Date: October 10, 2009

Title: I Know Why it’s Dark at Night. Do You?

Podcaster: Terry Johnson, Jeremy Lusk, and Friends

Organization: http://infinitewell.wordpress.com

Description: 100 years ago astronomers pondered a question that they knew would eventually be solved, but the technology of the time wasn’t capable of producing the answer. Olbers’ Paradox was logical at its core, which meant something was very wrong with the way we were viewing the universe. We are going to trace the steps that scientists and others took to solve this famous problem in a forum similar to NPR’s Car Talk.

Bio: Terry Johnson is a high school science and math teacher living in rural Arkansas. He has spent many nights under a dark planetarium dome as a writer, producer, programmer, and technician.

Jeremy Lusk is a physics major at the University of Central Arkansas. He is presenting at this year’s AAS conference and hopes to one day become a high school science teacher.

This podcast was produced with the help of several of their colleagues and friends.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Physics Department at Eastern Illinois University: “Caring faculty guiding students through teaching and research at www.eiu.edu/~physics/.

Transcript:

“Star Talk”
October 10, 2009 – Olbers’ Paradox

Jeremy: And welcome back to Star Talk. We’ve been talking to Jenny on the phone with a question regarding a meteor she saw?

Jenny: Ummmmm, well, I’m pretty sure it was a meteor. It flew right over my house and then blew a big hole in my lawn.

Terry: And by ‘blew a big hole’, was there a loud bang involved about the time of impact?

Jenny: Well, yeah. Sparks flew up and it was really smoky.

Terry: Do you think you know what it was?

Jeremy: I have an idea which I’m sure is entirely wrong. Do you…

Terry: Well, I think I know what it was, and I’m ready to throw my opinion out there.

Jeremy: You think you know? Just from that?

Terry: I think I do. In fact, I’m willing to go ahead and give my final analysis right now. Unless you have some more questions for Jenny here…

Jeremy: Oh no! I gotta hear this now! You go ahead.

Terry: Well, let me get this straight. You heard a hiss then a loud bang. You turned around to see smoke rising from a small hole in your front lawn.

Jenny: Yeah.

Terry: You found a rock nearby and assumed that it must have been the culprit.

Jenny: Right.

Terry: Yeah, what I think happened here was a classic case of cherrybombae explodicus.

Jeremy: You think a neighbor was pulling a prank on Jenny?

Terry: I think a neighbor was pulling a prank on our dear friend Jenny. What you actually saw, Jenny, was a cherry bomb thrown over your house by some rambunctious neighborhood kids!

Jenny: And not a meteor?

Jeremy: Yeah, my brother and I hardly agree on unusual phenomena, but in this case I’m going to have to go with his theory.

Terry: So we agree that it was not a meteor?

Jeremy: Absolutely. Definitely not something from the depths of space.

Terry: Yeah, I’d say more like something from the depths of some kid’s pocket!

Jenny: I see. Well, there is a 12-year-old living right behind me. I think I may need to give his mother a call.

Jeremy: You do that, Jenny! And thanks for the call!

Terry: Have a great day. Bye now.

Jeremy: Well, now we turn our attention on the brain teaser of the week, where we let the listeners try to answer a question posed by one of their own. This one comes from Aaron Christie from Bucksnort, Arkansas. Aaron writes, “I heard there’s, like, a gazillion stars in the sky. If that’s true, shouldn’t the sky be brighter at night?”

Terry: Wow. A gazillion, huh? That’d be a lot.

Jeremy: Well, I think we all know what he means. He’s saying that there are more stars than we can imagine, which is actually true. But that doesn’t mean that there’re an infinite number of stars.

Terry: Now hold on. How do we know that it’s not infinite? The universe is full of stars. And we haven’t seen the edge of the universe yet, have we? Well, then we can’t go about saying how many stars there are out there.

Jeremy: That’s not the point. The point is there are only a certain number of stars we can see. But that number is huge! And so I can see his question: “Why is the nighttime sky dark if all of these bright stars are out there shining down on us?” It’s a good question.

Terry: It’s a very good question. In fact, whether he meant to or not, Aaron has stumbled upon Olbers’ Paradox which states: If the universe is infinite and stars are distributed evenly throughout, then why isn’t the night sky bright?

Jeremy: Well, you’re not very bright. Are you a paradox?

Terry: Hey, hey… don’t make me come over there! Olbers was simply restating a question that had been raised by Kepler years before which made three assumptions: 1) The universe is steady-state, 2) Stars are evenly distributed throughout, and 3) The universe is infinitely big. If these things are true, then we should have a very, very bright nighttime sky.

Jeremy: Okay, so three things. Universe isn’t getting bigger or smaller.

Terry: Uh-huh.

Jeremy: Stars everywhere.

Terry: Right.

Jeremy: And they go on forever.

Terry: That’s the three things.

Jeremy: And we get a ridiculously bright sky.

Terry: It sounds ridiculous, but it’s completely logical. That’s why we call it a paradox. Evenly distributed stars across an infinite, static universe means that there’s no direction you could look and not see a star. It’s like being way out in the woods and everywhere you look all you can see are trees. And those three things were all assumed to be true by practically everyone when Olbers wrote it 180 years ago.

Jeremy: But since the universe isn’t ridiculously bright, at least one of those things must be wrong. And so I think we should throw it out to our listening audience for a reasonable answer.

Terry: Alright, let’s go to the phones. James, you’re on the air with Star Talk.

James: Hi, guys. I was listening to one of your earlier shows and you mentioned how there are dark nebulas out in the galaxy that block any light from coming through. Wouldn’t that take care of the problem?

Terry: Ah, good point. But what my brother probably didn’t mention, ‘cause he’s absent-minded like that, is that these dark nebulae eventually absorb enough light to start glowing themselves! But don’t feel bad, Olbers offered the same solution himself when he posed the problem.

James: Well, I had another idea.

Jeremy: Throw it out there, James!

James: Stars don’t live forever, so we’re not seeing all the stars at once.

Terry: Good point. Jeremy?

Jeremy: You’ve got a good idea, but I think your numbers are a little bit off. You have to consider that if enough stars could make the universe bright, then a fraction of that should make the universe somewhat bright, and it’s just not.

Terry: Wow. You shot him down pretty quickly.

Jeremy: Well, we had only 10 minutes for this podcast, so I don’t have the time to let him down easy.

Terry: Right! Next caller.

Jeremy: Well, next up we have Jim Zepowski, an expert in astronomical history.

Jim: Hey, guys. You know, it’s just a hobby.

Jeremy: Aww, now don’t be modest! So can you help us with this paradox?

Jim: I think I probably can. You know, this has been something we’ve puzzled over for centuries. At the time Olbers wrote his paradox, he wasn’t coming up with some random conditions. Those three conditions were assumed to be true by practically everyone – for both scientific and religious reasons – which is why his paradox has persisted.

Jeremy: And do you know which of those assumptions turned out to not be true?

Jim: Well, the universe certainly isn’t steady-state. Edwin Hubble made the astronomical discovery of the century when he found that it was expanding. Well, the news was so jarring that even Einstein refused to accept it. People were just so convinced that the universe had always been here.

Jeremy: Wow. People really get hung up on science not evolving, huh? So, does this take care of the paradox? One of the conditions fails; therefore the conclusion can’t be true.

Jim: Well, that’s not the end of it. Just because the universe is expanding doesn’t mean those multitudes of evenly spaced stars aren’t out there for us to see.

Jeremy: I see, I see. So what you’re saying is that you haven’t helped us a bit!

Terry: Yeah, umm, thanks for the call, Jim. It was really…

Jim: Now hold on. I’m not quite finished with everything. The fact that the galaxies are mostly speeding away from each other does help us out. Think about how long it would take for light to reach us from a star very far away. If the universe had always been smaller, those stars would be closer and more tightly packed. The sky would be much brighter.

Terry: So does that mean when the universe was younger it was brighter?

Jeremy: Well I’d say you were a lot brighter when you were younger!

Terry: Seriously, how many other brothers do we have that might make a good replacement for you? I’m sorry, we’re off track. Jim…?

Jim: I’m not sure about the universe getting dimmer, but it seems reasonable. Not only does the expansion spread things out, but it can actually remove objects from our sight.

Terry: You’ve got to say that again in a language that even my brother can understand.

Jeremy: Yes, please!

Jim: The Universe has a sort of ‘horizon’ that we can’t see beyond. This is the edge you guys were talking about earlier.

Jeremy: Ah-ha! So the universe does have an edge. And I, again, am right and you were wrong.

Terry: Now, now. Wait a minute! What’s this about an edge?

Jim: It’s not really an edge. It’s a horizon. It takes time for the light to reach us – a lot of time from objects way far away. Light has a speed limit but the expansion is even faster out there. Anything beyond a certain distance will never be visible to us because the light from those objects can’t outrun the space-time expansion. This prevents us from ever knowing how far the Universe actually extends, and keeps the visible Universe a finite size.

Terry: So the Universe could go on forever and we’d never know it.

Jim: Eh? Who knows? We might find a way of figuring out that answer some day. Scientists view a good question as a challenge.

Jeremy: And as far as good questions go, we have another caller on the line with a question for Jim! Lauren, welcome to Star Talk.

Lauren: Hi, guys. Hi, Jim. Jim, why did it take so long for people to connect the dots? I mean, obviously the sky isn’t bright at night, so why couldn’t some of this have been predicted as an explanation from the beginning?

Jim: Well, some folks did offer these explanations; they were just ignored because of generally accepted beliefs about the nature of the universe that we’ve since proven to be wrong.

Terry: Like the moon is made of cheese!

Jeremy: No one believed that.

Terry: I believed that. Apollo 11 was such a let down…

Jim: Hey, you guys mind if I handle this one? In the mid 1800s this would have been the question on people’s minds. It may not be as famous today, but just like we all have an opinion about the question of global warming, a hundred and fifty years ago folks liked to weigh in on their thoughts about the infinitely-bright-sky paradox.

Jeremy: And Hubble cracked the code.

Jim: Oddly enough, the solution was offered way back before Hubble was even born by Edgar Allen Poe.

Terry: What?

Jim: Edgar Allen Poe.

Terry: No way! The Raven guy?

Jim: Why do people always say that? Yes, the poet. Turns out he wrote a piece of prose called Eureka that explained the solution beautifully. In a nutshell the poem claimed that the Universe just isn’t old enough for all the light in it to have reached us yet. He did not directly contradict any of the suppositions – an infinite number of stars in an infinite, unchanging Universe – yet he offers a loophole. What this required was a beginning to the universe. His idea of the Universe having a beginning gives us a good starting point for the explanation. It also gives us the basis for having a universal horizon. He wrote this in 1848, 80 years before Hubble would show conclusive evidence for a beginning to everything.

Jeremy: So, if the idea that the universe had a beginning mostly solves the paradox, why did it persist through the rest of the century?

Terry: Well, I would think that such a claim going against conventional wisdom, especially if the source wasn’t a noted scientist, would be ignored by most people.

Jim: It was ignored by everyone! No one paid attention to Poe just like no one paid attention to Hubble at first because folks were so convinced that the universe had always been here. And there’s a whole discussion here of religion versus science that we don’t want to get into, but let’s just say that all were opposed to Poe’s idea. Which, we now know, was exactly right.

Jeremy: So that does it then! Every one of Olbers’ conditions failed. The universe hasn’t always been here, it undergoes constant change, stars aren’t uniformly distributed, and space isn’t infinite.

Terry: Or, at least it can’t be proven to be infinite! Whether it is or not, we don’t know because of limited speed of light.

Jeremy: Right, right. The visible universe isn’t infinite.

Terry: Yeah, that does it. Well, we’d like to thank our callers today – especially Jim with his near encyclopedic knowledge on the subject and, of course, Aaron from Arkansas who brought up such a fantastic question!

Jeremy: And there it is, you’ve wasted another perfectly good hour listening to Star Talk. Star Talk was written and produced by Terry ‘We don’t need no stinking badges’ Johnson

Terry: and Jeremy ‘We’ll figure out a funny nick later’ Lusk.

Jeremy: Cast includes Jenny ‘Stop Looking at me like that’ Temple, Kathleen ‘The Drama Queen’ McMurray,

Terry: James Aaron ‘I-for-one-welcome-our-new-robot-overlords’ Christie, James ‘Pocket Rocket’ Tidwell,

Jeremy: Lauren ‘The Hammer’ Jacobs, and Jim “These shoes are not girly” Ross.

Terry: And though Martin Radcliffe cringes when we say it, you have been listening to 365 days of astronomy dot org.

End of podcast:

365 Days of Astronomy
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