THE TIME TRAVELER'S GUIDE TO THE GALAXY
Simon Newcomb, an American mathematician and astronomer, said " Air flight is one of those problems that humans will never have to face ." Newcomb didn't believe it . Yet the first aircraft has a precise date: December 17, 1903, when the two American brothers Orville and Wilbur Wright created the first aircraft capable of lifting off the ground and flying in a controlled way thanks to an engine. Newcomb had time to see it, he would die a few years later, on July 11, 1909. And to think that NASA recently presented the first fully electric plane .
The New York Times, in 1920, released a long and detailed
article explaining how and whyno rocket could ever leave Earth's atmosphere .
In June 1969 they had to retract these claims. But let's be clear: time travel,
in the current state of things, based on our best understanding of the physics
and laws that govern the universe, especially travel into the past, is
absolutely impossible . Yet, as we have seen, our best understanding of the
physics and laws that govern the universe has been proven wrong many times
over.
The man who believed it
Those who believed in it (although not too seriously) was
the famous physicist Stephen Hawking , who on June 28, 2009 organized a party
for all time travelers who wanted to meet him: no one showed up , alas. Sure,
we would have liked to know if time travel will ever be a reality, and meeting
a potential time traveler would have been overwhelming, but if this event had
happened, you probably wouldn't be reading this: let's look at the glass half
full.
But why didn't anyone show up? The possible answers are
innumerable. Maybe in the future Stephen Hawking will not be a very popular
person, or at least not enoughfrom wanting to attend his parties, or maybe
there will never be any time travel , or again, they may have chosen not to
participate weakly in order not to run into any paradoxes ( even if there are
those who think it is possible without paradoxes ). There are many possible
answers. But what does physics think about time travel? To answer we must
necessarily take into consideration Einstein's theory of relativity.
The journey into the future
We owe everything we know to relativity, whose main merit
was in fact telling us " hey! Look, light moves at the same speed in every
frame of reference! ". A concept that in truth can even bother us, which
can be seen as a limit, because the speed of light is really very very small
when compared with galactic or intergalactic distances. Suffice it to say that
to reach Alpha Centauri, the closest star to our Sun, light takes four years (
NASA is planning to visit it). However, this fact that the speed of light is
always the same has led to a long series of logical and mathematical
consequences, first to the elaboration of special (or special) relativity and
then of general relativity. Special relativity states that time passes slower
(or faster) depending on the speed at which we are traveling. For example, the
faster we go, the slower time slows down .
That is: time is not absolute. This concept for us is
counterintuitive, so much so that we struggle to believe it, sometimes we
unconsciously minimize it thinking it is a matter of " perceived
time"and not a real effect to have to deal with. But it is a real effect,
there is no cosmic clock to which all the stars and planets refer, for
everything in the universe time flows differently (also for us.) This leads to
what is known to most as the " twin paradox ."
There are twins and one of them makes an interstellar
journey at the speed of light. He goes to a distant planet and returns home.
When he returns he will be much younger than the twin who remained on Earth
(that is, he will have traveled into the future compared to his brother who has
remained stationary). However, despite the misleading name, this is by no means
a paradox, but a real effect.
It really works, and the higher the speed you travel, the
more noticeable the effect becomes. In other word, the faster you go, the more
you travel into the future, the rest is all about engineering rather than
physics. Obviously this at low speeds (like those we are used to) is
negligible, but not absent. This means that technically, if you take your car
and hit the road, you slow down "your" time . However, external time
continues to flow normally. When you stop, by special relativity you will have
traveled into the future an infinitesimal fraction of time and absolutely
negligible, but measurable (in fact, we have measured it).
This means that the flow of our time is personal and unique,
we are in equal time points, but we get there by following very different paths
. How did we measure it? Through the Hafele-Keating experiment, performed in
1971 by Joseph C. Hafele and Richard E. Keating.
They used three atomic clocksperfectly identical and a very
fast aircraft. Atomic clocks are the most accurate and powerful tool for
measuring time that we currently have available. According to the special
theory of relativity, for a clock located at the equator, time is dilated and
therefore flows more slowly than a clock stopped at one of the two terrestrial
poles, as the first has a speed due to the rotation of the earth that in the
poles it is absent.
They then placed a third clock on a very fast plane, and
time is expected to flow differently than the two clocks mentioned above. In
particular, the differences in the passage of time of the clock on the plane
will be due to two effects : on the one hand the dilation of time due to
special relativity (which we have already talked about) and on the other the
opposite effect of acceleration of times compared to a clock on the ground due
to the lower intensity of the earth's gravitational field predicted by general
relativity (which we have not yet talked about).
By comparing the times, which are different in the three
clocks, the predictions made by both general relativity and special relativity
can be confirmed. We therefore know how to travel in time in the future , as
much as it may surprise you, we know how to do it (we do it, in a small way, on
a daily basis) and the limits for making bigger jumps are only and exclusively
technological, but this is a one-way ticket . There is no turning back, for
now. Let's see what we can say about travel to the past.
The journey into the past
Mathematically we know that the faster we go the more time
slows down . And slow down, slow down, slow down, to a complete stop for the
speed of light. A photon has no time, it does not age, in a single instant it
sees the entire universe pass in front of it and immediately arrives at the end
of time (if it exists or will ever exist) . Intuitively, with a very crude and
possibly wrong reasoning, however, it is to think that if we went so faster
than light ( like neutrinos? Like hell! ) Time could go backwards. The problem
is that to push something (even without mass) beyond the speed of light you
need infinite energy, which is neither practical nor realistic.
Special relativity is not enough, so let's see what we can
say by considering Einstein's second stroke of genius, general relativity . In
fact, Einstein took Newton's theory of gravity and completely overturned it,
stating that gravity is nothing more than the geometry of space-time , and not
simply a "mysterious force".
A real geometric structure, which as such can be modified,
curved, varied (all in a way that is anything but simple, as we will see).
Everything moves within this structure. Recall that it was only 2018 when the
award was presentedNobel Prize for the discovery of gravitational waves ,
another of the hundreds of correct predictions of this theory that manages to
explain the world around us in a surprisingly accurate way.
So let's look at one of the earliest achievements of general
relativity as far as time travel is concerned, the Tipler cylinder . The Tipler
cylinders was discovered as a solution to the equations of general relativity
by Willem Jacob van Stockum in 1936 and Kornel Lanczos in 1924, but it was only
thanks to an analysis by Frank Tipler in 1974 that its true theoretical
implications for travel in the time . Tipler in his study showed "
Rotating cylinders and the possibility of violation of global causality"He
imagined a portion of space-time containing an enormous cylinder, infinitely
long and rotating along its longitudinal axis.
Such a cylinder would create a dragging effect which would
have the result of deforming space-time in such a way that the light cone of
the objects in the vicinity of the cylinder would tilt, so that a part of the
light cone then points backwards along the time axis on a space-time diagram
(of which I bring you an example, in the photo).
To understand the (extremely complex) concept, let's take a
spiral staircase. We know that if we turn around 360 degrees we return to the
starting point, but in a spiral staircasethis does not happen, we are not in
the exact same point, but on the upper floor. Similarly, in a rotating
cylinder, if we choose the right direction and the right speed and make a
360-degree turn around the cylinder, we would find ourselves in the same
spatial point, but in a different time point . Future, or even past.
The problem with this methodology is: where do we find so
much material, enough to create a sufficiently dense cylinder? Moreover Stephen
Hawking has pointed out that for such an idea it would be necessary to create
either an infinitely long cylinder , or alternatively " negative energy
". In fact, matter with negative rather than positive mass. In short,
beautiful in theory, but nothing functional.
What if instead of turning space-time we create a hole
through space-time? The result is what is commonly referred to as a Wormhole .
In this way one could instantly travel from one point in the universe to
another, but also to another universe or, possibly, to another point in time .
Again, past or future. The problem with wormholes is that they are a great
idea, you can write the equations, the solutions come back and the predictions
come from general relativity, which as we have seen is extremely precise and
accurate . Unfortunately, this also tells us that such a system, as far as
possible, is hopelessly unstable. Even a single photon attempting to pass
through a wormhole would instantly collapse it.
It would take, once again, something that has negative
energy to be able to cross it. A negative mass. As you can see, the concept of
negative mass is inextricably linked to time travel in the past.
The ideas presented, however, are sufficiently old (decades
have now passed), and today there are very few (serious) physicists who are
concerned with seeking a solution to the problem of time travel in the past at
an experimental level. One of them is Ron Mallett, who through some articles
and debates, has even built the prototype of a machine that rotates very
powerful lasers with the intention of subsequently passing a particle through
it, which at the end of its path would find itself back in time.
The discussion is heated and there are many sensational
articles about it online. Obviously there are still no results and the
scientific debate concerns once again the length (infinite?) That this vortex
should have (which works in theory but not yet in practice) or the mass that
the particle passing through it must have ( negative?). In short, there are
recent researches but they are almost all theoretical, and the very rare
experiments carried out in this regard are still far from the realization of a
" time machine ".
On the other hand we could ask ourselves: but if time travel
will ever exist because the world is not inhabited by temporal tourists, people
interested in visiting our age? The reasons may be various, but the most
interesting of these is that if we take a wormhole as an example, this NEVER
allows us to reach a point in time prior to its realization.
We create a wormhole today, and in a hundred years we will
be able to go back to today, but we will never be able to use it to reach
yesterday, for example. That is, the reason we don't see temporal tourists
could trivially be " because time travel hasn't been invented yet, "
however stupid such a phrase may sound in that context.
For time travel fanatics, sorry ; maybe we will be able to
achieve the technology needed to get into the future, butwe will hardly ever
know how to go back . I ask myself: if you had the opportunity, would you
travel to the future without being able to return?
