Imagine you own a watch that let's you travel through time.
One day, you get the bright idea to travel back to the 1920's to mess with your grandparents.
You press a few buttons, and suddenly you're standing beside old Pappy who is taking
in the scenery at the precipice of the Grand Canyon.
Since you've never been the biggest fan of your grandfather, you decide to push him over the edge.
Gravity does its thing, and a few seconds later, no more Pappy.
But wait, if you killed your grandfather before he ever met your grandmother...
you wouldn't have been born to be able to go back in time and kill him!
Here's the problem. If you didn't exist and killed your grandfather, you couldn't have killed him...
Because you didn't kill him, he's still alive.
but that would mean that you yourself would be alive and now you can go back in time to kill him.
And so the endless loop goes on.
This is called the Grandfather Paradox.
And it's part of what makes time travel so interesting.
The grandfather paradox deals with time travel on a significant scale.
But why don't we start with something a little smaller?
Let's say you and I are wearing identical watches and we synchronize them right before I blast off into space.
In the space shuttle, I'm traveling at roughly 28,000 kilometers per hour relative to the Earth.
If I make a few orbits around the Earth before I return, when we compare watches,
we'll see that less time has passed for me.
A very, very tiny amount of time, but is there nonetheless.
This is called time dilation, and it simply means that according to Einstein's theory of Relativity,
Time measured along different trajectories is affected by differences in either gravity or velocity
– each of which affects time in different ways.
One example of time dilation is when we compare the passage of time on the ISS versus on Earth.
After spending six months on the Space Station, astronauts have aged approximately
.005 seconds less than those of us here on Earth.
Of course, this is a very insignificant amount of time, but it would be much more apparent if the astronauts were
able to travel to closer to the speed of light.
Interestingly, the effects of time dilation are fairly minor, even up to around 70% of the speed of light.
However, once we reach 75% or so, the effects become dramatic.
Now, bear with me here, because this is where things get a bit weird.
Possibly the strangest aspect of special relativity is the distances shrink in the direction of motion.
When we think of traveling to a point 10 light years away at 90% of light speed,
you'd expect it to take 11 years, right?
That's not exactly the case.
To a stationary observer, it would indeed 
seem to take your vessel 11 years.
But to the people inside the vessel, 
not only time, but distance would dilate
and you would reach that  point in only 4.4 years.
Want to play with time dilation yourself? I've added a link down below to a time dilation calculator.
Okay, that's all really fascinating, but is it really time travel? Perhaps not in the sense that we usually
think of it, but according to the nature of
space-time, it is.
If you were to return home from your 90% light speed trip, you would be almost 9 years older.
But everyone back on Earth would have aged 22 years.
So in essence, you've traveled 13 years into the future relative to your time in space.
let's look at another example. In the film Interstellar, Cooper and his team land on a large watery planet,
on which gravity is 30% stronger than on Earth.
This planet is situated very close to a
supermassive black hole called Gargantua, which it orbits at 55% of light.
Gargantua's mass is equal to that of 100,000,000 suns and spins at 99.8% the speed of light.
The combination of all these factors have the effect of slowing down time relative to the astronaut left aboard
the ship, by a staggering 61,000 times.
That means that 1 hour on the planet is equal to seven years aboard the ship.
When the team makes it back to the vessel after a little over three of their hours, their friend had aged by 23 years.
The craziest part is that this isn't just science fiction. Those calculations all check out.
That's exactly how it would happen in real life.
So in essence, the crew had traveled 23 years into the future.
Now so far, we've only explored traveling into the future.
Is it possible to travel backwards in time?
According to some theories, specific types of motion in space might allow time travel into the past and future,
if these geometries and motions were possible.
However, that's a big if.
It would require something called a closed timelike curve, or CTC for short.
A CTC is a closed loop in space-time which could theoretically allow an object to return to its own past.
The science behind time travel to the past,
is incredibly complex and speculative
and many scientists suspect that it is not possible at all,
because of the issue of causality.
That bring us back to the Grandfather Paradox,
if in fact these scientists are correct and assuming traveling to the past would cause a paradox
then we have our answer, time travel to the past is impossible.
If however, the Novikov Self-Consistency Principle is correct, then there is still a chance.
The Novikov Principle states that if an event exists that would cause a paradox or
any change to the past whatsoever, then the probability of that event is zero.
It would thus be impossible to create time paradoxes.
Unfortunately, for any would-be time travelers, the Novikov Principle is not widely accepted.
Another possibility would be the existence of wormholes, which are technically
permitted by general relativity.
- In order to travel through time using a wormhole,
it would have to be what is known as a
Transversable Wormhole.
Brace yourself for more strange 
space-time shenanigans here.
In order to be able to travel through time using a Transversable Wormhole,
its creation would have to be in one of two ways.
Option one, one end of the wormhole would have to be accelerated to a significant portion of the speed of light,
then brought back to the origin point.
time dilation would result in an accelerated wormhole entrance
aging less than the stationary one as seen by an external observer
Option 2 requires one end of the wormhole to be placed
within the gravitational field of an object with higher gravity than the other entrrance
and then returned to a position near the other entrance
This is a difficult concept to grasp
So think of it this way,
imagine you and I synchronize our watches to both display the year 2000
I hop in my ship and accelerate one entrance to the wormhole to near light speed
and then bring it back
My watch now reads 2004
And yours reads 2012
If someone were then to enter the accelerated entrance
they would now exit the stationary entrance in the year 2004
the same location, but 8 years in their past
It's incredibly complicated stuff
But an easier way to look at it
is by picturing by a piece of paper
This paper represent space time,
a wormhole is an area of warped spacetime with an entrance and an exit
which you can imagine as the paper being folded back on itself
with a hole poked through it
essentially you're simply skipping all the time and distance between the two points
The problem with transversable wormholes
is that you can't possibly travel back further than the initial creation of the wormhole
So...really it's more a path through time rather than
a device propels itself back and forth through the years at will
It could be useful for the people in the distant future
wanting to come observe our time
but if we created such a portal today
we couldn't use it to go visit the dinosaurs
One final method that could potentially allow an
individual to more or less travel through time
is cryopreservation
The branch of science concerned specifically
with preserving human is called cryonics
and the practice has been around since the late 1960s.
Cryonics is on the speculative edge of medicine
as its proponents suggest that
Death is not a singular event, but a process
it would have to be for the practice to be successful
because it's currently illegal for a human to be frozen before they die
One a person opted to be cryo-preserved
their bodies prepared minutes after official death
the theory is that by reducing the patient's body temperature
to around -130 degrees celcius
enough brain information will be retained in an accessible state
for doctors and scientists of the far future to revive the patient
temperatures of that low inevitably cause significant damage to the human body
regardless of prior-preservation safeguards
so the doctors of today are relying on the development of
future technology that will allow the brain to be repaired at the molecular level
and restored to functioning condition
if someday technology has advanced far enough
to revive cryo-preserved humans
they will have essentially time traveled to the future
it's not quite as easy as shows like futurama make it seem
but it is theoretically possible
and that's time travel in a nutshell
you really can move forward through time
by traveling at an immense speeds
or through cryo-preservation
But travel to the past is likely impossible
of course the science of quantum physics and time travel
is incredibly vast
and well beyond the grasp of most normal people
so we may be missing just one crucial element
to understanding exactly how spacetime works
but who knows?
maybe someone from the far future will show up in our time
and give us the key to unlocking the mystery
or...would that cause a paradox
as always, I've included sources and links for further reading in the description
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