Definition of a particle?

[Tim_Lou]

i have been thinking about this thing for a while.

 

seeing how fields are always not well defined approaching the position of a particle, perhaps a particle can be defined as a discontinuity of field (any kind of force field)

 

but then.... a photon, there is no discontinuity of electromagnetic field. perhaps this is something different.

 

as I am not so good with quantum mechanics... knowing that position is not very well defined, the position of the "discontinuity" wouldn't be very well defined as well....

 

What other ways can a person detect/define a particle otherwise? besides observing severe distortion of force fields? hmm.... and how would a person observe a particle without another particle? if a particle is only well defined in relation with another particle, what does it mean?

[Mercedes Benzene]

:hihi: Those are some good points...

I've never really thought of that before.

Good questions!

[ronthepon]

Well, for one thing, the photon is better defined as a travelling disturbance in the electric and magnetic feilds.

 

More like fluctuations in the magnitudes of the two feilds, a fluctuation that moves. (Transverse wave style, only that the crests and troughs are in the magnitudes of the feilds)

 

So it cannot be a particle in the same sense as the other ones.

 

I know that you know all of this... but we all tend to keep the elementary things in the back of our minds normally. Am I mistaken by any chance?

[Mercedes Benzene]

Well... I've only just recently started to study particle physics...

...so I do not claim to be an expert to any degree of the definition.

I mostly work with subatomic particles that are a result of radioactive decay...

...that area, I can claim some knowledge...

 

I'm interested in seeing how this thread turns out.:hihi:

[Tim_Lou]

Well, planck's energy distribution of black bodies can be derived perfectly from the assumption that energy exists in discrete packages. the only explaination for this is that photon is discrete particles, so that 1 energy package= 1-photon package, 2 enerage package= 2-photons package

 

planck's finding really is mind boggling... I'm still trying to understand it fully... i read the above in a book from barnes 'n nobles, it shows the how planck first "accidently" derive it when trying to improve Wilhelm Wien's distribution, then comin up with a explaination to it....:) :hihi: nice free learning from barnes n' nobles.

 

this has indeed shaken my understanding of continuum...it troubles me to think that one almost perfectly validated concept does not go with another almost perfectly validated concept...

 

Anyway, how is electromagnetic field defined? another test "particle"... well the existence of photon must still rely on other "particles" regardless of whether not it is a particle...

 

i think that something is missing regarding fundamental definitions here.

[Tim_Lou]

perhaps the existence of a particle can be defined by gauss law? (neglect strong and weak force as i have no idea what the equations behind them are)

 

as long as the surface integral of E field is non-zero, we say a charged particle exists within that region. or even better, using divergence (only for "density" though). (same for gravitational field)

 

but then the definition of "fields" have to be re-written (so that it does not use a "test" particle)

[ronthepon]

The gauss law definition of a particle won't hold for neutral ones.

 

The electrical feild is defined that way so that it becomes totally rigid. However, we can imagine it to be a property of that portion of space... And it's induced by the presence of particles with special properties nearby.

 

We merely simplify it by calling these nearby particles as 'charged'. However, if there is merely one charged particle in the entire universe, there is no significance of this concept. So, the test charge thing is just a process of clearing it up.

[Erasmus00]

In quantum field theory, all particles are defined like the photon: excitations in a field. Electrons are excitations in the dirac field just as photons are excitations in the Maxwell field.

-Will

[Tim_Lou]

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

 

so what exactly is this "excitation"?

[Turtle]

What other ways can a person detect/define a particle otherwise? besides observing severe distortion of force fields? hmm.... and how would a person observe a particle without another particle? if a particle is only well defined in relation with another particle, what does it mean?

 

String theory is an attempt to describe a "particle" as other than a speherical dimensionless point. It describes the particle as a vibrating string and/or membrane.

A person would not, could not, should not, observe a particle without another particle. Chirality rules duality.:)

[infamous]

 

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]

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.