”. Now for some background, Lubos Motl is the author of the well known blog:
where he is not shy to call out the “the Emperor is naked” in a most politically incorrect fashion. Now on quantum mechanics I sometimes found Lubos’ opinions out of touch or bizarre (like in supporting EPR=ER: forget publishing, just think if you would even manage to upload such a paper on the archive without risking reclassification to General Physics if you were not already a famous physicist). Still, his quantum opinions do echo the sentiment of most physicists not working in quantum foundations and there is a genuine gap of understanding between the community of physicists at large and the minority working in making new sense of quantum mechanics. Matt agreed to answer a few questions on his position, motivation, and ideas and I hope this will help bridge this gap.
So Matt, let’s start by clearing the air. You have found a
way to make a ton of money and get super filthy rich by selling the T-shirt in
the picture above. How many T-shirts have you sold?
As of today, 4. But let's get one thing straight, I
get only $2CAD for each t-shirt sold, and I have promised to donate my
commission to the Next Einstein Initiative of the African Institute for
Mathematical Sciences, so that means they'll get $8CAD so far. We can do
better than that. Please remember that if you live in a cold climate you
can also buy anti-quantum zealot hoodies, or you can buy a mug, or a onsie for
your little one. These are all perfect presents for the quantum
foundationalist in your life.
You stated that you did not like any existing
interpretations. (For the record and a bit of shameless self-promotion, I do
not like any of them either and that it is why I work on my own interpretation.
The correct one which will take over the physics world ;) ) What is wrong
with good, old fashion Copenhagen?
Or with modern Copenhagen like
consistent histories?
This is a complicated question because there is no one
single Copenhagen interpretation.
Some people call the type of interpretation one usually finds in textbooks by
the name "Copenhagen".
This is very different from the views of Bohr, Heisenberg et. al., i.e. the
interpretation that actually comes from Copenhagen.
I prefer to call the textbook interpretation the "orthodox" or
"Dirac-von Neumann" interpretation because it derives from the famous
books of Dirac and von Neumann. To my mind, the orthodox interpretation
is simply inconsistent. It treats the quantum state as a property of the
quantum system, which evolves unitarily in the ordinary course of affairs, but
suddenly jumps to a new state when a measurement is made. The latter is
inconsistent with treating measurement as a unitary interaction between system
and measuring device. Unless one is prepared to accept measurement as a primitive,
i.e. to divide up those interactions that count as measurements from those that
don't in advance, this is a reducto ad absurdum for the orthodox
interpretation. To my mind, it is simply wrong, and obviously so.
As an aside, I think a lot of the confusion about the interpretation of quantum
theory actually comes from setting up all of the problems in opposition to the
orthodox interpretation. For example, a lot of people will tell you that
the main problem we have to solve is the measurement problem, but the
measurement problem is a problem with the orthodox interpretation of quantum
theory, not with the theory itself. You need to believe that the quantum
state is a complete and literal description of reality in order to even set it
up. I think we should instead start from a much more minimal view of the
meaning of quantum theory that is non-committal about the status of the quantum
state, i.e. just start from the predictions for experimental outcomes that we
all agree on, and use that as the starting point for discussion.
Moving on to real Copenhagen, which
is most strongly associated with the views of Bohr, I don't actually have too
much of a problem with this. I think that some of its modern variants,
like QBism, are perfectly consistent, but I just don't think they are
correct. A lot of people will tell you that Bohr's writings are far too
unclear to extract a unique interpretation from them, and that is true, but I
think we can extract one or two key ideas. Firstly, unlike the orthodox interpretation,
the quantum state is not supposed to be a direct representation of reality in Copenhagen.
As Bohr says, it concerns not reality itself, but rather "what we can say
about Nature". This is a clear statement of a psi-epistemic
position.
The other important aspect of Copenhagen
is a split between the microphysical world, which we are to describe using
quantum mechanics, and the "classical" world, which we are to
describe using the concepts of classical physics. Some people seem to
think that this posits a definite cut that we have to put at a definite scale
somewhere between the micro- and macroscopic. However, in Copenhagen
it is clear that this cut is not supposed to have any definite location.
If you are concerned about whether a given physical system should be put on the
classical or quantum side, perhaps because you are uncertain about whether
quantum coherence plays a role in its operation, then Copenhagen
advises you to put it on the quantum side. In fact, you can, in
principle, move the cut as far up the chain as you like, putting more and more
things on the quantum side as needed, although in practice one does not have to
go too far up the chain to describe most real world experiments. The only
thing that Copenhagen insists on is
that the cut needs to be put somewhere. This is not because there are any
physical systems that are "fundamentally classical" and cannot be
described by quantum theory, but rather because the quantum formalism is not a
literal description of reality, and hence there must be some classical systems
around for its predictions to refer to, i.e. measuring devices and the
like. Bohr sometimes talks about the necessity of describing these
systems according to classical physics, i.e. Newtonian physics complete with positions
and velocities and the like, but elsewhere he only emphasizes the need to talk
about them in "ordinary language". I interpret this as meaning
that the "classical" systems must have unambiguous observable
properties that we can communicate to one another, i.e. things like pointers on
measuring devices pointing to specific readings, and we must assume that these
are objective properties of the world. This is more important than
positing that they obey exactly the equations of classical physics.
Read like this, I think Copenhagen
is fairly consistent. It needs a few refinements to properly deal with
experiments like Winger's friend, but I think the modern variants like QBism
can deal with that. I also think that the Copenhagen
advice on the moveable cut is pretty good advice for the practising physicist,
i.e. to put it as high as necessary and no higher, and we now have quantitative
tools like decoherence theory to help us decide exactly where it should
go. The main objection I have to Copenhagen
is that it does not seem to offer any advantages over a minimal statistical
interpretation in which we accept the predictions of quantum theory as given,
but are more non-committal about what it says about reality. I think that
would be less confusing for the practising physicist. Copenhagen
involves a lot of metaphysical claims in addition to this, e.g. claims that
certain questions are necessarily meaningless and that it is necessarily
impossible to achieve a deeper description of reality. There was no good
evidence for these claims at the time that Copenhagen
was first proposed, and it stalled investigation of these issues for many
decades. Perhaps, one could argue, that no-go theorems like Bell,
Kochen-Specker and PBR now provide some
evidence, but the Copenhagenists were willing to make these claims far before
we had such evidence and tried to shut down the avenues of inquiry that led to
these results. Quantum theory has always been beset by the problem of
quantum jumps, by which I mean that quantum physicists are always jumping to
conclusions, so I think we should try to avoid this, above all else.
The other thing I dislike about Copenhagen
is that it does not seem to tell specifically on quantum theory. By this
I mean that, if we had any physical theory at all and we were confused about
how it should be interpreted, then, so long as the theory made definite
predictions for the outcomes of experiments, we could always do a Copenhagen
job on its interpretation. I think one of the jobs of a good interpretation
is to uncover the explanatory structure of the theory, and that that this
should be useful for generalizing the theory beyond its current scope,. Copenhagen
seems to do a rather poor job of this. Something Copenhagen-like can
always be used as a fall-back position though.
Regarding consistent histories, it is a bit inaccurate to lump it in with Copenhagen
(at least I'll have to deal with another long email from Bob Griffith if I do
so again). I think Omnes looks at it this way, but Griffith
wants to view it as a realist interpretation, just with the
"single-reality" criterion thrown out. It is more difficult to
tell what Gell Mann and Hartle intend, particularly as they keep revising their
interpretation by adding exotic probabilities and such like. Furthermore,
in Saunders-Wallace many-worlds, the consistent histories formalism is used to
define what they mean by "worlds", so we could also think of it as a
type of many-worlds theory. Nonetheless, what we have is a broad class of
interpretations, based on a histories formalism and using the decoherence
conditions to decide when we have "classical" worlds to which
ordinary probabilities can be assigned.
The main problem I have with consistent histories is that I think it is ill
founded. In standard quantum theory, if we prepare a system in some state
and make a sequence of measurements on it then we get a formula for the
probability of the outcomes. Consistent histories takes this formula and
says that it applies even if we don't actually make the measurements (providing
the consistency conditions are satisfied), where now we are to think of the
projectors as representing properties of the unobserved system rather than
measurement outcomes. This is totally bananas, or at least an example of the
type of jumping to conclusions that I would like to avoid. If there is
one thing that we know about quantum measurements it is that they are not mere
passive observations of the system. Therefore, what justifies taking a
formula that applies to a necessarily invasive process and saying that it
applies even without that process? Doing so leads to some pretty bizarre
assignments of conditional probabilities, such as in the Aharonov-Vaidman
three-box paradox, where the consistent historian is forced to say that there
is a consistent set of histories in which the ball in definitely in box 1,
another in which it is definitely in box 2, but this is OK because the two sets
of histories have no common refinement, so their predictions should not be
combined. But such effects also crop up in classical models in which
measurement causes disturbance, in which they have a perfectly straightforward
explanation, i.e. the distrubance caused by the measurement can affect the
probability of the later postselection. If you applied a
consistent-histories like formalism to these classical models they would imply
a similar split into two incomparable but contradictory sets of histories,
which is clearly nuts, bananas, and whatever other combination of fruits and
vegetables you care to supply. So this, in short, is why I don't like
consistent histories.
The ontic camp of quantum mechanics interpretation justify
their position from Bell and his
opposition against measurement and seek to construct an observer-independent consistent
narrative of quantum mechanics. Why are you not in the ontic camp?
I am not sure here if you mean the psi-ontic camp, or the
realist camp in general. If it's the former, then none of these arguments
tell specifically on the psi-ontic/psi-epistemic distinction, so that's why we
needed theorems specifically targeted at that.
I am in the realist camp to a large degree, but I am not prepared to accept an
interpretation of quantum theory just because it has a well defined ontology,
if I think it has a lot of other problems. To my mind, de Broglie-Bohm,
collapse theories, and many-worlds all fall in this category, but I'm not going
to engage in a take-down of each one as that would take too long. I have
already ranted against consistent histories and I think a rant against one
interpretation per interview is probably enough.
I understand that the epistemic position appeals to you, but
you consider yourself a realist (and so you are a “standard” psi-epistemist).
Here is a hardball question. Isn’t this position discredited by the PBR
theorem? If not, is psi-epistemic position falsifiable? Is it real science, or
is it something like astrology?
One could equally argue that Bell's
theorem and the like discredits hidden variable theories, i.e. if you are
committed to locality then you need to come up with a more exotic type of
ontology or go neo-Copenhagen. The same is true of the psi-epistemic
position. If you are really committed to it then there are lots of things
still to try, such as retrocausality, relationalism, and many-worlds.
Given that all of these have already been proposed as responses to Bell's
theorem, I don't see that PBR poses an
especially new threat here. In fact, the idea that we are looking for a
psi-epistemic theory places new constraints on what these theories must look
like, so it might actually help in the search for a viable ontology.
No scientific idea is ever falsified on its own, but rather along with a
variety of other assumptions about the theoretical framework and the working of
experimental apparatus. One always has a choice about which to throw out
in the face of new evidence. I would argue that the ontological models
framework in which PBR was proved was
already on sketchy grounds due to the previous no-go theorems like Bell.
Therefore, it only represents a starting point on investigating the
issue. It may turn out that all of the proposed alternatives have their
own difficulties, or that we can prove psi-ontology within some reasonably
well-defined class of them. If so, I think the evidence will be strong
enough that I'd have to go psi-ontic or neo-Copenhagen (but, as I argued, Copenhagen
is an unfalsifiable idea if ever there was one, so do you want to call that
unscientific too?). I am not sure, at present, whether my realist
sympathies are stronger than my psi-epistemic ones, but I don't think I have to
make that decision just yet.
Not all ideas that are useful to science are directly falsifiable.
Instead, there are a pool of ideas and principles that get mixed together into
our theory construction. Some of them turn out to be important to the
future of science, and some of them turn out to be dispensable and get
jettisoned somewhere along the way. It remains to be seen what becomes of
the psi-ontic/psi-epistemic distinction, but it is far from astrology as it has
already led to rigorous theoretical results and experiments.
What is the difference between an anti-quantum zealot and a
crackpot?
Both of these are fairly difficult to define. An
anti-quantum zealot is a person that Lubos has decided to call an anti-quantum
zealot. Generally speaking, this will be anyone who promotes an
interpretation of quantum theory that is deemed "classical" by Lubo's
lights, which would include things like de Broglie-Bohm theory and spontaneous
collapse theories. It doesn't include all realist theories, as
many-worlders tend to be called idiots instead. It also seems to include
people, such as myself, who work on theorems about ontological models for
quantum theory, even if we do not actually believe these models are good
descriptions of reality, but are rather trying to investigate the differences
between quantum and classical, or using them as a relatively well-defined
starting point for something else. However, this is relatively
inconsistently applied by Lubos as, for example,
PBR
were not called anti-quantum zealots by him.
For crackpots I can do no better than John Baez
http://math.ucr.edu/home/baez/crackpot.html
Of course, some anti-quantum zealots are crackpots and vice versa, but
generally they are incomparable sets.
What is realism?
Broadly speaking, scientific realism is the idea that there
is a physical world that objectively exists and is independent of us, and that
the job of science is to attempt to describe it. The word
"attempt" is key here as, of course, we only have access to reality
indirectly via our measurements and sense impressions, and history has shown
that we often do a bad job of converting those into a picture of what reality
is like. Nonetheless, the realist asserts that our best physical theories
provide a better picture of the world than anything else we have, so we are
better off believing that the entities it posits really exist than we are not
doing so. So, for example, if the standard model posits entities like
quarks which we cannot directly observe, then thinking that quarks actually
exist is more accurate than thinking they don't.
This is in contrast with anti-realist positions, which only accept the reality
of what can directly be verified, and view all other entities as mere
theoretical constructs, ultimately to be analysed in terms of things that can
be directly observed.
Put this way, I believe that realism is a position that few physicists would
deny. However, in the specific context of quantum theory it often gets
conflated with narrower ideas, such as the idea that all observables must have
definite values all of the time, or that a model must be formulated within the
standard hidden variables/ontological models framework to be called
"realist". Given this, it is no surprise that a lot of
physicists call themselves anti-realists because they have these stronger ideas
in mind. With the proper understanding though, I think that most
physicists are probably realists.
What is epistemic?
Broadly speaking, anything that refers to knowledge is
epistemic. However, what we are trying to get at with the
psi-ontic/psi-epistemic distinction is the distinction between something that
is an intrinsic property of an individual system verses something that is
not. The archetypal example of the latter is a probability
distribution. Whatever your favourite interpretation of probability is,
there is still a distinction between probabilities and intrinsic
properties. A probability must be defined with respect to relative
frequencies, rational beliefs, conditions surrounding an experiment, or
something like that. Whether you call that "epistemic" or not
does not really matter, e.g. you would do if you were a Bayesian, but you may
prefer "statistical" if you are a frequentist.
People often get lost in the terminology, and the specific reference to
"epistemic" or "knowledge" can be misleading. What is
at stake is whether a quantum state is in closer analogy to a probability
distribution, or an intrinsic property like a phase-space point.
What is your intuition about quantum mechanics?
The strongest intuition I have about quantum theory (and
note that I deliberately eschew the term "mechanics" here) is that it
is best understood as a kind of nonclassical probability theory. This
view is extremely powerful and useful in many areas of physics. For
example, in quantum information and computation, if you want to understand how
classical protocols get generalized to quantum ones then probability
distributions become quantum states, stochastic maps become quantum channels,
etc. Additionally, quantum probability theory based on operator algebras
has been very successful in understanding statistical mechanics. Another
example is the classical limit of quantum theory, which is best understood as a
Liouville limit, where quantum states are used to derive probability
distributions over phase space obeying the Liouville equation, rather than the
Newtonian limit with definite trajectories. Finally, if you try to define
quantum chaos thinking that quantum states are like points in phase space you
will get very confused. Generalizing the classical definitions of chaos in
terms of probabilities, e.g. the entropic definitions, works much better.
I could go on. There are dozens more examples.
Given all this, it would be very puzzling if the quantum state were not
something more like a probability distribution than a state of reality.
That would make it a miracle that these probability based generalizations work
so way. Ultimately, I think this is where my psi-epistemic convictions
come from.
You are in an elevator with Edward Witten and he asks you to
give him the “elevator pitch” about your approach. What do you say?
Witten was visiting Perimeter at the same time we were
chatting away at the New Directions conference, so the course of events that
led to this interview actually prevented this from actually happening (not that
Witten would bother talking to me anyway).
If I had to emphasize one thing it would be that it is possible to make
progress in quantum foundations. It is not all about wishy-washy
discussions that never lead anywhere, but we can actually turn these debates
into precise questions that get resolved by rigorous argument and experiment,
just like in the rest of physics. Bell's
theorem is the best example of this, and it has taken us a long time to realize
we can investigate other aspects of quantum theory in a similar way, but we are
now doing this.
The elevator breaks down and you now have the attention of
Edward Witten for much longer time. How do you elaborate on your prior points.
To be honest, if I have a long time to spend with Witten, I
would be more likely to ask him about his work and what he finds interesting
than to go spouting on about quantum foundations. I may feel like quantum
foundations is very important for the future of physics, but that does not mean
that Witten's insights on quantum field theory and its connection to
mathematics are not even more important. So I feel that the best use of
the time would be for me to get all of the insights I can from him rather than
the other way round.
David Albert has already spent an afternoon discussing quantum foundations with
Witten. I think they talked
about Bell's theorem, the
measurement problem, many-worlds, and perhaps a few other things. Albert
told me that Witten said it was
refreshing and amusing to discuss these things, so he wasn't completely
anti-foundations, but he's not likely to drop what he's doing in favour of
foundations. That would be ridiculous. He is already very
successful with his own research agenda.
However, if I did have the opportunity to discuss one thing with any
non-foundational physicist, it would be Bell's
theorem, as it is our best example of progress. Most physicists know of
it, and maybe also know a proof, but they don't understand it well, or what its
applications are.
What is your approach on quantum foundations?
Today there are many quantum interpretations and no
single one manage to win universal acceptance. What does it take for a new
interpretation to be accepted by everyone?
I'm going to answer these two questions together because they are closely
related.
The slogan for my approach is my repeated ad nauseum joke about Schrödinger's
quantum jumps --- the problem of quantum jumps is that quantum theorists are
always jumping to conclusions. This obviously applies to the old
Copenhagen
hegemony, where people were prepared to say quite outlandish things about
quantum theory with little evidence, but it is also meant to apply to the
modern debates.
A typical history of a quantum foundations researcher up to the late 90's goes
something as follows. When they learned quantum theory at university,
they were confused. They were told all sorts of outlandish things about
the theory that did not seem to be supported by the evidence, and furthermore
their instructor shut down any attempt to inquire further about the foundations.
Then, at some point in their career they encountered an obscure approach to
quantum theory that did seem to make sense, be it Bohmian mechanics,
spontaneous collapse theories, many-worlds, etc. They then decided to
work on that approach and faced continual challenges for doing so. Maybe
it was hard to get a job, hard to get published, and they certainly encountered
bad arguments as to why their approach was completely and obviously
wrong. In this climate, it is only natural that such a person would
become a staunch defender of their theory, to the exclusion of almost anything
else, and develop a very aggressive attitude in arguing for their approach.
The story I have just told is a bit of a cartoon, but I think it explains some
of the sociology of the field. Namely, the traditional approach has been
to grab onto one very specific approach to the exclusion of everything else and
defend it to the hilt. Many of the people who do this are still around
and I do not want to criticize them too much. They were the torch-bearers
for the idea that thinking about foundations is a fruitful activity in an
environment where most people could not care less, and the modern field would
not exist without them. Nevertheless, I think we can now afford to step
back and critically assess what has been done so far, and hopefully come up
with new ideas that have a chance of leading to progress.
Overall then, I want to make a plea for more open mindedness in the foundations
of quantum theory, but we should not be "so open minded that our brains
fall out" (see
http://www.skeptic.com/insight/open-mind-brains-fall-out-maxim-adage-aphorism/ for
the origins of this quote). This means that we need to adopt a critical
attitude, properly weigh the evidence, make rigorous arguments, and be
absolutely clear about what we are trying to do.
One thing we should not be trying to do, at this point in time, is to solve the
measurement problem. As I said earlier, the measurement problem is really
a problem with the orthodox interpretation of quantum theory, and not with
quantum theory per se, and in any case we now have at least half a dozen
solutions to it. The fact that none of these alternative interpretations
of quantum theory has caught on as the mainstream view should give us some
pause for thought as to whether they are really going in the right direction.
As an aside, I recently listened to an FQXi podcast
http://fqxi.org/community/podcast/2015.04.18 in
which Jean Bricmont, an old-school quantum foundations researcher if ever there
was one, described the reasons that he thinks Bohmian mechanics has not taken
on as the mainstream view. His reasons are entirely sociological, having
to do with the
Copenhagen hegemony
and the irrational refusal of most physicists to entertain alternative
ideas. I will admit that, in the general population of physicists, one
more often encounters bad arguments for not accepting alternative
interpretations than good ones. For example, you will hear that
Bell
and/or von Neumann already proved the impossibility of theories like Bohmian
mechanics, or that its nonlocality means that it necessarily cannot be
generalized to relativistic field theory. This is a relic of the fact
that most physicists are still not that well educated in foundations. But
come on dude! There is a whole community of researchers in the
foundations of quantum theory, albeit a comparatively small one, who have
dedicated their careers to properly understanding quantum theory. These
people have thought about these matters much more deeply than most physicists,
and yet Bohmian mechanics has not gained uniform acceptance even within this
community. Furthermore, if Bohmian mechanics were really the correct view
of quantum theory --- if it really helped one to think as clearly as possible
about the meaning and application of the concepts of the theory --- then it
would have already proved essential to the future progress of physics and have
been accepted. The general physics community, whilst stubborn and skeptical
about non mainstream ideas, is not the socially dominated festival of cultural
relativism that Bricmont appears to think it is (ironically so for the
co-author of "Fashionable Nonsense"). Fruitful ideas only need
to be accepted initially by a small number of people. If they are
genuinely useful then the rest of the community will eventually see massive
progress being made and adopt them, perhaps slowly over a long period of time,
but they will gain acceptance eventually.
I think that last aside has already revealed one of my prejudices about
foundational enquiry. I do not want to make sweeping statements about the
nature of truth in general, but one thing that a scientific truth ought to have
is some pragmatic value. There may be a deeper notion of truth as well,
but in order to call something "scientific" it has to have some sort
of pragmatic utility. I define pragmatic utility quite broadly: it could
mean making a different prediction from existing theories that is later
confirmed, it could mean being essential to theory construction, or it could
just mean a helpful way of thinking that makes it far easier to derive some
result then it otherwise would have been. I am sure there are some other
things I have not thought of that could be included as well.
To make this point clearer, let's look at a non-quantum example that has this
kind of pragmatic value. In the foundations of probability, there are
various points of view including frequentism and various subjective/Bayesian
views. The frequentist view (as well as Popper's falsificationism) was a
heavy influence on classical statistics. The subjective view is a big
influence on Bayesian statistics. Whilst it is not impossible to pursue
either of these statistical methodologies independently of the foundations of
probability, foundational thinking continues to inspire new statistical
methodologies, which then go on to successful use in practical
applications. It would be fairly difficult to come up with such
methodologies and appreciate when and why they work work without some
understanding of the foundations. Further, the ubiquity of Bayesian
methodology lends at least some credence to subjective foundations, even if it
does not pin them down uniquely. Nobody can really defend the idea that
there is no truth to the subjective approach, even if they posit that some more
objective notion of probability is needed in addition.
It is this type of indispensability that I want for the foundations of quantum
theory. I believe that, in this sense, there is a correct foundation for
the theory and we will know when we find it via its vast array of successful
applications. For this reason, I reject the traditional distinction
between the practical aspects of quantum theory and its interpretation.
If we rope off the latter as its own independent activity then it will become
stale and drift further from the (scientific) truth as we only know that a
foundational idea is true through its applications. So, to answer your
second question, if an idea does have such an impact, then it will win
universal acceptance.
That said, I agree with Shelly Goldstein when he says we have to be clear on
what the theory is about. It is not just "anything goes".
There is no point in conducting a foundational investigation by merely futzing
around with equations. Leave that to the non-foundational
physicists. The point of foundational investigations is to achieve
clarity, not to muddy the waters even more. Shelly intends his point to
mean that we must start our investigation with a clear ontology, i.e. a clear
statement of what exists in the world and how it behaves. I agree that
this is our ultimate aim, but I disagree that this must be our starting
point. To me, operational ideas are also perfectly clear. Once we
have decided which systems we are going to call measurement devices,
preparation devices, etc. then it is perfectly clear what you are talking
about, and so perfectly fine to use that terminology to develop the
theory. So long as we are clear that we are only adopting an operational
*methodology* rather than adopting operationalism wholesale, and that we still
aim for ontological statements in the long run, there is no problem.
Operational methodology has proved so successful in the history of physics,
e.g. in thermodynamics and the development of both relativity and quantum
theory, that denying yourself these techniques would be a big handicap.
Now we are getting to the point where I can outline the kind of work I think is
promising. In much foundational work, we take the axioms of quantum
theory as laid down by von Neumann as gospel and only try to find an ontology
behind them. In contrast, I think that first reformulating the theory in
various ways will give us a better target to shoot at. Historically, the
same sort of thing happened in thermodynamics and statistical mechanics.
The original formulation of the second law directly in terms of the properties
of heat engines is pretty hard to derive from Newtonian mechanics +
probability, but once entropy is introduced into thermodynamics it has a clear
microphysical counterpart and the derivations can proceed much more
easily. Similarly, I think that reformulating quantum theory will lead to
new insights that make it much clearer which of our current interpretations are
ill-founded and need to be ditched, and I think the answer is probably all of
them.
The project of not treating standard quantum theory as a fixed target has
already been tremendously successful. Take, for example, the generalized
measurement theory of POVMs and quantum instruments, or the theory of
continuous quantum measurements. Most of quantum information theory would
be impossible without this and it is furthermore apparent that most
measurements we do are of the latter type. For example, when I look at
the tree outside my window, I am not doing a projective measurement on it, but
rather observing a some photons that are correlated comparatively weakly with
the properties of the tree. It is a rather noisy POVM rather than a
projective measurement. This means that if I am going to explain the
appearance of the classical world, i.e. why trees look like trees, it is going
to be in terms of generalized measurements rather than projective ones.
This is an important foundational insight that you would not get if you were
myopically focussed on solving the measurement problem within the standard
formalism.
In the future, I think that similarly important insights will come from playing
around with the causal assumptions of the operational approach. In the
usual approach, there is a quantum state, determined by a preparation, which
evolves forward in time, is subsequently measured and then collapses.
This makes it look like measurement is time-asymmetric. However, one can
alternatively formulate the measurement in a retrodictive formalism in which
everything goes in the opposite time direction, which shows that things are in
fact time-symmetric. It is only the direction of inference that makes
things look asymmetric, i.e. the fact that we asked a question about the future
based on knowledge about the past rather than the other way round. This
insight makes approaches that posit a time asymmetry due to measurement look a
bit suspicious, e.g. spontaneous collapse theories. Similarly, I think we
can make progress by not putting in causal structure by hand in advance but
simply saying that I have a bunch of variables I am going to treat classically,
which may be settings of preparation or measurement devices or measurement
outcomes but we are not going to say which they are in advance, and asking what
is the most general way that quantum theory says they can be correlated.
This is one of the things I and others are working on at the moment, and I
think it has the potential to yield a lot of foundational insights. For
example, I think it will make the primacy of unitary evolution look silly, and
hence the many-worlds picture may look less plausible.
That is just a flavour of the type of approach I favour. I could go on
much longer about other ideas, but perhaps that is enough for your readers for
now.
I asked you in the past to help classify my position and we
agreed I am a neo-Copenhagen (distinct from the other new-Copenhagen). Upon
further introspection my interpretation is both observer free and beable free
and therefore I do not fit in either the ontic or the epistemic camp. Moreover
I just heard Sheldon Goldstein this weekend stating that to be observer free
you must have beables. So one of us is wrong. If you were to bet one dollar on
me vs. Sheldon, how would bet?
Depends what the odds are. To be realist in any
conventional sense there has to be something that really exists out there and
you have to say what that is. I think this is what Shelly means by a
"beable" in this context and I agree with him. That is just the
meaning of conventional realism.
However, there are all sorts of subtle philosophical distinctions between
different kinds of realism, and if you try hard enough I am sure you can find a
sufficiently weak version of realism to call yourself a realist. I doubt
that such subtle distinctions have any relevance to quantum theory though.
PS: I want to thank Matt for all his answers. Initially I wanted to break up this interview into several parts, but it all makes sense much better together. If Lubos cares to reply, this blog is open to him (and oh boy-do I have cheeky questions for him?) .
