Expert: Steve Nelson - 5/25/2010

Question
QUESTION: Astronomers teach gravity has an effect over gas, is it true? We were taught gas is a substance structured to expand indefinatly.

ANSWER: Gas has mass density, so of course gravity affects it.  In the absence of a compressive force, gasses expand indefinitely, but can be contained by gravity.  Otherwise, we'd have no atmosphere.

---------- FOLLOW-UP ----------

QUESTION: We were tought gas with a high atomic Wt., e.g. Xenon, cannot pass (penetrate) through a gas with a lower atomic Wt. e.g. Hydrogen. That's why our atmosphere cannot "blow away".The ceiling of hydrogen holds our atmosphere "in"and gravity has no effect or influence over the natural molecular reaction.Please read www.aptheory.info it hypothisises, where our atmosphere came from. These facts were taught by a physics professor, was he wrong? sincerly A. Pettolino Author: www.aptheory.info

ANSWER: That's...all ridiculous.  Entirely, gravity is completely the reason why our atmosphere is held in, there's no "ceiling" of hydrogen. Thermal physics dictates the exponential falloff of pressure at altitude.  Whoever taught you this has no business teaching physics and, I suspect, has never had a modern thermal physics class.  In my vacuum technology class one of the first things I do is explain the falloff of atmospheric pressure with altitude to my students as a result of gravitation...how would gravitation not have an effect?  That's insane.  And what's this about a hydrogen ceiling?  The number of particles in the solar wind is absolutely ridiculously tiny compared to the atmosphere, it's certainly not some kind of a shell that holds the air in...where does this person teach?

---------- FOLLOW-UP ----------

QUESTION: Thank you for your e-mail; I'd like to learn more about gravity's attraction of our atmosphere. Please share your source of reference. Also, please site one experiment proving gravity can attract or influence gas. None have I seen.  In responce to your claimes:The ceiling (covering) of the entire universe is Hydrogen.Indeed true. All gas of lesser atomic wt., exists below the "ceiling" of Hydrogen. The Solar Winds flow 1 trillion miles out from the Sun with great force to the Heliosphere.Hardley "ridiculously tiny compared to the atmosphere". I awaite your references. Respectfuly   A. Pettolino   Author:  www.aptheory.info

Answer
I notice that you never said who taught you what you believe, or where.  Having gone to Case and Duke Universities to get a Ph.D., I have plenty of references...most at too high a level so I'll focus on two.

Everything about the idea that gravitation has no effect on gases is wrong, and there are no numbers or measurements on your website to support such a notion.  Go to www.spaceweather.com.  The number varies, but the current Solar Wind density averages 0.9 protons (hydrogen)/cm^3.  The best vacuums ever achieved by man on Earth are on the order of 350 atoms/cm^3, and that's about 10^-14 mbar, or 10^-17 times atmospheric pressure.  Reference: Modern Vacuum Practice, 3rd edition, by Nigel Harris...page 3.  That makes the density of hydrogen in the space near the Earth, as directly measured by the ACE sattelite, almost 10^-20 times less dense than the atmosphere here at sea level.  Atmospheric pressure drops (same reference for now) by 13 orders of magnitude as you go from 0-800 km, in an exponential drop as you would expect from statistical mechanics.  See Kittel and Kroemer's Thermal Physics textbook (2nd edition), pages 125 and 126 for calculations of pressure drop vs altitude.  Sure, there's hydrogen spread throughout the universe, but it drops all the way down to 4 particles/cubic meter in intergalactic space.  Even in the solar wind there's 1.4 trillion times less hydrogen density than there is in our atmosphere (air is measured to be, by volume, 0.0005% hydrogen) as there is out there...by your reckoning that would push our atmosphere up and away rather than locking it in place.  These are two commonly used references, and the science from them is quite provably correct with measurements.  

The masses of the protons, neutrons and electrons which comprise gas molecules are well known.  The masses of the molecules are well-measured and are, in fact, necessary to know precisely so that machines like mass spectrometers work at all.  Gravity affects everything with mass, everything.  That includes gas molecules.  Thermal physics and gravitation govern the behavior of atmospheres.  And what about atmospheres of planets like Jupiter, where the atmosphere is about 90% hydrogen?  Even in the freshman physics class I teach, challenge problem 18.92 deals directly with the variation of temperature and pressure of the atmosphere...and the solution involves gravity.  You can probably find it worked out if you're a premium member of cramster.com.  Want more references on thermal physics?  Go to Amazon.com and just search for thermal physics.

I went to your website to see it.  The artwork is fantastic, my compliments to the artist.  However, that's all that's there aside from some random numbers with none of the real cited sources which you seem to value so much.  No equations to predict anything.  No testable measurements.

Certainly there's no explanation there of how mankind has flown many deep-space satellites on gravitational trajectories, using gravity slingshot maneuvers from passing planets to get them out deeper into the solar system.  The Cassini space probe did this quite well, as an example, and is currently investigating Saturn.  If anything on your page were right and all scientists were so wrong, it would have never made the journey and wouldn't be sending us clear radio signals now.
http://saturn.jpl.nasa.gov/mission/gravityassistsflybys/

Science works.  We measure it, test it, use it to build things that work.  Spaceflight works.  Satellites orbit and give us things that work, like GPS.  There is no "hydrogen shell" and no evidence for such.  I doubt you'll believe me or tell me who told you that there was such a thing (is it your own personal theory?), but you're welcome to do so and to read my reference books.

----A follow-on---

Gravity does not spin the Earth.  The Earth spins because it has angular momentum and there's nothing in the very empty space around it to stop it.  It doesn't need anything to "make it spin," it was formed rotating and there's nothing to stop it.  Much like a bicycle wheel, if I pick it up and give it a spin, it will keep spinning (if it's a good bicycle and has low friction) for a long time without anything exerting a continuing torque on it to maintain its rotation.  Don't feel bad, Aristotle believed that it was simply in the nature of things to come to a stop because his everyday experience always involved friction.  Sir Isaac Newton came along and said that it was in the nature of objects to keep moving unless acted on by an external force.  

---Didn't mean to reject the last one, clicked the wrong button----
Dear Steven; Log on to the gas industry's website:Hydrogen,H2, Physical properties, safty,MSDS. In the mean time here's a quote from the site "Hydrogen is the lighest gas and is therefore NOT held by the Earth's gravity"... read more if you wish.Who now can I believe? Sincerly Angelo

Answer: Hydrogen is light enough, like helium, to be thermally able to escape the Earth's atmosphere. That is why its concentration is so low, that and its reaction with oxygen to form water. It will attach itself to the higher-mass objects in the solar system, namely gas giants and the Sun. Thermal physics governs the process of hydrogen escaping the Earth's atmosphere as well and is perfectly calculable and well-understood. It still doesn't form a shell holding gas in. You just pointed it out yourself, with the word "lightest." If it's the lightest, then it has a weight like other gasses with higher mass density, and therefore gases are indeed affected by gravity. Otherwise "lightest" and "heavier" wouldn't apply at all. Hydrogen thermally escapes the Earth's gravity molecule by molecule.

Read the actual physics books I pointed you towards to understand the real science, and don't make assumptions based on an MSDS that scientists like myself already understand very well.