The idea of travelling through time, similar to how an object or person can move between different points in space, is referred to as “time travel”. This concept is well-known in philosophy and fiction, particularly in science fiction. H.G. Wells’ novel “The Time Machine” from 1895 played a significant role in popularising the concept of a device for time travel. The notion of travelling through time has been present in various myths and ancient cultures for centuries.
Some scientists are sceptical about the feasibility of time travel and raise the possibility of encountering numerous paradoxes. They argue that the idea of time travel is filled with logical contradictions, also known as temporal paradoxes or time paradoxes. Currently, the concept of travelling to the future is consistent with the principles of physics through the theory of relativistic time dilation. On the other hand, the idea of travelling to the past is associated with temporal paradoxes and is often used to support the argument that time travel to the past is impossible.
In the field of physics, temporal paradoxes can be classified into two categories: consistency paradoxes, such as the grandfather paradox, and causal loops. There are also variations of the Fermi paradox and paradoxes related to free will, like Newcomb’s paradox, that arise from the concept of time travel. Now, let’s examine the theoretical methods for time travel.
Time Travel through Extreme Speeds
Einstein’s theory of special relativity states that when one travels at speeds close to the speed of light, time appears to slow down relative to the outside world. This effect is not just a theoretical idea but has been scientifically confirmed. Experiments with twin atomic clocks, where one clock was taken on a jet flight and the other remained stationary on Earth, have demonstrated that the clock in motion ticks slower due to its velocity.
When it comes to aeroplanes, the impact on time is negligible. However, if you were on a spacecraft travelling at 90% the speed of light, you would experience time passing 2.6 times slower than it would on Earth. The closer you get to the speed of light, the more pronounced the time dilation effect becomes. Theoretically, time travel to the past is possible in specific spacetime geometries defined by general relativity, such as cosmic strings, traversable wormholes and Alcubierre drives, which allow for travel faster than the speed of light.
General relativity theory provides a scientific foundation for the possibility of reverse time travel in specific atypical circumstances. However, semiclassical gravity theories suggest that when quantum mechanics is combined with general relativity, these potential loopholes may be eliminated. These semiclassical arguments prompted Stephen Hawking to propose the chronology protection conjecture, which states that the fundamental laws of nature prevent time travel. However, physicists are unable to reach a definite conclusion on the matter until a theory of quantum gravity, which would combine quantum mechanics and general relativity into a single unified theory, is developed.
Time Travel through Extreme Gravity
A highly anticipated method for time travelling into the future involves slowing down or stopping one’s bodily processes and then resuming them later, effectively altering the perception of time. General relativity considers the effects of acceleration and gravity to be equivalent, and it demonstrates that time dilation also occurs in gravitational fields, where clocks deeper in the field tick more slowly. This effect is taken into account when adjusting the clocks on satellites in the Global Positioning System, and it could result in substantial differences in the ageing rate of observers at varying distances from a large gravitational field, such as a black hole and neutron star. Any object that has sufficient matter density can produce gravitational waves that could bend and twist the fabric of spacetime.
For example, a time machine that operates on this principle could be a spherical shell with a diameter of 4 meters and the mass of Jupiter. An individual at the centre of the shell would experience time passing four times slower than individuals observing from a distance. It is not believed to be possible for humanity to achieve the technological capability to compress the mass of a large planet into such a small structure in the near future. With current technology, it is only possible to cause a human traveller to age a few milliseconds less than those on Earth after several hundred days of space travel.
Time Travel by Stimulated Light System
In physics, a device for time travel is referred to as a closed timelike curve (CTC). Essentially, this involves an object travelling through spacetime in a loop, enabling it to interact with its past self. A team of researchers recently published a study in the journal Nature that simulated the potential effects of a time machine using polarized light. Since it wasn’t possible to send a beam of light back in time, the team used two separate beams, with one reflecting the previous state of the other. Their objective was to examine the impact a CTC could have on quantum computers.
An alternate concept for time travel using stimulated light was proposed by American physicist Ron Mallet. He suggests using a rotating cylinder of light to manipulate spacetime. Objects placed inside the cylinder could, in theory, be pulled through space and time, similar to how a bubble moves around in a cup of tea when stirred with a spoon. Mallet believes that the correct geometry could allow for time travel into both the past and the future. Since presenting his theory in 2000, he has been seeking funding for a proof-of-concept experiment, which involves dropping neutrons through a series of spinning lasers arranged in a circular pattern.
Time Travel through Wormhole
According to the theory of General Relativity, it’s possible to create shortcuts through spacetime known as wormholes. These could potentially provide a connection between distant locations that are separated by billions of light years or different points in time. A wormhole is essentially a tunnel between two points in the universe that significantly reduces the time it takes to travel from one location to the other. Instead of taking millions of years to travel from one galaxy to another, it may be possible to use a wormhole to reduce the travel time to mere hours or minutes under the right conditions.
The concept of wormholes was first introduced in 1916, though it wasn’t referred to by that name at the time. Scientist Ludwig Flamm discovered this idea while reviewing the solutions to the relations in the General Relativity Theory of Albert Einstein provided by another physicist. He came to the conclusion that another solution was possible and described it as a “white hole,” a theoretical opposite of a black hole in terms of time. The entrance to both black and white holes could be connected by a spacetime pathway.