xHMg
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The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
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x6YWpe9xHMg-t273.0
|
x6YWpe9xHMg
|
about minimizing a path. What does it mean to minimize something? Minimize the action.
| 273
| 278.84
|
https://youtu.be/x6YWpe9xHMg
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The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
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x6YWpe9xHMg-t278.84
|
x6YWpe9xHMg
|
Minimize in math terms, how do we find the minimum of any function? Okay, so let's imagine
| 278.84
| 287.88
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https://youtu.be/x6YWpe9xHMg
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The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t287.88
|
x6YWpe9xHMg
|
the much simpler function f of x as a function of x. So this is not x, the position is just x
| 287.88
| 295
|
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The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t295.0
|
x6YWpe9xHMg
|
sum variable, we don't know what it is. And someone has a function that does something like this.
| 295
| 298.84
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t299.55999999999995
|
x6YWpe9xHMg
|
How do we know where the minimum of that function is? Well, you can see it. There it is. You and I
| 299.56
| 304.92
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t304.92
|
x6YWpe9xHMg
|
can see it just by looking, but math can't see it or a computer can't see it. How would we
| 304.92
| 310.84
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t310.84000000000003
|
x6YWpe9xHMg
|
mathematically figure out where the minimum is? Again, I should have said this a long time ago,
| 310.84
| 315.56
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t315.56
|
x6YWpe9xHMg
|
but the point is we know that the slope of that curve is the derivative, the change in f as we
| 315.56
| 324.68
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
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x6YWpe9xHMg-t324.68
|
x6YWpe9xHMg
|
change x. And the point about the minimum is that at the minimum, the slope d f dx equals zero. That's
| 324.68
| 335.08
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t335.08
|
x6YWpe9xHMg
|
the mathematical way of saying where the minimum is. The minimum is the point, if it's a minimum,
| 335.08
| 341.32
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t341.32
|
x6YWpe9xHMg
|
then at the minimum, the slope is zero. So the derivative is zero. So to solve for the minimum
| 341.32
| 346.76
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t346.76
|
x6YWpe9xHMg
|
mathematically, you take the function f, you differentiate it, you find its derivative, you set
| 346.76
| 352.52
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
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x6YWpe9xHMg-t352.52
|
x6YWpe9xHMg
|
the derivative equal to zero, and then you solve that equation. That's an equation, some expression
| 352.52
| 356.84
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
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x6YWpe9xHMg-t356.84
|
x6YWpe9xHMg
|
in terms of x, the whole thing equals zero, you solve it, you find the minimum. So that is what
| 356.84
| 362.52
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t362.52
|
x6YWpe9xHMg
|
you do up here, of course, for the least action principle, you take the action, but the action
| 362.52
| 368.04
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t368.03999999999996
|
x6YWpe9xHMg
|
is not a function of one variable, like x. It's a function of all the different paths you could
| 368.04
| 372.68
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t372.68
|
x6YWpe9xHMg
|
go on, right? Here's another path you could have gone on. So you have to do a much more complicated
| 372.68
| 379.16
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t379.16
|
x6YWpe9xHMg
|
integral. This is the subject called the calculus of variations, the idea that you have an infinite
| 379.16
| 385.08
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t385.08000000000004
|
x6YWpe9xHMg
|
dimensional space, the space of all possible paths. But nevertheless, you can find one path and then
| 385.08
| 390.76
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t390.76000000000005
|
x6YWpe9xHMg
|
take the derivative of some quantity, like the action, as you change the path by a little bit. In
| 390.76
| 396.28
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t396.28000000000003
|
x6YWpe9xHMg
|
fact, the mathematically rigorous way to define it is to basically turn that infinite dimensional
| 396.28
| 401.16
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t401.16
|
x6YWpe9xHMg
|
thing into a finite dimensional thing, where you just change the path in one direction at a time,
| 401.16
| 406.68
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t406.68
|
x6YWpe9xHMg
|
and set that equal to zero everywhere. So you can do this little bit of calculus, this little bit
| 406.68
| 412.2
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t412.2
|
x6YWpe9xHMg
|
right here. Look at this, I'm learning to change colors. So this little bit of calculus can be done
| 412.2
| 418.12
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t418.12
|
x6YWpe9xHMg
|
on the space of all paths, not just on individual curves. So that's how you find the path of least
| 418.12
| 424.76
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t424.76
|
x6YWpe9xHMg
|
action. But then the way that we do this, how do we, there is a cute mathematical way of
| 424.76
| 430.2
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t431.64
|
x6YWpe9xHMg
|
letting you know that we're differentiating with respect to an infinite dimensional thing,
| 431.64
| 435.56
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t435.56
|
x6YWpe9xHMg
|
rather than a finite dimensional thing, which is that rather than using the letter D for derivative,
| 435.56
| 440.44
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t440.44
|
x6YWpe9xHMg
|
we use the Greek letter delta, the lowercase delta. So the functional derivative, as we call it,
| 440.44
| 446.92
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t452.04
|
x6YWpe9xHMg
|
of the action is written d s s is the action. I don't know if I said that. I don't know again,
| 452.04
| 460.04
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t460.04
|
x6YWpe9xHMg
|
why is the letter S used? There's probably some reason lost to history, but d s and I'm not going
| 460.04
| 466.28
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t466.28000000000003
|
x6YWpe9xHMg
|
to delta s rather, delta path. So for path, I was going to write the word path, but that's a little
| 466.28
| 472.84
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t472.84000000000003
|
x6YWpe9xHMg
|
bit too casual even for me. Let me write x of t. Okay, this is a fancy language for simply saying
| 472.84
| 482.12
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t482.12
|
x6YWpe9xHMg
|
we differentiate with respect to all possible paths, that number called the action, we set that
| 482.12
| 486.92
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t486.92
|
x6YWpe9xHMg
|
equal to zero. And the point is we get an equation, okay, we get an equation, and that equation turns
| 486.92
| 494.04
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t494.04
|
x6YWpe9xHMg
|
exactly into Newton's laws. That's how you prove the equivalence between the least action way of
| 494.04
| 501.8
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t501.8
|
x6YWpe9xHMg
|
doing classical mechanics and the Newtonian way of doing classical mechanics. You have this
| 501.8
| 506.2
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t506.20000000000005
|
x6YWpe9xHMg
|
formalism that sounds very abstract, you have all these paths and infinite dimensional space of all
| 506.2
| 510.68
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t510.68
|
x6YWpe9xHMg
|
the different possible paths, you figure out how to differentiate with them, there's a lot of math
| 510.68
| 514.68
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t514.68
|
x6YWpe9xHMg
|
involved, you take math classes, good for you, but at the end, the equation that comes out from
| 514.68
| 520.76
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t520.76
|
x6YWpe9xHMg
|
minimizing the action is the statement that at every single point along the trajectory, the particle
| 520.76
| 527.08
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t527.0799999999999
|
x6YWpe9xHMg
|
obeys Newton's laws of motion. Okay, so that's a way of easing your anxiety about the fact that
| 527.08
| 534.84
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t534.8399999999999
|
x6YWpe9xHMg
|
the particle seems to know ahead of time which path to take. It's not that it thinks about every
| 534.84
| 539.96
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t539.96
|
x6YWpe9xHMg
|
single path and chooses the right one, it's that the condition that follow, that the path you follow
| 539.96
| 545.4
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t545.4000000000001
|
x6YWpe9xHMg
|
be the least action one is the same as the condition that you're obeying Newton's laws at every moment.
| 545.4
| 551.32
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t551.32
|
x6YWpe9xHMg
|
Okay, that's one thing to keep in mind. Here's another thing that you're probably thinking already,
| 551.32
| 557.56
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t557.5600000000001
|
x6YWpe9xHMg
|
as you watch me write these things, sure, the minimum of this function has the feature that
| 557.56
| 565.16
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t565.16
|
x6YWpe9xHMg
|
its derivative is zero, but guess what, a point where the function is a local maximum also has that
| 565.16
| 572.28
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t572.28
|
x6YWpe9xHMg
|
feature where the slope is exactly zero. So the condition that the action have zero derivative,
| 572.28
| 580.04
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t580.04
|
x6YWpe9xHMg
|
zero slope in the space of all possible paths, doesn't pick out the minimum uniquely, it just says
| 580.04
| 586.68
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t586.68
|
x6YWpe9xHMg
|
it's a place where the function isn't changing instantaneously momentarily, it could be a minimum
| 586.68
| 592.28
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t592.28
|
x6YWpe9xHMg
|
or a maximum or in higher numbers of dimensions, it could be what we call a critical point where
| 592.28
| 597
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t597.0
|
x6YWpe9xHMg
|
in some directions it could be going down, in other directions it could be going up, but it's
| 597
| 601.4
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t601.4
|
x6YWpe9xHMg
|
momentarily stationary. So sometimes this is called a saddle point, if you think of literally a saddle
| 601.4
| 607.64
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t607.64
|
x6YWpe9xHMg
|
that is sort of this kind of shape in two dimensions, right, then if you're right here, you're at a
| 607.64
| 614.04
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t614.04
|
x6YWpe9xHMg
|
critical point where everything is flat, you're at a flat location on the surface, but in this
| 614.04
| 618.6
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t618.6
|
x6YWpe9xHMg
|
direction things are going up, in this direction things are going down, right, so you're at a mixed
| 618.6
| 625.16
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t625.16
|
x6YWpe9xHMg
|
point where in some directions it's up, some directions is down, but momentarily everything is flat.
| 625.16
| 629.4
|
https://youtu.be/x6YWpe9xHMg
|
The Biggest Ideas in the Universe | Q&A 3 - Force, Energy, and Action
|
2020-04-12T15:32:41Z
|
x6YWpe9xHMg-t630.44
|
x6YWpe9xHMg
|
How do you know that the actual path you're going to take is not just some extreme, some
| 630.44
| 636.2
|
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