In the motion pictures, people who goes back and forth through time regularly step inside a machine and — poof — vanish. They then instantly reappear among dinosaurs, knights, or cowboys. Time teleportation is essentially what these films depict.
Scientists don't think this idea is likely to happen in the real world, but they also don't think time travel is crazy. Although the devil is in the details, chronological hopping may actually be permitted by physics.
Time traveling to the not so distant future is simple: You are currently doing it at a rate of one second per second, but physicists claim that this rate is subject to change. The speed at which you move is a factor in how quickly time passes, according to Einstein's special theory of relativity. The passing of seconds slows down the faster you travel. Also, Einstein's general theory of relativity says that gravity has an effect on clocks: Time moves more slowly the closer gravity is to you.
"Close to monstrous bodies — close to the outer layer of neutron stars or even at the outer layer of the Earth, despite the fact that it's a small impact — time runs more slow than it does far away," says Dave Goldberg, a cosmologist at Drexel College.
According to Goldberg, a person might spend a few hours near the edge of a black hole, where gravity is enormous, while someone on Earth would spend 1,000 years. The person who was close to the black hole could have effectively traveled into the future if they had returned to this planet. That has a real effect, he claims. That is without a shadow of a doubt.
However, going back in time can be challenging—more so than being ripped to shreds inside a black hole. Time travel paradoxes in general relativity have been known to scientists for decades, and they have proposed a few possible scenarios. An early time travel solution, according to physicist Fabio Costa of the Nordic Institute for Theoretical Physics, began with a 1920s scenario. This concept involved a massive, long cylinder that twisted spacetime with it and spun quickly, like straw rolled between your palms. In the 1970s, a few decades after scientists had discovered a phenomenon known as "closed timelike curves," it was only realized that this object could be used as a time machine to travel to the past.
According to Costa, "a closed timelike curve describes the trajectory of a hypothetical observer that, while always traveling forward in time from their own perspective, finds themselves at some point at the same place and time where they started, creating a loop." In a region of spacetime that is warped by gravity and loops back into itself, this is possible.
He adds, "Einstein was very disturbed by this idea after reading about closed timelike curves." However, the phenomenon sparked subsequent research.
In the 1980s, science began to take time travel seriously. In 1990, for example, Russian physicist Igor Novikov and American physicist Kip Thorne worked together on an examination paper about shut time-like bends. " They began investigating not only how one might attempt to construct a time machine but also how it would function, Costa claims.
But just as important, they looked into the issues with traveling through time. For instance, what if you tossed a billiard ball into a time machine, causing it to travel to the past and then collide with its past self in a way that prevented its present self from ever entering the time machine? That seems like a paradox, says Costa.
According to him, there has been intermittent interest in the subject but no significant breakthrough since the 1990s. The field isn't exceptionally dynamic today, to some extent in light of the fact that each proposed model of a time machine has issues. " According to University of Rhode Island researcher Gaurav Khanna, "it has some attractive features, possibly some potential, but then there ends up being some kind of a roadblock when one starts to sort of unravel the details."
For example, most time travel models require negative mass — and consequently bad energy on the grounds that, as Albert Einstein uncovered when he found E = mc2, mass and energy are indeed the very same. Theoretically, mass can be either positive or negative, just like an electric charge can be, but there has never been a case of negative mass. For what reason truly does time travel rely upon such intriguing matter? Holding open a wormhole—a general relativity-predicted spacetime tunnel connecting two points in the universe—requires it frequently.
Gravity would result in the collapse of this tunnel if there was no negative mass. According to Goldberg, it can be thought of as preventing the positive mass or energy from passing through the wormhole.
Although Khanna makes the observation that some quantum phenomena, such as negative energy on very small scales, show promise, both Khanna and Goldberg concur that it is highly unlikely that matter with negative mass even exists. However, according to him, that would be "nowhere close to the scale that would be needed" for a genuine time machine.
These difficulties explain why Khanna initially discouraged Caroline Mallary, then a graduate student at the University of Massachusetts Dartmouth, from working on a project involving time travel. Mallary and Khanna proceeded in any case and concocted a hypothetical time machine that didn't need negative mass. The idea of Mallary's involves two parallel cars, each made of regular matter, in its most basic form. A closed timelike curve will form between them if you park one and zoom the other extremely fast.
Simple, right? However, while Mallary's model eliminates the requirement for negative matter, it introduces a further obstacle: it requires endless thickness inside the vehicles for them to influence spacetime in a manner that would be helpful for time travel. Inside a black hole, where gravity is so strong that it compresses matter into a mind-bogglingly small space called a singularity, there is infinite density. Each car in the model needs to have such a unique feature. According to Mallary, these constraints are one reason why there isn't much ongoing research on this topic.
A wormhole—a spacetime tunnel connecting one location in the universe to another—has been used in models of time travel developed by other researchers. Goldberg describes it as "a kind of shortcut through the universe." Imagine sending one end of the wormhole back to where it came from after it was accelerated to close to light speed. He asserts, "Those two sides are no longer in sync." The other has passed; one is from here on out." You'll experience time travel if you walk between them.
You could achieve something almost identical by moving one finish of the wormhole close to a major gravitational field — like a dark opening — while keeping the opposite end close to a more modest gravitational power. On the big gravity side, time would thus slow down, allowing a particle or other mass to reside in the past relative to the other side of the wormhole.
However, the dreaded negative mass and energy are needed to create a wormhole. Gravity would cause a wormhole made of normal mass to burst. According to Goldberg, "most designs typically have some similar kinds of issues." They are theoretically possible, but there is currently no practical way to make them, similar to a calorie-free, delicious pizza.
In addition, the late physicist Stephen Hawking held the belief that it is impossible to do so except on microscopic scales, so perhaps the issue is not only that we do not know how to construct machines that can travel through time. The chronology protection conjecture that he proposed: Time travel is not possible because the universe does not permit changes to the past. In a 1992 paper published in Physical Review D, Hawking wrote, "It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians." Part of his reasoning involved the paradoxes that time travel would create, such as the situation with a billiard ball and its more well-known counterpart, the grandfather paradox: You can't be born, you can't time travel, and you couldn't have killed your grandfather if you went back in time and killed him before he had children. Yet you are there.
Agustin Rayo, a philosopher at the Massachusetts Institute of Technology, is interested in these complications because the paradoxes challenge more than just causality and chronology. Additionally, they make free will appear shaky. Why can't you kill your grandfather? Physics says you can go back in time. What prevents you? He states Is it safe to say that you are not free?
However, rayo suspects that time travel is steady with freedom of thought. " He asserts, "What is past is past." Therefore, if my grandfather actually lived long enough to have children, going back in time will not alter that. If I try, why will I fail? I'm not sure because I don't know enough about the past. I do know that I will fail in some way.
In other words, you might trip over a banana or miss the bus if you tried to kill your grandfather. According to Costa, "it's not like you would find some special force compelling you not to do it." You would neglect to do it for completely commonplace reasons."
Together with Germain Tobar, Costa's then-undergraduate student at the University of Queensland in Australia, in 2020, they worked on the mathematics that would support a similar concept: that there are no paradoxes and there is freedom of choice when traveling through time.
In some ways, Goldberg agrees with them. According to him, "I definitely belong to the category of thinking that if there is time travel, it will be constructed in such a way that it will produce one self-consistent view of history." because it appears that is how all of our other physical laws are constructed.
What the future holds for time travel to the past is unknown. Additionally, no time travelers have yet shown up to inform us about it.
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