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We found VY Canis Majoris, a star so big it can't even be seen in scaled illustratations with the sun itself.

However, we stop at particle physics, or quantum mechanics, i.e. particles, subatomic, etc.

If stars keep getting bigger, and we keep finding things bigger, and the size of the universe still is unknown, why does the size (oppositely) have a limit?

Like, why is the atom the "smallest mass", but the "largest mass" is indeterminate?

I hope you get my point. If not, please tell me how I should improve.

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What is the "smallest mass" then, going by these same measures? – Strong Oct 19 '13 at 19:27
That is going solely by mass, not volume. For example, photons may have no mass, but they can be evidently smaller than others things which do. Are you calculating "mass" as measurable weight, energy, or volume? – Strong Oct 19 '13 at 19:29
And "0" is a number that says nothing. However, numbers are abstractions. – Strong Oct 19 '13 at 19:30
Saying that the photon has zero mass has a very specific and precise meaning. – dmckee Oct 19 '13 at 19:32
@Sancho The problem with saying the total mass of the Universe is that it's not defined in General Relativity:… – jinawee Oct 19 '13 at 19:45

It is not like there is lower limit of mass and not the upper limit. as far as I know we are still struggling to discover things at both extremes.

As per the question, I believe you are talking about lower limit as if the rest mass of photon is zero, but then who has found photons at rest till now.We may have gotten to extremely small scales in terms of mass but nobody is saying that a certain number is the least mass you can have. As atleast I can not think of something with absolute 0 mass in practicality.

You can not be sure that there is going to be the lower limit of mass as 0 (even after disregarding the sci-fi negative masses) because as closer we will get to absolute zero, we would look even closer and find newer limits going into continuum for reaching the zero.

Hence I would say that we do not have either the upper or lower limit of mass.

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the neutrino poses a lower limit of rest mass for massive particles, however (and photons have zero rest mass) – Jan Dvorak Oct 21 '13 at 10:26
I am not saying that photon do not have zero rest mass, I am simply saying that it is a theoretical concept. I have not heard of anh experiment where photons are found at rest, there is rest has simply been extrapolated till now. Can one be sure of what will happen when they do stop entirely ? also on the neutrino front, are you sure that we are never going to find something lighter still ? – Rijul Gupta Oct 21 '13 at 11:11
Rest mass isn't necessarily determined by stopping the particle (since of course a photon can't be at rest). We can use other methods, like measuring energy and momentum and applying $E^2-p^2=m^2$. Countless such experiments have been made, and nobody doubts that photons are massless (within current error, anyway). – Javier Jun 2 '15 at 20:15

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