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Over one hundred years ago in 1915, Albert Einstein changed the world forever
with his theory of general relativity.
This new groundbreaking theory solved several mysteries
including the precession of Mercury. But people probably could not have anticipated
all the other things this theory would subsequently tell us.
Some of the seemingly crazy predictions that have been confirmed by observation, and that we’ve
also talked about in prior videos, are things like infinite mass density objects called black holes,
gravitational waves, and light bending around massive objects.
In addition to these confirmed phenomena, scientists have been
able to find mathematically consistent (if not creative) solutions to Einstein’s field equations
that can show for example, how to build a spacetime bubble or warp bubble,
that could theoretically travel faster than light.
Now the question I want to ask is whether any solutions
to these same equations of general relativity can show us a way to travel in time - not forward
in time, because we’ve kind of figured that out, but backward in time?
In a prior video I looked at how it could be done by traversing
a time-like curve around a naked singularity, which is an object arising from certain
solutions to Einstein’s field equations. It’s like a black hole without an event horizon.
Another solution to the same field equations predicts an Einstein Rosen bridge, which is
a type of wormhole. This is a fascinating possibility, because if wormholes are real,
they could also allow us to travel back in time.
How can we form wormholes and how would they allow time travel backward? That’s coming up right now…
Worm holes have been part of science fiction for a long time, from Star Trek to Interstellar.
Now, while some of these depictions take quite a bit of artistic license,
some are inspired by actual solutions to Einstein’s theory of general relativity.
Einstein’s field equations are nothing but a tool to understand how matter
affects the curvature of spacetime and how the curvature of spacetime
affects matter. So we can, in principle, imagine almost any creative spacetime curvature
that results in a worm hole, and then by solving the field equations figure out
how the matter in this spacetime must be configured to obtain that desired curvature.
So by using a specific configuration of matter and energy, we can form a spacetime curvature
that is like a tunnel or shortcut between two points in space. These different points can in
principle be very far apart in space, or they can also be far apart in time.
If you can travel between two distant points in space using such wormholes,
then you can effectively travel from one end of the wormhole, point A, to the other end, point B,
faster than the speed of light. The key is that the worm hole just gives you a much shorter path,
because it connects two distant points in space, via a curvature
such that you don’t need to physically travel within space faster than the speed of light.
Keep in mind that these depictions are for visualization purposes only.
A three dimensional wormhole would have entrances and exits that
are three dimensional spheres rather than two dimensional rings
like the mouths of a funnel as you see here. It is usually shown this way so that we can visualize it
by its 2D analog. The actual higher dimensional wormhole is not possible to visualize.
So now the question is, how would this allow you to go backward in time, since you are just
going from point A to point B very fast. You might say, I’m just traversing space, not time.
Caltech physicist Kip Thorne came up with a thought experiment to achieve this.
By the way, Kip Thorne was also the technical consultant on the movie "Interstellar,"
a good movie which I think got a lot of the science right, because of him.
So here is what Kip Thorne’s thought up: Imagine sometime in the future, let’s say the year 2200,
we create a stable and traversable wormhole in a lab on earth. We then take one end of the wormhole
and place it on a spaceship that can travel nearly at the speed of light, let’s say 99.5%
the speed of light. This is 200 years in the future after all.
The other end of the wormhole stays in the lab on earth.
So the spaceship leaves earth and travels 99.5% the speed of light.
A wormhole is such that the astronaut on the spaceship could in principle
shake hands through the wormhole with the scientist in the lab,
regardless of how far apart the two ends of the wormhole are located.
Now the spaceship travels at 99.5% the speed of light for a year.
According to special relativity, the clock on the ship containing the astronaut and the wormhole
will run slower compared to the clock on earth, in fact about 10 times slower. This is time dilation.
So by the time the astronaut returns to earth, 10 years will have gone by for the scientist
in the lab, but only one year will have gone by for the astronaut and the wormhole on the ship.
So now the astronaut has traveled 10 years into the future. The year is 2210, but biologically
he is only 1 year older. No wormhole is needed for traveling into the future, just time dilation due to speed. But,
because the wormhole in the lab 10 years earlier would have its clock synchronized
with the wormhole on the spaceship, even though the astronaut is physically in the year 2210, he
could step into the wormhole and land in the lab in the year 2201 because only one year will have
gone by for the two synchronized wormholes. The astronaut would be able to travel back in time.
In addition, the scientist in the lab in 2201 would be able to step
into her wormhole and land 9 years into the future. She would travel forward in time.
By the same token, the scientist in the lab in the year 2210
would be able to step into the astronaut’s wormhole and travel back in time to 2201 as well.
Now, you might ask, what about the wormhole in the lab in the year 2210? What would you see
if you looked into that wormhole. Well, presuming both the lab and spaceship maintained the wormhole
and kept it open, that wormhole would take you 9 years into the future to the year 2219. This would
of course require both wormholes to be maintained open for the duration of this time frame.
Now, if the spaceship could travel even faster, then it could potentially end up hundreds of years
or even thousands of years into the future, allowing future humans to go back in time to
point where the wormholes were first created, and for humans in the past to go to the future.
Note that you would not be able to travel further back in time than the point when the wormhole was
first created. In other words the future humans could not travel further back than the year 2200.
So this may answer the question, if time travel is possible in the future,
why don’t we see time travelers all over the place? The answer could be that it’s because
a wormhole time machine has not been invented yet.
So now the question you might have is, this looks great on paper.
What’s the catch? How realistic is this wormhole concept?
Well, no wormholes have been observed. But it is possible that what we observe as black holes,
may actually be wormholes. Scientists are working out how they could tell the difference.
Wormholes are a lot like black holes. Both objects are extremely dense
with strong gravitational fields for their size. The main difference is that no
object can come back out after crossing the event horizon of a black hole,
not even light. But things entering a wormhole could theoretically come right back out.
In a new paper, published in the Monthly Notices of the Royal Society, Russian astronomers suggest
that what we believe to be black holes at the centers of some bright galaxies, may in fact be
wormholes. They propose that the way to tell the difference would be in the light signatures we
would detect. They reason that matter entering one mouth of a wormhole could slam into matter
entering the other mouth of the wormhole at the same time, a kind of event that would never happen
with a black hole. This would produce a spectacular display of gamma rays surrounding
the object in a giant sphere. Whereas black hole gamma rays are ejected in the form of a jet.
If we don’t ever observe any wormholes, then wormholes may only exist if we
artificially create one. Given unlimited technology and resources in the future,
could we do this? One theoretical issue is that when you create a wormhole, the high gravity
near the center, where the tunnel connects the two mouths causes it to collapse in on itself, as soon
as anything with mass or energy enters it. This means that even a photon, a particle of light,
could potentially collapse the wormhole. In order to hold it open, it requires
exotic matter, in other words matter with negative energy density, or negative mass.
What the heck is negative energy and mass?
Negative mass is like negative 1 kg for example, instead of positive 1 kg. What does this even
mean? Negative matter should not be confused with antimatter, which is just like matter,
except with the opposite charge. Antimatter has positive mass and energy. Negative matter
would curve spacetime in the opposite direction of regular matter. Two negative matter objects
would repel each other instead of attract each other. It is not known to exist,
but this doesn’t mean that exotic matter isn’t out there somewhere waiting to be discovered.
Negative energy densities, however, have been shown to exist in the Casimir effect,
where two smooth uncharged plates are attracted to each other due to a negative pressure differential
created by virtual particles in the vacuum of space. This, however would be a very small
amount of negative energy. The amount needed to hold up a worm will be many magnitudes higher
than this. Whether enough negative energy could be created for a wormhole is a question mark.
There are some proposals by scientists on how to create wormholes. For example, in a 2020 paper by
Juan Maldacena and Alexey Milekhin from Princeton university,
they suggest wormholes could be made if they are charged (yes black holes can have a charge,
but it’s probably rare) and if they can get a hold some something called dark sector,
which is a kind of exotic matter that has not been discovered
yet. So this could be a viable solution.
There are other scientists that have also proposed creating wormholes by connecting
two oppositely charged black holes with cosmic strings to hold their mouths open.
Cosmic strings are leftovers formed in the first fractions
of a second after the Big Bang. They are exotic objects, no wider than a proton. But one inch
of their length would outweigh Mount Everest. These have also not been confirmed to exist.
So we are not sure wormholes exist. If we create one artificially, we may not be able to produce
enough exotic matter to hold it open. But even if some future version of us could overcome all
the technical barriers to create one, we know that at least using the Kip Thorne time machine,
no one could travel further back in time than the moment when the wormhole was first formed.
If you think about it, thismakes our existence in the here and now
so much more precious. Does it not? Because we will never be able to relive the moments
we are living right now. The beauty of earth, the sky, the moments with your
friends and family will never be revisited by any future version of us. The now is precious
because it will be lost forever in the sands of time.
I’d like to give a big shoutout to Wondrium, today’s sponsor. I was inspired to make this video
after watch a lecture series called, “How science shapes science fiction” by professor Charles
Adler of St. Mary’s college. It’s a fascinating series looking at the science of popular movies
like Avatar, The Avengers, and Interstellar which I talked a bit about in this video.
Prof. Adler also thought that movie got a lot of the science right. Although
he didn’t think the story was that good. I thought the story was great.
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You’ll even see my testimonial at the bottom of Wondrium’s home page.
It’s really easy to sign up right now because they are offering a free trial,
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so there’s really nothing to lose. If you want to support this channel, and greatly
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And if you have any questions or comments for me, please leave them in the comment
section because I try to look at all of them. I’ll see you in the next video my friend.