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Today I want to talk about a peculiar aspect of quantum measurements that you may have
heard of.
It’s that the measurement does not merely reveal a property that previously existed,
but that the act of measuring makes that property real.
So when Donald Trump claims that not testing people for COVID means there will be fewer
cases, rather than just fewer cases you know about, then that demonstrates his deep knowledge
of quantum mechanics.
This special role of the measurement process it an aspect of quantum mechanics that Einstein
worried about profoundly.
He thought it could not possibly be correct.
He reportedly summed up the question by asking whether the moon is there when nobody looks,
implying that, certainly, the question is absurd.
Common sense says “yes” because what does the moon care if someone looks at it.
But quantum mechanics says “no”.
In quantum mechanics, the act of observation has special relevance.
As long as you don’t look, you don’t know if something is there or just exactly what
its properties are.
Quantum mechanics, therefore, requires us to rethink what we even mean by “reality”.
And that’s why they say it’s strange and weird and you can’t understand it and so
on.
Now, Einstein’s remark about the moon is frequently quoted but it’s somewhat misleading
because there are other ways of telling whether the moon is there that do not require looking
at it in the sense of actually seeing it with our own eyes.
We know that the moon is there, for example, because its gravitational pull causes tides.
So the word “looking” actually refers to any act of observation.
You could say, but well, we know that quantum mechanics is a theory that is relevant only
for small things, so it does not apply to viruses and certainly not to the moon.
But well, it’s not so simple.
Think of Schrödinger’s cat.
Erwin Schrödinger’s thought experiment with the cat demonstrates that quantum effects
for single particles can have macroscopic consequences.
Schrödinger said, let us take a single atom which can undergo nuclear decay.
Nuclear decay is a real quantum effect.
You cannot predict just exactly when it happens, you can only say it happens with a certain
probability in a certain amount of time.
Before you measure the decay, according to quantum mechanics, the atom is both decayed
and not decayed.
Physicists say, it is in a “superposition” of these states.
Please watch my earlier video for more details about what superpositions are.
But then, Schrödinger says, you can take the information from the nuclear decay and
amplify it.
He suggested that the nuclear decay could releases a toxic substance.
So if you put the cat in a box with the toxin device triggered by nuclear decay, is the
cat alive or is it dead if you have not opened the box?
Well, it seems that the cat is somehow both, dead and alive, just like the atom is both
decayed and not decayed.
And, oddly enough, getting an answer to the question seems to depend on the very human
act of making an observation.
It is for this reason that people used to think consciousness has something to do with
quantum mechanics.
This was something which confused physicists a lot in the early days of quantum mechanics,
but this confusion has luckily been resolved, at least almost.
First, we now understand that it is irrelevant whether a person does the observation in quantum
mechanics.
It could as well be an apparatus.
So, consciousness is out of the picture.
And we also understand that it is really not the observation that is the relevant part
but the measurement itself.
Things happen when the particle hits the detector, not when the detector spits out a number.
But that brings up the question what is a measurement in quantum mechanics?
A measurement is the very act of amplifying the subtle quantum signal and creating a definite
outcome.
It happens because if the particle hits the detector it has to interact with a lot of
other particles.
Once this happens, the quantum effects are destroyed.
And here is the important thing.
A measurement is not the only way that the quantum system can interact with many particles.
Indeed, most particles interact with other particles all the time, just because there
is air and radiation around us and there are constantly particles banging into each other.
And this also destroys quantum effects, regardless of whether anyone actually measures any of
it.
This process in which many particles lose their quantum effects is called “decoherence”
because quantum effects come from the “coherence” of states in a superposition.
Coherence just means these state which are in a superposition are all alike.
But if the superposition interacts with a lot of other particles, this alikeness is
screwed up, and with that the quantum effects disappear.
If you look at the numbers you find that decoherence happens enormously quickly, and it happens
more quickly the larger the system and the more it interacts.
A few particles in vacuum can maintain their quantum effects for a long time.
A cat in a box, however, decoheres so quickly there isn’t even a name for that tiny fraction
of a second.
For all practical purposes, therefore, you can say that cats do not exist in quantum
superpositions.
They are either dead or alive.
In Schrödinger’s thought experiment, the decoherence actually happens already when
the toxin is released, so the superposition is never passed on to the cat to begin with.
Now what’s with viruses?
Viruses are not actually that large.
In fact, some simple viruses have been brought into quantum superpositions.
But these quantum superpositions disappear incredibly quickly.
And again, that’s due to decoherence.
That’s what makes these experiments so difficult.
If it was easy to keep large systems in quantum states, we would already be using quantum
computers!
So, to summarize.
The moon is there regardless of whether you look, and Schrödinger’s cat is either dead
or alive regardless of whether you open the box, because the moon and the cat are both
large objects that constantly interact with many other particles.
And people either have a virus or they don’t, regardless of whether you actually test them.
Having said that, quantum mechanics has left us with a problem that so far has not been
resolved.
The problem is that decoherence explains why quantum effects go away in a measurement.
But it does not explain how to make sense of the probabilities in quantum mechanics
for single particles.
Because the probabilities seem to suddenly change once you measure the particle.
Before measurement, quantum mechanics may have said it would be in the left detector
with 50% probability.
After measurement, the probability is either 0% of 100%.
And decoherence does not explain how this happens.
This is known as the measurement problem in quantum mechanics.
Thanks for watching, see you next week.