# Thread: Why can't we tell exactly location of a electron moving through out space but only its probability?

1. Hi I am learning quantum mechanic, but I can't understand the behavior of particle:How can a partical behave like a wave? Can anyone explain to me?
And I need some document about quantum mechanic/

2. Originally Posted by johnzxcv
Hi I am learning quantum mechanic, but I can't understand the behavior of particle:How can a partical behave like a wave? Can anyone explain to me?
Hi johnzxcv - Let me be the first to welcome you to the forum. Welcome!

A particle does not behave like a wave and a wave does not behave like a particle. Quantum mechanics teaches us that particles such as photons and electrons sometimes behave like a wave and sometimes behaves like a particle. That's called the Wave-Particle Duality.

Originally Posted by johnzxcv
And I need some document about quantum mechanics
I don't know your level of understanding of math or physics. That's something we'd need to know in order to make a good recommendation. If you've studied physics perhaps in high school or a few courses in college then you'd probably already know this stuff. However just in case, try reading An Introduction to Quantum Mechanics by A.P. French and Edwin F. Taylor. You might be able to find it or at least something like it at Electronic library. Download books free. Finding books . It's a wonderful site to download virtually any text that you could think of.

For now you can try reading the Wikipedia article on the wave-particle duality and an application at

Wave

Double-slit experiment - Wikipedia, the free encyclopedia

I hope that helps. Please feel free to ask anything else that you feel I can help with.

3. I think the result of the Double-slit experiment is caused by the propulsive force between each electron, not "electron behave like a wave". Why didn't anyone think about the propulsive force between each electron.

4. Do you mean the repulsive force? It's not the explanation as this experiment also works with neutral particles.

5. Originally Posted by johnzxcv
I think the result of the Double-slit experiment is caused by the propulsive force between each electron, not "electron behave like a wave". Why didn't anyone think about the propulsive force between each electron.
First of all the double-slit experiment uses photons, not electrons. The famous wave-particle experiments for electrons is the Davisson–Germer experiment. They diffracted electrons off the surface of a nickel crystal. The pattern of the diffraction cannot be explained using Coulomb's law of attraction between charged particles. In fact when you send one electron in at a time then according to your idea there should be no diffraction at all, yet when many electrons are sent in one at a time and a patter is accumulated then that's when the wave-nature of the electron appears.

Mere repulsion could only explain why the beam would diverge. I cannot explain any interference pattern. Believe me, as Feynman explains how things were done in the early days of quantum theory all of these things have all be thought out very carefully using all sorts of strange schemes.

6. Originally Posted by johnzxcv
I think the result of the Double-slit experiment is caused by the propulsive force between each electron, not "electron behave like a wave". Why didn't anyone think about the propulsive force between each electron.
You "think" do you? And everyone else does not "think" do they? snort

7. Nobody did this experiment with neutral particles.
neutral particles have no Electric charge so no way they can accelerate the neutral particles.

8. How do you know The pattern of the diffraction cannot be explained using Coulomb's law of attraction between charged particles? moreover. you can't send one electron at one time.In the experiment they always send alots of electron not one.

9. Oh yeah . You will see my theory

10. Originally Posted by johnzxcv
Nobody did this experiment with neutral particles.
neutral particles have no Electric charge so no way they can accelerate the neutral particles.
Photons, neutrons, helium atoms, hydrogen molecules, and buckyballs are neutral particles for which diffraction has been experimentally observed.

Such particles do not need to be charged because their motion can be induced thermally in the gas phase. The use of a velocity filter obtains a monochromatic beam of particles that are suitable for diffraction.

11. Originally Posted by johnzxcv
How do you know The pattern of the diffraction cannot be explained using Coulomb's law of attraction between charged particles?
Because diffraction is a consequence of Huygens-Fresnel principle and is independent of charge.

Originally Posted by johnzxcv
moreover. you can't send one electron at one time.In the experiment they always send alots of electron not one.
By greatly reducing the intensity of the beam of particles, one can ensure that statistically there is only one particle at any time. The interference pattern then is the accumulation of many particles over time.

12. Originally Posted by johnzxcv
Nobody did this experiment with neutral particles.
neutral particles have no Electric charge so no way they can accelerate the neutral particles.
That's incorrect. Neutron diffraction experiments were done using, of course, neutrons which are electrically neutral. The neutrons are produced by a nuclear reactor which are created why heavy nuclei decay.

Originally Posted by johnzxcv
How do you know The pattern of the diffraction cannot be explained using Coulomb's law of attraction between charged particles? .
Because we know using the laws of electrodynamics exactly what happens in any conceivable situation with any conceivable charge and current distribution.

Originally Posted by johnzxcv
moreover you can't send one electron at one time.In the experiment they always send alots of electron not one
That's hardly true at all! See Double-slit experiment : Research & Development : Hitachi
Electrons are emitted one by one from the source in the electron microscope.
That electron microscope use diffraction using a low intensity beam of electrons. As it says, electrons are emitted one at a time.

You really should do a search on the internet before you make these claims. That way we won't have to point these mistakes out to you.

13. Originally Posted by Physicist
That's incorrect. Neutron diffraction experiments were done using, of course, neutrons which are electrically neutral. The neutrons are produced by a nuclear reactor which are created why heavy nuclei decay.

Because we know using the laws of electrodynamics exactly what happens in any conceivable situation with any conceivable charge and current distribution.

That's hardly true at all! See Double-slit experiment : Research & Development : Hitachi

That electron microscope use diffraction using a low intensity beam of electrons. As it says, electrons are emitted one at a time.

You really should do a search on the internet before you make these claims. That way we won't have to point these mistakes out to you.
That is how I learn: learning from mistake . Thanh you. But you still didn't answer why do partical behave like a wave? And can you tell what It's wavelenght depend on?

14. Dear John,

When I wrote You really should do a search on the internet before you make these claims. That way we won't have to point these mistakes out to you. it seemed fine to me since I merely wanted to point out that if you do a search using Google first then you can verify whether your assumptions are valid or not. However when I look back at the way I phrased it that's not the way I appear to come across. I almost seemed a bit rude. So I wanted to let you know that I never intentionally say something rude to people so please don't take it that way, okay?

Originally Posted by johnzxcv
But you still didn't answer why do partical behave like a wave?
Nobody knows why. That's why it's referred to as a postulate. See Louis de Broglie - Wikipedia, the free encyclopedia

Referring to de Broglie Wikipedia writes
In his 1924 PhD thesis he postulated the wave nature of electrons and suggested that all matter has wave properties. This concept is known as wave-particle duality or the de Broglie hypothesis. He won the Nobel Prize for Physics in 1929. The wave-like behaviour of particles discovered by de Broglie was used by Erwin Schrödinger in his formulation of wave mechanics.
Originally Posted by johnzxcv
And can you tell what It's wavelenght depend on?
Yes. It depends on the magnitude of a particles momentum, p. I.e. wavelength = h/p.;

I did a bit of searching myself and found this - Electrons behaving like a particle and a wave: Feynman's double-slit experiment brought to life -- ScienceDaily

15. Originally Posted by johnzxcv
But you still didn't answer why do partical behave like a wave?
A particle behaves like a "wave" when it has a definite momentum. It behaves like a "particle" when it has a definite position. The so-called "wave-particle duality" is about the relationship between momentum and position as observables.

16. Originally Posted by KJW
A particle behaves like a "wave" when it has a definite momentum.
I believe that to be phrased poorly. In my humble opinion you should have phrased it something like An electron, photon etc. has a definite wavelength when the momentum is well defined. If you say that as you did then you’d be asserting that A particle behaves like a "wave"… which contradicts the wave-particle dualism which states that an electron etc can behave either a particle or a wave but never both at the same time. See wave-particle duality, uncertainity principle
It is one of the strange, but fundamental, concepts in modern physics that light has both a wave and particle state (but not at the same time), called wave-particle dualism.

17. Originally Posted by Physicist
Originally Posted by KJW
A particle behaves like a "wave" when it has a definite momentum.
I believe that to be phrased poorly.
I did consider this before posting. However, rather than using the somewhat esoteric term "quantum state", I chose to use the more familiar term "particle" as it occurs in the term "subatomic particle". Indeed, I placed "wave" and "particle" as in "wave-particle duality" in quotation marks to help distinguish this use of the term "particle" from the other use of the same term. Placing these terms in quotation marks also reflects my view that these are somewhat loose terms for the nature of these objects.

18. Originally Posted by KJW
I did consider this before posting. However, rather than using the somewhat esoteric term "quantum state", I chose to use the more familiar term "particle" as it occurs in the term "subatomic particle". Indeed, I placed "wave" and "particle" as in "wave-particle duality" in quotation marks to help distinguish this use of the term "particle" from the other use of the same term. Placing these terms in quotation marks also reflects my view that these are somewhat loose terms for the nature of these objects.
Yeah. I was leery about saying anything in fact. But I wanted the reader to be sure about this because it can be so tricky. Thank you kindly, sir, for explaining that for me. It's greatly appreciated.

19. Originally Posted by Physicist
I believe that to be phrased poorly. In my humble opinion you should have phrased it something like An electron, photon etc. has a definite wavelength when the momentum is well defined. If you say that as you did then you’d be asserting that A particle behaves like a "wave"… which contradicts the wave-particle dualism which states that an electron etc can behave either a particle or a wave but never both at the same time. See wave-particle duality, uncertainity principle
You said "an electron etc can behave either a particle or a wave but never both at the same time". That was wrong.
They has done a experiment that prove photon can behave like a wave and a partile at the same time.
Photons Observed as Particles, Waves Simultaneously | Research & Technology | Nov 2012 | photonics.com

20. John, not quite! The experiment concerns 2 entangled photons. It detect one as a particle and lets the other do the wavy bit. The same photon does not behave in both ways.

21. Originally Posted by johnzxcv
You said "an electron etc can behave either a particle or a wave but never both at the same time". That was wrong.
They has done a experiment that prove photon can behave like a wave and a partile at the same time.
Photons Observed as Particles, Waves Simultaneously | Research & Technology | Nov 2012 | photonics.com
John - Scientific principles that have been around for a century like the one I just mention and you're now talking about, aren't discarded by the first claim made in a paper by someone. Such experiments have to be repeated and everything considered before its tossed out. So no. Just because you found one paper that claims otherwise doesn't mean that I'm wrong.

22. Originally Posted by Physicist
John - Scientific principles that have been around for a century like the one I just mention and you're now talking about, aren't discarded by the first claim made in a paper by someone. Such experiments have to be repeated and everything considered before its tossed out. So no. Just because you found one paper that claims otherwise doesn't mean that I'm wrong.

23. I have been thinking why electron behave like a wave . And my first claims "the propulsive force between each electron" was horribly wrong .But when I read stephen hawking book He said'' In space paricles can simply disappear and reappear at somewhere else'' perhaps that why particles can behave like a wave.

24. Originally Posted by johnzxcv
I have been thinking why electron behave like a wave . And my first claims "the propulsive force between each electron" was horribly wrong .But when I read stephen hawking book He said'' In space paricles can simply disappear and reappear at somewhere else'' perhaps that why particles can behave like a wave.
Quantum mechanics does not have an answer as to why there is a particle wave duality. As Richard Feynman wrote in The Feynman Lectures on Physics - Volume - III on page 1-1
Newton thought that light was made up of particles, but then it was discovered that it behaves like a wave. Later, however (in the beginning of the twentieth century), it was found that light did indeed sometimes behave like a particle. Historically, the electron, for example, was thought to behave like a particle, and then it was found that in many respects it behaved like a wave. So really it behaves like neither. Now we have given up. We say: "It is like neither."
There is one lucky break, however - electrons behave just like light. ...
...
We cannot make the mystery go away by "explaining" how it works. We will just tell you how it works. In telling you how it works we have told you about the basic peculiarities of all quantum mechanics.
Regarding your thought about electrons repelling each other. That would never be able to account for wave phenomena because that includes interference. Consider a plate with two holes in it which are closely spaced together. Block one hole and some of the electrons coming out of the other hold will hit the screen at position x. Now block that hole and open the other and the same will happen, i.e. some electrons will hit the screen at position x. If you were right then when both holes were open then there's be less electrons arriving at x than when either hole was open but in reality there are more. That's there the waves add.

You can download that volume of the Feynman lectures at The Feynman Lectures on Physics 3 Volume Set) Set v | Richard Phillips Feynman | digital library BookOS I highly recommend that you download it and read chapter one. It's exactly what you're looking for.

25. No that not what I mean.
I want to said that electrons can be disappear from this point and reappear somewhere else .Perhaps that the reason why electrons behave like a wave.

Anyway thank for the book

26. Originally Posted by johnzxcv
No that not what I mean.
I want to said that electrons can be disappear from this point and reappear somewhere else .Perhaps that the reason why electrons behave like a wave.

Anyway thank for the book
Hi John, it an appealing idea and one that I too have thought about over the years. However if this happened in real spacetime there would be evidence for it. So if it happens at all it is outside our dimensions.

27. Originally Posted by johnzxcv
No that not what I mean.
I want to said that electrons can be disappear from this point and reappear somewhere else .Perhaps that the reason why electrons behave like a wave.
For something like that to be meaningful you'd have to be able to be able to measure the position of a particle as a function of time in a continuous manner. But that's impossible. All you can do is measure the position of a particle at distinct instants of time. When you're not measuring where it is then it can't even be said to even have a position. Odd but true and that's been experimentally verified in the lab.

28. Actually I read that in Stephen Hawking book.
He said that particle such as proton really can appear at random, stick for a while and then vanish again.
That why they don't have a position.

29. Originally Posted by johnzxcv
Actually I read that in Stephen Hawking book.
He said that particle such as proton really can appear at random, stick for a while and then vanish again.
That why they don't have a position.
What was the name of the book and what page did you read that on?

30. Originally Posted by Physicist
What was the name of the book and what page did you read that on?
Actually That is a document movie
Stephen Hawking Grand Design Did god create the universe.
Stephen.Hawkings.Grand.Design.S01E03.Did.God.Creat e.the.Universe.HDTV.XviD-AFG.avi | Sockshare

31. Particles are a form of energy which can be seen or touched.(Theoretically at least)
There isn't a clear line of distinction between a fundamental particle and energy.(mass is negligibly small)
When these particles move they become energy(partially), and since energy can't be seen or touched we are unsure about it's position.
We only know the probability of finding the particle at some point in space.
As mass of a particle increases it's particle behavior also increases.
Mass less particles at all times exist as energy.

Particles jumps in and out of existence... that is true.
When we try to measure a particle it's wave function collapses and it becomes a true particle.
There is no way to know if it's the same particle that we are measuring,because all fundamental particles(eg electrons) are identical to each of other.
Even reality has it's limits.

32. johnzxcv: You said "an electron etc can behave either a particle or a wave but never both at the same time". That was wrong.
They has done a experiment that prove photon can behave like a wave and a partile at the same time.
Photons Observed as Particles, Waves Simultaneously | Research & Technology | Nov 2012 | photonics.com

Physicist:John - Scientific principles that have been around for a century like the one I just mention and you're now talking about, aren't discarded by the first claim made in a paper by someone. Such experiments have to be repeated and everything considered before its tossed out. So no. Just because you found one paper that claims otherwise doesn't mean that I'm wrong.

cinci: The article doesn't claim that particles can be observed as wave an particle at the same time. The article says the same particle can be observed as a wave and a particle at different times. As near as I can tell, the experiment takes photons that have been through a splitter and displayed wave behavior and measures them in a device that shows them to be particles.

33. Originally Posted by johnzxcv
Hi I am learning quantum mechanic, but I can't understand the behavior of particle:How can a partical behave like a wave? Can anyone explain to me?
And I need some document about quantum mechanic/
John - I don't think that I really answered your question here. The answer is that you can measure exactly the location of a electron moving through space. However when you measure it you've changed its momentum in an unpredictable way during the process so you can't predict where it will be later.

34. I think an explanation on how an interference pattern builds up in the double slit experiment is warranted here.

One photon will not show up as interference fringes. It will show as a dot on the photosensistive surface. A dot from a particle.
A large quantity of photons emitted quasi simultaneously will build up a multitude of dots on said surfcae. Dots from particles.
These dots accumulate in a pattern of interference fringes.
Photons "shot" one at a time say one every nanosecond, microsecond, millisecond, second, minute and so will build up up a multitude of dots in a pattern of interference fringes. It just takes a longer time. An individual photon interferes with itself. One at a time. An individual photon does not form bands of fringes by itself. it just hits where a part of a fringe would be.
Classical particles of macrosize would form two bands each centered on a slit. Two normal guassian curves. Not an interference fringe pattern.

An electron orbiting a nucleus, not radiating away energy and not collapsing into the nucleus. Classical physics states an accelerating charged particle radiates enegry (emitts photons). But electrons swirling around an atomic nuclei do not emit photons.

Richard Feynman would say "If you say you understand this then you don't" He would also say "These are all the same thing"

35. Originally Posted by pikpobedy
An individual photon interferes with itself.
Photons don't interfere with themselves. What you've neglected to note is that its the ensemble of measurements that the probability refers to, whether that ensemble is measured in the same set of or on different ones. For example. Consider 10,000 double slit experiments set up at the same time but separated by 1/4 mile from its nearest neighbor. If only one photon is emitted from each of the sources that comes with the double slit experiment then only one will ever be detected on the screen of the same experiment. It's only when you all get together an compare results by adding up where each of the photons from all the experiments landed do you see the interference patter. No two photons are interacting with each other. The cause of the wave patter is the potential barrier itself, i.e. the geometry of the double slit.

36. Physicist: Photons don't interfere with themselves.

cinci: We may never know for sure. But many quantum physics people say exactly this. maybe it's just loose talk, but they do say it.

One explanation I read for the two-slit phenomenon was that when the particle is in the slit, it's location is constrained so, by the uncertainty principle, its momentum becomes less certain. But this doesn't explain why you need two slits to get the wave appearance. But I'm more comfortable with that than self-interference, but quantum physicists will tell you that applying logic from every day models is dangerous in QM.

37. It helps me to think of it as a field, rather than a particle so it goes through both slits and the field nterfere with itself. The particle nature only becomes manifest on interactions/absorptions. If you try to detect it you effectively localise it and the interference pattern is destroyed.

38. Originally Posted by cincirob
Physicist: Photons don't interfere with themselves.

[B]cinci: We may never know for sure. But many quantum physics people say exactly this. maybe it's just loose talk, but they do say it.
Please show me a few places where you've them say that.

39. Originally Posted by Jilan
It helps me to think of it as a field, rather than a particle so it goes through both slits and the field nterfere with itself.
How do you think of it like that when only a single particle goes through and when you compare the results of an ensemble of different identical experiments also using one photon you get an interference pattern? After all, its this fact that makes it funky.

40. Please. Keep the colloquialisms to a minimum of zero.

But seriously, it is said and has been said that we can consider photons as if they interfere with themselves.
I do not understand why you have never seen this nor found it in texts or electronic docs.

41. Originally Posted by pikpobedy
Please. Keep the colloquialisms to a minimum of zero.

Originally Posted by pikpobedy
I do not understand why you have never seen this nor found it in texts or electronic docs.
Probably because I've been reading the textbooks written for physicists from undergraduate to graduate level and none of them say any such thing. It wouldn't have any meaning to say such a thing. However to appease your curiosity I sent the author of the quantum mechanics text used at MIT an e-mail and asked him this question. Never let it be said that I'm closed minded.

In the meantime I await your quote where it says what you claim. Please find it in a college textbook and not a book written for the layman. I won't respond to this subject again until you've done so.

42. Funky is a colloquialism , I was kidding around, I thought you would realize that.

As for photons interfering with themselves. Interfering with themselves being a shortcut term, conjecture, an appearance or a play on words; none the less the words are said and written.

Single Photon Inteference by Justin Winkler
I am but a professional engineer who rubs elbows with some physicist Phd. My fields are material science, probabilities and statistics, reliability and qualty in the primary and secondary sectors.

Any clarification to our discussion is appreciated.

43. Originally Posted by pikpobedy
May I asked whether you actually read this entire paper? I was actually looking for a textbook quote since many physicists make errors on quantum mechanics and I only trust serious QM textbook authors such as Feynman, Sakurai, Griffiths, Cohen-Tannoudji et al, etc.

I'm in a hurry to read this right now but I notice that he says
This single photons apparently interferes with itself.
Note the word "apparently". At the end he writes
It is worth noting that this does not constitute a single photon source. Even though the average spacing of the photons may be on the order of a meter, one would not be able to obverse antibunching, as instead photons can form multi-photon bunchs. Even though this is not the case of a single photon interfering with itself, the photon rate should still be suciently low to allow a basic demonstration of the particle nature of light.
What does this tell you?

The author
I asked for a textbook reference since many physicists make mistakes about quantum mechanics. Compare what your friend said compared to Wheeler and Feynman

Wheeler
A clear and simple explanation can instead be obtained in the framework of absorber theory, starting from the simple idea that each particle does not interact with itself.
I have another friend who wrote the text that MIT used to use to teach QM. I'll ask him too.

44. You know fully well the whole paper is not the point. I consider this closed.

45. Originally Posted by Physicist
I asked for a textbook reference since many physicists make mistakes about quantum mechanics. Compare what your friend said compared to Wheeler and Feynman

Wheeler
I don't think interaction and interference are the same thing. The wavefunction of a single photon can go through both slits and interfere with itself. An issue arises when you try to think of the photon as a particle going through one slit or the other. But it doesn't exist as a particle between emission and absorption, it only appears to be so when it is absorbed or emitted. In between times it is a spatially extended disturbance in the EM field. It can go through both slits at once and so can interfere with itself.

46. Originally Posted by Jilan
I don't think interaction and interference are the same thing. The wavefunction of a single photon can go through both slits and interfere with itself. An issue arises when you try to think of the photon as a particle going through one slit or the other. But it doesn't exist as a particle between emission and absorption, it only appears to be so when it is absorbed or emitted. In between times it is a spatially extended disturbance in the EM field. It can go through both slits at once and so can interfere with itself.
There is the concept of Feynman path integrals where you add the amplitudes from different paths. Perhaps this is what he meant by interfering with itself.

47. Originally Posted by pikpobedy
You know fully well the whole paper is not the point.
No. I don't know that. That's why I asked. Sometimes if you take a "sound bite" out of a paper like that they you get the wrong idea and the paper itself might be saying something else. Glad to see you're satisfied.

By the way. If anybody ever asks you again for a textbook referral tell them to look under most QM texts under the Feynman path integral treatment.

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