universe - Are we big or small?

How does the size of humans compare to the size of other objects in the universe? Are we among the relatively large or the relatively small things?

My very preliminary research suggests that the smallest thing in the physical universe is the quantum foam which should be about Planck length or 10^-35 m. While the universe is about 10²⁶ m. This would seem to bias us toward the big side by about 9 orders of magnitude.

I am skeptical of this estimate because it is pretty close to the middle of the scale and I am always skeptical of measures that put humans in the center. So is there another way of thinking about/ quantifying this question?

Edit: it seems that the smallest thing we have actual evidence of is a neutrino which is 10^-24 m, which would put us even closer to the center!

Answer

The best you will get is ‘middle/small’, assuming you treat humans as a whole. Here’s why:

• Usually, ‘large scale’ physics as given by GR (or even SR) does not apply to us: The gravitational force between two humans is small and the curvature in spacetime caused by a human being is absolutely negligible.


• At the same time, ‘small scale’ physics, described by quantum mechanics, does not apply to us either: You can run against a wall as often as you like, but you won’t get through it. However, this point relies on the description of humans as a whole, rather than made up of ‘small’ atoms. This is only partly true for us, as many biological systems rely - at least to some extent - on quantum mechanical (non-classical) effects. Were minor parametres (magnitude of the Van der Waals force, for example) to change suddenly, we certainly would notice (and die).

Planetary objects, stars and black holes nearly do not rely on such small-scale effects. Nuclear fusion will probably even take place if some forces double or triple and a black hole won’t stop existing just because quantum fluctuations change slightly.

Hence, as we rely ‘more’ on small-scale physics than on large-scale physics, we are certainly more on the ‘small’ side of stuff, but since we are much larger than actually ‘small’ things (atoms), ‘middle/small’ is probably the best you can get.

But really, ‘large’ and ‘small’ only make sense in relation to some other object: We are about as large as trees, smaller than the moon, much smaller than the sun and much larger than atoms. I guess you knew that already ☺.