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#1
Old 06-01-2011, 10:59 PM
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Quick question: Is light a form of matter?

I was always under the impression that light is not a form of matter, but rather something else. Is this true? Or am I wrong? Also, if you have the time, could you point out some other things that aren't matter. Thanks.
#2
Old 06-02-2011, 12:13 AM
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I Am Not A Physicist, but my (no doubt flawed) understanding is that light is both matter and energy depending on how you look at it. And strangely, how you look at it influences how it behaves.

http://en.wikipedia.org/wiki/Double_slit_experiment
#3
Old 06-02-2011, 12:23 AM
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One working definition is that all the fundamental fermions and any composite particles made up of those fermions is matter. This would be quarks, charged leptons (electron, muon, tau), and neutrinos, plus composite objects like protons, neutrons, and (say) a brick. Photons would not be matter under this definition. But, "matter" hasn't proven itself as a useful enough term to earn a specific definition, and there are numerous ways that the word "matter" actually gets used, depending on context.

(Or, in other words: the scientific community at large doesn't assign any specific definition to the word "matter". As long as you define what you mean in any given context, though, you're okay to use it as a word.)

Last edited by Pasta; 06-02-2011 at 12:26 AM.
#4
Old 06-02-2011, 02:18 AM
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I rather suspect he means something like "does it have a physical presence, a reality, or is it just pure energy without form".
#5
Old 06-02-2011, 02:29 AM
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Light exerts force like matter.
#6
Old 06-02-2011, 03:20 AM
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Originally Posted by RadicalPi View Post
Also, if you have the time, could you point out some other things that aren't matter. Thanks.
Ghosts
Ideas
Music
Christmas
#7
Old 06-02-2011, 04:13 AM
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Originally Posted by inkling View Post
I Am Not A Physicist, but my (no doubt flawed) understanding is that light is both matter and energy depending on how you look at it. And strangely, how you look at it influences how it behaves.

http://en.wikipedia.org/wiki/Double_slit_experiment
The particle-wave duality exists for electrons, too. But we tend to consider electrons part of matter.
#8
Old 06-02-2011, 05:36 AM
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I thought matter was generally defined as everything from atoms up - once you start talking about fundamental particles in their own right, the term 'matter' (which I understand to be a general one, with chief utility in talking about the composition of physical objects) doesn't seem so useful.

Does light occupy volume, even?

Last edited by Mangetout; 06-02-2011 at 05:37 AM.
#9
Old 06-02-2011, 06:15 AM
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Originally Posted by Mangetout View Post
I thought matter was generally defined as everything from atoms up - once you start talking about fundamental particles in their own right, the term 'matter' (which I understand to be a general one, with chief utility in talking about the composition of physical objects) doesn't seem so useful.
That's one definition, though like others it has problems at the extremes. e.g. it implies that a neutron star contains no matter, even though they are typically about 1.5 times the mass of our sun.
#10
Old 06-02-2011, 08:30 AM
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There are two fundamental types of particles: fermions, and bosons. There is a very intuitive distinction between them owing to the fact that fermions can emit/absorb bosons, but bosons cannot emit/absorb fermions. What this means is that fermions can be said to 'interact' via the exchange of bosons. So there is a sense in which bosons are 'messenger' particles, and fermions are 'matter' that interacts with other matter through the exchange of bosons. Light is an example of a boson, a messenger particle. It's role in life is to go back and forth between fermions (like electrons), and mediate how they interact with each other. Ultimately the word 'matter' it is just an arbitrary definition, but I've found that generally the term is used by particle physicists to refer to fermions as opposed to bosons.
#11
Old 06-02-2011, 09:22 AM
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Originally Posted by Pasta View Post
One working definition is that all the fundamental fermions and any composite particles made up of those fermions is matter. This would be quarks, charged leptons (electron, muon, tau), and neutrinos, plus composite objects like protons, neutrons, and (say) a brick. Photons would not be matter under this definition. But, "matter" hasn't proven itself as a useful enough term to earn a specific definition, and there are numerous ways that the word "matter" actually gets used, depending on context.

(Or, in other words: the scientific community at large doesn't assign any specific definition to the word "matter". As long as you define what you mean in any given context, though, you're okay to use it as a word.)
The problem with this distinction is that you can combine fermions to get bosons, like mesons or Cooper pairs, for instance, or much more complicated things, like (famously) rubidium-87 nuclei, which one would probably ordinarily like to think of as matter... So a hard-and-fast divide is hard to come by.

That said, when it comes to elementary, rather than composite particles, I'd tend to think of fermions as matter, as well, and thus, would not count the photon as a matter particle.
#12
Old 06-02-2011, 09:33 AM
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Incidentally, I remember reading about an approach to describe the photon as a bound state of neutrinos (by de Broglie?), but in the end, it didn't work out...
#13
Old 06-02-2011, 09:44 AM
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Originally Posted by Mangetout View Post
Christmas
I suppose you think "Seasons Greetings" *are* matter? The war against Christmas has officially extended to the world of physics.
#14
Old 06-02-2011, 10:17 AM
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Does having mass factor into the definition at all?

I thought that it was that the force particles are not matter and have no mass (even if the Higgs boson exists and is what gives the property of mass to other particles). Force particles are all bosons but not all bosons are force particles. Photons, gluons, w and z particles, and if every proven to exist, gravitons and/or Higgs, are force particles, not matter. Mesons are bosons but not force particles, have mass, and are matter, for example.
#15
Old 06-02-2011, 10:37 AM
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Ws and Zs do have mass (at least on energy scales on which electroweak symmetry is broken), however, and neutrinos were originally thought to have none, and one of them still might not have mass, even with neutrino oscillations.

Last edited by Half Man Half Wit; 06-02-2011 at 10:37 AM.
#16
Old 06-02-2011, 10:55 AM
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Huh. Did not know that.

How about then defining that anything that is not a force particle is matter and force particles are not? It seems to capture common usage even among physicists ...
#17
Old 06-02-2011, 11:04 AM
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Originally Posted by Mangetout View Post
. . . - once you start talking about fundamental particles in their own right, the term 'matter' (which I understand to be a general one, with chief utility in talking about the composition of physical objects) doesn't seem so useful. . .
At the subatomic level you get to use the cool quote from Raymond Hall, Fermilab: " Stuff is made of particles. Therefore, particles can not be made of stuff."

(I may have found the quote here on the board. But when I search to see, it fills the memory and errors out, so I can't give attribution.)
#18
Old 06-02-2011, 11:14 AM
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Light (and electromagnetic radiation in general) is made up of photon, and differs fundamentally from leptons (electrons, muons, and the like), hadrons, andthe like. They have no rest mass and travel at the speed of light, which sets them apart from most other elementary particles (neutrinos have a small mass, and don't travel at lightspeed. Gravitons, if they exist, are massless)

Photons, despite having no rest mass, can nevertheless carry momentum and exert force. Light clearly interacts with matter, but in an odd way -- since it carries no electric charge, it doesn't directly interact with electric or magnetic fields (electro- and magneto- optics effects are really due to the electric or magnetic field interacting with the medium through which the light passes, or by which it is absorbed or emitted). There is a Feynman diagram for the interaction of light with light, but it requires each photon to create virtual particles which then interact with each other via more standard interactions, and the probability of such interaction is extremely small. For practical purposes, it doesn't exist -- you can send extrmely powerful laser beams through each other in vacuum and not see any interaction.


By most common-sense definitions, then, light isn't matter. In order to get some aspect of it to act like matter, you have to create a pathological situation. In ordinary human experience -- even in most human laboratory experience, light is a very different animal.
#19
Old 06-02-2011, 11:16 AM
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Originally Posted by RadicalPi View Post
if you have the time, could you point out some other things that aren't matter
I would, but ...

[sunglasses]

they don't matter.

[cue music]
#20
Old 06-02-2011, 11:19 AM
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Originally Posted by Askance View Post
I rather suspect he means something like "does it have a physical presence, a reality, or is it just pure energy without form".
There is no such thing as "pure energy without form" except in bad science fiction.

As Pasta notes, modern particle physics and the Standard Model does not have a rigorous definition of "matter", unless by that you mean fermions (particles that obey Fermi-Dirac statistics and thus, obey the Pauli exclusion principle). Photons do occupy a space in the sense that it consists of a field, but the same can be said for fermions as well. In fact, all matter and energy are, at least from the point of view of quantum field theory, just fields of quantized energy that interact by a defined algebra.

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#21
Old 06-02-2011, 12:10 PM
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As has been noted 'matter' is not rigourously defined enough in physics to give a definitive answer.

I'd say most of the time you see the term 'matter' used generally it's definition, on balance of probabilty, won't include light, for example 'matter-filled universe' vs radiation-filled universe' in general relativity. However seeing 'matter' used in a way that includes light or is ambiguous as to whether light is included is common enough as to be completely unremarkable.
#22
Old 06-02-2011, 12:50 PM
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Originally Posted by DSeid View Post
It seems to capture common usage even among physicists ...
I would scratch that sentence, replacing it with "There is no clear common usage among (particle) physicists."

Just for kicks: another use for the word "matter" is as an antonym for "antimatter". This can specific, referring only to baryons (versus antibaryons), or it can be more general, referring to electrons (vs. positrons), quarks (vs antiquarks), etc. Since a photon is its own antiparticle, it can't be catergorized as "matter" or "not matter" under this usage (as "not matter" would incorrectly imply "antimatter", in the same way that "Joe is not left-handed" implies that Joe is right-handed.)

But, again, this is just another of many context-dependent usages you can find in ithe wild.
#23
Old 06-02-2011, 01:23 PM
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Quick question: Is light a form of matter?
The question might be quick, but generally there are no quick answers in physics if you have enough physicists in the room.
#24
Old 06-02-2011, 01:41 PM
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Eh, it's not really a physics question. If the question were "Is a photon a fermion?", or "Does a photon have mass?", then there'd be a quick answer (no, in both cases). But "Is light matter?" is a terminology question, not a physics question, and terminology is squishier than physics.
#25
Old 06-02-2011, 03:20 PM
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Originally Posted by Chronos
If the question were "Is a photon a fermion?", or "Does a photon have mass?", then there'd be a quick answer (no, in both cases). But "Is light matter?" is a terminology question, not a physics question, and terminology is squishier than physics.
Why isn't "Is a photon a fermion?" a question of terminology? It just seems that the definition of "fermion" has been nailed down much better than that of "matter".
#26
Old 06-02-2011, 03:45 PM
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Originally Posted by Bytegeist View Post
Why isn't "Is a photon a fermion?" a question of terminology? It just seems that the definition of "fermion" has been nailed down much better than that of "matter".
The properties of a fermion (half-integer spin, obey canonical anticommutation relations algebra) are well established, and a photon does not satisfy this definition. Matter, on the other hand is a term that "everybody knows", but isn't really well defined in a particle physics sense, and indeed, may not be a fundamentally sensible question.

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#27
Old 06-02-2011, 03:52 PM
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In elementary school I was taught that matter is defined as anything that has mass and takes up space. Of course "mass" was defined as a measure of the amount of matter in something, so clearly there were some terminological issues going on. I'm not sure what “takes up space” means (does an electron take up space? how much?) but I thought that having rest mass was more or less definitional.

Did I learn anything in elementary school that was true?
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Old 06-02-2011, 06:02 PM
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"Takes up space" can be interpreted as a quick and dirty statement of the Pauli Exclusion Principle, I suppose.
#29
Old 06-03-2011, 12:10 AM
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What about e=mc2?

If light exerts force, then it has/is energy. If it has/is energy, frankly that is the equivalent of a certain amount of matter. Or mass anyway- mass in the space of a Cartesian point?

A better question might be 'is no-matter possible?' or maybe 'is there any such thing as empty space?'.
#30
Old 06-03-2011, 03:43 AM
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Originally Posted by Try2B Comprehensive View Post
What about e=mc2?

If light exerts force, then it has/is energy. If it has/is energy, frankly that is the equivalent of a certain amount of matter. Or mass anyway- mass in the space of a Cartesian point?
None of these statements make any sense.

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Originally Posted by Try2B Comprehensive View Post
A better question might be 'is no-matter possible?' or maybe 'is there any such thing as empty space?'.
All of space is filled with fields, and even isolated vacuum is hypothesized to contain a foam of virtual particles being constantly generated and annihilated at energy levels too small to detect normally and for intervals that are not observable.

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#31
Old 06-03-2011, 05:41 AM
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Originally Posted by Pasta View Post
One working definition is that all the fundamental fermions and any composite particles made up of those fermions is matter. This would be quarks, charged leptons (electron, muon, tau), and neutrinos, plus composite objects like protons, neutrons, and (say) a brick. Photons would not be matter under this definition. But, "matter" hasn't proven itself as a useful enough term to earn a specific definition, and there are numerous ways that the word "matter" actually gets used, depending on context.

(Or, in other words: the scientific community at large doesn't assign any specific definition to the word "matter". As long as you define what you mean in any given context, though, you're okay to use it as a word.)
I had assumed that it did have a specific definition, the one you just said about the quarks and the leptons, but it seems that I was wrong. It seems that the term “matter” is a sort of catchall with a meaning that depends on what you’re trying to do. Thanks to you and the others for pointing that out to me. Ignorance successfully fought.

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Just for kicks: another use for the word "matter" is as an antonym for "antimatter".
This raises another question for me, though. If we don’t really know what matter is, how do we know what antimatter is? Is there a clear way to distinguish the two, or is it just that the particles that we loosely call matter are just the ones that happen to be common and antimatter is the stuff that isn’t? Or is this another thing that doesn’t come up enough to be useful in physics. Like that two particles are either antiparticles or not, and the "team" you would put them on (the Regulars versus the Antis) isn't really important. But maybe it is, since the lack of antimatter is, if I'm not mistaken, something that needs to be explained that hasn't been explained yet.

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Originally Posted by Askance View Post
I rather suspect he means something like "does it have a physical presence, a reality, or is it just pure energy without form".
No, this is not what I meant. I was wondering if things like dark matter or antimatter or energy or some other things along those lines were considered to be matter, but I didn’t want to bring them up and embarrass myself.

Last edited by RadicalPi; 06-03-2011 at 05:44 AM.
#32
Old 06-03-2011, 01:39 PM
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There are many particle-antiparticle pairs for which one is significantly more common than the other, but then again, there are also many such pairs for which both are equally common. Where things get especially confusing is with the mesons, which consist of a quark and an antiquark. Everyone agrees that the pi+ (an up quark and an antidown quark) and the pi- (a down quark and an antiup quark) are antiparticles of each other, but how do you say that one's the "particle" and the other the "antiparticle"? The usual convention in such cases is to call the negatively-charged one the "antiparticle", but that's just a convention, and anyway there are also some neutral ones.

Quote:
I was wondering if things like dark matter or antimatter or energy or some other things along those lines were considered to be matter, but I didn’t want to bring them up and embarrass myself.
In any context in which one is dealing with dark matter, it would be considered "matter", even though it might or might not be fermions. Antimatter, it depends on the context, and most forms of energy would usually not be described as "matter".
#33
Old 06-03-2011, 02:29 PM
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Unless of course "dark matter" turns out to actually be antimatter oriented in the extra curled up dimensions in such a way that it is only partly observable by "matter" which turns out orient itself differently in those dimensions - in such a case gravity would be the only interaction able to spill through the multiple dimensional space.
#34
Old 06-03-2011, 02:57 PM
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Unless of course "dark matter" turns out to actually be antimatter oriented in the extra curled up dimensions in such a way that it is only partly observable by "matter" which turns out orient itself differently in those dimensions - in such a case gravity would be the only interaction able to spill through the multiple dimensional space.
That's what you always say!
#35
Old 06-03-2011, 05:23 PM
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Originally Posted by Alan Smithee View Post
That's what you always say!


Sooner or later that concept will catch on! And I am just gonna keep at it!
#36
Old 06-04-2011, 02:32 AM
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Originally Posted by RadicalPi View Post
This raises another question for me, though. If we don’t really know what matter is
Be careful with that, since we really don't know what <i>anything</i> is. Pick nearly any topic at all, and you can say we don't really understand it 100%, since no concept is imperfect and all have limits, and there are some slightly fuzzy areas and unknowns. Wait a few hundred years, and we'll have much better understanding. But for most practical purposes, we already understood matter a hundred years ago.

Also, think what happens whenever we ask questions about "x" but without providing our working definition of the word "x." It's a recipe for endless arguments and clouds of confusion. Paraphrasing Feynman: if nobody agrees on what word "Wakalixes" means, then it's not a good idea to start extensive discussions about "Wakalixes."


But yes, the "regulars" are simply the ones which are common, and the "antis" don't survive long unless they can be kept in a good vacuum and prevented from contacting any "regulars." You can make anti-hydrogen gas using anti-protons and positrons, but you can't store it in a jar.

Further thought about original post: In grade school we learn about matter and energy. We'll possess a solid understanding... as long as we stay away from more advanced classes!

Also, is a magnetic field a form of matter? How about an electric field? Are radio waves a form of matter? So matter can travel right through walls, right through plastic and glass? (After all, light is also waves of EM fields.) Or going farther, is sound a form of matter? Sound after all is quantized as phonons. Are waves on a rope a form of matter?
#37
Old 06-04-2011, 07:34 AM
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yes and no. If you mean wavelength that light has and how it varies then no.

But the light is particles reflecting off objects at different wavelenths creating colour as each colour is a differnt wavelength in the visible spectrum. But it is made of photons which are particles. That is why the mere viewing of an experiment on the sub-atomic level changes its aoutcome.
#38
Old 06-04-2011, 12:08 PM
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Originally Posted by mla1 View Post
yes and no. If you mean wavelength that light has and how it varies then no.

But the light is particles reflecting off objects at different wavelenths creating colour as each colour is a differnt wavelength in the visible spectrum. But it is made of photons which are particles. That is why the mere viewing of an experiment on the sub-atomic level changes its aoutcome.
That's not really right. You're referring to the 'quick-and-dirty' explanation of Heisenberg uncertainty ('Heisenberg's microscope'), which was indeed reportedly how it was first derived, but it is actually much more fundamental than that -- for any observables that don't commute with each other, it's the case that they can't be measured to arbitrary accuracy simultaneously. This isn't just an effective experimental limitation, as it would be in the 'disturbing photon' thought experiment, but rather, it's physically fundamental -- the information just does not exist.

Also, this whole 'observation changes the outcome' is a poorly popularized version of the idea of wave-function collapse -- that from a superposition of many possible states for a system to be in, measurement selects a specific one to become 'actualized'. This is in itself a somewhat contentious concept, with long standing debates raging over whether or not it actually happens, is a real, physical process or just something akin to the Bayesian updating of outcome probabilities when new knowledge about the system is gained, but the essence of it is that a system has a well-defined, unique state only in the context of measurement -- so it's not that 'observation changes the outcome', but rather that observation (which, btw, doesn't imply anything like a conscious, human, or even (very) macroscopic observer) ensures the existence of any unique outcome at all.
#39
Old 06-05-2011, 08:40 AM
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When i said that the observation changes the outcome what i was refering to is that for us to view anything we need photons as it photons bouncing off particles that create an image for a human eye. But these photons have the POTENTIEL although not always to change the outcome of an experiment.
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Old 06-05-2011, 09:07 AM
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Originally Posted by mla1 View Post
When i said that the observation changes the outcome what i was refering to is that for us to view anything we need photons as it photons bouncing off particles that create an image for a human eye. But these photons have the POTENTIEL although not always to change the outcome of an experiment.
Except one can do interaction-free measurements (the most famous form of which is the so-called bomb tester experiment) in quantum mechanics, such that, for instance, one can build up an image of an object without ever having any photons interact with it; yet even here, Heisenberg uncertainty is preserved. But this is rather tangential to the topic at hand.
#41
Old 06-05-2011, 09:09 AM
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Interesting i thought you needed the photons.
back to the question though or else RadicalPi won't like us. How can light be a wave or particle depending on the experiment?
#42
Old 06-05-2011, 11:41 AM
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What's the problem with that? I'm an uncle, a son, and a brother, depending on the person describing my relationship. I guess you could say I have uncle-brother-son duality (triality?). But an uncle, a brother, and a son are three different things, and there is no single person to whom I'm both an uncle and a brother.
#43
Old 06-05-2011, 11:43 AM
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yes but science is different don't you think. waves and particles behave differentley.
Why would they not do an experiment that turns out one reult that can be made fact.
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Old 06-05-2011, 12:37 PM
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yes but science is different don't you think. waves and particles behave differentley.
Why would they not do an experiment that turns out one reult that can be made fact.
Because light (and other fundamental objects) are neither particles or waves in our normal everyday conception of them. Instead, they have properties that are both particle-like (discrete properties) and wave-like (they interact stochastically based upon a probability distribution and interference). This gives rise to the supposed paradox of particle/wave duality, but in fact, the paradox is only in our definition, as we have chosen to call elementary objects "particles" instead of using a neologism like "blobicles" to identify their unique properties. (The reason for this is historical and grounded in an effort to attempt to describe elementary particles in terms of classical phenomena.) Chronos's analogy is appropriate; depending on the type of interaction you are modeling, you may treat a photon or electron as a discrete particle or a very tightly controlled waveform (called a wave packet). Like Chronos being a brother and uncle, the difference is in from what perspective you are looking at the interaction.

As for why interference with a system is always via photon, that is very simple: photons are bosons that carry the electromagnetic force, and this is the only force by which we can directly observe (and hence, interact with) other objects. Every sense that we, as human beings, experience, is some variation of a transfer of energy and information via the electromagnetic force; light (direct impingement on photon-sensing organs), heat (via radiation, convection, and conduction), sound (conduction), smell (electrochemical interaction with free-floating elements and molecules), and touch (electrostatic repulsion). Although gravity is definitely a part of our everyday world, it may be a surprise to many that we can't directly observe it, either on a discrete or gross level; individual gravity wave packets (gravitons) or small gradients in the field are too low of an energy level to sense, and the effects we feel or observe from the effects of gravity such as falling onto pavement or watching a ball arc in a ballistic curve are inferred from electromagnetic reactions. The nuclear interactions happen on such a scale that we never directly observe them in normal conditions, and the amount of energy required to create a medium of free fundamental nuclear particles (quarks) and the more energetic leptons like the muon and tau particles) is far larger than exist in anything other than exotic high energy environments or particle accelerators.

Half Man Half Wit is also correct when he states that the multitude of interpretations that try to make something mystical about observations and their effects upon the outcome of an experiment are missing a fundamental point. While an unperturbed system will show distributed, wave-like phenomena, i.e. the two slit experiment without prior observation will show a distribution of electron that is consistent with wave interaction (even if you only allow one electron through at a time, precluding hypothetical interactions between electron), detecting the presence or absence of the electron "forces" it to take on path or the other, because the act of detecting it (or not) has influenced its behavior. This may seem counterintuitive, as we expect that the same action happens whether we are there to observe it or not per the normal rules of causality. However, causality is broken or at least severely traumatized on the quantum level, and interactions can occur that are distinctly non-causal. This causes a lot of distress for people who expect "particles" on the fundamental level to behave the same way as particles in everyday observation, but in fact, they're two different types of objects. The lack of causality (or at least, local causality) on the quantum level isn't really problematic, as for reasons too complicated to explain no useful information can be conferred by such channels and the sum of all events in a system are such that such non-causal events are loss in the noise (decoherence) such that everyday objects like a baseball approximate causal interaction down to any level of resolution that looks at the system or any major part of it. For that reason, gedankenexperiments like the Schrödinger's Cat concept serve to highlight the absurdities of applying quantum phenomena to macroscale world behavior rather than present any genuine paradoxes. Or in other words, "things work different down there."

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