If you've been watching any of my
videos on quantum mechanics, then you know that the smallest particles in
nature behave in some very strange ways. These particles include things like
photons, electrons, atoms, even some molecules. You know that observation is
synonymous with measurement. The double slit experiment shows that they behave
like waves of probability when not measured. And quite strangely, when measured,
these probability waves collapse so that they become distinct particles. We also
see from experiments like the delayed choice quantum eraser, that two particles
can become entangled, meaning that they appear to be somehow intimately
connected through the sharing of one probability wave, even if the particles
are separated apart in space-time. And this behavior seems really strange to us
because this is not something that we observe in our macro world. But why is
that? Why don't we see this kind of strange and mysterious behavior in
our everyday lives? Why doesn't this tennis ball behave like a photon, or an electron,
or an atom? At what point does the quantum behavior stop, and classical
behavior of everyday objects like this begin? And why is there cutoff? The
explanation is coming up right now...
Scientists have shown quantum behavior in particles as large as molecules
containing hundreds (*thousands as of late 2019) of atoms. How large a particle can you get, and still show
quantum behavior? What if I take this tennis ball and throw it at a double
slit, with a screen behind it that records a position of the ball when it
hits the screen? What if I take thousands of these tennis balls and throw it at a
double slit? Will I see the same kind of behavior that electrons or atoms have?
No, of course not! I'll just see two patterns on the back wall. Why doesn't this thing
behave like electrons or atoms? It's made up of atoms and electrons around it - six
trillion trillion atoms. Why doesn't the whole behave like the individual
particles it's made up of? To answer this we have to understand some of the basics
of quantum mechanics. Let's look at how the double slit experiment works with
light first. The results are the same with electrons or atoms. Richard Feynman
called this, "the central mystery of quantum mechanics." When we shine a
monochromatic light, that is light of only one wavelength, through two slits, as
light hits the screen, we see an interference pattern on the other side,
because as the waves spread out, the crest of one wave hits the crest of another, it
becomes stronger. Where a crest hits a trough, it will cancel. This should be no
big mystery. This is not quantum mechanics. This is just a property of
waves. It'll happen with water waves too. Now,
what if we lower the intensity of the light, and keep lowering it until our
light source emits only one photon at a time?
Surely this should not create a wave pattern! When we shoot one photon at a
time, it forms a dot on the screen. But if you fire enough of them, thousands or
millions of photons, what you find is that collectively, the photons form the
same interference pattern. So somehow the photon either interacts with itself, or
somehow is able to communicate its part in the interference pattern when fired
one at a time. So since we want to find out what's going on at the slits, what if
we put a detector there to determine whether the photon goes through slit 1
or slit 2? This is the most startling result of quantum mechanics, because when
you try to find out which path the photon took, the interference pattern
disappears! The photon appears to now behave like a particle, instead of a wave.
So what's going on? Let's talk in the language of the Copenhagen
interpretation, which is the most accepted interpretation of quantum
mechanics. This was conceived by Niels Bohr and Werner Heisenberg at the
University of Copenhagen around 1927. This interpretation basically says that
these so-called particles are not particles at all, but really like clouds.
They are in multiple states at the same time. They're like waves of probabilities.
The waves become distinct, like particles, only when they are measured. At the point
at which the probability wave becomes a distinct particle is called a "collapse"
of the probability wave. What scientists have found is that at any point when the
which-path information of any particle is measured, its probability wave collapses.
For the particles to remain probability waves, their path information must remain
in absolute secrecy. If at any point, any kind of measurement is made, whether it
is seen by anyone or not, the particles become distinct, and do not display
wave-like behavior. The same behavior has been found in subsequent years with
electrons, atoms and even molecules composed of as many as 800 atoms (*2000+ as of 2019).
The same interference pattern forms, and the same collapse occurs if their which-path
information is measured. A measurement always collapses the wave. So what is a
measurement? A measurement is a formation of any physical record of which path the
particle takes. It's not about conscious observers. It's about a record made in
the universe even if no one is looking. It is such that a forensic exam, if we
had even the most sensitive quantum tools, could establish which path the
particle took. You never have to look at the information. You can even destroy the
information, before you look at it, like burn it, or destroy the hard drive that
it's on. The wave will still collapse, because the universe already recorded it.
Why is it recorded even if you destroyed
it?...because theoretically, you could go back
in time and see the path. The universe will say,
"hey, I already know the path. Now it's a particle. You can't have your wave back."
The path has to be taken in absolute secrecy. The particle has to be
informationally isolated. The interference can only occur if it is
impossible, even in principle, to find out which path the particle took. Now, as I
said, this happens with photons, electrons, atoms, and even large molecules. Now, what
if we do this with really large particles, like grains of sand, which by
the way, are typically composed of quintillions of atoms, or tennis balls?
Will we see the same pattern?...of course not. We'll just see two lines on the other
side of the double slit. Why don't we see this behavior with tennis balls? The
reason is that large objects are nearly impossible to isolate informationally
from the outside world. What do I mean by this? Let's look and see what it would
take to isolate a tennis ball. Remember, to do this, we have to make sure that
there is no record made anywhere in the universe of which path the tennis ball
follows. First we have to remove all the air and photons in the experiment. Why? If
a photon or air molecule bounces off the tennis ball, then it has potentially
recorded the path of the tennis ball. For example, if the photon reflects off the
ball, then that could be a measurement, because the path of the photon would be
changed, and the bounced path of the photon has recorded the path information
of the ball. Potentially, someone or the universe could examine the paths of all
the photons in the room, find out how they were affected by the path of the
tennis ball, and find out from that information, the path that the tennis
ball took. And the same thing applies for atoms. We'll also need to cool this
tennis ball to near absolute zero, because a ball that has any temperature
will emit photons due to something called "blackbody radiation."
This is a property of all bodies with the temperature above absolute zero,
which is by the way negative 273 degrees Celsius. The emitted photons will have
recorded the information about the tennis ball, such that someone
examining the photons in the room, could determine the path of this tennis ball.
We even have to worry about the small amount of gravitation that the tennis
ball will have. Why?...because this gravitation of the tennis ball will
affect nearby atoms. This, in principle, can allow someone to determine the path
that this tennis ball took. The movements of nearby atoms can also form a record
of the path of that tennis ball. We have to completely isolate the tennis ball
because any information that is leaked to the universe about its path will
create a record, even if this information is recorded in one atom. It is still
information which has been captured in the universe and the tennis ball will
not be in superposition, will not be a cloud. It'll be a distinct entity.
Macroscopic objects like this are very difficult, if not impossible, to isolate
informationally, so in our everyday experience,
we will not see quantum superposition or wave-like behavior of macro objects. But
one principle of quantum mechanics is that it applies to all objects,
regardless of size. Could the tennis ball behave like an atom if we could isolate
it informationally? This may be true, but there's a measurement problem. We can
look at this mathematically. The wave of an object is described by the de Broglie
wave function. Lambda equals Planck's constant over mass times velocity. Since
the Planck constant is so tiny, and the mass "m" so huge, the wavelength will be
extremely small. So the interference pattern, for all intents and purposes,
will approximate no interference. The quantum mechanical behavior of the
tennis ball converges to classical behavior. Its interference is so small
that it probably cannot even be measured. So the tennis balls and other large
objects, like our bodies, are going to be about as real and here as you're going
to get. This is the reason you and I, and the cat and Schrodinger's experiment are
not in superposition. The cat is not dead and alive at the same time. That's a myth.
And there's no possibility of interference patterns being formed in
our normal everyday interactions. And for the same reasons, macroscopic objects like
this, cannot be entangled in the way photons can be entangled, for example. So
what we see macroscopically is a very different world than what we see
microscopically. and this is why it's a mistake to extrapolate our observations
and equations in quantum mechanics to our everyday experiences. They behave in
completely different ways. They are two different worlds. Qantum mechanics is
what happens when no one is looking. This is what makes it so mysterious. It's as
if it only happens when you're not allowed to look and find out how it
happened. Another way to look at this is that macro objects like tennis balls and
humans are very intimately connected to the universe. Our every move and path, and
even thought, is recorded in the particles around us, and within us. In
this way, our whereabouts and even thoughts, become part of the fabric of
the universe. There's no escaping our intimacy with the forces and particles
that we are made up of and surrounded by. And in this way, we are, at least
informationally, indeed one with the universe!
You can enjoy lectures from some of the best professors and educators in the
world. I just finished several lectures by
professor Benjamin Schumacher on some of the crazy implications of quantum
mechanics. One of his lectures in fact was an inspiration for this video. If
you're intrigued by quantum physics, and want to get more in depth, you're gonna
love Dr. Schumacher, and his teaching style. Some of the topics in this course
are like "Quantum Computing," "Quantum Cloning," and "Entanglement."
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