In 2009, British physicist Stephen Hawking hosted a party for time travelers - the trick was that he sent out invitations to the party a year later (none of the guests showed up).
Traveling back in time is most likely impossible. Even if this possibility existed, Hawking and others argue that you can never get to a point in time until the moment your time machine was built.
But a journey into the future? This is a different story.
Of course, all of us time travelers are racing in the flow of time from the past to the future at a rate of one hour per hour.
But like a river, the flow of time flows at different speeds in different places. Modern science offers several ways to bring the future closer. Here is a summary of their essence.
Image caption: A journey through a time-space tunnel as seen by Bossinas Forest for NASA
Speed
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The easiest and most practical way to get to the distant future is to move very quickly.
According to Einstein's theory of relativity, when you travel at a speed close to the speed of light, time slows down for you in relation to the outside world.
It's not just a hypothesis or thought experiment - it's a measurement result. With the help of two identical atomic clocks (some flew in a jet plane, others remained stationary on Earth), physicists proved that flying clocks tick slower due to speed.
In the case of an aircraft, the effect is minimal. But if you were aboard a spacecraft traveling at 90% the speed of light, time would pass 2.6 times slower for you than on Earth.
And the closer your speed approaches the speed of light, the more extreme time travel becomes.
The highest speed achieved thanks to human technology can be called the speed with which protons rush around the Large Hadron Collider - 99.9999991% of the speed of light. Using the theory of relativity, one can calculate that one second for a proton is equivalent to 27,777,778 seconds or, in practice, 11 months for us.
Surprisingly, particle physicists take deceleration into account when dealing with decaying particles. In the laboratory, muon particles typically decay in 2.2 microseconds. But fast-moving muons, which are produced when cosmic rays reach the upper atmosphere, decay 10 times longer.
Image caption: Gravity can slow the passage of time
Gravity
The following method is also inspired by the work of Einstein. According to his theory of general relativity, the more you feel gravity, the slower time moves.
For example, as you get closer to the center of the Earth, the force of gravity increases. Time passes more slowly for your legs than for your head.
Again, this effect has been measured. In 2010, physicists at the US National Institute of Standards and Technology (NIST) placed two atomic clocks on shelves, one 33 centimeters taller than the other, and measured the difference in their ticking speed. The clock on the shelf below ticked more slowly because it was slightly more subject to gravity.
To be in the distant future, all we need is a place with extremely strong gravity, like a black hole. The closer you get to the border, the slower time moves - but this is risky, because crossing the line, you can never return.
In any case, the effect is not that strong, so the trip is probably not worth it.
Let's say you have the technology to travel long distances to get to a black hole (the closest is about 3000 light years away). During the travel itself, time will slow down much more than during travel through the black hole itself.
(The situation described in Interstellar, where one hour on a planet near a black hole is the equivalent of seven years on Earth, is too extreme and completely impossible for our universe, says Kip Thorne, the film's scientific advisor).
Perhaps the most amazing thing is that GPS systems must take into account the effects of time dilation (both due to the speed of the satellites and the gravity that acts on them) in their work. Without these corrections, the GPS on the phone will not be able to determine your position on Earth, even within a radius of several kilometers.
image caption: how the future is shown in the TV series "Lost in Space"
Anabiosis
Another option for traveling to the future is to slow down the perception of time by slowing down or stopping the life processes of your body, and then restarting them.
Bacterial spores can live for millions of years in suspended animation until the right temperature, humidity and food conditions start their metabolism again. Some mammals, such as bears and squirrels, can slow down their metabolism during hibernation, which greatly reduces their cells' need for oxygen and food.
Will people ever be able to do the same?
Although the complete stop of the body's metabolism is not yet subject to modern science, some scientists are working towards achieving the effect of short-term "hibernation" lasting several hours. This may be enough time to help the person survive, for example, during cardiac arrest, before they can be taken to the hospital.
In 2005, American scientists demonstrated a way to slow down the metabolism of mice that do not hibernate. During the experiment, they were given small doses of hydrogen sulfide, which is perceived by the same cell receptors as oxygen. The overall body temperature of the mice dropped to 13 ° C and the metabolism decreased 10 times. After six hours, the mice were resuscitated without side effects.
Unfortunately, a similar experiment conducted on sheep and pigs was unsuccessful, which gives food for thought: perhaps this method is not suitable for larger animals.
Another technique that puts the body into a hypothermic "hibernation" - replacing blood with cold saline - has worked in pigs and is currently undergoing clinical trials in humans in Pittsburgh.
image caption: this is how the artist Kjordand embodies his idea of the space-time tunnel
Space-time tunnels
General relativity also allows for the possibility of rapid travel through time-space tunnels, which could help cover distances of billions of light years or simply different times.
Many physicists, including Stephen Hawking, believe that space-time tunnels, constantly appearing in different places of the quantum shell, are much smaller in size than atoms. The trick is to grab one and expand it to human proportions - a feat that will require a tremendous amount of energy, but can only be possible in theory.
Attempts to prove such a method have failed, ultimately due to the incompatibility between general relativity and quantum mechanics.
Use of light
The idea, put forward by American physicist Ron Mallett, is to use a rotating cylinder of light to fool the space-time continuum. All objects trapped inside a swirling cylinder can theoretically be dragged through space and time, like a bubble that has risen to the surface of coffee after stirring the drink in a cup.
The right configuration can help travel past and future, Mallett said.
Mallett published his theory in 2000 and has since been trying to raise funds for an evidence-based experiment involving passing neutrons through a circle of rotating lasers.
However, his ideas did not receive support from the physicist community, as lacking originality.
Olga Melnik