Going back in time may not be possible. But is there a way to travel into the future?
The artist's impression of the wormhole
In 2009, British physicist Stephen Hawking threw a party for time travelers — but a year later he sent out an invitation (no guests showed up). Traveling back in time is not possible. Even if it were possible, Hawking and others would argue that you would never be able to go back in time until the time machine was built.
But travel to the future? Proof is feasible. Of course, we are all time travelers, because we are flowing through time at a rate of hour and minute by second, from the past to the future. But, just like a river, water flows at different speeds in different places. As we know, science allows multiple methods to be used to bring the fast lane into the future. It's a concept.
Scientists envision wormhole travel
1. Time travel by speed
According to Einstein's special theory of relativity, when you travel at nearly the speed of light, your time slows down relative to the outside world. It's not just a conjecture or thought experiment, it's already measured. Using a binary atomic clock (one flying on a jet and the other stationary on Earth), physicists have shown that the ticking of a flight clock is slower because of its speed. In the case of aircraft, the effect is minimal. However, if you're in a spaceship flying at 90% of the speed of light, you'll experience the passage of time 2.6 times slower than when you return to Earth.
The closer you get to the speed of light, the more extreme time travel becomes.
The maximum speed achieved by human technology may be 99.9999991% of the speed of light whizzing around the Large Hadron Collider. Using special relativity, we can calculate that one second of a proton is equivalent to our 27,777,778 seconds, or about 11 months. Surprisingly, particle physicists have to take this time dilation into account when dealing with decaying particles. In the lab, meson particles typically decay within 2.2 microseconds. But fast-moving mesons, such as those produced when cosmic rays hit the upper atmosphere, take up to 10 times longer to decompose.
2. Gravity time travel
The next time travel method was also inspired by Einstein. According to his theory of general relativity, the stronger the gravitational pull you feel, the slower time moves. For example, when you are close to the center of the earth, the gravitational intensity increases. Your feet are slower than your head.
This effect was measured again. In 2010, physicists at the National Institute of Standards and Technology (NIST) placed two atomic clocks on a shelf, one 33 centimeters taller than the other, and measured the difference in their ticking speeds. The lower one ticks more slowly because it feels slightly stronger in gravity.
All we need to do is a region with extremely strong gravitational pull, like a black hole. The closer you get to the event horizon, the slower time moves. But it's risky, crossing borders, and you can never escape.
Gravity can slow down time
Anyway, the effect isn't that intense, so it might not be worth a visit.
Assuming you have the technology to travel long distances to reach a black hole (the nearest distance is about 3,000 light-years), the time expansion caused by traveling will be much greater than any time expansion caused by orbiting the black hole. (The scenario described in the movie "Interstellar," according to film science consultant Kip Thorne, is equivalent to seven years back on Earth on a planet close to a black hole.) ）
Perhaps the most exciting thing is that GPS systems have to take into account the effect of time expansion (due to the speed of satellites and the gravity they feel) in order to work. Without these corrections, your phone's GPS features won't be able to pinpoint your location on Earth to within a few meters.
3. Time travel in suspended animation
Another way to travel time into the future might be to slow down your perception of time by slowing down or stopping your bodily processes and then restarting them again.
Bacterial spores can survive in suspended animation for millions of years until the right temperature, humidity, and food conditions restart metabolism. Some mammals, such as bears and squirrels, slow down their metabolism during hibernation, greatly reducing the cell's need for food and oxygen.
Can humans do the same thing? While stopping metabolism completely may be far beyond our current technology, some scientists are working to achieve a short-term hibernation state that induces at least a few hours. This may be just enough for a person to experience a medical emergency, such as cardiac arrest, before arriving at the hospital.
How the movie Lost in Space looks to the future
In 2005, American scientists demonstrated a way to slow the metabolism of mice (which they do not hibernate) by exposing them to trace amounts of hydrogen sulfide, which binds to the same cell receptors as oxygen. The core body temperature of the mice dropped to 13°C and the metabolism decreased by a factor of 10. After 6 hours, the rats can be revived without adverse effects. Unfortunately, similar experiments in sheep and pigs have not been successful, suggesting that the method may not be suitable for larger animals. Another method of inducing low-temperature hibernation by replacing blood with a cold saline aqueous solution already works in pigs and is currently undergoing human clinical trials in Pittsburgh.
4. Time travel through wormholes
General relativity also allows for shortcuts through space-time, namely wormholes, which may be able to span a billion light-years or more, or different points in time.
Many physicists, including Stephen Hawking, believe that wormholes are constantly appearing and disappearing at the quantum scale, much smaller than atoms. The trick is to capture one and inflate it to human scale — a feat that requires a lot of energy, but in theory that might just be possible.
Attempts to prove this in either way failed, ultimately because of the incompatibility between general relativity and quantum mechanics.
5. Use light for time travel
Another time-travel idea proposed by American physicist Ron Mallet is to use a rotating column of light to distort space-time. Theoretically, anything that falls inside a rotating cylinder can be dragged in space and time, just as bubbles flow over coffee after spinning a cup with a spoon, and the right geometry can lead to time travel to the past and the future.
Since publishing his theory in 2000, Mallet has been trying to raise money to pay for a proof-of-concept experiment involving the delivery of neutrons through a circular arrangement of rotating lasers. However, his idea did not attract the attention of others in the physics community, who believed that one of the hypotheses of his basic model was plagued by singularities, which is the "impossible" claim of physics.