# Can an electron's spin change? **URL:** \ **Category:** Factual Questions\ **Created:** [October 9, 2016, 5:45pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078 "2016-10-09T17:45:49Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![keeganst94](https://avatars.discourse-cdn.com/v4/letter/k/a6a055/32.png) [@keeganst94](https://boards.straightdope.com/u/keeganst94)\ **Post date:** [October 9, 2016, 5:45pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/1 "2016-10-09T17:45:49Z")
So electron’s have spins of either plus or minus one-half, but can they change spin from plus one-half to minus one-half? If you have a hydrogen atom with one electron with a plus one-half spin, and you transfer another electron with plus one-half spin, they cannot both occupy the 1s orbital. So will one of them change to a minus one-half spin, or will it just occupy the 2s orbital? If it does occupy the 2s orbital, is that fraction of the hydrides much more reactive due to it’s radical nature? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 9, 2016, 6:15pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/2 "2016-10-09T18:15:22Z")
Yes, they absolutely can and do change spin, provided that the angular momentum of something else in the system changes accordingly, to keep the total constant. That’s how we know that spin is a form of angular momentum. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 9, 2016, 9:34pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/3 "2016-10-09T21:34:47Z")
Spin flipping of electrons in hydrogen atoms produces what’s known as the [21-centimeter](https://en.wikipedia.org/wiki/Hydrogen_line) line in radio astronomy. It’s an extremely useful emission line since it allows detection of neutral hydrogen, a very plentiful substance in the universe. ---
**Author:** ![joema](https://avatars.discourse-cdn.com/v4/letter/j/a9adbd/32.png) [@joema](https://boards.straightdope.com/u/joema)\ **Post date:** [October 11, 2016, 12:52pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/4 "2016-10-11T12:52:18Z")
> [@keeganst94](#): > > So electron’s have spins of either plus or minus one-half, but can they change spin from plus one-half to minus one-half?.. Technically the spin of an electron never changes, but the orientation of that spin can change: [What exactly is the 'spin' of subatomic particles such as electrons and protons? Does it have any physical significance, analogous to the spin of a planet? | Scientific American](https://www.scientificamerican.com/article/what-exactly-is-the-spin/) A longer explanation by science writer Chad Orzel: [Electron Spin for Toddlers | ScienceBlogs](http://scienceblogs.com/principles/2010/07/26/electron-spin-for-toddlers/) Since we currently believe electrons are point particles without physical size, “spin” is more of a quantum mechanical state than physical rotation about an axis. This video summarizes the history of how spin was discovered and how viewing it as physical rotation is misleading: [![](https://img.youtube.com/vi/cd2Ua9dKEl8/maxresdefault.jpg "What is Spin? | Quantum Mechanics") ](https://www.youtube.com/watch?v=cd2Ua9dKEl8) ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [October 11, 2016, 3:04pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/5 "2016-10-11T15:04:12Z")
> [@joema](#): > > Technically the spin of an electron never changes, but the orientation of that spin can change: This is incorrect. The spin changes, full stop. Spin is a vector quantity, and a change in the magnitude or direction constitutes a change in spin. Quantum mechanically these two pieces are often dealt with separately (_e.g._, through spin quantum numbers like _S_[sup]2[/sup] and _S_[sub]_z_[/sub]), but that technical detail isn’t correctly handled by saying “spin never changes”. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [October 11, 2016, 3:39pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/6 "2016-10-11T15:39:37Z")
To be specific, in QM angular momentum is a vector quantity, but there is an uncertainly relationship (like the position-momentum one) between the three components of the vector. I.e. if the classical angular momentum vector is (l[sub]x[/sub], l[sub]y[/sub], l[sub]z[/sub]), then in QM, pinning down one of these components exactly, makes the other two components maximally uncertain. Just like how in the classical Hiesenberg position/momentum relationship, pinning down position makes momentum uncertain, and vice versa. But unlike the position/momentum relationship, there is one more thing you can fix about the angular momentum vector – its length: sqrt(l[sub]x[/sub][sup]2[/sup] + l[sub]y[/sub][sup]2[/sup] + l[sub]z[/sub][sup]2[/sup]). So out of this three dimensional vector, there ends up being only these two numbers you can figure out. For the spin angular momentum of an electron, the “length of the vector” number is fixed. It is just a property of being an electron that your spin vector has that certain length – if that changed, you would no longer have an electron. Which is what **joema** means by “spin can’t change” The “single component you can fix” of the vector can be changed, but it is constrained by the fact that no single component of the vector can be bigger than the vector’s length. For electrons that fact (plus the fact that it is quantized) means that that component can be one of two states. So that component _can_ change, but only in a very restricted way. ---
**Author:** ![Some\_Call\_Me.Tim](https://avatars.discourse-cdn.com/v4/letter/s/439d5e/32.png) [@Some\_Call\_Me.Tim](https://boards.straightdope.com/u/Some_Call_Me.Tim)\ **Post date:** [October 11, 2016, 4:48pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/7 "2016-10-11T16:48:29Z")
The famous example of this, which was the primary example cited when Heisenberg got the Nobel for QM, is the [ortho- and para-](https://en.wikipedia.org/wiki/Spin_isomers_of_hydrogen)spin states of molecular hydrogen. The two can and do convert from one to the other, and this has implications for the cooling of hydrogen. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [October 11, 2016, 6:34pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/8 "2016-10-11T18:34:31Z")
Shades of Principles of Physical Phenomenon. Once, in answering a question, the Professor said, “I am trying to not simply say the same thing, slowly and more loudly.” ---
**Author:** ![keeganst94](https://avatars.discourse-cdn.com/v4/letter/k/a6a055/32.png) [@keeganst94](https://boards.straightdope.com/u/keeganst94)\ **Post date:** [October 13, 2016, 2:26pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/9 "2016-10-13T14:26:46Z")
Just to see if I have this correct; electron spin is a vector, and what changes is the direction of the vector, not the length. When electron spins change, their angular momentum changes, and something must compensate to keep the total angular momentum of the atom constant. And there is a small energy difference between the two spin states, so a transition between them causes the release of a low-energy photon. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 13, 2016, 7:41pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/10 "2016-10-13T19:41:03Z")
Yes, except for one detail: > [@](#): > > …and something must compensate to keep the total angular momentum of the atom constant. The compensating change can also be something outside of the atom, such as the photon it emits, or another atom that bumps into it. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [October 14, 2016, 2:48pm UTC](https://boards.straightdope.com/t/can-an-electrons-spin-change/768078/11 "2016-10-14T14:48:08Z")
> [@keeganst94](#): > > And there is a small energy difference between the two spin states To expand on this, if the electron is alone in the universe, there is no energy difference at all between its various spin states. It is the electron’s magnetic interaction with the proton (which has a dipole moment that the electron’s spin can be either parallel to antiparallel to) that makes the different states energetically different.