If we know the position and motion of a basketball at one instant, and all the forces that act on it,
we can work out from the classical laws of motion exactly where it will be at a later time.
But quantum mechanics doesn’t let us do that for a particle. All it tells us
is where the particle could be at the later time, and what’s the probability we’ll find it there.
So how does the indeterminate world of quantum mechanics, where the future isn’t fixed,
become the classical world we experience, where at least some things are predictable?
There’s a traditional answer to that – but we now know that it’s not a very good answer.
In fact, quantum researchers have been arguing about it ever since quantum mechanics was
invented in the early twentieth century. I’m going to tell you the new story,
or at least as much of it as we now understand.
And at its essence, it’s really a story all about time,
and the boundary between becoming and being, between the possible
and the actual. That’s coming up right now…
First, let me explain what I mean about the quantum world being indeterminate.
When a woman is pregnant, she and her partner can choose to know in
advance whether the baby is a boy or girl, or they can wait for the surprise at birth.
All they need is an ultrasound scan. Once you look, you know.
But if you don’t look, you don’t know.
Now imagine a quantum particle that has been put in a state where, if we look at it,
we might find with 50:50 probability that it is either this or that – not a
male or a female particle, of course, but we might
find it spinning either one way or the other, which physicists call spin-up or spin-down.
You might think that here too the particle is always definitely one or the other,
and we’ll just find out which it is by looking – by making a measurement. But that’s not the case.
Quantum mechanics seems to insist that the spin has neither one direction nor the other
until we look, until we measure it. The very act of measuring forces the universe to make that choice.
If the classical world was like this, it would be as though the baby is neither male nor female
until we perform the ultrasound – until we look. Then, before we look,
we’d have to say that the chances are 50:50 not because we don’t know the answer – not because
of ignorance – but because there genuinely is no answer. There are just probabilities.
When we look, the probability then seems to switch abruptly
from 50:50 to 100:0 one way or the other: the baby becomes definitely a boy or girl.
This sounds nonsensical for a baby. But it’s precisely how it is for a particle. Particles are
in what’s called a superposition prior to measurement. What does this mean?
All the variables that characterize the properties of elementary particles,
such as its position and momentum, are encoded in a mathematical expression called a wave function.
And while the particle isn’t measured, the wave function is just a sum of all the wave
functions of the states that the particle could be found in. This is called a “superposition.”
By itself, the wave function doesn’t have any intuitive meaning. But the square of the wave
function gives us the probability of finding that quantum object in any particular place.
This picture seemingly makes no sense from a classical perspective, because then it looks
as though, before a measurement, the particle must be in all those places and states at once.
And although I myself and guilty of characterizing it that way, that’s probably not what’s happening.
Prior to measurement, quantum mechanics doesn’t actually tell us where the particle is, or what
it is like, at all. It just tells us the chances of what we’ll see – if measure it, if we look.
So now you might ask, “How do we know this is really true, if
we only see one value of those variables when we look?” We know it because even though those
superposed states cannot be observed directly, they can, and do, interfere with each other.
In the famous double slit experiment, for example, where particles like electrons
are fired at two slits in a screen, we see an interference pattern on the far side
caused even by electrons fired one at a time.
It’s as if each of them goes through both slits at once and interferes with itself.
These superpositions, and the interference they cause, can only persist as long as we don’t try
to find out where the particle “actually” is, or what state it is “actually” in.
If we do, these quantum effects vanish and the particle becomes like a classical object,
with definite position and properties. And that’s the real problem:
quantum theory doesn’t tell us how the switch from probabilities to certainties happens.
All quantum mechanics can do is describe the particle before we do the measurement, when
the chances of being spin-up and spin-down are, say, 50:50, or some other set of probabilities.
According to quantum mechanics, there’s no way a particle’s wave function can abruptly switch
from the 50:50 probabilities to the 100%
certainty we get when we make the measurement. There is no theoretical basis for this switch.
So quantum physicists have to add in that step by hand,
as something extra to quantum mechanics itself. This is often called the “collapse” of the
wave function, and was first introduced by the Hungarian mathematical physicist John von Neumann
in the early 1930s. But it’s just a kind of botch for conjuring the classical world of this or that
from the quantum world of everything-at-once probabilities.
This is not satisfying. In the past several decades, though, quantum researchers have realized
that what we really need to do to understand this thing we’ve called collapse of the wave function,
is to think more carefully about what goes on in a measurement.
It’s not magic, after all. Whatever the quantum object is that we’re measuring – let’s say we’re
trying to find out the spin of that lone particle – we need some way of getting it to interact
with atoms in its environment, especially those in our great big measuring device.
According to quantum mechanics, what that means is that the quantum state that the particle is in,
becomes mixed up – the technical word is “entangled” – with the
states of the atoms in the environment. If the particle is in a superposition,
this superposition then spreads to the atoms it interacts with, through this process of
entanglement. They all become part of one big superposition. It’s one big entanglement party!
The more of its environment the particle interacts with,
the further the superposition spreads. So it’s still a quantum system,
it’s still in a superposition, but it becomes ever harder to see in that huge crowd
any synchrony between the waviness of the original particle’s superposed quantum states.
It’s like a line of kids on swings: they might start off going back and forth in synchrony,
but gradually that synchronization is lost. This is called decoherence.
What you’re left with looks like a mess of unrelated waves. If now you want to see the
original superposition, you would have to look at the quantum behavior of all those entangled atoms
to get the full picture. But pretty quickly, this becomes impossible.
It’s literally like trying to keep track of the effect a floating dust grain
in air has on all the atoms in the air as they collide with it and then with each other.
So as entanglement spreads, it inevitably leads to ever more decoherence. This, you might say,
dilutes the quantumness, hiding the superposition from plain sight. You can’t see it any longer in
the original particle. Instead, it turns out that the environment gets imprinted
with the effects of just one or other of the possible spin states, selected at random
in much the same way that a needle on its tip will fall in one direction at random.
The difference is that where the needle falls is, in theory, predictable in advance
if we know all the molecular interactions and forces acting on the needle.
But this is not the case for our quantum object.
We cannot know, even in principle, before it happens, which way the decoherence will
tip the balance. There’s now a pretty good understanding of how decoherence happens,
and how it creates a classical outcome – a definite result – from a quantum measurement.
So in place of the old, sudden and slightly mysterious idea of wave function collapse, we now
have a theory that, at least in some simple cases, can show us how decoherence produces classical
from quantum. It’s a real physical process, and takes time, although usually a very short time.
Researchers at Yale University have
even been able to track and record this so-called collapse process as it unfolds.
Does this mean then, that we now understand what quantum measurement is all about – and that it’s not
really “collapse” at all, but just a gradual leaking and filtering of information about the
quantum object into the measuring apparatus? Well, not quite. There’s a critical thing still missing.
You see, as entanglement causes decoherence and spreads the quantumness ever more thinly
throughout the environment, there’s no obvious end to that process. Every atom that interacts
with an atom that interacts with another atom that interacts with the original quantum superposition
in effect gets tainted with a little bit of that superposition. It goes on and on spreading,
not only in the local environment of the quantum object, but throughout the entire
universe! This means that it very quickly becomes practically impossible for us to
keep track of how far the spreading goes and which atoms it entangles.
But not in principle. If we were some God-like being that could watch
every atom in the universe, then we could at any moment identify all those that have
become entangled by the act of measuring – of observing – the original particle.
If we only look at a few of them, we won’t be able to see the entanglement
because decoherence has left their quantum waviness looking all jumbled and out of step.
But if we could inspect all of them, we can make out the pattern
and deduce that they are in fact all in a kind of massive, entangled state of superposition.
In other words, they are all still in a state of both this and that. They’re still quantum!
What’s more, it could be possible in principle to then reverse the whole process:
to disentangle it all and focus all the quantumness back on the original particle,
recreating the superposition it started in.
This would be like keeping track of every molecule in a drop of ink dispersing in a glass of water,
and carefully guiding it back to reconstitute the drop. It sounds absurdly hard,
but actually there have been some experiments in which this sort of “recoherence” has been
possible for a quantum object interacting with just a few particles in its environment,
before decoherence has gone too far for any backtracking.
That would mean undoing the measurement, and erasing any information we gained about it.
Forgetting where our needle pointed. Almost like reversing time itself.
So here’s the question: Is there ever a point where the measurement process switches
from being fully reversible in principle by this sort of recoherence,
and becomes irreversible? Is there a point of no return?
Quantum experts don’t agree about this. The theory that describes a measurement by decoherence and
imprinting the result on the environment doesn’t generate any such point of no return.
But some believe that a point like that is needed to go from quantum to classical.
As long as the process is reversible, it’s still quantum. It’s only when it becomes irreversible
that it becomes classical – and it’s only then that we can truly say “what happened”:
which way the needle pointed, what the outcome was. In other words,
it’s only then that an outcome is real.
Physicist Lee Smolin has recently suggested that this is in fact what distinguishes the
past from the present. The past, he says, is completely classical: it consists of things that
definitely happened, and can never unhappen. The present, though, is quantum: it’s still unfolding.
What seems to separate them, then, is not just whether they are reversible or not,
but whether they are knowable or not: whether they have become
actual knowledge, something of which we can say “it was like this”.
According to Smolin, it’s the change from a quantum, indefinite present to a classical,
definite past that defines the very arrow of time itself, pointing it always in the
forward direction as the quantum present constantly churns out a classical past.
We can never go back – we can’t change the past. In fact, the past no longer really exists at all.
“Once something is definite”, says Smolin, “its job is done and it is gone.”
But what about the future? The future, says Smolin, is a quantum future:
It’s a place only of possibilities, not certainties. Not everything is possible in the future though,
but only things that can emerge from the present through the laws of quantum mechanics.
Like the past, it doesn’t truly exist – but for a different reason. The past is not real
because its job is done and it can have no further influence on the world. The future
is not real because nothing in it can be made concrete until the present,
forever moving forward in time, reaches it.
In this view, we live constantly in the moment in which probabilities are becoming actualities,
in which reality – what is, what has happened and been imprinted on the world – is at the point of
condensing, of changing from quantum to classical. We live in the borderland. But if the future is
quantum, then what happens to all those futures that aren’t selected by the decohering present?
Where do those alternative possible futures go? Do they, as some think,
pop up in an alternative universe? Or should we trust what our intuition seems
to tell us loud and clear: that there’s only one reality? That’s a question no one can answer…yet.
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And if you have a question on anything you saw in this video, please feel free to leave it in the
comments and I will do my very best to answer it. I will see you in the next video my friend.