How time bends, warps, and changes throughout the universe

Most of us think of time as pretty straightforward. It moves onward at a steady pace, never stopping or changing for anything. A second is a second. A day is 24 hours. A year is one trip around the sun.

But once you leave Earth, time loses that straightforward nature. A day on another planet won’t last 24 hours, an astronaut will age at a different rate in space than back home, and a telescope’s lens acts like a window into the past.

So why exactly do these things happen? Let’s explore.

Time isn’t constant

The first thing we need to understand is that there is a surprising connection between time and space, a discovery that goes back to one of the most well-known scientists in history: Albert Einstein.

By developing his theories of relativity, he was able to show that our measurement of time was dependent on motion and gravity. Time was, in fact, not constant.

Relativity comes in two varieties: special and general. In short, special relativity shows that the faster something moves, the more slowly time passes for it. Meanwhile, general relativity establishes that strong gravity will also slow the passage of time.

Special relativity: The faster you go, the slower time passes

Chances are, you’ve probably heard of Einstein’s famous equation E = mc². It’s used in his theory of special relativity to show that energy (E) and mass (m) are closely connected and essentially interchangeable.

This theory also revealed that the speed of light (roughly 300,000 kilometres per second), is perceived to be constant, no matter what. Light can never speed up or slow down in a vacuum, so to keep the speed of light constant for every observer, space and time must adjust instead.

Imagine a pair of 15-year-old twins. One boards an incredibly fast spaceship travelling close to the speed of light, while the other stays back on Earth. After what feels like five years aboard the spacecraft, the travelling twin returns home. She’s only aged five years, but her twin sister on Earth will have aged decades.

If you’ve ever watched a superhero like The Flash race around while the rest of the world appears frozen in slow motion, you’ve seen a fictional version of this same idea: time dilation.

A photon (the particle that makes up light) experiences time dilation at its most extreme. A photon travelling at the speed of light from the Sun to Earth takes an almost 150 million kilometer journey. From our perspective, that journey takes the photon about eight minutes. According to relativity, however, from the perspective of the photon, no time passes at all. Time dilation is at its maximum, and it’s as if the entire journey happened all at once.

General relativity: Gravity can bend time too

Velocity isn’t the only thing that can alter time. Einstein’s theory of general relativity shows that massive objects such as stars, planets, and black holes can warp both space and time around them.

Picture this: you’ve placed a bowling ball in the middle of a trampoline, creating a dip in the fabric. If you were to then roll a marble across the trampoline, its path would bend towards the bowling ball, beginning to circle around it. Massive objects bend space around them in a similar way with their gravity, and can even cause light to follow a curved path.

If light can be affected by gravity, that means time can be affected by gravity as well. The stronger the gravity, the more time bends and the slower it travels.

Imagine that the bowling ball weighs so much that the trampoline fabric touches the ground below it. The marble would fall towards this hole that’s been created, but not before rotating around it several times. The rotation would get faster and faster as it approaches the hole until the marble becomes a blurry image to our eye.

That’s similar to what it would be like to get sucked into a black hole. To a distant observer, the falling person would appear to slow down dramatically, becoming increasingly dim, and seeming to freeze near the event horizon of the black hole. Meanwhile, the person getting sucked in would be experiencing time at a regular speed from their perspective.

Relativity helps make sure your GPS works

It’s important to remember that relativity can affect our daily lives, though on a smaller scale compared to travelling at the speed of light or getting sucked into a black hole.

GPS (global positioning system) satellites orbit thousands of kilometres above Earth and carry incredibly precise atomic clocks. Because they’re moving quickly, special relativity makes these clocks run slightly slower than those on Earth. And because they’re farther away from Earth’s gravitational pull, general relativity also makes them run faster.

When both effects are combined, the clocks gain about 38 microseconds every day compared to clocks on Earth’s surface. This may sound like an insignificant difference, but since radio signals travel at the speed of light, even being one microsecond off could translate to roughly 300 metres of positioning error for a GPS.

To correct for relativity, engineers have to set satellite clocks to tick at a different rate before launch, making sure they stay synchronized once they’re in orbit. After launch, the systems then have to continually apply relativistic corrections. Without doing these, your phone’s GPS would become wildly inaccurate.

Mars has its own clock

Even without relativity, time in space can get confusing, fast. Planetary motion makes timekeeping very different on planets besides Earth. If humans ever settle on Mars, for example, they’ll have to set a completely different schedule for themselves.

A day on Mars (commonly called a sol) is just a tad longer than Earth, due to the red planet rotating slightly slower. The 24 hours and 39 minute sols of Mars might seem like just a small difference by comparison, but they add up over time.

When NASA’s rover missions first arrived on Mars, engineers at the Jet Propulsion Laboratory operated on mars sols for the first 90 days. This meant their workdays shifted about 39 minutes later every Earth day. Someone who started work at 7 a.m. Earth time would be reporting for duty around midnight a couple weeks later, then working through the night, before eventually cycling back to daytime again.

NASA eventually developed an app called Mars24 that keeps track of local time at different locations across Mars, including where the rovers are exploring.

The rovers also carry a device called a MarsDial. Unlike an ordinary sundial, it uses specially designed markings and image analysis software to help determine local solar time and calibrate colour and lighting in rover images, based on the shadows that are cast.

A day lasts longer than a year on Venus

By comparison, Venus has a far more peculiar schedule than Mars.

The second closest planet to the sun rotates so slowly that one full day there lasts about 243 Earth days. However, it only takes 225 Earth days for Venus to complete a full orbit around the sun.

If you were somehow able to set up a camp at one location on Venus, you’d experience a full year on the planet before a single day had passed.

Some planets never even see sunset

Meanwhile on other planets, a camper may never see the night sky come over them, no matter how long they waited.

Certain exoplanets (planets outside our solar system) are tidally locked, meaning they find themselves so heavily influenced by the gravity of the star they’re orbiting that their rotations become synchronized.

Our moon is a perfect example, as it’s tidally locked to the Earth. This is why we only ever see one side of it.

For a tidally locked exoplanet, one hemisphere would experience permanent daylight while the other would remain in eternal darkness. The star it orbits would appear frozen in the same place in the sky at all times, never rising or setting.

Galaxies keep track of time in their own way

It’s not just planets that have their clocks impacted by gravity and relativity. Entire galaxies have their own timescales too.

Unlike planets, our Milky Way galaxy isn't orbiting some even larger object. Instead, it is locked into a slow gravitational dance with our neighbour: the Andromeda galaxy. The two galaxies are gradually moving toward one another and are expected to collide in roughly 4.5 billion years, eventually forming one enormous galaxy.

The Milky Way rotates around like a pinwheel. In its roughly 13.5 billion years of known existence, the Milky Way has completed approximately 50 to 60 rotations. Since our solar system formed 4.6 billion years ago, the Milky Way has made about 20 rotations.  And since the dinosaurs disappeared 66 million years ago, our galaxy has completed about a quarter of a single rotation.

Looking into space is looking into the past

As you look beyond our own galaxy, whether that’s through a supercharged telescope or up at the stars above, you’re actually peering into a time machine.

The explanation for this is simple: light takes time to travel.

When you watch fireworks, you’ll often see the explosion before you hear the sound. This is because light travels much faster than sound; nearly 875,000 times faster, to be precise.

This same principle applies all across the universe. Moonlight takes about 1.3 seconds to reach Earth, meaning you’re always seeing the Moon just as it looked over a second ago. At the farthest end of our solar system, the light from Neptune takes roughly four hours to reach Earth.

For faraway galaxies, the travel time of light becomes millions or even billions of years. When astronomers observe a galaxy a billion light-years away, they are not seeing it as it exists today, but as it looked in the distant past.

This is why powerful telescopes like the James Webb Space Telescope are so revolutionary. They don’t just produce sharper images, they collect light that has spent billions of years travelling across space. This allows astronomers to look almost all the way back to the dawn of the universe.