So, at the end of April this year, I was honored to be invited to Wolfson College in Oxford
to document a series of informal presentations and conversations concerning the role of probability
in physics. This so-called unconference was sponsored by the Utopia Foundation.
Perhaps the most polarizing issue to emerge over the course of the discussions was the
many worlds interpretation of quantum mechanics, which seemed to pitch some of the more practically
minded experimental physicists like Marcus aren't from the University of Vienna.
Against those big picture theoretical physicists like David Deutsch from Oxford,
who were more willing to embrace this grand view of ever-branching parallel universes.
So, for the sake of this video, I asked Marcus and David if they wouldn't mind
hashing out their differences over their lunch break in one of the corridors at Wolfson.
And I began by asking Marcus how he conceived of the tensions here.
The fundamental problem that we all try to solve is why is there this unitary evolution
of quantum mechanics, which seems to explain everything very naturally.
And out of a sudden, during a measurement this evolution has to be reduced, collapsed in the
Copenhagen interpretation and that's I think something that David doesn't like.
He wants to have everything in the same mathematical formalism.
But if you follow it through, it leads to realities which seem to multiply.
And then my question to you is, what is really the meaning of reality to you?
Because I experience only my single reality here.
Yes, you put it in terms of how do we make sense of the unitary evolution
compared with what we see at a measurement and so on.
I think we want to understand the world, we want to understand how the world is.
And that is not necessarily what we perceive, our perceptions are at the end of a long
chain of physical processes of which themselves we only have scientific knowledge or indirect
So I would start with the question, how do we explain quantum phenomena like interference?
It's not how do we make sense of quantum theory which gives the right prediction.
But first, before that, how do we explain quantum phenomena?
So there's an interference process and we have an interference pattern which we can see without
any quantum mechanics that the result of the experiment cannot be explained by the events that we see.
Now this is not very unusual, this happens a lot, you know, in physics and ultimately
So we have to infer things that are not there, although it infers is the wrong word.
But what makes you then certain that there must be a parallel world in that sense,
or many, infinitely many parallel worlds, instead of either some hidden variables,
though of course there's an experimentalist, I know that some kinds of in variables
And do we need the notion of reality more than the notion of information?
It's such a say the many worlds thing, an information world or a reality world in a thingy sense.
Yes, so that's a very interesting point about the relationship of information to reality
because many people think that the concept of information is somehow prior to physics,
that the laws of information like mathematical theorems, they must be so.
Whereas I tend to the view that what information does and what it can do in the world is determined
by physics and therefore in regard to physics, physics is a theory of the world of the
reality of the world, not about the information. There is this view that quantum mechanics
only tells us what we will see. And it's silent about everything else.
Everything else is just mathematical formalism.
But I think that ultimately leads to solipsism and it is no good philosophically.
But even more important maybe for a physicist, it's no good for finding out what the next theory
will be. If you just think a theory is predictions of experiments,
then we would never have got from Kepler's theory to Newton's theory from Newton's theory to Einstein's
because they differ from each other in what in predictive terms is a tiny amount.
But in explanatory terms it's an enormous amount changes our whole view of the universe.
But in terms of predictions coming back to the many worlds and the predictive power of the many
worlds. I'm sitting here, I can only probe my local world. Even within my local world I seem
to experience all these funny quantum superpositions where you would probably say another
part of the multiverse is branching into me again or into my world again. But this process is
kind of experimentally at least at the moment very difficult to access. And the question is,
Yes, so I mentioned interference experiments. So interference in an interference experiment we don't
even with this notion of seeing things by through the explanation. We only see that the let's
say the photon exists in two instances rather than rather than that the whole world does.
And when you talk about your experience at the moment, as you say, we can only probe whether
an experimental outcome is caused by a single history or by multiple histories of an atom,
a molecule, I mean you in your talks you get wonderful example of very large objects
from an atomic point of view exhibiting interference or existing in more than one instance.
When we have quantum computers we will be able to have very large, very complex entities
existing in superpositions. So in principle I suggested a long ago before this was remotely
on the cards experimentally that if we had a quantum computer on which an artificial intelligence
program was running, say with a human level artificial intelligence, then this entity would be able
to experience interference in its own consciousness.
Well some people would say that your consciousness would collapse your reality in a certain point.
So if that happened that would refute the average interpretation or as I would say would refute
quantum theory. And that would be a very interesting problem and that's one of the reasons why
scaling up both the size and the complexity and the mass of phenomena that are experimentally
observed but can only be explained by quantum theory is very important.
And we just need to close the gap between that and the AI because the AI would not be having
this conversation or at least the AI would not be able to make the argument that you just made.
It would have to say I've only got evidence of many worlds on the scale of my mind but not bigger.
Yeah but there's something in the formulation of phrasing of our sentences where I'm getting
also dealt with it is when I talk about these things about superpositions I always make these
quote unquote when I say a particle is at the same time here and there because there are two
words that I don't understand three words that I don't understand. First the word is so reality.
The second what does time really mean and the third one what does space really mean and we don't
have any experimental evidence that the particle is at the same time here and there we just have
the physical description the quantum mechanical description that the way function behaves as if
and how can we make the step to the many worlds then?
I think we have something slightly more than that. Again you come from the theory but I think
prior to the theory we have the experience that this thing cannot be explained by single
trajectories. We don't have to believe quantum mechanics to see that so we rule out single
trajectory explanations and that we have before we have quantum mechanics. If we didn't have
quantum mechanics it would be a mystery. We would say there simply is no explanation.
Well hypothetically could this maybe be explained by some very weird folding of space time in
this case so not really making another multi-verse or another branch of the multi-verse but really a
new 3d or 40s space time or another 11 general space time. Could that jump through a shortcut
through another higher dimension so to say from there to the other? Yes it's the answer. Calling the
multi-verse many universes is a bit of a misnomer because the whole point of it is interference
many universe parallel universes would indicate separate universes or these universes that our
universe splits into two every time a quantum event happens whereas actually it's only the
electron or the photon or something that's splitting. Now if this can be explained in another way
then by quantum theory then another thing we know from just you know without theory with just
elementary reasoning about the experiments is that this other thing has to be immensely complicated.
It's sort of two to the power of the complexity of what we see and again when we have fully
fledged quantum computers we will have computations going on whose results cannot be explained by
any history of the computer that has it single valued so when that number is greater than the
number of atoms in the universe which will easily be attained as soon as we have quantum computers.
So yes there could be another explanation in terms of folded universes or other dimensions
or what but those things would have to be as complicated as the many universes and as some
people were saying that the other ever a conference these things would contain
other they would contain other instances of people or they would they would contain things
whose shapes or other instances of people and so on and other instances of the quantum computer
which would be interacting with our instance and so on. So it's really a matter of
terminology then whether you call that a multiverse or parallel universes or a much higher
dimensional reality than in classical concept. And in your multiverse concept
well there again zillions of branches and hypothetically you could also be here and in this other
part of the multiverse you could be in a different place, different time, different internal
state. What does that tell us about your identity? Does that affect anything about how you feel
as a human sort to say? It certainly has to be taken into account but it's logically
it's the same issue as am I the same person that I was 10 years ago that I will be in 10 years
time. If we go back to when I was a baby I certainly was not the same person as I am today
there is a continuity between me then and me now but that doesn't mean I'm the same person
because a car person I have changed drastically but the relationship between the past and the
present is one kind of thing the relationship between the present and future is a different kind
of thing the relationship between one branch ever at branch and another is a different kind
of thing but in all cases what makes us say that those things are real is the explanations that
we have for here and now. Although other scientists would say shut up and measure for the
calculator actually don't talk about things that you cannot see and the other part of the
other branches you don't see so by to you they are not too shut up. Yes well first of all I think
that that attitude involves saying that there are certain questions about reality that you're not
allowed to ask. You're allowed to ask how the experiment was prepared, you're allowed to ask what
will the results be. You're not allowed to ask how were the results brought to bounds by the
preparation so that's what's the therefore it's not an explanation in my terms but and as for
shut up that's really another way of trying to evade the consequences in terms of reality like
my favorite example is of dinosaurs in the past so there are people who say nobody ever saw
dinosaur nobody ever will and therefore it's just a frivolity to say that they really exist
that most we can say fossils behave as though dinosaurs existed but no paleontologist would
accept talking that way even though there is no experimental way of disproving that that
manual speaking so and that's because paleontologists are only interested in paleontology
because they want to know what really happened not they're not you know if they were
so interested in fossils they would be geologists.