Can humans travel through time?

In books and on films, our favorite characters have time machines to travel to the past or to the future. But in real life it is not so easy.

Journey to the past

Traveling to the past is impossible. Even sending information about the past is hard to imagine because this will change what happened, but no one can change what happened. For example, suppose you played a swing on a curved bridge and fell off your arm. What if you could turn back time and tell yourself not to play that game? If you do so, you will not fall and not break your hand. But if you didn't break your hand, you wouldn't need to go back to the past to tell yourself not to swing. So what happens to your arm, is it broken?

If you find that thinking like that very complicated and complicated, many people will find that way too.

In the film Harry Potter and the prisoner of Azkaban, the characters Harry and Hermione use the time machine to return to the past.

Time travel is a difficult idea to imagine for most people. That's because when we think of time, we think of it as a straight line with many things happening one after the other. 

If we could go back to the past and change something that happened, we would change the order of that line. Thus breaking a rule called 'causality'.

Cause and effect is a rule about a cause (such as your action) that occurs before an outcome (as a result of your action) occurs. In the example you play a swing, the reason is that you fell and as a result, you broke your hand and your broken hand was because you fell.

Cause and effect is one of the unbreakable law of the universe. Breaking this rule will have unintended consequences for the universe and all of us. According to experts, because the universe has this rule, returning to the past is impossible, otherwise this rule will always be broken.

Traveling to the future

If going back to the past is impossible, can we go ahead to the future?

Technically, we always go to the future, because time always passes. Every second we advance one second to the future. But this happens to everyone, so it's not called time travel, right?

Believe it or not, the truth is that each person can sense the passage of time at different speeds. Time passing with a person moving very fast is very different from a person standing still. This is a very complex idea, called 'time dilation'.

A person flying from Hanoi to Da Nang will feel that time passes faster than a person waiting for him at the airport during the flight. So why do we not notice this difference?

That's because it takes a lot faster to fly than the plane to start paying attention to the time of expansion. Even if you fly around the world, the time you feel different is only 1 millionth of a second compared to people sitting at home.

The only way for scientists to know the elongation time is through extremely accurate experiments that can be measured.

However, time expansion cannot help us travel time. If you fly around the world for more than 4 million years, the people on the ground will only spend more time than you have 1 second. 

How fast can we go?

If it all depends on speed, the answer must be to go faster . If you can go fast enough and long enough, through hundreds of ordinary years on a short trip, you will probably feel like you are traveling to the future.

Unfortunately, the 'fast enough' speed here must be equal to the speed of light , which is a lot faster than any means available to humans today. Light travel is about billions of kilometers per hour.

The fastest man-made object ever created by a man was the U.S. Space Agency's Solar Explorer spacecraft. This ship was launched in August 2018. But its fastest speed is only 0.064% of the speed of light, which means that light travels 1,000 times faster than this ship.

The US Space Agency's Parker Sunship can travel at speeds of 692,000 km / h.

All of this shows that if people want to reach the future, there is still a very, very long way ahead.

Retrospective

So we can't travel in time. But we can still look back on the past every night.

Light has a fixed speed. It travels extremely fast, but the objects in the universe are so far apart that the light takes a long time to travel from our stars and distant planets to Earth. Light from the Sun also takes 8 minutes and 20 seconds to reach Earth, which means we saw the Sun in the past 8 minutes 20 seconds. But remember never to look directly at the sun as this can damage your eyes.

The galaxy closest to our Milky Way galaxy is the Dwarf Galaxy Dog, which is 25,000 light-years away. This means that it will take 25,000 years for the light to reach Earth. When we look at this galaxy through a telescope, we see it 25,000 years ago. Therefore, even if we cannot travel in time by ourselves, we can look up into the sky every night and look at the past.