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Turning Ab Into My Personal Blog, #1: So What Is Particle Physics Anyway?
I was hoping to post semi-regular updates about what I'm doing, partly because it's good to keep AB science active, partly because it's helpful for me to think about what I'm doing, and partly because hopefully it's even interesting. At some point I'm bound to deliver talks on it, albeit to a more specialist audience, but communicating science is always important.
So, anyway. The project I'm working on to stay with is related to "the lifetimes of singly/doubly charmed baryons". I simply couldn't see a point in going any further than that without trying to define all the words in that sentence, to set the scene a little. In other words, what even is Particle Physics?
The best way to begin is with atoms. As everybody knows, hopefully, atoms are the building blocks of molecules, they represent all the known elements, and discovering and understanding them is one of the triumphs of modern science. Everybody hopefully also knows that atoms have structure: a tiny, tiny nucleus, made of protons and neutrons, orbited at a distance by electrons. What's mind-blowing about all this already is the scale: even the largest atoms are only a few ten-billionths of a metre across, and the nucleus is another hundred thousand times smaller still!
It's worth stepping back in time to try and appreciate this. It's now a staple of A-level physics to present the ground-breaking experiments that revealed the size of a nucleus as something matter-of-fact, but of course at the time (around 1910) people like Ernest Rutherford had to be geniuses to interpret what was going on. But anyway, physicists soon realised that atoms had structure, and set about trying to understand it. Particle physics is the result of this quest.
It has led to some crazy places, and what's most staggering to me about those places is how unnecessary they all seem to be, at least at first glance. For example, if all life on Earth is made from protons, neutrons, and electrons, then why in hell do you need anything like a muon, which is a particle exactly the same as an electron, only 200 times heavier, and so unstable that it always breaks up after a couple of hundred thousandths of a second?! Likewise, why is antimatter an actual thing, when it is similarly short-lived? I'll try to answer these questions, at least partly, later. Moreover, it isn't just those two new things, but hundreds more, apparently without end. It wasn't until the 1960s that people finally made sense of all of this and realised that, in reality, there are really a few fundamental building blocks.
The key breakthrough was the prediction of quarks. These are the (six) particles that make up protons, neutrons, everything like them. The "baryons" in my project title are anything made from three quarks. The names of the quarks are: up, down, strange, charm, bottom, and top. There is no reason for these names other than that physicists like them and in the 1960s everybody was also clearly on LSD. That answers the second question about my project title: I'm going to be looking at baryons containing one charm quark (or possibly two).
The "lifetime" of the title speaks to the fact that these baryons do not last very long at all (roughly 0.0000000000002 seconds) on average. It's worth stressing that the lifetime is similar to a radioactive half-life, and is more useful when you are seeing hundreds or thousands of these particles as opposed to a mere handful. This is why particle detectors like CERN have to run their experiments for so long -- you need to create and observe a *lot* of these particles to get anything meaningful, especially when there's so many other things that could be created.
continued shortly...
So, anyway. The project I'm working on to stay with is related to "the lifetimes of singly/doubly charmed baryons". I simply couldn't see a point in going any further than that without trying to define all the words in that sentence, to set the scene a little. In other words, what even is Particle Physics?
The best way to begin is with atoms. As everybody knows, hopefully, atoms are the building blocks of molecules, they represent all the known elements, and discovering and understanding them is one of the triumphs of modern science. Everybody hopefully also knows that atoms have structure: a tiny, tiny nucleus, made of protons and neutrons, orbited at a distance by electrons. What's mind-blowing about all this already is the scale: even the largest atoms are only a few ten-billionths of a metre across, and the nucleus is another hundred thousand times smaller still!
It's worth stepping back in time to try and appreciate this. It's now a staple of A-level physics to present the ground-breaking experiments that revealed the size of a nucleus as something matter-of-fact, but of course at the time (around 1910) people like Ernest Rutherford had to be geniuses to interpret what was going on. But anyway, physicists soon realised that atoms had structure, and set about trying to understand it. Particle physics is the result of this quest.
It has led to some crazy places, and what's most staggering to me about those places is how unnecessary they all seem to be, at least at first glance. For example, if all life on Earth is made from protons, neutrons, and electrons, then why in hell do you need anything like a muon, which is a particle exactly the same as an electron, only 200 times heavier, and so unstable that it always breaks up after a couple of hundred thousandths of a second?! Likewise, why is antimatter an actual thing, when it is similarly short-lived? I'll try to answer these questions, at least partly, later. Moreover, it isn't just those two new things, but hundreds more, apparently without end. It wasn't until the 1960s that people finally made sense of all of this and realised that, in reality, there are really a few fundamental building blocks.
The key breakthrough was the prediction of quarks. These are the (six) particles that make up protons, neutrons, everything like them. The "baryons" in my project title are anything made from three quarks. The names of the quarks are: up, down, strange, charm, bottom, and top. There is no reason for these names other than that physicists like them and in the 1960s everybody was also clearly on LSD. That answers the second question about my project title: I'm going to be looking at baryons containing one charm quark (or possibly two).
The "lifetime" of the title speaks to the fact that these baryons do not last very long at all (roughly 0.0000000000002 seconds) on average. It's worth stressing that the lifetime is similar to a radioactive half-life, and is more useful when you are seeing hundreds or thousands of these particles as opposed to a mere handful. This is why particle detectors like CERN have to run their experiments for so long -- you need to create and observe a *lot* of these particles to get anything meaningful, especially when there's so many other things that could be created.
continued shortly...
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and smoking....
//Well ,done Jim ( are you sure that your real first name isn't Sheldon)//
take it as a compliment - umm sneeringly said that of me a few years ago - the alternative was David Icke
I would have preferred Kripke - he published his first paper at 16 and put modal logic back on the map(*)
keep on blogging - - and foo the foo-ers!
and good luck with serb - the only nationality that doesnt have a vowel in it CPB - cyrillic
(*) I never knew if they realised all the characters in Bing Bang are named after famous scientists and philosophers
//Well ,done Jim ( are you sure that your real first name isn't Sheldon)//
take it as a compliment - umm sneeringly said that of me a few years ago - the alternative was David Icke
I would have preferred Kripke - he published his first paper at 16 and put modal logic back on the map(*)
keep on blogging - - and foo the foo-ers!
and good luck with serb - the only nationality that doesnt have a vowel in it CPB - cyrillic
(*) I never knew if they realised all the characters in Bing Bang are named after famous scientists and philosophers
TTT's first question...
First let's set the scene. It's the early 1900s, and JJ Thomson recently discovered the electron (the first subatomic particle to be identified), while Einstein and others have finally convinced the world that atoms are real. So the race is on to discover their structure. At the same time, Marie Curie has investigated radioactivity and people have learned how to harness it so that they can create alpha particles at will.
From the point of view of early 20th-Century physics, an alpha particle is a tiny, positively-charged, super-fast, high-energy bullet. It doesn't travel very far before hitting something, but the beauty part of that is that you know where it hits and you know where it's heading**. The other point is that, by this time already, it's well-known that if two things hit each other then you can use well-founded principles like "conservation of momentum" and "conservation of energy" to deduce something about the collision. So this leads to an idea that is the basis of all modern particle physics experiments: fire two things together and see what happens when they smash into each other.
The Geiger-Marsden-Rutherford experiment is one of the first of these. They hit upon the idea to try and work out what atoms look like by firing alpha particles at an atom and see what happens. By this point, as I say, it was already known that atoms have electrons inside them, but also that atoms are electrically neutral, so there must be a positive charge in the atom *somewhere*. But where, and how is it spread out?
The most reasonable first idea is that, because an atom is a hard ball and because the electrons are tiny, then they might swim around in a "sea" of smeared-out positive charge. On this scale, the positive charge would be so smeared out that any speeding bullet should burst through it, perhaps with a small deflection/glancing blow on the way. If that's the case, then you should see alpha particles on the other side of any (thin) target.
And, mostly, this is what was observed. So far, so good. But... luckily, the scientists involved thought to set up a detector capable of seeing if any alpha bullets were deflected by more than just a slight angle. And, astonishingly, some were! Not many, but also far more than anybody could expect. The classic quote is along the lines of firing a cannonball at tissue paper and watching it bounce back into your lap.
The two key observations, then, are:
1. Some alpha particles fired at a thin target (gold foil) deflect almost all the way back on themselves;
2. Most alpha particle pass straight through the foil.
The way Rutherford reconciled this was to conclude that perhaps the positive charge in the atom was concentrated into a tiny spot! This works, because:
1. Positive charges repel, so if the alpha particle heads directly towards the concentrated charge then it should be repelled backwards;
2. Most of the atom is empty space, allowing the alpha particle to pass straight through unimpeded.
The particular genius of Rutherford is not just to describe this model but also to use it to make a testable prediction. Actually, his second genius was to publish this model knowing that, according to the then known laws of physics, it made no sense at all.*** He pressed ahead because it fit with experimental data (a key distinction, for anybody lurking -- the hold standard of useful science is its ability to make predictions and then to have those predictions be verified in experiment!). But it took a few years before the model made sense.
**Obviously, from a modern perspective, this was too optimistic (see the Uncertainty Principle), but this is a world before Quantum Mechanics, so I don't care.
***No room, or time, to explain why, except to note that if like charges repel and opposite charges attract, then why don't the electrons fall into the nucleus?
First let's set the scene. It's the early 1900s, and JJ Thomson recently discovered the electron (the first subatomic particle to be identified), while Einstein and others have finally convinced the world that atoms are real. So the race is on to discover their structure. At the same time, Marie Curie has investigated radioactivity and people have learned how to harness it so that they can create alpha particles at will.
From the point of view of early 20th-Century physics, an alpha particle is a tiny, positively-charged, super-fast, high-energy bullet. It doesn't travel very far before hitting something, but the beauty part of that is that you know where it hits and you know where it's heading**. The other point is that, by this time already, it's well-known that if two things hit each other then you can use well-founded principles like "conservation of momentum" and "conservation of energy" to deduce something about the collision. So this leads to an idea that is the basis of all modern particle physics experiments: fire two things together and see what happens when they smash into each other.
The Geiger-Marsden-Rutherford experiment is one of the first of these. They hit upon the idea to try and work out what atoms look like by firing alpha particles at an atom and see what happens. By this point, as I say, it was already known that atoms have electrons inside them, but also that atoms are electrically neutral, so there must be a positive charge in the atom *somewhere*. But where, and how is it spread out?
The most reasonable first idea is that, because an atom is a hard ball and because the electrons are tiny, then they might swim around in a "sea" of smeared-out positive charge. On this scale, the positive charge would be so smeared out that any speeding bullet should burst through it, perhaps with a small deflection/glancing blow on the way. If that's the case, then you should see alpha particles on the other side of any (thin) target.
And, mostly, this is what was observed. So far, so good. But... luckily, the scientists involved thought to set up a detector capable of seeing if any alpha bullets were deflected by more than just a slight angle. And, astonishingly, some were! Not many, but also far more than anybody could expect. The classic quote is along the lines of firing a cannonball at tissue paper and watching it bounce back into your lap.
The two key observations, then, are:
1. Some alpha particles fired at a thin target (gold foil) deflect almost all the way back on themselves;
2. Most alpha particle pass straight through the foil.
The way Rutherford reconciled this was to conclude that perhaps the positive charge in the atom was concentrated into a tiny spot! This works, because:
1. Positive charges repel, so if the alpha particle heads directly towards the concentrated charge then it should be repelled backwards;
2. Most of the atom is empty space, allowing the alpha particle to pass straight through unimpeded.
The particular genius of Rutherford is not just to describe this model but also to use it to make a testable prediction. Actually, his second genius was to publish this model knowing that, according to the then known laws of physics, it made no sense at all.*** He pressed ahead because it fit with experimental data (a key distinction, for anybody lurking -- the hold standard of useful science is its ability to make predictions and then to have those predictions be verified in experiment!). But it took a few years before the model made sense.
**Obviously, from a modern perspective, this was too optimistic (see the Uncertainty Principle), but this is a world before Quantum Mechanics, so I don't care.
***No room, or time, to explain why, except to note that if like charges repel and opposite charges attract, then why don't the electrons fall into the nucleus?
I can't help thinking that this is one of the things AB should be good at. Naturally, it's a lot to take in at one go. I for one shall be re-reading this thread several times.
The subject matter may not be "user-friendly" for us laymen, but the way you present it definitely is.
Thanks for this Jim. I can't help thinking that if you were my physics teacher at school, I would have got a much better grade :o)
The subject matter may not be "user-friendly" for us laymen, but the way you present it definitely is.
Thanks for this Jim. I can't help thinking that if you were my physics teacher at school, I would have got a much better grade :o)
Well done jim, keep up the good work.
May I point out something you will know, that a significant part in the discovery of the quark was played by the recently deceased, aged 90, John Polkinghorne - erstwhile Professor of mathematical physics at Cambridge, who spent time working at CERN & was an ordained Anglican priest.
I thought that might interest some ABers, (notably the atheist cohort).
https:/
May I point out something you will know, that a significant part in the discovery of the quark was played by the recently deceased, aged 90, John Polkinghorne - erstwhile Professor of mathematical physics at Cambridge, who spent time working at CERN & was an ordained Anglican priest.
I thought that might interest some ABers, (notably the atheist cohort).
https:/
/// I thought that might interest some ABers, (notably the atheist cohort).///
Not in the least. I did wonder how long it would be before someone came along to try to derail this factual and very interesting thread with reference to mumbo-jumbo. Let's hope the Ed takes it away as "off topic".
Keep up the good work jim360, it's fascinating.
Not in the least. I did wonder how long it would be before someone came along to try to derail this factual and very interesting thread with reference to mumbo-jumbo. Let's hope the Ed takes it away as "off topic".
Keep up the good work jim360, it's fascinating.
Atheist: I have no wish to derail jim's thread, but I think it is a non-trivial observation that Polkinhorne; someone who has looked deep into matter & made the discoveries (quarks) which jim is involved in now studying further, became a priest. Does it not interest you why?
I think I can answer that though.
I think I can answer that though.
As long as Khandro (or anybody else) isn't trying to draw a causal link, there's no derailment. I'm sure I know at least one physicist friend who's Christian. So what? I haven't talked about it with them but I'm sure they'd be the first to say that the two views of the Universe are, and need to be, separate (and, in any case, whether God exists or not, the laws of physics are the same; it matters not who, if anyone, wrote them).
Haha, interesting idea TTT, but I'm not *that* special and I'm not sure I'll be posting *that* often -- although it would at least finally (almost!) validate the "twinned with CERN" claim! I still don't work there but all of the work I've done is related to experiments there (and in a Japanese facility, BELLE).
Jim, I'm merely being selfish, I've have a lot to ask you and it'd be great to have place where it's all together. You see I've never been classically trained in particle physics, my own interest was really encouraged from a physics lesson in sub o level school where my teacher was sick and the lesson was actually taken by an English teacher who had an interest in astrophysics. It was him that started me on things like fusion, mass spectrometry, black holes, neutron stars and thats the first time I discovered that matter is almost non existent, I was 13. It just blew my mind and started a life long, amateur obsession with the nature of matter, all be it possibly from the the wrong end. I have read hundreds of books and I don't pretend to understand them all but my hunger to understand has never waned. So forgive me if I hassle you for more.
