Q and BA Episode 6: I Am Your Density

By Phil Plait | March 11, 2007 8:17 pm

Space is a vacuum, right?

Well, not exactly. It’s all pretty empty on a cubic centimeter by cubic centimeter basis, but how empty is it?

Find out in this week’s episode of Q & BA: I Am Your Density.

Viewing options:

Watch it right here, right now!

Watch it on YouTube.

Watch it on Google video.

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Show notes

The Question:

The question was sent in by Walter L. Williams:

When we talk of the vacuum of space, is it a total vacuum or "pretty much" a vacuum? Could you elaborate, perhaps telling us how much matter might be in say a "liter of space"?

Images and Links

The Orion Nebula and Carina cloud images are courtesy of NASA/Hubble/AURA

Barnard 68 is from the European Southern Observatory.

The marshmallow came from my baking drawer. When I was done, I ate it.


Comments (31)

  1. Phil,

    Do you mind scrubbing the audio versions from the video feed, so I don’t end up downloading the same episode twice? It seems redundant, as well, now that you have a separate feed for the audio-only versions.

  2. Melusine

    LOL!! I needed to start that over because I was laughing too hard at the couch the first time. You’re too much, Phil. :-)

  3. Joshua, I’m not sure what you mean. I created two separate feeds, one for audio and one for video. On the LibSyn page they are separate. How are you downloading them?

  4. autumn

    Phil, I have a nine year old stepson, and he loved the first coulple of episodes. In fact, after I showed him the first one, he cornered me a week later and reminded me that I had told him that there would be one every week. I skipped last week, as it seemed to be beyond his ken, but if he asks I’ll certainly show him. This one may be a little past his understanding, but its brevity and the marshmallow have sold me. I will just have to give a little introductory lecture to make sure he understands density and such. You may have created a future astronomer with your congeiniality and wit.
    P.S. I showed him some stuff on P.Z.’s site, pictures of invertebrates and such, and his comment was “that guy is weird”.

  5. Lauren

    I LOVE this podcast! Astronomy + humor = a great thing. The vacuum in the corner also made me chuckle.

    That was a good question too. 1×10^19 particles is a lot compared to .1-1 particles, or even 1 million particles. I’ll have to keep this episode in mind next time I hear that some dust cloud, etc. is dense.

    I subscribed on iTunes, and both versions of Q and BA get downloaded each week. It may be redundant, but it’s easily fixed. I just delete the audio only version.

  6. Mark Martin

    If this were about a municipality such as, say, New York or Tokyo, it could be titled “I Am Your Dense City”.

  7. ABR

    Nice title. Hardly anyone quotes the early writings of G. McFly these days!

  8. A followup question. While I know space, even interstellar space isn’t devoid of matter, has a true vacuum ever been created in a laboratory? Also, you gave us values for the density of interstellar space, what about intergalactic space. It seems logical that this would be even less dense. Is there any data on this?

  9. Ah, I think that if you subscribed via iTunes before I split the feed, you’ll get both. Just unsubscribe, then resubscribe using the links in the blog entry.

    Everyone, thanks for the nice comments.

    Thomas, I looked up intergalactic density but got weird numbers, depending on the reference and such. Most were for density between galaxies in clusters, so that didn’t help much. That’s why I didn’t go into it.

  10. arcraig

    Just for reference, about what’s the best artificial vacuum that can be created here on Earth?

  11. Thomas Says: “has a true vacuum ever been created in a laboratory?”


    arcraig Says: “what’s the best artificial vacuum that can be created here on Earth?”

    In the laboratory, it sort of boils down to “how good do you want it and how much are you willing to pay?” The standard measure of vacuum is in Torr (a mm of mercury) named after Torricelli, the inventor of the barometer. Vacuum starts as you drop below standard atmospheric pressure (760 Torr). This is where Phil started with his “10 quintillion particles/cm” If you pull 99% of them out of a chamber, you’ll get down to unitary Torr numbers. That’s a good enough vacuum to do some industrial processes with, like vacuum bagging or sucking the air out of ground coffee packets to make them hard as bricks. By Phil’s measure, that’s still 100 quadrillion particles/cc. Vacuum engineers, call this a “soft vacuum.”

    The next stage of vacuum drops a few more orders of magnitude. If you need a vacuum from 1 Torr down to 1 milliTorr (that’s 1/1000 of a Torr), you can do it with a fancy mechanical pump that uses sliding vanes or sealed lobes sweeping the molecules out. To vacuum engineers, this is a “hard vacuum.

    Going further, the terminology changes a little, as do the physics. Things are still measured in Torr, but there get to be so many zeros in front of the number that you just use the negative exponent as a reference. For example a milliTorr (10^-3 Torr) is called “the three range.” Also, gasses stop acting as fluids and the individual molecules start bouncing around like ping-pong balls. This is called “molecular flow.”

    Continuing down to about the six range (a microTorr), pumping becomes a statistical game. Since the molecules are starting to get further apart, you can spin a pump rotor all you want and all you do is bat them around; they don’t herd nicely down the pump exit. This is a region called “high vacuum” (HV) and the actual pressure depends on the size of the chamber. HV starts when the statistical probability of a molecule making it all the way across the chamber without hitting another molecule drops below 50-50. The bigger the chamber, the lower the pressure has to be. This is the pressure range needed to do things like plasma etch computer chips.

    The pumps to achieve this have to work with the molecules one at a time. The most common is a “turbomolecular” pump, usually just called a “turbo pump.” This has a rotor section that looks exactly like a jet engine compressor with multiple rows of blades angled to bat the molecules down the direction of the exit. They have to spin very fast (50,000 to 80,000 RPM and faster) in order for the blades to be moving as fast as the molecules. Any slower and they will just bounce off in random directions. Even at this level, though, there are still about 10 billion molecules/cc (using Phil’s units).

    Continuing downward, we get into “ultra high vacuum” (UHV). This one has an even more esoteric definition than “high vacuum.” A vacuum is said to be UHV when a freshly cleaved surface takes more than 20 seconds to acquire one monolayer of molecules on its surface. This takes us down into the 10 range (10^-10 Torr) and these pumps get even stranger.

    The most common is a “cryopump” that has a multi-stage compressor using helium as a working fluid. By compressing and expanding the helium, sort of like freon in an air conditioner, the cryopump gets the helium down to about 15 Kelvins. This is pumped through a heat exchanger called the “15K array.” Any gas molecule that runs into the array instantly has all of the heat (i.e. kinetic energy) sucked out of it and it freezes to the surface. As you can see, the cryopump isn’t truly a pump, but an accumulator, and they have to be periodically “regenerated” to drive off the accumulated gasses. This is the pressure range needed to do physical or chemical deposition on chips or lens coatings. At this point, we still have about 1 million molecules per cc. All hatches and ports into a UHV chamber have to use metal seals (the type that have a knife edge that cuts into a copper gasket) because at this pressure the air molecules from the outside will burrow right through elastomeric O-rings.

    Beyond UHV we enter the realm of laboratory research. There are “getter” pumps that chemically remove molecules once you get down to these vacuum levels. These, though are like cryopumps in that they just remove them from circulation, but they’re still in there. You can getter down to the 15 range or so, which still leaves you about 10 molecules/cc.

    This is about the level that Phil said was in interstellar space. We can’t do much better here, and even if we started out in space with it’s near perfect vacuum, we couldn’t do much more to lower things to an absolute nothing-at-all vacuum.

    – Jack

    PS – the pressure where the space shuttle flies is in the six range. Sometimes they need lower pressures for some experiments, so they put them on tethers and place them behind the orbiter (away from the direction of travel). The shuttle itself acts as a plow, clearing the molecules out of the way. This lowers the pressure about one order of magnitude, down to the seven range.

  12. Walter Brameld IV

    You might have mentioned for the benefit of the questioner that to convert from particles / cc to particles / liter you just multiply by 1000.

  13. it took me a while to stop laughing at the title long enough to watch this episode! hilarious!

    also thought to mention that a black hole is an object predicted by general relativity to have infinite density.

  14. YinYang0564

    Thank you, Jack Hagerty for a clear and concise answer to a question I have wondered about for years.

  15. wright


    Concise, articulate and funny, as always. You explain the basics in a very acessible, nonpatronizing way. Love the trivia text too.


    And thank you for a fascinating followup. As a SF fan, I grew up reading about the “industrial applications” of “hard vacuum”, without having more than the vaguest concept of what those might be. Your examples, like Phil’s, were instantly comprehensible to this layman.

  16. DrFlimmer

    Like all the time: Very nice and I enjoyed it very much! Astronomers like big and huge numbers. I think you are right!

    I just heared a lecture about physics of the atmosphere and of the heliosphere. And it was funny that we started calculating with roughly the mention five or ten particles per ccm! On the earth this is a very perfect vacuum, but we just started with that to calculate the solar wind, etc. This is really fascinating!

  17. peenworm

    Did you *have* to use the keyhole nebula as a demonstration? My kid watches this show! I don’t want him seeing nebulae like that.

  18. If space is a vacuum, who changes the bag then?

  19. Mark Martin


    It’s not that the black hole itself has infinite density. The mass at the center, at the singularity, has infinite density (in non-quantum mechanical general relativity). The hole, on the other hand, is the black, spherical region surrounding the singularity. By some criteria the hole itself may have zero mass-density. (It does have an energy density, since it has gravitational potential. This does correspond to relativistic mass. However, classically the hole can be utterly empty all the way down to the infinitesimal thingamabober at the center.)

  20. XRayDog

    Wow..where did you get that Falcon 9 hat? I want one!

  21. Thanks Phil (and Jack too, for the additional info). When looking at photos of seeminly dense molecular clouds I always wondered that if they were made of oxygen, would it be possible to breath, given a comfortable temperature. Sounds like the answer is “Not on your life.”

  22. YinYang0564 Says: “Thank you, Jack Hagerty for a clear and concise answer to a question I have wondered about for years.”

    You’re welcome, but “concise”? That was a thousand words!

    wright Says: “thank you for a fascinating followup. As a SF fan, I grew up reading about the “industrial applications” of “hard vacuum”, without having more than the vaguest concept of what those might be. Your examples, like Phil’s, were instantly comprehensible to this layman.”

    Thanks. That makes it worth staying up to 3 AM typing that piece!

    BTW Phil is still a newbie at this, he’s only been doing it about a decade :-) I’ve been doing tech writing for laymen since the early ’80s. Phil, though, has “the knack” (for you Dilbert fans), and this blog and website reach a vastly greater audience than my little hobby magazine articles.

    For some of my other stuff, just click on my name at the top and follow the “Spaceship” tab.

    – Jack

  23. peenworm Says: “Did you *have* to use the keyhole nebula as a demonstration? My kid watches this show! I don’t want him seeing nebulae like that.”

    I used to keep a print of that image on my cube wall. I called it the “bird nebula.”

    – Jack

  24. Thomas Siefert Says: “If space is a vacuum, who changes the bag then?”


  25. Finally, someone noticed the hat! I have minions everywhere, doing my bidding.

    Actually, it’s a long story, but when a friend gave it to me, they didn’t say anything. I stared at it for a second, and the stylized falcon snapped into focus, and I realized what I had on my hands. Too cool.

  26. Irishman

    Phil, Q&BA is excellent. You’re doing great, even if I have to now mess with this video stuff.

    Jack, kudos to you, too!

  27. J. D.

    What happened to the framed poster that used to sit on the floor in the left-hand part of the picture?

    And do you light the bed behind you with a black light? It really stands out relative to the dark wall.

    J. D.

  28. Chip

    Another question comes to mind. Are there areas in space (but not in a planet’s atmosphere,) where the density of particles is almost as thick as Earth’s at sea level? Like deep in a nebula where a star is already forming?

  29. ioresult

    Chip, I think that kind of density doesn’t exist for very long. If you find that, then it’s probably well into gravitational collapse. Temperature and pressure would be rising in a matter of days to intelorable levels. Ok I know nothing of these matters, I guess it could realy be hours, minutes or seconds. Or weeks.

    Reminds me of a book by Larry Niven: The Smoke Ring. A torus of gas around a neutron star dense enough to breathe. Yeah. Sure. Now I know better.

  30. Mark Martin

    There was an episode of Rocky Jones, Space Ranger, in which there were two remote planets connected by a dense filament of interplanetary gas. The denizens’ technology was pre-rocket, but they had aircraft, so they flew their airplanes between the two planets in the cloud of gas. 😉

  31. Chip

    I recall there was a Voyager episode involving “liquid space” too. (That one was over the top for me.)

    Following up on what ioresult wrote – maybe there could be a transitory place during early stellar formation where collapsing gases temporarily become dense enough to rival an atmosphere in space, but I too have no sense of the time scale.


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