Is time travel possible?
In true time travel fashion let’s start with the conclusion. Yes, time travel is possible and has in fact been done before. Going forward in time is actually relatively easy, it’s going forward a considerable number of years or travelling back in time that proves difficult.
Take the International Space Station as an example, resident Sergei Kirkalev was a resident of the ISS for 803 days, 9 hours and 39 minutes whilst travelling around the Earth at a speed of 7.66 km/h. When he arrived back on earth he arrived 0.02 seconds into the future. This is known as time dilation.
But what else do we know about travelling through time?
Speed of Light
Albert Einstein’s Theory of Relativity told us that everything is experienced relative to the observer, including time. So travelling at half the speed of light (186,000 mi/sec being the full speed of light) in the direction of light itself would appear no different to those travelling, but it would be faster to those observing. This went against Sir Isaac Newton’s theory that time is a straight arrow, pointing in one direction at all times and only going forward.
So, what does that mean? Well, it means two things, that time travelling forward short bursts of time is entirely possible
as you don't need to be at, or faster than, the speed of light do so. For example, if you travel at 90% the speed of light then your mass is doubled (E=MC²), meaning you experience double the time of those observing (10 minutes for them and 20 for you).
Significant time travel, including into the past, would require travelling at the speed of light or faster, which is a problem. Einstein’s mass-energy equation calculates that to travel at the speed of the light you would achieve infinite mass and require infinite energy. Infinite mass isn't possible and infinite energy is highly improbable, at least where we are now. So nothing of mass can travel at the speed of light. Photons, which are light particles, have no mass to begin with hence why they don't encounter the mass-energy issue that organic matter would
If you want to get around the whole ‘speed of light’ issue you can always just perch yourself on the edge of a black hole. Black holes are celestial bodies which cause gravitational time dilation; they are believed to have a conical shape to them, the most compact concentration of matter in the universe with the ability to curve spacetime . At the edges, before you’re sucked in but when you’re caught in the gravity at the horizon of the black hole, extreme time dilation can be theorised. So, if you were to sit on the horizon of a black hole for five minutes from your perspective and then were somehow pulled free of the black hole’s gravity, you could have travelled hundreds or thousands of years into the future depending on the mass and energy of the black hole.
Wormholes are another theory for time travel, sitting prominently with faster than light drives in many a sci-fi show. The wormhole theory posits that in a universe governed by Einstein’s theory of general relativity, space-time can be warped, forming what we call a wormhole.
The problem with the wormhole theory isn’t so much that it’s not possible, there are scientists who agree that in the beginnings of the universe, milliseconds after the big bang, a wormhole could be formed, but it would require exotic matter to hold it open and keep it stable.
This exotic matter (not made up of protons and neutrons like ‘normal’ matter has been reproduced and found in tiny amounts in laboratories, at the Large Hadron Collider in 2014 but is yet to be found in the large quantities needed to sustain a theoretical wormhole.
A wormhole could take you back in time, but never further than back to when the wormhole was created, it would simply be the joining of two points in time and space so the destination would be fixed.
Going backwards in time
A man who loved the music of Mozart went back in time to meet him, but when he got there no one had ever heard of Mozart or his music. Distraught at the thought that the world could be denied the beauty of Mozart’s music, and armed with the sheet music the man had brought for Mozart to sign, he decided he would play the music to the world and subsequently become Mozart.
This is a bootstrap paradox, a time loop that goes over and over but has no beginning. Who originally wrote that music? It is just one of many paradoxes attributed to travelling backwards in time.
The butterfly effect paradox; you go back and step on a butterfly and when you come back the whole world is ruled by dinosaurs and you never existed…but if you never existed how did you go back in time to step on the butterfly and create the future?
Even if travelling backwards in time was possible, which given our inability to travel faster than light, is improbable, it would give you one mighty headache.
Even if time travel to other points in time isn’t entirely possible, there are instances of a certain type of time travel in nature.
First of all, we all travel through time every day because we live in a universe where time, or at least what we perceive as time, exists. Our cells decay with each passing minute as a result of the forward momentum of time and so we can say that forward time travelling is possible just as we experience it normally. It may not seem strange to us, but imagine a being outside of our universe fixed in one point of time, to them it would seem as though we were travelling through time.
Then there are the creatures we live with who can regenerate their cells to make them appear young again, regrow limbs and live a very, very long time.
Lizards can grow new tails, spiders can regrow their legs, starfish can regrow their limbs and sea cucumbers can regenerate their entire bodies. All of these animals ‘turn back the clock’ to a previous point in time by regrowing what they’ve lost and negating the consequence of the future.
The most spectacular of all the regenerative animals is the Immortal Jellyfish (Turritopsis Dorhrnii). This species is a small and biologically immortal creature found in the Mediterranean waters of Japan. It starts life as a Polyp on the ocean floor and slowly grows to become sexually mature. Once it reaches maturity and becomes sick, threatened, stressed or just old it can revert itself back to its polyp stage. Theoretically, it can do this indefinitely, over and over again for as long as it has a habitat. The jellyfish goes back in time biologically, so where the passage of time itself isn’t altered the jellyfish is, once again, at infancy stage.
This may interfere with evolution, which is arguably not needed in this case, but evolution itself, although it takes thousands and millions of years is perhaps a testament to why it’s best not to be looking at going backwards but instead focus on what’s ahead, at a normal pace.