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The fastest man-made object ever made was a space probe called Juno, sent to orbit the planet Jupiter. During its orbit, it skimmed deep into the planet’s gravity well and reached a top speed of 265,000 km/h or 165,000 mph.

The Juno space probe. Image credit: NASA/JPL-Caltech

Let’s imagine that we made a spacecraft, called Juno II, that could consistently travel this fast. At this speed, it could travel the distance from Earth to the Moon in just an hour and a half, a journey that took Apollo 11 three days.

If we wanted to use Juno II to go to some more exotic locations in space like another star, or another galaxy, how long would it take us to get there? Let’s look at the numbers.

  • The nearest star to the Sun is ‘Proxima Centauri’. At top Juno speed, we can reach it in 17,500 years.
  • The nearest (potentially) habitable exoplanet is Gliese 667. At top Juno speed, we can reach it in 96,000 years.
  • The nearest galaxy is Andromeda. At top Juno speed, we can reach it in 10 billion years.

That’s… a disappointingly really long time.

But if we really wanted to get there, what’s the fastest that we can theoretically go? How far away is Juno’s top speed from the speed limit of the universe, the speed of light?

The speed of light is about 4,000 times faster than Juno II at 300,000 kilometres, or 186,410 miles per second. It travels in less than a second what Juno would in an hour. In 1.2 seconds it would reach the moon.

There’s a gif below to put it into perspective.

Let’s look at those numbers again, this time using the speed of light.

  • Travelling to Proxima Centauri at the speed of light would take 4.3 years.
  • Travelling to Gliese 667 at the speed of light would take 24 years.
  • Travelling to Andromeda at the speed of light would take 2.5 million years.

That’s still a pretty long time for the fastest speed in the universe. More than anything this shows how unbelievably massive space is.

Getting to a planet outside our solar system, like Gliese 667, is going to take a really long time.

But why is the speed of light a speed limit? Why can’t we go faster? It’s a question that reveals something fundamental about our universe.

To get the answer, we need to spend a second on the idea of spacetime, how it’s made up of four dimensions, and how we travel through it.

Oddly enough, we can learn a ton about spacetime from Speed, a 1994 action thriller starring Keanu Reeves.

If you haven’t seen it, the premise is that Keanu has to drive a bus at a constant 50 miles per hour (80 km/h) through Los Angeles. If he speeds up or slows down, then a bomb that’s rigged to it will explode.

This is where you come in.

Imagine you’re in a sequel for Speed, and you and Keanu are driving a speeding car along a huge salt flat. The same rules apply; you have to drive it at 50 miles per hour without speeding up or slowing down or it explodes, but you can drive it in any direction you like.

Although it seems like we have bigger problems going on right now, we have to spend a second looking at the geometry of the situation. Let’s start with the possible directions that you can drive the car in.

You can drive it north or south in a line. This line is called one ‘dimension’. You can also drive it east or west. That’s two dimensions.

If you’re driving north, then you’re travelling north at 50 miles an hour. If you turn northeast you’re… still travelling at 50 miles an hour.

But if you’re going northeast, can also see it as travelling north at 25 miles an hour, and east at 25 miles an hour. It’s a little unintuitive at first but makes sense when you think about it. But most importantly it’s not going to set the bomb off.

At some point in the movie after a lot of driving, a dramatic rescue happens.

A helicopter appears on the horizon and soars over the salt flat towards your speeding car. It pulls up directly above and drops a rope down. Keanu grabs the rope and your arm, and the two of you are hoisted up and into the helicopter.

Down below, the car slows and explodes from the rigged bomb. That was close!

As you’re sitting in the helicopter thinking about your miraculous escape, you realise that something doesn’t feel right. The bad guy is supposed to learn something from the prequels, and this escape was too easy. Then you see it. Strapped onto the helicopter is another bomb!

Once again the rules are the same. You have to keep the helicopter moving at the same speed and you can move north/south and east/west, except this time you can also move up and down. This makes three dimensions. These three dimensions are sometimes referred to together as ‘space’.

The helicopter rockets upwards, still travelling at 50 miles an hour, but going north at 25 miles an hour and upwards at 25 miles an hour.

Here’s the kicker.

Try to imagine two more directions: futureward and pastward.

You can’t point in those directions like you can with the others, but it shouldn’t be too hard for you to understand them intuitively. Futureward is the direction in which tomorrow lies; pastward is the direction in which yesterday lies.

This ‘line’ of time is the fourth dimension.

There’s something important to know about this time dimension. All of us, right now, are hurtling through it. Travelling through the line of the time dimension is our experience of seconds ticking past, and of tomorrow becoming today.

If we slowed down how fast we travel along this line, then the seconds would drag out and might even stop ticking altogether.

Time seems like it’s a separate thing from the other three dimensions but it’s actually been there the whole time, just like the up/down dimension was there when you and Keanu were driving on the salt flat. Another term for this four-dimensional idea is ‘spacetime’, and it is a real description of how the universe actually is.

The wormhole scene from Donnie Darko. The ‘wormhole’ is the line through time that he’s going to take. But it wouldn’t actually look like this because everything is moving through time not just the main character.

Now we have a new dimension to play with, though the same rules apply. The faster we travel in one dimension (like north) the slower we have to travel through a different dimension (like time).

The awesome thing is, you don’t need some kind of special vehicle like a time machine to do this. You just need to be able to travel very fast.

A quick side note: The main difference between the time dimension and the other ones is that you can’t travel pastwards. Physicists have no idea why, and if you work it out there’s a Nobel Prize waiting for you.

This is where the final, dramatic rescue of the movie happens.

Keanu takes the controls of the helicopter and pitches it upwards. After a few minutes the atmosphere around you begins to thin, and the helicopter’s engine starts to stutter. Keanu grabs you by the hand and the two of you leap from the helicopter at the edge of space.

The two of you hurtle through the atmosphere hand in hand.

Just before the helicopter stalls and explodes, you think you see a glint of light at the edge of your vision.

Before you know it, Juno II, the super fast spacecraft is underneath you. You clamber on board and mutter something about how this rescue is more miraculous than The Matrix: Revolutions. Keanu agrees and says Juno II should be renamed Deux Ex Machina.

A thorough inspection of Juno II/Deux Ex Machina reveals that it has no bomb on board. You are free to travel as fast or as slow as you like.

Well, kind of.

The truth is that everything in the universe has a bomb that keeps us moving constantly at the speed of light.

You’re free to steer in any of the four dimensions. Most of us are just sitting around, and in doing so have allocated most of our speed to moving along the time dimension. We’re hurtling towards the future at top speed.

If the bomb is the speed limit of the universe, I suppose in this analogy that Keanu Reeves is God.

When you hop on your bike, you’ve started to move through space (the first three dimensions) a little faster. You’ve steered slightly away from the line of time, which means you will get to the future more slowly. But because the speed of any bike isn’t very much compared to the speed of light, you’re not going to feel much of a difference; but there will be a difference.

Even Juno II’s top speed isn’t that close to the speed of light. If you travel in it for three years, you’ll find that for everyone else three years plus one second had passed.

It’s not much, but when you think about time like this then all of us are time travellers.

If you ever manage to speed up and travel through space at the speed of light, then time will slow to a stop. As all of your motion is through space, there is none left for you to move through the line of time.

For as long as you are travelling at this speed from one point to another, it will feel like you teleported.

This is what life is like for a photon, a massless particle that travels at the speed of light through space. One may start its life in the middle of the Sun, will zoom across space and down through Earth’s atmosphere and hit a blade of grass. This may take years or decades from our perspective, but for the photon, it all happened simultaneously.

This is also why nothing can go any faster than the speed of light. There’s no speed left!

If we ever work out how to travel across space faster than light can, it has to come from manipulating space rather than adding more speed.

We may be able to warp space, which would mean using massive amounts of energy to contract the space in front of a spaceship and expand space behind it. This causes a wave that the spaceship can surf, effectively letting us travel at 10 times the speed of light.

A doughnut-shaped warp drive could let a spaceship travel through space effectively faster than light.

So when we talk about the speed of light and warping space itself to overcome the laws of the universe, there’s only one thing left that we can say.

If you have any questions about this article, please submit them to our open Ask Me Anything.


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Ben McCarthy

Ben McCarthy

Ben is the Founder of Discover Earth and the author of the Big Ideas Network.