How Long is Forever? A Physics and Astronomy Perspective

Gilah Langner: So you all know me, and I’m delighted to open the fourth program in our “Science Meets Judaism” series. So we’ve learned about creation and restoration ecology from Dr. Betsy von Holle. We’ve explored animal sentience in the Bible and in the wild with Dr. Diane Sharon and Mike Jawer. And we’ve dipped our toes into artificial intelligence and Jewish ethics with Jeremy Epstein and Dr. Rebecca Epstein-Levy.

Today our panel is called “How Long is Forever?” with two noted speakers, Doctor Michael Dine and Rabbi Melanie Aron, who I understand do not usually appear on panels together. So I’m glad we are the beneficiaries of this experiment.

And now I have the pleasure of introducing our panelists. So I don’t usually get to cheat like this, but if I can quote the Wikipedia entry for you, Michael Dine is an American theoretical physicist specializing in elementary particle physics, supersymmetry, string theory, and physics beyond the standard model. His PhD is from Yale University, and he spent a number of years doing research at the Institute for Advanced Study in Princeton – yes, that is the one that Einstein was noted for – and the Stanford Linear Accelerator center in California before becoming a distinguished Professor of Physics at the Santa Cruz Institute for Particle Physics at UC Santa Cruz. He’s been a Sloan Fellow, a Guggenheim Fellow, a fellow of the American Physical Society, a recipient of the 2018 Sakurai Prize for Outstanding Achievement in Particle Physics theory, an elected fellow of the American Academy of Arts and Sciences, and an elected member of the National Academy of Sciences. Dr. Dine works on the phenomenology – that is, experimentally testable models for low energy, of super symmetric extensions of the standard model and of super-string theory.

I am literally lost in space after this, but I would note that Dr. Dine works on one of my favorite concepts: the imbalance of matter and antimatter in the universe, and especially that whole dark matter stuff. And with Ian Affleck, he has proposed the Affleck-Dine mechanism that might provide a candidate for dark matter particles.

Dr. Dine is the author of a recent book, This Way to the Universe: A Journey into Physics, that offers a surprisingly readable introduction to modern physics. I highly recommend it. We are awed and honored to have you with us, Michael, to tell us what science has to say about the nature of the universe and eternity. So I think we should hold questions until after your presentation and then we’ll proceed from there.

Michael Dine: Okay, so I took time as my subject. Melanie will deal slightly differently with some of this. But time is a crucial concept in science. It is also somewhat elusive. And even though it’s a little bit tautological, time is a thing defined by the laws of nature. The laws tell us how things behave in time. Observing such behaviors allows us to define time and to measure it.

Newtonian Physics and Time

So, time in the cosmos – I’m thinking of the universe as a whole, whatever that may be. And Newton already thought about this question – so Newton formulated, sort of gives us, our notion of the laws of nature. And having put forth a set of basic laws of nature, he tried to think about the universe as a whole. He encountered many puzzles; time was definitely one. His statements on this are, at best, obscure. He was clearly troubled.

Newton, on time, says, “Absolute, true and mathematical time, of itself and from its own nature, flows equably without relation to anything external… Absolute space in its own nature, without relation to anything stable, remains always similar and immovable.”

I have actually no idea what he’s saying, and I don’t think he did, either. I think he was trying to sort of cover up. And he even invokes God in the picture. So that seemed appropriate for today. He says, “Absolute time is not an object of perception… The Deity “endures forever and is every where present; and, by existing always and every where, he constitutes Duration and Space.”

So again, this was clearly a kind of a little cheat. So while it feels that Newton is unwilling or unable to squarely face the issue, something like this viewpoint, as far as I can tell, would dominate science, particularly physics and astronomy, for almost three centuries.

It is Einstein who confronts the question of time, both within natural laws and in nature as we observe it. And that’s where nature is. If we observe it, it’s where this question of “Is time forever?” [is]. So, contemplating the universe as a whole, prior to Einstein’s General Relativity in about 1915, the observations of Edwin Hubble, the famous astronomer, and others, [we had] no real idea of what the universe looked like on very large scales. And that’s kind of what interests us. What does the universe as a whole [look like]?

The existence of galaxies, much less larger structures, was not really appreciated. And there were paradoxes – for example, if the universe were the same in all directions and extended to infinity, why would the night sky be dark? Light from stars as far away as one likes would light up the sky. So this was the famous paradox.

General Relativity

Now, this changes with Einstein’s general relativity. Einstein’s relativity allows scientists to ask sensible questions about time and about the universe as a whole over time. First, a little bit about Einstein – so, Einstein, unlike anyone in physics, except perhaps Newton or maybe Niels Bohr, is a figure who looms really large. Prior to Einstein, there were Newton’s laws of thinking about nature, thinking about astronomy, thinking about the universe. One had Newton’s laws of motion and gravitation, and these explained the motions of planets with great precision, and they could understand the motion of nearby stars.

But the universe as a whole was a challenge. And Einstein reshaped our understanding of space and time itself. There are precursors to Einstein and what Einstein did. So Mach, Ernst Mach, who was one of Einstein’s early heroes, who was a kind of scientist and philosopher, wrote that “Absolute time is a useless metaphysical concept. It cannot be produced in experience.” And he said Newton “acted contrary to his expressed intention only to investigate actual facts.” And I think that’s consistent with this quote I read you from Newton, where he’s kind of clearly fudging a bit. And Poincare wrote – he was a great French scientist – “Not only do we have no direct intuition of the equality of two times, we do not even have one of the simultaneity of two events occurring in different places.”

And this is remarkable. This is something that really precedes Einstein and was really Einstein’s great discovery. And it’s something where time becomes sort of more elusive, and a kind of relative concept.

So Poincare, in fact, had much of the mathematics of special relativity, as did Lorentz, but neither made the final leap. So, Einstein’s first step in this direction was special relativity. Einstein’s special relativity was sort of two relativities. He had one which is referred to as “special,” and one which is general, the “general.” The special relativity is the theory, really, of how time looks and how space looks. And general relativity is the theory of gravitation, which is relevant for thinking about the universe as a whole.

Now, by Einstein’s time, Maxwell’s equations – the equations which describe the phenomena of electricity and magnetism – were well-established. And studying these equations, Einstein realized that they do not admit an absolute time. There is also no action at a distance. Newton’s theory of gravity has the feature that the motion of a planet here instantaneously affects the motion of a planet over here, or the sun over here. And Newton knew that didn’t really make a lot of sense, but […] it was just a feature of his theory. And his theory worked pretty well, so he was happy with it. But in Maxwell’s theory of electricity and magnetism, there is no action at a distance. And events at one time affect those at other times through the emission and absorption of light.

Special Relativity

Einstein then developed this theory of special relativity. So, with special relativity, absolute time is lost. Different observers measure different times on their clocks. Events which are simultaneous for one observer are not simultaneous for an observer moving relative to the first. And crucial to the working of all this is that Maxwell’s equations involve electric and magnetic fields, quantities which exist everywhere in space and time, not just where charged objects are. And this framework is what comes to be called space-time.

So, Einstein developed a special relativity in 1905, but while this resolved the question of action at a distance, it did not fix the same problem in Newton’s theory of gravity. Solving this would take Einstein the better part of a decade and would undermine more completely our naive notions of time.

So he had certain guideposts to developing a theory, to trying to figure out how to make a theory of gravity which would be consistent with the principles he had enunciated – his theory of special relativity.

And so, for slow motion – for masses [that are] not too large – Newton’s law should be valid. There should be no action at a distance. And really critical for Einstein was to focus on something called the equivalence principle – the equivalence of the mass, which appears in Newton’s laws of motion, and the mass, which gives rise to the gravitational force. This inertial mass is related to how we define what we mean by time. It’s related to how things move. So, time gets related to the course of gravity.

In 1915, Einstein publishes his general theory. After a long struggle, he realized that the equivalence principle required reformulation of gravity in terms of the geometry of spacetime. The mathematics of the theory was challenging – this was a big part of his struggle. But mathematics remains a challenge for even a contemporary (graduate student), but the underlying concepts are not so hard to understand, and are discussed in this book, which I mentioned.

Einstein’s Limitations

So he proposed this theory of what shows up in the “real” world. So the predictions of Einstein’s theories, in most situations, are almost the same as Newton’s.

For most of the 20th century, there were only three tests of Einstein’s theory. There was something called the perihelion procession of mercury, a slight anomaly in the motion of the planet not accounted for by Newton’s laws. And it accounted for that.

There’s something called a gravitational redshift – a shift in the wavelength of light as it passes through a gravitational field near a massive object, a star or a planet.

And there’s a bending of light by the sun.

But the more dramatic phenomena were known to be possible – black holes, collapsing stars, which might yield objects so dense that light could not escape, but the gravitational force would actually hold light in.

The universe as a whole could be studied with this theory. At some point in its history, the universe was necessarily extremely dense, its space-time highly curved. The “Big Bang,” originally a disparaging term coined by the astronomer Fred Hoyle, who was the advocate of a “steady state” model, caught on. So there’s a term for this.

The theory also predicted gravitational waves, much like light waves – gravitational ripples in space-time, spreading through space-time. In the late 20th and early 21st century, black holes were observed and studied, and also dramatic events occurred in our understanding of cosmology, where cosmology, here, means sort of “the history of the universe as a whole.”

The “Fossil” the Big Bang Left

So what was studied was the cosmic microwave background. We’ll see a little more about this as we go along – a fossil of the Big Bang discovered in the early 1960s. Some very detailed studies were made in the late 20th and early 21st centuries. And the structure of the universe on very large scales, the composition of the universe, is something that came to be understood and known. It’s worth mentioning that your GPS devices are a day-to-day technological application of general relativity. General relativity actually enters in achieving the required accuracy for these devices.

Gravitational waves were first inferred from pulsar timing. Pulsars are particular star systems, but they were actually detected in 2017, and there was a Nobel Prize awarded in 2017 [for research] involving emitting by colliding neutron stars and black holes. Very massive objects approaching very close to each other would emit these gravitational waves.

So, in fact, among the black holes, there’s a black hole at the center of the Milky Way. And I like to tell a story. So, it was suspected for some suspected for a long time that there was a supermassive black hole [at the center of our galaxy], a black hole with a mass of many thousands of suns. In 2000, actually, I spoke at a conference right after a NASA administrator who talked about a huge NASA project to image this black hole, to develop a picture of it – a very glitzy presentation. But shortly afterwards, it was imaged by a beautiful small-scale experiment.

So this is the woman, Andrea Ghez, who’s in the center of this picture. She was a winner of the MacArthur Prize and won the Nobel Prize in 2020. She provided evidence for a black hole near the center of the galaxy from its effects on the motion of nearby stars. So she looked at stars near the center of the galaxy and was able to infer the existence, from their motions, of a very massive black hole.

Now, another thing that Einstein’s equations did, and Einstein himself did, was open up the possibility of thinking about the universe as a whole. So, with Einstein’s equations, we can contemplate the universe as a whole, [but] we need some starting assumptions, and the basic one is something called the cosmological principle that, on very large scales – viewed in a very gross way, in a very smoothed-out way – the universe looks the same wherever you are – that’s called homogeneity – and it looks in the same whichever direction you look. That’s something called isotropy. That seems a bit crazy because the universe around us doesn’t look like that at all. But it is the simplest assumption which one can make, and it turns out to actually be remarkably accurate when it speaks about the universe on a large scale.

So, with an assumption about what the universe is made of at different epochs, we could write a history of the universe starting with the Big Bang and evolving to the present time and into the future.

Einstein was, in fact, startled to discover that his equations implied a very dense – infinitely dense, in fact – starting point, followed by a steady expansion. He didn’t like this; he knew nothing like that from observation. So he tried to modify his equations to fix that. But Hubble, shortly afterwards, discovered that, in fact, the universe is expanding. He discovered that if you look in all directions, that stars are moving away from us, and more and more rapidly the farther away they are.

In fact, this picture has settled down, and we have a notion of certain key epochs in cosmic history. So there are a few things that we really test. At a time less than about three minutes after the Big Bang, light elements form – nitrogen, helium, lithium – and the ratio, the amount of these, is one of the ways that one, in fact, tests these theories.

At times less than about 100,000 years after the Big Bang, matter was tightly packed together. The universe was very hot, and atoms and molecules were broken into their constituents – so, electrons and protons and neutrons, for example, all floating around. At a time bout a billion years after the Big Bang, stars form and begin to burn. And here we are – our present moment is about 12 billion years after the Big Bang. The long-term future is something about which we can speculate.

So, what’s the evidence for this? First, the observed expansion of the universe. The second is this cosmic microwave radiation background. It’s a sort of “fossil” of this very early time, and it’s from a time at which the electrons and protons joined to form neutral atoms. So this time is about 100,000 years after the Big Bang. Remember, we’re at about 13 billion years now, and the abundances of light elements, which I mentioned. So, as I said, this cosmic microwave background is a fossil from about 15 billion years ago. It was discovered by Penzius and Wilson. I sort of mentioned Penzius because his daughter is a rabbi in Santa Cruz. And this is just an illustration of the spectrum of the background, which fits the theory extremely well – so, it gives a measure of the temperature. The temperature today of the universe, in some sense, is about 3º Kelvin, which is very cold.

And this has been studied with greater and greater precision – relatively recently, with something called the WMAP Satellite. And what’s measured here is the temperature of this background radiation. This is an exaggerated picture. The variation is very small. It’s one part in about 10,000. But the colors indicate these slight differences in temperature.

We know the energy budget of the universe now and in the past. So we know what the universe consists of – today, about 4.5% or so is sort of ordinary stuff, the stuff we’re made of. There’s something called dark matter. We don’t know exactly what that is – something I spend a lot of time thinking about. It’s about 25% of the energy. And there’s something called dark energy, which is rather mysterious. It’s actually related, probably, to one of the things Einstein considered when he tried to avoid expanding the universe. It doesn’t do that, but it is something he proposed then, something called the cosmological constant. It’s a form of energy with negative pressure, and it’s about 72% of the universe. And we know of the energy density of the universe, and we know that from detailed measurements – mostly studies of supernova explosions at great distances. And we’ve known that for now about 20 years.

Energy and Time

And we also know the energy budget at early times from Einstein’s equations. We can run the clock backwards – so, getting back towards time. And so I just sort of indicated here somewhat – the dark energy is irrelevant at early times. This is about nearly Big Bang. The universe is, in theory, about half a billion years old. Dark matter is a larger percentage of 63%, and photons, particles of light, or 15% and atoms are about 12%. So mainly, they’re larger because of this dark energy – this dark energy is actually only becoming important roughly now in the history of the universe.

Let’s say dark matter is about 30% of the universe. And one of the spectacular pieces of evidence for dark matter is something called the bullet cluster. So, this is a picture of the bullet cluster – two colliding clusters of galaxies. As they collide, they pass through each other. They provided evidence that the dark matter does not interact with normal matter or with itself. So they’re two clumps of stuff, and they just pass through each other.

So what about the long-term future of the universe? That’s sort of one question we started with. Because of the dark energy, as I said – a form of energy with negative pressure, which constitutes about 70% of the energy of the universe – it appears that the universe will go on expanding forever, and now it expands more and more rapidly because of this. And it breaks up into regions, each about the size of our observable universe – 15 billion light years across – which can’t communicate with each other.

So, there is the universe eternal? That’s sort of the question we started with. So there are speculative ideas about the Big Bang in which the universe doesn’t have a beginning. There are also speculative ideas about how the universe might have an end. And there’s some evidence for some of these ideas, but they’re hard to seriously test.

And before three minutes, at this very early time, we do know some things. We know there was a period in which the universe expanded very rapidly. And this was dubbed, when it was first considered, something called “inflation,” for an obvious analogy. And there’s indirect evidence for it, but there’s much we don’t know – for example, exactly when – we know it’s seconds or hours or something like that after the Big Bang. But we would like to know more precisely when around this time, the matter of the universe was created. This was this possibility that was mentioned – both the ordinary matter and the dark matter. And much we’d like to know about this.

And did time have a beginning? And this is where much of the current research focus sits. There are very early times where things were very close together. It’s sort of an intersection of our questions about very large-scale structures in nature and the very small-scale structures.

So we need to understand – this is sort of what has driven me in this area – this question of what the laws are of nature are  that govern things on the very small scales. This is sort of this particle physics idea, and how that’s connected with the universe as a whole.