Could we travel to the nearest stars in a human lifetime? Yes. If we could harness the full power of the sun, it would be far less unpleasant than you think. Welcome to this week's "Ask A Physicist" column.

Before we get started, I wanted to put out a special request. A few recent questions in my inbox have motivated me to do a special column on superpowers in the next few weeks where I'll answer a few shorter superhero-related questions. Supervillainy is also allowed. As always, send me your questions.

Today's questions are a two-fer, from readers Curtis Wright:

Is long distance space travel ever going to be possible?

And from Tim Raveling:

But why is it that the spaceship experiences this time dilation and not the people of Earth? If positions and vectors are relative, couldn't it be equally said that the Earth is moving at relativistic speeds away from a stationary spaceship?

A few weeks ago, I ruined your prospects for designing an FTL drive. Today I want to make it up to you, and give you a game plan for traveling to another star system. I tackle this one in my book, but from the opposite perspective: Could aliens visit us if they wanted to? You can either treat what follows as an exciting prospect for the human race, or a grim warning that we could be conquered by an alien menace just in time for the summer blockbusters.

But first, a caveat: even though the actual interstellar journey won't be as bad you might think, the energy requirements are downright absurd. I've crunched a few numbers, and for the trip I'm going to describe, it would take the equivalent of about 100 quadrillion gallons of gas to reach the nearest star; that's about the same amount of solar energy as hits the entire earth every 7 minutes. And mind you, that's all supposing that we're just sending a single person through space in a cardboard box with "spaceship" written on the side. Real spaceships are considerably heavier and would require even more energy, so even under the best of circumstances our civilization is a very long way from interstellar travel. There's also the harsh reality of moving through space ¬– let alone at speeds close to that of light. There are high-energy particles out there, which will almost certainly damage to your no-no bits.

Sorry, Curtis.

Let's ignore the sad fact that ours is not a supercivilization (and other genetic issues), and plan a trip to the nearest star, Proxima Centauri, about 4 light years away. Someone will doubtless point out in the comments section that it's actually more like 4.2 light years (and that it's probably not a particularly promising destination anyway), but humor me for the sake of simpler numbers.

Supposing your ship is pressurized and you have power for your Wii, the trip is actually much more comfortable than you might suppose. For the first two light-years, you set your thrusters to accelerate the ship at one g. The means that if you stand with your feet toward the back of the ship, it feels just like home. In a curious coincidence, it takes almost exactly one light-year's worth of acceleration to get you moving up to relativistic speeds, and by the time you're halfway to Proxima Centauri, you're moving at 94.5% the speed of light.

By then, it's time to start decelerating, so you don't crash into the star when you arrive. You pick up your crap, and the front of the ship now becomes "down." I've made a fairly crude animation (mostly for giving talks about time travel) that you can see here.

You simply reverse course to find yourself back on earth. If you run the numbers, the entire trip takes only about 11 years, 2 months, 22.3 days, give or take. I don't know about you, but I was pleasantly surprised at how short this is. After all, you've traveled a total of 8 light years in just a little over 11 years and the whole time you've got your artificial gravity going. Not bad.

It gets even better. Remember that throughout your trip you're traveling at close to the speed of light, which means that time, travels slower for you than for the good folks back home. You only experience 6 years, 11 months, and 2.5 days. You've gone 8 light years in what seems (to you) to be less than 8 years. This does not mean that you've broken the speed of light barrier, by the way. Just as your clock slows down, you also see lengths around you shortened. Throughout your trip, Proxima Centauri seemed closer than it would have otherwise.

You know the rest. If you'd left an identical twin back on earth, you'd then be over 3 years younger than him/her upon your return. This brings us to Tim's question. When I brought this up a few weeks ago, I said that your spaceship clocks should appear to be running slow from the perspective of people looking on the outside. However, I also said that special relativity tells us that there's no way to tell whether you're moving or standing still. These two statements seem to be a contradiction (that's the "paradox" in "twin paradox"), since we know which twin was moving: the younger looking one (aka "you", aka "the one who flew away in a spaceship.")

After all of this, my answer to Tim is pretty anti-climactic. The rule about not being able to tell if you're moving only holds if there are no accelerations involved. You know that you're going to be younger than your twin if your artificial gravity flips every few years. Of course, those few extra years are probably scant consolation for the damage that's been done downtown.

*Dave Goldberg** is the author, with Jeff Blomquist, of "**A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty.**" (Become a fan on **facebook**!) He is an associate professor of Physics at Drexel University. Feel free to **send him your questions about the universe**.*

## DISCUSSION

OK, but given how close Proxima is to Earth what would continuous video audio communication look like to a traveler leaving Earth at relativistic speeds (until you were too far away to receive a useful video signal). And what would it look like (once you were again within range) as you decelerated toward Earth from relativistic speeds?

I assume that the pitch and speed of the continuous video would lower and slow down respectively and grow worse as you accelerated and got further away. and the reverse would occur as you decelerated toward Earth.