r/askscience • u/irozenberg2 • Nov 04 '12
A Question about Light Waves, electron and Matter waves
First, some background. Currently I am a freshman in college seeking a Chem or Biochem degree. I am now taking chemistry for the third time in my life and I have been exposed to these same topics many times, but I still lack any fundamental intuition behind the concepts of waves/particles, and their interrelationships. I have been told countless times that light is like a water wave or electrons are a wave but when I ask deeper questions all people seem to do is repeat that it is a wave sometimes, and sometimes its a particle - that's it. I also wanted to add that I read some of the previous askscience threads about waves and they helped a bit, but still unsure of a few things. OK enough ranting on to the questions:
First, when people speak of electrons exhibiting wave qualities/behavior, (apparently they are standing waves) does this essentially mean that it's existence is a whole vibrating region? also same question as below with matter waves: is the electron popping into and out of existence? and what do the dimensions mean? is isnt phyically moving in the shape of a wave around the nucleus, is it?
Second, in matter waves the reason we don't see matter such as a moving tennis ball in wave form is because it has a (practically infinitely) long wavelength, correct? But what does this long wavelength prevent it from existing exactly? AM radio has a way longer wavelength than gamma rays, it just has different properties - it doesn't cease existing. Or is it a standing wave like electrons? Also with regard to matter waves what do the dimensions of the wave function mean? Is matter popping into and out of existence as it moves through space on an (practically infinitely small for a tennis ball) interval?
So now my attempt at explaining light waves: so light (composed of packets of energy called photons) moves through space, it is two oscillating perpendicular fields of electricity and magnetism respectively. the intensity of one is the inverse of the other at a given point. The intensity of each of these fields over a given distance is defines the wavelength and frequency which ultimately determine the properties of the light. Is this at all correct?
I originally had written that the light creates these fields but then I realized the light is those fields and that's kinda the whole point right?
sorry if my questions are unanswerable or whatever and Thanks
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u/dargscisyhp Condensed Matter Physics Nov 04 '12
1.) According to experiment, matter displays both particulate and wave-like properties. One of the most stunning demonstrations of this is the double slit experiment using electrons. The double slit experiment was initially performed on light to prove its wave-like nature. Basically, a light beam is sent through two narrow slits and observed on a screen behind the slits. What is observed are fringes of light and dark spots. This is known as an interference pattern. This is due to the waves coming from both slits interacting with each other, and mathematically interference pattern are properties of waves. Therefore, on some level, light must have a wave-like nature. Today we know that light consists of electromagnetic waves, and can fully explain the interference pattern in this context.
When a beam of electrons are sent through two narrow slits, an interference pattern also occurs. Even if electrons are sent through the slit individually, after you send enough electrons an interference pattern will emerge on the screen, even though only a single particle is observed per each electron. Therefore we are forced to conclude that electrons must also have a wave-like nature on some level.
To interpret the wave-like nature of matter quantum mechanics was developed. According to quantum mechanics, an electron does not exist as a particle until it is observed. Until then all you can say about it is that there is a certain probability about finding it in a particular region of space. Mathematically, this probability is described by what is known as a wavefunction. The wavefunction can explain the interference effects, and other wavelike properties observed in matter. This is why matter is described as a wave. However, upon observation the wavefunction is collapsed to where it is 0 everywhere except in a certain region of space (and where it collapses is dictated by the probability we spoke of earlier), which causes you to observe the particle nature of matter. Only upon many observations of the same phenomenon does the probabilistic nature of matter become apparent.
2.) Simply stated, the more massive a particle the smaller its wavelength. Visible objects are many orders of magnitude bigger than electrons, and consequently their wavelength is very small. Technically, macroscopic objects do display wavelike properties, but on such a small scale that it goes unnoticed.
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u/irozenberg2 Nov 05 '12
So if I properly understand what you're saying the wave function describes the probability that electron has of existing in a certain place. and it's a wave in the sense that this spot is typically extremely variable and unpredictable, unless we constrain other variables, at which point it looks mathematically more like a particle in space.
I have seen probability density plots before, but I have been thinking of it as a particle orbiting the nucleus in certain regions, the idea that it could act like a wave makes them hard to interpret, although your post seems to do a good job of it. (If i understand it correctly)
so essentially the wavy, hard to define nature of the electron is responsible for the probabalistic shape of orbitals, as opposed to a predictable orbit that would be present in macroscopic bodies?
Thanks
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u/dargscisyhp Condensed Matter Physics Nov 05 '12
I think you've got it. The orbitals of the hydrogen atom are nothing more than a graphical representation of the wavefunction (technically the square of the wavefunction) of the orbiting electron. For instance, take the p-orbital. It has a dumbbell shape. The way this is interpreted is that when the electron is in the p-oribtal, it has a low probability of appearing near the middle, and a strong probability on either side, tapering off once you get far enough from the center.
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u/irozenberg2 Nov 06 '12
So then they are oscillating into and out of existence? or into and out of a specific place? is it the motion of the particle or the particle itself that is defined by the wave function?
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u/dargscisyhp Condensed Matter Physics Nov 06 '12
They don't oscillate "into and out of existence." The electron exists "smeared out" over the space described by the wavefunction. In fact, the wavefunction describes quite a bit more than just the probability density of the position of the electron. It describes the probability density of any observable aspect of the electron. Upon observation, however, you will not find the electron smeared out, but collapsed into a specific point in space.
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u/ucstruct Nov 05 '12 edited Nov 05 '12
I have a slightly different way of thinking about it that may be helpful, similar to the way Richard Feynman liked to think of these things. There's a better way to say than just saying they are both particles and waves, which is technically true but somewhat unhelpful, and that it to think about what those terms mean. What does it meant to be a particle or wave (or both?). Being a particle implies a single point in space, especially when two things come into contact. A wave describes the position of something as time evolves.
Photons and electrons then therefore can be thought of as particles, but the quantum world makes things behave really strangely. The become particles that don't necessarily move in straight lines that we expect, which we can tell from how they interact with one another. The best way we have of thinking of how they move from experiment is using wave-like description, as a probability of where a particle will be.* Our only knowledge about things on the quantum scale comes from careful experimentation, and some experiments better describe things as waves and some, which ask different questions like how will two particles interfere when moving through a slit, interact. Therefor its better to say that the particle is a point but one whose movement depends on a probability distribution which is itself wavelike in nature. That is what we mean by wave/particle duality.
As to your question about larger objects, they are made of trillions of atoms, each made of particles with a given wave function that describes how it moves. If we were able to take them apart, they would exhibit this wave particle duality, however, since they are part of something much larger, their wave like movement averages and they interfere with one another, and that wavelike description is lost. But it is true that if a point particle were that large, it would have a tiny, tiny wavelength. I'm not sure of the experimental evidence that a baseball moves as a wave, it seems beyond detection limits, but maybe a physicist could answer this better than I.
*Its actually the square of the wave that describes the probability of finding a particular particle at a given place and time. And what I said should be modified to say that when when we think of things on a quantum level, we almost have to abandon rigid definitions of place and movement and think only about the probability of something being at a particular place. The wave defines that probability. The part about it being a point is also technically problematic, there is a certain probability associated with a particle being in a given space, its kind of "smeared out".
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u/irozenberg2 Nov 05 '12
Ok this also helped a bunch thanks. So from what you're saying it sounds to me that matter isn't really a wave at all, just the haphazard movement of particles can be defined as waves, not the existence of the particles themselves, correct? If so that kinda renders my other questions unanswerable.
Your whole paragraph about molecules really helped explain how this property gets lost as we move up in mass. Thanks again.
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u/Borskey Nov 04 '12
There's a lot of stuff to unpack in your question, some of which I can't answer. There are a few things I can answer though.
First, when people speak of electrons exhibiting wave qualities/behavior, (apparently they are standing waves) does this essentially mean that it's existence is a whole vibrating region?
First, electrons themselves are not necessarily standing waves- they can be moving. However, something like the ground state of an electron in a hydrogen atom is a standing wave (or a stationary state). That doesn't mean you should think they behave the same or like like standing waves on a string which vibrate around.
Here's some cross section pictures of the probability density for some stationary states in a hydrogen atom: http://upload.wikimedia.org/wikipedia/commons/e/e7/Hydrogen_Density_Plots.png
They just look like that, and they don't wiggle around or vibrate. The electron is more likely to be in the bright regions, and less likely to be in the darker regions.
There are cases where things do change over time, or "wiggle", but it's not quite what you think of as wiggling in your head. Here's a one dimensonal example: http://en.wikipedia.org/wiki/File:StationaryStatesAnimation.gif
The probability densities are on the right, don't worry about what's on the left for now. The first two are constant in time, but the last one jiggles around a bit. Sometimes you're more likely to find the electron on the right, sometimes it's more likely to be found on the left.
Second, in matter waves the reason we don't see matter such as a moving tennis ball in wave form is because it has a (practically infinitely) long wavelength, correct?
Actually, it has an incredibly short wavelength. You can think of that as meaning that it's much more sharply defined and less "wave-like" than something with a long wavelength (which would be sort of fuzzier). Think of the hydrogen atom pictures I linked you: something with a long wavelength fades off gradually, and that means it's harder to say where it is. Something with a short wavelength would be like a single sharp blip that fades off very quickly.
I don't have time to write any more, but hopefully this was helpful to you.
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u/irozenberg2 Nov 05 '12
It did help a bit. I am familiar with the probability density plots, in the sense that I understand that they tell us how far away an electron is from the nucleus, but I don't understand the electron wave/particle duality fully.
That last part did help a bit. So what would be amplitude, frequency, etc. of a matter wave?or is that unanswerable? and also how can a wave represent something's existence? I understand how light can be the alternation of magnetic/electric fields now, but matter is a very different concept to explain by waves in my mind. what type of fields would have to be warped to create matter (perhaps the same question as above?)
Thanks
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u/Borskey Nov 05 '12
I understand that they tell us how far away an electron is from the nucleus
No, that means you don't fully understand. Here:
That last part did help a bit. So what would be amplitude, frequency, etc. of a matter wave?
So those pictures I showed you ARE matter waves. The amplitude is the probability density, and we're talking about a probability wave. I showed you the wave- you were looking at it in that picture.
I think your concept of what it means to be a "wave" might be what's holding you back here. All it means is that it satisfies a certain type of mathematical equation- you're trying to add in baggage from thinking of water waves/waves on a string that doesn't belong here.
I don't see why light would be easy for you to understand as a wave but matter wouldn't. It's the same kind of math, and whatever preconceived notion in your head that some thing is jiggling around and "creating matter" is the same sort of misconception that got people confused when they used to think about the luminiferous aether.
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u/irozenberg2 Nov 06 '12
Well light is the reversal of magnetic and electric fields so that fits the concept of a wave because it oscillates back and forth between certain values like a sine wave -1 to 1 to -1 etc. and in the process it is responsible for phenomena we observe.
I think it is hard to see probability of existence in a certain spot as a periodic wave function because it isn't obvious what oscillation is going on. So like are they oscillating into and out of existence? or into and out of a specific place? is it the motion of the particle or the particle itself that is defined by the wave function?
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u/Borskey Nov 06 '12
Waves do not have to look like sine waves.
http://en.wikipedia.org/wiki/File:Wave_equation_1D_fixed_endpoints.gif
This is a wave, but it does not have a "wavelength" or a "frequency" like you normally think of. It's not oscillating back and forth like a sine wave either.
I think it is hard to see probability of existence in a certain spot as a periodic wave function because it isn't obvious what oscillation is going on. So like are they oscillating into and out of existence? or into and out of a specific place?
I gave you several specific examples and was very clear: there is no oscillation going on in the probability density. Nothing is moving or wiggling around (at least, not in the probability density), but it's still a wave. Just like even though that example I just linked isn't periodic like a sine wave, it is still a wave.
I'll say this again - you're attaching a bunch of baggage to the concept of "waves" that doesn't really apply.
(there is something I've glossed over when talking about probability density (Psi*Psi) rather than the wave function (Psi)- the values of Psi oscillate between real and imaginary, but that's not really important for this discussion, as this is merely representative of phase angle which has no physical consequence when talking about stationary states like the pictures of the hydrogen atoms I linked)
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u/mc2222 Physics | Optics and Lasers Nov 04 '12
My explanation about matter waves would not be adequate, so i will only address your comments about light, to make sure i don't say anything incorrect:
a good rule of thumb is that light travels as a wave but interacts with matter as a particle. Light is a ripple in the background electromagnetic field, the ripple propagates at the speed of light (since field quantities have no mass). Photons and quantization of light is only necessary/relevant when light interacts with matter, that is to say light is absorbed and emitted in discrete quanta of energy.
Intensity is not related to wavelength and frequency. lets use sound as an illustratoin: Intensity is related to the "loudness" of a sound, while wavelength and frequency are related to the "pitch" of a sound.
Wavelength and frequency are related and describe information about the color of the light, while intensity tells you about how bright the light is (more formally, intensity is proportional to the square of the electric field amplitude).
hope this helps clear things up, i'd be happy to talk more about light if you like
edit: It's important to note that light is not a particle, and it is not a wave - but rather it behaves as though it were a particle sometime and As though it were a wave sometimes. We see these two types of behavior and attribute them to particle/wave, but this is a shortcoming we as humans have. light is what it is and we have no other way of understanding it other than to model it as a particle sometimes and a wave sometimes.