Waves as Particles

[noz92]

I've been reading the book A Brief History of Time , by Stephen Hawking, in some of his chapters he's mentioned that :all particles are also waves", or something like that. How can something be both a wave and a particle? Particles are packets of energy and matter, and waves arn't in any neat little package, but rather spread out. To me they seem to be very different things.

[timo]

If the wave was small, could you distinguish it from a small particle ?

[P_Rog]

all things in motion have waves, called matter waves. light is is both waves and particles, called wave-particle duality, which einstein proved. the de Broglie relationship says that wavelenght= plank's contstant/mass*velocity. Since it is inversely proportional to the mass, the bigger the object, the smaller the wave. a baseball traveling at 60 mph has a wavelength of 1.6 x 10^-22 picometers. so small, it's not noticeable.

[algore]

It's a good question noz92. A few decades ago it wouldn't have come up because back then physicists explained the whole picture .. today physics instruction has become pure dogma. "Particle is also a wave" "big bang universe was infinite in extent and also condensed into a point" "dark matter exists even though it's impossible to detect, just because we say so", etc. It's exactly like "God is a trinity, both 3 and 1" or "the Pope is infallible", or "6,301 angels can dance on the head of a pin" etc. You must have faith in your priest / physicist, who is the fount of all mystical wisdom. If you have any doubts, genuflect before a cross, or a picture of Stephen Hawking, and admit you're a miserable sinner

 

Anyway, (now that I've gotten that off my chest) here's the answer to your question.

 

You're right that if a baseball were literally, simply, a wave, you could tell. After a while, as you watched it, it would start to spread and get fuzzy round the edges. Obviously that's not so, and I'm sure Hawking didn't say precisely that. Instead, the position (and momentum, energy etc) of a particle is specified by two things: a <i>probability</i> wave (described by Schroedinger's equation) and the <i>collapse</i> of that wave function, which happens when the particle is observed or measured. (BTW no one knows precisely how that collapse happens). So, suppose you have an observed particle in an exact location (because it has just been observed), then you allow it to evolve without observation. Its position starts to become uncertain as the probability wave function starts to spread out according to Schroedinger. If you could see it (which you can't) it would look just like any wave you're familiar with. After a while the probability wave will spread out over an arbitrarily large area. At this point, conceptually, there is no particle per se, just a large area wherein the particle <i>might</i> be. Now, you observe or measure it (by, for instance, shining a light on it). At that instant the wave "collapses" - completely disappears (well, MWI says it continues to exist in "infinite parallel universes" but let's not go there, requires too much faith ) - and the actual particle is seen to be somewhere within its spread. According to QM there is absolutely no way to know where it will be, but it's more probable to be found where the wave function was "higher" (again, see Schroedinger's equation for the details).

 

To clarify, consider the famous 2-slit experiment. (If you're not familiar with it look it up on the net). We fire a particle (from, for instance, an electron gun like the one in a traditional TV set) at a barrier containing two slits. On the other side is a screen (like the face of the TV's cathode-ray tube) which "observes" (records) where the particle hits. We do this again and again and see a pattern build up on the screen from the successive hits (which remain marked on the screen by a little dot). Until it hits the screen the "particle" has traveled as a spread-out wave function, so (just like a water wave) it passed through both slits, and these two paths interfered with each other on the other side. So instead of seeing the image of two slits on the screen, opposite the two slits in the barrier, as would have happened if the particle were just simply a particle, we get an interference pattern on the other side.

 

So you see it's not as simple as "the particle is a wave". Rather, the particle's position is determined by a probability wave function, which we can never directly observe. All we can ever observe is an actual particle, solid and distinct, after the collapse of the wave.

 

Why do you never see anything like this happen with a baseball? According to the most sensible "quantum ontology", it's continually being observed, so its wave function is continually being collapsed. That may be because you're constantly looking at it, or perhaps the wave function is spontaneously collapsed by something like gravity (according to Penrose) or whatever - but the point is that with large objects it's just about impossible to let the wave evolve long enough to detect its effects. However it's easy enough to do it with small things like photons (eg, the diffraction fringe that happens to light going around a sharp corner, which was known in Newton's time). With very careful experiments we've observed these effects with objects as large as alpha and beta rays.

 

Hope that helps.

[swansont]

It's a good question noz92. A few decades ago it wouldn't have come up because back then physicists explained the whole picture .. today physics instruction has become pure dogma. "Particle is also a wave" "big bang universe was infinite in extent and also condensed into a point" "dark matter exists even though it's impossible to detect' date=' just because we say so", etc. It's exactly like "God is a trinity, both 3 and 1" or "the Pope is infallible", or "6,301 angels can dance on the head of a pin" etc. You must have faith in your priest / physicist, who is the fount of all mystical wisdom. If you have any doubts, genuflect before a cross, or a picture of Stephen Hawking, and admit you're a miserable sinner

 

Anyway, (now that I've gotten that off my chest) here's the answer to your question.

[/quote']

 

Utter crap. I have 100 years of physics experiments that show that matter has both wave and particle properties. Evidence. Not dogma.

 

 

However it's easy enough to do it with small things like photons (eg' date=' the diffraction fringe that happens to light going around a sharp corner, which was known in Newton's time). With very careful experiments we've observed these effects with objects as large as alpha and beta rays.

[/quote']

 

Atoms have been made to diffract and interfere.

[Woxor]

How would we know about dark matter if we couldn't detect it? You don't disbelieve in black objects just because you technically can't see them, do you?

[algore]

Swansont, I didn't say that matter doesn't have both wave and particle properties. In fact I explained exactly how it works: Schroedinger's eqn describes the probability wave, while observation collapses it so that the particle is seen. I even mentioned the 2-slit experiment. You can do this yourself at home using sunlight, exactly as Thomas Young first did it 200 years ago - it's very instructive and convincing. My point was, the way the particle-wave duality is presented to a layperson (like noz92) is often dogmatic. Simply "it's both a particle and a wave - believe it because I say so". The real, complete story is both more complicated and more interesting. I admit I could have made the point more clear; rereading my first parapgraph, it does sound like perhaps I'm denying this elementary fact of QM. However if you had read the rest of my post it's obvious that I understand the Copenhagen ontology. Unless you think there's something wrong with my explanation? If so, please tell me what it is.

 

A couple more points: note that we actually have a full 200 years of experiments proving wave-particle duality, not just 100. And, I didn't say it hasn't been observed with atoms; in fact, you're right: it has.

 

Woxor, at this time I neither believe nor disbelieve in dark matter. We definitely can't detect dark matter directly (ie, using any form of electromagnetic radiation). The evidence is all indirect, consisting entirely of certain gravitational anomalies. In various situations (expansion of the universe, gas leaving galaxies, etc) cosmological objects are receding more slowly than we expect. There are 3 possible reasons: 1) calculation of mass (of the universe, or of galaxies) is too low due to some error; 2) General Relativity equations are incorrect at these cosmological scales; or 3) there is some extra matter we can't detect which supplies the missing mass. The "dark matter" theory assumes #3; my personal best-guess is #2; nobody knows the right answer yet.

 

We definitely can see (ie, via electromagnetic radiation) normal black objects, in three ways: reflected EM radiation, absorbed (or blocked) EM radiation, and black-body radiation. It's well established that "dark matter" can't be seen in any of these ways, which is why it remains such a puzzle.

 

Have you ever heard of "zodiacal light"? Here's the story .. in 1845 Leverrier established that the perihelion of Mercury's orbit precesses 43 sec / 100 years more than Newton's equations predicted. At the time he said this apparently indicated that Newton could be wrong under these extreme conditions, which are so far from normal experience, and that "future researchers" would probably someday find tiny modifications to Newton that would explain the discrepancy. It took 70 years, but finally a guy named Einstein proved him right. In the meantime, however, other physicists, dogmatically holding to Newton, came up with a far-fetched explanation. They calculated that if there were some nebulous gasses between the Sun and Mercury, "zodiacal light", in just the right configuration, it would explain the precession anomaly without disproving Newton. True, they couldn't detect these nebulous gasses in any way .. now, we can see that they were dogmatic and wrong.

 

I suspect today we're in an analogous situation, and "dark matter" is this generation's "zodiacal light". But of course I don't know - the resolution of this problem is left to future researchers.

 

Swansont, I must say you've illustrated my larger point very well. A non-dogmatic scientist, after carefully reading my post, would calmly and rationally point out what he thought were my mistakes. OTOH a religious believer whose dogma is questioned would read a few sentences and dash off an impassioned reply. Which of these two do you sound like?

[swansont]

My point was' date=' the way the particle-wave duality is presented to a layperson (like noz92) is often dogmatic. Simply "it's both a particle and a wave - believe it because I say so".

[/quote']

 

"Sounds dogmatic" and "is dogmatic" aren't the same thing. If the explainer gives details of experiment, how is that then dogmatic? And it's a lot better than "go study physics and math for three years so you can understand the relevant details. Some guy on the internet says I can't just quote you the answer." to a casual inquiry. As far as actual physics instruction, in which colleges or universities is the duality being presented without supporting evidence?

[algore]

swansont: "Sounds dogmatic" and "is dogmatic" aren't the same thing.

 

- Agree. Similarly, "accidentally sounds rabidly iconoclastic when sounding off a bit" and "is rabidly iconoclastic" aren't the same thing either!

 

swansont: As far as actual physics instruction, in which colleges or universities is the duality being presented without supporting evidence?

 

- No one, I'm sure, instructs actual physics students about this duality without presenting the evidence. However laypersons are, I think, often instructed dogmatically. I took the trouble to look up the reference to Hawking's book originally mentioned by noz92. To my amazement, Hawking devotes 3 pages to it (pp 58-61 of the "Updated and Expanded 10th Anniversary Edition") without ever mentioning wave collapse! He even goes through the 2-slit experiment, but the whole presentation is just pure magic without this crucial information. No wonder noz92 was confused .. shows he's reading carefully.

 

Meanwhile, on a subject like non-baryonic dark matter, I suspect that a dogmatic presentation is given even to advanced physics students, although it's been so long since I was in school I can't say. Can any physics students out there tell me: when your professors go through such topics as that, or wormholes or black hole photon orbits or etc, do they ever throw in this caveat: "BTW this depends on the accuracy of GR. If GR doesn't hold exactly at cosmological scales (where of course we have no experimental evidence) then all of this could be pure fantasy"? If not, I would say they're being dogmatic, just like the old proponents of "zodiacal light", ether, phlogiston, divine creation, the 4 humors, alchemy, and countless other failed theories which were dogma in their day.

[Severian]

A lot of things in undergraduate physics (including dark matter) are taught without (proper) evidence these days. This is mainly because the details of the experiement are too hard to describe to undergraduates, either in physics terms or just that the experiments are so complicated that there is insufficient time in the course. There ia usually some attempt to describe the evidence but it is very wishy-washy hand-wavy and wouldn't (shouldn't!) convince anyone.

 

How can you calculate the relic density of a neutralino in class? You have to solve the Boltzmann equation numerically, making model assumptions for the mass and co-annihilation cross-sections. While this could concievably be done as a class project, not everyone can do it, and there are plenty of more things like this...

 

This is true even for professional particle physicists these days. The community is now split into 'theory' and 'experiment'. The experimentalists have to believe what the theorists tell them without understanding all the background, while the theorists have to have faith that the experimentalists have taken all the backgrounds, errors, biases, triggers etc into account. There is just too much for one person to follow it all.

 

I really can't stand Hawking's method of sensationalising physics, telling half-truths or making bogus statements just to sell his books....

[algore]

Hi Severian, note that the divergence you describe between theorists and experimenters is a recipe for disaster. There's a crying need for more "generalists" who take an overall approach rather than specializing in narrow areas. I don't think it's impossible for "one person to follow it all". If a person of ordinary intelligence can master one branch of physics (ordinary in this context might be IQ around 140) then a real genius will be able to master the whole ball of wax (since an IQ like 200 is dozens of times more capable). The problem is that such people are steered into the most advanced theoretical maths, where they publish incredibly brilliant, elaborate and dense papers on incredibly picayune and physically meaningless topics. BTW this is just my opinion - no offense is meant to anyone - perhaps I'm wrong.

 

I know what you mean about Hawking but I consider him less offensive than many other popularizers. His best characteristic is that he often throws in a caveat like "if GR is right, then" as in "assuming GR is correct, I was able to show there had to be a big bang", etc. Nobody else (like Davies, Weinberg etc) ever does this. Actually I've given up recommending any of these books to friends who want to understand what's going on - they're too misleading. From now on I'm going to tell them "why don't you just post your dumb question on SFN"?

[Severian]

I disagree - there are lots of geniuses working on quite phenomenological physics. Did you know that Witten started off as a phenomenologist but turned to string theory because phenomenology was too hard? There will be renewed interest in 2007 when the LHC switches on, but still no-one will know all the details of theory and experiment.

 

Consider the LHC...

 

We don't even have a proper understanding of hadronization - the process by which quarks reform into hadrons like the proton. QCD is so bloody difficult that we can't solve it at low energies (at high energies we make an approximation which works reasonably well, and allows us to calculate things) and have to construct sophisticated models to describe it instead. This is on ongoing research topic of its own.

 

Then there is the details of the beam - we don't just collide one particle, but collide bunches of particles, two of which hit head on. So we need to know all about the bunch of particles - their spread in energy etc.

 

And the interaction of the particles after the collision with the detectors. It isn't bubble chambers anymore, with easy to undertand mechanisms. The workings of hardon calorimeters is extremely complicated, and I personally don't have a clue how they work. I have to rely on the experimentalist who builds it.

 

Even the data read out will be a challenge. It is already acknowleged that the data will come in too fast to be processed, so a lot of it has to be thrown away. How do we know which data should be discarded and which should be kept (this is called the 'trigger')? It has been realised that we don't have enough processing power in any computer in the world to handle the data flow. A new field of research has developed around this, called the Grid Project, which aims to use a special high speed internet to connect different computers around the world to link them up as one huge supercomputer. (Often the grid is hyped as the new web.)

 

I could go on and on... The challenges of the LHC are far too many for ne person to underatnd everything.

[algore]

What you say is very encouraging. It sounds like my view is out of date. I could defend the position that 2-3 decades ago (when I was still a theoretical mathematician, with a strong interest in physics) experimenters were looked down upon, but who cares? Apparently today they're finally getting the attention, respect and funding they deserve. I'll have to read up on LHC, about which I know nothing ..

 

As for how much one person can or can't understand, if a young John von Neumann (or Leonardo da Vinci, or etc) comes along, he might be able to shed new light on the subject. Witten is brilliant, but he's no von Neumann, as I'm sure he'd agree. Certainly around 1970 we would have laughed at the suggestion (although not every great mind is a prodigy). Anyway, it's definitely not worth arguing about.

[CPL.Luke]

I believe that everything needs to be questioned by every student.

 

as schrodinger stated long ago, quantum mechanics is not an accurate representation of the real world. if something, (no matter how many times is explained to you) doesn't make sense then think about it. believe it but still think about it, eventually you might be lying awake at night and come up with a different brilliant answer that changes physics

[cyeokpeng]

OK,

 

The electron diffraction is a strong example that electron can behave as a wave. We also know that if we use a light detector to observe which slit the electron goes into, we collapse the wavefunction and there will be no interference pattern anymore. However, there is something in the electron diffraction experiment, we can use delayed choice in 'tricking' the electron, by switching on the light detector only after the electron has passed through the slits. This is weird. The moment you switch on the light detector, the electron wave function collapses but it has already passed through the slits as waves!!! As we know from the result of the experiment, you will not see the interference pattern the moment you turn on the detector. But the electrons have already passed through the slits as waves, so why not the interference pattern being observed on the screen???

 

Does it mean that the quantum system can somehow predict the future that you are going to switch on the detector, and so it will collapse the wavefunction passing through the slit even before you turn on the detector???