## Neil DeGrasse Tyson Says Scott Kelly Aged More Slowly In Space, And He’s Right

Astronaut Scott Kelly got countless “welcome home” messages on Twitter following his return to Earth Tuesday after spending almost a year in space.

But a tweet from Neil deGrasse Tyson stood out for referencing Albert Einstein’s theory of relativity — and for noting one particularly mind-bending implication it has for Kelly, given his long stint in orbit:

Welcome back to Earth, Scott Kelly. After a year in orbit, Relativity says you’re 1/100 sec younger than you’d otherwise be.

— Neil deGrasse Tyson (@neiltyson) March 2, 2016

As the bearer of a Ph.D. in astrophysics from Columbia University, Tyson knows a thing or two about spaceflight and relativity. But is he correct that time passed more slowly for Kelly in space, thanks to what’s known as the time-dilation effect? And did he get the math right?

According to Dr. Jeffrey Bennett, a Colorado-based astrophysicist and author of the 2014 book *What is Relativity?*, Tyson nailed it.

“Relativity tells us that when we (in this case meaning all of us on Earth) observe someone/something else that is moving relative to us (in this case the International Space Station), we will see time running slower for the moving people/things,” Bennett told The Huffington Post in an email. “It’s called time ‘dilation’ because you can think of the moving person’s time as having ‘expanded’ (dilated) since less time passes for them than for us.”

Bennett said the following equation can be used to calculate the time dilation between Earth (the “rest frame” in the equation) and Kelly’s body aboard the ISS (the “moving frame”):

Bennett explained that the “v/c” term represents the space station’s speed (v) as a fraction of the speed of light (c). The space station’s orbital speed is about 17,000 miles per hour, Bennett said, —> Read More