This is KZSU Stanford.

Welcome to entitled opinions.

My name is Robert Harrison.

We're coming to you from the Stanford campus.

[music]

Our intellectuals have failed us as value creators, Randolph-Borne.

And taste is after all the only morality Randolph-Borne again.

Those of you who heard the first show of our new season know that Randolph-Borne,

the early 20th century American essayist,

is one of the new trustees of this radio show,

joining the ranks of our adopted ancestors.

The few dozen of them who thought legacies and words permeate the spirit of entitled opinions.

Let me recall what Borne said about opinions.

I quote, "In the scientific attitude, there is no place for belief.

We have to act constantly on insufficient evidence on the best opinion we can get.

But opinion is not belief.

Belief is dogmatic.

While opinion has value only when it is tentative and questioning.

In modern thinking, the attitude of belief has given place to what might be called higher plausibility.

There you have it. On this show, we don't traffic in beliefs,

but place ourselves in the sphere of higher plausibility.

If you're looking for absolute truth, we can't help you.

But if you're looking for the most expansive opinions around,

stay tuned in title opinions coming up.

And that's way of on KCSU doing its thing.

When it comes to the really big issues in science and in human affairs,

generally, we are almost always dealing with insufficient evidence.

Being men and not gods, things reveal themselves to us only partially,

never completely, if only because we dwell in space and time,

which a manual can't call it our A priori forms of intuition.

I don't really know what time is exactly,

but I know it's what prevents everything from happening at once.

And I know that space gives a separation, distance, and these local finite horizons in which we live.

We don't live everywhere, but somewhere.

The space-time continuum is my home for better or worse.

Even if its nature is beyond my comprehension,

it makes room for me and gives me my hour in the sun.

I'm not bothered by the fact that I can't grasp the true essence of the universe,

that I can approach it only in the mode of higher plausibility,

because if it were otherwise, the universe would lose that mysterious attraction

that binds me to it according to the laws of spiritual gravity,

if I may call it that.

Albert Einstein did more than any modern scientist to transform our understanding of space and time,

and I'd like to think that he would sympathize with what I just said about the ultimate mystery of the space-time continuum.

He did not think that the universe is unknowable.

On the contrary, he thought that the fact that the universe lends itself to human comprehension

renders it all the more incomprehensible.

Science has a wondrous ability to uncover matters of fact,

yet what too often gets overlooked in scientific inquiry is the encirclement of facts by a fringe of impenetrability.

For scientists like Einstein who are alive to the world,

matters of fact received into the mystery of matter itself,

which calls on us to relate to it in another mode than that of knowing only,

it's called its submission, wonder, awe, or even terror.

I quote Albert Einstein,

"The most beautiful thing we can experience is the mysterious.

It is a source of all true art and science.

He to whom the emotion is a stranger who can no longer pause to wonder and stand wrapped in awe is as good as dead.

His eyes are closed. The insight into the mystery of life,

coupled though it be with fear, has also given rise to religion."

In today's show, we're going to talk to a person who has eminently entitled opinions about Albert Einstein

and his insights into the nature of the physical world.

Professor Thomas Reichman is a philosopher of science,

specializing in the physical sciences,

and is a professor of philosophy here at Stanford.

He has authored more than 70 papers and is book called The Rain of Relativity, Philosophy, and Physics,

1915-1925, published by Oxford University Press in 2005,

traced the confluence of various currents in mathematics, physics, and philosophy that shaped 20th century philosophy of science.

He has also written a new book to be published in 2016,

which looks at the philosophical principles underlying Einstein's well-known major accomplishments in physical theory.

This new book also examines the principles and philosophical vision behind what is widely regarded as Einstein's tragic failure,

namely his refusal to accept quantum mechanics as a fundamental theory.

I, for one, am looking forward to what our guest has to say today, Tom, welcome to the program.

Thanks very much Robert. It's a pleasure to be here.

The first question I would like to ask of you is about Einstein as a philosopher.

In what sense could we or should we call him a philosopher?

And if he was indeed a philosopher, does that make someone like Newton a philosopher as well?

Right, so let's start with Newton if we might.

You know that Newton practiced something that at the time was called "natural philosophy."

And that name continues today in the University of Cambridge,

where the chair from which Stephen Hawking retired was the Lucosian professorship of natural philosophy.

That's the chair that Newton held, actually.

And Newton practicing natural philosophy is partly responding to the philosophers of his time,

in particular to Descartes, and to a work that Descartes published in the 1640s.

We all know that Descartes starts with clear and distinct ideas,

but Descartes had very strong ideas about the Constitution of the physical world.

It's a world that really the first manifestation of what you might call the mechanical world view.

Newton responding to this very strongly developed a better way of doing natural philosophy,

that sense Newton has been called classical mechanics.

Einstein, I think, is a philosopher in the sense of natural philosophy,

but in a different sense, I think that Einstein is a philosopher because he recognized more so than many scientists of his time,

and certainly more so than a lot of scientists today, that to do theoretical science,

one is doing also philosophy, one might not be aware of it, and the philosophy may be not so good,

but it's there. And Einstein was always concerned that a good, productive, fruitful philosophy would guide natural science.

And I think that's why I think he merits the title of a philosopher as well as the world's most leading theoretical physicist.

I'm not sure what the term philosophy now, how to understand it.

I know if I think back to Descartes, the discourse on method, why he has to find a metaphysical foundation for the new method that he had mathematical method that he was responsible for, call it algebraic geometry or whatever.

And therefore, the ontological proof for the existence of God on which basis he has then produced and find a criterion for his clear and distinct ideas all in order to say that whatever I come up with through my mathematical method,

I can count on as being true in the actual physical world, therefore that is where philosophy comes in to serve as a ground for a particular method, a scientific in nature.

In Einstein's case, is it the same thing? Does he look to philosophy define foundations for his scientific practice?

He did, but not in the same sense of foundation. In Einstein's case, he knew that the fundamental principles that guided his productive life were possibly false, possibly true only of certain times in the history of the universe that the energies that we can explore, and maybe not true at other times.

But nonetheless, those principles transcend the empirical evidence that we have for them. And they're in a sense axioms for Einstein.

And what this amazing about Einstein is how productive he was to use this approach, using these principles that possibly were wrong, certainly are fallible, to build up physical theories that have such astonishing empirical consequences that we observe.

Do you see the author of these principles?

Well, in one or two cases he was, yes.

So which principles are we talking about exactly?

Well, in the case of general relativity, we're talking about the principle of equivalence.

And basically, this is a principle that says that a freely falling observer, that is somebody in a gravitational field,

will experience the same laws as an observer who's accelerating in the opposite direction in a region of space without gravity.

And Einstein was able to exploit that incident. Now, of course, that principle is generally just as I just stated it as false.

But it's true in a very limited circumstance, and he was able to show from using that principle the necessity of the geometry of space time now as being

a curved, not a Euclidean geometry.

So, and that's a principle he was responsible for coming up with.

He came up with that in 1907, and he called it the happiest thought of his life.

So, Tom, can I ask what then is the difference between a principle like that and a theory?

It sounds like the principle is essentially theoretical, no?

Well, it is a theoretical principle in a sense.

I mean, I'm talking about freely falling observers and accelerating observers and things like this.

But at the same time, it's not restricted to any one particular theory.

It should apply to all of physics.

And that's the thing about principles.

They have much wider scope and applicability than just particular theories,

which are usually targeting particular domains of phenomena.

But if a principle is going to have this kind of universal reach,

doesn't the scientist in question or the philosopher in question have to prove or demonstrate that it has this kind of universal application?

Well, sure.

Sure.

So, the demonstration consists in what inferences can I draw from using this principle about phenomena that we can possibly observe?

And the great thing that Einstein showed was that beginning with that principle and a couple of other ones that are involved in general relativity,

you could predict a very slight deviation in the orbit of Mercury.

That was known.

It was an empirical anomaly.

It was an anomaly of Newtonian planetary theory.

And it couldn't be accounted for by any Newtonian explanation.

And Einstein derived that exact empirical value in 1915 and showed that his theory get it got it without any fudging.

And it just popped out of the theory.

Out of the theory, not the principle.

Well, the theory, but the theory was, after all, was as it were the downstream of the principle.

I see.

Got it.

So, should we talk about some other principles or would you prefer to backtrack?

And I for one would like a kind of clear and distinct account of what is the difference between special relativity and general relativity,

maybe we want to continue on the question of principles first?

Well, let me pick up the former question first.

And that'll lead us to, again, some of the principles involved in general relativity that are particularly associated with Einstein.

So, the principle of relativity is not something Einstein invented.

It was basically invented by Galileo in 1632.

You can read it in his famous two dialogues book.

It was known to Newton. It's a corollary of his laws of motion that basically the Newtonian laws are going to be the same for observer at rest or for one who's traveling uniformly.

That is a relativity principle.

Now, that holds for Newtonian mechanics, and it's been known to hold for Newtonian mechanics since Newton since 1687.

What Einstein showed, and this was remarkable at the time, was that electromagnetism, which was the other big theory of his time back in 1905,

was thought widely that it couldn't obey the relativity principle.

And the reason was there's a fixed speed of light in the theory of electromagnetism.

How could the speed of light be the same for an observer who's at rest and one who's traveling uniformly?

It doesn't make any sense. But Einstein showed that it did make sense, and that the theory of electromagnetism and so the classical mechanics, which was already known, both obeyed the principle of relativity.

And so Einstein's postulate in 1905 is that the laws of physics have to obey a relativity principle.

Now, that transcends all evidence, because what we knew in 1905, that there were phenomena that were not accountable by either mechanics or elective mechanics.

Or electromagnetism, people were just had just discovered, for example radioactive decay.

It was very clear that that was not going to be a simple electromagnetic phenomena, and it certainly didn't appear to be mechanical.

But Einstein says the laws of physics have to obey a relativity principle.

You see this scope of the principle. Now, transiting on to general relativity, if I may.

General relativity was the result of eight years of struggle. It started with what we began with, with the principle of equivalence in 1907.

And Einstein essentially completed the theory in November 1915, in the middle of World War I.

And you can think of general relativity as the principle of relativity applied now not just to observers who are either at rest or moving uniformly,

but also to observers that might be rotating or freely falling. These are accelerating observers.

Now, in the way that I stated it, that's not quite true, but at guided Einstein.

And he based his theory of gravity as a generalization of the principle of relativity on the principle of relativity,

that the laws of motion or the laws of physics are going to have to obey a relativity principle, the principle of equivalence, and one other principle.

And this is particularly associated with Einstein, although it doesn't have his name and has the name of somebody that he greatly respected, the famous physicist Ernst Mach.

Mach, in a book in the 1880s that Einstein knew backwards and forwards, had written a historical critical account of the rise of mechanics.

And in the latter chapters of that book, he discusses Newton, and he discusses the problem of Newton's, basically, attempt to explain a particular phenomena called the rotating bucket.

You put a full bucket full of water, you hang it from a rope from a beam, you twist the rope, you let the bucket go, you observe various things.

You observe, first of all, that the surface of the water is flat, and then the bucket is spinning, of course, the water recedes up the sides of the bucket, and at some point, there's no longer any relative motion between the water spinning in the bucket and the sides of the bucket.

So, there's no relative motion.

But what's responsible for the recession of the water from the axis of rotation in the bucket?

That's what's called the force of inertia.

And Newton essentially said that that force has a cause, and that cause is absolute space.

And this is what Mach objected to in the 1880s.

He wrote a very strong criticism of very famous Einstein knew it backwards and forwards, it even quotes it verbatim from time to time.

And Mach's idea, which Einstein followed, and this is the root of Mach's principle, is that inertia is not due to absolute space, that is to motions with respect to absolute space.

But inertia is due to other masses.

It's a relation between masses only.

There's no such thing as absolute space.

And that's what Einstein tried to build in to his general theory of relativity.

Now, it turns out that's really difficult to do.

And he doesn't quite succeed in doing that.

But you see again, it's the force of the principle.

It's the idea, the beautifully simple idea that's wider than the theory that makes the theory as wonderful as it is.

You mentioned often giving an account of the principles of the role that the observer plays in establishing a principle.

Why is the observer so important in these principles?

Because this really has to do, I think, with one way of, I think, the right way of reading Einstein, which is that, of course, you know, physical theories, the general theory of relativity,

and evokes very abstruse concepts, things that you have to use advanced mathematics to talk about and to think about.

But ultimately, it's about what you can observe.

And the test of any theory is ultimately an empirical test.

This is an important thing for Einstein, you say, well, of course, that's what science does.

But actually, in the contemporary world of physical theory, this is a criterion that people are beginning to,

fudge a little bit and to have second thoughts about because basically physical theory today has outstripped our ability to test it.

Okay, well, we're going to talk in a moment, you know, about quantum mechanics and its relation to general relativity.

But if I could stick with general relativity for a moment, the theory is really a theory of gravitation, do I get that right?

That's right.

And gravitation is not a force in Einstein's theory, it's space-time geometry.

The planets when they orbit the sun are basically following the laziest trajectories that they can in space-time geometry.

Does that have directly to do with the Mach principle that we're not dealing with absolute space, but we're dealing only with relations between masses?

Is gravitation reducible to that equation that it's relation between masses or is it something else?

No, but it's not reducible to that.

It's one way of understanding what that curvature of space-time is.

Curvature of space-time is caused in the sense of causation that you have from the Einstein field equations, just by matter, energy densities.

How dense, how energetic is something that causes space-time to curve?

The classic example is putting a heavy marble, say, on a rubber sheet that's stretched tightly, and you see that the heavy marble, which we can think of as the sun, is depressing the center of the sheet and some area around it.

You can think about the planets as just spinning around in that little ovoid depression that the marble creates.

And they're not falling toward the sun because they have their own motion that keeps them in their inertial trajectory around the sun.

Well, here's where some non-businesses type of mind like mine has a very hard time trying to understand what space-time is, as you use the analogy of the rubber sheet, and we've all seen diagrams of the way there's these indentatures, but that means that space-time does it has a

material, a material, a materiality? Is it something that is like the nets that are used in those images to describe space-time? Is it something or is it nothing?

What is space-time apart from the bodies that curve it?

Well, let me start with space because time is a little more difficult. Space is definitely not nothing for Einstein. Everywhere even in empty space, it's absolutely vital in Einstein's theory of gravity that there's some quantity there called the value of the metric field.

Now, that's just general relativity. Quantum theory comes along and says that look, there's no such thing as empty space. Also, there's something out there called vacuum point energy and the density of empty space, the energy density of empty space.

This is one of the great problems of contemporary field theory in the quantum sense and also of cosmology. Just what is this vacuum energy density that's out there in empty space and we think it's somehow related to dark energy?

So we have space-time curvature and there is something actually which is curving.

No, there's something that is creating a condition of the manifold if I can use that slightly technical term, such that any body that has mass or even no mass of photon of light travels through that manifold, it will trace out if nothing is interacting on it. It should take a straight line.

Well, the straight line in a curved manifold is not a Euclidean straight line.

We've talked about a couple of principles, the principle of relativity, the principle of equivalence, there's also the principle of energy conservation.

In your forthcoming book, you examine Einstein's use of these physical principles as top-down constraints that any adequate and empirically adequate theory must satisfy. I'm really reading from you here.

You say one builds the theory to satisfy the principles and then tries to derive consequences that can be tested.

This seems quite interesting, but you've mentioned now more than once the way in which quantum theory or quantum mechanics has put into crisis somewhat the Einstein's theory of relativity at least what he still belongs to in terms of belonging to what we would call the classical model of doing physics rather than the quantum model.

Can you speak a little bit about what is quantum mechanics or what is quantum theory and what was Einstein's relationship to that?

And then, as you said, there might be a tragic failure at the end of Einstein's life because he was not able to account or did not want to embrace quantum theory as a physical theory as such.

There are a lot of pieces there, so let me try to start at the beginning and then you can remind me of what some of the latter pieces were.

So, this idea about principles.

I think Einstein pretty much invented the methodology, if you like, of theorizing, starting from principles.

And this is taken over by quantum mechanics, of course the principles are different.

The principles in quantum mechanics and in quantum field theory are things of symmetries.

And the symmetries involve complicated group theory and things of this kind in quantum mechanics.

And they're not all symmetries of space and time like a relativity principle.

So, I really think that Einstein is the person who first practiced that method of doing physics.

And it's been, it would be impossible to understand subsequent physics without that method of doing physics.

Of course, probably somebody would have come up with it anyway.

That method being the method of Einstein.

The method of Einstein is starting with these principles and building a theory that must satisfy.

And then, so Einstein's principles led him, well, let's start with this.

I think that why start with these principles?

How do you choose these principles?

I mean, some of them are more or less empirically manifested principle of energy conservation.

For example, we think that's manifested by just an energy.

There's no such thing as a perpetual motion machine.

People have been trying to build them. They still do. You can find great websites about people cataloging all the attempts to build perpetual motion machines.

So some principles are more or less close to what we can observe and others are quite far away, like Mox principle that we talked about before.

And it's, well, I guess one could say that the idea of a theory, guided by these principles, is for Einstein, it really goes back to what science is supposed to be doing.

If you're a theoretician, okay, there's lots of forms of science, right?

They're banging around test tubes and this kind of thing.

No, as a theoretician, what are you really trying to do? You're trying to explain the world.

You're trying to make the world, what is explaining it? It means make it comprehensible.

Make it so that you can start in a certain place and you can derive consequences and those consequences are going to match the phenomena that you observe.

That's explanation.

And Einstein thought that this was the mission of a theoretician. It's to make the world comprehensible.

And along comes quantum mechanics in 1925 and 1926 Heisenberg and Boren, Jardon, Dirac, Schrodinger.

And they give us a theory that works incredibly well for micro physics, but is completely incomprehensible.

And people will say that today. You use quantum mechanics. It's a perfect algorithm.

It's the most accurate theory in science. But does anyone really understand quantum mechanics?

And I think anybody who says that they do is lie.

Well, we're in trouble then because I'm not the one who's going to try to seek some clarity.

And I don't even really know what quantum mechanics means.

And so it does it mean that it's a theory of motion because classical mechanics is the laws of motion in Newton.

Yes, you want the mechanics a theory of motion? Right. So originally you start with looking at things that were known in the 1920s.

You knew, for example, by 1925 that light was particle as well as the wave.

We can talk about wave particle duality if you like.

We knew about electrons. We knew that when an atom radiates, it gives off a photon.

That's the radiation. And then the radiation comes from the electron in an orbit.

And then the electron falls to a lower orbital after giving off the photon.

And atoms also can absorb radiation.

Photon hits an electron. The electron pops up to a higher orbital, more energy.

And it was trying to understand that what seems to be mechanical process that the system of quantum mechanics came about.

Are these motion to describe fluctuations? No.

We'll talk about fluctuations later.

No, these are just straightforward theory of radiation.

And there is a theory of radiation in Maxwell's theory, but it involves only waves.

And not these discrete transitions between orbitals that were happening.

This was the gradient advantage.

As we were walking over here, we were talking about Bohr's atom.

And that was what the Bohr atom did. It gave these classical orbits a quantum character by saying that the transitions between orbitals was spontaneous and discrete.

There was no continuous transition from one orbital to another. It just was a jump.

And so what was it about quantum mechanics that made Einstein nervous on the one hand and defensive on the other?

Well, it wasn't just that the theory is a probabilistic and not a deterministic theory, although that played a role.

Einstein has very famous arguments, well known today because they've led to further progress in foundations of quantum mechanics that don't vindicate Einstein, but vindicate some problems that he saw in quantum mechanics that just happened to be features of the physical world.

And Einstein's criticism of quantum mechanics was that it was an incomplete theory.

And here's an example.

The famous example that he came up with in 1930 at the Solvé Conference in Brussels.

And you put a particle in a box.

And then somehow you put a partition in the box and you take the left half to Paris and the right half to New York.

Which half of the box has the particle?

Well, you can open the box in Paris or you can open the box in New York and see it's either empty or the particles there.

Fine. You think so. Okay. There's a 50/50 probability that the particle is in one side or the other.

Quantum mechanics gives you a definite probability. The formalism requires that the particle is in both boxes until it's opened.

Yeah, the observer. Yeah, there's the observer. And Einstein thought that that was an incompleteness of the theory.

There's a definite answer when you observe, but until you observe there's no definite answer.

And so why did this bother him so much?

Because he thought that it was a way of doing science that he thought was too cheap and easy.

Because it was probabilistic.

Because it was probabilistic because there should be in the physical world picture that he has.

A reason why the particle is in one box or in the other when you open it.

And quantum mechanics doesn't give you that reason. It only gives you the probability that it's in both.

Nehel es, cine razione. That's that famous. Leibnizio in principle.

There is nothing without reason.

And I guess he was committed to that. He was. And I think classical physics is committed to that principle.

You know, Leibniz was also a proponent of, well, not so much of classical mechanics, but much of what he did contributed to the classical world view.

And quantum mechanics must give up the principle's vision reason because, according to quantum mechanics, well, there is no reason.

We just calculate.

So Thomas, far as I know, the last time I checked, there were four fundamental forces in the universe.

No, electromagnetism. The weak force is strong force and gravity.

And again, as far as I understand, quantum mechanics is perfectly equipped to account for and give a remarkably accurate account for three of the four forces.

Right. But it's gravity. That is the obstacle in the case of quantum mechanics because there is still no quantum theory of gravity.

That's right.

And therefore, Einstein's theory of general relativity still applies to the case of gravity.

Right.

And therefore, how do you see the relationship between quantum mechanics and Einstein's physics today and whether quantum mechanics is on the path towards

the eventually coming up with an adequate theory of gravity or will Einstein always hold his ground when it comes to gravity?

Well, let's be clear that for most of the physics that's done in high energy physics, gravity doesn't matter.

It's a very weak force, if dealing with small bodies. But as you get to higher and higher energies, which basically means shorter and shorter distances between things, gravity becomes the strongest force.

And the problem there is to, we can't with any technologies that we have today or even can foresee, we're not going to be able to probe those energy scales and short distances in accelerators.

So the evidence that we need for quantum theory of gravity has got to come if it's going to come from somewhere else.

And so the question is, well, what can we learn from looking at astrophysical phenomena and things of this kind?

I mean, astrophysics has been called the, you know, poor person's accelerator. You can learn things about particle physics from astrophysics.

But even here, we're kind of stumped.

And the difference between the quantum theories, which are summarized in something called the standard model of elementary interactions, everything, the gravity.

And general relativity is that they're very different mathematically. They're not really consistent with one another.

They're not conceptually consistent with one another.

And if you try to quantize gravity in the usual way that you quantize other theories, you get nonsense results. This is called non-renormalizability.

So there is one program which deserves mention called string theory, where string theory starts with these tiny little objects that vibrate in various ways at very small scales, tend to the minus 33 centimeters.

And so it's funny like that. And one of those little strings vibrates in just the way that a quantum particle of gravity should be vibrating. So we think.

So it's always been claimed that string theory from the beginning has gravity as part of it.

Now, that remains a claim to be proven because there are no empirical tests right now of string theory as around.

So here is a kind of program. It's not just one theory or another. There are many different ways of doing string theory. And it's a complicated thing.

But basically, that program has no solid empirical evidence for it right now.

Well, can I read you from a review that your colleague Freeman Dyson wrote of a book that came out on Einstein. Let me get the title of it exactly. It's called Einstein is Space and Times by Stephen Gimble.

And in the New York Review of Books, he in reviewing it, he comes to this conclusion about Einstein as a philosopher and the problem of the relationship between his theory and quantum mechanics.

He says that to summarize the present situation, there are three ways to understand philosophically our observations of the physical universe.

The classical philosophy of Einstein has everything in a single layer, obeying classical laws with quantum processes unexplained.

The quantum only philosophy has included everything in a single layer obeying quantum laws with the astonishing solidity and uniqueness of the classical illusion unexplained.

The dualistic philosophy gives reality, the dualistic philosophy I think is referring there to Bohr who had the compatibility theory anyway.

The dualistic philosophy gives reality impartially to the classical vision of Einstein and to the quantum vision of Bohr with the details of the connection between the two layers unexplained.

And then he goes on to write that all three philosophies are tenable and all three are incomplete.

And he prefers the dualistic model because I give equal weight to the insights of Einstein and Bohr. I do not believe that the celestial harmonies discovered by Einstein are an accidental illusion.

Do you believe that those celestial harmonies are an accidental illusion?

Not at all. I think that those harmonies are there and there's evidence for them.

But I think that the problem with the dualistic philosophy is the problem with Bohr's Copenhagen or complementarity approach to quantum mechanics which makes a sharp divide between the quantum and the classical world.

The classical world is the world of the observer, the world of the measuring apparatus, the world of the laboratory.

The quantum world is the system that you're trying to investigate. And basically what Bohr is telling us is that we can't say anything about the system that we're interested in, the quantum system, unless we use the categories and concepts of the classical world.

And this kind of thing I think is not as widely shared as it was. Dyson, of course, I have enormous respect for him, but he is of a much earlier generation.

And the current physicist that I know very few of them follow Bohr and complementarity at one time.

As Einstein said, it was a soft pillow on which the quantum physicists could rest their head and forget that there were foundational problems.

Now people try to do a little better than that. And the reason is there's very good reason to think that the world is fundamentally quantum. There's no dualism.

Everything is ultimately quantum, even if we don't have a quantum theory of gravity yet. And we may never have because we may not be able to test such a theory or proposals for such a theory.

And if the world is fundamentally quantum, then you have this great problem. Where does the classical world come from? I mean you and I and Puneal are right all not in the state of superposition.

We are all people with macroscopic properties and the classical concepts describe us perfectly well.

And how does that emerge, that classicality emerges from the quantum domain? This is one of the most challenging and interesting aspects of research in fundamental quantum theory.

But sadly, it's very complicated and there's only toy models. These are called decoherence models to show that there's a certain process that takes place where the quantum multifaceted state, you know,

suddenly just decohears into a classical state. Well, I began in my introductory remarks, you know, he voking the fact that we live in a space time continuum that we have our experience of the world is temporal, it's spatial in the most basic, let's say classical sense. And that if God is beyond space and time as he is in someone like Dante's case, then he belong,

there are a completely different set of laws were to presumably apply there. But given that we whether we like it or not, we keep returning to the kind of classical world of experience.

Would it be misguided for me to propose at least by analogy, the Kantian theories that we are, we have access really to the phenomenal realm and not the numinal realm, the numinal realm being what nature or

the world would be in itself independently seen from our subjective forms of intuition, especially space and time as these a priori forms of intuition.

And is it the case that perhaps who we are as human beings structured the way we are?

And condemns or you could say more optimistically guarantees for us that we will always be living in a more or less classical world and not a quantum world.

Well, the sounds a lot like Bohr actually with his, you know, postulate about the necessity of classical concepts. So classical position, classical momentum, they're complementary, you can't have them both simultaneously,

but they're both needed. I actually think we can't really understand even certain features of the classical world without thinking about the quantum origins of the classical world.

And I think that so any kind of clear break between phenomenal numinal, whatever isn't going to work well.

I do think that physics has the theoretical physics and the wonderful technology that we have to test physical theories has the ability to probe below the, as it were, the surface phenomena of the observation and give us a quite compelling story

of how that observation that we make has been dynamically produced by some physical process.

And I think if you give up that, you really give up the attempt to try to explain the world, which is Einstein's attempt and many others.

Should we be grateful for decoherence? As far as I understand it, it's what allows the quantum waves to kind of disappear very quickly and leaves us the kind of stable world with celestial harmony.

Well, that depends on if you're an optimist or a pessimist. If you think human beings are a good thing, you should be grateful.

If you don't think they're a good thing, you shouldn't be grateful.

Well said. Finally, Tom, we've been talking a lot about the general relativity of Einstein, but his achievements are, you know, go beyond that.

And I just wanted to ask, I don't know if we can integrate it into this otherwise very coherent path we've been following as far as I'm concerned, about the EPR, the entanglement fluctuation theory.

Oh, well, that's two different things. Let's talk about EPR. This is one of the most cited papers in all of 20th century physics.

He, Einstein, P, Podolsky, R is Rosen. And Einstein wrote this paper. It was the second paper in English that he ever wrote in the United States.

He wrote in 1935 in Princeton with two research associates, young man who were just working with him.

He had the idea of Einstein's. And this has to do with this very wonderful phenomena of entanglement that seems to be part of the quantum world.

Einstein discovered something called entanglement. And basically, the idea of entanglement is you start with a system, say, a two particle system that interacts at some point.

And then the two parts of the system fly off in different directions. And the quantum theory allows you to write down what's called a state function for the system at the interaction and then after the interaction.

So, with two parts. But if you make a measurement on one part of the system, I say you choose to measure position, you can immediately infer without measurement what the position of the distance system is.

You can also measure momentum on the first system. And you can infer what the value corresponding value of momentum is on the other system with 100% accuracy.

Correlation.

Now, quantum mechanics says you can't measure position and momentum at the same time. But according to the scenario I just outlined, you can infer that the distance system has position and momentum at the same time.

So, overall, why should it matter to the distance system what you measure on the system you over here? It should make any difference.

And the fact that therefore, our two different state functions assigned to the distance system, depending on which measurement you make on the system and your hands, Einstein thought was in completeness.

So, that correlation across great distances and these distances can be as much as 27 or 28 kilometers. These experiments have been done. This is the phenomena of entanglement.

Once quantum systems interact, if nothing else interacts with them, they remain correlated.

Well, I know that there's a drive in theoretical physics to arrive at a complete theory and the Einstein's objections to quantum mechanics, if I understood you correctly, is that it suffers from incompletion.

And the same could be said on the other hand, I can't help but be grateful to the fact that our science is not going to arrive at some kind of complete theory.

So, then I think the mystery that I evoked at the beginning of my show, that this kind of wonder of that this universe is that we actually belong to is also beyond somehow our complete grasp, always reminding us, therefore, that we, the ones, the observers, the ones who have to do that measurement, the ones who project ourselves into this rather alien world that we are finite creatures that have our own mortality and that somehow,

maybe it's our mortality, that opens up this whole universe to contemplation and consideration and calculation, because without us in the picture, I don't know if the universe has any sense of itself whatsoever.

Well, that's probably something I would agree with. I think I would just add that what is remarkable is, goes back to what you said at the very beginning of the show, is that we do understand as much as we do.

It's a big universe out there. I mean, there are billions and billions and billions of galaxies and it's amazing that we, this little insignificant species and some tiny insignificant planet and some insignificant galaxy,

knows what we do know. I think with some confidence about the shape and dynamics and the history, if you like, of the universe. Now, some of the details going back to the really early universe are still not known and the future of the universe is not known, but it's kind of amazing what we do know about the universe with some confidence. It's part of mainstream physics now.

And for those of us nourished in the phenomenological tradition, of course, it's not by chance that we are able to comprehend at least partially the universe, but there's something about our mode of being.

And again, our finitude that projects us beyond ourselves, that we disclose, we open up the space in which intelligibility happens in the first place.

And without us in the picture, there is no intelligibility whatsoever. And you have certainly advanced our understanding of what, for most of us, is highly intimidating and obscure kind of matter, which is where to think stand in terms of physical theory and physics today.

So I want to thank you for that. I'm remind our listeners we've been speaking with Professor Thomas Reichman from the Department of Philosophy here at Stanford. I'm Robert Harrison for entitled opinions. Thanks for coming on Tom.

My pleasure.

And we'll be with you next week. You take care.

Thank you.

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A long edge of this airfield.

It will propch our left and the other's stand. All to meet us reading the road.

For most memories, lingering.

Night's a cold on this airfield.

I see the low watch the rain guy.

Lord, don't go wavelength, call in the lead.

But it's slowly into the screen.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

I see the rain.

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