When you look up at the sky in a clear night, you see a bunch of stars shining in the sky. All the starlight are unknown worlds waiting for us to study. After reading The Illustrated Brief of The Time by Steven Hawking, I saw the mysteries of the space and time. Albert Einstein’s formula E=MS shut the door of time travel down. It means that we need more energy to reach higher speed. Time travel requires light speed to activate. Which mean we are going to spend all our fuels before the speed reach light speed. Steven Hawking open another door for us about time travel: the black hole. He thought that the reason why the back hole is black in our vision is because it twisted the light around it. He thought that we can achieve time travel by go through the black hole. I think time travel is possible in the future, but not the time travel we saw on the movies.
If we want to travel through time, we need to know “what is time?”. Most people think time as a constant like every day, every week. Physicist Albert Einstein think it is relative, it depends on the observers. Einstein think of time as the fourth dimension. It provides the concept of time travel. We can travel in other three dimensions, why not in the fourth dimensions?
We travel in time every day. We can say it in another way: we are traveling in time by the speed of 1 hour per hour (NASA). That’s how the time work in this universe. Space and time are two dimensions that human is related most. People can move any direction as they wanted. This kind of freedom will not exist on time. We can move freely in the direction of space, but we cannot control time at will. Time is like a long river, everything seems to be floating in the river, can only drift. If time is a river, can there be ships in it? Floats can only drift, but ships can cut through the waves. If there were ships in the river of time, we could travel through time in them, looking into the future, returning to the past, and perhaps changing history. In science fictions, this imaginary ship is called a “time machine”.
We know that in Newton's absolute view of time and space, time and space are unaffected by any matter or motion (this is the main meaning of “absolute”). Obviously, in this view of time and space, time travel does not have a theoretical basis, its existence is just an illusion. But the introduction of special relativity brought about a major change in the view of time and space. In special relativity, time and space are no longer absolute concepts but closely related to the choice of frame of reference. His theory of special relativity says the speed of time is depending on how fast you go. In particular, the passage of time slows down in motion frames of reference, a well-known time-delay effect whose existence has been confirmed by numerous physical experiments. This new result of special relativity opens up the first theoretical possibility of time travel: the possibility of future-oriented time travel (Elizabeth, 2017). We may not be able to travel backward. It is possible to travel to the future.
According to Einstein's theory of special relativity, if someone wants to travel to the future. All he needed was a spaceship that could travel at speeds close to the speed of light. The farther into the future you want to go, the higher the speed you need to reach. This is the theory bases of future-oriented time travel. Human technology may achieve it in the future. Special relativity opened the door to future-oriented time travel but failed to provide the same theoretical feasibility for time travel back in time. If the mathematical framework of special relativity had to be interpreted in a general way, only faster-than-light motion could cause a reversal of time in a particular frame of reference. But special relativity itself sets up a light-speed barrier. So, within the framework of special relativity, time travelers can travel to the future, but not to the past, which is obviously a long way from the free movement we have in space. Moreover, future-oriented time travel does not necessarily require a time machine to do so, it can be accomplished by freezing travelers for years and then thawing them. The real unique value of the time machine is in going back in time. If we talk about travel in time in different speed, it is totally possible. But only to the future (Elizabeth, 2017).
There are theories about travel backwards in time. Ten years after the special theory of relativity, Einstein came up with the general theory of relativity. In general relativity, time and space depend not only on the choice of frame of reference, as in special relativity, but also on the distribution and motion of matter. An important consequence of this is that, unlike special relativity, our definition of “the future” is no longer absolute and is affected by the motion of matter. The future may point in different directions at different times and places. This is a curious result, suggesting that space-time, like a fluid in some sense, can be tugged by the motion of matter, and that even the direction of time can be shifted by the pulling (Elizabeth, 2018). The general theory of relativity opens up the door of travel backward in time in very strict, very few situations. But it always ends up in require breaking other known basic physics.
In 1949, the famous logician Kurt Godel discovered a very strange solution in general relativity to describe a whole rotating universe now known as Godel universe. In such a universe, the rotation of matter drags in the direction of time, and the further away from the center of the rotation, the greater the drag. At a sufficient distance, the drag action is sufficient to form a closed time-like curve. Thus, time travel could in principle be achieved in the Gödel universe by moving the spacecraft in some orbit far from the center of the rotation. (Gödel, 1949)
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Unfortunately, the Godel universe is not compatible with astronomical observations. First of all, we don't live in a universe where there is a complete rotation. Second, in Godel's universe the cosmological constant is negative, whereas the cosmological constant we observe is positive. The universe we live in is clearly not a Godel universe. Quantitative calculations also show that not only that, even if we live in a really Godel's universe, it is hard to achieve time travel, because the Gödel closed time-like curve in the universe run the time needed for a week and mass density of the universe, for the matter we observed density, run along the closed curve of class a week at least need to tens of billions of years. So the Godel universe has no practical significance for time travel. (Malament, 1984)
The discovery of the Gödel universe suggests that general relativity does allow for closed time-like curves, though it has no practical significance.
In 1974, Frank J. Tipler, a physicist at Tulane University, studied the space-time outside an infinitely long rotating cylinder and found that the drag of such a cylinder on the outer space-time was sufficient to form a closed time-like curve, as long as the rotation was fast enough. In 1991, astrophysicist John Richard Gott III of Princeton university found that two infinitely long parallel universe strings that pass each other at close to the speed of light also form a closed time-like curve around them. Unlike the rotating cylinders introduced by tipper, the existence of cosmic strings is something that many leading physics theories predict, though there is no clear experimental evidence for their existence. Gott's result is a step forward in the theoretical possibility of a time machine (Gott, 2001).
But both Tipler and Gott introduced an infinitely long distribution of matter for mathematical convenience, which is obviously impossible to strictly implement in the real world. If the distribution of matter is not infinite, can you get similar results? Physicists are studied, but the situation is not optimistic: in 1992, the famous physicist Stephen Hawking gives a depressing results, that is if the energy density of non-negative everywhere, so trying to build a time machine in any limited time and space area efforts to be successful, will have to produce the most don't want to see things physicists - spacetime singularity. Spacetime singularity is another unsolved problem in general theory of relativity (Gott, 2001).
In 1992, Hawking even proposed the famous “Chronology Protection Conjecture”, arguing that the laws of nature would not permit the construction of time machines (Hawking, 1992). So far, this is only a hypothesis, and hawking's argument is not perfect. Physicists who are optimistic about the theoretical feasibility of the time machine have come up with several models to break hawking's ban on the time machine. Discussions in this regard are continuing.
Even through Hawking put a STOP sign on time travel for now. There is many theories about time travel are popular. The Wormholes was first proposed by Austrian physicist Ludwig Flamm in 1916 and hypothesized by Einstein and Nathan Rosen in 1930 when they were studying the equations of the gravitational field, believing that they could be used for instantaneous space transfer or time travel. It also called Einstein — Rosen bridge (Elizabeth, 2017). Wormholes are tiny tubes of space-time that connect distant regions of the universe. Dark matter keeps the wormhole exit open. Wormholes can connect parallel universes to infant universes and offer the possibility of time travel. A wormhole is also a space-time tunnel that connects a black hole to a white hole.
The black hole theory is another popular theory in time travel topic. “Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be traveling through time”, physicist Stephen Hawking wrote in the Daily Mail in 2010 (Elizabeth, 2017).
Time travel is been talking about for many decades since Einstein open that door to us. And it will be the hot topic forever until people made it. Because people will never give up on pursuit the mysteries universe. The space, time, spacetime. We need to study it more to figure it out. Scientist are having more theories to support their arguments. Time travel is possible in the future. Even though we will not travel it in reality. We will travel through time in some ways like the light speed spaceship. But, in relativity, we do travel in time every day. It only depends how you see it.
Works Cited:
- Elizabeth, Howell. Time Travel: Theories, Paradoxes & Possibilities. SPACE.com. https://www.space.com/21675-time-travel.html
- Elizabeth, Howell. Is Time Travel Possible? Scientists Explore the Past and Future. SPACE.com. https://www.space.com/40716-time-travel-science-fiction-reality.html
- NASA. Is time travel possible? https://spaceplace.nasa.gov/review/dr-marc-space/time-travel.html
- Malament. David B. “‘Time Travel’ in the Gödel Universe.” PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association, vol. 1984, 1984, pp. 91–100. JSTOR, https://www.jstor.org/stable/192497
- J. R. Gott III, Time Travel in Einstein's Universe (Houghton Mifflin Company, 2001).
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