Travel Light-Years in a Lifetime

  • By Greg Kestin
  • Posted 09.05.18
  • NOVA

The Andromeda Galaxy is 2.5 million light-years away—meaning even light, the fastest thing there is, would take 2.5 million years to get there from Earth. But if you could travel near light speed, your time would slow down enough that you could get there easily in your lifetime. Find out how in this episode of What the Physics?!

Running Time: 05:28


Travel Light-Years in a Lifetime

Published September 5, 2018

Greg Kestin: This is the Andromeda Galaxy. Wouldn't you love to vacation here? It's our closest neighboring galaxy. But even so, it would take 2.5 million years for the fastest thing in the universe--light--to reach it. So is there any hope that you could reach it in your lifetime? Yes.

Possibly the weirdest part of Einstein's theory of special relativity is time dilation. What it says is that the faster you go, the slower time moves for you. If you were moving at the speed of light, your time would essentially freeze.

As you approach the speed of light, your watch will tick slower, your heart will beat slower, your neurons will fire slower; everything will go slower. It's kind of the fountain of youth of physics. Undergo time dilation for just a few years and you'll look decades younger than your friends because you will be decades younger than your friends. Time dilation actually doesn't affect everyday events because we're not usually going fast enough. But if you could travel near the speed of light--say, on a rocket to your vacation at the Andromeda Galaxy, then things might go something like this.

You start just above Earth and you set your boosters to accelerate at a rate of 1G. That's about the rate of ludicrous mode on a Tesla Roadster, which is 0-60 in just 2.7 seconds. If you continue to accelerate at that rate for a year, you'll travel 3 trillion miles, or half a light-year. A light-year is just the distance that light travels in one Earth year, which is 6 trillion miles.

OK, so you still have 2.5 million light-years to go. You've reached 75% the speed of light, but time has already gotten weird. You left Earth in summer 2018 and traveled for a year, but back on Earth it's now autumn 2019. A year and two months have passed. Your time is already moving slower. If you continue to accelerate at that rate for four more years, you'll reach 99.99% the speed of light. And you've traveled 72 light-years.

OK, something weird must have happened. How did you travel 72 light-years in just five years? Because light can only travel five light-years in that same period of time. Did you go faster than the speed of light?

Well, no, because at this speed, time dilation is strong. Even though five years have passed for you on the rocket, 74 years have now passed back on Earth. And most of that time, you were traveling at about the speed of light, so you've traveled 72 light-years. This is a good time for me to remind you to take anyone you care about and love on this vacation with you.

Alright, so after 12 years of accelerating, you're now at 99.99999999992% the speed of light. You've traveled 80,000 light-years and you're 3% the way to Andromeda. At this speed, your clock is going 80,000 times slower than the clocks back on Earth. You could even stop accelerating, because at this speed, you travel 80,000 light-years in every year that passes for you. So, you could just coast along for 30 years and you'd pretty much be at Andromeda.

Of course, you'd need to decelerate so you don't smash into Andromeda at nearly the speed of light, but that'll only take about 12 years. So now, after your 54-year-long journey, you are at Andromeda. Of course, because of time dilation, it's 2.5 million years later back on Earth, but who cares about Earth? You have a whole new galaxy to explore.

OK, so up until now, this has all been theoretical. But you can actually do an experiment that proved to scientists that this time dilation stuff is actually real. Just like people, subatomic particles have a lifetime. And just like how our aging and inevitable decay is slowed by moving really fast, that's also true with particles. So, take a look at the streaks in this cloud chamber. And by the way, you can do this experiment at home and I'll put the link to the video explaining how to do that in the description.

Many of the particles creating these streaks are called muons. They're just like electrons but heavier and more ethereal. When stars die, they can explode, shooting pieces of atoms across the universe. Some eventually reach Earth's atmosphere. Here, 10 miles above the Earth's surface, they collide with molecules in the air, bursting into showers of subatomic particles, many of which are muons. Now, muons have a lifetime, or half-life, that's less than a millisecond. It's 10^-6 seconds. This means that 99 percent of them decay just over a mile into the atmosphere.

On average, you should see only about one every nine hours. Instead, well, see for yourself: there are dozens every second. Since they're moving at 98% the speed of light, time--and their aging--has slowed. Now, of the thousands that are colliding with the atmosphere several miles above the detector, 5% of them make their way down to the cloud chamber before they decay, in the same way that you made it to Andromeda before you decayed.

It's pretty cool that you can look into a jar and imagine a future where intergalactic travel is actually possible.

For more on why that happens, check out my previous episode, where I explain how time dilation actually makes up for the fact that light must always move at the same speed.

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Host, Producer
Greg Kestin
Greg Kestin
Research, Writing
Greg Kestin
Samia Bouzid
Editing, Animating
Daniel Thomson
Editorial Input
Julia Cort
Ari Daniel
Media: Shutterstock
Special thanks
Entire NOVA team
From the producers of PBS NOVA © WGBH Educational Foundation
Funding provided by FQXi
Music provided by APM

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