Black holes are so massive that they warp space and time around them.

The CANs and CAN’Ts of Time Travel

  • 13th Feb 2024
  • Author: Caitlin Hauxwell

Time travel has long-been a topic of great discussion, debate, and experimentation in the world of physics. Famously, the brilliant Stephen Hawking threw an entire party for potential time travellers in 2009. He ordered in finger food, champagne, and balloons (all the makings of a great party) and sent out the invites. The catch? Hawking sent the invites out after the party was finished. This way, the only people who could show up were those who could travel from the future into the past. Sadly, nobody made it to Stephen Hawking’s shindig.

The results didn’t come as a surprise to many scientists. Unfortunately, backwards time travel is something that our current understandings of the universe don’t allow – and there are all sorts of problems and “paradoxes” (more on this later) that appear when we begin to think too hard about it.

That said, through some clever physics pioneered by Albert Einstein, there are still ways that we could travel to the future, age slower than the stars, and see the entire history of the universe in a single lifetime.

General Relativity – big words for a big idea!

When you think of time, what do you think of? Seconds, minutes, hours, perhaps. But to physicists, time is so much more.

Einstein showed that time and “space” (that is our 3D universe – including your house and my garden and the planet Earth and our galaxy…and everything in between them and beyond!) aren’t totally different things, but are actually intertwined. They form something called “spacetime,” and spacetime is like an infinitely large bowl of soup that everything in the universe sits inside.

Now, bear with me. If spacetime is like soup, then everything in the universe is floating in this soup like chunks. Some of these chunks are bigger than others – like a big pea! Or a crouton! These are the heavy parts of the universe. Things like humungous stars! These heavy things have a lot of gravity, and for anything that is travelling through spacetime, these chunkier, thicker parts of the soup make travelling muuuuuch sloooooower.

Time Dilation

In the universe, objects with very strong gravity can cause time to slow down around them. While in parts of the universe that have weak gravitational influences (the smooth, thin, lighter parts of the soup), time passes much quicker.

This effect – where space and time are warped wherever matter is denser – is called Time Dilation, and it’s part of what Einstein proved when he coined the term “General Relativity” (the idea that time – in general – seems to move at different rates, relative to where you are in the soup of spacetime).

Time dilation is at its most extreme at the densest (thickest) parts of spacetime (soup) – black holes. These are the enigmatic corpses left behind when the largest, most volatile stars in the universe go bang! Black holes have such strong gravity that they suck in everything around them, and not even light can escape their power. Side note: this is why black holes appear black – none of the light that hits them can ever return to our eyes or our telescopes.

Spacetime is so warped around these massive objects that, hypothetically, if you were to fall into a black hole, time would be slowed to such a degree that millions of years outside the black hole could pass in what felt like a blink of an eye to you inside it.

This is why it is entirely possible that from your perspective, inside the black hole, you could watch the whole future of the universe unfold before your eyes.

(Click the image on the left to play the animation - see how space is warped around the mass and how time passes at different rates dependent on the distance from that dense region of matter).

Time flies

However, it’s not just gravity (remember – thick, chunky points in the soup) that can slow time down. In fact, the faster we travel through spacetime soup, the “thicker” it feels. Imagine the difference between trying to stroll across a swimming pool, versus trying to sprint through it. Once again, this can make time feel like its passing verrrryyyy slooooowly.

Scientists have actually proven this effect right here on Earth! When a very fast plane flew through the atmosphere on earth, we discovered that the on-board clock had ticked slower than an identical clock back down on the ground. This was due to how quickly the plane was travelling relative to the ground; the people on board would have experienced time passing ever-so-slightly slower than the people on the ground.

Technically, then, this means that if you could hop aboard an incredibly fast spaceship, and travel super-super quickly out in one direction of space, then travel super-super quickly all the way back to Earth, what may have felt like a few months to you could actually have been DECADES passing here on Earth.

By the time you arrived home, even though you hadn’t been gone for long (from your perspective), it would be years and years into the future. To the people you were coming home to, you would be an immortal time traveller!

(Click the image on the right to play the animation - see how time passes slower when travelling faster i.e. on a plane).

The Spooky Paradoxes of Backwards Time Travel

You may or may not have heard of something called a paradox. These are situations that seem impossible yet possible, both true and false, and both wrong and right, all at the same time! For example, if I say to you – “I am lying.” – am I lying, or am I being honest? Think about it!

Time travel is infamous for creating mind-boggling paradoxes like this, and they are a simple way to demonstrate why backwards time travel is one of the trickiest concepts for physicists to wrap their heads around. But, if your brain isn’t too mushy already from all the general-relativity-talk, I’d like to introduce you to some of these paradoxes.

I’d like you to imagine a time traveller. Anyone you like. Maybe they have a big moustache. Or a tiny hat. Or red shoes like a clown wears. Now, we’re going to put our hypothetical time traveller in some interesting scenarios, and watch the complicated chaos which unfolds…

  1. The RMS Titanic, which sank in 1912.
    public domain (wikipedia)

    Predestination Paradox

    When past events affect future time travel which then affects past events.

    Our time traveller goes back to stop the sinking of the Titanic, but if the Titanic was prevented from sinking, then our time traveller (from the future) would have had no reason to travel back in time – because there was never any tragedy to begin with! But if they never went back in time, then there was nobody to stop the sinking, and so the ship would have sunk. Which means, in the future, our time traveller would go back in time to stop the tragedy from happening, but then if no tragedy happened, they’d never have gone back in time in the first place, and…my head hurts!

  2. Jane Austen, the prolific writer.
    public domain (wikipedia)

    Bootstrap Paradox

    When information or objects seem to appear out of nowhere.

    Our time traveller goes back to the 1700s to ask the real Jane Austen to autograph all of their books, but it turns out the real Jane Austen is illiterate! So, our time traveller hands over all their copies of Mansfield Park, Pride and Prejudice, and Emma to Jane Austen and tells her to copy them all down and have them published – and that’s exactly what Jane does! But all this raises the question: who wrote those stories? It couldn’t have been Jane Austen, like everyone throughout history believes (she only copied the books) and it certainly wasn’t our time traveller.

  3. Retro vinyl records.
    public domain (pexels)

    Grandfather Paradox

    When time travellers write themselves out of existence.

    Our time traveller, a big fan of old-fashioned Rock and Roll, travels to the 1950s, and who do they meet except for their very own grandfather! Excited, they strike up a conversation with their young grandfather. The trouble is – in that moment, while the two were chatting, the grandfather was meant to be meeting our time traveller’s grandmother! Our time traveller is holding him up! So, if that meeting never happened, then the grandfather and grandmother would never get married, and our time traveller would never have existed! But if the time traveller doesn’t exist…then there was nobody to stop the grandparents from meeting…and the loop continues.

While machines like the TARDIS may still be steadfast figments of science fiction, the reality of the physics around us is equally fantastic. Time isn’t as fixed as we might have once thought, and physicists are still uncovering the limits of what the “soup” of spacetime allows. However, if you ever do find yourself in control of a time machine…best not to stop any tragedies from unfolding – and never meet your heroes!


Full references / credits:

(Banner image) Black holes are so massive that they warp space and time around them. Credit: NASA/JPL-Caltech

(1) Objects with large gravity cause spacetime to warp around them. Credit: NASA

(2) Conceptual illustration of both 3D space and time distortion near a mass where matter is denser. Credit: Lucas Vieira Barbosa CC BY-SA 4.0 DEED 

(3) The faster we travel, the slower time seems to pass. Credit: NASA/JPL-Caltech

(4a) The RMS Titanic, which sank in 1912. Credit: public domain (wikipedia)

(4b) Jane Austen, the prolific writer. Credit: public domain (wikipedia),_from_A_Memoir_of_Jane_Austen_(1870).jpg

(4c) Retro vinyl records. Credit: public domain (pexels)