Particle Physics: Why do it? And why do it *that* way?

  • Who really cares about particles anyway?  Why are particle physicists so interested in them?

It’s not really the particles that are interesting in and of themselves.

Analogy time: Imagine that you became fascinated by Roman cities and how they functioned.  This might lead you to learn more about the architecture of the Romans.  Perhaps you personally might find yourself intrigued by how their buildings and aqueducts were constructed.  You might then naturally jump to an interest in the durability of their arches and structural supports, and from there to the properties of their bricks and their mortar.  Now bricks and mortar are not really what you’re interested in; they are a means to an end.  You want to view them within the larger questions of how the Roman buildings were designed and built, why they look so attractive, and how it is that they could last millenia.

Nature is the most profound and most ancient of architectures.  We live surrounded by glories and mysteries — oak trees and volcanoes, vibrant sunsets and powerful thunderstorms, a lovely moon and the uncountable sands of the beach.  A couple of centuries ago, scientists inferred that this diversity of architecture could in part be understood if matter were made from atoms of a variety of types — the “elements.”  And so they became interested in atoms, the “elementary” building blocks of nature as they were then understood to be.

This was only the beginning, however, because there turned out to be many dozens of types of atoms, with wide variations in their chemical transformations and in their ability to emit light.  In trying to make sense of the diversity and behavior of atoms, scientists realized that they, too, were forms of architecture, built from smaller particles: electrons surrounding atomic nuclei, held in place by the mortar of electrical forces.  In the nuclei themselves lay yet more architecture, with protons and neutrons held together by the mortar of the strong-nuclear force.  Along the way yet another force was uncovered, the weak nuclear force, often more of an erosive force than a constructive one.

Learning these new levels of architecture not only brought about explanations of basic chemistry and how light is emitted and absorbed but also access to other mysteries, such as the workings of stars and the oddity of radioactivity, and the promise and vast danger hidden inside the energy of the nucleus. The bricks and mortar approach was the key to unlocking secret after secret during the 20th century.

[By the way, what I am saying here is  a quasi-historical sketch, not a careful historical account; the real story is, of course, much richer, more complex, and far beyond what I could do properly.]

By the 1950s, it was known that the protons and neutrons of the atomic nucleus had many cousins: other “hadrons” with names like pions, kaons, Deltas, rho mesons, etc.  This complexity was a sign that they too were a form of architecture.  In the early 1970s a picture of these particles — as complicated objects built from quarks, antiquarks and gluons, and themselves held together by the strong nuclear force — came into focus.

You can think of particle physicists as those scientists interested in nature’s architecture at the level of bricks and mortar, durability and erosion.  What are the basic building blocks, we ask, and what holds them together, or breaks them apart?  How are they organized together to form the foundation of the huge diversity of structures that we see in the universe?

Starting in the 1960s, it was gradually understood that the properties of the world that we inhabit require that there be something that pervades the universe — a non-zero field, which we call the Higgs field by definition — that alters the properties of many of the particles of nature.   Without a Higgs field of this type, the architecture that we see around us would collapse.  Understanding what this field is and how it works is one of the central projects of particle physicists today, and the main justification for building the Large Hadron Collider (LHC.)  What secrets (if any) will be unlocked in the process?  No one yet knows.

  • Why do particle physicists have to build giant “atom smashers” to do their work?

Oh!!!  I hate the term “atom smashers”!  We’re not smashing atoms, we’re smashing subatomic particles: protons, which are 100,000 times smaller than atoms [in radius, mind you], or electrons, which are at least another 1000 times smaller than protons!  It’s like confusing a collision of two planets with a collision of two [speeding!] oil tankers or of two bullets.

  • Ok, ok, calm down.  Why do particle physicists have to smash protons or other subatomic particles to do their work? Can’t you think of something less destructive to do with your time?

The analogy is often given that doing physics with “colliders” (a more technical term for “subatomic-particle-smashers”) is like smashing fine watches together and trying to figure out how they work from the pieces that come flying out!  There is merit to this analogy, but there’s something very important that is left out.

The act of colliding subatomic particles at very high energy is not merely a destructive act; it is, more profoundly, a creative one.

It is a remarkable property of nature that when sufficient energy is crammed into a sufficiently small space, particles that were not previously present can sometimes be created out of that energy.  This is, in fact, why we do high-energy particle collisions.  The extremely-compressed-energy technique is the only one we know that can allow us to create heavy or exceedingly rare particles that humans have never previously observed.  We have no other way to make Higgs particles, for instance.

So it is not the smashing of fine watches that we are interested in.  In fact we already know a lot about the watches — the protons that are smashed in the Large Hadron Collider (LHC) are reasonably well understood already.   And what we hope to discover is not something contained within the watch — we have studied quarks and gluons, the bricks and mortar of protons, already in much detail.  No, we must modify the analogy.  It is more as though we smash watches together in hopes that a cellphone will appear out of the collision energy.

Said that way, it sounds mildly insane.  But nature is curious and fascinating, and the LHC makes rare heavy particles every day.  (Some explanation of how this works is given in the video clips from a public talk I gave in March 2011.)  It is in order to create Higgs particles, and perhaps other unexpected phenomena, that we sacrifice protons on the altars of the LHC.

14 responses to “Particle Physics: Why do it? And why do it *that* way?

  1. Random physics nerd.

    Wow…. That’s heavy, doc! Those analogies I mean.
    This sort of reminds me of the clip of Feynman talking about his conversations with his artist friend. Recapping, the artist stated how ‘you scientists’ take a beautiful looking flower and take it all apart making it dull and such.
    Basically Feynman was saying scientists can also appreciate a flower for its beauty, though maybe we are not so aesthetically refined. But taking it apart is also very interesting. We get to see the complex workings of the flower under a microscope, and it is also interesting how insects for example; as to why they are attracted to the flower, and its certain colours. This adds another question: Is this aesthetic appreciation also present in a low dimension, and did the flower evolve this way to attract the ants ect ect? So on. You get the jist.

    Basically my point is…. Feynman is the man!

  2. Hi Matt

    Reading some of the posts on your website (which is very helpful), I can assure you I’ll be one of the most ‘lay’ of the laymen to post a question…. and I have one that is probably ridiculous (here goes anyway).

    The question is regarding what you were describing above about particles being created from energy being forced into a small area.

    If you had a situation where there was a small area of condensed energy (nearly enough to turn into a particle, but not quite), would somebody travelling past that small area at a high enough speed (or vice versa) see that area of energy turn into a particle because of the increase in energy the area would have gained (relative to the observer) due to the motion?

    (Obviously I’m assuming the observer has got a sub-atomic particle magnifying glass at hand to see this happen (or some really good glasses).)

    Thanks

    Pete

    • Pete — you have pointed out an ambiguity in what I said. The issue to keep in mind is that the minimal energy you need to create a new particle is M c-squared… and if it is moving, you need more. So if I want to make, say, a Z particle, I need to squeeze energy equal to M_Z c-squared into a small volume (not “area” really). But that volume should not be moving relative to me — because if it were moving, the resulting Z particle would also be moving, in which case it would have motion energy as well as mass energy, and so more energy would be required to create it in the first place! If I wanted to make a moving Z particle, I would need more energy than M_Z c-squared.

      So to answer your question directly, suppose I did not have quite enough energy to make a Z particle at rest. From a point of view of an observer going by to the right with speed v, I would indeed have more energy available, but not quite enough energy to make a Z particle that, from that moving observer’s point of view, is itself moving with speed v to the left. And so things would be consistent between me and the observer moving relative to me — we would agree that there is not enough energy for me to make a Z particle.

  3. From my understanding (please correct me if I’m wrong), Einstein’s work (building on Planck’s) clearly showed the quantal nature of emitted light, and because also of limitations in modeling transfer of energy from waves to particles, this was interpreted as “particle-like” behavior of light that gave rise to modern quantum theory. However, there are other phenomena that may be more consistent with a wave-like property of subatomic particles (e.g., re-emission of low-energy bands in atomic spectra when higher energy absorption bands are reached – destructive interference?). Was that ever considered? And if so, was there good reason to discard that idea?

  4. Just as a follow up, as I understand it, another reason Einstein’s work called into question the wave theory of light was that it was assumed that a wave model must be continuous (i.e, all wavelengths would exist, therefore not “quantal”). However, so long as a mechanism existed to explain how emitted light might be restricted to specific wavelengths, this assumption does not necessarily seem to be justified. For example, if charged particles had rotational motion of an asymmetric charge around an internal axis, this would create a “standing wave”. If one assumes that only the magnetic component of a light wave (not the electric component) acted additively, only when the frequency of external light matched that of the rotation, one would predict constructive or destructive interference leading to increases or decreases in rotational velocity. I’d welcome any thoughts on that, particularly if there is a fatal flaw in the concept.

  5. Hello sir I have a doubt that what is the mystery behind ‘charge’ ???

    • Can you be a little more specific?

      We do understand many things about how electric charges and electric forces work. Our equations for them work extremely well. So in that sense most mysteries are solved. There are other, similar types of charges that control how the weak nuclear and strong nuclear force work, and similar equations work very well for them too.

      On the other hand, I cannot tell you why there are electric forces, weak nuclear forces or strong nuclear forces in the world. So in that sense there are still mysteries.

  6. If time stands still on a photon travelling at the speed of light, how does the photon remember what colour it’s supposed to be? Presumably each colour has a frequency which implies a vibration of something over TIME, which no longer passes.

    • Your error here is a misinterpretation of relativity and frequency. If time stood still for a photon, we wouldn’t be able to see them move. And color of a wave isn’t something that has to stay constant.

      Instead, while we can still see the photon move (at the speed of light) we don’t see it age. That is the sense in which time doesn’t pass. From the photon’s point of view, the lifetime of the universe goes by in an instant.

      The frequency of light on the other hand depends on its source. That source, having mass, can’t travel at the speed of light, and thus is affected by relativity. Thus, If you move towards the light source, you see more ‘crests’ of the waves per second and perceive it as higher wavelength than if you were stationary relative to the source. This is known as the Doppler Effect.

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