In physics, what is a ‘point particle?’
Google says:
A point particle (ideal particle or point-like particle, often spelled pointlike particle) is an idealization of particles heavily used in physics. Its defining feature is that it lacks spatial extension: being zero-dimensional, it does not take up space.
Interesting, but then I have to ask myself how the three dimensional world can be made-up of stuff that is dimensionless?!
A point particle is a mathematical model of a particle where you pretend it is infinitesimally small. No width, length, or height - like a point in Euclidean geometry.
In classical mechanics, massive bodies are often modeled as point masses. For example, in calculating orbits, you can consider the star and the planet to be points in Euclidean space each with a certain mass attached.
In particle physics, the same idea can be used to model electrons (or positrons) which can be thought of as points with a certain charge attached.
In quantum mechanics, the point particle idea breaks down, because of the uncertainty principal - you can’t say a particle has zero volume because that would imply that you know its precise location.
Still, point particles are useful as a model for lots of calculations.
This is where the can of worms lies. How can the real number line have a length when every number is represented by a dimensionless point? For that matter, how can it be continuous?
The answer is: these things are abstract models which provide tools for learning about the real world. Nothing in the real world is actually dimensionless. But just for shits and giggles we can come up with rigorous ways to derive these structures to be sure that our math is legitimate. (That’s what mathematicians do.) But mathematical legitimacy is not a necessary prerequisite to apply a model to the real world to make predictions (that’s what physicists do.)
Okay, so it’s mathematically expedient to consider items as point particles under certain circumstances. I kinda get it.:dubious:
Under Newtonian mechanics your motion with respect to a sphere of uniform density is independent of the size of the sphere so long as you are outside the sphere and depends only on the mass. So you get the same results if you assume all the mass concentrated at a single point. I don’t know what happens in general relativity. I suspect it is false.
Actually, for fundamental particles like electrons and quarks, the point particle model does not, so far as we know, break down. We’ve tried to measure their size, and all we’ve ever been able to find is “no greater than _____”. We’ve come up with theoretical lower bounds for their sizes, that it wouldn’t make sense for them to be less than for some reason or another, and still found them to be smaller than that anyway. They really are either infinitesimal points, or so small that we can’t tell the difference, and if our theories say that that’s nonsensical, then we have to scrap those theories.
Any interaction between particles has what’s called a cross section, which is a measure of area that basically says that the particles have to pass within that area of each other in order to interact significantly. For macroscopic, non-point particles like billiard balls, the cross section is in fact just the size of the particles. So why can’t we call it the size for fundamental particles, too? Because it turns out that the cross sections depend on just which particles are involved, and what energies they’re at, and there are some interactions with arbitrarily small cross sections.
Actually, that holds in GR, too. The Schwarzschild metric was originally developed to describe the spacetime outside of a spherical object like a star. But then people noticed that if you continue that metric inwards, for smaller and smaller objects, you eventually got something weird, what we now call a black hole. Outside of a star or other spherically-symmetric object, though, spacetime has exactly the same shape as it would at that same distance from a black hole of that mass.
In quantum field theory, too, particles are still “points,” described by quantum fields of course. It is only in a theory like string theory where the particles have nonzero dimension, though presumably these are very, very small, so that the particle will still be pointlike as far as you can tell.
How come I’ve read that particles only come into being when they are measured? Is that true?
No.
Really it is as simple as that.
Not sure why some of this weird mystical misunderstanding keeps being repeated.
Perhaps some strange misconception about Heisenberg’s Uncertainty Principle, or collapse of wave function.
The idea they only exist when measured make no logical sense anyway. So they didn’t exist before? So what were we trying to measure? This is no better than wondering if you exist if there is no-one to see you.
Hmmm…okay, well, I don’t want to seem argumentative but I’m a little perplexed.
The reason is because I read this: “Australian scientists have recreated a famous experiment and confirmed quantum physics’s bizarre predictions about the nature of reality, by proving that reality doesn’t actually exist until we measure it - at least, not on the very small scale.”
Things aren’t in a definite state until we measure the state. But even if things aren’t in a particular state, the things still are.
Hmmmm…
Reading any press report on new science needs to be done after reading this seminal comic.
The problem with the report linked above is that the first few lines were clearly written by a reporter who had no clue, but who had a good idea about what would make a nice headline, especially one that meets the public expectations for the weird and wonderful world of quantum physics. The public seems to have been convinced that the world of QM is one where philosophical concepts of reality are challenged, and reporting seems to have become a game of one-upmanship to get even more “shocking” headlines. This is physics as click bait.
The experiment is supposed to prove the existing science that " by proving that reality doesn’t actually exist until we measure it". Bollocks. QM says no such thing and breathlessly claiming that an experiment proves it is just stupid. And when you get further into the article, it is clear that the experiment makes no such claim.
I think it’s more subtle than that. These experiments seem to show that what is done in a future event can effect what has already happened, in other words that the future can alter the past. I know this is only at the quantum level and does not apply to the classical world, but it illustrates how we have two realities: the classical one and the quantum one and they seem to be irreconcilable at present. The objective of physics is to find an underlying theory that is able to account for the behaviour of both words and is usually termed ‘A theory of everything.’ The quest is to discover a mathematical equation that covers all the observational data that we have in physics, though whether that will ever happen is speculative. I hope it does. It is not a question of there not being any reality before we measure it or otherwise manipulate it, there clearly is, but the point is what kind of reality can depend oh how you interpret it by setting-up special experimental situations.
Why should we assume, as many nineteenth and twentieth century scientists did, that the universe acts like a ‘clockwork’ mechanism? Newton made this mistake and was shown to be in error by Relativity. In turn, even Einstein, could not accept the results of the quantum word and in a way he reacted like Newton in being unable to relinquish commonsense notions about reality. Why should the universe behave in ways that we, human beings, find ‘logical’ and sensible? My view is that reality has various layers and science is at present attempting to strip away such layers but in doing so is having to abandon preconceptions that are rooted in our biological and evolutionary past. I’m also inclined to think that we cannot completely eliminate our place as observers as part of the mix, although pursuing this too much will only lead to unresolvable speculations, as we have already seen. 
Well yes and no. No in that there is no new science in this experiment. It is validating QM theories that are many decades old. Nobody is surprised at the results. It is however a neat experiment, and it extends the validity of the the theories even further.
Yes, in that there is subtlety. But the person that wrote the article had no idea what that subtlety was. They pure and simple got it wrong. There is nothing subtle about that. QM does not say that reality does not exist before it is measured. It never has, and this experiment does not change things.
Einstein railed against the idea that there was pure randomness in QM. He won the Nobel prize for contributing one of the most important early components of QM. I find it remarkable how few people know that he didn’t win a Nobel for relativity (either special or general) but did win it for the photoelectric effect. [Nitpick. Newton’s ideas about a clockwork universe were not in any way challenged by relativity. Relativity continues to allow for a perfectly deterministic universe, (except perhaps when we get to issues of information deletion in black holes.)] There remains a rump group of physicists who adhere to ideas of “hidden variables”. They still believe in a clockwork universe. Underneath it all, I suspect Roger Penrose is one such.
The way we think the universe runs is for physicists quite logical, and so far rather a lot of what we know is covered rather well by QM and GR. We know our knowledge is not complete. But that understanding does not behove us to abandon everything we know so far. Sure, much of modern physics is counter-intuitive, but this does not suddenly let us say that our understanding of reality can be dismissed.
You talk of reconciling classical and quantum worlds as if this was a problem. It isn’t. Classical physics is understood to be wrong. It is however also very useful. Physicists use classical approximations wherever they think they can get away with them because it simplifies the mathematics. It simplifies the mathematics a lot. You will hear of semi-classical models. Basically using QM or GR where it is needed, and keeping as much classical as possible to stop the maths blowing up.
What you call the classical world is really the mezoscopic world. The world of things at scales we are used to. So we see neither the effects of the very small or the very large, but live in a comfortable zone where physics is nicely approximated by comfortable continuous and largely linear effects.
Where you will get discussion is about the interpretations of QM, and I suspect this is where you are getting a lot of the distorted ideas about the nature of QM and how it is interpreted from. This is where you get a lot of popular coverage of QM. People writing about the mystical elements of how QM might involve parallel universes (it doesn’t, that is a misunderstanding of the many worlds interpretation), pure randomness, hidden variables, pilot waves, and so on.
But there is a very important question that is probably what is a the core of where you are finding these notions. The question of causality, and its apparent bending in some quantum effects. Here is where Einstein wasn’t happy. The EPR paradox was created to show there was a problem with QM. It seems to suggest that there may be a violation of causality*. But it doesn’t violate it in any manner that breaks relativity. It seems to break it in a strange way, that preserves relativity, but also is not consonant with hidden variables. So, everyone understands that there is a yet to be discovered and possibly very important extra bit of physics here.
- This is the bit about the future affecting the past. The manner in which causality is possibly violated is very specific. It does not allow information to travel faster than light, and it does not allow information to travel backwards in time. The future cannot affect the past, but there is something that allows the separated events to remain entangled in a manner that cannot be currently explained. And no, you can’t change the kind of reality based upon how you set things up. That is quite specifically disallowed, and nobody expects otherwise.
The entire EPR paradox is based in, as I said, decades old theory. Experiments have shown that, well, the experiment does actually do what it says on the tin. This tells us, not as EP&R perhaps hoped, that QM is wrong, but rather that there is a core bit of reconciliation remaining. The fact that the theory is quite clear that information cannot be send at superluminal speeds, consistent with relativity, is a very big deal.
“QM does not say that reality does not exist before it is measured.”
No but what it does say is that we can never predict what reality will be because it is a matter of probabilities. Are you really saying there already exists a ‘set’ reality before any measurement is made that is independent of our measurement? If so, this has found to be false. This is why our place as observers seem to play an important role in what is measured. An electron both exists and does not exist when unmeasured but settles on being an electron when measured. If you mean by ‘reality’ something or other does exist before we look at it then yes, but after we make an observation it is no longer exactly what is was before we ‘interfered’ with it. You have to remember that we are part of reality too so it’s impossible to separate the quantum world from us - it would be a false dichotomy.
“* This is the bit about the future affecting the past. The manner in which causality is possibly violated is very specific. It does not allow information to travel faster than light, and it does not allow information to travel backwards in time. The future cannot affect the past, but there is something that allows the separated events to remain entangled in a manner that cannot be currently explained. And no, you can’t change the kind of reality based upon how you set things up. That is quite specifically disallowed, and nobody expects otherwise.”
In a way you can because it is the fact that the experimenter is conscious of the information that seems to be causing the effect. Don’t ask me how because it seems very mysterious but I come back to the fact that we, as observers, seem to play a vital role in these results and it may be consciousness itself that is causing such anomalies. In fact, this seems quite logical to me because how could a quantum object be ‘aware’ of what the experiment is doing? It does seem to come back to human intervention. I think it’s probably as well to point out that we do not really understand what consciousness is so I don’t think we should automatically poo-poo this idea. You don’t seem to understand that a particle can become a wave by manipulating the experiment in the particle’s future, thereby, causing a retroactive effect. Remember, the particle has already been set as a particle and ordinarily you would expect it to remain so but this has been disproved. Are you saying that when the particle comes into existence it isn’t real? If not the the particle is real and can be made to change its ‘reality’ by a future event.’ We are all made of quantum objects so this effect should have implications for us all in some way or another. Do we really know what space and time is anyway? The answer is probably no.