The idea of time travel has fascinated humans for centuries. We dream of going back in time to witness monumental events or interact with historical figures. But according to our current understanding of physics, going back in time appears impossible. There are several reasons why backwards time travel will likely remain fiction.
The laws of thermodynamics
The laws of thermodynamics suggest time can only move in one direction – forward. The second law of thermodynamics states that entropy, a measure of disorder, always increases over time. If entropy were to decrease, it would imply time reversing itself.
For example, an egg breaking on the floor increases entropy as the egg goes from an ordered state to a disordered state. The egg remnants will not leap back onto the counter and reassemble into an egg. That process would require entropy to decrease, which does not occur.
The arrow of time
The “arrow of time” refers to the fact that time moves in a unidirectional manner. It only points from past to future. This aligns with our everyday experience – we remember the past but cannot “remember” the future.
There are several explanations for the arrow of time:
Thermodynamic arrow of time
As mentioned earlier, the second law of thermodynamics requires entropy to increase over time. This gives time an inevitable direction, sometimes called the thermodynamic arrow of time.
Cosmological arrow of time
Evidence shows the universe is expanding from an initial state of high density. Cosmological models thus have an inbuilt direction of time, pointing from the hot dense past towards the cooler expanded future.
Quantum arrow of time
Certain subatomic interactions operate asymmetrically in time. For example, a neutron can decay into a proton and electron, but the reverse does not occur. This directionality gives rises to an arrow of time at the quantum scale.
Causality
In daily life, causes precede effects – we flip a switch which turns on a light. Effects do not happen before causes. ThisRelationship between cause and effect contributes to our perception of the arrow of time.
Time travel paradoxes
Many time travel theories give rise to logical paradoxes, suggesting backwards time travel is implausible.
Grandfather paradox
Perhaps the best known is the grandfather paradox. Imagine you travel back in time and accidentally prevent your grandparents from meeting. This would mean your parents were never born, which means you were never born. But if you were never born, how could you travel back in time in the first place? This is an inconsistent loop which brings causality into question.
Bootstrap paradox
Also called an information paradox, this occurs when information or an object is sent back in time, and ends up creating itself.
For example, imagine a time traveler gives their past self a revolutionary mathematical equation. The past self then grows up to discover the equation and eventually sends it back to their past self – the equation exists in a causality loop with no clear origin point.
Ontological paradox
This arises when a time traveler performs an action that becomes the cause of the traveler’s existence in the first place.
For example, a time traveler builds a time machine and gives it to their past self. The past self uses the machine to eventually become the time traveler. The time machine has no origin point.
Lack of empirical evidence
Thus far, there is no empirical evidence that time travel to the past has ever occurred. If even one instance of time travel to the past occurred, the effects and evidence of it should ripple through the present. The lack of any undisputed evidence is another reason backwards time travel appears implausible.
Time travel to the future
While backwards time travel faces huge challenges, time travel to the future is allowed by physics. That’s because it only requires traveling at relativistic speeds or taking advantage of time dilation near a strong gravitational field.
Time dilation from velocity
Einstein’s theory of special relativity showed the passage of time slows down for objects moving near the speed of light. An astronaut traveling at 99% the speed of light for 10 years would return to find much more than 10 years had passed on Earth. In effect, they traveled to the future.
Gravitational time dilation
Gravity also slows the passage of time. Clocks tick slower in strong gravitational fields like near black holes. An astronaut orbiting close to a black hole would also return to an earth much older than they are. Einstein showed gravity and velocity slow time itself, allowing travel to the future.
Requirements for backwards time travel
While travelling to the past appears implausible, physicists have speculated about requirements for backwards time travel to be possible:
Traversable wormholes
Wormholes are shortcuts through space-time allowed by Einstein’s general relativity. But they likely exist at quantum scales and getting one large and stable enough for human travel poses immense challenges.
Alcubierre drive
This speculative propulsion concept contracts space-time in front of a ship while expanding it behind. Effectively this moves the ship in a warp bubble. But immense amounts of exotic matter are likely needed.
Cosmic strings
These are hypothetical 1-dimensional topological defects in space-time. Cosmic string loops might allow for closed timelike curves permitting backwards time travel. But the loops may collapse too rapidly.
Time machines
Rotating black holes and infinitely long rotating cylinders have been theorized as potential time machines. But engineering megastructures on this scale is an immense challenge, if possible at all.
Overall, the likelihood of manipulating space-time to travel to the past looks overwhelmingly difficult, if not impossible.
Can quantum mechanics allow time travel?
Some have proposed time travel may be possible in the quantum realm. Two examples are:
Quantum teleportation
This involves transmitting quantum information such as the exact state of a particle. However, this does not allow faster-than-light communication or travel. Measurements still cannot occur faster than the speed of light.
Quantum retrocausality
Some interpretations of quantum mechanics allow influences from the future to affect the past at a quantum particle level. But this does not violate causality or allow macroscopic objects to travel through time.
So unfortunately, quantum mechanics appears to offer no shortcuts for backwards time travel.
Simulation hypothesis
Some propose our universe may be a sophisticated computer simulation. If so, perhaps the programmers could allow time travel capabilities. However, there is no direct evidence our reality is a simulation. It’s a philosophical proposition that cannot be tested. So the simulation hypothesis does not strongly imply actualized time travel abilities.
Conclusion
With our current understanding of physics, going back in time appears extremely implausible. The thermodynamic and cosmic arrows of time, quantum dynamics, causality, and lack of empirical evidence all suggest time has an inevitable forward direction. While we can travel to the future under relativity, the past remains out of reach. Time machines or teleportation back in time will likely remain fiction. The alluring idea of revisiting history or undoing regrets continues to fascinate humanity. But actually making it a reality faces immense challenges across physics.