Black holes bring up possibilities as crazy-sounding as time travel, tunnels through space called wormholes, even parallel universes. It's amazing that when I started my career in the 1980s, we weren't sure if black holes existed. At the time, they were a solid prediction of general relativity, the theory of gravity that Albert Einstein developed almost 100 years ago. But we didn't have a "smoking gun," convincing evidence that they had been observed in nature. And in science, even what to us may be beautiful and compelling ideas need to be tossed away if nature doesn't cooperate.
Now, of course, things are different. Astronomers have compelling evidence that most galaxies harbor a huge black hole in their centers, a kind of eye of the storm. Our own Milky Way has a behemoth with mass equal to that of some 3 million suns. How can we be sure without going there? Stars near the galactic center move as if they were attracted by a huge mass with practically no volume. The only explanation we have for the culprit is a black hole, a region of space where gravity is so intense that not even light can escape. (Presumably, they do emit electromagnetic radiation with a rate inversely proportional to their mass, the so-called Hawking radiation. Although we haven't detected it, it does seem to be a very reasonable consequence of bringing quantum effects into black hole physics.)
In astronomy — as in any other branch of science where the object of study is too remote, too small or too far removed in time — we must construct explanations based on the best description that fits the data. We won't land on another galaxy, grab an electron with our hands or ride a dinosaur. But we can be quite sure that these entities exist(ed) based on the evidence we collect.
Wormholes are the next step, essentially tunnels in space that can act as shortcuts between distant points. The fun part is that, depending on the type of wormhole, the points can be separate spots in this galaxy, in different galaxies or, get this, even in different universes. Contrary to black holes, though, wormholes remain highly speculative. Their first incursion into physics came in a 1935 paper by Einstein and his collaborator Nathan Rosen. Their wormhole is called an Einstein-Rosen bridge, connecting a black hole to a "white hole," the mirror image of its black counterpart: In a white hole everything comes out and nothing gets in, a sort of cornucopia in space spilling out matter.
We have no evidence that white holes exist; quite the contrary, it seems they don't. Fortunately, other types of wormholes don't call for white holes at one extreme. These are the ones used in the movies Contact and Interstellar. Kip Thorne, the scientist behind Interstellar, explains them in his book The Science of Interstellar, the companion to the movie where Kip attempts to explain some of the movie's more complex scientific ideas.
Replying to my review of Interstellar here a few weeks back, Kip wrote to me in an email: "Hopefully, the end will seem a lot less wacko when you read in my book the chapters Tesseract and Messaging the Past, and the technical notes on those chapters. The chapters on Singularities and Into Gargantua may also be helpful."
Kip's message expresses his hope — and that of many sci-fi fans — that such exotic ideas will pan out in the future. And that, one day, we will be traveling across interstellar distances to search for other worlds, other earths, possibly meeting other intelligences. (Or even ourselves in a different time ...)
We like to make analogies between the exploration of the Earth and that of space; I am guilty as charged, having done it many times. However, we must be very careful. Often, we focus on the challenge of the distance itself. Just to our neighbor stellar system in Alpha Centauri it's 4.4 light years, a distance that would take our fastest spaceship now some 100,000 years to cover. Even if we traveled at 10 percent of the speed of light — something that, in principle, is possible using the pressure of light as a kind of wind in spaceships that would have huge sails like flying galleons — the time travel is still very long, 44 years.
Add to the distance the following: lethal radiation abounds in space and is, in fact, a major deterrent even for us to get to Mars. To shield it is difficult and makes the spaceship heavy. Also, what kind of fuel can we use to travel for such distances and for such a long time? And what about hibernation? We always see people going to sleep for years in these movies and getting reanimated in a jiffy. It turns out that to induce a long-term cryogenic sleep is extremely complex and still not feasible due to the fact that water expands when it cools: To lower the temperature of a human body is to make the freezing cells explode. Given that we are over 65 percent water, substituting our water with some nonexpanding nonpoisonous fluid remains quite a challenge. Let's not forget collisions with micrometeorites, a possibility that, in a long trip, becomes quite high and a major problem.
Why am I being such a party-pooper? While I enjoy the sci-fi speculation like most people, I think there also is a chance here for us to learn something about ourselves and our planet. We just saw the headlines from the Lima Conference on Climate Change, that the rules will now change so that countries will set there own carbon emission limits — a disastrous outlook. As Naomi Klein wrote in her urgent book, This Changes Everything, why are we failing so enormously to change the course of history and curb emissions to protect our environment?
It's the economy, of course. Interstellar is, in a sense, a call to arms. By making interstellar travel look so far fetched, it makes our situation here and now all the more urgent. We have a single planet, a single chance to make this right. Whatever the possibilities of our future technology and cutting-edge science, we are bound to stay here for a very, very long time. We should thus heed to Edward Wilson's eloquent plea in The Meaning of Human Existence, that I repeat here:
"We alone among all species have grasped the reality of the living world, seen the beauty of nature, and given value to the individual. We alone have measured the quality of mercy among our own kind. Might we now extend the same concern to the living world that gave us birth?"
Let fiction guide your imagination. Meanwhile, the lesson is clear: Be the future of your planet. It is the only one we have and will have, it's our only viable home. It's not going to be pretty when 10 million New Yorkers have to move inland. Will you open your door for them?
Marcelo Gleiser, a world-renowned theoretical physicist and cosmologist, is professor of natural philosophy, physics and astronomy at Dartmouth College. He has published over 100 peer-reviewed papers and is the author of dozens of essays and four books, including The Island of Knowledge: The Limits of Science and the Search for Meaning. You can keep up with Marcelo on Facebook and Twitter: @mgleiser