Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

This question already has an answer here:

I read that the number of atoms in the entire observable universe is estimated to be within the range of $10^{78}$ to $10^{82}$.

Does the Universe have finite number of particles? If so, how could it be determined?

share|improve this question

marked as duplicate by Pulsar, Kyle Kanos, John Rennie, Brandon Enright, Emilio Pisanty Feb 11 at 9:00

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
Can you tell us about sources where you read that figure? –  Sachin Shekhar Feb 10 at 16:14
1  
My guess on how it's determined, if it is determined -- figure out the average density of an observable region of space and figure out the volume of that same region and then assume it's uniform enough everywhere else to get a number. –  tpg2114 Feb 10 at 16:18
    
    
1  
@kenn I mean the physical universe in the traditional sense (without speculating about mutiverses or other exotic hypotheses). Since the curvature of space is very close to zero, it is very likely that the entire physical universe is much much larger than the part that we can observe. In principle, it could be infinitely big. See also the wiki article on the shape of the universe. –  Pulsar Feb 10 at 21:19

3 Answers 3

up vote 0 down vote accepted

The universe must contain a finite energy - sum of all matter and fields - or the mass-equivalent would collapse within its own gravitation. Said mass-energy is fractionally partitioned among elementary particles and their agglomerates. One then strongly expects there are a finite number of particles including extremely low energy photons and neutrinos.

share|improve this answer
    
First of all, it's not me who downvoted your answer. Your approach makes sense indeed. –  kenn Feb 10 at 17:35
    
It appears that somebody(ies) is(are) hunting my answers. You will know a man by his fears. –  Uncle Al Feb 10 at 18:23
    
I'm new to this site, I also noticed that expressing your ideas is not welcome here. –  kenn Feb 10 at 18:48
1  
@kenn: As per the help center, personal theories are correctly not welcome here. –  Kyle Kanos Feb 10 at 18:50
1  
@kenn: Not true; if Newton published his works (which he did do), it could be discussed here. If your pet theory is not published it is, by definition, off-topic here. If your theory is published in a reputable journal, it is on-topic. –  Kyle Kanos Feb 10 at 19:10

Particles in physics in current terminology is "elemenntary particles", which are the building blocks that form atoms molecules and radiation that we observe macroscopically in bulk. These can be created and destroyed during the processes of stellar evolution, and particularly photons, which are bosons, have no limit on their number at all. There is no limit to the number of particles currently or at any time after the Big Bang.

If you mean if "the number of atoms is finite" a tentative number could be estimated from the mass of galaxies, the number of galaxies in galactic clusters and the number of clusters in a homogeneous universe ( ignoring dark matter since we know next to nothing about it) One could come out with a number by dividing the average galaxy mass by the average atomic weight estimated for the distribution of atoms in a galaxy.This number would be just an estimate and would not be constant because stars evolve, sometimes becoming black holes, sometimes atoms fuse into heavier atoms, sometimes atomic nuclei decay to two or more fragments etc.

By both definitions of particles , their numbers are not constant.

share|improve this answer
    
Anna, thank you for taking the time to post an answer. Logic suggests that number of particles in the universe must be finite, but concepts of universe and observable universe confuse me. –  kenn Feb 10 at 19:27
    
Classical physics type logic. Quantum physics works differently. Observable universe is what we see and fit our models to it. Universe is the universe as the model extends it/ describes it mathematically outside the variables we can see and measure. –  anna v Feb 10 at 19:43
    
I mean "including also the range outside the variables we can see and measure" –  anna v Feb 10 at 20:02

That depends on whether the Universe is finite or not, which we don't know and probably will never find out.

share|improve this answer
    
I don't mean it by size, I mean particle level counted number. –  kenn Feb 10 at 15:43
    
You mean if the observable universe has a finite number of particles? –  Thriveth Feb 10 at 15:56
    
Actually term of observable universe is obscure too, according to Big Bang theory it must be determinable. I mean mass and particle numbers in the universe must be determinated. –  kenn Feb 10 at 16:02
2  
The observable universe is finite. –  Pulsar Feb 10 at 16:30

Not the answer you're looking for? Browse other questions tagged or ask your own question.