The Naked Scientists

The Naked Scientists Forum

Author Topic: Why does an atom not collapse?  (Read 9841 times)

Offline Murchie85

  • Sr. Member
  • ****
  • Posts: 124
    • View Profile
Why does an atom not collapse?
« on: 07/06/2010 15:33:37 »
Ok this is quite a simple one I have seen and read in the popular science that claims they will go on to explaining it but end up just quoting the exclusion principle or what not.

Basically in a stable atom of say hydrogen, the electron orbits the proton, they are opposite charges and there for should be attractive although the electron never falls into the proton.

Why is this? Also can particles actually collide in a real physical sense and not just electrical repultion, say for example a positron and electron.

Thanks in advance

Adam


 

Offline daveshorts

  • Moderator
  • Neilep Level Member
  • *****
  • Posts: 2583
  • Physics, Experiments
    • View Profile
    • http://www.chaosscience.org.uk
Why does an atom not collapse?
« Reply #1 on: 07/06/2010 16:54:00 »
An electron is a wave/particle. The probability of where the electron can be found is defined by its wave function. If the atom was 2D you could imagine a circular piece of string around the atom with wave travelling on it, if the wavelength is a whole fraction of the circumference a standing wave will build up. This has lots of amplitude, so you are quite likely to find an electron there it is an allowed state. Other places and the wave will cancel itself out -> no amplitude -> no electron.

The minimum distance this will happen at is when the circumference is 1 wavelength this would be the lowest state the electron can be in, it couldn't get any closer.

A 3D atom is a lot harder to think about, but the allowed states are there for a very similar reason.
 

Offline graham.d

  • Neilep Level Member
  • ******
  • Posts: 2208
    • View Profile
Why does an atom not collapse?
« Reply #2 on: 07/06/2010 17:20:17 »
If an electron collides with a proton with sufficient energy it can create a neutron and an anti-neutrino. It needs a fair bit of energy though. It is the reverse of a neutron decay into a proton, electron and neutrino.

Electron-positron collisions result in high energy em radiation (in the gamma ray region I think).
 

Offline Murchie85

  • Sr. Member
  • ****
  • Posts: 124
    • View Profile
Why does an atom not collapse?
« Reply #3 on: 07/06/2010 17:41:28 »
Dave I understand the principle of quantum physics describing the path of the electron in probabilities and the minimum distance. But I don't see why this is the case other than the mathematics used to explain it because we don't see anything to counter it. There is a minimum distance yes, and this can be calculated yes, and the wave function is zero near or on the proton in the middle... but why? Why is this the case? Why is there a minimum distance involved?
 

Offline lunar11

  • Jr. Member
  • **
  • Posts: 40
    • View Profile
Why does an atom not collapse?
« Reply #4 on: 08/06/2010 23:03:53 »
Let me have a try!

Initially, the issue with the Rutherford model of the atom was that the electron should spiral into the nucleus, as it accelerates, so would emit radiation (classical physics).
But, Bohr model stated that the electrons were confined in orbitals with a discrete energy; besides this was a quantum approach, so there was no need for concerning the radiation emission by the electron.
Shrodinger came up with the wave idea. Imagine you tie a 2 metre thin rope to a wall, and begin vibrating the rope up and down. Soon you will form a standing wave, the simplest one will have one loop (1st harmonic)the wave appears stationary. This is analogous to an electron in the 1st energy level. Now, if you increase the vibration then the wave vanishes but as you increase the vibration you will reach the 2nd harmonic having 2 loops, again the wave appears stationary, which is double the vibration as the 1st harmonic. This is analogous to the 2nd energy level. This also, neatly, clears up the idea that the electron cnnot just have any energy (or wavelength), but specific values to form the beautiful standing waves.
Hence, consider the electron as a wave on a string and not a particle.
Lunar.
 

Offline Murchie85

  • Sr. Member
  • ****
  • Posts: 124
    • View Profile
Why does an atom not collapse?
« Reply #5 on: 09/06/2010 13:31:13 »
Thanks a bunch lunar, I guess I am just having trouble in picturing the electron as a hoop or a ring that has a minimum length as I always note the attractive force pulling it into the centre.
 

Offline KSI_UA

  • First timers
  • *
  • Posts: 1
    • View Profile
Re: Why does an atom not collapse?
« Reply #6 on: 15/04/2013 09:34:55 »
Let me have a try!

Shrodinger came up with the wave idea. Imagine you tie a 2 metre thin rope to a wall, and begin vibrating the rope up and down. Soon you will form a standing wave, the simplest one will have one loop (1st harmonic)the wave appears stationary. This is analogous to an electron in the 1st energy level. Now, if you increase the vibration then the wave vanishes but as you increase the vibration you will reach the 2nd harmonic having 2 loops, again the wave appears stationary, which is double the vibration as the 1st harmonic. This is analogous to the 2nd energy level. This also, neatly, clears up the idea that the electron cnnot just have any energy (or wavelength), but specific values to form the beautiful standing waves.

Well - and, more physically, the wave-like steady flow rather then some speculative "waves of probability" or something alike. Outside the atom, the electrons will behave as confined wave trains instead (i.e. particles, as a matter of fact). And, when colliding with small regularly spaced objects they could be diffracted like common waves.
 

Offline flr

  • Sr. Member
  • ****
  • Posts: 302
  • Thanked: 2 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #7 on: 15/04/2013 16:08:27 »
A macroscopic negative charge will eventually collide with a macroscopic positive charge, while an e- will form stable states around proton.
 
What keeps the e- from collapsing is a quantum effect that does not have a classical equivalent. It is explicitly postulated in Bohr model and in my opinion it is implicitly postulated in Shrodinger theory. The results that came out of Shrodinger equation were in agreement with experimental spectroscopy.
It may be more difficult to see where discreteness  comes from in Shrodinger equation  but when making that steady wave assumption+associate higher orbits with higher harmonics of the standing wave that implicitly forced a discrete quantification because harmonics are integer multiples of a fundamental frequency and that will introduce a integer number which will translate in discrete energy levels when solving that Shrodinger equation.

Now does the quantification in Shrodinger equation comes from math? I believe the discreteness  is not a mathematical result, but rather it is implicitly postulated from the beginning in that steady wave assumption, i.e. things were poised to be be discrete when associated to waves higher orbites were harmonics of the wave described the groundstate. 

-----
I guess the answer to your question has to do with the discrete nature of things at quantum level, which is experimentally proven.

 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11999
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: Why does an atom not collapse?
« Reply #8 on: 15/04/2013 19:01:45 »
Eh, they have been photographed too :)
As 'particles' if I remember right.

Welcome to the duality.
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11999
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: Why does an atom not collapse?
« Reply #9 on: 15/04/2013 19:45:06 »
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: Why does an atom not collapse?
« Reply #10 on: 15/04/2013 22:42:32 »
Basically in a stable atom of say hydrogen, the electron orbits the proton, they are opposite charges and there for should be attractive although the electron never falls into the proton.

Why is this? Also can particles actually collide in a real physical sense and not just electrical repultion, say for example a positron and electron.

Thanks in advance

Adam
I'm sorry to say that all the above responses are incorrect. They are based on an incorrect model, the Bohr model. The Bohr model does predict a non-collapsing atom with very accurate energy levels but there are things which it predicts that are wrong such as a non-zero probability density at varying values of r wherein the probability density varies continuosly

You might be visualising the atom orbiting the nucleus of an atom like a satelite orbits a planetary body. That image is wrong. If you're visualising an electron in a standing wave around the nucleus then that is also wrong. The first is called the Rutherford model and the later the Bohr model. Both are wrong. The Rutherford model predicts that the electron will radiate energy and then spiral into the nucleus while the second predicts a non-zero angular momentum for the ground state. THe quantum mechanical model predicts that in the ground state the probability density is non-zero at r = 0. It also predicts a zero orbital angular momentum for an electron in the ground state.

In fact the nucleus can capture one of its innermost orbital electrons turning one of ther protons in the nucleus into a neutron. The process is called "K-capture."

The reason that an atom doesn't collapse is because physics at the atomic level is descriibed, not by classical Newtonian physics, but by quantum physics. If Newtonian physics is used to describe an atom then electrons move on classical trajectories and accelerates around the nucleus and thus radiates energy inf the form of electromagnetic energy, In quantum physics one does not describe motion by a particle moving on a trajectory but by using a quantum state |psi>. In one dimension, when one takes the "inner product" of the "position bra" <x| and the "state ket" |psi> you get the wave function psi(x), the magnitude of which gives a probability density. This view is very different than the Newtonian view and does not predict that atoms will collapase by radiatoing energy. The wave function is described by the quantum mechanical Schrodinger equation rather than a Newtonian equation such as F = ma of F = GMm/r^2.

So rather than electrons orbiting like satellites they are descxribed by what is referred to as an "electron cloud" where only probability density is determined. The exist in three dimensions unlike the Bohr and Rutherford model. The Bohr hydrogen atom is a 3D flat object whereas the quantum mechanical hydroden atom is 3D cloud of electrons. In the ground state there is a finite probability that an electron can be found at r = 0 whereas the Rutherford-Bohr Models predict that can't happen.
 

Offline flr

  • Sr. Member
  • ****
  • Posts: 302
  • Thanked: 2 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #11 on: 16/04/2013 02:34:44 »

 But there is still a question left:

 Is actually the nature of the discrete character at quantum level  understood?
 
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #12 on: 16/04/2013 03:50:59 »
Define "understood." We understand very well how things behave at that scale. We don't have an easily grasped model in terms of macroscopic intuition... both then why would we expect to?
 

Offline flr

  • Sr. Member
  • ****
  • Posts: 302
  • Thanked: 2 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #13 on: 16/04/2013 20:33:21 »
Quote
Define "understood."

It is proven (i.e. it comes as a result of a theory) and not postulated from empirical evidence.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: Why does an atom not collapse?
« Reply #14 on: 16/04/2013 20:42:20 »
Quote
Define "understood."

It is proven (i.e. it comes as a result of a theory) and not postulated from empirical evidence.
Science does not have the ability to prove theories right. We only have the ability to prove things wrong, make predictions and then test them. Proof is not a result of theory.Theory is a result of empirical evidence, not the other way around. Observation comes first, i.e. empirical evidence. Then from that one uses inductive logic to derive theories. Those theories are used to make predictions. Those predictions are then used to create experiments. If the experimental results are inconsistent with what was predicted then the theory has to be modified or disposed of. If the results are consistent with theory then we have a reason to believe the theory. The more predictions which are consistent the more we believe the theory. We have great number of reasons to believe quantum theory.
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11999
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: Why does an atom not collapse?
« Reply #15 on: 16/04/2013 21:33:03 »
Maybe Pete, but I would like to think that philosophy also drives science, and choice of experiments, with the science produced from those ideas and experiments in its turn driving philosophy. As science change so do philosophy. Depending on views there was different science produced historically, with those fitting the experiments at the time defining new philosophy. Just look at Stanford.edu, and the wealth of good scientific philosophy it contains.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #16 on: 16/04/2013 22:19:15 »
Basically in a stable atom of say hydrogen, the electron orbits the proton,
They have already answered you that the electron doesn't "orbit" the proton; I would only like to ask you if you believe that the electron in the H atom is a little corpuscle.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #17 on: 16/04/2013 22:26:34 »
Basically in a stable atom of say hydrogen, the electron orbits the proton,
They have already answered you that the electron doesn't "orbit" the proton; I would only like to ask you if you believe that the electron in the H atom is a little corpuscle.

Considering that this post is from 2010 and Murchie was last active in 2011, I suspect he might not answer you.  :p
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #18 on: 16/04/2013 22:33:21 »
Quote
Define "understood."

It is proven (i.e. it comes as a result of a theory) and not postulated from empirical evidence.
Science does not have the ability to prove theories right. We only have the ability to prove things wrong, make predictions and then test them. Proof is not a result of theory.Theory is a result of empirical evidence, not the other way around. Observation comes first, i.e. empirical evidence. Then from that one uses inductive logic to derive theories. Those theories are used to make predictions. Those predictions are then used to create experiments. If the experimental results are inconsistent with what was predicted then the theory has to be modified or disposed of. If the results are consistent with theory then we have a reason to believe the theory. The more predictions which are consistent the more we believe the theory. We have great number of reasons to believe quantum theory.

Pmb put it well, and to reiterate: quantum theory is about the most accurate theory we have in terms of what it predicts and how well our measurements match with predictions.  So its "understood" in the sense that we have a theory that has stood up incredibly well to experimental tests. 
 

Offline Bill S

  • Neilep Level Member
  • ******
  • Posts: 1828
  • Thanked: 12 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #19 on: 17/04/2013 19:38:27 »
Pete, whilst I am happy to accept that the earlier explanations are wrong, they are the sort of explanation that one finds in P S books.  I feel, therefore, it is quite important that counter explanations should be such that the average reader of P S books (hitch-hikers on the journey of scientific understanding) should be able to understand them.

For example; what is a "position bra" (under-wired ?  :) ), or a "state ket", and what is the significance of < | and | >. 
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #20 on: 17/04/2013 20:00:56 »
Basically in a stable atom of say hydrogen, the electron orbits the proton,
They have already answered you that the electron doesn't "orbit" the proton; I would only like to ask you if you believe that the electron in the H atom is a little corpuscle.

Considering that this post is from 2010 and Murchie was last active in 2011, I suspect he might not answer you.  :p
Gasp! I haven't noticed.
Thank you.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #21 on: 17/04/2013 20:12:27 »
Bill, the "bra" and "ket" are fancy words for shorthand notation used to express the rather complex mathematics of quantum mechanical models.  The short answer is that you can interpret them in this case as mathematical descriptions of the particle's waviness.  That waviness describes where you're likely to find the particle if you look for it as well as describing how it interacts with other objects.  The difference with the Bohr model is that Bohr thought the wave was basically a sine wave wiggling around a circle about the nucleus, so that you could be certain the particle was somewhere on that circle. (http://en.wikipedia.org/wiki/Bohr_model) That worked out OK for some things, but wasn't very accurate.  Others came along and worked out in detail the physical model that governed the wave's behavior and its much more complicated than a simple sine wave: http://en.wikipedia.org/wiki/Atomic_orbital 
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #22 on: 17/04/2013 20:16:39 »
For example; what is a "position bra" (under-wired ?  :) ), or a "state ket", and what is the significance of < | and | >. 
It depends on the level of the answer  :)
A ket is a state vector, a bra is a linear functional which associate a complex number to a ket.
But I suspect you would prefer something simpler.

In qm you use "wavefunctions" which are complex valued functions of x, y, z, t and which describe the qm system (for ex. a particle): the wavefunction codify the informations about the state of the system. But you need to make the scalar product of two wavefunctions f(x,y,z,t) and g(x,y,z,t); this is represented with the symbol: <f,g> and it is usually computed as:

<f,g> = Integral[-oo,+oo] f*(x,y,z,t) g(x,y,z,t) dxdydz

where f* indicates the complex coniugated of f.

It turns out that the integral is much better viewed as a function which associate a complex number (the result of the integral) to the function g, given the function f, so it is a "functional", that is something which take a function and returns a scalar (instead a normal function takes a scalar and returns a scalar).

Dirac invented this notation: the scalar product can be viewed as the functional <f| applied to the function |g> and becomes, simply: <f|g>. Since it was formerly represented with a "bracket": (f,g), he said that the firs part is the "bra" and the second the "ket"  :) and so the name.
« Last Edit: 17/04/2013 20:20:13 by lightarrow »
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3822
  • Thanked: 19 times
    • View Profile
Re: Why does an atom not collapse?
« Reply #23 on: 17/04/2013 22:55:01 »
It is strange how old topics come back to life and that this should appear when I opened a relevant discussion on the Schrodinger equation
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: Why does an atom not collapse?
« Reply #24 on: 20/04/2013 03:37:08 »
Maybe Pete, but I would like to think that philosophy also drives science, and choice of experiments, with the science produced from those ideas and experiments in its turn driving philosophy. As science change so do philosophy. Depending on views there was different science produced historically, with those fitting the experiments at the time defining new philosophy. Just look at Stanford.edu, and the wealth of good scientific philosophy it contains.
I'll say this. From one of my favorite texts - Ignoring philosophy in physics means not understanding physics. - Fritz Rohrlich
 

The Naked Scientists Forum

Re: Why does an atom not collapse?
« Reply #24 on: 20/04/2013 03:37:08 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums