I read that some physicists think that you can can divide any particle into other particles (including quarks) whilst others think that a particle that cannot be divided exists.

1/2 a neutrino is nonsensical.

When we have evidence otherwise ... I'll change my opinion.

Principle of charity, DA. I think Rob means that all particles are comprised of even smaller particles.

I understood what he intended.

But lacking evidence that such exists such discussions are as meaningless as my invisible purple rhinoceros.

As I said ... the day someone figures out a logical meaning for 1/2 a neutrino or 1/2 a photon ... I will embrace the possibility.

An open mind is important. But an open mind does not mean believing in invisible purple rhinoceri.

What's the mass of a photon? It's not zero, I remember that much. So just divide that mass by two. Now, this new number that you have cannot be the mass of a photon, it has to be the mass of something else. Right?

Errr...?

Does invisible purple rhinoceri = things we cannot possibly know thus are not worth the energy discussing or thinking about? I think so.

They are kind of like road blocks. The road blocks that protect travelers from driving off the cliffs of insanity.

I'm going to picture them from now on as protective giants, these purple things holding signs with big orange letters, "YOU SHALL NOT PASS!"

Leptons are point particles. The lowest mass leptons in any family have neither internal strucure nor anywhere to go smaller. They do not decay and they cannot be composite particles.

Rob wrote:
"What's the mass of a photon? It's not zero, I remember that much. So just divide that mass by two."

Neutrinos have a mass expressible with the use of numbers. That does not make them divisible.

Pi = 3.1415926...

Divide it by 6.293 and you will get a result. A totally meaningless result.

Some theorist believe that space is quantized on a much finer level than we are accustomed to thinking about. If this turns out to be true, then even "elementary particles", e.g., leptons like the electron, may well have structure. Structure usually means: "it has parts." These theorist are speculating on ways to unite quantum theory and gravitation theory and the final word is far from in. There are several theories and the one that I have in mind is called Loop Quantum Gravity or LQG for short.

Have a look at this for starters

http://en.wikipedia.org/wiki/Loop_quantum_gravity

A book on the subject is: Three Roads to Quantum Gravity by Lee Smolin. This is a popular treatment of the subject.

Right. I've been monitoring the results of this poll and it seems that a lot of you believe in a non-divisible particle. Answer this; can a particle that has zero mass exist?

The photon - any photon, all photons - have zero rest mass. The photon is the virtual vector boson for electromagnetism, and that is infinite in range. The graviton - if it existed - would have zero mass. The graviton would be the virtual tensor boson for gravitation, and that is infinite in range. All infinite range forces, if they have quantized propagators, must have zero rest mass quantized propagators.

Rob this may come as a shock to you ... but science does not work by, and nature does not conform with, the expectations of the many.

And, of course, what Al said ... but you are confusing things. Zero rest mass has absolutely nothing, nada, zilch, to do with whether something is divisible.

so what proof is there that a photon is a particle and not a wave?

Rob asked "so what proof is there that a photon is a particle and not a wave?"

I don't know if "proof" is the right word to use since physics is not math. In math all that is needed is logical reasoning. In science we need to used inductive reasoning. But you were not asking about method, so ---

In the 1880s Hertz was trying to produce the electromagnetic waves predicted in Maxwell's electromagnetic theory. He noted a pesky effect interfering with his experiments. He observed that electrodes should be cleaned frequently to avoid the problem and suggested that someone look into the physical basis the problem.

That got done by Philip Lenard in 1902 and is now called the photoelectric effect. Lenard's efforts were experimental and quantified the effect without offering an explanation. It was totally inexplicable on any classical basis. A real head scratcher, in fact.

Three years later Einstein developed a theory that accounted for all the odd info concerning the effect. He used the quantum of action discovered by Planck in 1900 and the assumption that light is composed of discrete quanta. (These quanta are now called photons, after G. N. Lewis 1924.)The theory was regarded with caution at first, but it works and gives one of the first insights into the explantion of a whole bunch of head scratchers. It was the first quantum theory of anything.

From an inductive point of view the whole idea of a photon works. That's all the proof that you get that light is a particle.

As for showing that light is not a wave. Well that's just not true. Particle - wave? Wave - particle? To really do this question justice we would need to conside duality/complementraity and the correspondence principle.

dr_rocket wrote:
"Rob asked "so what proof is there that a photon is a particle and not a wave?"

I don't know if "proof""

And quite frankly there is none and never will be any. Because a photon is not a wave. And a photon is not a particle. And a photon is not some new-age mixture of the two.

A photon is a photon is a photon. All of these words like "wave" and "particle" refer to analogies in mathematic that help us describe and predict the behavior.

No physical entity changes its form to comfortably comply with our wishes.

A wave can act like a wave and like a particle. A time-independent localised electron-wave has a centre of mass just like a particle, and when observed from an inertial reference frame moving relative to the time-independent wave, the centre of mass will move like a "point-particle". What happens to a photon when it excites an electron? It disappears. This is strange behaviour for a photon is a photon is a photon; suddenly it is a "nothing". What happens is that the photon entangles with the electron wave, thus increasing its energy so that it has to morph, within a time delt(t) as determined by Heisenberg's uncertainty for energy and time, into another atomic orbital (wave).

Now the photo-electric effect: the incoming photon entangles with the electron-wave thus increasing its energy. If this energy is equal to the workfunction, the electron-wave morphs into a free electron wave that is stationary relative to the metal substrate. If the photon has more energy, the electron wave it entangles with, morphs into a free electron wave with kinetic energy relative to the metal substrate. You do not need "particles" colliding; every interaction can be described in terms of waves entangling, or waves that superpose. A photon is a wave, a wave a wave, which can change its shape and size. Only its energy is quantized NOT its locality. It has to localise in order to entangle with an atomic electron wave.

The quantum-mechanical energy of a free stationary electron wave is its mass; i.e. it has potential energy. Thus solving a wave equation (whether Schroedinger's or Dirac's) by setting the potential energy of a free electron equal to zero, is a futile calculation. You must have a potential energy term that accounts for the rest mass of the electron. The energy of an atomic electron orbital is thus less than the rest mass of the electron. The difference is the energy required to set the electron free and ionise the atom.

I was talking about matter waves; specifically electron waves. Nonetheless, most waves, except a light wave, which move along a single direction, will be stationary within a reference frame travelling with it; for example, if you have non-dissipating water waves which travel along a single direction, and you travel with the waves, you will observe them as being stationary. If the wave is a soliton-wave and you travel with it you might conclude that it is a type of stationary "particle" on the surface of the water. If it is a localised time-independent electron-wave, you might interpret it as a particle "with size" having a centre of mass. When such a wave moves relative to you, it will appear as a particle with size moving as a a particle should; i.e. along the path followed by its centre of mass. Thus although it is a localised field, one would interpret it as a "point particle". The size of a wave is determined by the applicable boundary conditions. When these change, for example when the electron enters a solid material, the localised electron wave can delocalise to spread over the volume of the solid; i.e. it can becaome a Bloch wave.

I would like to be more rigorous but it is difficult on a BB when one wants to make a statement in as short a space as possible.

Johnny Boy wrote:
"I was talking about matter waves; specifically electron waves"

You still don't get it. There is no such thing as electron waves: There are only electrons. Electrons ARE electrons. They are not something else.

This is where we disagree. Electrons are localised waves they are not particles "waving" their tails only when you look for their tails. Try and think outside the Copenhagen paradigm; it is wrong just like Einstein expected. Oh where is Einstein? We need his brains!

Johnny Boy wrote:
"Electrons are localised waves they are not particles "waving" their tails"

Please assume that this DUH is 72 point font: Bold faced.

You still don't get it.

Electrons are not waves.
Electrons are not particles.
Electrons are electrons.

A map of the state of New York is not the state of New York. A map of the andromeda galaxy is not a galaxy. And an analogy of electron properties is not an electron.

I understand your reasoning, which is the standard dogma leading to the non-causal absurdities when interpreting quantum mechanics. I am arguing that your reasoning is outdated, and should never have been believed. This is the bad legacy that a brilliant scientist called Heisenberg left behind. As Einstein anticipated, this reasoning is just plain wrong. How can an atomic orbital represent a moving charged-entity (whatever it might be) without having to radiate EM radiation? The Heisenberg uncertainty relationship for position and time relates to the relationship between position and k-space which every wave-function has to have; IT IS NOT UNCERTAINTIES IN THE POSITION AND MOMENTUM OF A POINT PARTICLE OWING TO INBUILT STATISTICS IN THE UNIVERSE (GOD REALLY DOES NOT PLAY DICE!!!). To say that an electron is an electron is an electron is acceptable as a religious mantra, but is not good science!

Dear D A, Do you always have to go off on a tangent? You know well that the "standard dogma" that I have been referring to is the interpretation of quantum physics; NOT the validity of physics. Can you explain to me in detail how the the Copenhagen interpretation of quantumm mechanics, as such, has been instrumental in placing a man on the moon?

This interpretation, introduced by Bohr, Heisenberg and Born, married physics to metaphysics. Physics is based on two questions: "why? and how?". Bohr's statement of complementarity stated that in the quantum world it is futile to ask questions like: "Why does an electron act as either a particle or a wave?" or "How does it manage to do so?" No wonder those people practising metaphysics and mysticism welcome this interpretation with open arms, and mouth the mantra: an electron is an electron is an electron! Maybe it IS a mystic world, but as a scientist I refuse to accept that there are aspects of Nature about which I should not ask "how" or "why"; even when my head is bitten of by you and Uncle Al when I do so.

Now let us consider Heisenberg's interpretation of his own uncertainty relationship for position and momentum. According to this interpretation both position and momentum of a free electron cannot manifest simultaneously; however, all experimental data on the free electron can be accurately modelled as the movement of a particle with a centre of charge and a centre of mass! This is the experimental fact. No experiment has ever shown that an electron ejected by a cathode becomes "uncertain". All the electrons follow well defined classical paths which can only manifest if momentum and position manifest simultaneously for their centres of mass. If this were not the case, the movement of a free electron would have been akin to Brownian movement. Has this ever been observed? If it were the case we would not have had electron microscopes or electron accelerators. What happens when the electron moves with a constant velocity? Since it follows a classical path, it means that its centre of mass MUST be stationary within the inertial reference frame moving with it! Now let us in addition, consider a "vacuum foam" of energy. As Uncle Al pointed out, an electron cannot be stationary when one has such a vacuum energy; however, this is at variance with experiment (as outlined above). How did Heisenberg, address this problem? He stated that the "electron's path only comes into existence when it is observed". Another metaphysical concept. It implies that if there is no observer, there would be no path; however, an electron in free flight within an electron accelerator is not "observed" but it still follows a definite path which requires the manifestation of both momentum and position at all instants of time. But notwithstanding this experimentally verifiable fact, Heisenberg postulated that it is impossible for both momentum and position to manifest at the same time (unless the electron is observed). Is observation not a "measurement"? So this implies that when you measure you CAN observe both the position and momentum of the centre of mass of a free electron. This is also the experimental fact. As I scientist I rather believe experimental results than an interpretation that is at variance with experiment. My model of the electron as a localised wave with a centre of charge and thus a centre of mass, removes these contradictions. It also gives a possible reason why the mass of an electron consists entirely of electromagnetic energy.

"And, of course, what Al said ... but you are confusing things. Zero rest mass has absolutely nothing, nada, zilch, to do with whether something is divisible"

Okay, so maybe I didn't research the proper scientific meaning of mass. Consider this then, a simple, easy to visualise example. I weigh 62Kg -on earth; divide that by two till you reach a ridiculously small number, call it x. Now let's say that x is what the smallest particle ever or ever-to-be discovered weighs -on earth. x is without a doubt greater than zero, therefore it can be divided by two to produce a positive number. So when I do this and get a new number, z, I am asking what z is the weight of -on earth.

How sad, you wont answer my question.

what z is the weight of -on earth?

This is unbelievable! Why won?t anyone answer my question?

perhaps because your question, as stated is unanswerable. While it's true that numbers can be infinitely divided, objects cannot. A piece of cherry pie can only be divided down so far before you come to molecules of cherry pie. At some point, the division has to stop because the fraction of a molecule of cherry pie is not cherry pie. so it is with quarks. Thus while you numerically can divide indefinitely, with objects there are limits.

I hope that helps.

Amaranth

there have been many experiments that have indicated that it is a particle instead of a wave.

there have been many experiments that have indicated that it is a wave instead of a particle.

there was even an experiment at one point that indicated that it was neither, but something else that had characteristics of both. few ppl agreed with the test result of this.

the only thing test have proven: if you believe its a particle, you ll find evidence of it. if you believe its a wave, you ll find evidence of this too. neither side is willing to give up.

if you follow one of the theories of the early universe (first few seconds: yes da, i know that they don't "know" what it was like but many ppl have made theories about it, many of which are opposing) it suggest that before their were particles there was just energy. the corollary of this is that if you divide particles up enough, you end up with energy. and of course there are scientist that say that that particular theory is a bunch of malarkey

"so what proof is there that a photon is a particle and not a wave?"

Photons striking a surface have an impact which can be measured.
The impact is proportional to the energy carried,
(inverse to the wavelength), and indicates the
energy, (by E=MC<2), is all the mass there is.

However photons also demonstrate interference,
a wave property.

Wave? Particle? Wavicle? - Pick your choose.

I am a self confessed waveist and my arguments are thus:

a) On the macro scale we know all about particles so it's natural to "force" the world we can't see to look like the one we can.

b) The evidence for the "particle" nature of matter generally comes from interactions involving atoms, essentially the movement of an electron from one orbital to another. Atoms are most successfully described by assuming they consist of waves.

b.1) The description of a photon passing through a transparrent medium involves the exitation (vibration or distortion and relaxation) of an array of electron orbits (all wave mechanics) nullifying the original photon and retransmitting it, typically to give a slower speed of light. If the photon happens to be absorbed by a single atom during this process (see complexity theory as to why this is likely to happen) then it seems just as remarkable that the movements of all the electron work together to make just one atom get exited (as opposed to the photon being trasmitted unaltered*).

c) People like the probability view of quantum mechanics because it restores the idea of free will. It also adds a "mystery" factor a.k.a. accept this as an act of faith.

The best knowledge we have about electrons is that they are neither particles nor are they waves. The best knowledge we have is that sometimes they have behaviours that can be explained with math that describes particle beahviour. Sometimes they have behaviours that can be explained by math that describes wave behaviour.

And sometimes they have behaviours that can NOT be explained by either. For example:
http://polyticks.com/psi/orbitals.htm
and
http://www.physlink.com/Education/AskExperts/ae619.cfm
Look for the phrase "all possible paths."

I suppose I agree with you in the sense that electrons (and all matter for that matter(!)) are not waves as we know them "up here" in the macro world, neither are they particles. However I think the accepted probability expalantion where things swap from wave to particle is a fudge to let us do the maths.

Particleness is an emergent property of the correct wave equation (possibly never to be discovered).

I agree with you it is a fudge. It is possible to derive a clearly defined interface which demarcates when one can consider an electron as a "particle" (which it is not) and when wave-superpositions (i.e. QM) has to be used. When an electron-wave do not overlap sufficiently with other surrounding electron-waves, it experiences the other waves as point particles. This is simply the result one expects from Gauss' law when one views a distributed charge from outside. Similarly the electron waves around a nucleus do not overlap with the waves of the nucleons; therefore an orbital electron experiences the nucleus as a point charge. The amount of overlap that can be tolerated, while still experiencing other surrounding waves as points, is determined by the uncertainty in position (delta)r around the centre of mass (charge) of the wave, and is given for identical adjacent waves as 2*beta*(delta)r; where it seems that beta is equal to the square root of 2. It is at this distance between the s-orbitals that two approaching hydrogen atoms bond chemically. The two s-orbitals cannot experience each other as point charges anymore; their distributed charges coalesce and the covalent bond comes into existence owing to this entanglement of the two electrons.

It is zero, QED demands it and experiment has confirmed it. The CURRENT upper limit on the photon mass is 6*10^-17 eV.

Yes, it is the photon.

that is assuming that a photon is a particle and not an energy wave. last i saw neither side had yet to prove sufficently that the opposite camp conceeded to the arguement.

Mass is inertia; and inertia means that a "particle" can be stationary within an inertial reference frame. The photon ALWAYS move with speed c relative to ALL inertial reference frames. Therefore it has no mass but only kinetic energy. The photon IS divisible. In Compton scatterring a part of it can be absorbed so that a part with less energy remains. I also believe that at a double slit it splits up in two equal parts to move through both slits at the same time so that the two parts can interfere with each other on their way to the screen. At the diffraction screen the interfered wave collapses in order to be absorbed by one of the atoms (the statistical spread of points for different photons collapsing is determined by Heisenberg's Uncertainty Relatiionship for energy and time). If just behind the slit one places a detector to determine through which slit the photon came, the split-photon collapses to a point in order to be "seen" by the detector. The two parts does then not exist anymore and the interference pattern cannot manifest.

Remember, Planck's relationship only says that light with a frequency nu cannot have a lower energy that h times nu. It says NOTHING about the spatial extent of a lightwave. The latter is determined by the boundary conditions.

JB wrote:
" I also believe that at a double slit it splits up in two equal parts to move through both slits at the same time so that the two parts can interfere with each other on their way to the screen."

This one part of your post is at odds with most physicist's understanding of what happens. I believe the most reasonable, if you can call it that, explanation is that the photon takes ALL POSSIBLE paths to the target.

I have my personal ideas about this but they are the stuff of which laughter is often the result.

I believe that this is exactly where we all went wrong. It gives me pain to say this because I am a GREAT ADMIRER of Feynman; however, he is not totally incorrect (he is not even wrong). Just as one can create music from digital code, one can create some semblence of reality from Feynman diagrams; however, this is creating "reality" from "virtual reality". The photon is NOT a "particle" taking ALL paths; it is simply a wave which can "morph" into different shapes depending on the boundary conditions it encounters. Thus it can be stetched right across the Universe as in the case of the micro-wave background radiation; but when you observe it, it near-instantaneously "collapses" to be absorbed by the detector. Once one accepts the fact that a wave entity (which we call a photon) can be stretched over the Universe and "collapse" near-instantaneously to fill a small volume so that it can be absorbed by a detector, all "mysteries" in physics become causal.

Reasonable enough that I'll give you a peek under the covers of my thinking.

I think we need one spatial dimension that is disconnected from space-time and stands on its own. One place that an entity can be in which the concept of time is irrelevant.

If such exists it solves numerous problems in physics. The slit being one of them. But another being the old "spooky action at a distance" conundrum.

interesting concept, care to expand on it?

You have got it!!!! I have done the calculation for an electron. Within the "essence" of the electron wave you have a four-dimensional Euclidean space; and therefore time does not exist within the wave. The wave is in "immediate" contact with itself no matter how large it is or how small it is. The latter is determined by the boundary conditions. The free electron can then be modelled as a localised wave that forms within the field of a virtual positive charge, which is present along the fourth spatial dimension. Therefore the wave is localised even in free space. Furthermore, the rest mass of the electron is then nothing more than the potential energy of an electron that is stationary within its proper inertial reference frame. Furthermore, since the electron is a dipole along the fourth dimension, there cannot be an electric-energy field around a solitary electron. Renormalisation is not required. In addition there is an extra energy component along the fourth dimension which could be the dark energy we are looking for. When accelerating the electron the electron charge and the virtual charge do not cancel the electric field in three-dimensional space anymore; therefore EM radiation is generated.

AS you can see I am in total agreement with your postulate that there is a fourth spatial dimension. Outside the "essence" of the electron wave time exists. This is most probably so because the fourth dimension is now not perpendicular to our three-dimensional space. I believe that this bending defines a three-dimensional surface which constitutes "our" three-dimensional vacuum (space).

I will be away for two days. If you want to take this discussion further, and I hope you will be, then I will respond at latest on Sunday.

I'd like to take it further ... but not here at SAGG for what should be obvious reasons.

Contact me directly: damorgan@x.washington.edu

Replace the "x" with a "u".

BTW: I will be in Minneapolis-St. Paul next week teaching classes and lecturing at a conference so I may or may not have time to respond.