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.

If this subject fascinates you like it does me,  

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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.