What is the universe expanding into? How could we possibly make trips to other star systems? What happens when two black holes meet? Is a "theory of everything" within reach? Such are the questions that Cosmic Log readers posed for Stephen Hawking, arguably the world's most famous physicist as well as the world's most famous quadriplegic.
We've shipped off a selection of queries for Dr. Hawking to consider, but we can already address the questions we've just listed, as well as other questions relating to his favorite music - and even his favorite episode of "The Simpsons."
The questions we sent Hawking's way via e-mail focus mainly on the big mysteries: God, life, the universe and everything - plus space travel and weightlessness, of course. It'll take weeks for the good doctor to reply, and there's no ironclad guarantee he'll actually find the time to do it.
But in the meantime, Hawking's pronouncements continue to pop up in the news. Just today, for example, he caused a stir by observing that human activities are affecting Earth's climate in ways that "may forever change life on Earth." Some of the answers to frequently asked questions can be gleaned from Hawking's past statements, and others can be pieced together based on current cosmological theory.
In that vein, then, here are the likely answers to some of the deep (and not-so-deep) questions. If some of these answers aren't quite right, or up to the standards you'd expect from Stephen Hawking, that's my fault alone - and I'll look forward to your corrections and amplifications in the comments section:
N. Anthony: "Where does the universe end? Is it infinite? I've heard that the universe expands at the speed of light, but what is it expanding into?"
Here's what Hawking has to say on his Web site about the nature of our universe in four-dimensional space-time:
"... James Hartle of the University of California Santa Barbara, and I have proposed that space and imaginary time together, are indeed finite in extent, but without boundary. They would be like the surface of the Earth, but with two more dimensions. The surface of the Earth is finite in extent, but it doesn't have any boundaries or edges. I have been round the world, and I didn't fall off."
Like other cosmologists, Hawking would say the idea that the universe is expanding "into" something gives the false impression that we can perceive that "something." We often think of the expanding universe as the surface of an inflating balloon - but this analogy is imperfect, because we're trying to think of our three-dimensional space as a two-dimensional surface. Physicist Michio Kaku provided an explanation of all this a few years ago - an explanation that's actually a condensation of a longer answer he provided as part of the "Stephen Hawking's Universe" project at PBS.
Based on Hawking's no-boundary proposal, it wouldn't make sense to ask what came before the first instant of the universe's existence, or what will come after the last instant - just as it doesn't make sense, at least technically, to ask what on the earth's surface is north of the North Pole, or south of the South Pole. (Yes, I know there's "up" and "down," but I hope you see what I mean.)
Also, there would be no "edge" or "end" to the universe. If you extended a straight line in one direction, that line would theoretically come right back to the starting point. Of course, you'd never be able to check that out experimentally because the universe is so mindbogglingly big.
Gene Seawright: "If the universe is ever expanding, why will the galaxy Andromeda eventually collide with the Milky Way galaxy?"
On the largest scales, the expansion of the universe indeed is accelerating. But on smaller scales, galaxies are moving to and fro within local groups, influenced in part by gravitational interactions. The expected collision between the Milky Way and the Andromeda Galaxy - in about 3 billion years or so - is the result of such interactions.
Tim Nixon: "If the human race must move beyond Earth and colonize other planets in order to survive, how can we overcome the vast distances of space and the limitations of traveling at the speed of light?"
Hawking addressed this part of the question not all that long ago, in his interview with the BBC:
"Sooner or later, disasters such as an asteroid collision or nuclear war could wipe us all out. But once we spread out into space and establish independent colonies, our future should be safe. There isn't anywhere like the Earth in the solar system, so we would have to go to another star.
"If we used chemical fuel rockets like the Apollo mission to the moon, the journey to the nearest star would take 50,000 years. This is obviously far too long to be practical, so science fiction has developed the idea of warp drive, which takes you instantly to your destination. Unfortunately, this would violate the scientific law which says that nothing can travel faster than light.
"However, we can still within the law, by using matter/antimatter annihilation, at least reach just below the speed of light. With that, it would be possible to reach the next star in about six years."
Moreover, if we could travel that close to the speed of light, the trip would seem to take less time to the travelers onboard their matter/antimatter-powered craft, thanks to relativistic time dilation. This little applet demonstrates how the travel time would be shorter for the folks on board than for the folks watching from Earth, depending on how fast you travel.
Check out this archived article for more about antimatter drives and other exotic ideas for interstellar propulsion.
Steven Vanhee: "What happens if two black holes meet? Will the 'strongest' consume the other one, eventualy reducing the universe to one, triggering a second big bang?"
Physicists suspect that black holes do indeed collide with each other, setting off huge blasts in the process. But in Hawking's view, black holes don't last forever. In fact, one of Hawking's biggest contributions to physics is the view that black holes eventually fizzle out, due to a phenomenon known as "Hawking radiation." So the scenario of all the black holes being swept up into one big monster would be highly unlikely.
John B.: "Do you believe string theory is the Holy Grail of modern physics and if so how has it or will it impact our understanding of the universe both great and small?
In his BBC interview, Hawking stuck to his view that physicists could arrive at a "theory of everything" within 20 years - and that such a theory might allow scientists to "read the mind of God." However, in his book "A Briefer History of Time," Hawking speculates that there might not be one single theory to explain the whole universe. Instead, we might use a collection of theories to navigate the cosmos at different scales - just as we use maps at different scales to find our way around town or around the globe.
Brian: "What are your views about the current debate in the U.S. regarding fully funding stem-cell research, and what would you say to those who oppose such research that might lead to a cure for your condition?"
A few months ago, Hawking told The Independent that banning the use of human embryos for stem-cell research would be like banning the use of organs from accident victims:
"The fact that the cells may come from embryos is not an objection because the embryos are going to die anyway. It is morally equivalent to taking a heart transplant from a victim of a car accident."
Later, he told The Guardian, "We throw away many embryos in IVF [in-vitro fertilization] and no one objects. Isn't it better to use a few embryos to save lives?"
Of course, the debate over stem cells is more complicated than the organ transplant issue, in that an embryo can't give its consent for stem-cell extraction.
Thomas Ashby: "What is your favorite music? Who are your favorite musicians?"
Hawking's Web site provides the straight scoop: "I mainly listen to classical music: Wagner, Brahms, Mahler etc., but I like pop as well. What I want is music with character." Hawking said he went with his son to a Depeche Mode concert, "and my ears were ringing for the next 24 hours."
Robert LaNicca: "What is your favorite 'Simpsons' episode?"
One of his favorites - if not the favorite - would have to be "They Saved Lisa's Brain," the 1999 episode in which Hawking himself comes to Lisa's rescue. You'll find a screenshot from that episode posted on Hawking's Web site. Here's a snippet of dialogue from the episode:
Lisa: Oh, Dr. Hawking, we had such a beautiful dream. What went wrong?
Hawking: Don't feel bad, Lisa. Sometimes, the smartest of us can be the most childish.
Lisa: Even you?
Hawking: No. Not me. Never.