# Leaving the Milky Way

If we had a hypothetical spacecraft, would it be possible to exit our galaxy, wait for some time and enter back into another section? Will this method be more efficient that a more traditional flight path?

• Yes. Hypothetically you can go anywhere with Hypothetical spacecraft. But the speed of that hypothetical spacecraft cannot be greater than speed of light. So if we consider the spacecraft having the speed equivalent(not exact equal) to speed of light then you will require 500 Light year to go away from the disk of our galaxy as thickness of our Milkyway is 1000 light year or so and we are in the middle of the disk. – AmitG Mar 31 '15 at 21:53
• Actually, due to the Lorentz Contraction and time dilation, you actually CAN travel long distances in minimal time, even "faster than light", but only because the distance gets smaller. For example, you can reach the Andromeda galaxy in 28 years: math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html – user21 Apr 1 '15 at 17:12
• @barrycarter: That's 28 years ship time; it's still about 2.5 million years for the folks back home. – Keith Thompson Apr 1 '15 at 21:44
• Theoretically if you plan to cross a fair distance through the milky way, a gravity assist around a fast moving orbit could be beneficial. Using stars rather than trying to fly outside the Milky way would probobly be the best approach. It would still be a crazy-long journey though. – userLTK Apr 2 '15 at 10:31
• @userLTK I wonder how large the acceleration would be during such a gravity assist... – gerrit Jul 28 '15 at 13:45

Not really, for the same reason that you cannot travel west by jumping up in the air and let Earth rotate underneath you, such that you land a little farther to the west.

The reason is that standing on Earth's surface, you already have a velocity toward the east which matches exactly the speed of the surface. Thus, in the reference frame of Earth, you simply jump up and down.

In the same manner, if you use your mad hypothetical spacecraft to fly "up", i.e. away from the Galactic plane, you already have a velocity of $\sim$250 km s$^{-1}$ in the direction of the rotation of the plane, so that in the reference frame of the plane, you simply fly straight up.

Apart from this, in order to leave the Galactic plane, you need to fly roughly 500 lightyears. This will take a long time. You should stay home.

• Actually, you do travel west when you jump up and land, but only by a miniscule amount, because you travel at a constant velocity but the Earth's surface accelerates (moves in a circle). physics.stackexchange.com/questions/48287/… for more – user21 Apr 1 '15 at 17:09
• Good point! Negligible for most purposes, but nevertheless true :) – pela Apr 1 '15 at 19:16

It depends greatly on the nature of your "hypothetical spacecraft".

Any spacecraft travelling through interstellar space will have to deal with interstellar gas and dust. At sufficiently high relativistic velocities, running into a hydrogen atom is like being hit by a high-energy cosmic ray, and running into a dust particle could seriously ruin your afternoon.

The intergalactic medium has substantially less matter than the interstellar medium within the galaxy. A route that exits the body of the galaxy by going "galactic north", then travels parallel to the disk, then re-enters the galaxy at another point will obviously be longer than a straight-line route, but it will encounter significantly less matter along the way. If the maximum safe velocity of the USS Hypothetical depends strongly on the density of the medium through which it's travelling, then the indirect route could be faster.

Whichever route you choose, this will take a long time. You should bring a book and plenty of snacks.