# If everything in the universe doubled in size overnight, would it be noticeable?

By my understanding, if everything doubled in size, such as the Sun and the Earth, and because the space in between them (which is nothing) can't expand, would the gravities greatly change and the Earth be pulled into the Sun?

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What do you mean with everything ? If the electron charge/charge distribution is different but same "distance"(whatever it means, you should know that there is no knowable dynamic position of an electron) then there is nothing we could conclude by physics, because your suppositions break physics(it was obtained from a different world than yours) –  HDE Dec 20 '12 at 14:21
yeah there's a big difference if it's just the big stuff like Sun and Earth doubling in size and whether the elementary things like proton, neutron and electron double in size –  raindrop Dec 20 '12 at 14:22
Well I meant if everything with a physical mass I guess. I'm a noob with physics but I just want to understand –  dizzytri99er Dec 20 '12 at 14:33
I think if all subatomic particles doubled in size, there would be no observable change in size of the Earth. If you double the size of protons and electrons for instance, because force between them is determined by the central position of these objects, the force between them will remain the same, and therefore the radius of the atom will not change. –  Mew Dec 20 '12 at 14:40
What do you mean the space in between can't expand? Expanding is the only thing empty space is always doing –  Jim Oct 15 at 14:41

It doesn't really matter to the atoms if things double in size. It's like the Moon becoming the size of the Earth, it's something 'normal' that happens often in our universe: things become bigger and smaller. Another example is gases of clouds becoming giant stars, that isn't just 'double' but more like increasing $10^{30}$ in 'size' (from separate molecules to forming giant stars)

Note that in some superdense objects (like neutron stars) the strong gravitational force has a direct effect on distances on the molecular scale.

On a side note, remember, not all equations are linear. Equations such as that for (classical physics) gravity, $$F_g=GMm/r^2$$ are non-linear. If the distance between two objects double, and their volume doubles while their density remains constant, we have a problem.

(need to add some equations on what happens when doubles)

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If everything is expanding then that would mean everything. In our point of view the expansion would be imperceptible. As our atoms expand we expand along with everything else including space. This expansion is, in my humble opinion, the reason we cannot accomplish time travel. If everything expands except for our time traveler, who is moving beyond time, within a short time period he would be so much larger or smaller than the rest of the world as to either be larger than the planet or be imperceptible depending upon the direction of his travel.

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Space-time expansion is not currently occurring in the manner you describe (the atoms in our bodies are not expanding, even though the universe does expand on very large scales). See this question: physics.stackexchange.com/questions/2110/… –  kleingordon Dec 22 '12 at 5:57

I had a similar question in mind. If everything double in size we wouldn't be able to perceive that. If we have two identical objects and one expands keeping everything other same then we will know that the object expanded because it was equal to the next one and it is bigger now. But if both of them and rest all expanded by same factor we won't be able to notice it.

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If I have two tennis balls and they each have a radius of 1 unit, and there is a gap of 1 unit separating the tennis balls. If the tennis balls were to expand their radius by 1 unit each, then the tennis balls that were previously not touching will now be touching. Thus we can perceive the expansion. –  Mew Dec 22 '12 at 5:08
Objects expand on heating. If a circular object with circular hole is heated, both the radius of hole and the object increase. –  Prakash Gautam Dec 22 '12 at 5:31
@Mew: "If the tennis balls were to expand their radius by 1 unit each, then the tennis balls that were previously not touching will now be touching. Thus we can perceive the expansion". Yep, that's called gravitation - equivalence principle. –  bright magus May 17 at 22:11
@brightmagus, if gravity was just due to expansion, then no planet would ever orbit the sun. –  Mew May 18 at 1:28
There still could be another force, repulsive - namely charge. –  bright magus May 18 at 5:46