Questions Show: Decelerating Black Holes, Earth-Sun Tidal Lock, and the Crushing Gravity of Dark Matter

Artist's impression of a black hole

Artist's impression of a black hole

This week we wonder if you can made a black hole by accelerating a mass, but then can you un-make it again? Will the Earth ever be tidally locked to the Sun? And can dark matter crush an unsuspecting space ship?
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. Please include your location and a way to pronounce your name.

  • Decelerating Black Holes, Earth-Sun Tidal Lock, and the Crushing Gravity of Dark Matter
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    Can you make a black hole by accelerating matter?

    Will the Earth ever be tidally locked with the sun?

    Would the halo of dark matter surrounding a galaxy ever crush a spaceship?

    Since Hubble shouldn’t be pointed at the Sun, why didn’t they put it in a LaGrange point?

    If the speed of light turns out to not be constant, does that imply that gravity is not constant and that the mass of objects far away have to updated?

    Why doesn’t the weak force cause us to decay?

    Pioneer and Voyager are on their way out of the galaxy — is there an galactic escape velocity?

    Are there similarities in the way planets revolve around the sun and electrons revolving around the nucleus?

    Does the Milky Way’s gravity move things in  the same galactic orbital plane?

    Is there a standard speed that planets rotate based on their mass or distance from the sun?

    If we landed on the Moon 40 years ago, why is it so hard to return?

    Transcript: Decelerating Black Holes, Earth-Sun Tidal Lock, and the Crushing Gravity of Dark Matter

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    Fraser Cain: Welcome to the Astronomy Cast questions show. Question show time Pamela.

    Dr. Pamela Gay: It’s questions, our listeners ask scary questions.

    Fraser: Oh yeah, this is the second time we actually have recorded this episode [Laughter] because they were such zingers that we spent time arguing and chasing particle physicists and we thought it would just be best to just start from the beginning. Thanks for that.

    This week we wonder if you can make a black hole by accelerating a mass and then can you unmake it again? Will the Earth ever be tidally locked to the sun? Can dark matter crush an unsuspecting spaceship?

    If you have a question for Astronomy Cast shows, please e-mail it in to and we’ll try and tackle it for a future show.

    The first question for the second time [Laughter]. So Arunus Gidgowdusk from Lithuania asks: “If you took a one kilogram mass and accelerated it close to the speed of light would it form into a black hole? Would it stay a black hole if you then decreased the speed?”

    So if you take a one kilogram mass and accelerate it – because I guess Einstein said that mass increases as you increase your speed and as you approach the speed of light – will your mass turn into a black hole?

    Pamela: This is where I broke because if you’re going along at a constant velocity at really close to the speed of light, if you think really hard about it in one way you end up with a combination of the mass that you get from the number of atoms that you’re made up of.

    The amount of mass that you get because you’re moving really fast and have a lot of kinetic energy and a lot of momentum you get all of these things put together to give you enough gravitational acceleration that depending on how you look at things you might have an event horizon.

    But you traveling at a constant velocity very close to the speed of light are utterly unaware that you have changed in any way. To you everything is normal, life is good.

    Fraser: Let’s see if I understand it. The mass does gain in gravity like when you increase the speed of a particle. Its gravity is both its mass and its energy all mushed up together, right?

    Pamela: Yeah in a lot of complex ways to take into account, kinetic energy, momentum, everything else put together.

    Fraser: I get to over-simplify this stuff, not you. That’s Hawking’s problem.

    Pamela: And Penrose.

    Fraser: Yeah and Penrose’s problem, it is Einstein’s problem. [Laughter]

    Pamela: He created the problem.

    Fraser: You will have gravity but you won’t necessarily turn into an actual black hole?

    Pamela: Right and this is where I just broke several people. So you’re zipping along and if light tries to get to close to you and it does it in just the right way which will become very complex if you’re moving at that fast of a constant velocity, it will orbit you and not be able to escape. If you shine a flashlight the light won’t be able to escape.

    Fraser: But you inside won’t necessarily…

    Pamela: Yeah, it’s not like you’ve condensed down to a quark super something according to the Gedanken experiment. I have done no math. This is so far beyond me it’s not even funny in terms of doing the math.

    Fraser: Just to be clear, this is so far beyond everybody. This is the problem. We researched and nobody has a good answer for if you actually turn into a black hole or not when you approach that speed.

    Pamela: I did find a reference to a journal article but unfortunately the reference I found had been pulled off of Archive X. I was very sad so I’m going to go searching for that later. There was sadness involved.

    Any of you out there who are listening – and I know there are some among you who are actually theoretical physicists – this would make a wonderful journal article. What’s cool about this idea is in theory you could, if there were given enough energy in the universe – and I don’t think there is – accelerate something such that it had an event horizon. Then decelerate it and watch all the stuff that’s gotten caught up around it fly away.

    That’s kind of cool to think about – maybe. It depends on how the math works out. Any of you crazy wonderful theoretical physicists out there who do relativity write the journal article and send it to us. We’ll talk about it on the show.

    Fraser: Or at least give us a hand with a more comprehensive answer. What we found out there was that it was confusing. I think in the original we were like, no let’s not do this question but it is so good. It is such a great question and you broke us. Congratulations you broke us.

    Pamela: You let me leave a trail of broken theoretical physicists in my department who were talking about this but then had to go teach class.

    Fraser: Yeah and will be talking about it tonight. If anyone wants to pitch in and give us a hand, that would be awesome. Just to show you that I guess we found one thing that you don’t know Pamela.

    Pamela: It was beyond my Google.

    Fraser: John 6:05 asks: “Will the Earth ever be tidally locked with the sun? What would this situation look like?”

    If I remember from an old episode tidal locking is the situation with the Earth and the moon where the moon always displays one face to the Earth and the Earth is still turning but the moon has stopped rotating on its axis compared to what we see from here on Earth.

    That is still completing one revolution every time it does a rotation but from our point of view, we always see the same side of the moon. That’s just because the Earth’s gravity is pulling on the moon a little differently.

    Eventually it just sort of turns into this kind of perfect spot. Could that situation be happening with the Earth and the sun?

    Pamela: The moon actually is the dominant force here. We don’t usually get to say the moon is dominant about anything. We have this moon going around and around the Earth and given long enough and the sun will make the inter-solar system completely crispy and consume chunks of it before this happens.

    Given a long enough period of time and given the opportunity to survive the destruction of the inter-solar system by the sun, the Earth will stop rotating any faster than the moon is orbiting. You’ll always end up with the same point on the planet Earth pointed at the same point on the moon and the two are going to go round and round like that.

    Because this isn’t a solid body situation, because these are two different objects going round and round each other, it is that round and round each other that’s going to prevent the sun from being able to grab on to any one particular part of the Earth and torque it toward the sun. So instead we’re going to end up with the dominant force being gravity from the moon.

    Fraser: There we go. But if there was no moon then in theory eventually the sun would be able to do it.

    Pamela: Maybe, then you have to start figuring out what are the torques that you get from all of the other planets in the solar system. That gets complicated as well.

    Fraser: It all comes down to what is the dominant torques, what is the dominant gravity that is happening. If you get too much interference then those rules are out the window.

    Dan McCrow from Everett, Washington wants to know: “Would the halo of dark matter surrounding the Milky Way crush a spaceship with its gravity?”

    We talk about the halo of dark matter being ten times the mass of the visible matter that we can see – that’s a lot of gravity going on. If you went right to the very middle of it, which I guess would be the core of the Milky Way itself [Laughter] would you get crushed?

    Pamela: No, not so much.

    Fraser: I mean not from the super massive black hole.

    Pamela: Imagine you could remove all the luminous matter in the Milky Way. No super massive black hole at the center, no stars, no nothing, just a cloud of dark matter. Here because dark matter doesn’t interact electromagnetically we don’t have to worry so much about two things being in the same place at the same time.

    Co-existence, happy, not futile but this dark matter if you’re in the center of it it’s pulling you in equal directions. Just like when we talked about going to the center of the Earth, you’re good. In general flying through dark matter you’re oblivious to it. You walk through dark matter all the time. It is such a diffuse thing, such a low density thing that in general it isn’t going to crush you.

    Now if you go to the center of some of the densest super clusters of galaxies, there it does start to get to high enough densities where you might have to worry. Within our own Milky Way galaxy if you remove all the normal stuff, if you’re in the very center because this isn’t something that’s going to interact with you via the electromagnetic force and you’re at the center of the gravitational field, you’re fine.

    Fraser: It’s almost like that same question if you fell down into the middle of the Earth would you be pulled apart from the Earth’s gravity? No you’d float around weightless.

    This is kind of the same thing you would balance out the gravity pulling you in all directions but it wouldn’t be overcoming the molecular bonds of your spaceship.

    Pamela: So you should be fine.

    Fraser: You should be fine. Go ahead and try it. [Laughter] Move all the stars, super massive black hole, pull everyone from the galaxy fly your spaceship to the middle of the galaxy.

    We’re pretty sure you’ll be alright but you let us know if there is a problem, but don’t blame us. [Laughter]

    Pamela: You’ll get accelerated nicely toward the center.

    Fraser: Ross German from Sunshine Coast, Australia asks: “I was wondering about the position of the Hubble Space Telescope. I saw in a documentary that it can cause severe damage if it is pointed towards the sun. I was wondering why they didn’t position it at the Lagrange point opposite the sun. Wouldn’t that have solved a lot of the associated thermal expansion problems?”

    Yeah, this is true. There are no pictures of the sun taken by Hubble. In fact they are very careful to not point it anywhere near the sun because it is bad for the equipment. Lagrange points, these positions around gravitational objects that are stable. You could put something there and it will sit happily. There is one on the opposite side of the Earth so in theory I guess what Ross is wondering is couldn’t you just use the Earth as a sunshield and hide behind it?

    Pamela: The problem with sticking the Hubble Space Telescope or anything out in the Lagrange Point that is on the other side of the Earth from the sun is that Lagrange Point is out past the moon. While we can get there, it is hard. Once we get something there it is hard to fix.

    The Hubble Space Telescope was originally designed to be taken up and down by the space shuttle on a regular basis to swap out the instruments. We can’t take a space shuttle beyond the moon unless we’re doing a really bad science fiction movie. NASA is not a really bad fiction movie for which we’re all grateful. That’s a good place to put stuff.

    That’s actually where they’re going to put the James Webb Space Telescope. It is not the place that the Hubble Space Telescope was designed to go. It is just a matter of what is the purpose of the craft you’re putting into space and what is its future.

    Hubble’s future was getting new instruments now and then. James Webb Space Telescope is going up and we’re not touching it once it is up there so it will go out there.

    Fraser: W-MAP is there. That was one of the reasons they wanted to keep it away from the Earth. They wanted to give it a nice cold dark place where it could have a nice clear view of the skies and not have all of the microwave radiation coming from the Earth to mess up its really nice clean sensitive view of the cosmic microwave background radiation. It’s a wonderful place to put spacecraft absolutely.

    Pamela: We’re going to be putting Planck there in a couple of weeks. Herschel is going to be launching and getting put there hopefully.

    Fraser: James Webb, yeah. I think you’re in the right direction. It’s just that we weren’t there yet with Hubble but we’re there now.

    Rick Johnson from Charlotte, NC asks: “If the speed of light turns out to not be constant does this implies that gravity has not been a constant and that the mass of objects further into the past have to be updated?”

    So we think that the speed of light is constant, right? It has been constant since the beginning of the universe.

    Pamela: That is what we think. There are people out there who think that maybe it has been variable. That’s not canon and we don’t have evidence of it that I can tell you.

    Fraser: Do we think that the force of gravity has been constant?

    Pamela: Yes, we do think that.

    Fraser: We have lots of evidence of that.

    Pamela: If the speed of light turns out to not be constant I don’t know if that means that gravity would not be constant. I don’t think the two of them are tied together that closely. Mass doesn’t have to do with gravity it has to do with the amount of stuff, the amount of coupling with the Higgs bosons within the stuff has to the scale or field that gives things mass.

    You can tweak the speed of light, you can probably tweak gravity. You’d actually need to tweak the Higgs boson in the coupling to the scale or field in order to change something’s mass. The three aren’t necessarily coupled to each other.

    Fraser: It is unknown if the speed of light has been changing over time although most people don’t think so. It is not sure if there is a connection between gravity and the speed of light; interesting question. We have no idea. This is the no idea edition of the questions show. [Laughter]

    Pamela: This is the Fraser and the listeners trying to kill Pamela edition.

    Fraser: We could just shorten this right up and go: “I don’t know.”

    Matthew Camp asks: “Why is it that the weak force doesn’t make us all spontaneously decay into other elements via beta decay?”

    This is the thing that has radioactivity, right? You get plutonium and it turns into a collection of other particles or other atoms and releases energy while this is happening. Why doesn’t it just happen all the time to all of our particles?

    Pamela: Luckily, the weak force is indeed very, very weak and the probability of anything decaying at any given moment once it is tied up into the nucleus of an atom is pretty high. In order for the weak force to be able to have an effect you need to have a bunch of neutrons hanging around.

    If you take a random neutron and stick it on the shelf and just let it sit there not tied up into an atom so I’m not quite sure it is sitting on the shelf. Pretend with me. After about 15.4 minutes – just to give you an approximation – it will due to the weak interaction decide it wants to be something else.

    Things like protons which have a slightly different structure appear to be for trillions of years completely stable. Most atoms are in fact completely stable. It is only the atoms that have either an unbalanced nucleus or way too many neutrons – which are a different form of unbalanced nucleus – that undergo the weak force and beta decay.

    It is just a matter of make things energetically stable, what makes them unstable and it is in the unstable case that you start getting a weak interaction actually able to overcome everything else going on in the nucleus. If you want to learn about the weak force there’s a whole episode on it.

    Fraser: Yes but you’re not going to fall apart into a sort of a spray of energy and particles. So just don’t worry. It hasn’t happened yet, right?

    Pamela: Right and don’t leave neutrons on shelves. They won’t be there 16 minutes later.

    Fraser: I’ll keep that in mind – short shelf life. [Laughter]

    Eric Magnussen from Sacramento, California says: “Pioneer and Voyager spacecraft are on their way out of our solar system. Is there such a thing as a galactic escape velocity and will Pioneer and Voyager escape the galaxy or be very long period galactic comets?”

    Pamela: They should actually stay within the galaxy.

    Fraser: So, there is an escape velocity of the galaxy?

    Pamela: Yeah and what’s kind of cool and kind of mysterious is we’d thought for a long time that the large and small Magellanic Clouds (which are two baby galaxies out on the edge of our own Milky Way galaxy) we thought that they were actually orbiting us.

    Now it looks like they’re actually going faster than the escape velocity of the Milky Way. They’re just going to fly right on by and leave behind a spray of stars in the process.

    Fraser: We’ve actually done articles about white dwarfs getting kicked by some partnership with a supernova and they have an escape velocity that will escape the galaxy. The title is like: “White Dwarf, Speeding out of Galaxy.”

    Pamela: Right and you can get three body binary systems in globular clusters that will fling things out and occasionally things will escape. But happy little Voyager and Pioneer, no they’re just going to stay within the Milky Way.

    Fraser: Would they make a long period comet? I guess they’ve obviously not balls of ice but [Laughter] will they return?

    Pamela: They will be just another thing orbiting the super massive black hole in the center of the galaxy.

    Fraser: Cool. David Farmer asks: “I’ve always wondered if there are similarities in the way the planets revolve around the sun and electrons revolving around the nucleus. Has there ever been any work done to compare the two?”

    You see this quite a bit in science fiction stuff that a solar system or even a galaxy with all of the stars orbiting around it is very similar to this proton, neutron core with electrons buzzing around. Are they the same thing? Are we living in an atom? [Laughter]

    Pamela: No. This is actually one of those things that led physicists working to try and unify all the forces down many a wild goose chase. When you look at the equations that describe the electrostatic force between two happily sitting on a shelf protons, a proton and an electron that aren’t orbiting each other. When you look at the force of these two non-moving balls of charge, the equation is very similar to the equation that you have to describe two masses attracting each other.

    You have a constant. You have some characteristic of the object, the charge, the mass, times the same characteristic of the other object to charge the mass divided by the square of the distance between them. We use similar vocabulary for the two. We talk about electrons orbiting the nucleus of an atom. We talk about planets orbiting stars.

    The catch is a planet can orbit at any distance that it has the speed to stay at going around a central star. I can stick a planet a few thousand kilometers from the surface of the sun. It will have to be moving really fast, it will get fried a lot. But I could do it. I can stick a planet 40Au from the sun and call it Pluto and then decide it is not a planet a few years later.

    Any of these different distances is perfectly happy, perfectly stable as long as you have the right speed. You can get gravitational orbits of all different ellipsoidal characteristics. You can have something that gets really close to the sun on one side and really far from the sun on the other side. That’s what comets do. You can have a variety of different energies of orbits.

    With electrons orbiting protons you have similar physics involved. In terms of you have an attractive force; you have to start worrying about the affects of being in a circular motion which means you’re constantly having an inward acceleration. But, electrons are only allowed to have very discreet energy levels. The energy is quantized.

    The other thing is you can’t just grab the electron in its orbit and make predictions of where it is going to be on the other side of the orbit. The orbits are actually probability functions where you say there is a probability the electron is going to be in this shell around the proton. There’s a probability it will be in this other shell. You have non-discreet locations and a probability function.

    Fraser: Right that picture of the bundle of protons and neutrons with the electrons making these little spinney orbits around it is not true.

    Pamela: No.

    Fraser: It is a gauzy cloud of possible locations for where the electrons could be. Although they have to be in a specific region as you said the energy level where they are is impossible to determine.

    Quantum theory is in many cases so different from classical gravity and momentum and all of that. Once you really delve into it, they’re just really, really different.

    Pamela: The problem is that the equations look very similar. We use similar words and a lot of people thought well they must be similar and have tried very hard to unify gravity and the electric force. As near as we can tell you can’t get there from here.

    Fraser: Right but I think this is people have looked at it said that looks similar. Let’s figure out if it is the same and have failed.

    Pamela: Yeah, it’s sad.

    Fraser: Moving on, Michael McLaughlan from Denver, Colorado wants to know: “Does the Milky Way’s gravity move planets in the same galactic orbital plane and direction of motion when you’re viewed from space outside the Milky Way?”

    The question was a little vague but I think what Michael wants to know is, are the systems lined up with the Milky Way itself. Imagine if we look at our own solar system. The orbits of the moons are in line with the orbits of the planets. The planets are all in one big flat disc.

    For the most part all of the motion in the solar system is all in one flat plane. Is that happening with the whole Milky Way? Are the stars that are in it aligned in the same way as the overall things? Is it records within records all lined up?

    Pamela: No. In fact you know this is true because the disc of the Milky Way in the sky doesn’t follow the ecliptic of the sun. If you look at all of the places in the sky that the sun appears, all the constellations of the zodiac – Sagittarius, Scorpio, Pisces, whatever, that’s not where the disc of the Milky Way is located.

    In fact we have within our own solar system examples of things that are out of tilt. If you take a look at Saturn its rings aren’t always aligned flat with the disc of the solar system. We do have things that have all different alignments.

    This is actually kind of cool when we’re looking out across to other solar systems scattered throughout. It means that we can find in all directions of the sky systems where the planets are passing in front of their stars.

    Fraser: And systems where we’re seeing them face on. And sometimes we’re seeing them along the top of the star. Sometimes we’re seeing it face on. Sometimes we’re seeing something in-between.

    Which is great because then we can see, times when we look right down jets and there are times when we see jets sideways. There are times when we see protoplanetary discs or aligned to us or from the side. I guess for science purposes it is great that we get to see everything differently.

    Pamela: While there are overall bulk motions within all the systems, there’s a bulk motion of everything orbiting in the same direction that’s big enough within our own solar system. There’s the occasional comet that decides it is going to go the opposite direction. While there are bulk motions within the Milky Way where pretty much all the stars are orbiting in one direction, there are exceptions.

    We have bulk motions but there are exceptions to everything. When you look at small things in any of the systems what are the moons doing around individual planets for instance. What are the planets doing around individual stars? There are all sorts of neat chaos and that’s what makes studying astronomy fun.

    Fraser: Amy Russell from Delaware asks: “Is there a standard speed that planets rotate based on size or whatever?”

    So I guess if we look how even in our solar system is there some speed that planets rotates based on their mass or their distance from the sun or anything?

    Pamela: Nope and in fact we can look out at Venus and it is orbiting disturbingly slow.

    Fraser: Backwards.

    Pamela: Yeah and Mercury is partially tidally locked. Then we have Jupiter spinning rapidly enough that we can watch over the course of the night the red spot drift across the surface of the planet. We have all of these different speeds across objects of a whole variety of different sizes.

    Fraser: Jupiter is the biggest planet in the solar system and it rotates the fastest. It is actually bulging out because it is rotating so quickly. There just is no rhyme or reason to it. I guess what contributes then to this speed that a planet is rotating?

    Pamela: It is the whole history of the object. What was the angular momentum that it got during the formation while it was part of the protoplanetary nebula? What was the series of interactions it went through with other bodies? How much angular momentum has been stolen by moons and close collisions with other planets?

    All of these different effects gang up to affect the rotational rate of a body. Then you have to start worrying about the tidal affects where it is thought that part of the reason that Venus rotates so slowly is because of the tidal affects that it experiences due to the other planets in the solar system.

    Fraser: So you’ve got the formation speed, the interaction changes, and collisions with objects speeding it up or slowing it down, there is no rhyme or reason to it.

    I think this is going to be our last question and our best one. Newt Gordon from Northampton, England asks: “If you landed on the moon 40 years ago, why does it seem like such a challenge for the space-going nations to get back when technology has evolved so much since then? Is it possible that we never landed there in the first place?”

    Pamela: We landed on the moon!

    Fraser: We landed there! We landed on the moon! Yes, so we’re not even going to go there.

    Pamela: And Buzz Aldrin will punch you in the face if you say it.

    Fraser: Exactly. Feel free to read Phil Plait’s books and browse his site and come over to the Forum. We’ve got a million conversations on it. We WERE there.

    Pamela: We could probably get back there in less than a year if we had infinite amounts of money. This is the problem of so in my house we made the mistake of having a leaky roof. Not really our fault, it just happened.

    We have the technology to repair the roof and re-drywall and put insulation up. We repaired the roof about a year ago and we had to tear all the drywall out of our attic down. It is a year later and our attic is one-third of the way re-dry walled and completely re-insulated. We have the technology, we have the money because we had to save it, and it has been a year. What we don’t have is the time to actually go and do it because we’re two humans.

    NASA has thousands of humans but they’re busy doing all sorts of different stuff. So only a small cadre of those people is working on figuring out how we get to the moon. It takes time to figure out how to build a new spacecraft. Then they have to actually do things like completely rebuild the launch pad. It takes time to completely rebuild that. Then they have to build the rocket. It takes enormous amounts of money and enormous amounts of time and large numbers of people to build the rockets.

    Yeah, we could do it. We could have done it in the 80s; we could have done it in the 70s. We have the ability, what we don’t have is the time, money or manpower.

    Fraser: And vision, they were building the space shuttle and the space station. They only had so much energy that they could put into those things. But other aspects of the space industry have actually become quite commoditized.

    Just think about how often rockets launch carrying communication satellites. They go up all the time. They go way out to geosynchronous orbit and there are these monstrous satellites that sit out there and communicate with them.

    If you wanted to launch one you could hire Hughes or Intelsat to make you a satellite. You could get it launched from a Boeing, a Delta. You could get it launched on an Atlas.

    It is all, well not commoditized exactly but aspects of the space industry have become a modern business that you can get this kind of stuff done. Sending people to the moon just isn’t something that a lot of people need done.

    Pamela: We don’t really have the money-making business plan currently. There are people working on coming up with it but right now you’re not going to turn a profit going to the moon. So yeah we could do it, we just don’t have time, money or manpower to do it quickly.

    Fraser: Yeah. Thanks Pamela. Sorry for all the zingers. I should have warned you that I [Laughter] went and picked all the toughest nastiest questions I could find in this list. Actually you know what it is?

    Pamela: The listeners are getting smarter.

    Fraser: These were the ones that I’ve actually been avoiding.

    Pamela: Oh gee, thank you all in one week.

    Fraser: And actually we could probably take those on. Pamela is hopped up on coffee. She says she’s had enough sleep so let’s have a shot at it.

    Pamela: Well we got through it. [Laughter]

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