About a week ago, I had a contest on io9 calling for questions about the universe. I got one that seems, on first blush, to be kind of silly, but on further inspection is kind of awesome. Cameron asks:
What impact will the expanding universe have on humans exploring space?
Locally, not at all, but if you wanted to travel to other galaxies — especially the most distant galaxies — everything gets crazy.
I’m going to do my actual (non-technical) column on io9 answering this this Thursday, but I wanted to give a heads-up to the truly devoted. There are at least 4 issues:
- Time slows when you’re going fast.
The issue is in how we might imagine traveling over interstellar distances. We’re human, which means that we need to slowly get up to speed, accelerating at a rate of 1g until we’re going close to the speed of light, and then decelerating at the tail end of the trip.
Of course, if we’re going close to the speed of light, then Einstein tells us that time appears to slow. I already did a technical post on this, but only to the nearest stars.
In other words, to travel 100 light-years, even though you’ll never hit the speed of light (let alone break it), according to your personal clock, much than 100 years will have elapsed.
- Distant galaxies are a moving target.
Just because a galaxy is 5 billion light years away now doesn’t mean that it would take 5 billion years to get there, even at the speed of light. Every day these distant galaxies get further and further away.
- An expanding universe drags your ship.
The expanding universe slows down your ship, just from the expansion of spacetime itself. It’s the same thing that causes light to get redder and redder (lower and lower energy) as time goes on. Just to keep at a constant speed, you need to keep your foot on the gas.
- In an accelerating universe, there’s an ultimate horizon.
We live in a universe of dark energy, which means that the universe is accelerating, and there is a particle horizon beyond which can’t ever reach, no matter how quickly we travel.
This is to say nothing of other potential weird effects like the fact that you can’t be sure of what you’ll reach when you get to your destination (or even if the galaxy hasn’t already been destroyed in the collision), or that in a curved universe, you and a friend might head off in different directions and eventually meet.
This question got me so excited that I did the detailed calculation to figure it out. This goes well beyond what I’d normally post here, but if you’d like to have a look:
There are some awesome results, including:
- You’d need a matter-antimatter drive the size of the moon to carry about 500kg of cargo to the edge of the universe
- It would only take about 45 years (ship time) to reach the edge of the universe.
The detailed numerical calculations were done using a Python code, which I’m happy to share with you.
This code generated the figure up at the top which shows (for arbitrary distances), how long it takes to get there according to both the rocketship, and to the cosmos generally.
I hope you’ve enjoyed my nerdsplosion, and keep an eye out on Thursday of this week!
Update: 6/20 My greatly expanded version of this is up on io9!