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Summary Quantum physics
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Quantum Physics Full Course |Academic Lesson
The main goal of this lecture is to explain why it ‘s necessary and second of all to put it in historical
context. I ‘ll show one of the most famous photographs in all of physics that really gives you a feel for
the brain power that went into the construction of this theory. Some things in quantum mechanics that
are deeply non-intuitive but if you approach them with an open mind quantum mechanics is a
fascinating subject there ‘s a lot of really fun stuff that goes on now to move on to the necessity for
quantum mechanics there were some dark clouds on the horizon even at the early 20 th century
michelson was n’t quite having a big enough picture in his mind when he said that everything was down
to the sixth place of decimals. The black body spectrum as I mentioned you can think of as the light that
‘s emitted just by a hot object. While hot objects have some temperature associated with them let’s call
that t the plot here on the right is showing very qualitatively the intensity of the light emitted as a
function of the wavelength of that light. If you try to explain this knowing what early 20 th century
physicists knew about radiation and about electrons and about atoms and how they could possibly emit
light you get a prediction and it works wonderfully well up until about here at which point it blows up to
infinity. The next experiment I mentioned is the photoelectric effect this is sort of the opposite problem.
1900 they were pretty much the only thing that we had to go on so if you wanted to understand some
physical process, you had to calculate the consequences of all the different possible physical laws and
then try and find a one that fit what you were observing. So quantum mechanics comes in as a way to
simplify all of this by taking into account things like uncertainty principle and quantum entanglement,
and these allow us to calculate the consequences of different possible physical laws without having to
worry about actually trying to find them all so that’s one of the benefits of quantum mechanics. Um
now. Another benefit is that it gives us a way to understand things that are really hard to understand.
Like the behavior of subatomic particles. So let’s take a look at an example. Uh let’s say that I have a
subatomic particle called an electron, and I want to know what its position is at any given moment in
time well. According to classical physics, if I ask you where the electron is at this moment in time, you
would be able to give me an answer without any trouble at all because classical physics tells you that the
electron has a definite location in space and that location is specified by its coordinates um. According
to quantum mechanics, however, the electron doesn’t actually have a specific location in Space it
doesn’t have a definite position. It’s instead in a state of uncertainty or indeterminacy, and this
uncertainty principle says that you can’t know both its position and its momentum at the same time. So
if I ask you where the electron is right now and also what its momentum is at this. Moment in time you
can’t simultaneously give me an answer to both questions Because if you did then I would know both its
position and its momentum, which is forbidden by the uncertainty principle. So instead we have to say
that the electron is located somewhere in between its position and momentum, which is known as its
wave function. So now if I want to find out what the electron’s position is right now. I can just look at its
wave function and see that it’s located somewhere in between zero and plus infinity. For momentum.
It’s located somewhere between minus infinity and plus infinity. So these are just some examples of
how quantum mechanics can help us understand complicated concepts in simpler ways, so let’s take a
look at one more example before we finish up today. Um suppose, I want to know what the energy level
of an atom is Suppose I want to know what atom I’m looking at has the highest energy level well.
, According to classical physics, I would have to ask each atom individually what its energy level is. But
according to quantum mechanics, I can look at the energy levels of all of the atoms in dark clouds on the
horizon for quantum mechanics at the turn of the 20th century. According to Kelvin, there were a couple
of unexplainable experiments that scientists were having a hard time explaining so quantum mechanics
was born to try and make sense of all this stuff. One of the first things quantum mechanics did was to try
and explain the black body spectrum. This is an experiment where you take an object and you heat it up
so much that all the energy that is radiated away from the object is in the infrared range and what you
end up with. Is a distribution of radiation that looks like a black body. So this was an experiment that
was very difficult to explain using classical physics and quantum mechanics was able to explain it in a
way that makes sense. This is just one example of how quantum mechanics has been able to expand our
understanding of the world around us. There are more things in heaven and earth than are dreamt of. In
your philosophy, so take an open mind when approaching quantum mechanics.. There are some things
about quantum mechanics that are non—intuitive,, but if you approach them with an open mind, it can
be a fascinating subject. Additionally, there were dark clouds on the horizon for quantum mechanics at
the turn of the 20th century.. However, quantum mechanics was born to try and make sense of these
experiments.. In general, quantum mechanics has been able to expand our understanding of the world
around us. The two metals with a wire so that when the electron hits the metal it knocks off a piece of
the metal and it leaves a this little trail ofmetal behind it and you can see the trail in the photoelectric
effect, so it’s not just light coming in and out it’s kind of interacting with the material. We see something
similar in the avalanche effect. If you hit a piece of ice with a hammer. It starts to break into little pieces
and as you keep hitting it. These little pieces start to fall down and eventually they all fall down to the
ground. This is sort of the same thing, but with electrons instead of ice and in the avalanche effect. You
also have a nucleus involved. So let’s take a look at one last experiment before we move on to talk about
what quantum mechanics actually said about these things. So here ‘s an X-ray tube and when you shine
an X-ray in on something like this you get these beautiful pictures of things inside of people that would
be impossible to see with just light coming in from the outside so in this case you’re shining X-rays into
somebody’s chest and you can see all these different structures inside of them like blood vessels and
bones and tissues and so on and so forth And what quantum mechanics says about this is that those
structures are not just there because light is hitting them they’re there because the X—rays are
interacting. Way we don’t really know exactly how but that’s what quantum mechanics says about them
all right so that’s kind of a high level overview of some of the experiments that were used to develop
quantum mechanics. Now let’s move on to talk about what quantum mechanics actually said about
them all right. So quantum mechanics is a very strange theory because on one hand it predicts things
that work really well for some things, but don’t work at all for other things and on the other hand, it also
predicts things that blow up to infinity, which is not really very useful so what did early 20 th century
physicists do when they came across something like this first they tried to interpret it in terms of
classical physics, which is what they knew best at the time, but that didn’t work very well because it
didn’t explain why some. Predictions worked well while others didn’t so then they tried to reformulate
quantum mechanics in terms of more fundamental notions like particles and waves, and those attempts
also didn’t really work very well because they still couldn’t quite explain why some predictions worked
while others didn’t so eventually, physicists gave up on trying to understand quantum mechanics in
terms of more fundamental notions, and just accepted that it was a.
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