Hi, anyone want to discuss quantum mechanics? please!

Which points are you struggling with? Read Feynmann's QED if you can, it explains quantum mechanics quite well to someone with a low level physics background.

Reply 2

OP

Thanks for the reply :smile:

there are De Broglie, black body radiation, double slit experiment, Bohr model, Planck's theory photoelectric effect etc and I'm trying to organize and relate the idea but can't :frown:

when all this started? by De Broglie, by double slit experiment, by photoelectric effect or black body radiation or Planck's theory ? what was the first thought that led to quantum origin?

Reply 3

Double slit was evidence of apparent wave nature of light, against the particle theory at the time. Blackbody radiation was when people started to realise something wasn't right in their understanding of physics. Bohr suggested the energy levels, but I think (could be wrong) he didn't suggest they were discrete levels of only certain values. Planck's theory was that energy levels in atoms are discrete, which explains blackbody radiation. Einstein's work on the photoelectric effect was the beginning of his journey into the exploration of QM effects, and supported quantisation of energy similar to Planck's work. De Broglie came last I believe and he found that QM effects were not limited to photons etc. but it was a universal effect on ALL particles. The reason we don't really experience them at a macroscopic level is explained by a few different theories (decoherence being the most popular/supported). Also, please quote me so I remember to check the thread.

Edit: I was mistaken about Bohr, he applied Planck's theory to replace Rutherford's model of the atom with classic energy levels. Bohr instead proposed the quantum mechanical theory of the atom. Also, I've just noticed this is an undergrad thread not A Level so my knowledge is probably quite limited compared to what you need. I'll try my best.

Edit2: De Broglie also did some work into explaining why Bohr's discrete quantum levels exist. This website is quite good at explaining it http://abyss.uoregon.edu/~js/cosmo/lectures/lec08.html

(edited 9 years ago)

Reply 4

jaffacake111

9

There is a saying that goes around QM circles, it is attributed to Richard Feynman and says: "If you think you understand quantum mechanics, you don't understand quantum mechanics".

I know I certainly don't - our species evolved on the plains of Africa to escape lions - but it fascinates me nonetheless :smile:

Reply 5

OP

Original post by TVIO

Double slit was evidence of apparent wave nature of light, against the particle theory at the time. Blackbody radiation was when people started to realise something wasn't right in their understanding of physics. Bohr suggested the energy levels, but I think (could be wrong) he didn't suggest they were discrete levels of only certain values. Planck's theory was that energy levels in atoms are discrete, which explains blackbody radiation. Einstein's work on the photoelectric effect was the beginning of his journey into the exploration of QM effects, and supported quantisation of energy similar to Planck's work. De Broglie came last I believe and he found that QM effects were not limited to photons etc. but it was a universal effect on ALL particles. The reason we don't really experience them at a macroscopic level is explained by a few different theories (decoherence being the most popular/supported). Also, please quote me so I remember to check the thread.

Edit: I was mistaken about Bohr, he applied Planck's theory to replace Rutherford's model of the atom with classic energy levels. Bohr instead proposed the quantum mechanical theory of the atom. Also, I've just noticed this is an undergrad thread not A Level so my knowledge is probably quite limited compared to what you need. I'll try my best.

Edit2: De Broglie also did some work into explaining why Bohr's discrete quantum levels exist. This website is quite good at explaining it http://abyss.uoregon.edu/~js/cosmo/lectures/lec08.html

Thanks for your reply and for the link :smile:

Let's take them one by one. I think the first thing was black body radiation. A body that absorb and radiate heat almost perfectly is known as black body. Scientists realized that in a perfect black body, temperature can be determined by its color. At moderate hot temperature, the black body glows red, at high temperature, it's yellow and at very high temperature if becomes bluish white. If we say that sun is yellow and earth is blue but sun is hotter because for the color, temperature relationship the body must be a perfect black body (a best radiator and absorber.) Sun is a radiator but earth is a reflector, so this case don't apply on earth color.
If we compare sun with a white star, the white star will be hotter than sun.
We noticed that increasing the temperature, wavelength decrease so that mean increase in temperature further cause the wave towards ultraviolet and so on. But that didn't happened, (ultraviolet catastrophe). Instead the curve goes further and further away from ultraviolet. Here comes Planck's theory and photon.

My knowledge is really limited too, so please guide me where I'm wrong and if a question arise in your mind please tell, we both will try to research and find the answer then we'll move to the next. what do you say?

Reply 6

Original post by Slearner

Thanks for your reply and for the link :smile:

Let's take them one by one. I think the first thing was black body radiation. A body that absorb and radiate heat almost perfectly is known as black body. Scientists realized that in a perfect black body, temperature can be determined by its color. At moderate hot temperature, the black body glows red, at high temperature, it's yellow and at very high temperature if becomes bluish white. If we say that sun is yellow and earth is blue but sun is hotter because for the color, temperature relationship the body must be a perfect black body (a best radiator and absorber.) Sun is a radiator but earth is a reflector, so this case don't apply on earth color.
If we compare sun with a white star, the white star will be hotter than sun.
We noticed that increasing the temperature, wavelength decrease so that mean increase in temperature further cause the wave towards ultraviolet and so on. But that didn't happened, (ultraviolet catastrophe). Instead the curve goes further and further away from ultraviolet. Here comes Planck's theory and photon.

My knowledge is really limited too, so please guide me where I'm wrong and if a question arise in your mind please tell, we both will try to research and find the answer then we'll move to the next. what do you say?

The idea of the photon was proposed by Einstein from Planck's work I believe. Right, so the curve DOES shift towards the UV end, but not as they predicted it would (where it would be polynomial/exponentially higher towards UV with greater temperature). Planck's theory was that the energy is released in quanta (discrete energy packets) and so that's why UV was very minimal. Photons are released when electrons drop energy levels, and the distance between the energy levels corresponds to the frequency released. But they're much more likely to drop smaller energy levels (or fewer which both result in less change in energy) so that's why the curve is as it is rather than just increasing off to infinity. Have you looked at absorption/emission lines?

(edited 9 years ago)

Reply 7

OP

Original post by TVIO

But they're much more likely to drop smaller energy levels (or fewer which both result in less change in energy) so that's why the curve is as it is rather than just increasing off to infinity.

Why is that so? would you please explain the point?
so, quanta is the packet of energy contained within an em wave and we'll call an energy packet as photon when it's released by electron drop from higher to lower energy level, right?
is electromagnetic wave a standing wave? does the electric and magnetic fields after inducing the other disappear or remains there? where does the quanta lies? or it's that the quantum travel in a wave like motion from one point to another?
also, how Planck's quantum explain the ultraviolet catastrophe? is it that as energy is quantized so, to increase temperature further a specific high energy packet is required, which is not possible to obtain so the value of UV wavelength was low? but I'm confused about the UV catastrophe, if curve w shifted towards UV, then which value didn't agreed with the theory? how can we describe it on atomic level?

Reply 8

Original post by Slearner

Why is that so? would you please explain the point?
so, quanta is the packet of energy contained within an em wave and we'll call an energy packet as photon when it's released by electron drop from higher to lower energy level, right?
is electromagnetic wave a standing wave? does the electric and magnetic fields after inducing the other disappear or remains there? where does the quanta lies? or it's that the quantum travel in a wave like motion from one point to another?
also, how Planck's quantum explain the ultraviolet catastrophe? is it that as energy is quantized so, to increase temperature further a specific high energy packet is required, which is not possible to obtain so the value of UV wavelength was low? but I'm confused about the UV catastrophe, if curve w shifted towards UV, then which value didn't agreed with the theory? how can we describe it on atomic level?

This curve shows actual vs predicted curves http://en.wikipedia.org/wiki/Ultraviolet_catastrophe#mediaviewer/File:Black_body.svg which illustrates what I was trying to say better. The quanta aren't really anywhere until they're measured which is a weird quirk of quantum mechanics (but true). As for why the energy drop is less likely to be big, I think it's because the electrons will drop energy levels as soon as they get promoted so don't really have a chance to go much higher (could be wrong on this, not certain). You're right about why Planck's quanta explain the UV catastrophe

Reply 9

OP

This is my understanding of black body radiation. Please guide me where I'm wrong.In a perfect black body, the electrons contain energy that they previously absorbed. When these electron radiate energy in the form of em waves, as the wavelength decreases, the intensity increases. So from infrared to red, yellow, bluish white, the intensity (the flow of energy) kept increasing. But when the curve shifts to ultraviolet, the intensity decreased and a point reached where there was no energy flow. This was in disagreement with the classical law that decrease in wavelength or increase in the frequency will cause more and more waves production as frequency =no. Of waves emitted per second and more waves need more energy. So, if infinite waves are emitted means there is infinite energy produced( But how? Confused here???) So, if that was true that waves could absorb any amount of energy, the waves will suck out even a little amount of energy, this can even cause the sun to freeze up. But that doesn't happen, as Planck's suggested, if a wave of specific frequency is to be produced then there much have specific amount of energy, the quanta.So, when we reach at higher frequency, quantum of specific high energy is required which is hard to achieve, that's why the number of electrons radiating high energy quanta are less and the intensity decreases, finally reaching to zero.

Original post by TVIO

The quanta aren't really anywhere until they're measured

Quantum is the measure of energy which is released or absorbed by an object. I think the wave is a wave but when this wave is radiated or absorbed, it act like a particle, am I right? and photon is the elementary particle with no rest mass but has relativistic mass. so, when an electron absorb a quantum, it goes to higher energy shell and an absorption spectrum (dark lines against bright background) are observed and when it de excite, a photon is released, showing emission spectrum.

(edited 9 years ago)

Reply 10

Original post by Slearner

This is my understanding of black body radiation. Please guide me where I'm wrong.In a perfect black body, the electrons contain energy that they previously absorbed. When these electron radiate energy in the form of em waves, as the wavelength decreases, the intensity increases. So from infrared to red, yellow, bluish white, the intensity (the flow of energy) kept increasing. But when the curve shifts to ultraviolet, the intensity decreased and a point reached where there was no energy flow. This was in disagreement with the classical law that decrease in wavelength or increase in the frequency will cause more and more waves production as frequency =no. Of waves emitted per second and more waves need more energy. So, if infinite waves are emitted means there is infinite energy produced( But how? Confused here???) So, if that was true that waves could absorb any amount of energy, the waves will suck out even a little amount of energy, this can even cause the sun to freeze up. But that doesn't happen, as Planck's suggested, if a wave of specific frequency is to be produced then there much have specific amount of energy, the quanta.So, when we reach at higher frequency, quantum of specific high energy is required which is hard to achieve, that's why the number of electrons radiating high energy quanta are less and the intensity decreases, finally reaching to zero.Quantum is the measure of energy which is released or absorbed by an object. I think the wave is a wave but when this wave is radiated or absorbed, it act like a particle, am I right? and photon is the elementary particle with no rest mass but has relativistic mass. so, when an electron absorb a quantum, it goes to higher energy shell and an absorption spectrum (dark lines against bright background) are observed and when it de excite, a photon is released, showing emission spectrum.

I think you're right about the blackbody part. The photons are always particles, but merely behave as waves often due to weird quantum effects is my understanding of it. They don't interfere with each other (unless entangled, which is a whole 'nother story...) but only with themselves. Also, when an electron absorbs a quanta it may have enough energy to escape the atom, and the excess energy goes into the momentum of the electron (if it doesn't have enough energy I'm not sure what happens, it might not be absorbed or the atom may just gain kinetic energy/heat)

Reply 11

OP

Original post by TVIO

The photons are always particles, but merely behave as waves often due to weird quantum effects is my understanding of it.

When a particle is in motion, it behave like a wave. When it's motionless, it behave like a particle. Similarly, when a wave in in motion, it behave like a wave, but when it's absorbed or radiated, at that point it behave like a particle i. e photon. (am I right?) So, talking about the double slit experiment, when a matter, say a marble is fired through single a double slits, it produced single or double shadows at the point of interaction between marble and the wall. When light is shined on single slit, it showed up on the wall at the maximum intensity, but when there were two slits, an interference pattern appeared on the wall with having dark and bright bands due to interference.(this experiment had proved the wave nature of light and rejected the corpuscular theory of light at that time.)When electrons are fired on a single slit, it act like the marble, creating a shadow at the point of interaction. But, in the presence of double slits, electron unexpectedly produced an interference pattern. This experiment shows the duel nature of electron.And the wave particle duality was hypothesized by de Broglie,who took the idea of relativity and photoelectric effect to show that as wave can behave like particle, similarly particle or matter can behave as waves as well. He was about to fail his PhD exam due to strong opposition to this idea, but Einstein realized its importance and supported him.

(edited 9 years ago)

Reply 12

OP

Original post by TVIO

Also, when an electron absorbs a quanta it may have enough energy to escape the atom, and the excess energy goes into the momentum of the electron (if it doesn't have enough energy I'm not sure what happens, it might not be absorbed or the atom may just gain kinetic energy/heat)

I think if the energy isn't enough to knock it out, the energy transferred will be absorbed, increasing it K.E and temperature. When light shines upon a metal surface, electrons are ejected. Classical theory failed to explain the experimental results as according to classical theory,
Change in intensity will affect the energy of the electrons, so shining brighter light will emit energetic electrons, but in reality, intensity didn't affect the energy but number of emitted electrons.
Change in light frequency wouldn't affect the energy of electrons but it does.
Without a specific colour, wavelength or frequency, no electrons are emitted from a particular metal surface.

Einstein explained this phenomena by applying Planck's quantum theory. Em waves occur in chunks called quantum and the quantum for visible light is named as photon.
As each photon has same specific energy, increasing the intensity just increase the number of photons along with the number of electrons knocked out. There's no change in energy of electron.
While increasing the frequency, increase the energy of the photons, so also cause change in the energy of the electrons. As E=h(nu). the number remains the same.
Also, for knocking out an electron, the photon must have energy:
E=hf-W
Where hf is the energy required to give momentum to the electron and W is the work function required to get the electron free from nuclear influence.

Reply 13