The Milky Way, the swirling mass of galaxy that we call home is 100,000 light years in diameter, even if we could travel at the speed of light it would still take 100,000 years to travel from one side of this great unknown to the other. But what if I told you there is a way you could explore almost, the entire Milky Way within your lifetime, without having to worry about achieving light speed. There’s just one small caveat, you must maintain a constant 1g acceleration, that’s the same force as Earth’s gravitational pull. If, however your hypothetical spaceship could maintain such a near impossible level of unrelenting acceleration, then you could, due to the effects of time dilation, travel to the centre of the Milky Way and back again in just forty years. But when you return to Earth, it would be 60,000 years in the future. Better yet, if you could maintain that elusive 1g acceleration, you could reach the edge of the observable universe in a single human lifetime. What you would do when you get there remains to be discovered.
It would be the longest hour of your life and the heaviest. In all of four hours and thirty-five minutes, a thousand lifetimes would pass on Earth, generations would be born and inevitably fade. Yet, to you, all of a single afternoon would pass you by.
This would be your experience if you spent just one day on an imaginary planet in the ghostliness of space. A thousand light-years from Earth. Let’s call this imaginary place ZR 42A, which is a modicum catchier than GQ Lupi b, currently the largest exoplanet we have discovered, estimated to be up to thirty-six times the mass of Jupiter. Let us imagine that ZR 42A is 1,000 times as massive as Jupiter and thusly 317,800 times as massive as Earth. Gravity increases linearly with mass so therefore gravity is 317,800 times as strong on ZR 42A as it is here on Earth.
With that amount of force acting upon your humble frame, your skeleton would be crushed into a pile of dust, faster than you could say “quicksand”. It would be equivalent to bludgeoning an egg with a sledgehammer. But let us throw that law of physics into a cautionary black hole for just a moment and imagine you could walk, uninhibited, upon this far away rock in space. In Einstein’s theory of relativity he proposed that time is not a constant but a material, interwoven with physical space. Spacetime as it has come to be known, is akin to an elastic band, which can be stretched and contracted at will.
Time therefore is relative to the mass, and consequently the gravitational pull of the closest and most massive object to you. There are Arctic Terns that could migrate to the other side of the planet before I could finish explaining special relativity and general relativity in suitable detail. 12,000 Miles they migrate, twice per year, if you were interested. But for now let us assume a basic premise, that if you were to stand on the imaginary ZR 42A, that time would progress slower for you than your brethren back here on Earth.
This phenomenon is called time dilation. As gravitational force increases so does the bending, warping and hence dilation of time itself. With a weaker gravitational field time flows faster, stronger gravity causes time to trickle slower, but only from your perspective. So if you were to travel from Earth and stand upon the surface of ZR 42A for a single afternoon, upon your return everyone you knew and loved would be long deceased. Your home planet would be unrecognisable, thousands of years would have passed, yet to you, the smallest flicker of an afternoon would be all you had experienced, an unfathomably surreal afternoon, but an afternoon nonetheless. This effect isn’t just perceptionary, it is as real as the butter upon my bread and yes this is a form of time travel. For in those short few hours you wouldn’t have aged at all, upon your return to an Earth many thousands of years older than that you left it that very morning, your body would have aged only a few hours.
In reality time dilation isn’t quite that powerful in the vicinity of most planets, this preceding example is knocking on the decidedly delicious doors of science fiction. But time dilation does exist. If you were to stand on the largest planet in our solar system, Jupiter, then for every second that ticked by on your watch, one second and twenty nanoseconds would pass by back on Earth. That’s not enough to affect the timing of a boiled egg, nevermind the age dynamics of your relations, but it is unquestionably present. Jupiter is large, really large. 1,300 Earths could be stuffed inside Jupiter, if it were hollow, which is most definitely isn’t, I checked, although it is rather mushy.
But a mass certainly doesn’t have to be unfathomably large for the effects of time dilation to start messing up your interplanetary schedule. Near the event horizon of a black hole, the point in spacetime where the gravitational pull of the black hole becomes so powerful that you can no longer escape it, time dilation could very well cause your minutes to turn into hours, possibly even years, back on Earth. All this means that if you were somehow able to defy the relentless pull of gravity and avoid the small inconvenience of massive spaghettification, you could travel forward in time by spending a few hours at this exact point in space, within the grasp of a black hole. If you are able to make it back to Earth that is. If we’re getting technical about it, you wouldn’t actually travel through time at all, but time would pass at a different speed back on Earth, relative to you.
Nothing is safe from time dilation, not even the Earth itself, or your ankles; let me explain. The pull of gravity gets stronger the closer you get to the incredibly dense core of our planet. A which core is approximately 2.5 years younger than the surface. But what about your vulnerable ankles? Well, they also feel the slow grind of time dilation, if you were to wear a watch on your ankle and one on your wrist, over a number of years, many thousands, the watch attached to your ankle would fall behind your wristwatch, simply because it is closer to the immense source of gravitational force that is the Earth beneath your feet.
If all these time phenomena I have so far discussed have not yet left you inebriated in a cerebral overdose of factual soup upon your very own floor, then prepare yourself for yet another heavy dose of factual medicine. Every effect I have talked about thus far is caused by gravitational time dilation. You see, time dilation is more a time-warping double act. It comes in two flavours, gravitational and velocity-induced. We have already outlined how huge masses can warp spacetime itself to slow down the flow of time, but so can velocity.
According, once again, to Einstein’s general theory of relativity, a person’s perception of time is relative to the velocity at which they are traveling. This effect, which is hard-coded into our universe, is most apparent when two objects are travelling at different velocities, especially if those objects happen to be timekeeping devices.
Time dilation has to occur as a fundamental, unbreakable law of the universe, in order to to keep the speed of light as a constant. The speed of light is the great constant, the greatest equaliser. Whilst the waist size of the latest Big Brother winner inevitably and frequently varies (as well documented in a selection of trashy magazines named after English greetings) the speed of light will never do as such. There are other constants, such as Planck’s constant, but none are as unshakable as c. In Physics c denotes the speed of light inside a vacuum (300,000 km/s).
The speed of light is the great comparator in many crucial equations. If the speed of light were to change by 0.0001% it would throw off everything we thought we knew and have as a species, worked so hard over the past millenia to uncover, about our current understanding of the universe and everything it contains. So time dilation exists as a fundamental property of our universe to ensure that if one’s velocity changes, then time must slow, to maintain the constant speed of light. The speed of light accommodates no one or no one thing; everything, including spacetime itself must make extra special accommodations for this universal equaliser.
To understand how time can differ so substantially depending on a person’s location and velocity you must take everything you imagine about how time behaves and then turn it on its head. Naturally we think of time as a river flowing downhill towards the heady, relentless pull of the future, inevitably and often unforgivingly so. Einstein posed that time doesn’t flow as we feel it does, the encroaching future that is so omnipresent within our life experience is no more than an illusion, an impressively sophisticated one, according to general relativity. Einstein famously said "The distinction between past, present, and future is only a stubbornly persistent illusion."
Instead think of time like a train with an almost infinite number of carriages, the first carriage is the moment and the location of the big bang, conversely the last carriage is the last point in space and time of our universe, the point of its invenitible death. You can only see the people and events happening in your carriage, this is your slice of time, your present, a moment in the spacetime continuum that you share with all the other passengers in your train carriage. But passengers in other carriages, behind and infront of yours will have a different perspective of the “present”. Their slice of time, their now, will be entirely different to yours, their present could be your past or your future, depending on their location within the train.
Einstein theorised that through the mechanisms of gravity and velocity induced time dilation we can move to a different carriage upon the train, and thus our anchor within space and time, our “now”, shifts by one or more carriages. Depending on the velocity with which we are traveling and the gravitational force of our current surrounding. This example suggests a potentially unnerving truth. If time does behave in this way it would also mean that time is not something that is to be decided and altered, it is a static medium which we simply move through.
This would mean that everything that has happened and everything that is yet to be, has already happened, every tale has been foretold, every fortune has been realised, the future is not ours to make, it is a destination to reach. We cannot change what lays ahead in the train carriages in front of ours, we can only await the point in time and space when we will naturally reach that carriage. Each and every day is a page in a book that has already been written. But before you fall into a an abyss of self pity over your suddenly immutable future, do bare in mind this is not fact but our current best working model for how the universe behaves based on popular theories.
Importantly Einstein’s equations indicate that time has no directional bias, it prefers to flow neither forwards or backwards, not even sideways, if it were so inclined. Yet we inextricably experience time going forward. There is a sensational difference between what we experience as the past and the future. Time has no prefered direction, so naturally Physicists have long pondered this, what seems like a deep or flaw within our universe.
It transpires that what we believe is the passing of time, the steady rolling of the wheels of the future, isn’t the future at all. There is no past and there is no future, because time has no direction, at least not within this realm of existence we call the universe. As Einstein said, time is an illusion, and in the words of a great science fiction writer, “lunchtime, doubly so”. The passing of time is no more than a psychological illusion, a trick the world around us plays upon our brain. What, in fact we are experiencing is the terrifyingly stubborn process of low entropy systems becoming high entropy systems.
In thermodynamics Entropy is the measure of the order or disorder within a system, it eloquently describes the nature of all things that will eventually succumb to decay and fall into disrepair. With all things in the universe, from a distant star to a ham sandwich, there are more ways the particles within said thing can become disordered and fall into a chaotic abyss, than there are ways it can remain ordered. The star will eventually die and implode into a dense singularity and the ham sandwich will less spectacularly but just as inevitably grow mould and decompose. Entropy explains why buildings will always decay over time, equally and unfortunately so does the human body.
Therefore what we perceive as time passing, is no more than the decay and unstoppable onslaught of chaos all around us. People and animals dying, buildings and nature decaying. All these examples of entropy fool us into vesting wholy into a grand illusion, that we are approaching some kind of future. When in actuality, if the popular science is to be believed, then the future is not a point in time, it is a point in the vast continuum of spacetime and it is not ahead or in front of us, it is in no particular direction at all, no matter how hard our brains toil endlessly to make it thus. Thanks for watching.