According to NASA, time travel is possible, just not in the way you might expect. Albert Einstein's theory of relativity says time and motion are relative to each other, and nothing can go faster than the speed of light, which is 186,000 miles per second.
The Second Law of Thermodynamics is the reason you can't go back to the past. The universe, like an unmixed cup of coffee, started in an extremely ordered state. Over time, the universe mixed together and became less ordered, like what happens when you stir the coffee. Going back in time is unmixing; it can't be done.
"The answer is embedded in the beginning of the Universe," says Carroll. "The answer is because the Big Bang had low entropy. And still, 14 billion years later we are swimming in the aftermath of that tsunami that started near the Big Bang. That's why time has a direction for us."
But time is different, it has a direction, you always move forward, never in reverse. So why is the dimension of time irreversible? This is one of the major unsolved problems in physics. To explain why time itself is irreversible, we need to find processes in nature that are also irreversible.
Both the Hindu and Buddhist texts say that time is infinite in both directions, and that events repeat themselves. If you roll a pair of dice many times, sooner or later the same pattern of rolls will repeat.
Although many people are fascinated by the idea of changing the past or seeing the future before it's due, no person has ever demonstrated the kind of back-and-forth time travel seen in science fiction or proposed a method of sending a person through significant periods of time that wouldn't destroy them on the way.
While the debate continues over whether travelling into the past is possible, physicists have determined that travelling to the future most certainly is. And you don't need a wormhole or a DeLorean to do it. Real-life time travel occurs through time dilation, a property of Einstein's special relativity.
In fact, according to Albert Einstein's famous equation, E = mc² , time travel is possible, at least in one direction.
Time travel is still impossible because all these theories can't be tested practically. Scientists are trying to make a time machine, but it all seems possible in science-fictional movies. You will find several people claiming to time travel, like Alexander Smith. But no one has concrete proof to support their claim.
Constant Speed
So what does this sentence really mean? Surprisingly, the answer has nothing to do with the actual speed of light, which is 300,000 kilometers per second (186,000 miles per second) through the "vacuum" of empty space.
Einstein's theory of general relativity mathematically predicts the existence of wormholes, but none have been discovered to date. A negative mass wormhole might be spotted by the way its gravity affects light that passes by.
Because space isn't curved they will never meet or drift away from each other. A flat universe could be infinite: imagine a 2D piece of paper that stretches out forever. But it could also be finite: imagine taking a piece of paper, making a cylinder and joining the ends to make a torus (doughnut) shape.
The laws of physics prohibit traveling backwards in time for many reasons. If we did travel backwards in time and changed the course of events, we would be altering the course of history.
We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day. For example, we use GPS satellites to help us figure out how to get to new places.
The fact that the universe is still around is ample evidence that time travel to the past is impossible.
In about 100 trillion years, the last light will go out. The bad news is that the universe is going to die a slow, aching, miserable death. The good news is that we won't be around to see it.
The trite answer is that both space and time were created at the big bang about 14 billion years ago, so there is nothing beyond the universe. However, much of the universe exists beyond the observable universe, which is maybe about 90 billion light years across.
The multiverse is the hypothetical set of all universes. Together, these universes are presumed to comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them.
Unfortunately, no one has ever observed a worm hole or even any physical evidence that they actually exist. Still, because the theory for their existence is so strong, astrophysicists assume they do exist.
No human has ever been inside of a black hole. Humans are not yet capable of interstellar travel. Even if a human was able to travel to a black hole, he or she would not be able to survive entering it. Black holes condense all the matter that falls into it into one point called a quantum singularity.
Don't let the name fool you: a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area - think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City.