An interview with Dave Goldberg, astrophysicist at Drexel University, who has just come out with his second book, "The Universe in the Rearview Mirror" (Dutton). We take the opportunity to ask him about antimatter, a neglected genius who taught at Bryn Mawr College, and whether "Terminator" or "Hot Tub Time Machine" got time travel right. (Read an excerpt here.)
Q: What's the great take away from "The Universe in the Rearview Mirror"?
Goldberg: The big idea - and this is not my big idea, I didn't invent this -is the concept of symmetry, the laws of physics are all built up from a very simple, elegant foundation. Symmetry basically means that the universe -- or at very least the laws of the universe -- will look the same no matter how you turn or move your experiment, whether you're looking at it in a mirror, and so on. Virtually all the laws of physics don't care if you look at them in the mirror. They'd still work just as well. That's a symmetry.
Or take the moon, the planets or the sun. The strength of gravity toward the sun will be the same in all directions. That's symmetry, too. It's only when we're standing on the earth that it doesn't seem that way, and we can tell down from up.
Q: But not everything in the universe is symmetrical, right?
Goldberg: Some of the most interesting things are almost symmetric. Antimatter is a big one. For people who are drawn to science fiction or popularized science, antimatter is something they will have heard of. It has a bad reputation: You have a lump of antimatter and combine it with matter. Put them together and it all gets annihilated. We have to go to great lengths to make [antimatter] in the lab. But when we do so, the laws governing it are almost exactly the same as ordinary matter. You could swap every atom in the universe for its antimatter version and vice-versa, and we'd never notice.
But here's where things get strange. When we create a lump of antimatter we also produce the exact same amount of matter, and when they meet they annihilate in equal amounts. My role with this book is largely connecting the dots. If matter and antimatter are created and destroyed in equal amounts, why are there are no anti-people or anti-galaxies? And there aren't. Believe me, if one galaxy collided with an anti-galaxy, we would see it across the cosmos because it would be really spectacular.
When we look at the nature of symmetry it opens up some very big questions. We don't know why we exist, at least as far as the fundamental laws of physics are concerned, since really, as near as we've been able to reproduce in the lab, the two should have destroyed each other completely in the first few seconds of the universe.
Or take another really strange and incredibly important symmetry, one that's surprising to people when they hear it for the first time: The laws of physics look the same even if you switch the arrow of time. So why do we remember the past but we don't remember the future? There seems to be a very clear demarcation between the two.
But at a fundamental level, the laws of physics can't tell the future from the past. Here's a simple example. If I throw you a ball - especially on the moon where there's no air resistance- it has a perfect parabolic arc. Take a movie of the ball as it moves. That movie will look just as natural if you watch the movie going forward or backward. It's the same in big particle accelerators. If we were to take a movie of the interactions, they would look valid going forward and backward... with tiny exceptions with the weak force. That turns out to throw all sorts of interesting complications into the mix.
Q: Who was Emmy Noether, why is she important, and why haven't we heard of her before?
Goldberg: Emmy Noether was almost an exact contemporary of Einstein's, an absolute genius in every way you could imagine. In Germany at the turn of the century, she was encouraged to become a language tutor rather than pursue additional education. Because she was a woman, she wasn't allowed to take classes at the University of Erlangen, where her father taught, or anywhere else in Germany. Instead, she had to audit classes on her own, and ultimately sit for the big graduation exam in neighboring Nuremberg.
In 1915, she was asked by (math superstars) David Hilbert and Felix Klein to join them at the University of Gottingen. The thing that was amazing, she wasn't allowed to lecture under her own name. It took years and years to get her to get even a modest salary for all the work she was doing. In the early 1930's, she came to the U.S. and to Bryn Mawr. Sadly, she died only 2 years later, in 1935.
To physicists, her major contribution is something known as "Noether's Theorem." Noether's theorem is very straight forward to describe, but physicists have great difficulty reading her paper. It's an intimidating piece of writing, but it can be boiled down. Unlike Einstein's papers, which were written like 'I'm going to walk you through this thing,' Noether is just: 'Here's the math, good luck.'
Q: What did it say?
Goldberg: The biggest theme in ("Universe in the Rear View Mirror") is symmetry. All these things I'm listing -- time, the fact the laws of physical laws are the same everywhere in space, the laws of physics always act the same in any given direction (there is no up in the universe) -- all those symmetries tell you something beyond a mere curiosity: Every symmetry turns out to yield a conservation law. That's Noether's Theorem.
The laws of physics are constant over time. And if they don't change, Noether showed that energy must be conserved (a fact that had experimentally been known for quite some time). She just didn't take it as an observation she found in the universe. She explained why energy can neither be created or destroyed. She proved it.
The laws of physics are the same everywhere in the universe. Momentum is conserved. Objects in motion stay in motion, objects at rest stay at rest. Newton knew this, but Noether gave a much deeper explanation of why.
What's incredible about this? The ideas of the next generation of physical laws - that are not yet proven and may in fact be wrong- they all stem from a direct or indirect sense from Noether's theorem. Grand Unified Theories, a possible Theory of Everything, they're all built around symmetries. They predict things like new particles and new interactions. Their importance, in that respect, cannot be overstated.
Q: You've written a lot about time travel. Will it ever be possible? And what's your favorite time travel movie?
Goldberg: If time travel is possible, and that's a big if, it's possible because of solutions to general relativity, Einstein's theory of gravity. When people "invent" a time machine, they basically solve Einstein's equations and come up with a universe that is entirely self-consistent.
In these models, there is (only) a single history. If you traveled back in time, then interacted when you arrived, you'd change nothing. Even before going back, people would remember you from the past. The time machine must already have existed. What's more, just as you need an entrance ramp and an exit ramp to get on a highway, you cannot just punch through space-time.
Anyone discussing time travel -- scientifically, at least -- should concede those points, though most pop-sci readers get up-in-arms to defend their favorite time travel model or story.
I wrote an article about it for io9.com but it was intended to be faux serious, totally tongue in cheek, in the persona of a physicist complaining to the writers and directors of "Hot Tub Time Machine," about all of the flaws in their time travel models. In the movie, the character played by Craig Robinson, he and his band perform the Black Eyed Peas song ("Let's Get it Started"). Forgetting that here's a song his character never wrote and yet he's performing it, who did hear it from? Presumably the Black Eyed Peas. But it hasn't been written yet. Try to follow the timeline of its creation, it hasn't been created by anyone yet. It's a ridiculous paradox. History is not a matter of being 'close enough.' History is what actually turned out. If you go back in time and change things, that's not a self-consistent loop.
"Terminator" is the best. and "12 Monkeys" is also a time travel movie that is self-consistent. "The Time Travelers Wife," while not a great movie, is good when it comes to time travel. And while "Back to the Future" may have been a great movie, the quality of time travel in it is pretty bad.
Getting back to your question, though, time travel may well be impossible.
One of the things that these solutions (to time travel) have in common, is that they rely on a distribution of energy we don't know can exist.
Wormholes (a very popular mechanism for theoretically building a time machine) are supposed to be held open by exotic energy, a totally hypothetical substance that essentially has negative energy density. It's not clear that exotic energy can exist, and we've certainly never detected it. That's just to keep the wormhole open. To open it in the first place would require punching a hole through space time. We don't know how we'd create it or maintain it.
Physicists can solve the equations for time machines, but the (required) distribution of energy is something that is likely impossible. Einstein is someone who was among the first to come up with a solution for a wormhole, for instance, but Einstein's solution would collapse before a single photon could go through it. We don't lack the will to build a time machine, it's just that each one seems to have a fatal flaw. If you come up with design after design, and they don't' work, it could be the entire enterprise is impossible.
Q: Someone dubbed you the "coolest nerd physicist." How did you feel about that?
Goldberg: That "someone" would be my publisher. It's really embarrassing, and isn't not a sentiment I share. My daughters and my mom are probably the only ones who would. And perhaps one or two of my students.
Q: Some final questions:
Where did Big Bang happen? Should I be afraid of black holes? And where can I find dark matter?
Goldberg: The Big Bang happened everywhere. There's a common perception that the universe expanded from a point. But the expansion happened here, over the Andromeda galaxy and over the cosmic horizon. About 13.8 billion years ago, those places were all at the same place.
Should you fear a black hole? You could, but only if someone were to launch you into one. If the sun were to collapse, you might freeze to death, but you're not going to be pulled in -- another common misconception. We would keep orbiting around. On the other hand, if a friend throws you into a black hole, you will fall in, and be stretched and stretched, in a process called spaghettification. In a period of 0.2 seconds you will go from being uncomfortable to having your atoms ripped apart.
And where can you find dark matter?
As far as we can tell, it's everywhere. It's going through you all the time. It doesn't react very strongly which is why we've never been able to detect a dark matter particle.
Q: Anything you'd like to say in parting?
Goldberg: I hope that people enjoy the book. It's written for people who are interested in the big questions. I see myself as being a guide in that direction. I try to make it very accessible. The book becomes progressively more sophisticated over the course of it, but it's absolutely not for scientists. They're allowed to read it, but it's not written for them.