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“Hey Google: can we travel back in time?”
Following the events of ‘Avengers: Endgame’ where *spoilers* reverse time travel was involved, the question arises, is it possible to return to the past?
The Avengers use the fictional ‘Quantum Realm’ to access the past, but how would this feat be achieved in the real world?
THE REAL LIFE ‘QUANTUM REALM’
Interestingly, the science behind the actual theory of time travel is not dissimilar to that laid out in ‘Avengers: Endgame’. In the Marvel Cinematic Universe (MCU), an individual can travel through time by shrinking down to a sub-atomic level, entering the Quantum Realm at a specific point and exiting it at another point. The exit point deposits the person at a past point in the timeline, as spacetime operates differently in the Quantum Realm.
This logic is similar to reality, where the cosmic phenomenon of a wormhole comes into the play. In our Universe, we have tiny, minuscule quantum fluctuations in the fabric of spacetime on the smallest of scales. A quantum fluctuation is a temporary change in the amount of energy at a point in space.
These fluctuations can occur in either a positive or negative direction. A strong positive fluctuation would curve space in one fashion, while a strong, negative fluctuation would curve it in an opposite manner. If you could connect these two curvatures, it would briefly create a quantum wormhole.
This wormhole could allow the transportation of a particle from one point in spacetime to another. In the MCU, persons travelling through the quantum realm must shrink to this size in order to enter the Quantum Realm and travel through it.
HUMAN SIZED WORMHOLES
Now, if we remove the shrinking factor from the equation, how is it possible for a full-sized human to enter a wormhole? Every known particle in our universe has positive energy and either positive or zero mass. To create a big enough wormhole, we would need to create matter with negative mass/energy.
Fortunately, according to the law of General Relativity, it is quite possible for a particle to have negative energy or mass. For example, if a supermassive black hole were created, and a counterpart mass with corresponding negative energy/mass were created, the two points would be inseparably linked. Therefore, we would have a traversable wormhole through space.
SPACE AND TIME?
According to the laws of special relativity, your motion through space and your motion through time are related by the speed of light. For example, if you had a destination that was 40 light years away and you travelled to this destination at 99.9% the speed of light, you might arrive in approximately a year, and return in the same time.
However, upon your return you would find that roughly 82 years had passed on Earth. This is due to two phenomena that are experienced in light speed travel; time dilation and length contraction. The amount of time travelled is dependent on your motion through space.
Although, this technicality can be avoided if we employ the aforementioned wormhole. Imagine that the first point of the wormhole is stationary at a point near Earth. The other end is sent on a relativistic journey at the high speed referenced earlier.
After a year, if one entered the rapidly-moving point and exited from the stationary point, forty years would have passed for you, while only a year would have passed for Earth.
Now, if forty years ago, one established the same entangled wormhole, and entered it at the Earth point, they would find themselves exiting into the past, at around 1980. The caveat is that you would not be able to return to Earth at this time, it would take longer to return.
This method of time travel makes the Grandfather Paradox impossible, which is the hypothesised event whereby a time traveller would meet his grandfather before he had been born. There would be no conceivable way for one could exist at the one end of the wormhole early enough to travel to a point before their parents’ birth.
In the MCU, actions committed by time travelling individuals that alter the established chain of events creates new, branching universes. This is similar to the ‘daughter universe theory’. This theory posits that, due to the nature of quantum mechanics and laws of probability, that for every outcome in a range of decisions, an alternate universe should exist to satisfy each outcome.
In conclusion, time travel in the MCU is not unlike that posited in our universe. The Avengers travel through quantum realm wormholes to return to the past, which is similar to the entangled quantum wormholes outlined in our understanding of physics. These theoretical wormholes even obey the laws of physics, making time travel fantasies all the more enticing.