definition of a particle?

[infamous]

Originally Posted by Tim_Lou

What other ways can a person detect/define a particle otherwise?

Your question about detection is one I'm not prepared to deal with at present, however, as I understand it, there exist many such detectors available today. An example would be the cloud chamber. Never-the-less, I do have my own opinion about a definition.
Because there is no space empty of field, I propose that particles are nothing more than Localized Orbital Energy Flux . As one wave front passes another at a different angle to itself, a vortex of circulation is induced resulting in particle production ......Example would be the photon pair production of the electon and positron. This event doesn't extablish a permanent particle pair because they immediately self distruct thru mutual annihilation. Understanding that matter, or any such particle, is really just a concentration of energy, as referring to the wave in a field, one must rationalize that a disturbance in the wave front is responsible for particle formation............................Infy

[Jay-qu]

I was thinking this question but from a different angle while I was reading one of the chemistry threads. When I thought what defines a particle, I was thinking: We call protons particles, also the quarks that make them up are called particles, so is a proton 3 particles or a particle... I know its fairly semantic and you approached it from a far more fundamental sense, but it all gets you scratching you head in the end!

[CraigD]

I think an essential summary of any quantum particle physics theory – and, thus, the definition of “particle” - consists of, roughly, the following “rules of the game”:

• Nothing exists except particles (time and space exist, but aren’t “things”, but attributes of things, having to do with their quantum numbers)
• Field effects are the result of particle interactions
• Which particles interact with which, and which can become which, is precisely defined. This requires the number of kinds of particles to be smallish, or it wouldn’t be humanly possible to precisely define their interaction.
• It’s a quantum theory, so every particle must have an associated time-dependent quantum wave function.
• Oh, and, of course, the theory must make predictions that match observed reality (“the hard part”)

The standard model almost plays by these rules, failing only in that it has no accepted particle to account for gravity. It has a name for this particle – the graviton – but nobody’s been able to figure out how to precisely define it, so it’s not included in the model. So, the standard model can’t account for gravity, which is OK for atom-scale phenomena, but nearly useless for astrophysics.

Note that while fields can be explained by particles, under the particle physics “rules of the game”, fields can’t be used to explain the action of particles. Only a particle can interact with a particle, because, by the first rule of the game, nothing else exists.

[ronthepon]

That's the problem with physics so far.

I see that it has been specified that what will happen, but nothing about why it happens.

I might add that exactly what a particle is... the concept is wrongly considered basic, elementary and understood while the thinking about them is being done.

[Erasmus00]

Originally Posted by Tim_Lou

and all massive particles are "excitations" in "gravitational" fields?

Not quite. Consider the electron has mass but is an excitation of the dirac field.

While there is no proven theory of mass, the great hope is the Higgs field. http://hepwww.ph.qmw.ac.uk/epp/higgs3.html This is the best non-mathematical description I've found for the idea of what causes mass.

so what exactly is this "excitation"?

How do you think about a photon? Its a tiny perturbation in the Maxwell field that travels. (For a good "intuitive" way of treating E/M waves see the appendix on fields of moving particles in Purcell's excellent book). Its just a sort of knot in the field.

All of this is, of course, not accounting for things like string theory.
-Will

[Tim_Lou]

let's say we have a "photon", associated with an electromagnetic wave function.

let's say this wave travels through a straight line, in the x direction. well, if we take a "look" just a little bit above that "wave function" (with a slightly different y-coordinate or z-coordinate), would the E/B field be non-zero? if so, wouldn't there be like infinite "numbers" of "photons". perhaps photons are just 3 dimensional disturbance of E/B fields.

Well, what exactly makes one "photon", one "photon? maybe "one" photon can be defined as a "disturbance" of E/B fields with total energy one hf (integrating the energy density through the whole 3D space)?

or a more general question, how must particle exists as integers? not 1.5, not 2.58323, not 0.2135446878647654... or as jay says, we will never know if particles are discrete or not.....


to my understandings, the only way one can define a particle is through operations.

If so then in an accelerated frame, it can be assume that gravity exists and a particle is causing it somewhere using general relativity and we should be able to interact with this imaginary particle (or mass)! if not then the equivalence principle is violated (am i missing something?)! i think this might be one of the problems GR faces.

[HydrogenBond]

If one looks at a particle in contrast to their various energy fields, the fields extends outward in space and time, while the particle aspect is highly restricted in space, but can have a variety of expressions in time. With respect particle expression in time, some particles, like electrons, last almost forever. While other particles, last but an instant.

Another observation is that particles appear to have a mass aspect, which, in turn, appears to have a connection to its restricted use of space. The photon has no mass but it is a particle. This particle is the origin of particles, with its particle mass=0. As the mass of this massless building block, called the photon, increases we get all the rest of the particles.

[CraigD]

As usual, HydrogenBond makes some intriguing assertions. I think it’s important to note that they are very unconventional, bearing little resemblance to the Standard Model or any of its variants.

Originally Posted by HydrogenBond

If one looks at a particle in contrast to their various energy fields…

Not according to the Standard Model. Some particles (bosons, such as the photon) are responsible for the interactions that cause field effects, while others (fermions, such as the electron) interact with those particles to experience field effects, but none of these or any other particles have “energy fields”.

… the fields extends outward in space and time, while the particle aspect is highly restricted in space …

Not according to the underlying Quantum Mechanics of the Standard Model. Every particle is associated with a wave function that is defined for all points in space, and can be used to calculate the probability of observing that particle in a particular region at a particular time.

Another observation is that particles appear to have a mass aspect, which, in turn, appears to have a connection to its restricted use of space.

In the sense of “restricted use of space” meaning roughly “can’t occupy the same space”, this is the Pauli_exclusion_principle. Bosons, such as the photon, don’t obey it, a practical consequence being that there’s no limit to the number of photons that can occupy a given volume of space. Fermions, like the electron, do, a consequence being a limit to the maximum density of any mater containing particular particles.

The photon has 0 rest mass, but not all bosons do. A large part of the mass in an atomic nucleus is in its bosons, in addition to its fermions (quarks).

[The photon] is the origin of particles, with its particle mass=0. As the mass of this massless building block, called the photon, increases we get all the rest of the particles.

Though a pleasingly simple hypothesis, this is not a prediction of the Standard Model or any of its variants. It has not been experimentally observed, and contradicts a large body of experimental data.

The Standard Model is not simple. Proposals to simplify it by declaring only one of its particles fundamental has not been experimentally successful. This is not to say that there is nothing more fundamental than the fundamental particles of the Standard Model – String Theory, and several others are long-standing efforts to do just that. They are proving, however, also not simple, and often frustratingly unable to make predictions verifiable by practical experiments.

[HydrogenBond]

Instead of energy fields I was talking of force fields. My mistake.

I see particles as the substantial aspect of wavefunctions that it defined only by restricted space. By this I do not mean it connected to the uncertainty principle, but if one could take a snapshot, there would be this substantial but tiny thing roughly in the middle surrounded by the wave functions associated with its force and/or energy related properties.

As a common sense experiment. If someone was to throw a rock at you, what is going to hurt is the momentum of the ionic (protons, electrons and neutrons) particles. If one took away all the central particle/masses and just threw the same rock with only all its wave functions, it would feel like a cloud hitting you, since it would have no momentum and all the forces are in balance. Yet this ghost is assumed more substantial than the tiny particle aspect. In a universe assumed dominated by wave function style ghosts, anything seems possible via mathematics. But in the case of the rock, it would do nothing without the particles.

[CraigD]

Originally Posted by HydrogenBond

… If one took away all the central particle/masses and just threw the same rock with only all its wave functions, it would feel like a cloud hitting you, since it would have no momentum and all the forces are in balance.

I think there’s something profoundly wrong with this argument. In quantum mechanics, you can’t separate the observed particle from its wave function. One is not substantial, the other “ghostly”. The “substantial” particle is just what we call a measurement of the wave function – “collapsing” it, in the terminology of the Copenhagen interpretation

Yet this ghost is assumed more substantial than the tiny particle aspect. In a universe assumed dominated by wave function style ghosts, anything seems possible via mathematics.

The math of Quantum Mechanics is just a necessary tool of it’s formalism. One can (and most people do, most of the time) comprehend and discuss quantum wave functions without doing the detailed math, but to make experimentally testable predictions, the math’s necessary.