Smallest Living Organism
The parasitic bacterium Mycoplasma Genitalium that inhabits the genital and respiratory tracts of primates is widely considered by scientists to be the smallest organism capable of independent growth and reproduction.
Smallest Non-Living Organism
Although there is still some debate about what is considered “alive” and what isn’t, most biologists would not classify a virus as a living organism due to the fact that it cannot reproduce or metabolize by itself. A virus, however, can get much smaller than any living organism including the bacterium we just saw. The smallest is the single stranded DNA virus Porcine circovirus you see here. It is only 17 nanometers across.
Smallest Thing Visible to the Naked Eye
The smallest objects visible to the unaided human eye are roughly .1mm long. This means that under the right conditions you would be able to see an ameoba proteus, a paramecium, and even a human egg.
Smallest Observed Object in the Universe
Over the last century science has made both great strides into understanding the vastness of space and its microscopic building blocks. When it comes to figuring what the smallest observable particle in the universe is, however, we run up against a bit of a wall. At one point we thought they it was the atom. Then scientists discovered the proton, neutron, and electron. It didn’t stop there though. Today we know (by smashing particles together in places like the Large Hadron Collider) that these can be further broken down into even more particles like quarks, leptons, and even antimatter. The issue with trying to figure out which is smaller is that on a quantum level size becomes a bit irrelevant as the rules of physics you are used to living by begin to break down (some particles have no mass, some even have negative mass) Unfortunately, trying to answer this question is something like dividing by zero…not really possible.
Smallest Hypothetical Object in the Universe
Keeping in mind what we just said about the idea of size being irrelevant on a quantum level, there exists something known as string theory and although it is slightly controversial it proposes that all subatomic particles are composed of vibrating strings that interact in order to create things like mass and energy. So, although these strings would technically have no physical dimensions, according to our puny human ability to reason one could say that these strings would be in some sense the “smallest” objects in the universe.