[extropy-chat] Explaining Time Travel via Wormhole

Lee Corbin lcorbin at rawbw.com
Fri Dec 29 05:37:19 UTC 2006

The passages in Kip Thorne's great book "Black Holes
and Space Warps" that concern the confusing topic of
time travel are perhaps not as clear as they should be.

I recently wrote the following for some friends as we
were attempting to clarify the ideas. The basic notion
is that if you travel by space-warp, then time travel is
possible!  If you send one end of a wormhole to the
stars and back, it's kind of "out of phase" time-wise
with local clocks.  More specifically:


Two clocks will run at the same rate if
(a) they are very close physically to each other, and
(b) relative to each other, they aren't moving quickly.

But these two conditions are true when you walk a few
feet through a wormhole to the other clock, yet they
are false if you consider physically catching up to
a moving spaceship, or lowering/raising yourself in
a gravity well.

To explain concretely, suppose that you stay home and
your identical sister goes on board a spaceship that
heads for Alpha Centauri at .999999 the cpeed of light.
Your sister has the other end of your wormhole in her
cabin, and you two chat the whole time. Even though
she's going near c, *through the wormhole* she's at
rest relative to you! In fact, she's just a few feet
away, and your conversation with her proceed normally
(no speedup perceived by either party). You even dine
with her in her cabin at one point, and later she
watches a video with you in your motel room.

By the ship-board clock, it takes two days to go to 
Alpha Centauri, although, of course, it takes four
years by Earth clocks. You and your sister talk
constantly during the two days following the ship's
departure. Staying cooped up in your motel room on
Earth, you only hear the news from her. So after two
days, you learn that the ship has landed on Alpha
Centauri, and you step through the wormhole. Sure
enough, you are on Alpha Centauri four years in the
future according to galactic time.

But when you and your new acquaintances from A. C.
step *back* through the wormhole, it's as though to
them they are stepping back in time four years. In
fact, it's still possible for them to see through
Earth telescopes that their ship is really only
about thirty times further away than Pluto, and
has a long, long way to go yet on its journey to
Alpha Centauri.

Your new friends lose themselves on Earth, and you
resume talking to your sister. You learn that the ship
is coming back! So according to ship-board time only
four days will have elapsed when the ship finds itself
back on Earth. Sure enough, two additional days later
you step through the wormhole to be with your sister,
and you are back on Earth at the spaceport, the ship
having been gone eight years! At the spaceport, the
calendars say that you are indeed eight years in the

But when you and she step back through the wormhole to
your motel room, it's relatively speaking eight years in
the past, and is only four days since the spaceship left
Earth, with its long, long, long trip still ahead of it.

Therefore, you may step through again to the future, read
up on stocks, return to your motel room to buy, buy, buy.
And encounter the usual paradoxes, of course. -Lee Corbin.

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