Black holes can be used to travel into the future only. So far as we know,
our universe prohibits traveling into the past.
According to Einstein's theory of general relativity, and to experimental
evidence here on earth assembled by Harvard physicists Pound and Rebka, in the
presence of a gravitational field, an external observer would see a clock in a
strong gravitational field tick more slowly. This is analogous to the famous
time dilation effect in special relativity, except that in the 'gravitational
redshift' effect no motion between the observer outside the gravitational field
and the clock located within the field, is required.
What this means is that if you were traveling into a strong gravitational
field and sending out pulses of light every second, an observer watching these
signals from a great distance would see the interval between the pulses
increase from seconds to minutes and then hours as the field got stronger and
stronger.
Black holes are fantastic sources of very strong gravitational fields. What a
distant observer would see as your clock got closer to the so-called Event
Horizon of the black hole is that the pulse interval would increase without
limit from one second to one month and longer. The frequency of the light pulses
would also get longer as the light lost more and more energy struggling to get
out from the vicinity of the black hole. As your friend finally entered the
black hole by passing across its event horizon, the last photon capable of
making it to infinity is emitted at almost infinite redshift, meaning that if
you originally emitted a gamma ray with an energy of 1000 billion electron
volts, buy the time your friend received it far away, it would have lost enough
energy to become a radio photon with an energy of 0.00001 electron volts! So,
if it took your friend 1000 hours to travel from where you are to the black
hole, the last photon he sent you just before entering the black hole, would
arrive at your location 1000 hours from now, but when you looked at the interval
between the last two pulses he sent, you would see that they are not the one
second interval you started out with, but say 1 or 2 minutes or more. But
here's the rub. According to your infalling friend, he/she is still sending the
pulses out once each second!
In other words, one second to your friend falling into a black hole is
several minutes to you and, in essence, your friend is aging more slowly than
you and is traveling into the future faster than you are. If he/she could manage
to put on the breaks just before crossing over the Event Horizon and escape to
rejoin you, you would note that his/her clock reads a much earlier time than
your clock. To your friend, only 2000 hours may have elapsed, however, YOUR
clock would read perhaps 10000 hours or several weeks have elapsed depending how
close to the Event Horizon your friend could get before escaping. The tidal
gravitational forces are enormous near small black holes the mass of the sun, so
your friend would be shredded into spaghetti within a few hundred miles of the
Horizon. For supermassive black holes of several billion solar masses, however,
the tidal forces near the Horizon are very small and survivable. This means you
could accidently find yourself passing across this one-way barrier, and only
realize your mistake when you tried to escape and found it impossible.
In principle, if you could get within a few millimeters of an Event Horizon
before escaping, you could essentially time travel years or millennia into the
future as measured by outside clocks. According to your clock, however, perhaps
only a few hour or days actually elapsed.
Note, all of the above numbers are pretty darn approximate and are given to
qualitatively show the magnitudes of the effects.
Return to Dr. Odenwald's FAQ page at the Astronomy Cafe Blog.