One of the fastest man-made objects was a space probe called Juno sent to survey the planet Jupiter.

During its orbit it skimmed deep into the giant planet’s gravity well and reached a top speed of 265,000 km/h or 165,000 mph relative to the Earth.

Moving fast

Let’s imagine we made a second spacecraft called Juno II that could travel at this top speed consistently. It could travel from:

• London to New York in a blinding 1 minute and 15 seconds
• Around the Earth in 9 minutes if it’s at the same distance as the International Space Station
• From the Earth to the Moon in just an hour and a half, a journey that took Apollo 11 three days.

That’s pretty fast, so let’s take it to the next level. If we wanted to use Juno II for space travel, like to visit like another star or galaxy, how long would it take us to get there?

• The nearest star to the Sun is ‘Proxima Centauri’. Juno II could reach it in 17,500 years.
• The nearest potentially habitable exoplanet is Gliese 667. Juno II could reach it in 96,000 years.
• The nearest galaxy is Andromeda. Juno II could reach it in 10 billion years.

Each of these is… a disappointingly long time. Future spacecraft is going to have to use some incredible technology to move through the cosmos on human timescales.

But let’s look at the theoretical upper bound of speed. How far away is Juno II from the speed limit of the universe, the speed of light?

The speed of light is 4,000 times faster than Juno II. It travels at 300,000 kilometres or 186,410 miles per second.

That’s London to New York in 0.019 seconds. Or Earth to the Moon in 1.2 seconds.

Suddenly London to New York in 1 minute and 15 seconds seems slow.

Let’s look at space travel again 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.

It’s a lot faster than Juno II, but it still seems to take a pretty long time for the fastest speed in the universe. It’s going to be pretty difficult to have a space civilisation.

Why is there a speed limit?

But 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 all the time.

Maybe you’ve seen Speed, the 1994 action thriller starring Keanu Reeves. If you haven’t seen it, the premise is that Keanu has to drive a bus at 50 miles per hour (80 km/h) or faster through Los Angeles. If he slows down, then a bomb that’s rigged to it will explode.

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 but with a minor change; you have to drive it at 50 miles per hour without speeding up OR slowing down or it explodes. 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. You can drive north to south in a line. This line is called one ‘dimension’. You can also drive east to west. That’s two dimensions.

Of course you can drive north at 50 mph or northeast at 50 mph; whichever way you want to go.

But if you’re going northeast, you can also see it as travelling north at 25 mph, and east at 25 mph. It’s a little unintuitive but it makes sense. Most importantly it’s not going to set the bomb off.

At some point in this sequel 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 climbs on top, 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. This escape was too easy. Then you see it… Bolted 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 just like the car. But this time you can also move up or down. This makes three dimensions. These three dimensions are sometimes referred to together as ‘space’.

The helicopter speeds away, still travelling at 50 mph, but going north at 25 mph and upwards at 25 mph.

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

Keanu takes the control 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.

A moment later the Juno II spaceship is directly underneath you!

It was finishing up its around-the-world time trials from the beginning of this article. You climb on board and mutter something about deus ex machina.

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

Well, kind of.

Here’s the kicker, and how it relates to the speed of light. Try to imagine two more directions: futureward and pastward.

You can’t drive or fly in those directions like you can with the others, but it shouldn’t be too hard 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 time is our experience of seconds ticking past, and of yesterday becoming today.

In the car and helicopter, we’ve been free to travel in any dimension but we are stuck with a total speed of 50 mph. All of us move through life in a similar way, except we’re all stuck at travelling at the total speed of the speed of light. But almost all of this speed is allocated to moving through the time dimension.

When you’re standing still, you’re moving into the future at top speed.

When you start to move in any other direction, say to drive north at 40 mph, you’ve taken a tiny bit of your speed through time and put it into travelling north instead.

The very, very strange result is that you’ll experience going through time slightly slower than everyone at home sitting in their couch.

This is called ‘time dilation,’ but it only starts to become noticeable when you reach speeds like what you would on Juno II. If you travel inside 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 travelers.

If we managed to get a spaceship to accelerate closer and closer to the speed of light, then the passengers aboard would have a very strange experience. They would have allocated most of their speed to travelling through space, and are now barely crawling down the line of time.

From their perspective, the outside world would be on fast forward. Years would pass in an instant, and decades would pass in the time it took to make a coffee. In a few hours everyone they knew back on Earth would be gone, replaced by the next generations.

If they reach the speed of light, time stops altogether. The spacecraft is still travelling vast distances, but the passengers aboard are not able to experience it. For them, it’s more like teleportation. They are frozen in time until the ship slows down and reallocates some of its speed back to moving through time.

Nothing can go any faster than the speed of light, because there is no speed left to reallocate.

Travelling across the universe

If we ever work out how to get to a destination faster than light can and without the time diliation side effects, it has to come from manipulating space rather than adding more speed.

One method may be through warping space by using massive amounts of energy to contract the space in front of a spaceship and expand the space behind it.

It’s like Keanu’s bus bunching the road into a pile and driving over it, while keeping at the 50 mph limit. This will let the bus travel across far more road but without setting the bomb off. It might just work.

A space civilisation may be possible after all.