One Hundred Years of General Relativity - The Enduring Legacy of Albert Einstein
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Transcript: English(auto-generated)
00:22
Thank you and good morning everyone. It's a pleasure to be here once again and I thought it would be appropriate this year when we celebrate 100 years of general relativity to talk about the enduring legacy of one of our greatest colleagues,
00:41
fellow Nobel laureates, Albert Einstein. Ah, here we are. So Albert Einstein of course is known to us all and around the world after over 100 years, still the most famous physicist
01:01
with the possible exception of Stephen Hawking. And we all know this picture of Einstein, the wise old man, the kind, passionate exponent of peace and harmony. This is Einstein actually 100 years ago
01:22
when he formulated his laws of gravity and of dynamical space time. And this of course is Einstein as a younger man at his office, the patent office, when he shook up the world in 1905 with his works on special relativity and quantum mechanics.
01:45
Ah, Einstein is known for many things. These pictures are of course iconic but what you're perhaps less aware of is how incredibly eloquent he was. His prose was exquisite.
02:01
His papers in physics are a joy to read and to all the young students, by the way, I urge you, go back and read the original papers in all parts of physics. They are usually so much better than the textbooks and Einstein is known for his quotes.
02:22
He's one of the most quotable people in the world. Here are four of my favorite quotes. Two things are infinite, the universe and human stupidity and I'm not sure about the universe. This is quite appropriate after the British.
02:44
And then insanity, doing the same thing over and over again and expecting different results. And then physics, God is subtle but he's not malicious. God for Einstein meant nature who is indeed subtle
03:03
but not malicious, we hope. And then the quote which I took as a guidance for making this talk, everything should be made as simple as possible but not simpler. Now Einstein burst onto the world scene of physics in 1905
03:24
with his theory of special relativity. He actually hated that name. He knew it would give ammunition to the postmodernist. He preferred a theory of invariance because what he really did in reconciling Maxwell's theory of electromagnetism
03:41
which he regarded as the paradigm of classical physics and Newton's laws of motion which together were simply inconsistent and had to be reconciled according to him and he did that by taking as the principle that there is no privileged observer,
04:01
no privileged reference frame. All observers have an equally valid description, the same description of physical reality. And in order to achieve that, there must be underlying symmetries of nature that allow one to transform the point of view of one observer to that of another.
04:23
It's interesting that this principle that there's no privileged observer, Einstein applied to all of life and to politics. There is no privileged nation, there's no privileged religion, there's no privileged race. What he took as fundamental was the principle of symmetry
04:43
and he wanted to call his theory the theory of invariance. He revolutionized the way we view symmetry in nature. His great advance was to put symmetry first, not to take the symmetry as a consequence of dynamical laws as his contemporaries,
05:01
Lorentz and Poincare were want to do but rather to make the symmetry principle as the primary feature of nature that constrains, restricts, inspires the allowed dynamical laws. A profound change of attitude which led by and large
05:22
to the realization that symmetries come first and we should search for new symmetries and use them in our description of nature which has largely guided, largely guided the development of our understanding of the fundamental laws of nature throughout the 20th century and that continues today.
05:42
A profound change of attitude. He also changed the way we think about space and time. As you know, he unified space and time. The symmetry transformations that were the basis of special relativity transformed space and time together
06:01
and consequently, there was no absolute notion of simultaneity. This was perhaps his most radical modification of our preconceptions that two events, well, one happens before the other or vice versa but in reality, there can be events
06:20
which are, we say, space-like separated, no signal could be transmitted between them with a velocity less or equal to the speed of light and therefore, no way of telling which came first and that depends on the observer, it's relative. In fact, much of the history of elementary particle
06:44
physics and the development of the standard model and attempts to go beyond it are looking for new hidden symmetries of nature. They must be hidden because otherwise, we would have seen them already that might explain and enlarge the scope of our theory.
07:01
One of the most exciting ideas that still has not been ruled out by experiment is the idea of supersymmetry, transformations of an enlarged notion of space-time, a space-time which contains extra quantum dimensions, dimensions measured with numbers that anti-commute
07:22
and the symmetry being rotations in superspace. This symmetry is beautiful mathematically and has the potential of answering many of the problems that we face beyond the standard model and it unifies the kinds of particles we have in nature,
07:43
bosons on the one hand and fermions on the other and we can look for it and we are. My colleagues are looking for it desperately at CERN, that Large Hadron Collider and they might very well find it. There's a little bump at 750 GV, it might be a sign of supersymmetry,
08:01
it might be nothing, who knows, we will find out. But if it turns out to be supersymmetry, we will have to accept the fact that we live not just in space-time but in superspace-time. Now, after 1905, there were two outstanding issues from Einstein's point of view.
08:21
The first was that Newton's theory of gravity was inconsistent with relativity. This was a contradiction, this was impossible. Everyone who accepted Einstein's special relativity knew that and many people tried to reconcile the two in obvious ways.
08:42
But Einstein followed his own path based on his other thing that bothered him after special relativity, namely he wanted to extend the principle that there's no privileged observer to accelerated observers as well. What is special about inertial observers moving with a constant velocity?
09:02
He wanted there to be no privileged observer in any sense. Many others knew that one was going to have to change Newton's theory, but Einstein also wanted not only to do that but to extend
09:22
the principle of relativity, and this led him, after much thought, to a program which he enunciated in 1907 in which he imagined that it would be conceivable to extend the principle of relativity to systems
09:42
that are accelerated with respect to each other. And in this famous paper, he based his strategy and his goal on Galileo's discovery that all bodies fall with the same acceleration. Or we would say that the gravitational mass
10:04
that is the source of the force of gravity is equal to the inertial mass that you must divide the force by to get the acceleration. This principle was enunciated by Galileo,
10:22
was of course known to Newton who did experiments. Newton established that the equivalence principle to one part in a thousand, this was improved over the years by Bessel, by Etwos famously, by my colleague in Princeton, Bob Dickey,
10:41
and has now been extended. We now know that the equivalence principle is correct to one part in a trillion, a million million. It is amazing. This is what we all love about physics. This is the only place in science where we measure quantities to a precision
11:02
of one part in a trillion. And some colleagues will spend their lives heroically trying to extend that by an order of magnitude or two, most likely failing. But what a heroic journey. And then suddenly, in 1907, sitting at his desk
11:24
in Bern, the patent office, he had an idea based on the equivalence principle. He thought what would happen if a man falling off a roof did experiments? He wouldn't, he would drop something which would fall and accelerate with him
11:42
with the same acceleration. He would conclude there's no gravity. This later is now called the elevator thought experiment because when questioned by reporters after Einstein became famous, they would all ask him, well, what happened to the man who fell off the roof?
12:02
So Einstein considered two systems. Let's use the elevator. One, a rocket going upwards with a constant acceleration, G, and then a elevator at rest in a uniform gravitational field minus G.
12:21
And he said, consider these two systems, S1 being accelerated with a acceleration G and S2 at rest in a gravitational field with an acceleration minus G. The physicists in each frame of reference and moving elevator, the stationary elevator
12:40
in a gravitational field, do all the experiments they can do, they get exactly the same results. There's a symmetry, a principle of invariance. This was the principle that he based his search for the relativistic laws of gravity on the equivalence principle.
13:00
As far as we know, the physical laws he says with respect to S1 aren't no different than those with respect to S2. Based on the fact that all bodies are accelerated equally in the gravitational field. And then at our present state of experience, notice as a good physicist, he qualifies it. Who knows, there might be a deviation
13:21
at the trillionth percent point. But at our present state of experience, we have no reason to assume that these systems differ from each other in any respect. And therefore, we shall assume a complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.
13:44
Now, if this were true, it would achieve his goal of extending the principle of relativity to accelerated observables. And conversely, would give away of understanding the origin of gravity, which you could always transform to a reference frame in which you didn't feel the force of gravity
14:01
and then going back, you could deduce what gravity looked like. This was what he pursued with stubbornness for the next eight years. Others tried to construct theories of relativistic gravity, but ignoring this principle, which he held on to like a bulldog.
14:22
The happiest thought of my life, he said. He knew now which direction to go. And it wasn't easy, it was very difficult. Well, its principle was enunciated in 1907. It wasn't really till 1912 that he realized that the field that transmitted, that mediated gravity,
14:43
the dynamics of space-time that was enabled that one to have this equivalence of accelerated observables was the metric tensor that determines the distance between points in a curved manifold. He had to learn differential geometry with the help of his mathematical friends.
15:04
And then it was difficult. Mistake after mistake, misconception, oops. But finally in 1915, and in the article published just a little over 100 years ago,
15:22
he announced Einstein's equations, which relate the curvature of space-time how space-time is a curved manifold, much like the surface of the Earth, a sphere is curved, to the source of gravity, which is mass.
15:43
Mass is the same as energy of a body at rest, of energy and momentum. The energy-momentum tensor is given by the Einstein tensor, which describes the curvature of gravity. This was announced November 25th, 1915 in one of four talks he gave
16:02
at the Prussian Academy of Science that year. Interesting, the three other talks are all on experiment. All the time he was finally getting to the final form of his equations, he knew that he had to compare the predictions of this with experiment. He had done so before.
16:22
In a previous version of his theory, which was incorrect, he had calculated the deflection of light by the sun. The sun would pull on the light, deflect its curve, you could measure that during a solar eclipse. He predicted what you could have derived from Newton's theory, 0.87 degrees, and that was wrong.
16:44
There was an expedition set out to measure that deflection of light, 1914, which, lucky for Einstein, on the one hand, the war broke out, the expedition was canceled. Unlucky for him because he was a pacifist living in Berlin
17:04
and notoriously opposed to the war. But after the war, well, in 1915, in this correct theory, he realized that the deflection would be twice that value. And indeed, after the war, an English expedition
17:23
led by Eddington confirmed that and made Einstein a worldwide famous figure. The other experimental verification of his theory was a post-diction of a phenomena that had been observed already in the 19th century,
17:42
the advance of the perihelion of mercury, a discrepancy with Newtonian gravity, an outstanding puzzle that many scientists had tried to understand. And he knew that his modification of Newton's theory would change that calculation. And in his previous version, he'd calculated
18:02
the advance of the perihelion and gotten the wrong result, but when he had his final equations, he sat down and rapidly did the calculation again and got exactly on those, the deviation. He must have been in 7.7. He predicted the redshift of light
18:21
in the gravitational field, which was only confirmed later by 1959 and is, of course, an essential part of GPS, makes all these things work. I wanna discuss the legacy of Einstein that persists till today and still shapes
18:41
the way we work in fundamental physics, dynamical space-time, the fact that after Einstein's theory, we've had to confront the fact that space-time is not just out there, rigid frame, it's dynamical, it moves, it fluctuates. The ability for the first time ever in physics
19:01
to construct a quantitative theory of the universe, of the cosmos, and the goal which he spent the rest of his life trying to achieve, but still guides us, the search for a unified theory. So Einstein's theory of gravity is based on the fact
19:22
that space-time is dynamical, the metric of space-time, and its curvature gives rise to what we call gravity. It obviously, at large distance, reduces to Newtonian gravity, as it must, because Newton's theory is still very good. That's how we plan, send rockets to the moon.
19:43
So our guiding principle for any advance in theoretical physics is always that it agree with great precision in some limit with the previous theory, and Einstein's theory is like that, but it has something new.
20:01
It has a field, the metric tensor of space-time, and the field, like any other field, can fluctuate, can oscillate. Those oscillations are gravitational waves. That's what makes his theory of gravity consistent with special relativity, because when you shake the sun,
20:20
it takes some time for the earth to respond. The waves of gravity spread out from the sun with the speed of light, and miraculously, this 1916 is also the year in which we finally observed those waves, at least two times now,
20:45
with an incredible experiment done by LIGO, the Laser Interferometry Gravitational Observatory. This is one of the facilities in, I think this is in Hanford, in Washington,
21:04
where we have an interferometer a few miles long, and the gravitational wave passing by through the earth changes the lengths of the arms, and you can measure the minute shift in distance
21:20
and compare with theory, with Einstein's theory, and the theory of black holes merging to form a bigger black hole, and these are the signals that are observed in the two observatories, one in Louisiana,
21:42
one in Washington. They are right on top of each other. They're much, much bigger than the ambient noise, and they agree precisely with the predictions of general relativity of Einstein, 100 years before, in a sense, so accurately that they can be used
22:02
to measure the masses of the black holes that merged, and the amount of energy that was radiated, which is immense, and this now will give science, astrophysics, a tool for exploring the universe
22:20
with new telescopes, these interferometers, which can see gravitational waves and measure and observe the properties of compact objects like black holes, neutron stars, and so on. Black holes were discovered 100 years ago, theoretically, by Schwarzschild.
22:42
They seemed rather strange. Einstein never believed in them. They had strange properties, but now they've turned out, as in this example, but many others, to be real astrophysical objects, and they continue to be, in addition, subjects of thought experiments, the kind, the Gedanken experiments
23:01
that Einstein loved to carry out, especially after Hawking's realization that in quantum theory, black holes aren't black. Black hole is a region of space where there's so much energy density that light can't escape, so they're invisible, but quantum mechanically, you can tunnel through,
23:21
and light does escape. Black holes radiate and disappear, and the conundrum, so I'll come to the conundrum, these theoretical objects which were disbelieved by Einstein, many theorists, and all observers until recently are now believed to be abundant throughout the universe at the center of every galaxy.
23:43
This is the black hole, and the Keplerian orbits around it that tell us what its mass is at the center of the Milky Way, and they presumably are the fuel gamma ray bursts, and they raise theoretical conundrums
24:02
because if you throw information into them, if you throw stuff into them that contains information, like half of a correlated quantum pair, you seem to be forced into a mixed state. You lose information, and this has provided
24:22
one of the strongest clues or problems, paradoxes, to those of us who have been trying to understand the reconciliation of dynamical space-time and quantum mechanics. Extrapolating Einstein's theory
24:41
to very short distances at very high energies provides another, many other paradoxes and problems, which are often the guiding principles for people in the kind of game that I am involved in looking for fundamental laws.
25:01
Since the extrapolation of Einstein's theory to very short distances gives fluctuations of space-time, quantum fluctuations that are uncontrollable, space-time foam, as it's sometimes called, we are sure that we will have to go beyond Einstein's theory, as he expected, and as he himself tried to do.
25:24
And we are faced with one of our major challenges today, my opinion, is to understand the true nature of space and time. Einstein taught us that space-time is dynamical, and at very short distances, it fluctuates in an uncontrollable way.
25:42
Many of us believe that it probably will be in a beyond Einstein theory, be best described as an emergent concept, good at large distances, large compared with 10 to the minus 33 centimeters
26:01
or something like that, but still not a fundamental concept in physics. We're asking, in a sense, what space-time is made of. Now I briefly wanna describe the other legacies. Physical cosmology is perhaps the most important. Before Einstein, cosmology was addressed by religion,
26:23
by philosophy, we have a lot of beautiful stories, but it wasn't science. But as soon as Einstein wrote down his equations, he realized, and others immediately, that the structure and the history of the universe is the subject of physics.
26:40
In fact, Einstein feverishly began to work on a mathematical model of the universe after 1915. And he constructed a model of the universe, which he thought should be static, it's a bad model. But he wrote to de Sitter in 1917,
27:01
from the standpoint of astronomy, of course, I've erected but aloft the castle in the air. For me, however, it was a burning question whether the relativity concept can be followed through to the finish or whether it leads to contradictions. You see, he had these equations which govern space-time and the universe is space-time.
27:23
He had to apply them. And he was worried that he would get nonsense. Nobody had ever, in the history of physics, tried to construct a mathematical theory of space-time of the universe. So he was worried. And although his model had problems
27:42
and was soon discarded, he was satisfied that I'm able to think the idea through to completion without encountering contradictions. Now I'm no longer plagued with a problem which previously gave me no peace. But he knew that he perpetuated something in gravitation theory which exposes me a bit
28:02
to the danger of being committed to a madhouse. What he really didn't like was introducing a parameter in his theory, the cosmological constant, which really was there. His fundamental principles allowed for it and he used it to construct a static universe.
28:20
Some people have called that his biggest blunder. I don't believe that. His biggest blunder was not predicting the expansion of the universe. He was convinced that the universe was static, unchanging. You know, you go out at night, the stars look today like they looked yesterday. He was convinced that the universe was static.
28:42
It isn't. He could have predicted the expansion of the universe. That was his biggest blunder. He also, by the way, developed the tools that allow us. See, he said he'd never believed that one would ever figure out really what the universe was.
29:01
And now we have. And partly with his aid, he showed how you can use the deflection of light around massive objects to map out the structure of matter through the universe. And this so-called bullet cluster, where two clusters of galaxy are colliding,
29:23
this stuff here is dark matter, which the blue stuff, which is measured, observed by astrophysicists who measure the deflection of light from these quasars behind the dark matter through it, and that's how you map out,
29:42
observe dark matter in the universe. And today, Einstein would never have believed this. After only 100 years, we have a complete, extremely detailed, quantitatively successful history of the universe from the very beginning
30:02
through a period of rapid accelerated expansion, just the normal expansion, structure formation of galaxies, of planets, and we now believe accelerated expansion dictated by his cosmological constant.
30:23
We still, however, don't know how it began, the Big Bang. And Einstein taught us that the universe is the history of space-time. And if you're going to solve this problem, if you have a solution to the theory
30:41
that explains the dynamics of space-time, it had better give a consistent description of the beginning or what happens at the boundary if there is a boundary and or at the end. And that is an issue that, again, science has never had to address,
31:00
and until now was the realm of religion and philosophy. But we can no longer avoid that question. And in discussing the structure of the cosmic microwave background and the theory of inflation, we must address what happened at the beginning. What is the initial condition for the universe?
31:23
What are the rules? Never asked this question. We don't even know what the rules are. And I see Lars rising, so I will end briefly with a search for a unified theory, which was Einstein's obsession over the years.
31:44
He always regarded his specific theory as provisional to be replaced by a more comprehensive unified theory of space-time and matter. He always, looking at his equations, his famous equations, thought the left-hand side was beautiful,
32:03
the consequence of this profound symmetry of space and time. And the right-hand side, ugly and arbitrary and singular, the structure of matter. And he labored for decades, unsuccessfully, to move the left-hand side to the right-hand side
32:21
and explain matter from geometry. Didn't succeed. Today we have an incredibly successful comprehensive theory of the forces and of the elementary constituents of matter, which describes the constituents of matter as made up of quarks and leptons
32:41
and the forces inside the atom and the nucleus as electromagnetism, my favorite, the strong nuclear force and the weak nuclear force. Together with the Higgs sector, this completes the standard model, which is incredibly successful, the most precise, quantitative,
33:02
successful fundamental theory we've ever had. It could, in principle, work from the Planck length where things tend to break down to the edge of the universe. Extrapolating this theory, we have hints that the forces unify at that energy scale together with gravity, and we pursue ideas of unification
33:23
like string theory, but it is very difficult to go from the large to the small, from the standard model to the grand unified theory, from now to the beginning. It's much easier to go from the small to the large, from unified to broken, from the beginning to now.
33:45
But Einstein gave us encouragement and warning. He said, the successful attempt to derive delicate laws of nature along a purely mental path by following a belief in the formal unity of the structure of reality encourages commitment,
34:00
continuation in the speculative direction, the dangers of which everyone vividly must keep in sight who dares to follow it. So we continue on that direction. We ask about space-time whose properties seem to be disappearing in this investigation
34:22
to be replaced by something else whose rules we don't really know yet. So Einstein's legacy, dynamical space-time, physical cosmology, unified theory continue to shape our exploration of the fundamental laws of physics.
34:42
Dynamical space-time, we now ask, what is space-time made out of? Physical cosmology, we're faced with a question, what is the initial and perhaps final state? And in our attempts to construct the unified theory, we continue to explore how the forces unify.
35:01
And I have, I will skip to Einstein's So in the, at the end of the 20th century, Time Magazine had to choose a person of the century. And as theoretical physicists, I'm sure we are specially,
35:23
but all physicists, all scientists, we're very proud that the person they chose as a person of the century was this theoretical physicist who not only was a great scientist, but a great humanitarian, and who used his fame and celebrity for the good of mankind.
35:42
Thank you.