# How do fundamental constants determine the size of material bodies? [duplicate]

The size of an object depends on the size of the molecules and atoms it is made of and these ultimately depend on the value of the fundamental physical constants. The simplest example could be the size of the hydrogen atom which is Bohr's radius $$a_{0}=\frac{\hbar}{m_{e}c \, \alpha}$$. Let's consider the fine structure constant (since it is dimensionless). Then the size of an H-atom is inversely dependent from $$\alpha$$. If it would be different than what it is, say twice as much its value, the size of the H-atom would be half as much as it is, and so on. The question is if this linear dependence would still hold for other atoms, molecules and much more complex structures? Would a doubling of $$\alpha$$ lead to half the length of a complex objects, such for example a table, be also inversely proprtional as $$1/\alpha$$ ? I tend to say that it is not obvious since the molecular orbitals are complicated to calculate and it seems to me not so straightforward that a linear scaling of $$\alpha$$, and therefore also of Bohr's atom, leads automatically to a linear scaling of the size of more complex structures as well. I suspect that could be determined only by complicated numerical solutions of Schrödinger's equation? But I'm not sure about that. I'm just trying to find out if there is an easier logical reason to argue how the change of one constant leads to a change in size. Any thought about that?

## marked as duplicate by Ben Crowell, GiorgioP, Jon Custer, Kyle Kanos, Rory AlsopMar 21 at 11:08

• This question sort of reverses the way the question is usually asked, but it amounts to the same thing. We can determine whether the fine structure constant changes over time, because it's unitless. We can't determine whether the diameter of a hydrogen atom changes over time, because that's a number that has units. Since it's undefined whether the diameter of a hydrogen atom changes, it's undefined whether changing the fine structure constant leads to a change in the diameter of the hydrogen atom. – Ben Crowell Mar 10 at 17:37
• Yes, but this is precisely why I asked. That would be true only if the change of the adimensional constant keeps fixed the proportionality of the size between the ruler and that of the Bohr's atom. But I don't feel this to be obvious. – Mark Mar 10 at 18:00

For a size change to make physical sense there has to be some measurable difference before and after. It is commonly assumed that the Bohr radius scales the size of molecules and everything else based on it.

However, it turns out that the scaling is not entirely linear as a function of $$\alpha$$. King et al. (2010) has shown that the bond angle of water decreases in molecular simulations if $$\alpha$$ changes, in turn reducing the dipole moment and hydrogen-bonding ability of water. This would at the very least make ice change in size, for the same mass. A larger $$\alpha$$ also increases the bond length in the water molecule despite the Bohr radius declining. Generally covalent bonds between light atoms become weaker, and this would no doubt affect the maximum size of many objects like trees while perhaps not affecting metals as much.

Even without these local changes we can compare the size of molecules to the size of gravitationally bound structures (regulated by $$\alpha_G$$ in addition to $$\alpha$$), or structures strongly affected by the proton-electron mass $$m_p/m_e=\beta$$. The mass of a person has for example been estimated to scale as $$\propto 10^2\alpha^{3/4}m_p^{-1/2}$$ and would hence scale very differently from stellar masses $$\propto m_p^{-2}$$ and asteroid masses $$\propto \alpha^{3/2}\beta^{3/2}m_p^{-1/2}$$. (Carr & Rees 1979) is a nice overview of how the basic dimensionless constants affect the relative sizes of various objects (above estimates are from there); see also Barrow & Tipler's The Anthropic Cosmological Principle and (Tegmark, et al. 2006) for more review.

• "The Bohr radius scales the size of molecules and everything else based on electromagnetic interactions." - I don't find this to be obvious. The Bohr radius scales as $1/alpha$. Does the size of a DNA molecule scale with the same proportionality in alpha? If yes, how do we know? – Mark Mar 10 at 18:34
• @Mark for complicated systems one would need the exact electromagnetic equations , model the DNA with the em equations and see whether the size of the DNA "scales". It might be affected by the density, the rotations etc, so it is not simple . QED is a well validated theory, and there is no reason to think one cannot, with a computer, or a future computer , calculate the DNA dimensions, but the simple concept of "scale" cannot hold for complex systems. For example one can form carbon crystals, and diamond crystals from the same atoms, and they will not scale. – anna v Mar 11 at 8:17
• Precisely. Therefore we can not claim that a change in the fine structure constant would go unnoticed. We have still to prove that the same scaling holds for every material body. – Mark Mar 11 at 8:34
• @Mark the scaling of α is in the equations fitting observations,not in space, except for simple systems there might be a scaling in space. α exists only within equations. – anna v Mar 11 at 8:55
• What do you mean by equations fitting observations? I mean the scaling in alpha of the equation of the size of an object. For the Bohr radius it is just 1/alpha. But how do we know that the size of a much more complex object such as a DNA macromolecule scales as 1/alpha as well? To know we should numerically solve the Schröedinger equation which such an almost macroscopic body for different alpha and see if it still scales as 1/alpha. This would be horrendously complicated. It could scale as 1/alpha^2 or sqrt(alpha) or whatever instead. How do we know? Seems to me not that straightforward. – Mark Mar 11 at 10:09