Fun story: How to build a time machine?

Since the publication of the British writer Herbert George Wells's sci-fi novella "The Time Machine" in 1895, topics related to time travel have been talked about with great relish and have remained popular for a long time. So, can humans achieve time travel by traveling through time and space?

What is Spacetime

Before building a time machine, we must first understand time itself and figure out what time is.

When you are enjoying the warm spring sun, have you ever thought that it is actually the sun from 8 minutes ago? Because even light needs about 8 minutes to "run" the distance of about 150 million kilometers from the sun to the earth. Therefore, what we see is the sun in the past.

For a long time, people generally believed that if it is 15:00 here, then it must be 15:00 on the moon, or even 15:00 everywhere in the universe. But in fact, it is impossible to calibrate your watch to the standard time of the universe.

Many people believe that the rate at which time passes is fixed, and that time flows at the same speed throughout the universe. In fact, this is not the case. Clocks in outer space tick slower than on Earth. Note: rate is not the same as speed. Rate is a scalar quantity, which has magnitude but no direction.

The GPS satellite navigation system works only because the clocks on the satellites are calibrated every day. Time does not have an absolute value, and it is different in different places. Therefore, those who want to time travel must abandon the traditional view of time and regard time as a dimension.

Most people think we live in three-dimensional space, but in fact we live in four-dimensional space. In addition to the length, width and height of the ordinary three-dimensional space, there is also an additional time axis. Of course, it is not easy for us to accept time as a dimension.

Simply put, when we make an appointment with a friend to meet on a certain floor of a certain building, we must also agree on a time to meet. The fourth dimension of time is closely intertwined with the other three-dimensional spaces, and physicists call these four dimensions "spacetime." Therefore, to be precise, the macroscopic world we live in is actually three-dimensional space and four-dimensional spacetime.

Now, most scientists believe that time, like space, is not invisible, and that they are structures that can be stretched and even tied into knots. If this is true, then time travel may not sound so unreal. In other words, if we can find a way to manipulate the structure of space and time, then we can achieve real time travel.

Speed ​​Time Machine

Most of us live in normal time and space, but it is not an exaggeration to say that there are already people among us who can time travel. In fact, astronauts on the International Space Station and the Chinese Space Station have already started time travel into the future. You must be curious, how do they do it?

They mainly rely on speed. The space station flies around the earth at a speed of more than 27,000 kilometers per hour, and some people have lived in the International Space Station for hundreds of days and nights.

When they return to Earth, they have already traveled about 1 microsecond, or one millionth of a second, into the future. Therefore, we can also regard the spacecraft as a time machine, whose super-fast speed can slow down time.

So, can time be manipulated?

Scientists hypothesize that if a "light clock" is placed on the dashboard of a car, in which light moves vertically up and down, and when the car passes by, the light in the light clock seen by passers-by moves diagonally. Therefore, people looking from outside the car see the light moving a longer distance. However, for people inside and outside the car, the speed of light does not change.

Time is equal to distance divided by speed, so the person in the car spends less time. In other words, time passes more slowly for the person traveling at high speed. Scientists call this phenomenon "time dilation."

The world's largest particle accelerator at CERN is located in Geneva, Switzerland. For nearly half a century, scientists from more than 80 countries around the world have studied and experimented with time travel here. At present, they have discovered a useful time travel particle called "meson". This high-energy particle comes from cosmic rays, which constantly "bombard" the earth's atmosphere, but its life span is only about one millionth of a second.

Scientists use particle accelerators to push muons to speeds close to the speed of light, which will extend their lifespan. In other words, the time corresponding to the muon is going slower and slower, while the time outside the accelerator remains the same as usual. In a sense, the muon has already entered our future.

However, using speed to build a time machine is still full of obstacles. It turns out that mass corresponds to a certain amount of energy. When the momentum of an object increases, its mass will also increase. If an object moves at a speed close to the speed of light, the increased mass will be close to infinity. For example, when a muon moves at a speed close to the speed of light, its mass is more than 7,000 times that when it is stationary.

Energy is therefore one of the biggest obstacles to our ability to time travel. Simply put, we cannot currently make a human-sized object move at speeds close to the speed of light. Even if we use up all the energy known on Earth, we cannot make a rocket travel at speeds close to the speed of light.

Gravity Time Machine

There is always a way out. Of course, we have another way to achieve time travel, and this method can not only take people into the future, but also take people back to the past.

The method generally recognized by scientists to leap forward in time, besides relying on speed, is to rely on gravity.

Simply put, gravity demonstrates how mass bends (or warps) the fabric of spacetime. If an object is massive enough to rival a star or black hole, it can bend spacetime significantly. The more massive an object is, the greater the effect it has on the fabric of spacetime. The Hubble Space Telescope has shown that the path of light passing through the galaxy bends, demonstrating that mass can bend spacetime. And the curvature of spacetime will help us build time machines because when gravity bends spacetime, it has a real effect on the passage of time.

Time goes slower on the first floor of a skyscraper than on the top floor. This is because the closer you are to the surface of the earth, the stronger the gravity is, the more time and space are curved, and time goes slower. Although this is a very small difference, if you stand on the top floor for 100 years, your watch will run 500,000ths of a second more than the watch on the first floor. So, in a sense, the earth itself is a time machine.

If we move to space, the effect is even greater. In orbit far from the Earth, time passes at a different rate than on the surface, which is important for global satellite positioning systems. We must synchronize the clocks on the ground and on the satellites for these systems to work properly. And because the gravitational field is weaker where the satellite is, time passes faster there, with a difference of about 50 microseconds in a day. If we don't correct for this difference in the rate at which time passes, satellite navigation will not work.

Earth's gravity and the distortion it causes to space-time aren't enough to help us travel back in time, but there are places in the universe where gravity has greatly distorted space-time. Space-time is mostly flat, but it's drastically curved in a black hole. The gravity of a black hole stretches and curves the space-time around it, making it an incredibly deep, conical hole. Because its gravity is so strong, time slows down dramatically in a black hole.

For those who want to time travel, entering a black hole may be a way to achieve their wish. However, in a black hole, you will definitely encounter a singularity, which is the densest point in the black hole. Therefore, you will be crushed to pieces at the singularity.

However, scientists soon discovered cosmic strings, which are thin, long, and highly energy-concentrated tubes that were hypothetically formed during the Big Bang. It is speculated that the mass of a 1-meter-long cosmic string is greater than that of the Earth, so it can also cause space-time to distort. Therefore, if there are two infinitely large cosmic strings passing by each other, a time machine can be created. Scientists assume that there is a spaceship that travels faster than the speed of light and orbits around two cosmic strings, so it can return before it sets out.

However, according to the current cosmic string theory, to go back one year, you need about half the mass of the Milky Way. And to control such a large mass, you must have the ability to dominate the energy of the entire Milky Way.

The various theories of time travel seem absurd, but for more than a century, scientists have combined knowledge and imagination to try to prove that time travel will happen. Some people may only go to the future of microseconds, but scientists studying time travel are still moving forward.

Because every great adventure actually begins with a small step.