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The complete set of Feynman Simplified: Quantum Mechanics has now been released. See below for more details. This is one topic covered in those books.
Quantum
mechanics (QM) is the weirdest theory in science. It began over 100
years ago, and its many bizarre predictions have been exhaustively
tested by tens of thousands of high precision experiments.
In all that time, QM has never been proven wrong, but physicists still struggle to understand what quantum mechanics really tells us about our world and our existence.
It
is generally thought that quantum mechanics only rules the micro-world
of atoms and subatomic particles. QM says particles like electrons can
be in different places with incompatible characteristics at the same time. Remarkably, despite seeming impossible, this is exactly what happens. So, with such strange behavior, can electrons truly be real?
Worse than that, quantum mechanics actually claims to describe all of nature, including everything that is made of these strangely behaving particles — including us. Are we real?
Consider
this thought experiment. (The nice thing about thought experiments is
that you can keep your hands clean and avoid high-voltage shocks.)
Imagine
I order a pair of gloves to be delivered in two boxes, one glove in
each box. I send one box to each of two friends: Albert and Niels. Not
surprisingly, Albert and Niels receive opposite-handed gloves. But if
these were entangled quantum gloves, each glove would
have half a thumb on each side, as long as they’re boxed. Each box would
contain a half-left-half-right glove, until it is opened. And when the
first box is opened, one glove randomly and instantly becomes totally
left-handed and the other automatically and simultaneously becomes
totally right-handed. While quantum mechanics says Albert and Niels get
opposite-handed gloves, as expected, its description of how this happens
profoundly shocks our deep-felt concepts of reality.
Einstein disdainfully called this “entirely peculiar” and “spooky.” In 1935, Einstein launched the EPR paradox, saying quantum mechanics is incomplete — it fails to recognize some as yet undiscovered, essential feature of nature.
For
more than 70 years, physicists struggled to understand whether quantum
mechanics describes reality or only our incomplete perception of
reality. How can we know if the gloves really are half-left-half-right before opening the boxes?
In 1964, Irish physicist J. S. Bell had a brilliant idea. I explain all this thoroughly in Feynman Simplified 3C, but here’s a taste.
Physicists
don’t actually mail quantum gloves. Instead, we send pairs of entangled
particles that spin left-handed or spin right-handed — or as QM claims,
one spins half-left-half-right and the other spins
half-right-half-left, until we look at them, when one switches to all
left and the other to all right.
Bell’s
great idea, like many other great ideas, sounds crazy: he said look at
the spinning particles halfway between left and right.
For
the next 40 years, physicists got better and better at looking halfway.
Finally, in the last decade, we have seen through nature’s fog. The
bizarre quantum story is correct: nature’s gloves really do have
half-thumbs on both sides until the instant we look at them.
In
quantum poker there is no need for shuffling. In a quantum deck, as
long as no one is looking, each card could be the ace of spades, or the
two of diamonds, or any of the other 50 possibilities. Indeed, each
player’s cards are not just unknown but are actually undetermined until
someone peeks.
That’s reality.
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