Through the Fabric of Spacetime: The Physics behind Time Travel
Swarnika Dabral, XQ
Introduction
Time travel is a concept loved by movie directors, writers and physicists alike, and is vividly portrayed in several books, cartoons, and movies. But is this fascinating concept just in theory, or is it really possible from the point of view of physics? Several scientists have succeeded in proposing hypotheses and theories which support the possibility of time travel, including time dilation, Lorentz transformations, blackholes, wormholes, and many more.
Time is described as the fourth dimension, and in our daily experience, it always moves forward. The arrow of time is a physics concept which refers to the one-way direction of time from past to future. This unidirectional flow is closely tied to how disorder in the universe tends to increase over time (entropy), which is why we experience time as irreversible.
Essentially, all of us are time travellers, travelling at every moment into the future. We all travel through time at the same rate: one hour per hour, towards the future. The real question is whether this rate can differ between two observers, and whether we can travel into the past. While time travel into the future has been experimentally observed through time dilation effects, backwards time travel still remains theoretical and is not supported by experimental evidence so far.
Time Dilation and Relativity
All ideas enabling time travel are based on the idea that time is not separate from space but is connected to it through spacetime. Spacetime can be thought of as a flexible fabric that can stretch, bend, and curve depending on the mass and energy of matter. This idea comes from Einstein’s theory of general relativity, where gravity is described as the curvature of spacetime itself. This is why, near objects with intense gravity, time is experienced differently.
Mathematical equations describing how measurements of space and time change between observers moving at different velocities are called Lorentz transformations. In these equations, measurements of space and time depend on the observer’s motion. Hence, space and time are interdependent instead of being absolute.
The ideas of time dilation and the relativity of time come from the Lorentz transformations. The relativity of time means that the passage of time from a person’s reference depends on their velocity and proximity to a gravitational field. Hence it is possible for time to run differently for two different people.
Einstein’s theory of special relativity explains that it is possible for the passage of time to be different for different objects. This is known as time dilation, a phenomenon where time appears to pass slower for an observer moving at a velocity compared to an observer at rest. One can say that this happens because the speed of light must be constant for all observers, hence space and time adjust in such a way that different observers measure different time intervals.
There are two types of times in relativity. The time measured by a clock moving along with the object is called proper time, while the time measured by an external observer is called coordinate time. These times may be different due to time dilation.
Fig 1: Time Dilation for Satellites: Is Interstellar's Time Dilation Really Possible?
Time dilation has been experimentally confirmed. GPS satellites orbiting our planet experience lesser gravity and have high orbital velocities. Hence, they experience time approximately 38 microseconds per day faster than clocks on the Earth. This accounts for both time dilation due to special relativity (difference in velocity) and general relativity (different in gravitational field). Without correcting for this error, GPS systems could accumulate errors of about 10 kilometres per day.
Particle accelerators also demonstrate time dilation. When particles are accelerated to very high speeds, their decay processes slow down relative to stationary observers, allowing scientists to study them for longer periods.
The effect of time dilation becomes significant at velocities approaching the speed of light. If you were in a spacecraft travelling at 90% the speed of light, time would pass approximately 2.3 times slower for you compared to someone at rest on Earth. In fact, at 99% the speed of light, every year spent on the spacecraft would correspond to about seven years on Earth. This can essentially be seen as travelling into the future, because you can skip to the future from the Earth’s reference by moving extremely fast.
Closed Timelike Curves (CTCs) and Cosmic Strings
There are several theories built around time travel into the past. One of them is that of closed timelike curves (CTCs). A CTC is a theoretical path in spacetime that allows an object to return to its own past, while practically still moving forward in its own time. So, in a CTC, you always move forward in your own time, but the spacetime fabric loops back, making you end up in an earlier event. In theory, CTCs could enable time travel into the past.
Some mathematical solutions from the theory of general relativity could allow the possibility of closed timelike curves. For example, the Gödel universe, which is a suggested model of a rotating cosmos that naturally contains CTCs. Another solution is the Tipler cylinder, an infinitely long, rapidly rotating cylinder which could also produce CTCs. Though, these solutions are considered hypothetical because they require ideal conditions such as infinite length or a rotating universe unlike our own.
Fig 2: Closed Timelike Curve; Source: Time Travel Simulation Resolves “Grandfather Paradox” | Scientific American
Empty space contains tiny quantum fluctuations. When quantum mechanics is applied to regions containing closed timelike curves, these fluctuations could become amplified, similar to reverberation of an echo in a large hall. Such effects could possibly destroy the time loop before it can be used as a time machine. Thus, quantum effects may act as a natural mechanism to prevent the formation of macroscopic time machines.
Cosmic strings are another topic, defined as one-dimensional defects in spacetime that may have formed in the early universe. If they exist, they would be incredibly dense and could produce strong gravitational effects which could cause bending in spacetime around them. In highly specific situations, if pairs of cosmic strings overlap each other, they can even distort spacetime enough to allow closed timelike curves. However, like CTCs, cosmic strings have never been observed.
Paradoxes and their solutions
The possibility of closed timelike curves violates the normal cause and effect relationships, or causality. A famous issue is the grandfather paradox; if someone travelled to the past and prevented their grandparents from meeting, they would never be born, which makes their time travel impossible in the first place. A similar problem is the bootstrap paradox, where an object exists in a closed loop without any origin. For example, a time traveller goes to the past and gives a copy of the book, “How to build a time machine” to their younger self. This means that the book exists, but it was never truly created by anyone.
Because backward time travel causes issues with causality, many physicists believe that it may be impossible. A hypothesis opposing backward time travel is the Chronology Protection Conjecture proposed by Stephen Hawking. He said that the laws of physics may prevent time travel on macroscopic scales, as certain quantum effects near CTCs may become so strong that they might destroy the conditions needed for time machines. Hence, if we are able to develop a complete theory of quantum gravity, it may disallow violations of causality and time travel.
A proposed solution to these paradoxes is the many-worlds hypothesis, given by Hugh Everett, which states that changing the past creates a new branch of reality instead of altering the original timeline. Every time someone time travels, they create a new completely different timeline from their point of reference. However, this interpretation is part of quantum mechanics and hasn’t yet been confirmed by experimentation on the macroscopic scale.
Another interpretation is the Novikov self-consistency principle, proposed by scientist Igor Novikov. It states that any events occurring in a time loop must be self consistent. This means that the past cannot be changed in a way that creates contradictions. Also, the actions occurring in the past, whether they happen originally or as a consequence of time travel, are decided beforehand and hence cannot be changed. This principle solves paradoxes but also raises philosophical questions on whether we truly have free will, or if all actions and their consequences have already been written by the laws of physics.
Spacetime Curvature and Gravitational distortion: Wormholes and Blackholes
Wormholes, also known as Einstein-Rosen Bridges are hypothetical structures in spacetime shaped like a tunnel. They can connect two different points in space and time by bending spacetime using extreme gravitational effects, providing shortcuts. Imagine folding a sheet of paper so two distant points touch and then poking a hole through them to connect them directly. Wormholes work in a similar manner, possibly allowing time travel.
Keeping wormholes stable is very important for matter to be able to pass through it. Though they are mathematically possible, keeping them stable would need exotic matter and huge amounts of energy. Exotic matter is matter with negative energy density, and it is something not yet confirmed to exist in nature. If a block of exotic matter were kept on a sheet of paper, instead of the sheet stretching downwards, it would be pulled upwards. In theory, stable wormholes could allow both forward and backward time travel.
Fig 3: wormhole as a swirling vortex of blue and white; Source: The Physics of Time Travel: Examining the Possibilities and Paradoxes | by Ram | Medium
Currently, no known physical process can produce stable traversable wormholes. Though, natural formation of wormholes is possible in extremely rare conditions such as the collision of two galaxies or collapsing of neutron stars.
The gravitational effects due to black holes can also be used for time travel. The intense gravity of black holes warps spacetime, causing gravitational lensing and time distortion. Gravitational lensing means that black holes can bend light and even time passing near it. This happens because gravity is the curvature of spacetime. Thus, time flows more slowly near massive objects like black holes causing time dilation. However, time doesn’t stop or reverse near blackholes. An object cannot move into its own past through blackholes, it can only change the rate at which time passes for it relative to distant observers. Near a black hole’s event horizon, time can slow significantly, allowing effective future time travel.
Travelling Faster than light
Because of the time dilation caused when objects travel fast, several ideas connect faster than light travel to time travel. Concepts like tachyons and the Alcubierre drive allow for such fast motion. Tachyons are hypothetical particles that would always travel faster than the speed of light and, in theory, cannot be slowed below it. However, they haven’t yet been discovered. The Alcubierre drive model suggests that a spacecraft does not need to move faster than light locally. It could instead remain inside a “warp bubble.” In this idea, spacetime contracts in front of the bubble and expands behind it, allowing faster than light motion of the bubble. However, this requires exotic matter and high energy conditions, making it purely theoretical.
Fig 4: Alcubierre drive; Source: Reddit - https://i.redd.it/fdaf28tzp5y71.jpg
In principle, faster than light travel could lead to apparent backward time travel. Travelling faster than light can reverse the order of events in some inertial reference frames and may cause violations of causality. Physicists believe that nothing can travel faster than light. If something could exceed this limit, it could lead to situations where the effect appears to occur before its cause. Light cones are diagrams in special relativity that show all the possible paths light and objects can take through spacetime from a given event. They define the limits of causality in spacetime. Faster than light travel violates these limits.
Despite many theories, all experimental evidence shows that objects with mass cannot reach or exceed the speed of light. Experiments in particle accelerators show that as objects approach the speed of light, energy needed to increase the speed further becomes very high, making it impossible to accelerate them beyond this limit.
Conclusion
Time travel is still a fascinating topic in modern physics. Concepts like time dilation near black holes have been confirmed by experiments, while ideas like wormholes and closed timelike curves are still only theoretical. These theories suggest that time travel might be possible, but they require extreme conditions and physics that has not yet been proven. So far, there is no experimental evidence for time machines or traversable wormholes. Scientists continue to study space and time, and future research, especially in quantum gravity, may show whether such ideas are possible or whether the laws of physics always prevent time travel. Although practical time travel is still far beyond our reach, future discoveries may unlock new possibilities and deepen our understanding of the universe in ways we cannot yet imagine.