Is time travel possible?

The short answer:

Although humans can't jump into a time machine and travel back in time, we do know that clocks on airplanes and satellites run at a different speed than clocks on Earth.

We all travel in time! For example, we travel one year between our birthdays. And we're all traveling in time at roughly the same speed: 1 second per second.

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away. It takes a long time for light from distant galaxies to reach us. So, when we look up at the sky with a telescope, we're seeing what those stars and galaxies looked like a long time ago.

However, when we think of the phrase "time travel," we usually think of travel faster than 1 second per second. This kind of time travel is something you'll only find in movies or science fiction books. Could it be true? Science says yes!

How do we know time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are interconnected. Einstein also said that our universe has a speed limit: nothing can move faster than the speed of light (186,000 miles per second).

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower time will seem to you. Scientists have done some experiments to prove that this is true.

For example, in one experiment two clocks were set to the same time. One clock stayed on Earth, while the other clock flew in an airplane (in the direction of Earth's rotation).

After the plane flew around the world, scientists compared the two clocks. The clock on the fast-moving plane was slightly behind the clock on the ground. Therefore, the clock on the plane was running slightly slower than 1 second per second.

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years back in time or into the future. This kind of time travel only happens in books and movies. But the math of time travel affects things we use every day.

For example, we use GPS satellites to figure out how to get to new places. (Watch our video about how GPS satellites work.) NASA scientists also use a high-precision version of GPS to locate satellites in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly, at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a tiny fraction of a second (similar to the airplane example above).

However, the satellites are also orbiting about 12,550 miles (20,200 km) above the Earth's surface. This actually speeds up GPS satellite clocks by a tiny fraction of a second.

Here's how: Einstein's theory also states that gravity bends space and time, slowing down the flow of time. Where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on the GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on the GPS satellites experience time at a speed slightly greater than 1 second per second. Fortunately, scientists can use math to correct for these differences in time.

If scientists don't correct GPS clocks, big problems can arise. GPS satellites won't be able to accurately calculate their position or yours. The errors will add up to a few miles every day, which is a big deal. GPS maps might think your house is farther away than where it really is!

Summary:

Yes, time travel is indeed a real thing. But it's not like you've probably seen in the movies. In some situations, it's possible to experience time passing at a rate different than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.