Is Time Travel Impossible?

**SPEAKER_1** (0:00)
This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

**SPEAKER_2** (0:31)
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**Matt O'Dowd** (1:00)
If time travel turns out to be possible, I hereby invite all future time travelers to join me on set. I'll post the address one year from today. Okay? Right. Now.
Well, that's a bummer.
Time travel stories are cool because both the past and the future are somehow more interesting than the present. And because everyone secretly wants a do-over. But so far it appears that we are doomed to live consumed by regret in the eternal boring present. Time marches on, inexorably and only forward. Or so we thought until Einstein came along. His special and general theories of relativity change the way we think about time forever. And believe it or not, their raw equations permit time travel. They even tell us how to do it. So let's review the possibilities and decide how possible time travel really is. The first approach to time travel uses only special relativity, which describes how intervals of time and space are stretched or contracted depending on relative speeds. A fast moving spaceship appears to experience a slower rate of time compared to someone waiting back on Earth. Do a trip around the galaxy at close to the speed of light, and very little time might pass from the perspective of the traveler. But they'll find a minimum of hundreds of thousands of years have passed when they get back to Earth. However, that's a one way trip in time, and it's really just traveling in the same temporal direction at different rates.
So the original Planet of the Apes style time travel is possible, but it'd be nicer to be able to go back in time. Actually, the math does sort of allow that. The spaceship's clock slows down as it speeds up, and it stops completely at the speed of light. And at faster speeds, time should actually tick backwards. So if you could travel faster than light, you could navigate a path to a point in space time before you departed. We saw how to construct such a path in a previous Challenge episode. Of course, we know that the laws of physics forbid faster than light travel.
Or do they?
In order for any object with regular mass to even reach light speed, it would need infinite energy, which could never be obtained. But notice I said regular mass. We can hack the equations of special relativity by allowing mass to take values in the weird realm of imaginary or complex numbers.
An object with imaginary mass is now restricted to only traveling faster than light, never slower. That means it can only travel backwards in time, not forwards. We call a particle with imaginary mass a tachyon. If we could control tachyons, then perhaps at least we could send information back in time. But do they exist? Does imaginary mass exist?

**SPEAKER_1** (4:00)
This episode is brought to you by Accenture. When your advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales, using automation, analytics and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

**SPEAKER_2** (4:31)
When you need to build up your team to handle the growing chaos at work, use Indeed Sponsor jobs. It gives your job post the boost it needs to be seen and helps reach people with the right skills, certifications, and more. Spend less time searching and more time actually interviewing candidates who check all your boxes. Listeners of this show will get a $75 sponsored job credit at indeed.com/podcast. That's indeed.com/podcast. Terms and conditions apply. Need a hiring hero? This is a job for Indeed Sponsor jobs.