Space travel in our accelerating universe: Sometimes you can’t get there from here
The discovery that our universe is not only expanding but accelerating in its expansion led to a trio of Nobel prizes in 2011, but what does it mean for you? Well, it turns out, according to Drexel University physicist Dave Goldberg, the acceleration of the universe would influence space travel — if we were somewhat more advance in the speed department.
For now, the latest science of the cosmos still has a considerable influence on how science fiction writers can correctly speculate about intergalactic trips made by civilizations more advanced than ours. No matter how smart alien civilizations might be, or how smart humans might become in the future, it turns out there are situations in which the laws of physics say you just can't get there from here.
The issue came up for Goldberg in a question someone posed for a column he writes, called "Ask a Physicist," for the science and science-fiction-oriented website io9. Goldberg, who is author of a brand-new book on symmetry in physics called "The Universe in the Rearview Mirror," decided to consider the effects of the universe's accelerating expansion on the very farthest trip possible.
So the first interesting thing is that there is a farthest trip possible — not a farthest trip possible within a human lifetime or within the time between now and the end of the universe but a farthest distance anyone can go according to the laws of physics. It's not that there's an edge of the universe, reminds Goldberg, but physicists often talk in terms of horizons — distances past which we can't see, or can't go.
Just as every ship on the ocean is in the center of her own horizon, so every planet is in the center of its own horizons in space — particle horizons beyond which inhabitants can't see, and event horizons beyond which they can't travel.
Even before the universe was known to be accelerating, scientists talked about particle horizons — distances beyond which we can't see because there hasn't been enough time for light to have gotten here from there. Since the universe is about 13.7 billion years old, we aren't ever going to see anything past 13.7 billion light years away.
But acceleration creates a different horizon, said Goldberg. Even if you were so smart you could make a ship that travelled at 99.9999999999999999998% the speed of light, you could never reach galaxies past about 16 billion light years away. Why? Because the space between us and them is expanding faster and faster. So let's say you want to go to galaxy X, which is 16 billion light years away. Once your advanced ship had travelled 16 billion light years from home, the space between you and your goal would have doubled, so your destination would still be 16 billion light years away.
(I should point out, parenthetically, that the expansion of the universe applies to the behavior of distant galaxies, but not to stuff within our own galaxy, let alone our own solar system, where gravity, thankfully, holds things together. So when we send a spacecraft to, say, Jupiter, we don't have to worry that it will be speeding away from us.)
On the positive side for potential space-faring creatures, Einstein's special relativity suggests the trip can go rather quickly if you get going close enough to the speed of light. Some of you might remember that old twin paradox, in which one twin goes flying off near the speed of light and the other stays home and when they're reunited, the homebody would be older than his or her sibling.
So let's say you pick an ambitious but not physically impossible trip to some point more than 99% of the way to this farthest possible distance. To make any headway, Goldberg said, you really need to get to a cruising speed of 99.9999999999999999998 percent the speed of light. Now for a while you'll have to press your foot hard on the antimatter gas pedal to reach that speed, so he proposes a comfortable acceleration rate just enough to stick you to your seats with a force equivalent to the Earth's gravity, g. If you do that, he said, you can get to your destination after about 20 billion years or so have elapsed back on your home planet, but for you in the spacecraft, only 45 years would have passed.
"I was very surprised by it," said Goldberg. The time elapsed for the astronauts came out on a surprisingly human scale. He also learned that someone has already written a science fiction story based on the concept. In "Tau Zero" by Paul Anderson, apparently people go flying faster and faster and time keeps dilating until the whole universe ends.
Note: Dave Goldberg's book "The Universe in the Rearview Mirror" is officially out on July 11.