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I'm trying to describe a few physics concepts for non-physicists, and one of the things that came up was the fine-structure constant in the context of multiverse theory. I want to make sure my understanding is correct:

The fine-structure constant of the universe is part of the definition of electromagnetic attraction. In a universe where the constant is a fraction higher, electrons would orbit closer to the atomic nucleus, which means atoms would never clump together; a fraction lower would make electrons orbit farther out and everything would glue together.

Is that an accurate statement?


To provide more context: The Wikipedia entry says this constant affects size of atoms, but doesn't state the macro scale impact of that. I have been trying to research the impact we would experience at our scale. The quote above is my best understanding of that impact, but I can't find any explicit statement of it. So I'm asking: is this correct?

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  • $\begingroup$ From en.wikipedia.org/wiki/Fine-structure_constant, The anthropic principle is a controversial argument of why the fine-structure constant has the value it does: stable matter, and therefore life and intelligent beings, could not exist if its value were much different. For instance, were α to change by 4%, stellar fusion would not produce carbon, so that carbon-based life would be impossible. If α were > 0.1, stellar fusion would be impossible and no place in the universe would be warm enough for life as we know it. Note that α = 0.0072, so 0.1 is a big change. $\endgroup$ – mmesser314 Dec 11 '16 at 16:03
  • $\begingroup$ @SRM Can you link to the specific Wikipedia page/section you read? $\endgroup$ – Emilio Pisanty Dec 15 '16 at 16:14
  • $\begingroup$ The one mentioned by mmesser314 earlier in comments: en.wikipedia.org/wiki/Fine-structure_constant $\endgroup$ – SRM Dec 15 '16 at 16:19
  • $\begingroup$ I should have been more specific. Which section, in particular, gives you that impression? The page is long and there isn't really anything there about $\alpha$ 'controlling' the size of atoms. $\endgroup$ – Emilio Pisanty Dec 15 '16 at 16:24
  • $\begingroup$ Have a look at this link en.wikipedia.org/wiki/… in conjunction with my answer here physics.stackexchange.com/q/291316 . If the relationship with the three basic constants is kept, then there is no change in the physics models fitting the data $\endgroup$ – anna v Dec 15 '16 at 16:34
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In the form you have phrased it, the statement is false or, at best, incomplete. In particular, the specific claim that

In a universe where the constant is a fraction higher, electrons would orbit closer to the atomic nucleus.

is essentially meaningless.

The problem is the assertion that, in a universe where X happened, the orbital radius would change, and it is problematic because the orbital radius is a dimensional quantity. What does the assertion really mean? it posits two universes,

  • universe $A$, with fine-structure constant $\alpha_A$, and
  • universe $B$, with fine-structure constant $\alpha_B\neq\alpha_A$,

but... how do you compare lengths between the two universes? You can't take a ruler from universe $A$ and use it to measure stuff on universe $B$, that's for sure.

You can try to 'port' lengths from one universe into another by specifying, say, $N$ times the wavelength of $X$ optical transition, or the speed of light times $M$ periods of the radiation from the $Y$ microwave resonance. However, what happens if you do this and the different lengths don't agree? Keep in mind that you're proposing changes in $\alpha$ so radical that the fundamentals of chemistry in your new universe would change considerably, so there might be no such thing as caesium or krypton in your new universe, or they would be altered beyond the point where you could reliably use them as standard rulers.


Instead, the way you make meaningful statements of this sort is to specify what it is you're keeping constant, or in other words to make predictions about the ratio of two different quantities (of the same physical dimension), which can be taken in-universe; you can then compare the answers (which are just numbers) from the two different universes.


Let me first stress that this is not a pointless insistence, and that there is a lot of active science research doing precisely this: the precision measurement of $\alpha$-dependent physical quantities, over several years, to see if they change. One good example is the paper

Frequency Ratio of Two Optical Clock Transitions in 171Yb+ and Constraints on the Time Variation of Fundamental Constants. R. M. Godun et al. Phys. Rev. Lett. 113, 210801 (2014), arXiv:1407.0164.

where it is crystal clear that the only observable which can be reliably used as a gauge for changes in $\alpha$ is the ratio between two commensurate quantities. In this specific case the two quantities being compared are the frequencies of optical radiation from a single trapped ion (because they can be measured to exquisite precision, and because there is strong theoretical evidence that those frequencies have a different dependence on $\alpha$), but the principle can be applied to the ratio of any two lengths, times, velocities - you name it.

I bring up this example to emphasize that the overall theme - asking what happens if $\alpha$ changes - is a perfectly reasonable question. However, you do need to use a more refined gauge on the effects of those changes.


OK, now let's bring it back to how you interpreted the control of $\alpha$ on the orbital radius, in the form

electrons would orbit closer to the atomic nucleus, which means atoms would never clump together.

The problem here is that the clumping together of atoms (i.e. the chemical bonds between them) is governed by the same physics that dictates the characteristic sizes of atoms. This is the quantum mechanical version of electrostatics, which is essentially ruled by the electron mass $m_e$, the Coulomb constant $e^2/4\pi\varepsilon_0$, and Planck's constant $\hbar$. None of these involve $\alpha$ directly (or, more precisely, it is possible to change $\alpha$ without changing any of these quantities), which means that the essentials of atomic physics and the fundamentals of chemistry remain unchanged.

This can be developed a bit further: in essence, saying that you care about the 'clumping' properties of atoms ultimately means that your lens is the dynamics that determines it, or in other words that you're willing to consider changes in $\alpha$ between universes $A$ and $B$, but these should be done by keeping $\hbar$, $m_e$ and $e^2/4\pi\varepsilon_0$ constant. This then tells you exactly what a change in $\alpha$ means, because we know that $$ \alpha = \frac{e^2/4\pi\varepsilon_0}{\hbar c}, $$ and the only constant we can change now is the speed of light $c$. This is the real effect of $\alpha$ as far as atoms and molecules go, and indeed in atomic units $1/\alpha\approx 137$ is exactly the value of the speed of light.

As far as atoms and molecules go, then, changing $\alpha$ means that you keep the essentials of the situation unchanged, but you change the strength of relativistic effects, like spin-orbit coupling and other fine structure terms. (That name, of course, is not a coincidence.) These are small changes in the specific details of how atoms behave, which is precisely what the paper I talked about earlier is measuring.


However, this is only one perspective, and changing $\alpha$ will change other realms of physics differently. In deeply relativistic quantum regimes, for example, it makes much more sense to make $\hbar$ and $c$ the benchholders of your physics, and you switch from atomic units to natural units; here $\alpha$ is more meaningful as a measure of the Coulomb constant $e^2/4\pi\varepsilon_0$, which determines the strength of the electromagnetic interaction between two elementary charges.

This is a different perspective, one among many others; they are all equally valid, and for each a change in $\alpha$ will affect physics in different ways. For all of them you need to choose some dimensionless ratio as a hallmark that physics has changed - but the reference quantity in that ratio will be different, depending on what physics you're exploring, so you always need to say what it is you're keeping constant.

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    $\begingroup$ A very good answer and a nice little lecture in how to properly think in truly physical terms, which greatly helps in "alternate physics" discussions. $\endgroup$ – Neuneck Dec 15 '16 at 16:49
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Your statement is not true.

Any bound state is defined by a balance of forces. In the case of the covalent binding between two atoms, the repulsive part comes from the positive charges of the nuclei repelling each other. This is balanced by the attraction of atom 1's electrons by atom 2's nucleus and vice versa. Since both forces are of electro-magnetic nature, the binding between atoms should not be affected by a change in the fine-structure constant.

The binding of the nucleus itself is a completely different story. Here, we have the repulsive force between the various protons in the nucleus. This is balanced by the strong force attraction among the protons and between the protons and neutrons. Here, a change of the fine-structure constant only alters one side of the equation.

At higher values of the fine-structure constant, the protons repel each other more strongly and at one point, the strong force attraction is not able to bind any more protons. This is why at higher values of the fine-structure constant heavy nuclei would not be stable. This basically affects, at what point a nucleus becomes radioactive by emitting α -particles. So at lower values of the fine-structure constant, super-heavy elements should be relatively stable.

The binding between two hydrogen atoms would be largely unaffected, though (except maybe for a slower reaction rate or higher energy of activation).

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  • $\begingroup$ Good answer for impact at higher values. What about at lower values? Would protons stick together so as to create an over abundance of heavy elements? $\endgroup$ – SRM Dec 15 '16 at 16:22
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    $\begingroup$ @SRM I think so, yes. This basically affects, at what point a nucleus becomes radioactive by emitting $\alpha$-particles. So at lower values of the fine-structure constant, super-heavy elements should be relatively stable. $\endgroup$ – Neuneck Dec 15 '16 at 16:26
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The first answer is wrong. Of course atoms are smaller if alpha is larger!

The unit of length in nature is the Planck length; with a larger value of alpha, the diameter of atoms would have fewer Planck lengths.

It is perfectly sensible to say that atoms would be smaller if alpha is bigger.

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  • $\begingroup$ What is the change of the fine-structure constant means the change of its relation to $h$, but not the change of $h$? I think, this is a hyphotetical "what if"-question with educational value, and thus all answers are okay if they reveal the relations in the background. $\endgroup$ – peterh - Reinstate Monica Jan 29 '17 at 18:31
  • $\begingroup$ If atoms shrink and everything else shrinks with them, what's the difference between saying "atoms have shrunk" and saying "the Planck length got bigger"? And why should one be favoured over the other? $\endgroup$ – Emilio Pisanty Feb 28 '17 at 10:27

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