Of Particular Significance

Most Particles Decay — But Why?

Why do most particles disintegrate [the technical term is “decay”] into other particles?

Particle physicists have discovered a slew of apparently elementary particles, and there may be more.  But most of these types of particles aren’t just lying around on the floor waiting for us to sweep them up; we’ve had to build special machines like the Large Hadron Collider to produce, discover and study them.  Why is that?  Because most of these particles — with the exceptions of the ones out of which we ourselves are made, and a couple of others — fall apart (“decay”) into other particles in a tiny fraction of a second.  I mean tiny: a millionth of a second is forever.  Some of these particles survive only a trillionth of a trillionth of a second, or even less!  (You may well wonder how we find such evanescent things!  That’s another article; or you can read about how physicists are trying right now (August 2011) to find the Higgs particle.)

In this little article, using some pretty decent though imperfect analogies, I’m going to give you some insights into why decay is the ultimate fate of most elementary particles.

You may recall (or you may want to read this article [coming soon] or  the first part of the Higgs Particle FAQ) that waves in a quantum world are actually made from particles; sound waves are made from phonons, light waves are made from photons, and so on.  Or you can just accept this and read on.

Particle decay is to particles as “dissipation” is to waves, which is something with which you are very familiar (though you may not know it yet — read on!)

[ dissipate: to become scattered or dispersed; be dispelled; disintegrate: The sun shone and the mist dissipated.]

Nothing lasts forever, including the sound of a plucked string on a guitar or violin, or a struck note on a xylophone.  In fact, before there is sound, there is vibration.  The guitar string or xylophone key vibrates back and forth.  Why do you hear a sound, even though the string is far from your ear?  You  hear it because the string, as it vibrates through the air, makes the air vibrate, creating waves that move through the air and reach your ears, making your ear-drums flap back and forth — a motion which your brain converts into your experience of a musical tone.

Why does the sound of the string gradually die off?  When you plucked the string, you exerted yourself a little bit, and some of the energy you used was turned into energy of the vibrating string.  Energy is conserved — it is neither created nor destroyed, though it can move from place to place and change from type to type.   Little by little the energy which manifests itself in the vibrations of the string is lost, converted to other things.  Some is lost to the vibrations of the air, and thus to the sound waves.  Some is lost to friction and thereby to heat, which involves microscopic vibrations of molecules in the string and in the pegs that hold the string in place.  This conversion of one type of vibration to many other types, and the transfer of energy from the large-scale motion of the vibrating string to other places, is called dissipation.   Dissipation happens because the vibrating string is in contact with — has some sort of interaction with — other things, in particular the air and the pegs at its two ends, and also because of its own internal structure.

At left, a string is plucked; the energy expended in plucking the string is turned into the energy of the string's vibrations. Right: the string's vibrations serve to make waves in the air, and to warm up the pegs at the end of the string as well as the string itself, through friction. In this way the energy of the vibrations gradually dissipates into waves in the air and vibrations of microscopic molecules.

Particles decay by a similar sort of dissipation, but this is where quantum mechanics comes in and makes things different.  While the vibrations of the string disappear gradually into broad waves of sound and the jiggling of hordes of atoms and molecules, a typical particle can decay suddenly into just two, or three, or maybe four lighter-weight particles.  This is just the quantum version of dissipation; it is the same basic idea, with a quantum twist.

For example, a Higgs particle may decay suddenly into two particles of light (“photons”); a Z particle may decay suddenly to a muon and an anti-muon.

Terminology: Particles that decay rapidly are called “unstable”; particles that never decay are called “stable”. Particles that take a long time to decay are often called “metastable” or “long-lived” — but CAUTION: “long-lived” and “metastable” are relative terms whose precise meaning is context-dependent.

Above: A Higgs particle, at rest. Below: The Higgs particle may spontaneously, rapidly and permanently disintegrate into two photons (particles of light.)

A small warning: I’ve had to white lie just a bit here.  The phenomenon of dissipation that particles undergo is a quantum version of the dissipation that waves are subject to — that is true.  But in order to appeal to your intuition, I have described a type of dissipation that you are familiar with, and though similar it is not quite the one that is responsible for most particle decays. 

Almost all particles known to us decay, most very rapidly.  The only known stable particles in nature (for reasons to be explained later) are

  • the electron (and anti-electron)
  • the lightest of the three types of neutrinos (and its anti-particle)
  • the photon (which is its own anti-particle)
  • the graviton (which has not yet been observed and won’t be detectable any time soon, though gravitational waves have been indirectly detected and probably will be observed soon)

Then there are some particles that might be stable but probably are just extremely long-lived — with lifetimes so long that only a small number of them have decayed since the Big Bang.   These probably-metastable particles include

  • the other neutrinos (and anti-neutrinos  … I’m going to stop mentioning the anti-stuff, it goes without saying)
  • The proton (which is not an elementary particle, see here)
  • Many atomic nuclei (the cores of all the types of atoms we see around us)

The other rather long-lived particle is the neutron, which when on its own, outside an atomic nucleus, lives just 15 minutes or so.  But neutrons inside many atomic nuclei can live far longer than the age of the universe; such nuclei provide them with a stable home.

What determines how quickly particles decay?  Well, let’s ask what determines how fast the waves on the vibrating string dissipate.  It  has to do with what objects the string interacts with (the air, the pegs on its ends, itself) and how strongly the string interacts with those other things.  Air is easy to push around, so a guitar string can ring for quite some time. But if you put the string in a bathtub, its vibrations would die away much faster, because the string, in making water ripples, would use up its vibrational energy much faster.  And you yourself can make the dissipation occur much faster if you put your finger right on the edge of the string.  This is because (as you can feel)  the atoms and molecules in your finger start to absorb the energy.   Since you are interacting more strongly with the string than anything else, you determine thereby how quickly the vibrations die out.  The harder you press on the string, the more strongly you interact with it, and the more rapidly the sound stops.

What is true for wave dissipation is true for particle decay.   Some types of particles interact with each other strongly, others less so.  For instance, photons interact with ordinary solid matter strongly, which is why the earth is opaque to light; neutrinos interact with ordinary solid matter very weakly, which is why they usually travel straight through the earth.   Quarks have very strong interactions with each other, which is why they are always stuck inside composite particles like protons.  But quarks interact with electrons rather weakly, so electrons can easily fly free of quarks — and this is why electrons in atoms are found orbiting at a relatively great distance from the protons and neutrons that make up the tiny atomic nuclei.

Suppose a particle of one type (the “parent”)  is able to decay to two or more particles of other types.  The stronger is the interaction between these types of particles, the more likely the decay is to occur — and thus the more common is that type of decay, and the shorter is the “lifetime” of the parent particle.  For instance, the Higgs particle interacts very weakly with light, which is why its decay to two photons is rare.  But it interacts much more strongly with W particles, and so, if it is heavy enough to decay to W particles, it does so most of the time.  See here or here for more details on the Higgs and its decays.

So now you know that the basic physics behind particle decay is a quantum version of what you see around you: the dissipation that is happening to vibrations of all types.   You know now that the speed of dissipation has to do with how strongly a vibrating object interacts with other objects; and that in an analogous way, particles that have stronger interactions will typically decay faster than those that have weaker ones.  But this is not the whole story.  Quantum mechanics influences particle decay in ways that are not intuitive from daily life, and determines why some particles do not decay at all, or decay only very slowly.  Fortunately these features can be mostly stated as rather simple rules.

78 Responses

  1. I love this article. Had to leave a comment to let you know your work is appreciated!
    One question though. Would a theoretical fully isolated particle or elementary particle ever decay?

  2. So in summary; the collider hurls a vibration or specific resonance at a target. The result of more propulsion shortening the time the soundwave/resonance exists and reducing its spray pattern. Speed alters the pitch, tone and measurable target area and uses. Particles/wavelength’s? Which are we studying?

  3. Thanks for the article. I have a doubt though.
    If a particular non-stable particle has to break down does it have to interact with anything? Suppose if I assume that I leave a particular particle (unstable) in empty space alone, with any other particle absent or is some billions of light years away, will it still decay into stable ones?
    Does relativity has any effect on decay in this scenario?

    1. One of the key ideas that emerge from modern physics is that there is no such thing as ’empty space’. What we call space could be said to consist of the fundamental quantum fields. As Matt explained in this and many other postings, a particle is a regular ripple in its corresponding field, often continually interacting with at least one other field. For example, an electron speeding through ’empty’ space is rippling through its own electron field, and also continually deriving its mass from the Higgs field and its unit of negative charge from the photon field. So we have to deduce that these fields occupy every point of space.
      The electron is stable, but the same principle applies to your question about an unstable particle. Suppose it is a muon. Rippling through space in the muon field, it is drawing 207 times as much mass from the Higgs field as its lighter cousin, the electron, while having the same charge interaction with the photon field. After about 2 microseconds, it interacts with the Weak field, which transfers its properties into new ripples in other fields, most likely the electron and neutrino fields.
      I hope this is a basically correct and helpful explanation. But, as a non-scientist, I would very much appreciate confirmation and more detail from experts. I am particularly puzzled by how every point of ‘space’ can everywhere contain all the fundamental fields…?

      1. Thank is for the reply. I was expecting no one to even see my comment. I am not a scientist too.
        After reading the comment I had the same question you raised at the end.

        I studied both quantum and classical field theory. Assuming large scale structure of space time to have point like locations, like crystal lattice structures, that have random particles (in multidimensional space) pop in and out of existence, I wonder what is the case with the thing that happens/exists between two such pops – possibly happening simultaneously !?! There has got to be some empty space concept or our entire premise seems to me like a meaningless premise.
        I think the concept is more like string theory adjusted to 3d space. Can it be possible?

  4. As a novice in this field I must say that your explanations were for me the most lucid and best distilled for meaning that I have read to date. Thank you for not assuming that anyone asking questions about particle physics must be already thoroughly versed in the basics.

  5. Matt is a total douche. I love people whose immediate reaction to a work they’ve written being criticized is to attack the other person with attitude. How about fucking just teaching Matt huh? Don’t be a dick, be a teacher. Stay in control of yourself at all times. Marcus Aurelius would be proud

  6. Hello there, professor. First off, I would like to congratulate you for your article. Exceptional as usual.

    I would like to know your opinion on whether the particle decay (and other known phenomena, like the radioactive decay) are uncaused, that is, do not have any particular cause. As far as I know, there is no known reason for a particular atom to decay, they are objectively random events. All you can know is some variables or parameters involved. For instance, in the radioactive decay the most you can do is address a half-life constant for every element that has radioactivity and predict when an ensemble of radioactive atoms will decay through statistical methods, although, it is impossible to know when, and if, a particular atom will decay.

    Consider the case of an incandescent lamp. Macroscopically, you can attribute the cause of the lightning of the light bulb as the passage of electrical current due to the switch activation. However, analyzing the same problem microscopically, the case is slightly different. When the switch is turned on, the lamp filament is excited by the passage of the current of electrons and the free electrons in the atoms become excited. The light is emitted, then, when the electron decays. But when does the decay occurs? The decay can happen at any time or place indefinitely. There is no cause for a particular electron to decay, only a probability chance of decaying at a particular instant of time, which can occur or not. That’s what is called “spontaneous emission” (that’s not the case for a laser beam, which is a “stimulated emission”, even though it starts spontaneously). However, as there are sextillion of excited atoms in the filament, the probability of decaying and emitting light, through the law of large numbers, turns out to be practically 100%, what gives the appearance of being deterministic.

    All you can affirm about the lightning of the lamp is the “condition” (not “cause”) for the lightning. Cause determines, condition allows. For light to be emitted it is necessary to the electron be excited, but being the electron excited, the emission can occur, or not, that is, the emission is not determined by any constraints.

    Sorry for any possible grammar typos. Thanks again for the excellent article.

    1. Particles decay because of the vacuum environment in space ! This vacuum eventually pulls particles apart aka the “Weak” nuclear force ! On the other hand The strong nuclear force is created by the gravitational & the electro-magnetic ! “Forced” electron migration is one of keys to the life cycle of our universe ! If you wish to want to more email or fb me !

      *Richard Edward Peters Jr.* *”Genius”@Large !*

  7. Dear Professor Strassler,

    first off let me say I love these articles, they are great!

    I am currently writing an article on the decay channels of the Higgs Boson myself and I was hoping perhaps you could help me with references you used in writing this articles, such as papers and books.

    I hope to hear from you soon.

    1. The reason we have atomic decay is the Weak Nuclear Force ! The Weak Nuclear Force is because of the vacuum of space ! The vacuum of space is in fact the Weak Nuclear Force !

  8. The “reason” for elemental decay is the Vacuum created during the “Big Bang” that brought our universe into existence ! This vacuum will become the greatest natural force & all matter in expression will return to a point of origin !

  9. Sorry, Matt, I attached the following question to ‘What’s a Proton, anyway?’, but now realise it better fits here under ‘Most Particles Decay’:
    I was getting really comfortable with your explanations that electrons stay in ‘orbit’ around the atomic nucleus because they cannot drop below their lowest permitted energy state, and that, while neutrons can spontaneously decay into protons, the reverse process, proton-to-neutron, only happens in the extreme pressures inside stars – or the LHC. Then I read somewhere that some isotopes of common elements display both types of decay. For example, in Potassium-40, neutrons change into protons, transmuting the atom to Calcium-40 and releasing an electron and antineutrino in about 89% of decays; but 10% of the time an electron drops from the inner orbital, is captured by a proton which transforms to a neutron and releases a gamma ray and a neutrino. Result: Argon-40. And there are even a small minority of cases where the proton turns directly into a neutron, releasing a positron and a neutrino. No need to wait for star-core collapse: the bananas in my fruit-bowl are breaking the laws of physics on a daily basis! This calls for one of your inimitable explanations!

    1. Since posting my question above, further reading has made me understand that, although proton-to-neutron change is virtually impossible for isolated particles, it is perfectly feasible when the total mass-energy of an atomic nucleus is involved. Just as a slight excess of neutrons causes beta minus decay into a proton releasing an electron and antineutrino, so an excess of protons causes beta plus ‘decay’ into a neutron releasing a positron and a neutrino. E.g. Cobalt (27protons) – 60 isotope can change into Nickel (28) – 60 , whereas Cobalt(27) – 56 isotope changes to Iron(26) – 56.
      Which leaves me with two hopefully more sensible questions:
      1. is Potassium(19)-40 unique in ‘swinging both ways’, having a 10% chance of transmuting to Argon(18)-40 instead of Calcium(20)-40, in which case, what is so special about a nucleus containing exactly 19 protons and 21 neutrons?
      2. What about Electron Capture? How come in some isotopes an electron can plummet from its inner orbital to collide with a proton, resulting in a neutron and released neutrino? Why doesn’t this happen more often, given the attraction of positive charges? (Don’t get me wrong, I’m very thankful it doesn’t happen more often!)
      I hope these questions from an amateur make some contribution to the theme of this thread.

  10. Well not sure if that’s a legit comment by ‘acne…’ but if you’re not aware of Google and SEO and deranking, you’re probably better off either learning this or hiring a dedicated webmaster who knows. Copyscape and Getty and such are somewhat useful tools for images or text BTW, but IMO total idiots when it comes to getting blacklisted from practically every major site as an abusive bot. Imagine you’re running a web spider like Google or YACY and you do this, in this order:
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  12. I thought I will be the first one to comment that this whole theory sounds like pure cattle business. Why do they need all of this architecture and just start realizing that this stuff is made from some magical type substance that no one just yet can conceptualize. A while ago during 10 hour per week chemistry lectures and lab experiments I saw something completely different about these particles than what the QM model describes.

  13. dead bodies are sometimes found on the side of the road. sometimes murderers will bury dead bodies to try to hide any evidence. we watched the documentary on the body farm where they study how bodies decay. in the chapter on soil, you learned about various horizons in the soil. discuss the speed of the decay process in each of the horizons. where do you think a body would decay faster? explain

  14. On particle decay: I believe in the conservation of energy law and see everything in existence as a form of basic, eternal energy. I believe reality has always existed and has a constant, eternal value of 1. In other words, all that exists within reality can never lose or gain value, no matter how many matter to energy or energy to matter exchanges occur within it. This is in agreement with the conservation of energy law. That law is telling us that something had to have always been in existence in order for anything to exist. Just what that basic “something” is and how it managed to do what we’re witnessing are the basic questions. Is basic energy intelligent or completely mechanical? That’s another question to seriously ponder.

    All particles may eventually be stripped of the properties that distinguish them from the eternal energy that constructed them. If this is so, the process is most likely reversed to allow reconstruction of universes within an eternal reality. If gravity is also eliminated in the decaying process, there must be some other force or forces involved in the reconstruction of matter and the corresponding restoration of gravity. This unconventional thought process may offer clues into how energy from decayed matter can possibly form singularities that eventually expand into universes once critical mass is reached. A “big crunch” of matter and its massive amount of collective gravity is not necessarily the only way a singularity can begin to form and reach the critical point necessary for a “big bang” to occur. Quantum mechanics may be the enabler of this endless cycle of decay and reconstruction if universes do decay to the point of gravity loss.

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  17. RobertC
    As will be obvious from the question, I am not a scientist. However I am baffled by the concept that fundamental particles decay into other fundamental particles. Surely one fundamental particle cannot transform itself into one or more other fundamental particles? Perhaps decay “into” is the misleading word. can you throw any light on this for me as I’m finding it difficult to move on in this most fascinating of topics.
    Many thanks

  18. If you get into the math, it becomes clear very quickly why force and energy have the same sort of relationship as amps and volts. The concept is Dimensionality.

    Force: 1 Newton = 1 kg*m/s^2 (kilogram-meter per second per second)
    Energy: 1 Joule = 1 kg*m^2/s^2 (kilogram-square-meter per second per second)
    Mass-energy: 1 Kilogram = 1*c^2 ~= 9*10^16 joules
    That extra mass part of the term makes for a world of difference!

  19. Sorry, I was brainstorming a concept of energy/force not by its definition but by what it does. I can’t see how my last sentence could possibly insult women physicists. If anything, women who compete with the man, in a man oriented fields of sciences (physics/ maths in particular) are forced to think like a man. She tries to demonstrate her competence by using cold logic in her reasonings. She is being forced into unnatural mold. Recent researches of a human brain show that the brains of the two sexes really do function differently. Women physicists should be appreciated for the unique aspect they can provide to sciences, their intuition. I may of said this in a very blunt way (English is my second language) but I never meant to insult anyone. That put aside, this is your website and I know when I’m not welcome, so this is my last post here. However, dismissing someone elses reasoning as absurd just because it challenges the mainstream idea demonstrates to me a sensitive spot in a mainstream theory of these very subjects. Just sayin’ …professor

  20. How can force perform a work/ a task if it has no energy? A dead person can’t exert any force because it has no energy. Concentration of photons into a small stream cuts through things. Is energy of photons doing the work or does the energy become a force when concentrated? The strong gravity inside the black hole destroys more energetic stars. How can we say a strong concentration of gravity in a tiny place will surpass the energy of the more massive star if it has no energy? A thunder bolt fries the object it hits. Do we see anything like that in space? Can electric bolt travel through vacuum of space? If it can’t because it needs a medium of low resistance as a pathway, what happens to all those negatively charged electrons that run into quadrillions plus there in space? Two nuclear bombs dropped on Hiroshima and Nagasaki burned everything and everyone within its epicenter. If nuclear force is something without the energy, why didn’t it merely pushed/ blow away things away from itself?
    The floor and the ceiling drawn on a paper of the house plan serves two different functions, but a 2D representation of the space it will occupy once the house is built is the same space. We could walk on the ceiling if we turned the box/ house upside down. You say a particle has energy you can assign a number to, but force has zero energy. It
    looks to me like we are walking on a ceiling already. A force is more powerful than the energy locked in the particle. So who is performing this work? Why is it so hard/ impossible for a physicist to consolidate the two as one? These things of nature don’t depend on values assigned to them by us.
    I am not challenging physics. I am demonstrating how the rest of us see the natural world. You male brain is better suited for tunnel vision and spatial comprehension, but my female brain is great for networking. It looks very probable to me that the two need each other to get the whole picture of things. A man should not strain his brain to think like a woman, and a woman should not strain her brain to think like a man. Just sayin..

    1. REgarding your last comment: you are demonstrating how YOU see the natural world, not how “the rest of us” see it. You have no idea how other people see it. And for you to wrap yourself in “female brain” propaganda is absurd. You have just insulted every woman in my field. End of conversation.

  21. Thanks for your reply. Yes, I do know this looks wrong to a physicist, and no, I didn’t mean to upset or challenge anyone. Still, I see force as a dispersion of enormous energy, hence the two are interchangeable. I do apologize for my non – expert delivery.

    1. If you see force and energy as interchangeable, then there’s no way to communicate — that’s what I’m getting at. The very definition of these two concepts makes that impossible. If I told you that I thought that in my view the floor of the room you’re sitting on is interchangeable with the chimney above your fireplace, could we have a serious conversation about houses? With the definition that is used in physics, force can be zero when energy is not; energy is a number whereas force has a direction — etc.

  22. Oh, you probably will never read this, seeing that it is an older article. Nevermind, question; why can’t we just say that all, and I do mean ALL particles are no more and no less than various strengths/potencies of energy that disperse into the medium (Higgs field, vacuum, surrounding space). The separation is caused by the interaction/interplay of a mix of forces that operate from within that blob of energy we call a particle. We could also say that energy is that which we refer to as a force. Hence, EM force is energy made of strings with two opposing charges. When these connect into a long line, we feel the force or fow of that energy. Nuclear force is the energy that carries or breaks up into three different points of connection thus allowing for 16 combinations or outcomes. Nuclear weak and strong aspects of it are merely the limit to how much interaction is possible before something gives in. We can fill the glass with water, but when it reaches its capacity, it will overflow. The number of these combinations is limited because the number of connections is determined by the nature of the energy that makes up the force. The two terms are interchangeable. Force is energy and energy is a force. They are one and the same thing. Too much force
    or energy results in a spillage of that energy, therefore particles of smaller mass (if that spill is at its very minimum than virtual particles). Vacuum is a medium with its own variables through which these energies vibrate. The nature of this medium is such as to push against the energy, not because it stronger then forces per square inch, but because there is more of it. Analogy to that would be an ocean and the submarine thou submarine will not spin but may get crushed by the enormous pressure of water or under the weight of it. I believe that this is the reason why the blobs of energy (particles) form into spheres and the struggle / war of push and shove between the two different substances cause it to spin. the particle can’t stand still (too much energy in there) so it has to keep moving but the movement is limited by the amount of the vacuum that has its mass or even weight, so it assumes an orderly way of coping with it, it start to spin. (no rotten eggs or tomatoes please)

    1. You can say it, but it will get you nowhere, because physics isn’t done with words. Show me equations that represent your words and lead to predictions that we can compare with experiment; then we will compare with experiment and see.

      My educated guess, looking at your words, is that it would be impossible for anyone to turn your words into equations that would agree with experiment. And I know *I* can’t do it. For instance, despite what you suggest, force is not energy and energy is not force; I can take a free particle that is exerting no force on anything, and I can measure its energy; all the forces involved are ZERO, but the energy of the particle is NOT zero. So that seems like a flat-out contradiction. The only way to turn your words into equations would be to redefine some of the words so that they don’t mean what they mean to a physicist. It would be better for you to learn the language of physics first, and translate your ideas into the language that physicists speak. Then maybe equations can be written down [if that is even possible], predictions made, and comparisons with experiment performed.

      And don’t forget that modern physics equations agree with data to very high precision, so you’ll have to explain why those equations are wrong, not just why yours are right.

  23. Thanks for the great article. Am I correct in thinking that although there are four elementary particles that can’t decay they can all transform in some other way. For example, photons can be absorbed and electrons can collide with positrons. Can neutrinos and theoretical gravitons also transform as well?

    I ask because it seems that although transformation and absorption are different from decaying, all three are forms of mutability; ways in which a particle is destroyed and replaced with something else. It seems to me that if Neutrinos or Gravitons can’t mutate in some way then they are somehow more fundamental than the others because everything would eventually end up as that particle.

    1. Gravitons can be absorbed and emitted just like photons (it’s just more rare) and neutrinos can annihilate with anti-neutrinos and can also convert to electrons when they scatter off each other or off of nuclei. So yes, mutability is a consequence of having any interactions at all.

      The only thing that makes *decay* different from other forms of transformation is that a single particle can (and will) decay entirely on its own. All other forms of mutability require two particles come into close proximity, and that is much less likely to happen spontaneously except in very dense environments such as cores of stars or particle accelerators.

      1. Thanks Matt. Does the rarity of these collisions mean that in a heat death of the universe we will just have electrons and neutrinos remaining?

        1. Photons, electrons, neutrinos, positrons, antineutrinos, gravitons — and possibly protons and other atomic nuclei (and their antiparticles) though these are expected to decay (very slowly). But dark matter particles might be stable too; there may be additional conversation laws that prevent their decay. Alternatively, they may decay very slowly.

  24. Most particles decay but why electrons do not??? This has been puzzling me. Is it because there is no particle that an electron can decay into (if so why?)? or there is extremely high (energetic?) barrier for an electron to climb up to decay into another particle (if so what is this barrier?)? or 1) the electron elementary charge cannot be split into anything smaller quanta (<- if so why?) and 2) particle decay always comes in a pair to conserve momentum. 1) and 2) prohibits an electron to decay? So, in the end, conservation of momentum and of elementary charge prevents electron decay? How is elementary charge and particle momentum related? First of all, what will happen if two electrons collide at close to speed of light? Does such a collision destroy the electrons?

    1. There is no particle that electrons can decay to. Yes, conservation of electric charge, energy and momentum assure that an electron could only decay if there were charged particles (elementary or not) that are lighter than electrons; but there is no such particle in nature. To say it another way: conservation of energy and momentum and electric charge assure that the lightest electrically charged particle must be exactly stable — and that is the electron.

      Electric charge and particle momentum are not related; they are separately conserved quantities.

      If two electrons collide at near the speed of light, they will bounce off each other — i.e., scatter at some angle — and typically they will radiate some photons, and might even produce a pair of new particles, such as another electron and a positron. The original electrons will survive the collision. The charge before the collision is -2e and afterward must also be -2e.

      If you collide an electron and a positron, however, they may do more interesting things, because the total charge is zero. It is possible for the electron and positron to turn into a top quark and a top antiquark, or two photons, or three photons, etc. Long story.

      1. hmm, so why there is no charged particle lighter than an electron in nature? and Are there (many?) charged (non-composite) particles that are heavier than an electron in nature? Can they posses any arbitrary amount of charge (such as -2e, -3e, -4.34e, +1/4e, +πe, +2.71828e, +10^100e etc.)? First of all, how is charge and energy/mass related? how much energy is required to create (associated with) “(per unit/elementary) charge”? Is a charged particle always “massive”?(if so why?). Why there seems no massless charged particle?

        If two electrons fall into a black hole and collide inside the event horizon or at singularity, do they (and their charge) still survive? and the black hole will be charged (possess electric charge)? and you can still observe/measure/feel its charge from the outside of the black hole? Can these electrons (or electric charge) inside the black hole still exert its effect (extend its electric field) on the objects outside of the black hole?

        1. There are many particles heavier than an electron in mass, both composite and apparently-elementary ones. They all have charge equal to an integer times e/3. Why? we don’t know, but there are examples of theories in which this would be automatic; grand unification theories (which unify the three known non-gravitational forces) typically have this as a prediction, so you can view this as one of their successes.

          Why isn’t there a charged particle lighter than the electron? Well, some particle has to be the lightest; that is going to the be the stable one; and not surprisingly, the stable particle will be the one we’re made from. So you are asking the wrong question. The only right question is: why isn’t there are massless charged particle? And the answer is — there could have been, and in fact there would have been in the absence of the Higgs field. Since we don’t understand the origin of that field, the ultimate answer to your question is that we do not know why there are no massless charged particles.

          Yes, electric charge is conserved, and if charged particles fall into a black hole, the black hole becomes charged, and that charge can be measured outside the black hole through the electrical forces it exerts. http://en.wikipedia.org/wiki/Reissner%E2%80%93Nordstr%C3%B6m_metric

      2. >There are many particles heavier than an electron in mass, both composite and apparently-elementary ones. They all have charge equal to an integer times e/3. Why? we don’t know, but there are examples of theories in which this would be automatic; grand unification theories (which unify the three known non-gravitational forces) typically have this as a prediction, so you can view this as one of their successes.

        If the incomplete GUT can inevitably predict the amount (elementary e or e/3) of charge and other quantized fundamental values such as spin of elementary particles from scratch, that would be fantastic success. But I do not understand how the GUT can do that or if indeed it does. Can you explain how the GUT is capable of predicting the observed charge value (of an electron etc.) in nature?

        >Why isn’t there a charged particle lighter than the electron? Well, some particle has to be the lightest; that is going to the be the stable one; and not surprisingly, the stable particle will be the one we’re made from.

        Yes, it is possible to think that way but I am looking for something more mechanistic than that. For example, when I see any value/property is quantized, I associate it with wave-like nature of the phenomenon (and some kind of restriction/boundary etc. imposed on the system.) such as classical quantum wave mechanics describing behaviors of atomically bound electrons etc. (i.e. wave and spacial/geometric confinement/restriction) For now, I can happily accept such an explanation that if, say, spin of elementary particle (So you are asking the wrong question.

        I am not so sure if this is the case. If I am asking a question about the phenomenon which intrinsically has another layer of mechanism then I am not asking the wrong question.

        >The only right question is: why isn’t there are massless charged particle? And the answer is — there could have been, and in fact there would have been in the absence of the Higgs field. Since we don’t understand the origin of that field, the ultimate answer to your question is that we do not know why there are no massless charged particles.

        ho! existence of massless charged particles possibly without Higgs field? Very interesting! Thank you. I hope physicists will reveal much more about the mechanisms of our universe in the next few years or so (via Higgs field research and otherwise) and make it available to the public as soon as possible.

        >Yes, electric charge is conserved, and if charged particles fall into a black hole, the black hole becomes charged, and that charge can be measured outside the black hole through the electrical forces it exerts. http://en.wikipedia.org/wiki/Reissner%E2%80%93Nordstr%C3%B6m_metric

        This is interesting too. But how can electrical forces propagate from the inside of the black hole toward the outside if the force carrier of electric field ((virtual?) photon) cannot escape the hole? or can they? If so how? via the bubbling boundary of event horizon??

        1. Grand Unified Theories predict strengths of forces and various charges, but not spins (except for the spins of the graviton and of the photon, gluon, W, Z and Higgs, which are already predicted by quantum field theory in general). I have not found a good reference on the web yet, which suggests I need to write one. [It’s very distressing that 4 out of 5 websites that come up in a google search of “grand unification” and similar keywords are complete scientific garbage.]

          Regarding something more mechanistic: As far as we are aware, from theory and experiment, I’ve given you the complete explanation. There is no further layer of mechanism; there are conservation laws and symmetries; there are fields and their particles and their interactions; everything else follows from this. Someday maybe we’ll learn there is something additional to talk about, but with current physical theory there’s nothing more to be said.

          I think you misread what I said. I said we do not know WHY there aren’t massless charged particles. But we do know from experiment that no massless charged particles exist. (Strictly speaking, there’s a limit on the maximum charge; not sure what it is but it is certainly stronger than e/1000. Unknown massless neutral particles might well exist.)

          Re: black holes: your intuition is wrong. Electric fields surrounded the charged particles and extended off to infinite distance before the particles fell in, and as they fell in, to the black hole; and the electric field never dissipates even after the particles themselves go behind the horizon. It’s not as though you took a neutral black hole with no electric field and flipped a switch so that the charge suddenly wasn’t zero and the electric field had to turn on. The electric fields were always there, around the particles initially and around the black hole later. It all follows from Gauss’s law, if you know that one.

      3. -Dr. Matt Strassler
        Where are your articles on the current GUTs that predict/explain how some of “charges” and “spins” arise (automatically?)?

        “They all have charge equal to an integer times e/3. Why? we don’t know, but there are examples of theories in which this would be automatic; grand unification theories (which unify the three known non-gravitational forces) typically have this as a prediction, so you can view this as one of their successes.”

        “Grand Unified Theories predict strengths of forces and various charges, but not spins (except for the spins of the graviton and of the photon, gluon, W, Z and Higgs, which are already predicted by quantum field theory in general). I have not found a good reference on the web yet, which suggests I need to write one. [It’s very distressing that 4 out of 5 websites that come up in a google search of “grand unification” and similar keywords are complete scientific garbage.]”

  25. Do particles decay faster if they are produced in colliders? I am confused because is the Higgs a force particle that gives most other particles mass? If all Higgs particles decayed as quickly as the ones produced in the experiments would particles no longer have mass?

  26. Hi Professor Strassler,

    I have a historical question. I can’t find the answer anywhere, which I find quite puzzling. I know the neutron was discovered by Chadwick in 1932. When was the neutron found to be unstable? It seems like that would be a dramatic discovery, yet I can find no record of it. I’m reading The Second Creation by Crease and Mann, and they quote Maurice Goldhaber as being surprised by a prediction (based on masses) that the neutron could decay, yet there is no description of an experimental finding of this fact. Do you happen to know when it was found experimentally that free neutrons are unstable?


    Steve Whitt

    1. First, Chadwick and Goldhaber measured the neutron mass well enough to predict that it would be unstable. That was around 1934 or 1935. Here is their first measurement, but this first one was not precise enough for this prediction: http://www.nature.com/physics/looking-back/chadwick2/index.html . Here is the paper with their more detailed measurements, http://ivanik3.narod.ru/Termojad/Gorjachiy/Chadwick1935-151-479.full.pdf . In the last two paragraphs before the Summary they predict that a neutron, on its own outside of an atomic nucleus, must be unstable.

      I am pretty confident that most nuclear physicists would have quickly been convinced by the mass measurement that the neutron was unstable, but calculating its lifetime would not have been easy [Correction: based on Fermi’s theory of beta decay from 1934, it was quickly predicted (not sure yet by whom) that the neutron lifetime would be something vaguely around 30 minutes, with a large uncertainty — which is quite consistent with the actual lifetime, about 15 minutes.], and measuring it was certainly difficult.

      The first actual measurement occurred only in 1948, with better measurements soon after.


      One reason cited for the delay is the intervention of World War II, but there were technological advances needed also.

      It is not unusual for confidence to arise long before the corresponding measurement. Indirect arguments made almost all particle physicists very confident that the top quark existed as far back as 1988, and reasonably confident well before that — but the top quark wasn’t actually discovered til 1995. This is because the equations that describe the behavior of the bottom quark, assuming the existence of a top quark, made successful predictions for phenomena measured in the mid-80s, while alternative equations that describe the bottom quark, assuming instead that there is no such thing as a top quark, gave incorrect predictions.

  27. Am I correct in understanding that all particle decays are reversable? So that if two energetic photons merged, it could produce a Higgs particle. I suppose entropy would become relevant because it is much more likely for a Higgs to decay than the reverse to occur.

    1. Yes you are right. Two photons can make a Higgs particle just as two gluons can; the Higgs can decay to two gluons as well as to two photons. But indeed, a Higgs will decay on its own without your help, whereas if you want to make a Higgs particle you have to actively slam high-energy two photons or two gluons together. Since gluons are plentiful inside a proton while photons are not, we make most of our Higgs particles from gluons, but in principle we could make them from photons. Or from collisions of other particles, such as a muon and an antimuon; a muon-antimuon collider is under consideration for this purpose, but big technical problems are not yet solved.

      1. Why don’t we collide photons to make Higgs bosons? Is it much harder to control a photon? Is it a problem with the energy levels?

  28. Hello Prof Strassler,

    Is there anywhere outside of a particle smasher that you will find unstable particles that decay like this? Would these perhaps exist during a star collapse, supernova, or maybe right after the big bang?


    1. Yes — definitely right after the big bang, and in supernovas, and also in other high-energy stellar environments; near black holes; and most relevant for us, when cosmic rays (high-energy protons, mostly) hit atoms in the top of the atmosphere and create showers of unstable particles that rain down on us from the sky. There is a muon from such a shower passing through your body (and doing a tiny little bit of biological damage that your body has to repair) every couple of seconds.

      Not to mention all of the slow-decaying radioactive nuclei — including uranium, thorium, and radon — found naturally inside the earth.

  29. Dear Prof. Strassler,

    I am trying to figure out how fast the Higgs decays and how far it travels in a detector at the LHC. Figure 2.5 in http://www.hep.lu.se/atlas/thesis/egede/thesis-node14.html gives decay widths of the Higgs as a function of its mass.
    For a Higgs of 125 GeV, it is 0.002 GeV. Using the Heisenberg Uncertainty eq. lifetime*decay width = h-bar/2, I find lifetime = 1 zeptosecond. Is this correct? What kind of assumptions can I make about its speed so that I could calculate distance travelled?



    1. Marek — you can assume that a Higgs produced at the LHC typically travels at semi-relativistic speeds — speeds in the range of, say, 0.1 to 0.95 c. With so much energy flying around, it is hard to make a Higgs that moves very slowly (though it will happen occasionally.) And since the Higgs is a bit heavy it is rare that its speed is very close to c, though again this will certainly happen. A zeptosecond (0.000,000,000,000,000,000,001 seconds) is about right. At these speeds, time dilation only adds a factor of 2 or 3 at most to the Higgs’ lifetime. The corresponding distance that the Higgs can travel is larger than an atomic nucleus but smaller than an atom. Only a very fast (and also lucky) Higgs will travel further than an atom’s radius.

  30. I will remind you, it sounds promising! So (I guess it is impossible, but theoretically speaking), if we knew the exact conditions at which every different decay products happen, there would be no pattern at all? How do you know that all the variables are being controlled? I mean, I have read somewhere that gravity could be such a weak force because it may spread throughout other dimensions (This was my interpretation at least… I may be wrong). If this is to be true, wouldn’t it be possible that another uncontrolled dimension(s) is/are affecting the particle?

    1. All of the probabilities in question can be calculated, if you have a set of appropriate equations to work with; then you can compare the probabilities you calculate with the probabilities you measure in data, and figure out which sets of equations do not describe the data, and which ones do a better job.

  31. This blog is great! It’s amazing that I can actually understand most of this without having any previous physics knowledge. So probably my question is rather silly, but here it goes:
    For what I can understand, the same particle can decay to different types of particles. As you said in a video a Z particle can decay to form a muon and an antimuon, an up quark and an up antiquark or other particles. How does the Z particle “decide” what to decay to? What factors influence the resulting particles of the decay process?

    1. Quantum mechanics suggests that the “decision” is a random process… part of nature’s inherent randomness. If that sounds strange, it is — and it appears to be true. There are many other examples. If you remind me after the dust settles this week, I can give you more familiar ones.

      1. In the article, it sounded like the Higgs’ decay “decision” is not random but shaped by the things it’s interacting with. Was that not the implication? Instead does the “strength of interaction” simply affect the probabilities of decay; e.g. Higgs and photons don’t interact much so there’s a low probability of decay?

        Thanks! Love the blog btw.

        1. Well, I’d assume that it has something to do with virtual particle pairs as well. Denser particles should be rarer. And since we can’t predict or choose those (AFAIK), it’s effectively completely random although NOT even – there is a heavy bias like mentioned in the article. Closest to influencing is the Casimir effect?

          See also Ultraviolet_catastrophe on Wikipedia.

  32. Well, let me then represent a general public. With string we can speak aboud two pieces of matter with different impulse (mv) properties. Their interaction is then an equalization of impulse with given effectiveness. As this process is less than 100% effective, we have transient process. Now particle, is considered as single object in empty space, first of all. So, from the string analogy, we can conclude, that instead if single particle there is already (before any decay) system of some objects and that these objects are not particles themselves yet and they has some energy properties that tends to be equalized. As we observe the decay, it can be noted that degree of freedom of the system is increased and concentration of energy decreased. And probability of the event replaced effectivness of macro world. That’s how it looks to non expert like me. Still not entirely clear why the same decays of same particles has different times depending on outer conditions, like neutron in the article.
    As to nonlinear process, may be social activity, like forming of pairs/families out of groups of individuals.

  33. To be honest, I see very little help from guitar string analogy for understanding ethropy. may be it’s me.

    1. I assume you meant to write “entropy”? Thanks for your comment, because it reminds me to emphasize that entropy is *not* the point. Indeed, if I had wanted to explain entropy, I would have done so differently. But particle decay is not fundamentally about entropy — which is why I didn’t use the word “entropy” in the post.

      While it is true that particle decay increases entropy (a tiny bit), this is a symptom, not a cause. The important point in the guitar string analogy is that vibrational energy is transferred from one object (the string) to a variety of others (air, molecules in the pegs), because of interactions between the first object and the others — not that entropy increases in the process (though it does). This is to be distinguished from some processes in nature for which the tendency of entropy to increase can be viewed as the main cause — as in some aspects of weather.

      Another way to see that entropy isn’t crucial is to imagine the dominant loss of energy in the guitar string were to sound waves. In this case, entropy would increase very little — but the dissipation of energy would still be the cause for the sound dying off, and the analogy to particle decay would actually be better.

      Experts will note that the real problem with the guitar string analogy is this: in particle decay, the transfer of energy involves non-linear wave equations, not linear ones. I am not currently aware of a process with which most people are familiar in which transfer of energy is through simple non-linear wave equations. Suggestions are welcome.

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