The Likelihood Of Time Travel According To Our Current Understanding Of Physics

In 1905, physicist Albert Einstein revolutionised our understanding of the universe with his theory of relativity. He proposed that space and time are intimately connected, forming a unified framework, thanks to observations that light always travels at a constant speed, regardless of the observer's own speed and motion. This implies that space and time can bend and stretch in ways we find non-intuitive, like a clock ticking slower when it's moving relative to you.

Despite Einstein's theory, there's a distinct difference between the spatial dimensions that we can move through freely, and time, which appears to have a strict forward-only rule. Several theories, including string theory, hint at additional dimensions, but these theories also imply a single, forward-moving timeline.

Time travel isn't just a question of "can we?" but also "what happens if we do?" Travelling back in time could result in logical contradictions known as paradoxes.

Perhaps the most famous example is the grandfather paradox. This occurs in a situation where you traveled back in time and somehow prevented your grandfather from meeting your grandmother, you would never have been born. But if you were never born, how could you have traveled back in time to prevent them from meeting?

Then there's the causal loop or bootstrap paradox. In this example, imagine you receive a book when you were young, then in the future you travel back in time to give that same book to your younger self. Who, then, is the original author of the book? The book seems to have no origin — it simply exists, looped in time.

Even if a causal loop were possible, there's the problem of entropy — the principle that things tend to move towards disorder. This means any object like the book would gradually degrade while cycling through such a loop. Eventually, it would become so degraded it couldn't continue the loop, creating another paradox.

The biggest problem that occurs when we think about paradoxes is the logical contradiction of changing the past. If we successfully change a past event, we might alter the conditions that led us to travel back in time in the first place. This is the paradoxical loop: the conditions for our journey back in time should exist in the first place for us to change them.

One theoretical solution to resolve these paradoxes is the concept of multiple timelines or multiple universes - the solution that caused all the chaos in 'Back To The Future II'. As Doc Brown explained: When we travel back in time, instead of altering our past, we branch off and create a new, separate timeline. While this theory is mathematically possible, it doesn't allow us to change our own past, it just creates a different version of history on another timeline.

The Novikov Self-Consistency Principle, named after Russian physicist Igor Dmitriyevich Novikov, suggests that any attempt to create a time paradox would always result in a "self-consistency." In other words, the events will somehow resolve themselves, such that the known laws of physics are not violated. According to this principle, you can't change the past in a way that would cause a paradox. For example, if you traveled back in time, the universe would prevent you from doing something like killing your own grandfather.

The Novikov Self-Consistency Principle raises questions about free will. If our actions in the past are predetermined to avoid contradictions, what does this mean for our ability to make free and independent choices?

Whether through multiple timelines or self-consistency principles, these theories attempt to resolve the conflicts that arise when we consider changing the past. Ultimately, though, many scientists believe that the universe safeguards itself against these paradoxes, forcing us to live our lives forward—not backward.

Renowned physicist Stephen Hawking proposed an alternative solution. He said the universe itself prevents time travel paradoxes from occurring. According to this conjecture, the laws of physics will not allow for time machines to be created, thereby preserving causality and the natural flow of time.

Though it might seem disappointing that we can't travel back in time to fix our mistakes or save our loved ones, this could be seen in a positive light. It means that each moment we experience is unique and invaluable. Our past, with all its imperfections, shapes who we are today, and we have the freedom and responsibility to shape our future.