Time Travel in Real Life
Time travel is a favorite theme in science fiction films. A character steps into a special machine that transports them to a specific time.
Of course, no such time machine exists in reality. However, according to modern physics, it really is possible to “travel” forward in time, even if not in the way we see in films.
Time Does Not Pass at the Same Rate for Everyone
In everyday life, we feel that time passes at a constant rate and that one second is the same for everyone. However, Albert Einstein’s theory of relativity, introduced in 1905, showed that this is not the case. The passage of time is influenced by two important factors:
- the speed of motion
- the strength of gravity
This phenomenon is called time dilation. It may sound a little strange at first, but it will soon make sense.
The Effect of Speed
According to the special theory of relativity, the faster someone moves relative to another observer, the more slowly time passes for them. At everyday speeds, the difference is so small that it can only be detected using extremely precise clocks. As an object approaches the speed of light, however, the effect becomes significant.
Now imagine an astronaut travelling through space in an extremely fast spacecraft for one year—at least, one year according to the astronaut’s own clock. They experience nothing unusual during the journey; they are simply travelling at an exceptionally high speed. After one year, the astronaut returns to Earth and discovers that an entire decade has passed there. Everyone who remained on Earth has aged by ten years, and according to their clocks, the astronaut left ten years ago. Time is therefore relative.
This is often called the twin paradox or the clock paradox. One twin sets off on a high-speed journey and returns younger than the sibling who stayed at home—in reality, the travelling twin has aged more slowly.
Based on this, astronauts orbiting Earth are also time travellers. Because they move at high speeds, time passes slightly more slowly for them. When they return, they have aged by a tiny fraction of a second less than the people who remained on Earth. Their speed is nowhere near high enough to produce a dramatic difference, but the effect can be measured physically.
The Effect of Gravity
Ten years after introducing special relativity, Einstein described gravitational time dilation in his general theory of relativity in 1915. Put simply, the stronger the gravity, the more slowly time passes.
On Earth, gravity varies with altitude. It increases as we move towards the planet’s centre and decreases as we move farther away from it. For example, if we climb to the top of a very high mountain, the gravitational force acting on us is weaker than it is at sea level.
Suppose we place one clock at sea level and another on top of a very high mountain, then wait for one year. Because the clocks are exposed to different gravitational forces, the clock on the mountaintop will be ahead by a tiny fraction of a second. Slightly more time will have passed. The difference is extremely small, but modern atomic clocks are capable of detecting even such minute variations.
In an experiment published in 2022, JILA researchers successfully measured the difference in the passage of time caused by a height difference of just 1 millimetre.
In theory, we could therefore travel into the future by spending an extended period near an extremely strong gravitational field. Imagine an astronaut approaching a very massive black hole while remaining at a safe distance, then returning to Earth after a certain amount of time. For the astronaut, the entire journey might have lasted only a few days or weeks, while people on Earth would experience a much longer period.
Beyond the Speed of Light
If approaching the speed of light slows the passage of time, an obvious question arises: what would happen if a spacecraft reached that speed, or perhaps travelled even faster?
According to Einstein’s special theory of relativity, the speed of light in a vacuum is not simply an extremely high speed, but one of nature’s fundamental limits. Based on our current understanding of physics, information, matter and energy cannot be transmitted faster than this. The speed of light is 186 282 miles per second (more than 670.6 million mph).
The closer an object with mass comes to the speed of light, the more energy is required to accelerate it further. Reaching the speed of light would therefore theoretically require an infinite amount of energy.
Consider what was explained earlier: if a spacecraft travelled at an extremely high speed, time would slow down for it. Does that mean time would stop if it reached the speed of light? What would happen if it then returned to Earth? If time had stopped for the spacecraft, would an infinite amount of time have had to pass on Earth? The speed of light can therefore only be approached, never reached.
At present, the fastest human-made spacecraft is the Parker Solar Probe, which was launched in 2018. It studies the Sun from extremely close range and continues to accelerate. In 2026, it holds the record at a speed of 428 000 mph. Although this may seem incredibly fast, it is only 0.064% of the speed of light.
The physical possibility therefore exists, but science is still a long way from achieving noticeable time travel.
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