Spoiler alert: This post discusses a major plot point in the 2022 Disney Pixar film Lightyear.

The new Disney Pixar film Lightyear is full of sentient robots, laser blades, and hyper-speed travel and there is plenty of cool science to go with all that fun fiction.

At the start of the film, we join Buzz Lightyear and his crew on a journey. They are marooned on a hostile planet 4.2 million lightyears from Earth and their only chance of escape is to develop a special fuel that will allow them to travel fast— very very fast. Buzz, the courageous Space Ranger, volunteers to test the fuel. After his first flight, he returns and discovers that while it had only been a few minutes for him, everyone else on the planet had aged several years. With every subsequent test flight, each achieving faster and faster speeds, Buzz returns to find more and more time has passed.

Buzz eventually learns he has experienced something called time dilation. This real phenomenon causes time to pass slower for someone moving, relative to someone staying still. And the greater the difference in speed, the greater the difference in time. If, like Buzz, the person moving travels back to their original location, this can make it feel like they’ve traveled into the future. Or, as Buzz explains, “The faster I fly, the further into the future I travel.”

Albert Einstein, with his theories of special and general relativity published in 1905 and 1916, showed that time dilation can be caused by two things: differences in speed and differences in distance from a strong gravitational field. If you were to approach a black hole, for example, with a very strong gravitational pull on the space around it, time would move more slowly the closer you got. Both types of time dilation have been shown in dizzying effects in movies and television for decades, where it offers a way to seemingly travel forward through time.

Time dilation is most noticeable and causes the biggest jumps in time at high speeds or in close proximity to a very strong gravitational field, but we all experience time dilation every day. In 2010, physicists at the National Institute of Standards and Technology used highly-accurate clocks to show that clocks will tick more slowly even at speeds as slow as 25 miles per hour and height differences as small as one foot. So, as you stand on Earth, the top of your body travels a tiny bit faster than the bottom because of Earth’s spin, but your feet feel a stronger gravitational pull to the Earth when compared to your head. Both effects of time dilation compete here, but gravitational time dilation is stronger, making your head actually older than your feet. If you fly from Boston to Los Angeles, you will age faster than people of the ground. Once you return home, however, and fly from Los Angeles to Boston, you will age more slowly, due to the extra speed you gain by moving with Earth’s spin.  

These differences in time are incredibly small, and you won’t notice the occasional change in ageing rate, but what about astronauts zipping around Earth in space? Those aboard the International Space Station (ISS) are orbiting Earth at 17,500 miles per hour. That’s almost 5 miles every second! Astronauts returning to Earth after 6 months on the ISS will have aged slower than those on Earth, but only by about 0.005 seconds. The National Air and Space Museum holds in its collection a Buzz Lightyear toy that flew aboard the ISS for 15 months in 2008 to 2009. Because of time dilation, Toy Buzz is about 0.015 seconds younger than he would have been without the trip into space. Toy Buzz may not have traveled as fast as Movie Buzz, but both got to experience the effects of time dilation.

Though orbiting Earth only provides a tiny difference in time, not enough for a human to notice, even at this speed, time dilation has a practical importance. When you use GPS to determine your location, you’re relying on a system of satellites speeding around the Earth. Your GPS calculates how long it takes to receive signals from each of the satellites, which it then uses to determine how far away you are from each. With that information, the GPS can compare those distances and calculate its position. Global-positioning satellites are equipped with extremely precise clocks to increase the accuracy of its relayed positioning. However, because of time dilation, those clocks don’t match clocks on Earth. To avoid timing differences of a few nanoseconds, which could lead to hundreds of feet errors in positioning, these satellites have to account for time dilation. While, we may not yet be able to travel at the super fast speeds required for time dilation to jump us years into the future, our understanding of time dilation makes the convenience of GPS a reality.