Of Particular Significance

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.

33 Responses

  1. So, this sentence really confused me at first, “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.” I was sitting here with minimal scientific background, and I’m just like, ‘Did the Law of Conservation of Mass AND the Law of Conservation of Energy just go out the window?!!’ Anyways, after reading the comments, I grew a better understanding and addition to more questions. Am I interpreting this the right way? Why are these laws still taught in schools? More importantly: HOW IS THIS POSSIBLE???
    thank you

  2. I apologise if this is not the appropriate place to ask this question, but I’m pushed for time for searching for a better option.

    If the matter in the Universe is formed from down and up quarks and electrons, what is the role of the other quarks?

  3. I believe that Human life is matter that is twisting into itself,smaller than the atom originally from space and will reverse like a rubber band twisted. We use the oxygen in this world to pass through us to help create particle reaction to have children and grow.

  4. Trabalhar na internet exige dedicação e muita persistência, não
    existe dinheiro fácil, mas existem informações que quando não temos acesso tudo se torna
    mais difícil.

  5. While particle physicists liming in imaginary world, Condensed matter physicists understanding nature more and more, while particles physicist getting far and far fro the soul of physics that its founder were Bohr, Einstein, Heisenberg, any many more the particle physicist try to be more helpful to understand ” Nature”. Particle physics developed to justify the behavior of nucleus but it could not and deviate to nasty games of mathematicians.

  6. hello matt, if particles can be created basically out of energy then are particles fundamentally just areas of energy condensed into mass?

  7. Select subjects give you an instant overview of gifts for special events or the relationships you have with
    individual recipients. Nothing much: Kuch khas nahi (कुछ खास नही).

    Rituals of Teej: Women undergo fasts; sing folk songs and dance in the name of Goddess Parvati.

  8. Interesting article.

    Thanks for writing this, I’m a high school with an intrest in particle physics; things like this are what will help me succeed in achieveing that goal, that is become a particle/theoretical physicist.

  9. An outstanding share! I’ve just forwarded this onto a friend who had been conducting a little homework on this.

    And he in fact ordered me dinner due to the fact that I discovered it for him…
    lol. So let me reword this…. Thanks for the
    meal!! But yeah, thanks for spending some time to talk about this topic here on your internet site.

  10. When I initially left a comment I seem to
    have clicked on the -Notify me when new comments are added- checkbox and now
    whenever a comment is added I receive four emails with the same comment.
    There has to be an easy method you are able to remove me from that service?

    Appreciate it!

  11. Only desire to express a person’s article will be as shocking. The understanding for your release is merely interesting and i could you might be a specialist during this theme. Good together with your agreement allow me to get hold of your nourish to hold up-to-date together with impending post. Many thanks thousands of as well as be sure to carry on a rewarding work.

  12. You have to throw the collected dirt from the container in the garbage can in your backyard or outside your house to prevent the dust and dirt
    from getting back into your house. As can be seen from the above applications, cleaners of this
    type have a wide variety of uses. Try to find brands
    which are highly regarded, instead of the
    higher-end models which will cause you to spend a fortune.

    1. Start with freshman-level physics — then relativity, and then quantum mechanics. At that point you are ready to learn the basics of particle physics, but following that you need to learn quantum field theory, the modern language for how this all works.

      I am planning to write a book that will make this path a little easier. But in the end, if you really want to know how this works, it takes some years of study.

  13. Dear Strassler

    I am math major interested in understanding some physics. I have studied fair amount of mathematical physics, but i dont have any clue about physical aspects of the theories. How does one view particles? ( I am not happy with top down approach defining them as some states of the underlying Hilbert space.) What physical considerations motivate choices of particular Hilbert space or operator algebras How do people realise all the symmetries and representation theory associated to quantum field theories practically; i mean what phenomenon they see in experiments lead them to believe in many natural mathematical constructs like gauge bundles, connections .

    My geometry and analysis background is pretty good. Can you suggest resources where I can see practical aspects are treated at the same level as geometric and spectral ideas in physics? I should able to atleat find toy models to explain physical observations without bothering about the exact details of the nature of equations or spaces– right simplications to get the picture , but not overly simplified.

    1. I suggest that you talk to graduate students or Professors from your Physics department. May be talk to experimentalists, try to connect what you know with what they seeming to be doing. I had a similar problem once, but talking to people really helps.

  14. Kids are under more pressure than ever to succeed
    in school. With school books, notebooks, computers, video games, cell phones (not to mention snacks and water bottles), all having become “must have” items.
    To do this I still put a large plastic bag in
    my backpack and pack everything in this.

  15. Thanks for the auspicious writeup. It in reality was a amusement
    account it. Glance advanced to more brought agreeable from you!
    However, how can we communicate?

  16. 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.

    1. 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.

    1. 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.

  17. 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.

  18. 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?

  19. 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).)



    1. 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.

  20. 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!

Leave a Reply


Buy The Book

A decay of a Higgs boson, as reconstructed by the CMS experiment at the LHC