The number duodecillion represents 10^39. I must concede that this number is large beyond imagination. I believe that the current estimate for the number of electrons in the known universe is 10^88 (a.k.a. ten-thousand quattuordecillion), which considerably outshadows the relatively miniscule duodecillion. You could square duodecillion, and not be at 10^88. 10^88 is the largest number that measures something "tangible," as in "represents a physical value," that I have come across in my studies as a physicist. To fully answer your question, we must try to define the word "measurable." If by measureable, we mean "able to make a direct measurement," then measureable values are considerably small. Obviously, no one went out one fine summer day and counted all of the electrons in the universe. This value was deduced by using the knowledge of the properties of the elements and compounds that exist in our universe, and their abundance. This definition of measureable would be "able to estimate using known facts." This opens up a larger window to use numbers that apply to large values that are indirectly measureable. In the strange realm of quantum mechanics, physicists deal with probabilities (not observations or measurements) that certain events occur. The largest (calculated) number that I have ever seen in this field is 10^(10^30), which deals with the probability (or should I say "improbability" in this case) of something called "quantum tunneling" on the macroscopic level. An example of quantum tunneling is that you could technically walk through a wall if all of the empty space in your molecular structure were temporarily lined up with all of the empty space in the wall's molecular structure, but this is an exceedingly improbable event.
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Uh as the others have pointed out, there is no 'largest' number. That being said, the largest number used in a meaningful proof would be Grahams number.
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the greatest number is either pi or 2287(the numbers that stand for the letters that spell the word cats)
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Ther is no greatest number in the world. Numbers go on forever. But you could just say infinity.
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Zero
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there isn't one.
Numbers go forever.
Numbers go forever.