# If you had to make a perfect vacuum, version #2

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sophiecentaur
Gold Member
Some of those are also theoretically impossible, a perfect vacuum isn't.
Of course it is.

Of course it is.

Theoretically impossible? No, there are tons of it everywhere.

sophiecentaur
Gold Member
Theoretically impossible? No, there are tons of it everywhere.
Tons??? = Mass = vacuum??
But how would you define your perfect vacuum? How big a volume and for how long would you have to have your vacuum exist? The only way you could actually check that it's there would be to measure the presence of atoms and your atom detector could have missed that final atom in the time (however long) taken for the experiment.
I'm sorry but it is really a nonsense. It's not Science. If you cannot measure something then you cannot say it exists.
You are dealing with statistics here and there is never a zero probability of finding an atom in a given region; it's just very low down on the skirts of a probability distribution. The more money you spend, the lower down you can go but that's still not zero.
Specify some particular conditions and you can get a sensible answer (albeit very ball park).

Tons??? = Mass = vacuum??

It's a 'joke", an expression, don't take it literally.

But how would you define your perfect vacuum? How big a volume and for how long would you have to have your vacuum exist?

Well, someone mentioned deep space. Out there you'd find less than an atom per cc. I'm not sure exactly how many there are in interstellar space, because sources seem to say from 1 atom per cubic meter to 1 atom per cc. Theoretically you can use something to keep atoms away for a while, or you can pick a smaller volume, and you should definitely be able to keep your "perfect vacuum" intact for some time. Apparently the average density of the universe is 5.9 atoms per cubic meter, so it's pretty vacant at many points.

The only way you could actually check that it's there would be to measure the presence of atoms and your atom detector could have missed that final atom in the time (however long) taken for the experiment.

Again, this is all hypothetical. Let's assume you had a perfect detector, or that you could leave it in there for very long.

Specify some particular conditions and you can get a sensible answer (albeit very ball park).

Alright then. How about getting less than 10 atoms per cc? Is that better? Less than 10 is practically nothing.

sophiecentaur
Gold Member
It's a 'joke",
Mine too.
Alright then. How about getting less than 10 atoms per cc? Is that better?
MUCH better. That takes us from nonsense to almost sense. Except for the 'perfect detector' and the 'very long'. In some fields of study, 'very long' could be 1ns.
Less than 10 is practically nothing.
Haha. You would need to talk to a Mathematician about that statement and it's all relative. But you are now talking in the terms that can actually get an answer.
I remember my boss once picked me up for using the word "several". He replied "do you mean Eleveral?" It gave me a 'thing' about being quantitative when I can be.

Further to the 'pasta' problem. 'Suck' is a funny quantity. You get virtually the same amount of Suck with a cheap and cheerful vacuum pump and a state of the art one because AP minus a small number is much the same as AP minus a really small number - for the purposes of sphagetti and general engineering. Otoh, a force pump (positive) pressure can make a serious difference to the situation.

Mine too

...oh... I feel a bit awkward now...

That takes us from nonsense to almost sense. Except for the 'perfect detector' and the 'very long'. In some fields of study, 'very long' could be 1ns.

Let's assume we have a detector that can always detect an atom if it's there, and does not cause issues like polluting it with more atoms. And let's call "very long" something "easy" at first, 100 seconds.

Haha. You would need to talk to a Mathematician about that statement and it's all relative

It's all relative, and that's why I said less than 10 is practically nothing. As you said, it's possible to not even detect it without our perfect detector.

AP minus a small number is much the same as AP minus a really small number

Are you asking me to leave things to chance like that??? I've sent the noodle to a lab to remove imperfections to a molecular level! Do you think this is a game?

Anyway, if I get to try the experiment I'll just use a vacuum cleaner. Right now the "vacuum" is the least of the issues.

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jbriggs444
Homework Helper
Do you think this is a game?
Of course it is. None of this has any practical application.

Of course it is. None of this has any practical application.

Are you telling me that performing an experiment involving sucking noodles with the utmost precision has no practical application? What are you going to say next, that it's useless to think of how many angels can dance on the head of a pin?

jbriggs444
Homework Helper
Are you telling me that performing an experiment involving sucking noodles with the utmost precision has no practical application? What are you going to say next, that it's useless to think of how many angels can dance on the head of a pin?
This is the vacuum thread, not the sucking noodle thread. That thread is much more than a game.

OmCheeto
This is the vacuum thread, not the sucking noodle thread. That thread is much more than a game.

Yes, it's very interesting. I just joked about using a perfect vacuum to test a model that was proposed there.

mfb
Mentor
Yes it is possible to fully evacuate a 1 cm3 volume.

The BASE experiment at CERN made a vacuum so good that they couldn't detect any remaining gas, and set an upper limit of 3 particles per cm3. Even if the density is at the upper limit (it is expected to be at least 1-2 orders of magnitude better), based on Poisson statistics, cubic-centimer-sized regions free of atoms occur all the time. Or, put differently, if their trap would be smaller there would be a reasonable chance to have 0 atoms in it.

The BASE experiment had antiprotons in their vacuum, they are not included in the particle number estimate of course. Those antiprotons were stored in the vacuum for more than a year without detectable annihilation.

The trick to prevent outgassing is the temperature: the whole vacuum chamber is cooled to 6 K. At that energy the atoms stay trapped in and at the walls. Even helium atoms stay attached to the walls as long as there is not enough helium to form a full layer.

Andreas C
The BASE experiment at CERN made a vacuum so good that they couldn't detect any remaining gas, and set an upper limit of 3 particles per cm3. Even if the density is at the upper limit, based on Poisson statistics, cubic-centimer-sized regions free of atoms occur all the time. Or, put differently, if their trap would be smaller there would be a reasonable chance to have 0 atoms in it.

So that sorts outgassing. How did they manage to evacuate the chamber so well?

mfb
Mentor
Well, at least a year, that's how long the antiprotons were in it.
How did they manage to evacuate the chamber so well?
Same approach: Cooling. Start with a very good vacuum, then seal the chamber completely, then cool it. All the remaining atoms freeze out at the walls.

Longer explanation: BASE publication, page 24. The reference given there has a detailed description:
Thompson, W. (1977). "Characteristics of a cryogenic extreme high-vacuum chamber". Journal of Vacuum Science and Technology. 14 (1): 643–645. Bibcode:1977JVST...14..643T. doi:10.1116/1.569168.

Staff Emeritus
2021 Award
nd set an upper limit of 3 particles per cm3. Even if the density is at the upper limit (it is expected to be at least 1-2 orders of magnitude better),

Where do you see this? I see 10-14 mbar as the limit of measuring. That's about 500 particles/cm3. Ref. [17] would be nice to look at, if it were complete or pointed at a preprint. From their proposal, they thought they could do ~50x better than this, so we're still an order of magnitude away. Even your 3 is not 1.

There are some complications. We're interested in density, and typically what is reported is a pressure. At very low densities, especially with cryogenics, you don't have thermal equilibrium and so pressure becomes a less and less good proxy for density. The other is that, because you are out of equilibrium, the density varies throughout the volume. The BASE design (and the LHC vacuum pipe design) takes advantage of this and has a lower-than-average density where the beam or target is. I think that's cheating: it is almost certain that there is a cubic centimeter of the LHC without any gas molecules. I just can't tell you which cubic centimeter it is.

nsaspook
Ref. [17] would be nice to look at, if it were complete or pointed at a preprint.

I'm not sure if it is what you're looking for, but it seems to refer to p. 24 of this: http://cds.cern.ch/record/2120817/files/SPSC-SR-177.pdf

It just mentions that the storage time is more than 1.08 years.

mfb
Mentor
nd set an upper limit of 3 particles per cm3. Even if the density is at the upper limit (it is expected to be at least 1-2 orders of magnitude better),
Where do you see this?
See the first link (post 41):
“Given that we have not observed any antiproton disappearance yet,” says Christian Smorra, a research fellow on the BASE collaboration, “we can say that there are less than three matter particles left per cubic centimetre.”

3 is not 1 and not 0 either, but see above: Poisson statistics.
I'm not sure if it is what you're looking for, but it seems to refer to p. 24 of this: http://cds.cern.ch/record/2120817/files/SPSC-SR-177.pdf

It just mentions that the storage time is more than 1.08 years.
That is based on 3 months, they have more than a year now.

Andreas C
That is based on 3 months, they have more than a year now.

Oh good! I certainly did not expect a vacuum so close to perfect to have already been obtained, that really changed my perspective.

OmCheeto
Gold Member
The vapor pressure of solid iron below its melting point, in Pa, is given by the equation

log (P/Pa) = 12.106 - 21723 / (T/K) + 0.4536 log (T/K) - 0.5846 (T/K)−3

where the temperature is in degrees K.
Interesting. I'm guessing this is why: "When astronauts return from space walks and remove their helmets, they are welcomed back with a peculiar smell. An odor that is distinct and weird: something, astronauts have described it, like "seared steak." And also: "hot metal." And also: "welding fumes."" [ref]

ps. Sorry to be so late to the thread, but I just saw it today, and I'd just heard about space smelling like metal a couple of months ago, and a hypothesis regarding the evaporation of metal was the first thing that popped into my mind.
Thank you CM!

256bits
Gold Member
3 is not 1 and not 0 either, but see above: Poisson statistics.
There is some more going on than just the particle density, such as mean free path and how long before an expected collision.
For intergalactic space, isn't an expected collision mean free path measured in light years.
For atmospheric air, in mm.
For an ultra high vacuum, in kilometres.
Cool the gas temperature down lowers the velocity of the particles, and the expected time between collisions should increase.

256bits
Gold Member
Interesting. I'm guessing this is why: "When astronauts return from space walks and remove their helmets, they are welcomed back with a peculiar smell. An odor that is distinct and weird: something, astronauts have described it, like "seared steak." And also: "hot metal." And also: "welding fumes."" [ref]

ps. Sorry to be so late to the thread, but I just saw it today, and I'd just heard about space smelling like metal a couple of months ago, and a hypothesis regarding the evaporation of metal was the first thing that popped into my mind.
Thank you CM!
Or cosmic rays encountering the metal/

mfb
Mentor
All the vacua we are discussing here have mean free paths way larger than the size of the lab vacuum.

Interstellar space has mean free path in the range of millions to trillions of kilometers. Large compared to planets and most stars, but small compared to interstellar distances.

All the vacua we are discussing here have mean free paths way larger than the size of the lab vacuum.

Interstellar space has mean free path in the range of millions to trillions of kilometers. Large compared to planets and most stars, but small compared to interstellar distances.

Do you know what the density of antiprotons is in there?

mfb
Mentor
Did you check the publication? It should have numbers for the antiprotons and the approximate volume of their storage. The total vacuum volume is 1.2 l but that won't be filled with antiprotons. They used a few thousand antiprotons for measurements, so initially the number of antiprotons clearly exceeded the number of regular atoms in the trap.

Andreas C