My bad: If atoms are mostly made up of empty space, why do things feel solid?

Physicist: Thanks to a comment in the original post, I did a little research and found that I was wrong, wrong, holy crap wrong.  Here’s some of that comment:

The source of the ultimate “excluded volume” forces is entirely quantum mechanical: it is the fact that electrons are fermions, hence (this is the Pauli exclusion principle) cannot occupy the same volume without being in different energy states. If you attempt to push the orbital electrons of two atoms into the same volume of space, most of the electrons will need to be promoted to much higher energy states. The requirement of a great deal of energy to move the atoms closer is what we interpret as a force. You might call it an “exclusion” force, because it comes from the exclusion principle —”

One of the nice properties to come out of the depths of quantum physics is the “Pauli Exclusion Principle“.  “Nice”, because it can be understood without a couple years of extra schooling.  Essentially, two “fermions” (which is a classification of particles that includes all ordinary matter: neutrons, protons, and electrons) can’t be in the same state.  That state includes (in the case of electrons in atoms) electron orbitals.

You could argue (successfully) that the Pauli exclusion principle is responsible for all of the complex chemistry of the universe.  If electrons didn’t “stack up” into higher and more complicated shells, then every element would have more or less the same chemical properties as hydrogen and helium.  Instead, as you move along the periodic table, every new electron tries to settle into the lowest possible state, but finds that many of them are occupied.  So, it ends up in a high energy state, because it can’t find any that are lower.

When two atoms are brought very close together their electron orbitals start to overlap and the atoms start to “share” electron orbitals.  In some cases this takes the form of a chemical bond (and the atoms get stuck together).  More often sharing an orbital means that the electrons already present are forced into higher energy orbitals.  By necessity, the effect kicks in when the atoms are close enough that you can’t tell which electron came from which atom (for a few electrons at least).

A good way to define a force is a system’s attempt to get rid of energy (specifically: \vec{F}=-\nabla U, which is just fancy speak for “stuff rolls downhill”).  If the atoms are too close together some electrons find themselves in high-energy states, and if the atoms are a little farther apart those electrons will be in lower energy states.  So, there’s a force that pushes the atoms away from each other, once they get very close together.

So, the repulsion effect isn’t directly caused by electric forces, it’s more of a side effect.  But, if you could somehow “turn up” the strength of the electromagnetic force in our universe, you’d find that the repulsive effect increased in kind, because the amount of energy tied up in the electron’s energy levels is proportional to the strength of the electromagnetic force.

That’s a bit subtle.  It’s a little like saying that when you push a car up a hill (while the car is trying to push you back down the hill), you’re not fighting gravity directly, you’re fighting the effect of gravity on the car.

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20 Responses to My bad: If atoms are mostly made up of empty space, why do things feel solid?

  1. Pingback: Q: If atoms are mostly made up of empty space, why do things feel solid? | Ask a Mathematician / Ask a Physicist

  2. Chris says:

    ” A good way to define a force is a system’s attempt to get rid of energy (specifically: , \vec{F}=-\delta U which is just fancy speak for “stuff rolls downhill”). ”

    Ah, as a grad. student in mathematics, I’d never thought of it like that, but that makes perfect sense in terms of potential energy, and certainly is intuitive when thinking about the “exclusion force.”

    Of course, this analogy only works for conservative forces, otherwise there is no potential, strictly speaking.

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  4. ajay says:

    why friction is independent in area of contact?But viscocity is depentdent on area of contact?

  5. The Physicist The Physicist says:

    Friction is proportional to both the contact area and normal-force-per-unit-area (the force pushing the surfaces together). Normal-force-per-unit-area is what makes the difference between someone stepping on your foot in running shoes, and the same person stepping on your foot in high heels. They weight the same amount (same normal force), but the normal-force-per-unit-area is different.

    For example, when you’re moving a chair there’s a fixed normal force (the weight of the chair), but the contact area and the normal-force-per-unit-area can change. If you half the area of the chair’s feet, then the normal-force-per-unit-area doubles. The nature of the friction changes (half the contact area grinding twice as hard), but the end result is the same. So, rather than write friction out as (coefficient of friction)x(normal-force-per-unit-area)x(area), it’s easier to just write (coefficient of friction)x(normal force).
    In fluids (in a nutshell) there is no normal force, so you’re left with something kinda like the first equation: (coefficient of friction)x(?)x(area).

  6. Erm…so the answer is what exactly? In layman’s terms, or better still, to quote Denzel Washington in Philadelphia “explain it to me as if I’m a 2 year old”. Thank you!

  7. The Physicist The Physicist says:

    For a 2 year old I’d say:
    “Matter feels solid because it doesn’t like being in the same place, and no you can’t have my ice cream because it’s my ice cream. If you wanted your own you should have said you wanted your own ten minutes ago, but you said you were full. Why are you crying?”

  8. John says:

    Could you explain why the Pauli Exclusion Principle happens? Or is the answer just “That’s the way our universe works.”? Why doesn’t it apply to Bosons?

  9. The Physicist The Physicist says:

    That’ll take a post. When it’s up you’ll see a link right here: _____!

  10. Gordon says:

    Physicist: Thanks to a comment in the original post, I did a little research and found that I was wrong, wrong, holy crap wrong. Here’s some of that comment:

    To add my two-cents…

    It’s not that you said you were ‘so’ wrong…to use the magnet analogy (touch is like the two-north poles of a magnet coming together and that’s why we say we feel solid things/stuff) it ‘IS’ a good analogy, for the layman; but then one needs to add to it that it’s not the total story…analogies ARE important and it is even interesting that we can postulate up such analogies. I’ve used the magnet analogy to just make it obvious to people (the layman out there) that ‘life’ as we know it and typically abuse it and take it for granted…is; too weird to be ignored by one being caught up in the ways of the world and to then waste time…with the silly ‘things’ in the world (we should sense our ability to be in the world but not of the world). It seems we are in and of a situation like a hologram whereby what is ‘stuff’ is/are: forces, information, and relationship. It is the abstracts that have value and the materialism of the world is a misleading endeavor to focus on (be that scientific or philosophic or even theologic). In a shocking manner it makes the materialist rethink his abuse of ‘his’ allotted time. So whatever one ‘believes’ it is now an interesting reversal that the phrase: “he with the most toys (at death) wins”. We are a strange creature in a strange situation and what is even stranger is that science can/should make us aware of it. Those who use the term: “why is there something rather than nothing” in a discussion of “what is life?” should begin to see (and feel!) that a reductionist method to discover what’s afoot…should also see and feel that we need to bring ‘knowledge’ together and stop the reductionist conclusions. If Hawkins and Guth want to find a G.U.T. then bringing knowledge together is wisdom.

    Thanks
    [email protected]

  11. Ashely Williams says:

    Can you explain atoms empty space?

  12. The Physicist The Physicist says:

    Not in words.
    The emptiness falls out of the solution of the Schrodinger equation for atoms.

  13. Alex says:

    If one is using the magnet analogy and that things feel solid because of the repulsivity of electrons, if one could create a neutral field around themselves would they then be able to move through “solid” objects? And if theoreticaly so, is it even posible to create a neutral field or space?

  14. The Physicist The Physicist says:

    Using the magnet analogy, yes; if you could neutralize the electric forces you could pass right through stuff. However, the Pauli exclusion principle doesn’t really depend on the electric force directly, so in that case; no.

  15. JESSICA says:

    i dont understand this can someone explain this is wrond and by the way what is mass please (scientific definition) thankyou :D

  16. The Physicist The Physicist says:

    There’s a somewhat frustrating post about that!

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  18. Alan Swinney says:

    If matter is in wave form when not being observed, why do you bump into things in the dark, or when you walk backwards? Why don’t you fall through your bed when you fall asleep?

  19. Pingback: Why We Can’t Walk through Walls | A Moment of Science - Indiana Public Media

  20. Olive Farmer says:

    Just postulation:
    Let’s assume there’s a creator.
    Let’s assume that the creator made this virtual reality.
    Within the coding of the VR are rules which we, the host of the observer, have to comply with.
    The signals we recieve from our senses are converted into electricity and the biological reasoning instrument we have and call our mind interprets those electronic signals according to the rules of such interpretation written into our programming.
    Our mind, of course, exists within and is part of the VR and made of the same (apparent) stuff.
    The question should not be why do things seem real when we touch them, even though they are made of mostly nothing? The question should be how do we win this game…?

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