Very soon after quantum mechanics developed, people started to worry very much
—and there were endless discussions between Bohr and Heisenberg and Einstein—
what does the theory mean?
and all these discussions were always—the issue was whether the theory is consistent or not.
But when people tried to understand more deeply what it means, they gave up.
Feynman, for example, said, Don't ever try to understand what quantum mechanics means
because you will fall into abyss and you will never get out of it.
Now, I, from the beginning, because I was led to think about physics
because I wanted to understand foundations questions,
I couldn't believe that there is no way to understand it better.
So, all my life was developed just to try to understand it better.
Classical physics is a theory that shows that
interactions in physics are always local,
meaning that, if I have a particle that feels any force
and, due to that, it changes its velocity according to the laws of Newton,
that happens only if the force acts at the same location where the particle is.
So that's called locality.
It turns out that, in quantum mechanics, that is not true.
There was a discovery made by me and my thesis advisor, Professor Bohm,
that showed that, in quantum mechanics, there is a possibility that
the force will act in one region of space, the particle is moving outside this region,
and nevertheless it is affected by it.
And that is a very important new property of quantum mechanics that shows that
in the world, the interactions—all the interactions in the world—have this kind of nonlocality.
All of them.
You have something that is called a solenoid.
A solenoid is a kind of current—a loop of current—that, inside it, there is a magnetic field, confined to that solenoid.
Outside, there is nothing there.
Now, you take a charged particle and you send it around the solenoid.
If it were a classical particle, it must go either one side or the other side.
A quantum particle can do the trick of splitting itself into two halves, and
one half of it moves here, the other half moves there—like a wave.
And then when the two of them meet together, there is something that is called interference, like interference of waves.
And that interference has maxima and minima—places where the two waves combine constructively
and places where they combine destructively—like two waves in water.
So, the places where the maxima and minima occur depend on how much flux was in the solenoid.
So, the solenoid affects nonlocally the particle, but in a way that could happen only quantum-mechanically.
At first, there was a great disbelief.
I remember that people told me that Niels Bohr was sure it that it cannot be right
because it was against the classical correspondence.
But then, luckily, there were two physicists at Harvard, Furry and Ramsey
(Ramsey, by the way, got the Nobel Prize for another reason),
and they published an article where they showed that the effect we predicted must be there
for the consistency of quantum mechanics.
Otherwise, it would lead to violations of the Uncertainty Principle.
When people saw this article, they became convinced that we must be right.
Parallel to that, there was experimental evidence already collected
from the first experiment that was done by Chambers, at Bristol University,
that showed that we are right.
But, at first, people couldn't believe that something like empty space could affect a particle.
The particle is moving completely in empty space, because the field is far away.
There is nothing there; it looks like vacuum.
And still the particle is affected nonlocally by something that is someplace else.
And the reason why it's possible in quantum mechanics
is only due to the fact that there are uncertainties.
If there were no uncertainties, then we would immediately violate causality.
Causality says that if you do something at one point,
it cannot affect the other thing immediately.
There must be time before it goes and affects something else.
So, imagine that we have a nonlocal interaction.
Then, I do something here; the particle is there, and suddenly it feels, very quickly, an effect.
So, we have shown that the only way that quantum mechanics allows it to happen
is because the thing that is affected in the particle is completely uncertain.
And because it's completely uncertain, there is no way to see the change.
No way to signal, yeah.
It is nonlocal, but causality is preserved.
We discovered now, by looking at this new way of quantum mechanics, a whole host of new phenomena
that have to do with nonlocality in space, nonlocality in time.
And a lot of these are very nonintuitive classically, but once you learn to think correctly,
about the nonlocality of quantum mechanics
it can become intuitive to you, too.
So that's my message.
