Questions Show – light speed, Andromeda galaxy, dark matter and black holes

Another week, another roundup of your questions. This week listeners asked: will reaching light speed destroy the Universe? When is Andromeda going to look really, really cool with the unaided eye? Why didn’t dark matter all turn into black holes? And there’s even more. If you’ve got a question for the Astronomy Cast team, please email it in to and we’ll try to tackle it for a future show.

  • Show – light speed, Andromeda galaxy, dark matter and black holes (17.07MB)
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    Will traveling at light speed destroy the Universe?

    Will Andromeda be ever be easily visible (and look really cool!) in our night sky?

    Will proton beams in the LHC be going twice the speed of light?

    Does dark matter collapse into black holes?

    Are black holes spinning?

    Is it possible the Big Bang wasn’t the beginning of the Universe?

    Transcript: light speed, Andromeda galaxy, dark matter and black holes

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    Fraser Cain: Welcome to the AstronomyCast Question Show. This is where we answer your questions about Space and Astronomy. All right – second week Pamela.

    Dr. Pamela Gay: Second week – we’re going to try and keep doing this as long as we can, so keep sending us the questions.

    Fraser: Yeah, the questions came pouring in. [Laughter]. We got quite a few in the last time. This week listeners asked: “Will reaching the Speed of Light destroy the Universe? When is Andromeda going to look really, really cool with the unaided eye? And why didn’t Dark Matter all just turn into Black Holes?”

    There’s more. If you have a question for the AstronomyCast team, just e-mail it in to and we’ll try and tackle it in a future show. Let’s get on with the first question.

    Mohammad Murro?: When you reach the Speed of Light the Mass of an object increases to infinity which means that you would create Infinite Gravity. Causing Infinite Gravity, objects would accelerate towards that body approaching the Speed of Light. Would this cause the Universe to implode?

    Fraser: I think I need to sorta break this up. We’ve got some Spaceship and it’s traveling faster and faster. As we know from Relativity as a Spaceship goes faster it takes more Energy, gains in Mass and as you approach the Speed of Light, the amount of Mass and Energy required approaches infinity. So I guess that’s our first problem, right?

    Pamela: [Laughter] it’s just a small one, no big deal.

    Fraser: Yeah, like is that even possible?

    Pamela: Well, no you can’t actually get to the Speed of Light because of the whole infinite relativistic Mass problem. There are two different ways to look at the Mass of an object. The first is – what is the Mass that comes out of the number of Atoms it’s made out of? What’s the Mass that just comes from the fact that it exists?

    Then the other Mass that you have to look at is the other Mass that comes out of its motion. You take the Energy of the object and that actually increases its Mass. So the more something is moving, the faster it is going the more relativistic Mass we talk about it having.

    The way the equations work out if your velocity just happens to reach the Speed of Light then you end up dividing by zero, which goes to infinity. Dividing by zero is always a bad thing. Luckily things with Mass can’t get there. It just takes too much Energy to accelerate to that velocity.

    Fraser: So the first part of the problem with this is that the premise is just not possible. If something could reach the Speed of Light then what would happen? Well, nothing can reach the Speed of Light so it’s really difficult to answer the “what would happen” because we’re way beyond what’s even possible by the Laws of Physics.

    That’s fine you know we’ll just throw the Laws of Physics out. [Laughter] Let’s say like “what if” some piece of Mass was going the Speed of Light. Would it have an infinite amount of Mass?

    Pamela: We think so according to the Math.

    Fraser: Okay, so then you would have an infinite amount of Mass, would it then have an infinite amount of Gravity?

    Pamela: Not so much. The equation says yes but what does that mean is the real issue. What it means is there is some sort of a Force and that Force is always pointing at this object that’s moving at the Speed of Light.

    But Gravitation travels at the Speed of Light so if it starts to break down and you have to start worrying about what is time doing and it just becomes really ugly. Even ignoring the ugliness, you’re moving really fast.

    Fraser: Sure, but the key is would the Universe implode? This is the big problem.

    Pamela: No, because where’s it going to implode to, you’re a moving target.

    Fraser: Right but I think the point here is that you are moving the Speed of Light, you have an infinite amount of Mass, and you have an infinite amount of Gravity. Everything in the Universe is going to start to be attracted to you as the speed of Gravity is reaching out from your Spacecraft, right?

    Pamela: The problem with this is though, as you’re zipping along it takes time for your pull to reach objects far out. So say you’re moving along, first the objects nearest you are going to start to move toward that line you’re moving along.

    Fraser: But wouldn’t they be pulled at almost the Speed of Light because your Gravity would be infinite?

    Pamela: But it takes time to communicate the idea.

    Fraser: I understand once the idea gets there…’re flying at Light Speed….

    Pamela: But they’re all going to get pulled in different directions because they’re going to perceive you at different places at different times. You’re going to end up perhaps yanking things around. But you’re not going to yank everything in the Universe to a single point. You are a moving target and it takes time for the information about where you are to get to distant objects.

    Fraser: Right so you can imagine almost this wash of Gravity emanating out from your Spaceship that is starting to yank on things but you’re still moving at the Speed of Light and Gravity is moving at the speed of light. I get it.

    Pamela: It’s really messy and ugly and can’t ever happen. Every time we throw out a new Law of Physics it still doesn’t help us get to the making the Universe implode problem.

    Fraser: Right, well I guess that’s the end of the thing. Even if you had an infinite amount of Gravity, even if you’re just like sitting somewhere you’re a Black Hole with an infinite…

    Pamela: And you can’t get there.

    Fraser: Sure, but let’s say you’re a Black Hole with an infinite amount of Gravity. You still wouldn’t actually destroy the Universe would you? Wouldn’t you just suck all the Matter towards you?

    Pamela: Are you still moving at the Speed of Light?

    Fraser: No, no let’s say we’re just sitting there.

    Pamela: So if you were just sitting there and you somehow have an Infinite Force of Gravity – not that we can do that – yeah, eventually you would crush the Universe down into yourself.

    But it would take a long time and I’m not sure that there’s stuff that wouldn’t be beyond the visible edge of the Universe that wouldn’t know about you.

    Fraser: Right because of the speed of your Gravity.

    Pamela: Then you start to get to questions of: “Are there things where the Universe’s expansion is carrying them away from you faster than you’re communicating toward them and you can just never get the information there.”

    Fraser: Right. But even so and you’ve got all this Matter that you’re crushing into a single point, but don’t you have Space itself still sitting there? It’s like you’re wiping the slate clean of stuff. [Laughter] But the slate is still there, right?

    Pamela: Right that’s true too.

    Fraser: So the Universe isn’t imploding, you just have a…..

    Pamela: You’re just vacuum cleaning.

    Fraser: Yeah, you’re cleaning up the Universe of everything, Light, Matter, you name it in an ever expanding sphere but the Universe is still there.

    If your Super Black Hole got destroyed you could just throw more Matter out there and everything would be fine.

    Pamela: Exactly. It will evaporate eventually.

    Fraser: Right. So let’s just run through this. You can’t move faster than the Speed of Light.

    Pamela: No.

    Fraser: So there’s no way to move a Spaceship faster. It’s the whole question really isn’t a problem. But even if you could be going the Speed of Light, yes your Gravity would increase but you wouldn’t be sucking the whole Universe towards you because you’re a moving target.

    And finally, even if you could pull the whole Universe in on yourself, you’d just be pulling the Matter and the Energy and Space itself would still be there.

    Pamela: Yes. You got all of it.

    Fraser: Okay, let’s move on then. So, sorry you can’t destroy the Universe this way, Mohammed, try a new way. [Laughter] This one comes from Noreen Gwilliam. She wants to know, right now with Andromeda it just looks like a little spot in the Sky that you can kinda just barely see when you’re not looking at it. But will there be a time when we would be able to see it as a beautiful grand Spiral with the unaided eye as opposed to its current little fuzzy bit?

    Pamela: Not really. It’s one of the really sad things about extended objects. Right now the Andromeda Galaxy takes up about three degrees across the Sky if you can trace it out really nicely with a telescope. We only see the very core of it, the very center because that’s the only part that is really bright enough for our eyes to see.

    The reason that we have trouble seeing the rest of it is this thing called Surface Brightness. You take all the light, smear it out over a large area and suddenly it’s not as bright. It’s sort of like when you take your flashlight and you un-focus it. If you have a Mag light you get a bigger beam, but that bigger beam is a lot fainter because you’re spreading the light out over a larger area. Well as Andromeda moves towards us it’s going to get bigger and bigger on the Sky. If you add the distance to it, it’s going to sort of kinda take up twice the angle on the Sky.

    I say sort of kinda because you’re supposed to only do this with Small Angle Approximations and I’m ignoring that. So you’re spreading out the light over a larger and larger area as it gets closer and closer to you. Because you’re bringing the object closer it does appear brighter. In this case it appears brighter as a function of the distance squared. If you have the distance it seems four times brighter.

    Andromeda is so faint compared to what our eyes can see when you start looking out at its arms that basically you reach the point that Andromeda is destroying our Galaxy before you start to be able to see its pretty Spiral structure. In the process of destroying our Galaxy, it’s destroying itself. In order for it to be close enough for us to see its arms really well, we have to destroy it.

    Fraser: So let’s imagine that we weren’t sort of on the Milky Way with all of its Gravitational structure and we were just flying in a Spacecraft. Would there be some moment when we would be able to look out the window at Andromeda and go: “oh, pretty Galaxy.” Without the telescope attached to the Spaceship.

    Pamela: So the way the Magnitude System works is you make something about a Magnitude brighter for every two and one half times closer you make it. Currently the arms have a surface brightness of about 13. To get them to Magnitude 6, and I’m just going to ignore the fact that the thing is getting bigger on the Sky.

    So let’s say that we were able to bring Andromeda closer to us and keep it the same angle on the Sky – which really wouldn’t happen. You’d have to make it 610 times closer to be able to see those arms. In reality it’s even worse than that because the thing is growing as it gets closer.

    But we’ll start to be able to at least make out individual Stars and so we’ll start to see an area of the Sky that just has extra Stars in it. You have to get it a lot closer to be able to see the arms. This is where telescopes are useful. They’re able to bring things closer in a lot of ways without having actually destroyed the Universe.

    Fraser: I think that people really don’t understand how much different the view of a telescope is from the view of even just your eyeballs. The telescope is brightening distant objects a tremendous amount. Even in a lot of the most powerful telescopes like hobby telescopes, you don’t really see the Spiral arms that well.

    Pamela: Not really.

    Fraser: You could see the Galaxy but it’s only when you start taking photographs and that’s when you’ve got a camera that is gathering up the light over the course of several minutes. In the case of Hubble it will just watch the same spot in the Sky for days.

    Pamela: And the thing to think about is the human eye is about half a centimeter by half a centimeter in the area that it’s collecting light. Your back yard telescope might have a diameter of 20 inches compared to that half centimeter – and I’m mixing units. That’s a huge difference. The amount of light you’re able to gather determines what you’re able to see.

    Fraser: I think a good example is that we’re inside the Milky Way. And still inside the Milky Way, we’re only a few tens of light years from the structures of the Milky Way. You still have to be in a very dark Sky to be able to see even a fuzzy bit in that direction. And it’s every where, right, all around us?

    Pamela: Right, yeah.

    Fraser: Well that is a bummer.

    Pamela: Yeah, but at least we have Large and Small Magellanic Clouds to keep us company.

    Fraser: But again we’re just never going to see much more than fuzzy bits.

    Pamela: Nope.

    Fraser: Let’s move on. The next question comes from Bruce Pulian13:57 and it is a question about the Large Hadron Collider. And everyone knows the Hadron Collider is not operational right now. It will be back in a couple of months.

    Pamela: [Laughter] it tried.

    Fraser: It tried. Bruce wants to know if you’ve got one beam going in one direction around the Collider at 99.9 percent of the Speed of Light and you’ve got the other one going in the other direction at the same speed, when they collide aren’t they going almost twice the Speed of Light? Does this break any rules?

    Pamela: No. This is one of the wonderful things about how Relativity works. If you’re an observer riding along on the Particles in the beam, time slows down for you. You still see even though you’re going 99.9 percent the Speed of Light relative to someone just hanging out standing beside the Detector, you still see light as traveling at the Speed of Light.

    The only way this is possible is if your clock slows down so that the little bit of difference between you and the Speed of Light, the relative velocity between you and the Speed of Light, you perceive that as the standard 300,000 kilometers per second.

    Since the observers on each beam of light see their time slowing down, they look at each other and they don’t see each other as going faster than the Speed of Light relative to each other, just because of changes in the clocks.

    Fraser: So if you could put clocks on the two Proton beams they would experience the time slowing down so that when they looked at the other Proton being moved towards them it would look as if it was coming at them at 99.9 percent of the Speed of Light.

    Pamela: And life would be good.

    Fraser: And life would be fine and so this is the whole thing with Relativity. The speed that you see objects moving depends on your speed and I guess their speed.

    Pamela: And all clocks vary. Time is not a constant.

    Fraser: Right, so time gives. Time will change to make it so that you can never see things move faster than the Speed of Light or even move at faster than the Speed of Light compared to you. Crazy.

    Pamela: Thank you Einstein.

    Fraser: Thanks Einstein. Alright Yu Chung16:21 asks “Given the recent evidence that seems to suggest that Dark Matter only weakly interacts with itself and Gravitation seems to be dominant, why didn’t the Dark Matter in our Universe all collapse into Black Holes?”

    So I guess this a good question. We think that most of the Matter in the Universe is Dark Matter. Like 10 times as much Matter in the Universe is Dark Matter than just the regular Matter like you and me and the Stars and so on. If regular Matter turns into Black Holes and Super Massive Black Holes, why didn’t all that Dark Matter just turn into Black Holes?

    Pamela: I think this is even more complex than that. I think he’s asking why is it that the Black Holes are unable to just suck up all of the Dark Matter. The answer is well, why did the Black Holes not suck up all the regular Matter too?

    Dark Matter does fall into Black Holes. So does regular Matter. But not everything falls in. You have to get too close to get sucked in. Luckily, most of the Dark Matter and most of the regular Matter hasn’t gotten too close to a Black Hole such that the Gravitational Force pulls it in within the Event Horizon.

    Originally the Universe was fairly smooth. The Dark Matter, the regular Matter, was all distributed pretty evenly, with just little tiny fractions of a percent variation in density from place to place. Over time in the first few million years of the Universe things started to clump up. We started to get Stars, we started to get Galaxies and we have data from a project called Cosmos that looks back through the current section of our Universe – the most recent parts of the Universe – and traces out what structures did both the Dark Matter and the Luminous Matter end up forming.

    What they find is the Luminous Matter for the most part is embedded in the scaffolding18:13 of Dark Matter. If we were able to look around and see the Dark Matter, we’d see it forms bigger structures. Those structures pretty much wrap themselves around the stuff that we can see.

    Not always, there are exceptions where there’s Dark Matter someplace that we don’t see Luminous Matter and where there’s over-densities in Luminous Matter that don’t totally track with the highest density regions in the Dark Matter. Some of it does fall in, but most of it is just out there providing us a structure to hang our Luminous Matter on.

    Fraser: But isn’t it also part of the problem that Dark Matter doesn’t have a cross-section that Matter does. Like when two pieces of Matter bump into each other they actually could bump into each other and that’s what can help them kinda get slowed down and drop into a Black Hole.

    But Dark Matter – based on some of the recent evidence – doesn’t seem to have any kind of cross-section. It doesn’t bump into each other.

    Pamela: There is evidence specifically from the Bullet Cluster that Dark Matter does have some sort of interactions. We do see it in colliding systems ending up forming Halos around the colliding Galaxy Clusters.

    But this collision is nothing compared to what we get with Luminous Matter. This implies that the cross-section is really, really small. The best way to think of this is to imagine you have a room full of people and you try rolling a beach ball which has a large cross-section through the room.

    The probability that you’re going to end up hitting a person with that beach ball is pretty high. If instead you try rolling a BB across the room, the probability you can get the BB across the room without hitting a person is much higher because it’s so small that it will be much easier for it to slip between the feet of the people.

    In this situation, instead of having a beach ball and a BB, we have something more like the size of a dust mite – except a perfectly round easy to roll dust mite – that’s trying to roll across that floor. The probability that it’s going to have a collision is really, really low. Collisions still happen.

    Yeah, it is easier for Luminous Matter to go through collisions to get driven into the centers of Galaxies during collisions and thus feed the Quasars at the center and you do see more Luminous Matter falling in because of these collisions and these interactions especially in things like Quasars. Mostly it’s just the Dark Matter is smart enough not to get close to the Black Holes but if it does it can fall in.

    Fraser: Right, so Dark Matter does fall into Black Holes and who knows what percentage of like the Super Massive Black Holes are made up of Dark Matter. There may very well be Black Holes or even Stars or gigantic objects made of Dark Matter. We just don’t know but the evidence seems to be that the Dark Matter is just diffused quite thinly across everywhere.

    Pamela: And we can’t make Stars out of Dark Matter because they don’t undergo fusion reactions.

    Fraser: I’m sorry, a ball of Dark Matter, held together by its mutual Gravity. [Laughter] Yeah. I did an article in Universe Today about that, about a Dark Star.

    Pamela: Cool.

    Fraser: How Dark Matter would snuff out the fusion reactions. Let’s move on to a two-part question from Britt Johnson. When discussing Neutron Stars, give the examples of a figure skater spins faster by reducing the diameter of the spinning form.

    This made Britt wonder about an object as small and dense as a Black Hole. They must be spinning at some obscene rate. “Do the mathematical models predict this sort of speed and is it any significant amount of the Speed of Light?” There is tons of research on this recently so it was a great question.

    Pamela: What’s cool is we’re actually even starting to look for observational evidence of this spinning. Yes, Black Holes should be spinning and this makes the math REALLY ugly because you have to start worrying about all of the effects you get from a spinning relativistic object.

    Fraser: But it’s spinning at an obscene rate?

    Pamela: Yeah. Well think about it, you have Neutron Stars that are spinning at a thousand times a second and we’re looking at things that are spinning way faster than that. I consider a thousand times a second pretty obscene, so yeah, Black Holes – really obscene rates.

    What’s cool is as they’re rotating they actually start to affect the Matter around them. We can’t see the Black Hole and information isn’t allowed to escape from the Black Hole – at least not in any easy to observe with a telescope sort of way.

    What we can see is how the Accretion Disk of material that’s in the process of falling into a Black Hole is affected. You get different temperature gradients. You get different density gradients depending on the rotation of the Black Hole and we’re starting to see these things.

    We’re doing models and we’re now able to prove: “yes, Black Holes rotate.” We just can’t actually measure how fast. The theories say yes, obscenely fast.

    Fraser: Well, if I recall, and I think we did some articles in Universe Today as well, there have been Super Massive Black Holes detected that are spinning at the maximum rates predicted by Einstein. So Einstein made predictions about how fast a Black Hole should be able to spin or how fast relativistically something is allowed to spin.

    There seem to be Super Massive Black Holes out there that are spinning at that speed. So, they essentially cannot spin any faster because of Relativity. So, yes obscenely fast, [Laughter] as fast as is allowed by the Laws of Physics.

    Pamela: And in this case obscenely fast just happens to boil down to: “yeah, it’s pretty near the Speed of Light.” And that’s kinda cool.

    Fraser: Very cool. Okay, then Britt had a second question which is going to be the one that blows our minds [Laughter] “When listening to your shows on the Big Bang and the expansion of the Universe, I was reminded of how Matter changes phase at different levels of density and pressure.

    Is it possible that the Universe’s Big Bang event was simply a point in time where the expansion of the Universe passed the point where it went from some theoretically unknown phase to the form that we recognize?”

    So, is it possible that the Big Bang wasn’t the beginning of the Universe but simply the beginning of the time where Matter’s phase was shifted to some way that we just can’t detect?

    Pamela: Yes.

    Fraser: And then everyone wants to know is this a nonsense question.

    Pamela: No, this is actually a really good question. A lot of different people have been trying to follow up on it. There is this idea that everything is made out of Waves and before our Universe, these Waves were lined up in a froth that didn’t really produce anything.

    But if they line up just right, if they collapse just right, out of all of that you can end up with the Big Bang. It’s sorta like rolling dice until the roll of the dice instead of being some random pattern of numbers ends up being pi, ends up being a perfect sequence of 1 2 3 4 5 6 1 2 3 4 5 6.

    Fraser: Right if you have like a Googolplex number of dice and you roll them and they all came up with one.

    Pamela: And so something happened that triggered the collapse of all of these probabilities in such a way that it produced the Big Bang. This is just one line of reasoning. There are just lots of different ways to try and explain what the heck happened prior to the Big Bang.

    We can’t get there observationally. We can’t get there from here. That’s where the Dragons are. But one of the ways of thinking of it is that wasn’t the starting point. That was just the point at which the Wave Function collapsed and we ended up with the Universe.

    Fraser: Right and once again – I feel like I’m plugging Universe Today – [Laughter] but we’ve done a bunch of those series as well. There are many different ideas and always the evidence is unreachable.

    Theoretically if there is a line that yes, the Universe was different and then something changed and now we have the Universe that we have. Maybe it’s cyclical and maybe the Universe will last forever. It’s really impossible right now to trace any of that line of reasoning for the Big Bang itself.

    Pamela: Turtles all the way down…

    Fraser: Right but why is it impossible to get past the Big Bang?

    Pamela: This is something I discuss on my blog, – the problem is we can’t observe anything before the Microwave Background. Prior to that the Universe was completely opaque and it’s like trying to stare with your eyes through a cement wall to see what’s on the other side. You can’t do it.

    We can’t see beyond that Cosmic Microwave Background to test what were the parameters. We can get certain amount information on the Microwave Background. We can learn a little a little bit out of it by looking at Polarizations by looking a the way the light is aligned in different ways, by looking at slight temperature variations from place to place.

    But you can’t actually go back to the first moment. We can’t get there from here because there’s a cement wall that we just can’t observe it through.

    Fraser: I think an analogy – did you ever watch the Mythbusters show – they show an explosion in really slow motion [Laughter] right? And you can see the explosion and they run it in reverse and you can see all of the stuff coming back from one little car blowing up.

    Then you get just about the car and the car is blowing up and it’s less blown up and less blown up and you get to the point where it’s almost a perfect car and then see the video cuts out. [Laughter] They’re pretty sure that’s where it came from, right? A car blowing up but you couldn’t actually see the first few moments of that.

    That’s the same kind of thing. I think Astronomers are very certain of what the Universe probably looked like in the first few absolute fractions of a second after the Big Bang all the way up until the Cosmic Microwave Background Radiation, but they can’t actually observe it. The Background is there to obscure everything that came before it.

    Pamela: Even with mathematics we struggle because a lot of our Physics doesn’t work when you get to earlier than ten to the negative 40 or something of a second. At those two small times we just don’t know how to get Gravity and Quantum Mechanics to talk to each other.

    The entire Universe just becomes a Probability Function where instead of being able to say: “aha! I see this moving in this direction, that means it came from over there.” Instead I can just say this object came from these 30 different locations and I don’t know which. That starts to get really hard because the Universe doesn’t work in a way our human mind is programmed to cope with.

    Fraser: But definitely the Universe could have been something different and changed to what it is today. We can’t observe it and that leads to the question of what was it before. And what was it before that?

    Pamela: We don’t know.

    Fraser: We don’t know and that’s what makes it so awesome. [Laughter] I love the fact that we don’t know. I think that’s just the greatest thing.

    Pamela: It gives us a job. [Laughter]

    Fraser: Totally and that lets me sorta see the new discoveries pouring in and think about the different ideas and try them on for size. I love it.

    Pamela: It’s a wonderful time to be in Astronomy.

    Fraser: Well, I think we’re out of time this week. We’ll get back next week and do more questions. Thanks Pamela.

    12 Responses to Questions Show – light speed, Andromeda galaxy, dark matter and black holes

    1. Vanmonde September 26, 2008 at 2:08 pm #

      Download is not working!

    2. Goran Nordstrom September 26, 2008 at 3:17 pm #

      ddownload this instead !!!

    3. Astronomy Cast September 26, 2008 at 4:10 pm #

      Sorry – stray character. Now fixed. – Pamela

    4. Kai September 26, 2008 at 5:55 pm #

      Both the link in the second comment and the link in the post are giving me a 500 – Internal Server Error page.

    5. Kai September 27, 2008 at 2:02 pm #

      And…it’s working now. Great show as always, guys.

    6. Dave B. September 28, 2008 at 5:34 am #

      Linky no worky

    7. Dave B. September 28, 2008 at 5:41 am #

      Sorry, I can in fact dl it– it appears to be a Firefox issue for me, works fine in IE.

    8. Nathan September 28, 2008 at 11:23 am #

      Dave B is right both links would not work in Firefox 3.0 but i could download it through IE6 without issue

    9. Andrew September 28, 2008 at 12:05 pm #

      Very interesting show! Thanks.

    10. Yoo September 29, 2008 at 3:14 am #

      Thanks for answering my question about dark matter and black holes! I think I get it now, but can you confirm or deny my understanding that it’s the lack of friction is why dark matter is more diffuse than ordinary matter?

    11. John September 29, 2008 at 1:36 pm #

      I’m not getting these downloaded anymore and my podcast aggregator (j river media center 12) says there’s an “xml field error”. Any thoughts?

    12. SF Reader October 2, 2008 at 12:54 pm #

      Whew! I had to stop listening to this at work and instead listen during my commute. Serious listening needed.



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