Time Travel: Physics Friend Or Foe?

Time travel is a concept that has fascinated humans for a long time, with many theories and ideas being explored in science fiction. But does time travel break the laws of physics?

The laws of physics, as we understand them today, do not support the idea of travelling back in time. The concept of backwards time travel breaks the causal structure of spacetime, allowing the effect to precede its cause. This goes against our empirical understanding of nature and would make it impossible to logically reason about science. The second law of thermodynamics, which states that entropy can only increase or stay the same, is also often cited as a law that would be broken by time travel.

However, it is important to note that our understanding of physics is always evolving, and there may be ways to reconcile time travel with the laws of physics that we have not yet discovered. Some theories propose the existence of multiple timelines or universes, where time travel could occur without breaking the laws of physics as we know them.

While time travel to the past may break the laws of physics as we understand them today, time travel to the future is theoretically possible through the principle of gravitational time dilation. By travelling close to the speed of light or by being in a strong gravitational field, time would pass more slowly for the traveller relative to an observer at a slower speed or weaker gravitational strength. This would allow the traveller to effectively move forward in time without breaking any known laws of physics.

Time travel to the future is theoretically possible using the principle of gravitational time dilation

Time dilation is a property of Einstein's special relativity. Einstein was the first to realise that time is not constant, but instead slows down as you move faster through space. As part of his theory, Einstein re-envisioned space itself. He coined the phrase "spacetime", fusing the three dimensions of space and one dimension of time into a single term. Instead of treating space as a flat and rigid place that holds all the objects in the universe, Einstein thought of it as curved and malleable, able to form gravitational dips around masses that pull other objects in, just as a bowling ball placed in the centre of a trampoline would cause any smaller object placed on the trampoline to slide towards the centre.

The closer an object gets to the centre of the dip, the faster it accelerates. The centre of the Earth's gravitational dip is located at the Earth's core, where gravitational acceleration is strongest. According to Einstein's theory, because time moves more slowly as you move faster through space, the closer an object is to the centre of the Earth, the slower time moves for that object.

This effect can be seen in GPS satellites, which orbit 20,200 kilometres above the Earth's surface. These satellites have highly precise clocks onboard that gain an average of 38 microseconds per day due to time dilation. While this time gain seems insignificant, GPS satellites rely on their onboard clocks to maintain precise global positioning. Running 38 microseconds fast would result in a positioning error of nearly 10 kilometres, an error that would increase daily if the time difference were not constantly corrected.

A more dramatic example of time dilation can be seen in the movie Interstellar when Matthew McConaughey and his crew land on a planet with an extreme gravitational field caused by a nearby black hole. Because of the black hole's intense gravitational influence, time slows dramatically for the crew on the planet, making one hour on the surface equal to seven years on Earth. This is why, when the crew returns to Earth, Matthew McConaughey's daughter is an old woman while he appears to be the same age as when he left.

So why hasn't humanity succeeded in making such drastic leaps forward in time? The answer to this question comes down to velocity. In order for humanity to send a traveller years into the future, we would either have to take advantage of the intense gravitational acceleration caused by black holes or send the traveller rocketing into space at close to the speed of light (about 1 billion km/h). With our current technology, jumping a few microseconds into the future is all humans can manage.

But if technology one day allows us to send a human into the future by travelling close to the speed of light, would there be any way for the traveller to use time dilation to return to the past and report her findings? "Interstellar travel reaching close to the speed of light might be possible, says Dr Jaymie Matthews, professor of astrophysics at the University of British Columbia, "but this voyage is one way into the future, not back to the past."

Travelling back in time would break the causal structure of our spacetime

The causal structure of spacetime refers to the causal relationships between points in a manifold, which describe which events in spacetime can influence other events. In modern physics, spacetime is represented by a Lorentzian manifold.

In Minkowski spacetime, the causal structure takes a simple form because the tangent space is also R^4, and the tangent vectors can be identified with points in the space. The classification of any vector in the space will be the same in all frames of reference related by a Lorentz transformation.

In special relativity, by causality, we mean that an outcome cannot occur from a cause that does not lie in the back (past) light cone of that event. Likewise, a cause cannot affect outside its front (future) light cone. These limitations are expected as causal impacts cannot propagate quicker than the speed of light and/or backward in time.

Time travel to the past would break the causal structure of our spacetime by allowing an effect to precede its cause. For example, if someone went back in time and killed their grandfather, they would prevent their own birth. This breaks causality, as the cause (the birth of the time traveller) would come after the effect (the killing of their grandfather).

While general relativity does allow solutions with time travel (closed timelike curves), these are just possible solutions to the way spacetime behaves mathematically. There is no way of knowing whether a real configuration of matter could do that to spacetime unless we observe it or create it.

Time travel would break the second law of thermodynamics

Time travel, specifically travelling back in time, is theorised to break the second law of thermodynamics. The second law of thermodynamics pertains to entropy – energy is constantly being consumed, and any effort to revert something to its original state results in a net loss of energy.

If an object were to travel back in time, it would need to take its entropy along with it, resulting in a decrease in entropy at the time of departure. This would break the second law, which states that the total entropy of the universe must increase with any process.

One solution to this paradox is that the entropy of the universe must increase somewhere else to compensate for the decrease in entropy caused by the time travel. This could be achieved through the creation of microscopic black holes, which would rapidly evaporate.

Another theory suggests that time travel in a single universe violates the conservation of energy and allows the creation of infinite-energy states. However, time travel in a branching multiverse does not violate the laws of thermodynamics, as these laws refer to closed systems, and time travel between different timelines would mean that the closed system is the entire multiverse, rather than any individual timeline.

Time travel in a single universe violates conservation of energy

Time travel is a concept that has fascinated humans for a long time, with many theories and ideas being explored in science fiction. However, the idea of travelling back in time, in particular, seems to break several laws of physics. One of the key laws that time travel in a single universe would violate is the conservation of energy.

The conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another. When considering time travel, it is important to note that time is often viewed as the fourth dimension, alongside the three spatial dimensions. Therefore, travelling through time can be thought of as moving through this four-dimensional spacetime.

If an object were to travel back in time within the same universe, it would essentially be adding its mass-energy to the past, resulting in an increase in the total energy of the universe at that time. This would violate the conservation of energy, as energy cannot simply appear out of nowhere.

One way to potentially resolve this issue is to consider the concept of multiple timelines or a branching multiverse. In this scenario, time travel would not violate the conservation of energy as the total energy within each individual timeline or universe would remain constant. However, travelling between these timelines or universes would still require a mechanism that complies with the known laws of physics.

Another possible explanation is that time travel could be achieved by exchanging matter-energy from one time to another. In this case, the law of conservation of energy would still hold as the total amount of energy in the universe across time would remain unchanged.

While these theories provide interesting possibilities, it is important to note that backward time travel remains a highly speculative concept. Our current understanding of physics suggests that travelling back in time may not be possible without violating certain fundamental laws, such as causality and the conservation of energy.

Time travel breaks causality

Time travel to the past would also violate the second law of thermodynamics. The second law states that entropy can only increase or stay the same. Going back in time would require you to obtain lost information and reverse enthalpy.

Time travel in a single universe violates conservation of energy and allows the creation of infinite-energy states. However, time travel in a branching multiverse does not violate thermodynamics as thermodynamics talks about closed systems, and if you can leave a timeline and enter a new one, the closed system is the entire multiverse, not any individual timeline.

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