Two views on gravity Part 1: Particle Zoo

Particle Tracks as from a cloud chamber

Particle Tracks as from a cloud chamber

For the past couple of weeks I’ve been busy teaching particle physics to two very different populations. First I work with working with little kids through the Davidson Institute, going over the ins and outs of making atoms. Then I took on particles again with my physics for poets class at SIUE. Between the two, its forced me to do a lot of thinking, and has reminded me how annoying gravity can be.

Here’s the problem: Einstein taught us that gravity is a manifestation of the geometry of space and particle physics says gravity comes from the exchange of bosons called gravitons that communicate the force of gravity, which is related to mass which is mediated by the hitherto undiscovered Higgs.

These particles couple particles force us to stare at two rather different ways of understanding and visualizing one of the most fundamental things in our universe: gravity. To get to this problem though, I want to give a bit of history, so this is going to be in two parts: Particles and Einstein’s gravity.

Let’s start with particle physics 101.

The idea that our universe is made of several very fundamental particles isn’t very novel. During ancient history, the idea that everything was made of either 4 elements (Earth, Air, Fire, Water) or 5 elements (where they added void) cropped up in Greek, Buddhist, Hindi, Japanese, Tibetan, and Chinese history. The varying attributes of everything we interact with were attributed to the almost infinite variety of ways these 4 (or 5) elements could be mixed.

This idea that fundamental elements was rebranded under the concept of atoms (atomos in Greek): or indivisible. The idea was simple: at a certain point, you can’t break something apart any longer, and that smallest bit of stuff you get is the atom. (While this idea is often blamed on dead white greeks, it first appeared in ancient India and was related to Jainism.

Alchemists, philosophers, and all manner of other forms of pre-scientist thinkers and experimenters worked to find definitive ways to classify and categorize materials by these elements, but the more they burnt, mixed and evaporated, the more they realized that there were rather more than 4 or 5 elementary pieces of stuff.

Making a long story rather short, this all came to a head in 1869 when Dmitri Mendeleev (and the ever forgotten Julius Lothar Meyer) published their periodic tables. In both cases, they arranged the elements in rows ordered by increasing mass, with columns of elements sharing similar chemical properties. This arranging of elements was the organization needed to set all of modern physical chemistry and quantum mechanics on a path to understanding how atoms are made of something even more fundamental.

As early as 1815, Wiliam Prout hypothesized that all atoms were made from Hydrogen. One of the problems with this idea was the weights of atoms aren’t nice, consecutive multiples of Hydrogen. If you look at just the most common versions of the first few atoms, you have atomic masses (in multiples of Hydrogen’s mass) of 1, 4, 7, 9, 11, 12. This seemingly random pattern of particle particulars led to a long period of confusion.

It wasn’t until 1919 that Rutherford sorted out that when rather bad things are done to innocent atoms, you can force out protons (he didn’t know that word, he simply noted the expulsion of particles that were identical to hydrogen nuclei when Nitrogen gas was bombarded with alpha particles). But atomic masses still didn’t make a lot of sense.

But at least it gave us a world consisting of protons and electrons. (Electrons had come somewhat easier, and somewhat earlier, since they’re at the heart of electricity. Here’s that story. You can read about it here.)

Understanding atomic mass required playing with radiation and discovering neutrons. This is one of those moments in science that to me always falls into the category of “How did the even think to try that??” It was realized that if one has the radioactive material Polonium-210 (most famous for killing writers), it will emit alpha particles while it undergoes radioactive decay (alpha particles are really just Helium nuclei with 2 neutrons, but saying alpha particles is just cooler). If these alpha particles are then directed at Beryllium, the Beryllium will then give off a stream of neutral particles. So far, so good. It is very odd that someone tried this, but pretty cool. The next part is fabulous though.

Marie Curie’s daughter and son-in-law were both active scientists and they started working with these strange neutral particles (assumed to be gamma rays actually). For reasons that lead to a great discovery, they placed paraffin wax in front of the neutral particles. They noted that this caused protons to be ejected from the paraffin.

They were, in all reality, playing pool with protons. Since the neutron has essentially the same mass as the proton, when a neutron hits one of the very many hydrogen atoms in the paraffin wax it is wacked out of the way and sent flying. Just as pool balls bounce so nicely since they are all the same-ish mass, so too do protons bounce nicely when hit with neutrons.

With neutrons understood, particle math became a possibility! Suddenly, atomic masses were made up of combinations of protons and neutrons, with variations in individual particles coming from the variations in the numbers of neutrons.

Finding all these particles was all just a start. It was quickly (on the scale of human history) realized that particles come in a vast assortment of unstable versions as well as all the stable versions. As quantum mechanics and particle physics were born, scientists started building rule sets that allowed what boiled down to particle math to be done.

  • energy must be conserved
  • charge must be conserved
  • classes of particles must be conserved
  • all the different types of momentum have to be conserved
  • And few other things…

    Trying to balance all these qualities led to speculation that other types of particle had to exist. Particularly problematic was the decay of neutrons.
    n â€šÃœÃ­ p + e + ?
    In this situation, we have for charge 0 = +1 – 1 (good), but the momentums and particle types are not conserved.

    Particles are in general divided into sets of families. Protons and Neutrons are classified as Baryons. Electrons and the unstable Muon and Tau particles are all Leptons. In the above equation, the electrons leptoness is not cancelled out, and some sort of an anti-lepton is required. To fill that need for cancellation, the neutrino was first postulated by Wolfgang Pauli in 1930. Its mass was finally detected in 1998 (and it still isn’t accurately measured).

    But then came the question of how exactly does a neutron become a proton? This in turn also led to the idea (eventually, in 1964 by Murray Gell-Mann and George Zweig) that Baryons are actually made of even more fundamental particles.

    Today, after filing in all the blanks, we have a particle zoo of
    Baryons: protons, neutrons, and a bunch of unstable stuff
    Leptons: electrons, electron neutrinos, muons, muon neutrinos, and tau particles with their matching tau neutrinos
    Quarks: Up and Down make stable things, and charm, strange, top and bottom contribute to making the world unstable at times.

    But this chart leaves off all the little worker particles. The photons, with their ability to make the electromagnetic force happen has been left off. The gluons that ever so strongly glue together all the little baryons are messing. And then there are the W and Z, and their determination to decay nuclei, that are also not on the list.

    Together, these force ferrying particles have been named Bosons, and have been given their own list.

    The Basis of Everything

    The Basis of Everything

    But if you look closely, and think hard, you’ll notice something is missing from our standard model of the particle zoo.

    This something is a particle for conveying gravity. (And, if you’re in to thinking of mass as a directionless – scaler – not-quite-a-force, then we’re missing a particle for that too).

    And this is where particle physics breaks me. In this particle physics zoo, my too much mass comes from the ability of my bodies atoms to ever so ably interact via the Higgs Boson with some scaler field that permeates all of space, giving me a quality that does require vector hat in math equations. It also means, that rather than seeing gravity as the side effect of things rolling down a hill through space and time toward high mass objects, we instead have little force carrying particles – gravitons – running back and forth at (we think) the speed of light, communicating “I’ve come from a mass, attract yourself this way now.”

    Suddenly, all of space is being organized by so many particles that are zipping around dictating where the forces are forcing (or not).

    Can’t you see it now, little yellow photons dressed as the universes traffic copes, forcing charged particles this way and that through all of space? Perhaps not, but this is still a very different way to imagine our universe than the gravity as geometry that that we were all spoon fed by Einstein.

    And this is where I’m in an emotional bind: I liked relativity as geometry. I know how to work within that model. But the particle physics idea is harder for me, and there is a non-logical part of my brain going: Um, haven’t found the Higgs, haven’t found the graviton, haven’t really proven they are required. But that part of my brain (along with the rest of me) took only 1 semester of graduate quantum mechanics, and I have to admit, I haven’t ever tried to solve out for the hole in the math that requires the particles be made up. And as a scientist, I have to know, if evidence is presented, I have to believe it.

    And with the Large Hadron Collider coming, I may be facing the discovering to the nemesis of my comfortable geometric way of thinking of the universe.

    But, this is what makes me a scientist: Even if I don’t like a discovery because it forces me to change my world view (and in the case of Dark Energy, all my cosmological calculations), I still have to accept the theories I don’t like. The truth isn’t required to be likable.

    Next stop – Gravity.


    1. Steven February 7, 2010 at 4:56 am #

      Nice summary of the history of particle theory!

      I agree with wanting to hang on to the geometrical picture of gravity, and that the picture of all these bosons being exchanged all overthe place seems messy. The thing is, neither is correct, they are both simple ways of us getting our heads around the maths in terms of approximations that work in different situations. The reality is something else, beyond our ability to imagine in a way related to our everyday experience.

      However, though I’m no expert in the details, there does seem to be hope for geometry. Although it would be nice to have all forces mediated by particles in the same way, the gluon does not naturally appear in our Standard Model of particle theory. It seems you have to go beyond thinking of point particles, for example in string theory, where things that look like gluons do naturally arise. Although I believe strong theory has a whole set of other difficulties to overcome, what’s interesting is that the physics are, at least partly, determined by the way in which (11-or-so-dimensional) space is all wrapped up. So it may ultimately all come down to geometry again!

    2. Todd Gonzales February 7, 2010 at 5:08 am #

      I believe this is what so interests me in the science of Astronomy, such a huge spectrum of unknown knowledge. It’s very exciting to know that tomorrow the universe as you know it could be completely and utterly different. I can’t help but wonder what it must of felt like when those spiral nebulas turned out to be separate galaxies…..a feeling that I anticipate will be a 100 fold when we figure out the real story behind gravity. My skin tingles. Great article!

    3. Carl February 7, 2010 at 12:11 pm #

      Thanks once more for being a great communicator!
      Gravity has always been an interest. What hits me on the graviton-particle theory is the idea of these moving at light speed. This would seem to imply that relativity should apply. Is there any evidence of this?

    4. Xerxes February 7, 2010 at 12:22 pm #

      You know, if you’re not too hung up about renormalizability, it’s perfectly ok to throw in a spin-2 graviton in the most naive way. If you toss in N=8 SuSy, you might even get your renormalizability back.

    5. pamela February 7, 2010 at 12:48 pm #

      Hi Carl: Gravitons would have no mass, so you don’t need to worry about mass related relativistic effects. The rest of the time, the can often just be treated like photons.

    6. Carl February 7, 2010 at 4:55 pm #

      I was thinking of aberration? – the driving through a rainstorm effect, where all the raindrops appear to be coming directly at you.
      At relativistic speeds most of the photons appear to be coming from in front of you.
      Wondered if this also might apply to gravitons?
      Seems like there ought to be a thought experiment in there somewhere!
      Have a great day and thanks again for your efforts.

    7. Steven February 9, 2010 at 4:13 am #

      Oops, wherever I said gluon in above post, I meant graviton. Doh!

    8. Evan February 10, 2010 at 3:49 am #

      i cant help but feel that gravity and wave particle duality are the first bit of insight into alternate universes,

      like the first time realizing how many types of light we cannot see,

      the more i think about it, gravity, like wave particle duality must be part of some sort of webway, in the sense that i dont think its necessarily travelling the speed of light or some insane speed, but that its more like stocking a store shelf with cans,

      if you have a full shelf with cans and wish to put another can on this shelf, you must unavoidable push another can off the other side taking its place,

      so i guess what im saying is that perhaps some sort of displacement propertys in effect with what must be a cool set of new rules and laws, i couldnt begin to say what math would be required but, with all the empty space in between atoms, thier is alot of room for parallel universe types. maybe im way off but great article.

      regardless im looking forward to part 2!!!!

      keep up the good work!

    9. Gord Davison October 11, 2010 at 8:01 am #

      Clear explaination. I think that they will never find the graviton. After studying the particle zoo and reading about string theory I think that there is strong evidence that gravity is not a force at all.

      I think that gravity is caused by the distortion of time. This distortion generates a dv/dt which is an acceleration, without motion, giving us the ‘a’ in the formula Force equals mass times accelleration thereby generating the force of gravity.

    10. tax attorney January 20, 2011 at 11:15 am #



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