Speaking of Hubble...

Telescopes as Time Machines

August 31, 2012 by Tracy Vogel
Credit: T. Rector and B. Wolpa (NOAO/AURA/NSF)

The light from our neighbor Andromeda is 2.5 million years old. CREDIT: T. Rector and B. Wolpa (NOAO/AURA/NSF)

Hi everyone. My name is Tracy Vogel, and I’m a writer and editor for the Office of Public Outreach at the Space Telescope Science Institute. I’m also the person who answers e-mail for HubbleSite. You know how you shoot off an e-mail to HubbleSite and expect it to get answered by some kind of automated robot? That’s me! I’m the robot! Hi! It’s very exciting to meet you all, and to finally be able to point out that Hubble, a non-sentient telescope floating through space, neither needs nor possesses a LinkedIn account. Though I’m sure it would thank you all for the many, many, many gracious invitations.

Unlike the other members of this blog, I’m not an astronomer. But I do spend a lot of time asking them questions. “Can Hubble be seen with the naked eye?” “Do we know the age of the universe?” “Are we scheduled to observe that giant invisible death planet that’s going to crash into the Moon?” (The answers are, respectively: “Yes,” “13.7 billion years,” and “Can I see your employee ID, again?”)

Why do I need to know these things? Well, it’s probably my unquenchable thirst for knowledge. Or my massive, overflowing inbox of e-mail queries. One of those! And since the questions we get are actually quite interesting, and at least a handful are not written in all-caps by people who appear to be typing by gnawing on the keyboard, we thought we’d share them with you. To elucidate you, the public, with your inquisitive minds and your curiosity-driven missives and your obvious homework questions that you’re clearly hoping I will do for you so you can get back to playing Mario Kart. (Yes, of course all the answers are “The Crab Nebula,” hopefulsixthgrader@gmail.com! You can trust me! I work for science.)

Anyway! Occasionally I’ll be jumping in on this blog to discuss some of our more common or uncommon questions, or Hubble news of note. Because they are paying me, and our public demands it. In between bouts of Mario Kart.

Today’s question is one we get fairly often: You keep talking about seeing galaxies in the past. How does that work, anyway? The past is, well, past — thus the name. How does having a telescope let you see something that happened millions of years ago?

To understand this, you first need to know something about how a telescope works. Telescopes provide you with a bigger lens than the ones you have in your eyes. Because it’s bigger, it can capture more and much fainter light than your eyes can on their own. But telescopes don’t reach out into space — they stay right here (or 353 miles above here, in Hubble’s case) and wait for the light to hit.

Light moves. It moves extremely fast — at about 186,000 miles per second — so it looks instantaneous when you flip a switch, but it takes time to travel across the vast distances of space. Light from the Moon takes about 1.3 seconds to get to Earth, so when you look up at the sky, you see the Moon as it was over a second ago. Light from the Sun takes 500 seconds to reach us; the sunlight hitting your face is over eight minutes old.

Other galaxies are so far away that we actually measure the distance to them in the time it takes light from them to reach us — the “light-year.”

The nearest large galaxy, Andromeda, is 2.5 million light-years away, thus its light takes 2.5 million years to reach us. When you look at Andromeda, you’re seeing ancient history. And if you lived in the Andromeda galaxy, and you were looking at the Milky Way right now, you’d be seeing this galaxy as it was 2.5 million years ago. Wave hello to Earth’s Glyptodonts, Andromedeans! They won’t wave back, as they’re deeply, extremely, enthusiastically extinct, but that’s no reason to be rude. Upon further reflection, it seems like a good reason to be extra polite. A moment of respectful silence, everyone, for the Glyptodonts. I think we can all agree that the world would be a better place today if it still had Volkswagen-sized armadillos. Thank you.

Anyway. The point is that the farther away an object is the longer it takes for the light to reach us. So the light from the most distant objects we can see started traveling billions of years ago, when the universe itself was young.

So that’s why Hubble can look at deep space and see objects as they were far in the past. Doesn’t seem quite so odd when you realize you’re doing it yourself, every time you look at the stars, does it?