Introducing 11 theories regarding time travelling in physics. From the neutron star theory to the Wormhole theory, you get an overview of all 11 theories about time travelling. Astrophysics #2!
Figure 1: Time travel
Introduction
Time travel in science fiction has always been a plot device both of great mystery and delight for its audiences. We’ve also seen conflicting theories on time travel and how time works in our universe. For example, we’ve seen the idea of a fixed timeline in movies like Harry Potter and the Prisoner of Azkaban and The Terminator, a dynamic timeline in movies like Back to the Future, and a multiverse timeline in movies like Star Trek (2009). With fixed timelines, all events remain at fixed points in time, and the actions of the traveller in the past have already become a part of history. In dynamic timelines, altered events in the past have definite impacts on the present, resulting in paradoxes like the grandfather paradox. Finally, in the multiverse interpretation of timelines, there’s an infinite number of parallel universes – travelling into the past causes a new divergent timeline from the first. While plenty of paradoxes arise, it may be particularly helpful to look into what physics has to say about how time travel works. While we’ve seen the concept of wormholes as a device to explain time travel in science-fiction, it isn’t the only way, theoretically.
Introduction to the Various Time - Travel Theories
According to theoretical physics, most physicists agree that there are at least 11 proposed time travel theories. In the anime Steins; Gate, the black hole theory is used most prominently to tell the exciting story of time travel. We will now explore these 11 theories below.
The Neutron Star Theory
A neutron star is the gravitationally collapsed core of a massive supergiant star, which had a total mass somewhere between 10 and 25 solar masses, possibly more if the star was especially metal-rich. To put that in perspective, these stars have a mass of around 466,000 times the mass of the earth, thus exerting a very high gravitational force (1.962×1012 m/s2, Earth's is 9.81 m/s2) although they have a diameter of 10-12 Km. According to Einstein’s special theory of relativity, time isn’t universal for every observer like a ticking universal clock that dictates all of the time. Rather, the larger the mass of an object, the stronger its gravitational field – the gravitational field really being the curvature of both space and time. The stronger the gravity, the more spacetime curves, and the slower time itself proceeds. The gravitational time dilation of a neutron star would pass 30% slower on the surface of such a star, meaning that if someone could travel to such a star and withstand this gravity, they could “travel” to the future. This theory only serves for time travel to the future, to the past wouldn’t be possible. Nevertheless, we actually experience this differing time dilation on earth – albeit to a significantly less degree.
The Black Hole Theory
Black holes, in addition to wormholes, are one of the most used resources in time-travelling novels, next to wormholes. The Kerr-black hole theory is the result of Roy Kerr’s calculations for relativity. A Kerr-black hole is a singularity that possesses mass and angular momentum but does not possess an electrical charge. This black hole spins around a central axis and has two event horizons, which contain a ring-formed singularity. Inside each of the two event horizons, time and space are reversed, so in a Kerr-black hole, this swapping occurs twice. It’s both possible to escape the ring-formed singularity as well as end up in “negative space” as you cross the singularity. Avoiding it would cause you to back in time while you are crossing the first event horizon.
The Light Speed Theory
Like the neutron star theory, the light speed theory, although unclear exactly what light speed theory refers to, makes several connections with other theories and some basic principles of Einstein’s theory of relativity. The first form of time travel that relates to light speed is when the matter is accelerated to close to light speed, causing time to slow down for it due to time dilation from Einstein’s relativity. This can only be used to travel to the future, not to the past. The neutron star theory demonstrated this example. While this first form of time travel is actually hard science and not science fiction, all it accomplishes is time travel to the future, not the past. The Light Speed theory, therefore, refers to a more broad definition of the other theories by which light speed travel is essential.
The Tachyon Theory
The Tachyon or a Tachyon particle is a hypothetical particle that always moves faster than light, Most physicists believe that faster-than-light particles cannot exist because they are not consistent with the known laws of physics. The existence of such particles, called tachyons, has not been totally ruled out, but several experiments have tried, without luck to detect them. If they did exist, and they interacted with ordinary matter, it would give us the means to communicate with the past. Tachyons could literally be sent outwards, bounce off a tachyonic mirror, and return before they were sent. Like most time travel theories, Tachyons also give rise to many logical problems – paradoxes.
The Wormhole Theory
A wormhole is a special solution to the equations describing Einstein's theory of general relativity that connects two distant points in space or time via a tunnel. Ideally, the length of this tunnel is shorter than the distance between those two points, making the wormhole a kind of shortcut. Though they are a staple of science fiction and have captured the popular imagination, wormholes are, as far as we know, only hypothetical. They are legitimate solutions to general relativity, but scientists have never figured out a way to maintain a stable wormhole in the real universe. To do so, you would need exotic matter – matter with negative mass – that can repel gravity, thus negating the supergravity of the wormhole that would otherwise collapse the wormhole as soon as it opened. The exotic matter, in itself, is also another proposed theory of time travel as we’ll see in the next theory.
The Exotic Matter Theory
Along with the way exotic matter can be used to stabilize a wormhole in order to make it traversable for time travel via wormholes, there are other methods of time travel using exotic matter. The most popularly discussed method is the Alcubierre warp drive, which requires exotic matter to create an area of space with negative mass, and achieves faster-than-light travel by warping the space-time continuum, contracting space in front of a spacecraft using the warp drive, and expanding space behind the spacecraft. Once the Alcubierre warp drive using exotic matter is achieved, faster-than-light travel can be used to go back in time, using the same mechanism as tachyons, as originally explained in Albert Einstein’s thought experiment of the tachyonic antitelephone that would use tachyons (subatomic particles that move faster than light speed) to send information back in time. The difference, however, is that you could actually build a spacecraft and it would produce an Alcubierre warp drive around it using exotic matter, in order to achieve physical time travel of ordinary matter back in time, including people inside a spacecraft and whatever else they want to bring along. So while tachyons would only be able to send data back in time, the Alcubierre warp drive could be used for physical time travel.
The Cosmic String Theory
The cosmic string theory for time travel was developed by the physicist J. Richard Gott is explained in the 2002 book How to Build a Time Machine by Paul Davies. As described in Gott’s book, a cosmic string is a string-shaped crack that has an extreme mass. You can think of the crack as something the width of an elementary particle, and at least the length of a galaxy. Because of their extreme mass, cosmic strings have the property of space-time distortion. Once you travel near the cosmic string, you experience a full rotation around the string in less than 360 degrees. In short, you can do something resembling a warp known as a space-time angular deficit. When you pass through an area of angular deficit, transit time becomes zero. Applying this, once the cosmic string moves approaching light speed, according to the theory of relativity, time will flow slower for the cosmic string in relation to its surroundings. Therefore, passing through the area of the angular deficit would cause the zero transit time to become negative. In other words, it will be the 'past' after transit. So, if you use two cosmic strings, you can do a space deficit jump. If you revolve back to your original location, you can return to the same time you started revolving. And that, roughly speaking, is time travel by means of cosmic string theory.
The Quantum Gravity Theory
In a rather short explanation compared to the previous theories, quantum gravity theory uses what’s known as Quantum jumps in the use of a “stale state” or a “dead state” where no movement is possible because of density.
The Cesium Laser Theory
At the NEC research institute in Princeton, Dr. Lijun Wang gave the world the first glimpse of multi-dimensional reality and the possibility of time travel in the future. Namely, Wang transmitted a pulse of light towards a chamber filled with specially treated cesium gas, and recorded its travel through the chamber at an accelerated speed of up to 300 times the speed of light. Before the pulse fully entered the chamber, Wang reported that it appeared at the same instant at a point 60 feet across the laboratory. In effect, it existed in two places at the same time. Thus Wang not only proved that objects can move at speeds exceeding the earlier prescribed limit of 186,000 miles per second, but he proved Einstein's theory that time slows when objects travel at a speed approaching (and exceeding) the speed of light. The implications of this are mind-boggling, hinting that time travel is quite possible.
The Elementary Particle Ring and Laser Theory
Elementary particle ring and laser theory is a time travel theory developed by Professor Ronald Mallett of the University of Connecticut in Storrs, Connecticut. It involves the use of a ring laser to generate a closed time-like curve or CTC for short. It is based on an older time travel theory called the Tipler cylinder except modified to be more practical, as Tipler cylinders would have to be infinitely long, so Ronald Mallett came up with a better way to generate CTCs using only a ring laser. The circulating light beams of a ring laser can theoretically twist spacetime and allow the dimension of time to be traversable just like the dimension of space is normally traversable. Then, one could theoretically walk through time in any direction, as one can do in space. It works by producing gamma rays or magnetic fields to warp time into CTCs. Theoretically, this could be used for physical time travel for human beings.
The Dirac Antiparticle Theory
Dirac antiparticle theory, more commonly known as Dirac hole theory, was the original theory to explain the existence of antimatter from 1929, postulated by Paul Dirac. It is derived from the Dirac equation formulated in 1928, a relativistic version of the Schrödinger equation that improved on the Schrödinger equation by making it compatible with Einstein’s relativity (which was rather important, since the purpose of the Schrödinger equation was to explain the behaviour of electrons, electrons typically move near the speed of light, and any equation describing things moving near the light speed that does not incorporate relativity produces erroneous results).
Dirac's antiparticle theory can also be used as an explanation for time travel. It is an extremely simple explanation. Antimatter is the same as matter, mathematically speaking, except if you mathematically negate time the values for time, and related quantities that use odd powers of time such as velocity, momentum, and angular momentum (quantities that use even powers of time like acceleration, force, work, and energy are unchanged since if you square a negative number you get a positive number). This explains things like how electrons have a spin of positive ½ but positrons (the antimatter equivalent to electrons) have a spin of negative ½, as spin is a measure of angular momentum which has an odd power of time, so if time is negated, spin is negated. Because of the way electric charge is derived from spin, electrons have a negative charge but positrons have a positive charge, which is what gives them the name “positrons”, short for “positively charged electrons”. antimatter travels backwards in time rather than forwards in time like matter, and actually, this is to be expected, since the laws of physics have many symmetries, and for time to only go in one direction rather than in both directions would break time symmetry, which is bad. This might seem to allow information to be sent to the past using antimatter
Conclusion
Although there are 11 widely accepted theories of time travel in the world, some are very impractical like the Dirac Antiparticle theory. It is, as far as modern science knows, impossible to send information back in time. In practice, this would mean, among matter particles, it is impossible for matter particles to send or receive information that comes from the future to the past, while among antimatter particles, it is impossible for antimatter particles to send or receive information that comes from the past to the future. Nevertheless, it’s still interesting to speculate on how scientists can achieve physical time travel. At least, for filmmakers, there are opportunities for even more time-warping and mind-knotting entertainment.
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