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Antimatter, a Bryn Mawr-ter and 'Hot Tub Time Machine'

What do they have in common? Astrophysicist Dave Goldberg connects the dots.

Dave Goldberg, Drexel astrophysicist.
Dave Goldberg, Drexel astrophysicist.Read more

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.

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.