Humans need matter with negative energy to build a time machine

Have you ever wished that you could go back in time and correct a mistake you made? Or go back to the loneliest and most lost time in your life to encourage yourself "Do your best, everything will be okay".

Many scientists have spent their lives working on this idea, but eventually realized: It is impossible to time travel and change the past. Accordingly, there are paradoxes like the "Grandfather's Paradox" that will prevent them from doing so.

But there are also scientists who have not given up. They have more and more new ideas about time travel, so as not to have to face grandpa. For example, we can go back to the past as a "view only", or the universe actually exists in different timelines, allowing machines to send people jumping between them.

In this article, we take a look at those supposedly possible time travel ideas.

Building a time travel machine has been the dream of generations of scientists.

To build a time machine, we need a form of matter with negative energy

Today, what we know about time is based on the theory of general relativity. In it, Albert Einstein combined space and time into a single entity - "spacetime". 

General relativity and the concept of spacetime have enabled scientists of the 20th century to explain most phenomena in the universe, in a way of considerable complexity that no previous theory can match. .

What Einstein wrote and predicted has also been verified over the course of 100 years, with the most recent discovery of the existence of gravitational waves. This once again confirms the indisputable correctness of relativity and the way time works within that framework.

So the challenge with the idea of ​​time travel is that it must pass the test of relativity.

If you want to time travel, you have to pass Einstein's test.

Over the decades, mathematicians have discovered that, it turns out, they can write equations that describe the idea of ​​time travel, and all of these equations are consistent with Einstein's theory.

But the point is that physics is not just math. All equations are meaningless if they are not compatible with reality. Accordingly, there are two problems that challenge mathematical equations to come true:

The first problem is that a time machine - if built - would require an exotic substance. This matter must have negative energy. Meanwhile, all the things we see in our daily life have yang energy.

Humans have not yet grasped negative energy matter at the present time. But again, another theory called quantum mechanics says that negative-energy matter does exist. They are still being produced in brief, explosive moments, on the quantum scale of extremely tiny pieces of matter.

Nor is there currently any equation or scientific evidence that says humans cannot create negative energy matter in large quantities. Therefore, this problem is probably just a limitation of current technology. At some point as we learn more about the quantum world, we might find a way to harness negative-energy matter.

Positive energy matter concave spacetime, negative energy matter convex spacetime

Solving the grandfather paradox

Moving on to the second hurdle challenges the idea of ​​time travel, which is that it seems to contradict logic and creates a series of paradoxes. The most famous of these is the "Grandfather's Paradox". This paradox says:

There was a man who time traveled to the past and killed his own grandfather, before he married his grandmother. As a result, his father will not be born. Moving on, it leads to the time traveler himself never being born.

Then how can he travel to the past? But if he didn't go back to the past to kill his grandfather, then his grandfather must be alive and that means he was still born, and then returned to the past to kill his grandfather. This paradox actually denied the existence of both cases.

An extension of the grandfather paradox statement is that it happens whenever an event leads to a change in the past, but the change itself prevents the event from happening in the first place. This is a purely logical paradox and it proves the existence of a time traveler is unlikely.

There is a common misconception in sci-fi movies that directors assume that paradoxes are something that humans "create". Therefore, to avoid time travel paradoxes, in the movie, the main character is always warned that when he returns to the past, he should not change any events that have happened.

We can easily see this logic in old movies, from decades ago like the Back to the Future trilogy. But unfortunately, in real life, a paradox is not an event that "happens" and people cannot by willpower control it.

The time travel paradox is a purely theoretical concept, it is a contradiction in theory itself and has never "happens" in practice, and may well never happen.

In other words, the paradox not only implies that time travel is a dangerous endeavor, but it also implies that going back to the past is simply something where no one, no civilization, no form of transcendental consciousness. which can be done.

Going back to the past is unthinkable.

This is one of the motivations for theoretical physicist Stephen Hawking to think that our reality and its past years are like a flow of time strictly guarded by physics.

Hawking also conducted a simple experiment to prove that time travel is impossible. He writes an invitation, specifies the place and time at which he holds a party, and sends it into the future thousands of years in the future to invite future people back to his party.

But how to send that letter to the future people thousands of years later? It turned out to be very simple, put the invitation in a sturdy metal box and bury it in the ground.

Thousands of years later, when future people dig it up, they will know where Hawking is and also his "GPS" position in the timeline. If there is a machine that travels back to the past, future people will definitely come to Hawking's party, because after all, he is a theoretical physicist with enough reputation for humanity.

Unfortunately, Hawking waited all day and no future person came to his party.

"Welcome Time Travelers"

Under what circumstances is time travel possible?

However, does the fact that none of the futurists return to Hawking's party prove that they can't travel back in time? According to a theory put forward by Igor Dmitriyevich Novikov, a Russian theoretical physicist in the 1980s, is unlikely.

It's called the Novikov principle of self-consistency, which says: "You can time travel to the past, but you can't change the past, in any way, even by appearing in front of you. Hawking at the party is also impossible."

Physics has always had a way of maintaining the consistency and history it has created. So if you could really travel to the past, you would probably just exist there as an observation, like someone with "view only" permission in a GoogleDoc file without "edit" permission. it.

But what's the use of time travel that can't change the past? At the very least, it allows us to confirm history. If you can't change history, do you at least know what really happened and verify what historians record?

According to theoretical physicist Igor Dmitriyevich Novikov, you can go back to the past as a "view only".

However, even the Novikov self-consistency principle alone cannot solve all the paradoxes. In a recent paper by physicist Barak Shoshany at Brock University, he and his students Jacob Hauser and Jared Wogan prove it and claim that humans return to their past in the form of " view only" is also not logically feasible.

Instead, Shoshany proposes a different idea of ​​time travel, in which there is a lot of history and we can actually change some history. In other words, Shoshany's idea is similar to what we've seen in the Marvel movies, that there are multiple timelines that exist side-by-side at the same time. This can solve any time travel paradox, including the grandfather paradox.

Shoshany said that when you exit the time machine, you will fall into a different timeline, not your own. In that timeline, you can do whatever you want including killing your grandfather without changing anything in your original timeline.

Since the grandfather in your main timeline is still alive, your dad will still be born and so will you. There will be no paradox here.

"After studying the time travel paradox for the past 3 years, I am increasingly convinced that time travel is possible, but only if our universe can allow multiple histories to coexist. So, is it possible or not," said Shoshany.

As it turns out, quantum mechanics actually implies the existence of parallel timelines in our universe, and also the existence of multiple universes. For example, whether Schrödinger's cat is alive or dead. It can live in one timeline and die in another. Therefore, we can say it is both alive and dead in our universe.

But in the end, these are just speculations. Scientists like Shoshany are still working to find a specific theory of time travel, given the many histories that exist alongside general relativity.

Of course, even if we tried to work out such a theory, this would not be enough to prove time travel is possible. But at the very least, it also means that time travel is not ruled out by paradoxes.

And so, people have every right to dream, and even seriously pursue the idea of ​​​​creating a time machine - even though it is exactly a machine that jumps between different timelines and universes.