why haven't we met a single time traveler yet?

And if we did meet one,

how would we know they weren't faking it?

In 1998, a gentleman

by the name of John Titor

arrived from the future.

Or so he said.

In his timeline, as he claimed,

General Electric had managed to invent

time travel in the year 2034.

He even showed and described

his time traveling device in great detail.

And then Titor vanished,

as abruptly as he appeared.

Did he finish his mission?

Or was he real at all?

If we were able to prove

that someone has traveled from a different time,

it would be very cool for science.

Although it might overwrite

Albert Einstein's theory of relativity.

Einstein approached time as a fourth dimension.

Space is a three-dimensional spectrum

that provides us with length, width and height.

Time offers direction.

Together, they form a space-time continuum.

And it can be affected by gravity.

According to Einstein's theory of relativity,

gravity is a curve in space-time.

And technically, because space and time are one,

gravity could bend time as well as space.

But you'd need something really, really big

to notice any changes in the movement of time.

If you somehow managed

to get yourself close to a black hole,

like Sagittarius A,

you'd be experiencing time at half-speed

compared to people on Earth.

This is because Sagittarius A

packs a mass of four million suns

into an infinitely dense point,

creating a very strong gravitational field around it.

Another way to travel in time

is to move really fast.

The closer you get to the speed of light,

the slower time passes for you.

If you were a passenger on a train

that somehow could travel at 99% of the speed of light,

for every year you spent on the train,

223 years would have passed back at the station.

That would make you a time traveler into the future.

Titor claimed that this little machine

is what made time travel possible in any direction.

The C204, as he called it, allowed him

to manipulate gravity with the help

of two microsingularities that were packed inside.

It also had gravity sensors

to lock the machine and time traveler

in a fixed place in space.

Titor had this machine installed in a car,

pretty much like the good old, time-traveling DeLorean did.

But it would only take him as far as

60 years from his time.

Proving that someone really is a time traveler

might be even harder than time travel itself.

Time travelers could make predictions about the future,

show their futuristic technology,

or even undergo some genetic testing

to support their story.

Maybe the best way to prove time travel is real

would be to take you with them on their next adventure.

Of course, there are easier ways

to prove the possibility of time travel.

In 2009, the world-famous

theoretical physicist Stephen Hawking

threw a party.

Not just any party,

but one to which only future time travelers were invited.

Hawking didn't send off invitations until after the party.

He believed that if travelers from the future

arrived before the invitation was released,

that would prove that time travel will become possible one day.

He sat there waiting for hours,

but nobody showed up.

That doesn't mean time travel is impossible.

There is a chance that time travelers from the future

will learn to use wormholes, or "bridges" in space-time.

We haven't seen one of those yet, but...

the theory of general relativity predicts that they exist.

The biggest problem with wormholes

is that they're microscopic and collapse too quickly

for a human traveler to go through.

But maybe the reason nobody attended Hawking's party

is that time traveling might not be that precise.

Maybe the guests from the future did arrive,

but they were too late for the party.

Or maybe going back in time

is strictly prohibited for time travelers,

because if they do,

they might change their timeline completely.

Perhaps they are just trying to avoid the risk

of spreading any diseases from the far future onto us.

Or maybe the information about time traveling

is so highly classified that

only a few people would be trusted with it.

Maybe traveling back into time isn't possible after all.

Maybe time can only move forward and never backwards.

Maybe John Titor was one big lie.

We have a few more years until 2034 to find out.

In a way, we already have time travelers in our lifetime.

Astronauts on the International Space Station

are moving at 8 km/sec (5 mi/sec).

This makes them age slower,

although the difference is measured in hundredths of a second.

But if you're listening,

you're invited to a reception for time travelers

hosted by Stephen Hawking.

The party will take place at the University of Cambridge, UK,

on June 28, 2009.

No RSVP required.

And hey, if you have a chance to jump in a spaceship

and enjoy the time traveling effects of a black hole,

Just make sure to not fall into that gravitational monster.

But that's a story for another WHAT IF.