Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How low can we Go?

  1. Apr 3, 2004 #1
    When talking about Vacuum what are we actually talking about?

    In automotive engineering terms a vacuum is the low pressure area created on the intake stroke.

    When talking about absolute vacuum what pressure is this?

    Some one has suggested that a true vacuum or maximum vacuum is the removal of one atmosphere of pressure.

    If we have a vessel that we keep removing space from ....how low can we go?
  2. jcsd
  3. Apr 3, 2004 #2
    im guessing an absolute vacuum is 0 PSI

    if you have a container with a seal/plundger, and you pull it up, it'll get harder and harder to pull up. i dont know what happens when the inside has nothing more to give.
  4. Apr 3, 2004 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    We can never obtain a true vacuum which would be a region of space in which no particles existed (including photons), as quantum theory tells us that there are literally thousands of particles popping in and out of existence all the time. This discussion came about when trying to obtain negative energy. We define a zero point of energy to be a vacuum yet we still have these particles that appear and disappear at random. IN one experiment two metal plates were placed very close together in a vacuum so that the gap was small enough to stop particles of a wavelength greater than the gap being formed thus creating an area of 'negative energy'. Try looking for negative energy on a search engine I'm sure they'll have a definition of vacuum there.
  5. Apr 3, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Kurdt, you're speaking of the Casimir effect. You'll have more luck searching for that.

    - Warren
  6. Apr 3, 2004 #5
    Yes that is true, alot of textbooks simplify absolute pressure as
    P(absolute) = P(gage pressure) + P(atmospheric pressure) or
    P(abs) = P(atm) - P(vacuum chamber pressure)
  7. Apr 4, 2004 #6


    User Avatar
    Science Advisor

    whitelighter - if you wanted to learn a little more beyond the above answers, maybe try a search on the internet for vacum pumps. When I did, I learned about the importance between -29.9 and -29.91 and -29.919 and so on inches of mercury (relative to one atmosphere at STP). This would be about -14.7psi with an obvious need for more decimal places. There is literally a little niche industry to service these different vacum needs. Also there are terms to truly define the different laboratory levels of vacum, how few air molecules are left in a given space at each level, and some experiments that require certain levels in order to work properly.

    For an automotive engine, things like cylinder heads are specified to have flow characteristics at 28.8 inches of water which would be -27.8 in of mercury or about -13.7psi relative to atmospheric. Funny how just about any vacum/boost gauge is offered in inches of mercury for vacum and psi for boost or just in terms of bars. Converting is actually easy once you figure out if its relative to atmospheric or absolute, here's link that you may find handy:

    Last edited by a moderator: Apr 20, 2017
  8. Apr 4, 2004 #7


    User Avatar
    Science Advisor
    Gold Member

    Gara. Most people believe as you stated. Pull a plunger up on a sealed vessel and it will get harder to pull the farther you get from where you started. This is NOT true. You speak of the inside 'having nothing more to give'. This really isn't the way it works. What the resistance you feel when pulling is is a WHOLE PLANET of air pushing against the SMALL plunger on a SMALL container. It IS small relative to the rest of the atmosphere. The atmosphere EASILY absorbes the air removed from the container. You could do this with a cylinder/plunger as large as you wanted and the pressure would always be the same until you have displaced a significant portion of the earths atmosphere.

    Put another way, what would happen if you took a zip-lock bag into space with you? If you got into your space suit and into the vacuum of space, and then sealed the bag, it would not behave as it does on earth. You could pull the bag into any position you wanted and it would behave as if it were on earth and NOT sealed. Do the same with a sealed bag on earth and the sides will bulge in when you pull it. Vacuum is simply a LOWER pressure than the atmosphere.
  9. Apr 4, 2004 #8
    Cliff J....and others thank you so much for your constructive comments.....

    Vacuum is a state of relativity I guess is the main point to come out.
  10. Apr 4, 2004 #9
    Relative energy density, I surmise.

  11. Apr 4, 2004 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Just to clarify supernova's comment (because it really misled me), he's talking about the case where the interior of the container is already essentially a total vacuum. The force required to pull the plunger will start off low, and keep increasing as you pull it out, but the force will approach a limit instead of going off to infinity.
  12. Apr 5, 2004 #11
    The question comes to mind " when you remove teh contents of the vessel is there more or less space in the vessel.

    Ahhhg!!!! you think what a stupid question....well maybe you are right....
    However the volume of the vessel stays the same but it has less in it. Does that mean we are creating space or are we just maintaining volume?

    How can volume be maintained when there is less in it with out creating space?

    A Logic riddle maybe??
  13. Apr 5, 2004 #12
    whitelighter said: "Vacuum is a state of relativity I guess is the main point to come out."

    If, by relativity, you mean that the word "vacuum" means different things in different situations, then that's probably correct.

    But in physics there is/are thoretical meaning(s) for the word. In classical physics (Newton + Maxwell) it means a volume of space with no massive particles. Light is allowed. In QM (as has been mentioned here) you have to be careful about whether you're allowing light, since the distinction between massive and non-massive particles is not absolute the way it is in classical physics.

    But those are theoretical definitions. In the real world, neither is attainable, at least not for very long or in very big volumes.

    On the earth I think the best we can do is about 10^-11 atmosphere's. But since cm3 of gas a t1 atm, has about 10^20 molecules in it, even 10^-11 of that ain't exactly empty!

    I think I remember reading somewhere that in the volume of space just behind objects in orbit around the earth there's a vacuum better than anything we can create at sea level.

    And there's a very good vacuum in interstellar space; I think it's around 1 H atom/cm^3 (about 10^-20 atmospheres). That's WAY better than anything we can achieve near the earth.

    By the way, in the 2nd posulate of Einstein's special theory of relativity (light always propagates through a vacuum at the same speed) he was talking about a region with no massive particles. Even though we can't achieve this in the real world, the atmosphere at sea level was empty enough for Michelson and Morley's experiment.

    So, after all that, I guess you had it right (in more ways than one :wink: ); "vacuum is a state of relativity"!
    Last edited: Apr 5, 2004
  14. Apr 5, 2004 #13
    ha ha...except my view was so simplistic......low pressure being only available by relating to higher pressure.....and vica versa.... there fore a relative state...........

    But BTW thanks for the answer.......taught me to use the word relativity a little more carefully....relatively speaking of course.....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook