# Physics question **URL:** \ **Category:** In My Humble Opinion\ **Created:** [October 7, 2022, 3:32pm UTC](https://boards.straightdope.com/t/physics-question/972902 "2022-10-07T15:32:31Z")\ **Posts on this page:** 17\ **Page:** 2
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [October 7, 2022, 7:52pm UTC](https://boards.straightdope.com/t/physics-question/972902/21 "2022-10-07T19:52:12Z")
Well…more absurd. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [October 7, 2022, 9:01pm UTC](https://boards.straightdope.com/t/physics-question/972902/22 "2022-10-07T21:01:18Z")
Maybe I can simplify the question a little bit. Let’s suppose a relative to me and known universe was the size of a pea. Would it appear to me as a solid Mass or would it be an invisible gas ---
**Author:** ![DCnDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dcndc/32/2842_2.png) [@DCnDC](https://boards.straightdope.com/u/DCnDC)\ **Post date:** [October 7, 2022, 9:47pm UTC](https://boards.straightdope.com/t/physics-question/972902/23 "2022-10-07T21:47:40Z")
It would look like a solid mass. Jupiter is just a gigantic ball of gas and it looks solid. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [October 7, 2022, 10:25pm UTC](https://boards.straightdope.com/t/physics-question/972902/24 "2022-10-07T22:25:18Z")
> [@HoneyBadgerDC](#): > > and made up of some form of matter that would not even interact with ours We already have that, roughly speaking: it’s called dark matter. It already comes in chunks more than 1000 light-years across. The only physical effect it has is gravitational. It can change the speed that galaxies rotate at, cause light to bend around it, possibly perturb some orbits, etc. But it is a very subtle influence and barely detectable. Certainly not detectable at any normal human scale. It doesn’t travel at the speed of light (let alone greater), but it wouldn’t matter much if it did. Neutrinos also barely interact with normal matter and travel at \>99% the speed of light, but they’re unnoticeable except via sensitive instruments (despite them being blasted out by the sun in vast quantities). ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [October 7, 2022, 10:42pm UTC](https://boards.straightdope.com/t/physics-question/972902/25 "2022-10-07T22:42:13Z")
Back of the envelope calculations suggest that if such an object had the 1% of the density of water, it would collapse into a black hole. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [October 8, 2022, 5:52am UTC](https://boards.straightdope.com/t/physics-question/972902/26 "2022-10-08T05:52:28Z")
I think it would colapse at a much lower density than that. According the the wikipedia page, the Schwarzschild density is (3c^2)/(8pi_G_r^2) In standard m, kg, sec units c=3_10^8 G=6_10^-11 and r=100,000 ly = approx 10^21 m So the density is less than 10^-16 kg/m^3, which to put things in perspective would be the equivalent of one mushroom spore every 10 cubic meters. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [October 8, 2022, 7:21am UTC](https://boards.straightdope.com/t/physics-question/972902/27 "2022-10-08T07:21:31Z")
To try to rephrase the question that I think the OP is trying to ask, suppose I shrunk the galaxy to human size and walked into it, what would it be like. The answer is that it would be like rarefied gas. (note its 3am as I’m writing this so math mistakes are possible) If we shrink 100 thousand light years to a disk 2 meters in diameter, that would shrinking by a factor of 5x10^15, it would be also only be about 2 cm thick. XKCD has a [nice essay](https://what-if.xkcd.com/83/) which points out that although 99% of the stars are small, the largest 1% make up 99% of the volume. So to make matters easy lets assume that of the 100 billion stars in the galaxy 1 billion of them are the size of Anteres and these are the only ones that count. So anteres at 4_10^8 meters would shrink down to 4_10^-7 meters, or about the size of a virus. Now with 1 billion stars, they would be separated by the distance of a human hair. So you would probably not even notice them. There would be a few other small galaxy’s scattered around, the closest one being 8 inches way. The next big one (andromeda) would be about 140 feet away. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [October 8, 2022, 10:34am UTC](https://boards.straightdope.com/t/physics-question/972902/28 "2022-10-08T10:34:14Z")
I blame my envelope (thanks for checking my work) ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [October 8, 2022, 10:47pm UTC](https://boards.straightdope.com/t/physics-question/972902/29 "2022-10-08T22:47:34Z")
> [@Buck\_Godot](#): > > Now with 1 billion stars, they would be separated by the distance of a human hair. Perhaps I have misunderstood your calculation, but I don’t think that’s correct. A 2 m x 2 cm disk has a volume of 63e6 mm^3. That’s 0.063 mm^3 per star (dividing by 1e9), which arranged in a cubic lattice would be 0.4 mm separation. That’s quite a bit more than a human hair. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [October 9, 2022, 12:00am UTC](https://boards.straightdope.com/t/physics-question/972902/30 "2022-10-09T00:00:56Z")
See the posting at 3AM disclaimer above. I think at that point I was just happy to get the right order of magnitude and arrived at 10^-4 meters (1/4 the size you came up with) which is mid ranged human hair width [(17-181 μm)](https://en.wikipedia.org/wiki/Orders_of_magnitude_(length)). ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [October 9, 2022, 12:07am UTC](https://boards.straightdope.com/t/physics-question/972902/31 "2022-10-09T00:07:05Z")
That’s fair. It’s not unheard of for me to compute things with zero significant digits; just the value of the exponent. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [October 9, 2022, 12:15am UTC](https://boards.straightdope.com/t/physics-question/972902/32 "2022-10-09T00:15:04Z")
i.e. [Fermi Estimation](https://what-if.xkcd.com/84/) (yes I’m working my way through Randall Munroe’s newest book.) ---
**Author:** ![Encinitas](https://avatars.discourse-cdn.com/v4/letter/e/ee7513/32.png) [@Encinitas](https://boards.straightdope.com/u/Encinitas)\ **Post date:** [October 9, 2022, 6:24pm UTC](https://boards.straightdope.com/t/physics-question/972902/33 "2022-10-09T18:24:21Z")
I love you guys! ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [October 9, 2022, 6:58pm UTC](https://boards.straightdope.com/t/physics-question/972902/34 "2022-10-09T18:58:54Z")
> [@Dr.Strangelove](#): > > That’s fair. It’s not unheard of for me to compute things with zero significant digits; just the value of the exponent. If there is a lack of significant digits, is that different from zero? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [October 9, 2022, 7:05pm UTC](https://boards.straightdope.com/t/physics-question/972902/35 "2022-10-09T19:05:35Z")
> [@eschereal](#): > > is that different from zero? Yes. We are talking here about 0.1, 1, 10, 100, etc. There is a leading 1, of course, but it is not _significant_, because it is always there–it carries no additional information. It could be some other number, as long as we were consistent, but staying with exact powers of 10 makes the mental math easy. Computers make use of this trick in floating-point math, where because they are in binary, the leading digit of the fraction part (called the mantissa) can only ever be 1, and therefore isn’t stored. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [October 9, 2022, 7:16pm UTC](https://boards.straightdope.com/t/physics-question/972902/36 "2022-10-09T19:16:34Z")
> [@Dr.Strangelove](#): > > Computers make use of this trick in floating-point math, where because they are in binary, the leading digit of the fraction part (called the mantissa) can only ever be 1, and therefore isn’t stored. Right. A 32-bit FP has a 24-bit mantissa stored in 23 bits with an implied high-order 1 – unless the value is denormalized (exponent is 0 and mantissa is non-zero). But in standard written scientific notation, a tiny number still has at least one significant digit (e.g., 1.0 x 10-50), otherwise it is indistinguishable from zero. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [October 9, 2022, 8:12pm UTC](https://boards.straightdope.com/t/physics-question/972902/37 "2022-10-09T20:12:44Z")
> [@eschereal](#): > > But in standard written scientific notation, a tiny number still has at least one significant digit (e.g., 1.0 x 10^-50), otherwise it is indistinguishable from zero. Well, zero is a special case (for floating-point math as well). For most Fermi problems, you can ignore it completely. If some part of the calculation did get multiplied by zero, or there was some kind of exact cancellation of terms, then you’d simply ignore that part. [Previous page](https://boards.straightdope.com/t/physics-question/972902.md?page=1)