# Finish this observable universe scale **URL:** \ **Category:** Factual Questions\ **Created:** [October 7, 2017, 7:55pm UTC](https://boards.straightdope.com/t/finish-this-observable-universe-scale/798255 "2017-10-07T19:55:51Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![Algernon](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Algernon](https://boards.straightdope.com/u/Algernon)\ **Post date:** [October 13, 2017, 2:28pm UTC](https://boards.straightdope.com/t/finish-this-observable-universe-scale/798255/21 "2017-10-13T14:28:49Z")
> [@digs](#): > > Our town (Madison, WI) has [the solar system laid out](http://www.spaceplace.wisc.edu/planettrek.htm). Starting near the middle of town is a two-foot diameter “sun” plaque, a block or so away is a sign explaining Mercury and showing its relative size (a small ball). > > Then, following the commuter bike path, comes Venus, Earth, Mars, then a long ride (but they threw in Ceres\*) to Jupiter, followed by Saturn (basketball size)… by the time you get to Uranus, you’re out of town, and by Neptune you’re past the next town… > > And marble-sized Pluto is 23 miles away (in Mount Horeb, where you can put your kickstand down outside The Grumpy Troll Brewpub…). Cool. I did not know this. Though I now live near Milwaukee, Madison is my home town. I wonder how long ago this was constructed? ---
**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, 2017, 4:09pm UTC](https://boards.straightdope.com/t/finish-this-observable-universe-scale/798255/22 "2017-10-13T16:09:56Z")
I’m pretty sure I stole that line from somewhere, but I don’t think it was Borges. I think it was a comedian. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [October 13, 2017, 4:19pm UTC](https://boards.straightdope.com/t/finish-this-observable-universe-scale/798255/23 "2017-10-13T16:19:59Z")
Steven Wright told a similar joke, but it was a map of the United States, not the solar system. > **[A quote by Steven Wright](https://www.goodreads.com/quotes/101896-i-have-a-map-of-the-united-states-actual-size)** > > I have a map of the United States... Actual size. It says, 'Scale: 1 mile = 1 mile.' I spent last summer folding it. I hardly ever unroll it. People ask ... ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [October 13, 2017, 11:27pm UTC](https://boards.straightdope.com/t/finish-this-observable-universe-scale/798255/24 "2017-10-13T23:27:21Z")
> [@septimus](#): > > (many physicists object to defining the “size” of subatomic particles.) I feel compelled to object explicitly. 🙂 There _are_ small-scale objects whose size is fuzzy. For those, it’s perfectly reasonable to throw in scare quotes or apply weasel words about it being a rough estimate and all (although I feel one needn’t be so apologetic in these cases). Can you talk about the size of an atom’s electron cloud? Sure. Size of a proton? Sure. These objects have real spatial extent, and the weasel words are fine for stopping nitpickers from complaining that a definition of their size has some level of arbitrariness baked in. But this flatly doesn’t apply to elementary particles. I can make some guesses about what your table might be quoting for each particle, but in no case is that the size of the particle. The fundamental particles are understood as having no spatial extent. They _may_ have a spatial extent that’s way smaller than we’ve been able to probe so far, but they may not. And an experimentally determined upper limit is decidedly _not_ the size of the particle. It’s a measure of how good experiments are so far at setting a limit on the size. Analogy: Image we find evidence of life on a far distant planet thanks to radio signals from intelligent life or through optical measurements of the chemical composition of the atmosphere. And imagine that our telescopes can spatially resolve things only at the level of the planet’s radius, give or take. If someone asks, “What is the typical size of a living organism on this planet?” the answer is certainly not “10,000 km”, even though that’s the upper limit we can set on the size. (Some of your entries appear to be of this type.) The neutrino appears a few times with what looks like the square root of an interaction cross section. A cross section is not a measure of the spatial size of a particle but rather relates to the probability of interaction with some target. Of note, it isn’t even definable without introducing some other particle to interact with and without specifying the kinematics – two decidedly extrinsic properties. Perfectly natural neutrinos interacting on perfectly natural materials in the universe span a good twenty orders of magnitude here. But that doesn’t matter anyway, since it’s ill-conceived to equate the square root of a cross section to a physical size. Example 1: Protons are 10[sup]-15[/sup] m across, so their physical cross sectional area is 10[sup]-30[/sup] m[sup]2[/sup]. If I send a 1 GeV neutrino through a cloud of protons, the neutrino will physically intersect a proton (in the classical sense we’re trying to force here) with a probability governed by the proton cross sectional area. Yet, the actual interaction cross section in this case is twelve orders of magnitude smaller, and that’s because physics size doesn’t relate to interaction probability. Example 2: A gamma ray passing through carbon has an interaction cross section of 10[sup]-27[/sup] m[sup]2[/sup]. Does that mean that the photon (or a carbon atom?) is 10[sup]-13.5[/sup] m across? Certainly not. The atom is 2000 times larger than this, and the photon is point-like as far as we can tell. The interaction cross section does not tell you about the physical sizes of the participating particles. They have related units, and sometimes that’s a sign that you can relate two things, but it’s not _always_ a sign. Example 3: Photons can scatter off other photons.\* At 10 GeV or so, the cross section for this interaction is 10[sup]-35[/sup] m[sup]2[/sup]. This does not mean that photons are 10[sup]-17.5[/sup] m in size. To prevent this chart from begetting future charts with the same bad entries, I’ll say to any future chart makers reading this: If you want to include length scales from the subatomic world, do so, but just label those length scales with what they really are. And what they really _aren’t_ are the sizes of the fundamental particles. [sup]\* Though a straightforward prediction of the Standard Model, photon-photon scattering was only first observed this year.[/sup] ---
**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 14, 2017, 1:26am UTC](https://boards.straightdope.com/t/finish-this-observable-universe-scale/798255/25 "2017-10-14T01:26:42Z")
> [@](#): > > - Though a straightforward prediction of the Standard Model, photon-photon scattering was only first observed this year. Whoa, really? I’d assumed, from how often it shows up in textbooks, that it had long been experimentally verified. Another possible “size measurement” (which doesn’t actually imply a size) for subatomic particles would be the Compton wavelength. And for charged particles like the electron, yet another would be the classical self-energy radius. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [October 14, 2017, 9:58pm UTC](https://boards.straightdope.com/t/finish-this-observable-universe-scale/798255/26 "2017-10-14T21:58:57Z")
> [@Chronos](#): > > Whoa, really? I’d assumed, from how often it shows up in textbooks, that it had long been experimentally verified. Yeah, it’s a nice pedagogical case study since photons clearly can’t interact at tree level, but because of that it’s a challenge experimentally. At viable laser energies (x-ray, say) the cross section is 15 orders of magnitude smaller than the example above, and in the collider setting it’s the usual needle-in-a-haystack problem when looking for rare events. Here’s the ATLAS measurement: [technical paper](http://www.nature.com/nphys/journal/v13/n9/full/nphys4208.html) and [a random lay article](https://phys.org/news/2017-02-scientists-evidence-light-by-light-standard.html). [Previous page](https://boards.straightdope.com/t/finish-this-observable-universe-scale/798255.md?page=1)