SCIENCE IS NOT
ABOUT CERTAINTY: A PHILOSOPHY OF PHYSICS
A Conversation with Carlo Rovelli [5.30.12]
I seem to be saying two things that
contradict each other. On the one hand, we trust scientific knowledge, on the
other hand, we are always ready to modify in-depth part of our conceptual
structure about the world. But there is no contradiction, because the idea of a
contradiction comes from what I see as the deepest misunderstanding about
science: the idea that science is about certainty.
Introduction
by Lee Smolin
by Lee Smolin
Carlo Rovelli is a leading contributor to
quantum gravity, who is also made influential proposals regarding the
foundation of quantum mechanics and the nature of time. Shortly after receiving
his Ph.D he did work which made him regarded as one of the three founders of
the approach to quantum gravity called loop quantum gravity-the other two being
Abhay Ashtekar and Lee Smolin. Over the last 25 years he has made numerous
contributions to the field, the most important of which developed the spacetime
approach to quantum gravity called spin foam models.These have culminated over
the last five years in a series of discoveries which give strong evidence that
loop quantum gravity provides a consistent and and plausible quantum theory of
gravity.
Rovelli's textbook, Quantum
Gravity has been the
main introduction to the field since its publication in 2004, and his research
group in Marseille has been a major center for incubating and developing new
talent in the field in Europe.Carlo Rovelli's approach to the foundations of
quantum mechanics is called relational quantum theory, he also, with the
mathematician Alain Connes, proposed a mechanism by which time could emerge
from a timeless world called the thermal time hypothesis.
–
Lee Smolin
CARLO ROVELLI is a theoretical physicist,
working on quantum gravity and on foundations of spacetime physics. He is
professor of physics at the University of the Mediterranean in Marseille,
France and member of the Intitut Universitaire de France. He is the author
of The First Scientist: Anaximander
and His Legacy; and Quantum
Gravity.
SCIENCE IS NOT ABOUT CERTAINTY: A
PHILOSOPHY OF PHYSICS
[CARLO ROVELLI:] We teach our students: we
say that we have some theories about science. Science is about
hypothetico-deductive methods, we have observations, we have data, data require
to be organized in theories. So then we have theories. These theories are
suggested or produced from the data somehow, then checked in terms of the data.
Then time passes, we have more data, theories evolve, we throw away a theory,
and we find another theory which is better, a better understanding of the data,
and so on and so forth.
This is a standard idea of how science
works, which implies that science is about empirical content, the true
interesting relevant content of science is its empirical content. Since theories
change, the empirical content is the solid part of what science is. Now,
there's something disturbing, for me as a theoretical scientist, in all this. I
feel that something is missing. Something of the story is missing. I've been
asking to myself what is this thing missing? I'm not sure I have the answer,
but I want to present some ideas on something else which science is.
This is particularly relevant today in
science, and particularly in physics, because if I'm allowed to be polemical,
in my field, in fundamental theoretical physics, it is 30 years that we fail.
There hasn't been a major success in theoretical physics in the last few
decades, after the standard model, somehow. Of course there are ideas. These
ideas might turn out to be right. Loop quantum gravity might turn out to be
right, or not. String theory might turn out to be right, or not. But we don't
know, and for the moment, nature has not said yes in any sense.
I suspect that this might be in part because
of the wrong ideas we have about science, and because methodologically we are
doing something wrong, at least in theoretical physics, and perhaps also in
other sciences.
Let me tell you a story to explain what I
mean. The story is an old story about my latest, greatest passion outside
theoretical physics: an ancient scientist, or so I would say, even if often je
is called a philosopher: Anaximander. I am fascinated by this character,
Anaximander. I went into understanding what he did, and to me he's a scientist.
He did something that is very typical of science, and which shows some aspect
of what science is. So what is the story with Anaximander? It's the following,
in brief:
Until him, all the civilizations of the
planet, everybody around the world, thought that the structure of the world
was: the sky over our heads and the earth under our feet. There's an up and a
down, heavy things fall from the up to the down, and that's reality. Reality is
oriented up and down, heaven's up and earth is down. Then comes Anaximander and
says: no, is something else. 'The earth is a finite body that floats in space,
without falling, and the sky is not just over our head; it is all around.'
How he gets it? Well obviously he looks at
the sky, you see things going around, the stars, the heavens, the moon, the
planets, everything moves around and keeps turning around us. It's sort of
reasonable to think that below us is nothing, so it seems simple to get to this
conclusion. Except that nobody else got to this conclusion. In centuries and
centuries of ancient civilizations, nobody got there. The Chinese didn't get
there until the 17th century, when Matteo Ricci andthe
Jesuits went to China and told them. In spite of centuries of Imperial
Astronomical Institute which was studying the sky. The Indians only learned
this when the Greeks arrived to tell them. The Africans, in America, in
Australia… nobody else got to this simple realization that the sky is not just
over our head, it's also under our feet. Why?
Because obviously it's easy to suggest that
the earth sort of floats in nothing, but then you have to answer the question:
why doesn't it fall? The genius of Anaximander was to answer this question. We
know his answer, from Aristotle, from other people. He doesn't answer this
question, in fact. He questions this question. He says why should it fall?
Things fall toward the earth. Why the earth itself should fall? In other words,
he realizes that the obvious generalization from every small heavy object
falling, to the earth itself falling, might be wrong. He proposes an
alternative, which is that objects fall towards the earth, which means that the
direction of falling changes around the earth.
This means that up and down become notions
relative to the earth. Which is rather simple to figure out for us now: we've
learned this idea. But if you think of the difficulty when we were children, to
understand how people in Sydney could live upside-down, clearly requires some
changing in something structural in our basic language in terms of which we
understand the world. In other words, up and down means something different
before and after Anaximander's revolution.
He understands something about reality,
essentially by changing something in the conceptual structure that we have in
grasping reality. In doing so, he is not doing a theory; he understands
something which in some precise sense is forever. It's some uncovered truth,
which to a large extent is a negative truth. He frees ourselves from prejudice,
a prejudice that was ingrained in the conceptual structure we had for thinking
about space.
Why I think this is interesting?
Because I think that this is what happens at every major step, at least
in physics; in fact, I think this is what happened at every step, even not
major. When I give a thesis to students, most of the time the problem I give
for a thesis is not solved. It's not solved because the solution of the
question, most of the time, is not solving in the question, it's just
questioning the question itself. Is realizing that in the way the problem was
formulated, there was some implicit prejudice assumption that was the one to be
dropped.
If this is so, the idea that we have data
and theories, and then we have a rational agent that constructs theories from
the data using his rationality, his mind, his intelligence, his conceptual
structure, and juggles theories and data, doesn't make any sense, because what
is being challenged at every step is not the theory, it's the conceptual
structure used in constructing theories and interpreting the data. In other
words, it's not changing theories that we go ahead, but changing the way we
think about the world.
The prototype of this way of thinking, I
think the example that makes it more clear, is Einstein's discovery of special
relativity. On the one hand there was Newtonian mechanics, which was extremely
successful with its empirical content. On te other hand there was Maxwell's
theory, with its empirical content, which was extremely successful, too.
But there was a contradiction between the two.
If Einstein had gone to school to learn what
science is, if he had read Kuhn, and the philosopher explaining what science
is, if he was any one of my colleagues today who are looking for a solution of
the big problem of physics today, what would he do?
He would say, okay, the empirical content is
the strong part of the theory. The idea in classical mechanics that velocity is
relative: forget about it. The Maxwell equations, forget about them. Because
this is a volatile part of our knowledge. The theories themselves have to be
changed, okay? What we keep solid is the data, and we modify the theory so that
it makes sense coherently, and coherently with the data.
That's not at all what Einstein does.
Einstein does the contrary. He takes the theories very seriously. He believes
the theory. He says, look, classical mechanics is so successful that when it
says that velocity is relative, we should take it seriously, and we should believe
it. And the Maxwell equations are so successful that we should believe the
Maxwell equations. He has so much trust in the theory itself, in the
qualitative content of the theory, that qualitative content that Kuhn says
changes all the time, that we learned not to take too seriously, and so much
faith in this, confidence in that, that he's ready to do what? To force
coherence between these two, the two theories, by challenging something
completely different, which is something that is in our head, which is how we
think about time.
He's changing something in common sense,
something about the elementary structure in terms of which we think of the
world, on the basis of the trust of the past results in physics. This is
exactly the opposite of what is done today in physics. If you read Physical
Review today, it's
all about theories that challenge completely and deeply the content of previous
theories: so theories in which there is no Lorentz invariance, which are not
relativistic, which are not general covariant, quantum mechanics might be
wrong…
Every physicist today is immediately ready
to say, okay, all of our past knowledge about the world is wrong. Let's
randomly pick some new idea. I suspect that this is not a small component of
the long-term lack of success of theoretical physics. You understand something
new about the world, either from new data that arrive, or from thinking deeply
on what we have already learned about the world. But thinking means also
accepting what we've learned, challenging what we think, and knowing that in
some of the things that we think, there may be something to modify and to
change.
What are then the aspects of doing science
that I think are under-evaluated, and should come up-front? First, science is
about constructing visions of the world, about rearranging our conceptual
structure, about creating new concepts which were not there before, and even
more, about changing, challenging the a-priori that we have. So it's nothing to
do about the assembly of data and the way of organizing the assembly of data.
It has everything to do about the way we think, and about our mental vision of
the world. Science is a process in which we keep exploring ways of thinking,
and changing our image of the world, our vision of the world, to find new ones
that work a little bit better.
In doing that, what we have learned in the
past is our main ingredient, especially the negative things we have learned. If
we have learned that the earth is not flat, there will be no theory in the
future in which the earth is 'flat.' If we have learned that the earth is not
at the center of the universe, that's forever. We're not going to go back on
this. If you have learned that simultaneity is relative, with Einstein, we're
not going back to absolute simultaneity, like many people think. This means
that when an experiment measures neutrinos going faster than light, we should
be very suspicious, and of course check and see whether there is something very
deep that is happening. But it is absurd that everybody jumps and says okay,
Einstein was wrong, just for a little anomaly that shows so. It never works
like that in science.
The past knowledge is always with us, and
it's our main ingredient for understanding. The theoretical ideas which are
based on 'let's imagine that this may happen because why not' are not taking us
anywhere.
I seem to be saying two things that
contradict each other. On the one hand, we trust the knowledge, and on the
other hand, we are always ready to modify in-depth part of our conceptual
structure about the world. There is no contradiction between the two, because
the idea of the contradiction comes from what I see as the deepest
misunderstanding about science, which is the idea that science is about
certainty.
Science is not about certainty. Science is
about finding the most reliable way of thinking, at the present level of
knowledge. Science is extremely reliable; it's not certain. In fact, not only
it's not certain, but it's the lack of certainty that grounds it. Scientific
ideas are credible not because they are sure, but because they are the ones
that have survived all the possible past critiques, and they are the most
credible because they were put on the table for everybody's criticism.
The very expression 'scientifically proven'
is a contradiction in terms. There is nothing that is scientifically proven.
The core of science is the deep awareness that we have wrong ideas, we have
prejudices. We have ingrained prejudices. In our conceptual structure for
grasping reality there might be something not appropriate, something we may
have to revise to understand better. So at any moment, we have a vision of
reality that is effective, it's good, it's the best we have found so far. It's
the most credible we have found so far, its mostly correct.
But at the same time it's not taken for
certain, and any element of it is a priori open for revision. Why do we have
this continuous…? On the one hand, we have this brain, and it has evolved for
millions of years. It has evolved for us, for basically running the savannah
and run after and eat deer and try not to be eaten by the lions. We have a
brain that is tuned to meters and hours, which is not particularly well-tuned
to think about atoms and galaxies. So we have to get out of that.
At the same time I think we have been
selected for going out of the forest, perhaps, going out of Africa, for being
as smart as possible, as animals that escape lions. This continuous effort that
is part of us to change our own way of thinking, to readapt, is a very part of
our nature. We are not changing our mind away from nature; it is our natural
history that continues to change that.
If I can make a final comment about this way
of thinking about science, or two final comments: One is that science is not
about the data. The empirical content of scientific theory is not what is
relevant. The data serves to suggest the theory, to confirm the theory, to
disconfirm the theory, to prove the theory wrong. But these are the tools that
we use. What interests us is the content of the theory. What interests us is
what the theory says about the world. General relativity says space-time is
curved. The data of general relativity are that Mercury perihelion moves 43
degrees per century, with respect to that computed with Newtonian mechanics.
Who cares? Who cares about these details? If
that was the content of general relativity, general relativity would be boring.
General relativity is interesting not because of its data, but because it tells
us that as far as we know today, the best way of conceptualizing space-time is
as a curved object. It gives us a better way of grasping reality than Newtonian
mechanics, because it tells us that there can be black holes, because it tells
us there's a Big Bang. This is the content of the scientific theory.
All living beings on earth have common
ancestors. This is a content of scientific theory, not the specific data used
to check the theory. So the focus of scientific thinking, I believe, should be
on the content of the theory, the past theory, the previous theories, try to
see what they hold concretely and what they suggest to us for changing in our
conceptual frame themselves.
The final consideration regards just one
comment about this understanding of science and this long conflict that has crossed
the centuries between scientific thinking and religious thinking. I think often
it is misunderstood. The question is, why can't we live happily together, and
why can't people pray to their gods and study the universe without this
continuous clash? I think that this continuous clash is a little bit
unavoidable, for the opposite reason from the one often presented. It's
unavoidable not because science pretends to know the answers. But it's the
other way around, because if scientific thinking is this, then it is a constant
reminder to ourselves that we don't know the answers.
In religious thinking, often this is
unacceptable. What is unacceptable is not a scientist that says I know, but
it's a scientist that says I don't know, and how could you know? Based, at
least in many religions, in some religions, or in some ways of being religious,
an idea that there should be truth that one can hold and not be questioned.
This way of thinking is naturally disturbed by a way of thinking which is based
on continuous revision, not of the theories, of even the core ground of the way
in which we think.
So summarizing, I think science is not about
data; it's not about the empirical content, about our vision of the world. It's
about overcoming our own ideas, and about going beyond common sense
continuously. Science is a continuous challenge of common sense, and the core
of science is not certainty, it's continuous uncertainty. I would even say the
joy of taking what we think, being aware that in everything we think, there are
probably still an enormous amount of prejudices and mistakes, and try to learn
to look a little bit larger, knowing that there is always a larger point of
view that we'll expect in the future.
We are very far from the final theory of the
world, in my field, in physics, I think extremely far. Every hope of saying,
well we are almost there, we've solved all the problems, is nonsense. And we
are very wrong when we discard the value of theories like quantum mechanics,
general relativity or special relativity, for that matter. And throw them away,
trying something else randomly. On the basis of what we know, we should learn
something more, and at the same time we should somehow take our vision for what
it is, a vision that is the best vision that we have, but then continuous
evolving the vision.
If this is science, if science works or in
part works in the way I've described, if this which I've described, it's some
relevant aspect of the way science works, this is strongly tied to the kind of
physics I do. The way I view the present situation in fundamental physics is
there are different problems in fundamental physics. One is the problem of
unification; it's providing a big theory of everything. The more specific
problem, which is a problem in which I work, is quantum gravity. Quantum
gravity means simply doing the quantum theory of gravity, how things fall, i.e.
the gravitational field.
It's a remarkable problem because of general
relativity; gravity is space-time; that's what we have learned with Einstein.
Doing quantum gravity means understanding what is quantum space-time. And
quantum space-time precisely requires some key change in the way we think about
space and time. Now, with respect to quantum gravity, in my opinion there are
two major research directions today. Which is the one in which I work, loops,
and strings. There are not just two different set of equations, but they are
based on different philosophies of science, in a sense.
The ones in which I work is very much based
on the philosophy I just described, and that's somehow what forced me to think
about the philosophy of science. Why? Because the idea is the following: the
best we know about space-time is what we know from general relativity. The best
we know about mechanics is what we know from quantum mechanics. There seems to
be a difficulty in attaching the two pieces of the puzzle together: turn them
around, and they don't fit well. But the difficulty might be in the way we face
the problem. The best information we have about the world is still contained in
these two theories, so let's take quantum mechanics as seriously as possible,
so believe it as much as possible. Maybe enlarging a little bit to make it
general relativistic, or whatever.
Let's take general relativity as serious as
possible. General relativity has peculiar features, specific symmetries,
specific characteristics. Let's try to understand them deeply and see wether as
they are, or maybe just a little bit enlarged, a little bit adapted, can fit
with quantum mechanics to give a theory. Even if the theory that comes out then
contradicts something that is the way we think.
That's the way quantum gravity, in the way
of the loops, the way I work in, and the way other people work in, is being
developed. This takes us in one specific direction of research, a set of
equations, a way of putting up the theory. String theory has gone in the
opposite direction. In a sense it says, well, let's not take too seriously
general relativity as an indication of how the universe works. Even quantum
mechanics has been questioned to some extent. Let's maybe imagine that quantum
mechanics has to be replaced by something very different. Let's try to guess
something completely new, which is some big theory out of which somehow the
same empirical content of general relativity and quantum mechanics comes out in
some limit.
I am distrustful of this huge ambition
because we don't have the tools to guess this immensite theory. String theory's
a beautiful theory. It might work, but I suspect it's not going to work. I
suspect it's not going to work because it's not sufficiently grounded in
everything we know so far about the world, and especially in what I think or
perceive as the main physical content of general relativity.
String theory's a big guesswork. I think
physics has never been a guesswork; it has been a way of unlearning how to
think about something, and learning about how to think a little bit different
by reading the novelty into the details of what we already know. Copernicus
didn't have any new data, any major new idea, he just took Ptolemy, in the
details of Ptolemy, and he read in the details of Ptolemy the fact that the
equants, the epicycles, the deferents were in certain proportions between them,
the way to look at the same construction from a slightly different perspective
and discover the earth is not the center of the universe.
Einstein, as I said, took seriously
Maxwell's theory and classical mechanics to get special relativity. So loop
quantum gravity is an attempt to do the same thing: take seriously general
relativity, take seriously quantum mechanics, and out of that, bring them
together, even if this means a theory where there's no time, no fundamental
time, so we have rethink the world without basic time. The theory, on the one
hand, is very conservative, because it's based on what we know. But it's
totally radical because it forces us to change something big in our way of
thinking.
String theorists think differently. They say
well, let’s go out to infinity, where somehow the full covariance of general
relativity is not there. There we know what is time, we know what is space,
because we're at asymptotic distances, at large distances. The theory's wilder,
more different, more new, but in my opinion, it's more based on the old
conceptual structure. It's attached to the old conceptual structure, and not
attached to the novel content of the theories that have proven empirically
successful. That's how my way of reading science matches with the specifics of
the research work that I do, and specifically of loop quantum gravity.
Of course we don't know. I want to be very
clear. I think that string theory's a great attempt to go ahead, done by great
people. My only polemical attitude with string theory is when I hear, but I
hear less and less now, when I hear 'oh, we know the solution already, certain
it's string theory.' That's certainly wrong and false. What is true is that
that's a good set of ideas; loop quantum gravity is another good set of ideas.
We have to wait and see which one of the theories turns out to work, and
ultimately to be empirically confirmed.
This may take me to another point, which is
should a scientist think about philosophy, or not? It's sort of the fashion
today to discard philosophy, to say now we have science, we don't need
philosophy. I find this attitude very naïve for two reasons. One is historical.
Just look back. Heisenberg would have never done quantum mechanics without
being full of philosophy. Einstein would have never done relativity without
having read all the philosophers and have a head full of philosophy. Galileo
would never have done what he had done without having a head full of Plato.
Newton thought of himself as a philosopher, and started by discussing this with
Descartes, and had strong philosophical ideas.
But even Maxwell, Boltzmann, I mean, all the
major steps of science in the past were done by people who were very aware of
methodological, fundamental, even metaphysical questions being posed. When
Heisenberg does quantum mechanics, he is in a completely philosophical mind. He
says in classical mechanics there's something philosophically wrong, there's
not enough emphasis on empiricism. It is exactly this philosophical reading of
him that allows him to construct this fantastically new physical theory,
scientific theory, which is quantum mechanics.
The divorce between this strict dialogue
between philosophers and scientists is very recent, and somehow it's after the
war, in the second half of the 20th century. It has worked
because in the first half of the 20thcentury, people were so smart.
Einstein and Heisenberg and Dirac and company put together relativity and
quantum theory and did all the conceptual work. The physics of the second half
of the century has been, in a sense, a physics of application of the great
ideas of the people of the '30s, of the Einsteins and the Heisenbergs.
When you want to apply thes ideas, when you
do atomic physics, you need less conceptual thinking. But now we are back to
the basics, in a sense. When we do quantum gravity it's not just application. I
think that the scientists who say I don't care about philosophy, it's not true
they don't care about philosophy, because they have a philosophy. They are
using a philosophy of science. They are applying a methodology. They have a
head full of ideas about what is the philosophy they're using; just they're not
aware of them, and they take them for granted, as if this was obvious and
clear. When it's far from obvious and clear. They are just taking a position
without knowing that there are many other possibilities around that might work
much better, and might be more interesting for them.
I think there is narrow-mindedness, if I
might say so, in many of my colleague scientists that don't want to learn what
is being said in the philosophy of science. There is also a narrow-mindedness
in a lot of probably areas of philosophy and the humanities in which they don't
want to learn about science, which is even more narrow-minded. Somehow cultures
reach, enlarge. I'm throwing down an open door if I say it here, but
restricting our vision of reality today on just the core content of science or
the core content of humanities is just being blind to the complexity of reality
that we can grasp from a number of points of view, which talk to one another
enormously, and which I believe can teach one another enormously.
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