Hello everyone, today here I've got a stack of physics books that I got out from the library,
and they all deal with ideas relating to time or space and time travel. I thought it would be
really interesting to see how these books go about describing and illustrating time travel because
time travel is something that usually belongs in fiction books or sci-fi and is treated in a
very fanciful way but these are non-fiction books and some are written by quite serious scientists,
so I thought it would be interesting to see how they unravel the same ideas,
and how time travel is treated by people who are dedicated to studying it.
The first book in our lineup today is Geometry, Relativity and the Fourth Dimension by Rudy
Rucker. Now if you've been following my short videos about the fourth dimension you'll
have met some ideas from this book already but tucked away in its chapters there's a compelling
discussion on time travel. This diagram here is to get us thinking in terms of space-time,
it shows these flat slices of our world that are moving forward in time. In this particular
example there are two characters; a square and a triangle, living on a flat plane. This is what
their world looks like at 12 o'clock but five minutes later at the slice 12:05 the triangle
has moved towards the square maybe to say hello and then five minutes later again at 12:10 the
triangle has moved away meaning that in space-time you could draw the triangle as a worm that snakes
over here and then back again whereas Mr. Square remains stationary the entire time and their world
line is just this straight line. This just shows two characters but you could try to represent
the entire world or Universe using these world lines and it would be a vast tangle of worms.
You could represent every atom by one line and because every atom in a person's body is
replaced every seven years or so there wouldn't actually be a single thread that goes the whole
length of a person's life. A living individual is a persistent pattern of world lines rather than
a particular collection of particles. Drawing world lines helps you think of space-time as a
structure all of its own where the universe at any instant is a 3D cross-section but
that does make us wonder why we can't see the past or future if they really exist.
One idea is that maybe all instances of time do exist but because our memories
only relate to events that happened at previous time steps to our present moment,
we get the illusion that we are moving forwards in time always.
The author then makes a slight departure from physics but this next diagram helps
me to understand meditation better than any yoga class I've been to. He asks if there is a way to
be conscious in a static space-time instead of always moving forwards. Well one trick to
achieve the feeling of timelessness is to stop thinking new thoughts because without that it's
hard to distinguish one moment from the next. To achieve this timelessness you could either
think nothing at all which is not easy or concentrate your attention on a repetitive
thought loop like a mantra. Thinking nothing, you can't differentiate one instant from the
next and doing a mantra you can't differentiate one repetition of the mantra from the next.
This next diagram here shows a way to time travel to the past although it only works
in a universe where time is circular. In this universe the time dimension is wrapped into
a cylinder and space extends infinitely out to the sides. The circle here represents the world
line of the earth. If we say that it takes one trillion years for time to repeat itself,
so that is the circumference of this world line, to time travel we could leave the Earth speeding
off to the side for say a quarter of a trillion years then we could turn around and travel back
this way for say half a trillion years then turn around again and take another quarter
of a trillion years to get back to where we started but because time is circular
we will intersect the world line of the Earth in its past. Ideas of circular time have been
proposed in physics but there are plenty of practical reasons why a journey like this
taking a trillion years and needing to travel close to the speed of light is not going to
work. You'd need a rocket the size of a galaxy to have enough fuel and you'd probably get lost.
This next book is Paul Davies: About Time. In a chapter on time reversal there is a discussion
about antimatter and how there are things called positrons which are like anti-electrons. They
have the opposite charge but maybe they also go back in time. Electrons and positrons are
created together in pairs, usually it's from a photon colliding with an atom. They fly off
and hope to never see each other again because if a positron were to run into an electron then
the pair would instantly annihilate and reverse this pair creation process giving back photons.
This is a space-time diagram where here a photon coming into point A creates an
electron-positron pair. The positron then meets with another electron and annihilates giving
back a photon. At this time T an observer would see three particles; an electron,
a positron and another electron, but according to Richard Feynman this zigzag track could be
viewed as the worldline of just one particle, an electron that travels back in time between
these annihilation and creation events. That's why the arrow here for the positron is pointing
backwards. Feynman's theory was that this is all just one electron and it travels back in
time for that brief period where it is considered a positron. This idea could be extended to include
many more electrons and positrons and you could say that all the electrons in the universe are
just the same one particle. That there is just one electron making up the entire universe. It
offers a nice explanation for why all electrons appear to be identical but that theory comes with
problems of its own. It would also predict that the universe is made exactly of half matter and
half antimatter but we don't see nearly as much antimatter as we do normal matter and the arrow
of time in our universe appears to point forwards rather than having a symmetry pointing backwards.
Another way to time travel to the past is detailed in this next book; Time Travel in
Einstein's Universe by J Richard Gott. Gott mentions something called cosmic strings,
which are thin strands of high-density material left over from the early Universe. This diagram
is designed to show what space-time looks like around a cosmic string. It's like a piece of
pizza missing a slice. If we take out that slice and stick it together we get a cone. Here my
pencil is the cosmic string and the cone is what a horizontal plane of space-time around the string
looks like. Suppose that here's the Earth, here's the cosmic string and here is a quasar split
across the region of the missing slice. These straight lines show the path that light would take
from the quasar to reach Earth and here's what those paths look like in the conical space-time.
To us on Earth it will appear as if there are two different quasars even though it's the same object
and even though the light is traveling on the shortest straightest path to us from the quasar,
one of the paths will be shorter. This is the principle of gravitational lensing and may be
what helps us to find cosmic strings. The light traveling along path two has beaten the light
traveling along path one and if it was a rocket traveling along path two, traveling at nearly the
speed of light, it could have also beaten light path one. If cosmic strings exist then you could
travel in a spaceship and outrun a light beam by taking the shorter of two paths around a
cosmic string. When the rocket arrives at Earth, the light showing its departure from the quasar
won't have arrived yet so you could look back and see yourself still on the quasar about to depart.
On another page in this book there's instructions on how to build a time machine that can visit the
future. To construct this time machine you need to find the mass of the planet Jupiter
and construct it around yourself into a spherical shell. That will create a huge gravitational well
around you but because it's a sphere it will perfectly cancel out in every direction and
inside the shell you will feel unaffected. We're just teetering on the limit of mass
concentration that would create a black hole so you don't want to go overboard with this.
Just outside of the spherical shell you would be torn apart by tidal forces generated by
its gravitational attraction but inside the sphere you would be perfectly safe.
This diagram simplifies the situation down to just a two-dimensional shell so it's drawn around
this time traveler here as a circle but for us it would actually be a sphere. If you imagine an ant
falling into something this shape it would require a lot of energy to crawl back out. The same thing
will happen with our photons. Photons emitted by us will lose a lot of energy on the way out and be
redshifted turning into longer wavelength light. Photons from outside of the well that fall in will
pick up energy as they fall. The person inside of the well would see these waves oscillating
faster than they did on the outside and if the people outside the well had been using a clock
which relied on the oscillation of these waves, the person inside of the well would see these
disassemble the well he would step out of his time machine and be younger than those around him.
Another way to time travel to the future is by moving really fast. Usually running away
on a rocket going near the speed of light, leaving Earth for a bit then turning around
and coming back and being younger than everyone around you. I think this diagram does a good job
of explaining why a clock on a rocket ship moving fast actually ticks more slowly than
one that's left on Earth. If you construct a clock that uses the bouncing of light off
mirrors to measure the time then this is what our clock on Earth would look like. The light
would predictably bounce between the two mirrors and the clock would tick every time it hit one.
Give that same clock to an astronaut on a moving rocket and if we observed that astronaut from the
outside of their space station this is what we would see. The mirrors are still separated by
the same distance but because the rocket is moving past us we will see the light bounce
off the bottom mirror and then bounce off the top mirror but the distance between the mirrors will
no longer be the initial three feet separation. It will be longer because it's traveled on this
diagonal line. Because light travels at a constant speed, if it's traveling a greater
distance then there'll be a greater time delay between these two ticks of the clock. So this
astronauts clock will tick more slowly compared to ours. It will happen to everything on board.
Even the astronaut's heart is a kind of clock and that is why when the astronaut returns to earth
after their journey they won't have physically aged as much as someone who was left on Earth.
Most books that mention time travel will have one of these, a Minkowski or a space-time
diagram. These diagonal lines represent an object moving at the speed of light.
Anything less steep than that will be inside of these cones and will be from either the
past or the future of Point P. Because things can travel from anywhere in this region to P,
anything in this cone could have affected P, it is its past light cone. Same with the future,
anything from P could affect anything in this region. But these points A and B are
outside of the light cones, this is the region of Elsewhere. Whilst A and B might happen at the
same time as P they cannot affect P because light could not travel from here to here.
All observers agree on the light paths because light travels at the same speed
for all observers but depending on how fast you are moving yourself the axes
x and t will actually move and change based on your reference frame.
And now here are some highly commended extra entries. Space-time breaking up,
where at a mini-microscopic level quantum effects are so violent that they start to tear space and
time apart completely creating a sponge-like structure of wormholes and bridges. This
activity would be unnoticed by even a subatomic particle which would be larger than the wormhole.
This one shows a rocket stuck inside of a black hole. The rocket is always
dragged back down and so are light signals that they try to send out,
there's no escape and no way to let anyone know that you are here.
This figure shows black and white holes. A is a collapse to a black hole and B is
an explosion from a white hole. This one's too complicated for me to understand enough
to explain but the study of black and white holes looks very cool.
Here's an interesting model called a self-creating universe where the universe has a time loop at its
beginning meaning the question of what happened before the universe existed doesn't really apply
because according to this the universe is its own creation and if you were to go back in
time throughout the history of the universe you would end up intersecting with your own tail.
Thanks for joining me on this look through some physics books.
Thanks to my patrons for making this video possible and a special shout
out to today's Patron Cat of the Day Freeway.