There may be no other concept that captures the imagination
more than the idea of time travel -- the ability to travel to
any point in the past or future. What could be cooler? You
could jump into your time machine to go back and see major
events in history and talk to the people who were there! Who
would you travel back to see? Julius Caesar? Leonardo da
Vinci? Elvis? You could go back and meet yourself at an
earlier age, go forward and see how you look in the future...
It's these possibilities that have made time travel the
subject of so many science fiction books and movies.
It turns out that, in some sense, we are all time
travelers. As you sit at your desk, doing nothing more than
clicking your mouse,
time is traveling around you. The future is constantly being
transformed into the past with the present only lasting for a
fleeting moment. Everything that you are doing right now is
quickly moving into the past, which means we continue to move
Ideas of time travel have existed for centuries, but when
Albert Einstein released his theory of special
relativity, he laid the foundation for the theoretical
possibility of time travel. As we all know, no one has
successfully demonstrated time travel, but no one has been
able to rule it out either.
In this edition of How
Stuff Will Work, we will learn about the concept of
time and the different theories surrounding the viability of
Sagan had it right when he said that time is
"resistant to simple definition." Lots of us think we know
what time is, but it is hard to define. You can not literally
see or touch time, but you can see its effects. The evidence
that we are moving through time is found in everything -- our
bodies age, buildings weather and crumble, trees grow. Most of
us feel the pressure of time as we are pushed to meet
deadlines and make appointments. Our lives are often dictated
by what time we need to be somewhere.
Ask most people to define time and they are likely to look
at their watch
or a clock.
We see time as the ticking of the hands on these devices. We
know that there are 60 seconds in a minute, 60 minutes in an
hour, 24 hours in a day and 365 days in a year. These are the
basic numbers of time that we all learned in grade school.
Time is also defined as being the fourth dimension of
our universe. The other three dimensions are of space,
including up-down, left-right and backward-forward. Time
cannot exist without space, and likewise, space cannot exist
without time. This interconnected relationship of time and
space is called the spacetime continuum, which means
that any event that occurs in the universe has to involve both
space and time.
According to Einstein's theory of special
relativity, time slows as an object approaches the speed of
light. This leads many scientists to believe that
traveling faster than the speed of light could open up the
possibility of time travel to the past as well as to the
future. The problem is that the speed of light is believed to
be the highest speed at which something can travel, so it is
unlikely that we will be able to travel into the past. As an
object nears the speed of light, its relativistic
mass increases until, at the speed of light, it
becomes infinite. Accelerating an infinite mass any faster
than that is impossible, or at least it seems to be right now.
But time travel in the other direction is not as difficult,
and the future may one day be a possible destination...
While writers have produced
some great ideas for time machines over the years, a real-life
time machine has yet to be built. Most theories of time travel
don't rely on machines at all. Instead, time travel will
likely be done by way of natural phenomena that will transport
us instantly from one point in time to another. These space
phenomena, which we are not even sure exist, include:
- Rotating black holes
- Cosmic strings
We'll take a look at each of these in the following
that are more than four times the mass of our sun
reach the end of their life and have burned up all of their
fuel, they collapse under the pressure of their own weight.
This implosion creates "black holes," which have gravitational
fields so strong that even light cannot escape. Anything that
comes in contact with a black hole's event horizon will
be sucked in. The event horizon is the boundary of a black
hole at which nothing can escape.
You can think of the shape of a black
hole as similar to an ice cream cone. It is large on top
and tapers into a point, called a singularity. At the
singularity, the laws of physics cease to exist and all matter
is crushed beyond recognition. This kind of non-rotating black
hole is called a Schwarzschild black hole, named after
the German astronomer Karl Schwarzschild.
Another type of black hole, called a Kerr hole, is
also theoretically possible. Kerr holes are rotating black
holes that could be used as portals for time travel or travel
to parallel universes. In 1963, New Zealand
mathematician Roy Kerr proposed the first realistic theory for
a rotating black hole. In his theory, dying stars would
collapse into a rotating ring of neutrons that would produce
sufficient centrifugal force to prevent the formation of a
singularity. Since the black hole would not have a
singularity, Kerr believed it would be safe to enter it
without being crushed by the infinite gravitational force at
If Kerr holes do exist, it might be possible to pass
through them and exit out of a "white" hole. A white
hole would have the reverse action of a black hole. So,
instead of pulling everything into its gravitational force, it
would use some sort of exotic matter with negative
energy to push everything out and away from it. These
white holes would be our way to enter other times or other
Given the little we know about black holes, Kerr holes may
possibly exist. However, physicist Kip Thorne of the
California Institute of Technology believes that the laws of
physics prevent such a formation. He says there is no such way
to enter and exit a black hole, and that anything attempting
to enter a black hole will be sucked in and destroyed before
it even reaches the singularity.
Thorne believes there could be
another type of tunnel-like structure existing in the universe
that could be used for a time travel portal. Wormholes,
also called Einstein-Rosen Bridges, are considered to
have the most potential for time travel if they do exist. Not
only could they allow us to travel through time, they could
allow us to travel many light-years
from Earth in only a fraction of the amount of time that it
would take us with conventional space travel methods.
Wormholes are considered possible based on Einstein's
theory of relativity, which states that any mass curves
spacetime. To understand this curvature, think about two
people holding a bed sheet up and stretching that sheet tight.
If one person were to place a baseball on the bed sheet, the
weight of the baseball would roll to the middle of the sheet
and cause the sheet to curve at that point. Now, if a marble
were placed on the edge of the same bed sheet it would travel
toward the baseball because of the curve.
Imagining space as a curved,
two-dimensional plane, wormholes like this could be
formed by two masses applying enough force on spacetime
to create a tunnel connecting distant points in the
In this example, space is depicted as a two-dimensional
plane rather than the four dimensions that actually make
up spacetime. Imagine that this sheet is folded over, leaving
a space between the top and bottom. Placing the baseball on
the top side will cause a curvature to form. If an equal mass
were placed on the bottom part of the sheet at a point that
corresponds with the location of the baseball on the top, the
second mass would eventually meet with the baseball. This is
similar to how wormholes might form.
In space, masses that place pressure on different parts of
the universe could eventually come together to form a tunnel
-- this is a wormhole. We could then travel from Earth to
another galaxy and back relatively quickly (within a
lifetime). For instance, let's picture a scenario in which we
would want to travel to Sirius, a star that's seen in the
Canis Major constellation just below Orion. Sirius is about 9
from Earth, which is about 54 trillion miles (90 trillion km).
Obviously, this distance would be far too great for space
travelers to traverse and return in time to tell us about what
they saw there. So far, the farthest people have traveled into
space is to the moon, which is only about 248,548 miles (about
400,000 km) away from Earth. If we could find a wormhole that
connected us to the space around Sirius, then we could cut the
time considerably by avoiding the trillions of miles that we
would have to cross with traditional space travel.
So how does all of this relate to time travel? As we
discussed earlier, the theory of relativity states that as the
velocity of an object nears the speed of light, time slows
down. Scientists have discovered that even at the speeds of
shuttle, astronauts can travel a few nanoseconds into the
future. To understand this, picture two people, person A and
person B. Person A stays on Earth, while person B takes off in
a spacecraft. At takeoff, their watches are in perfect sync.
The closer person B's spacecraft travels to the speed of
light, the slower time will pass for person B (relative to
person A). If person B travels for just a few hours at 50
percent the speed of light and returns to Earth, it will be
obvious to both people that person A has aged much faster than
person B. This difference in aging is because time passed much
faster for person A than person B, who was traveling closer to
the speed of light. Many years might have passed for person A,
while person B experienced a time lapse of just a few hours.
Find out more about this twin paradox in How
Special Relativity Works.
If wormholes could be discovered, it might allow us to
travel to the past as well as the future. Here's how it would
work: Let's say the mouth of the wormhole is portable. Then
person B in the example above, who traveled at 50 percent of
light speed into space for a few hours, could carry one
wormhole mouth into space, while the mouth at the opposite end
of the wormhole would stay with person A on Earth. The two
people would continue to see one another as person B traveled
into space. When person B returned to Earth a few hours later,
a few years may have passed for person A. Now, when person A
looks through the wormhole that traveled into space, that
person will see him or herself at a younger age, the age he or
she was when person B launched into space. The cool thing
about it is that the older person A would be able to step into
the past by entering the wormhole, while the younger person B
could step into the future.
Yet another theory for how we
might travel back and forth through time uses the idea of
cosmic strings, proposed by Princeton physicist J.
Richard Gott in 1991. These are -- as their name suggests
-- string-like objects that some scientists believe were
formed in the early universe. These strings may line the
entire length of the universe and are under immense pressure
-- millions upon millions of tons.
These cosmic strings, which are thinner than an atom,
would generate an enormous amount of gravitational pull on any
objects that pass near them. Objects attached to a cosmic
string could travel at incredible speeds, and because their
gravitational force distorts spacetime, they could be used for
time travel. By pulling two cosmic strings close together, or
one string close to a black hole, it might be possible to warp
spacetime enough to create closed time-like curves.
A spacecraft could be turned into a time machine by using
the gravity produced by the two cosmic strings, or the string
and black hole, to propel itself into the past. To do this, it
would loop around the cosmic strings. However, there is still
much speculation as to whether these strings exist, and if
they do, in what form. Gott himself said that in order to
travel back in time even one year, it would take a loop of
string that contained half the mass-energy of an entire
galaxy. And, as with any time machine, you couldn't go back
farther than the point at which the time machine was created.
Problems with Time Travel
If we are ever
able to develop a workable theory for time travel, we would
open up the ability to create very complicated problems called
paradoxes. A paradox is defined as something that
contradicts itself. Here are two common examples:
theory regarding time travel brings up the idea of parallel
universes, or alternative histories. Let's say that you do
travel back to meet your grandfather when he was a boy. In the
theory of parallel universes, you may have traveled to another
universe, one that is similar to ours, but has a different
succession of events. For instance, if you were to travel back
in time and kill one of your ancestors, you've only killed
that person in one universe, which is no longer the universe
that you exist in. And if you then try to travel back to your
own time, you may end up in another parallel universe and
never be able to get back to the universe you started in.
- Let's say, for the sake of argument, that you could
travel back to a time before you were born. The mere fact
that you could exist in a time before you were born creates
a paradox. If you were born in 1960, how could you exist in
- Possibly the most famous paradox is the grandfather
paradox. What would happen if a time traveler went back
and killed one of his or her ancestors before the traveler
was born? If the person killed his or her grandfather, then
how could that person be alive to go back and kill his or
her grandfather? If we could change the past, it would
create an infinite number of paradoxes.
The idea here is that every action causes the creation of a
new universe, and that there are an infinite number of
universes that exist. When you killed your ancestor, you
created a new universe, a universe that was identical to your
own up until the time you changed the original succession of
Confused yet? Welcome to the world of time travel. Just
imagine how complicated the ticket prices will be.
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