# Do electrons really perform instantaneous quantum leaps?

There are so many questions about this and so many answers, but none of them says clearly if the electron's change of orbitals as per QM can be expressed at a time component or is measurable (takes time or not), or is instantaneous, or if it is limited by the speed of light or not, so or even say there is no jump at all.

Quantum jump of an electron

How do electrons jump orbitals?

where Kyle Oman says:

So the answer to how an electron "jumps" between orbitals is actually the same as how it moves around within a single orbital; it just "does". The difference is that to change orbitals, some property of the electron (one of the ones described by (n,l,m,s)) has to change. This is always accompanied by emission or absorption of a photon (even a spin flip involves a (very low energy) photon).

and where DarenW says:

A long time before the absorption, which for an atom is a few femtoseconds or so, this mix is 100% of the 2s state, and a few femtoseconds or so after the absorption, it's 100% the 3p state. Between, during the absorption process, it's a mix of many orbitals with wildly changing coefficients.

Does an electron move from one excitation state to another, or jump?

where annav says:

A probability density distribution can be a function of time, depending on the boundary conditions of the problem. There is no "instantaneous" physically, as everything is bounded by the velocity of light. It is the specific example that is missing in your question. If there is time involved in the measurement the probability density may have a time dependence.

and where akhmeteli says:

I would say an electron moves from one state to another over some time period, which is not less than the so called natural line width.

the type of movement in electron jump between levels?

where John Forkosh says:

Note that the the electron is never measured in some intermediate-energy state. It's always measured either low-energy or high-energy, nothing in-between. But the probability of measuring low-or-high slowly and continuously varies from one to the other. So you can't say there's some particular time at which a "jump" occurs. There is no "jump".

How fast does an electron jump between orbitals?

where annav says:

If you look at the spectral lines emitted by transiting electrons from one energy level to another, you will see that the lines have a width . This width in principle should be intrinsic and calculable if all the possible potentials that would influence it can be included in the solution of the quantum mechanical state. Experimentally the energy width can be transformed to a time interval using the Heisneberg Uncertainty of ΔEΔt>h/2π So an order of magnitude for the time taken for the transition can be estimated.

H atom's excited state lasts on average $10^{-8}$ secs, is there a time gap (of max 2*$10^{-8}$ secs) betwn. two consec. photon absorpt.-emiss. pairs?

So it is very confusing because some of them are saying it is instantaneous, and there is no jump at all. Some are saying it is calculable. Some say it has to do with probabilities, and the electron is in a mixed state (superposition), but when measured it is in a single stable state. Some say it has to do with the speed of light since no information can travel faster, so electrons cannot change orbitals faster then c.

Now I would like to clarify this.

Question:

1. Do electrons change orbitals as per QM instantaneously?

2. Is this change limited by the speed of light or not?

• You seem to ask a lot of these questions, and the answer is almost always some version of "it depends on how you're defining the word in question." – probably_someone Jun 28 '19 at 8:43
• We should not debate what this or that person posted here. Did you investigate any scuentific literature on this? – my2cts Jun 28 '19 at 12:49
• @my2cts en.wikipedia.org/wiki/Atomic_electron_transition "Atomic electron transition is a change of an electron from one energy level to another within an atom[1] or artificial atom.[2] It appears discontinuous as the electron "jumps" from one energy level to another, typically in a few nanoseconds or less. It is also known as an electronic (de-)excitation or atomic transition or quantum jump. The damping time constant (which ranges from nanoseconds to a few seconds) relates to the natural, pressure, and field broadening of spectral lines. – Árpád Szendrei Jun 28 '19 at 12:54
• @my2cts "However, in 2019 it was demonstrated that the evolution of each completed jump is continuous, coherent and deterministic." nature.com/articles/s41586-019-1287-z – Árpád Szendrei Jun 28 '19 at 12:54
• From what I can tell, none of the quoted answers are actually saying that the jump is discontinuous. If you want to reconcile them all in your head, the only ingredient you really need is to remember that making an energy measurement takes time. If you accept that, I don't think any of your quotes contradict each other, and I don't think any of them make a claim about instantaneity. – Jahan Claes Jun 28 '19 at 19:51

Do electrons change orbitals as per QM instantaneously?

In every reasonable interpretation of this question, the answer is no. But there are historical and sociological reasons why a lot of people say the answer is yes.

Consider an electron in a hydrogen atom which falls from the $$2p$$ state to the $$1s$$ state. The quantum state of the electron over time will be (assuming one can just trace out the environment without issue) $$|\psi(t) \rangle = c_1(t) |2p \rangle + c_2(t) | 1s \rangle.$$ Over time, $$c_1(t)$$ smoothly decreases from one to zero, while $$c_2(t)$$ smoothly increases from zero to one. So everything happens continuously, and there are no jumps. (Meanwhile, the expected number of photons in the electromagnetic field also smoothly increases from zero to one, via continuous superpositions of zero-photon and one-photon states.)

The reason some people might call this an instantaneous jump goes back to the very origins of quantum mechanics. In these archaic times, ancient physicists thought of the $$|2 p \rangle$$ and $$|1 s \rangle$$ states as classical orbits of different radii, rather than the atomic orbitals we know of today. If you take this naive view, then the electron really has to teleport from one radius to the other.

It should be emphasized that, even though people won't stop passing on this misinformation, this view is completely wrong. It has been known to be wrong since the advent of the Schrodinger equation almost $$100$$ years ago. The wavefunction $$\psi(\mathbf{r}, t)$$ evolves perfectly continuously in time during this process, and there is no point when one can say a jump has "instantly" occurred.

One reason one might think that jumps occur even while systems aren't being measured, if you have an experimental apparatus that can only answer the question "is the state $$|2p \rangle$$ or $$|1s \rangle$$", then you can obviously only get one or the other. But this doesn't mean that the system must teleport from one to the other, any more than only saying yes or no to a kid constantly asking "are we there yet?" means your car teleports.

Another, less defensible reason, is that people are just passing it on because it's a well-known example of "quantum spookiness" and a totem of how unintuitive quantum mechanics is. Which it would be, if it were actually true. I think needlessly mysterious explanations like this hurt the public understanding of quantum mechanics more than they help.

Is this change limited by the speed of light or not?

In the context of nonrelativistic quantum mechanics, nothing is limited by the speed of light because the theory doesn't know about relativity. It's easy to take the Schrodinger equation and set up a solution with a particle moving faster than light. However, the results will not be trustworthy.

Within nonrelativistic quantum mechanics, there's nothing that prevents $$c_1(t)$$ from going from one to zero arbitrarily fast. In practice, this will be hard to realize because of the energy-time uncertainty principle: if you would like to force the system to settle into the $$|1 s \rangle$$ state within time $$\Delta t$$, the overall energy has an uncertainty $$\hbar/\Delta t$$, which becomes large. I don't think speed-of-light limitations are relevant for common atomic emission processes.

• Upvote for "are we there yet?" (and for the rest, too). – Peter - Reinstate Monica Jun 28 '19 at 8:46
• @Maxter The expected energy of the atom is continuously changing, that's true. Meanwhile, the state of the electromagnetic field goes from $|0 \text{ photons} \rangle$ to $|1 \text{ photon} \rangle$ via continuous superpositions. So the expected energy of the electromagnetic field also goes up continuously, though the states we are superposing each have a whole number of photons of discrete energy. – knzhou Jun 28 '19 at 15:30
• Okay, but: 1. changing amplitude between two orthogonal physical states is really not at all like physically moving along a continuum of states which have a natural norm, as in your road trip analogy. Just at the level of pedagogy, I think that it leads to more confusion when you imply that $|x_1\rangle + |x_2\rangle$ is the same as $|(x_1+x_2)/2\rangle$ (Maxter's comments are an example). – Rococo Jun 28 '19 at 15:34
• @Rococo 1. I thought I was implying the exact opposite: the only reason one would have a problem is if one didn't apply the idea of superposition + unitary evolution consistently elsewhere, i.e. to both the atom and the field. 2. Sorry, I was unclear here, and just edited a bit to address this. I'm not trying to say anything about how measurement works, I'm saying that the way we measure can naively lead to the (incorrect) conclusion that unitary evolution in between measurements also contains jumps. – knzhou Jun 28 '19 at 15:44
• @Rococo I think you're aiming at a level of precision above the context of the OP's question: to 99.99% of people who will ever hear the phrase, "quantum leaps" refers to the wrong pictures I'm arguing against, while to the 0.01% that do study open quantum systems, it will be clear that what I'm saying isn't in contradiction to what they're learning! So that's why I chose to simplify this way. But it would also be valuable if you were to write an answer from the more advanced perspective. – knzhou Jul 1 '19 at 23:28
1. No. Instantaneous state transfer violates causality, which is a premise of all rational deterministic theories in natural philosophy. Like two magnets clicking together once they are in close proximity, the state transfer can occur very quickly relative to our perception and so can be considered "approximately" instantaneous, but this approximation only applies to systems that do not take time periods of this finer granularity into account. The term "instant" is often hyperbole, as it depends on your measurement interval--all that it conveys is that the event occurs within a lapse of time too small to be measured using the present apparatus.
2. I don't see why the speed of the transfer would be limited by the perceived speed of light.
• Don't superluminal speeds lead to causality violation? – Peter Shor Jun 29 '19 at 20:03
• Clearly not. Causality is more general and fundamental than any supposed limitation on the speed of matter. Deterministic models can exist that do not include the assumption of such a limit. – pygosceles Jul 2 '19 at 17:44
• Why does instantaneous state transfer violate causality when superluminal state transfer doesn't. Especially since, for any superluminal state transfer, there's a reference frame in which it is instantaneous. – Peter Shor Jul 2 '19 at 18:01
• My argument does not depend on instantaneous state transfer, it refutes it. Unless I am mistaken, the premise you are using is based on a flavor of relativity that defies definition of simultaneity and hence cannot opine on the subject of instantaneousness. The behavior of superluminal matter would be undefined in that framework since manifestly a different set of laws would have to be applied to it. – pygosceles Jul 2 '19 at 18:49

(An edit for all you editors out there: I know the 'How to edit' says 'fix grammatical and spelling errors'; but before you start correcting '-sing' to '-zing', please check with a dictionary whether this is in fact British English spelling. We ain't all from the States;-)

This is a good question, certainly in the sense that it asks something we are not yet equipped to answer with much certainty; thus it provokes us to think harder. I can't give you a better answer than what knzhou has already done.

However, I think it bears repeating, that QM is very hard to understand, not least because it tries to explain observations that are made by, in a sense, throwing a lot of particles rather forcefully at something very small, from very far away, and then seeing what happens. We have no way of observing an electron moving around a nucleus, even if it does; our methods of observation force us to think in purely statistical terms about what really amounts to 'steady states': electrons smeared out over an orbital.

QM does a marvellous job, without a doubt, but I think it is reasonable to ask whether this is because it offers genuine insight into what individual particles do or are; or whether this is instead due to the use of clever statistics. In analogy, think of how we can't predict what any individual person will do over the course of a day, but we can make very good predictions of what a population is likely to do.

Edit on 2 July 2019

The whole subject of quantum mechanics continues to be an area of contention, which to me is a sign of good health; science is at its heart about scrutinising and challenging theory. That, however, also means that we can never pronounce with absolute certainty, that we know the truth - it lies in the logical nature of the empirical method: experiments, however sophisticated, can, even in the ideal situation of perfect measurement, only ever disprove a theoretical prediction with absolute certainty. "Past Performance Is No Guarantee of Future Results" is even more true in science than in the world of investment.

So, about downvoting; I don't mind it, but please leave a comment to explain why, don't be an anonymous coward. I think those of us who take the time and make the effort to answer the sometimes very difficult questions that people ask, deserve better, for one thing. And of course, if you have an insight, why not share it?

@TCooper: I fully agree with your sentiment - people who are interested in science, are attracted exactly to the unanswered questions; they are curious and feel excited about the things that are yet to be discovered.

"thermomagnetic condensed boson": kzhou's answer is very much the orthodox, correct QM answer, but there is a lot of uncertainty about why it makes logical sense to talk about wavefunctions in the first place. It is in fact important to understand this part, not least because on the one hand, we know that QM and General Relativity aren't compatible, and on the other hand, wavefunctions as simply complex valued functions do not make good sense when space is not flat; they will at least have to be sections of the complex bundle over the space-time manifold.

• I'd like to know why this answer was down voted? It seems like it's the only answer acknowledging our (human populations current scientists on the frontier of particle physics, not me) knowledge of this really isn't absolute or definitive. Would someone who down voted explain why? I'm a layman without a doubt, just love reading about these things from time to time - but this answer did make a lot of sense to me..? – TCooper Jun 29 '19 at 0:33
• @TCooper Maybe because it isn't logical. It says we are not yet able to answer with much certainty and at the same time that he cannot give a better answer than kzhou's. But kzhou's gave a very concise answer, a big "NO". That goes against the first sentence of this answer. Furthermore this answer isn't actually an answer to the asked question, but just a pack of philosophical words with a faulty logic. – thermomagnetic condensed boson Jun 29 '19 at 12:09
• Is there hard evidence, you're certain will never be disproved, that shows knzhou's answer is 100% correct? This post acknowledges that knzhou's is the current the best answer, but highlights the limitations in the methods used to arrive at that conclusion. It leaves open that our current best understanding will more likely than not seem ancient like the ideas knzhou mocks in his answer at some point in the future. What if in the future the time it takes an electron in a hydrogen atom to fall from the 2p state to the 1s state will be the contant used to define one instant? – TCooper Jul 1 '19 at 22:27
• I apologise for not responding for a while - I have been away. Please see my edit - my comment is going to be too long for the space allowed here. – j4nd3r53n Jul 2 '19 at 7:41
• -1: "Throwing a lot of particles at something from very far away, and seeing what happens." This is not quantum mechanics, this is high energy physics (which does indeed use quantum mechanics, but isn't all of quantum mechanics). One can do very-well-controlled AMO—atomic, molecular, and optical physics—to study quantum mechanics without throwing very small pieces of matter at each other at enormous speeds. – Peter Shor Jul 20 '19 at 14:46