It's 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 scientists to transform our understanding of space and time and I'd like to think that he would sympathise 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 income prehensile 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 call it submission wonder all or even terror. I quote Albert Einstein the most beautiful thing we can experience is the mysterious it is the 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 his book called The reign of relativity philosophy and physics 915009 125 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 and 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 1 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 1st 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. And the University of Cambridge where the chair from which Stephen Hawking retired was the look. For of 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 was really the 1st 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 Also 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 of the world's most leading theoretical physicist. I sure what the term philosophy now how to understand it I know if I 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 is then presumes to 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 be to serve as a ground for a particular method scientific in nature in I instance cases it's the same thing as does he do look to philosophy to find foundations for his scientific practice he did but not in the same sense of foundation and Einstein's case he knew that the fundamental principles that guided his productive life were possibly faults possibly true only of certain times in the history of the universe 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 there in the 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 will see the author of these principles well and in one or 2 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. 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 a an observer who is accelerating in the opposite direction in a region of space without gravity and Einstein was able to exploit that instead now of course that principle is generally just as I just stated its faults 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 that's a principle he was responsible for his how do you know he came up with that in 1007 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 I fear e. . It sounds like the principle is essentially theoretical No Well it 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 plug ability than just particular theories which are usually targeting particular domains of phenomena but the 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 source 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 generality of 80 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 Imperial value in 1915 and show that his theory Get it got it without any fudging it just popped out of the theory out of the theory not you know principle Well the theory that the theory but the theory was as after all was as it were the downstream of the Prince of icing. So should we talk about some other principles or you would you prefer to backtrack and I I for one would like a kind of clear and distinct. Account of what is a difference in special relativity and general relativity but maybe we want to continue on the question of principles 1st well let me pick up the former question 1st and that'll lead us to again some of the principles involved in general it of any that are particularly socially with Einstein so the principle of relativity is not something I'd Stein invented it was basically invented by Galileo in 1632 you can really read it in his famous 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 is traveling uniformly That is a relativity principle. Now that holds for Newtonian mechanics and it's been been known to all for Newtonian mechanics since Newton and $1687.00 what Einstein showed and this was remarkable at the time was that. Electromagnetism which was the other big theory of his time back in 1000 No 5. Was thought widely that it couldn't obey the relativity principle though and the reason was there is a fixed speed of light in the theory of electromagnetism How could the speed of light be the same for an observer who is at rest and one who is traveling uniformly doesn't make any sense but Einstein showed that it did make sense and that the theory of electromagnetism and so that the classical mechanics which was already known both of the principle of relativity and so Einstein's postulate in $1005.00 is that the laws of physics have to obey a relativity principle now that transcends all evidence because what we knew in $1005.00 was that there were phenomena that were not accountable but either 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 the scope of the principle now transiting on to general relativity if I may. General relativity it's the result of 8 years of struggle it started with what we began with with the principle of equivalence in $1007.00 and Einstein essentially completed the theory in November in 1915 in the middle of World War one. 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 following these are accelerating observers. Know in the way that I stated that that's not quite true but it 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 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 Mark. Mark in a book in the 880 s. That Einstein knew backwards and forwards at 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 phenomenon called the rotating bucket. You. Put a feel of 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 1st of all that the surface of the water is flat and then as the bucket is spinning of course the water recedes up the sides of the bucket and at some point there is 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 causes absolute space and this is what Mark objected to in the 880 s. He wrote a very strong criticism of very famous Einstein knew it backwards and forwards and even quoted verbatim from time to time and Mark's idea which Einstein followed and this is the root of Mark's principle is that inertia is not due to absolute space of that is to motions with respect absolute space but inertia is due to other masses it's a relation between masses only there is no such thing as absolute space and that's what Einstein tried to build in to his general theory of relativity now 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 that the beautifully simple idea that's wider than the theory that makes the theory as wonderful as it is you mention. Often giving an account of the principle 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 invokes 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 2nd thoughts about because basically physical theory today has outstripped our ability to test. Ok 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 and Einstein's theory it's spacetime geometry the planets when they orbit the Sun are basically following the laziest trajectories that they can in spacetime geometry. Does that have. Directly to do with the mark 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 be 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 you can think of the classic example is putting a heavy marble say on a rubber sheet that 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. 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 and around the sun but here is where some business this type of mind like mine has a very hard time trying to understand what space time. Is if as you use the analogy of the the rubber sheet and we've all seen diagrams of the way there is indentured chairs but that means that space time does it has does it have a material I'm going to tear reality is it something that is like the nets that there are used in those. Images too to just describe spacetime is it something or is it nothing what what no on earth 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 and 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 and this is one of the great problems of contemporary. Field Theory and 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 Ok so we have spacetime that curvature and there is something actually which is curving. No it's. There is 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 a photon of light travels through that manifold it will trace out a if nothing is interacting on it it should take take a straight line right while the straight line in a curved manifold is not a Euclidean straight line. Or talks and we talked about a couple of principles the mark principle the principle of relativity principle of equivalence is also the principle of energy come to conservation and in your forthcoming book you examined Einstein's use of these physical principles as top down constraints that any adequate empirically adequate theory must satisfy and 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. And 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 if not theory of relativity at least what. He still belongs to in terms of blowing to what we would call the you know the classical model of doing physics rather than the quantum model right can you speak a little bit about how what is quantum mechanics or what is quantum theory and what was Einstein's relationship to that end 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 theory. Of physical theory as such Ok there are a lot of pieces there so let me let me try to start at the beginning and then you can remind me of what some of the latter pieces were but. So. This idea about principles. It's 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 that of symmetries and symmetries involve complicated. Group Theory and things of this kind in quantum mechanics and they don't they're not all symmetries of space and time like a relativity principle so I really think that Einstein is the person who 1st practice that method of doing physics and it's been. Would be impossible to understand subsequent physics without that method of doing physics course probably somebody would have come up with it anyway that method being the method of Einstein and the method of Einstein of starting with these principles and. Building a theory that must satisfy. And then so Einstein's principles let him well let's let's let's start with this I mean I think that why start with these principle how do you c