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Caesar's Last Breath
The Physics behind the Folklore
You are breathing in molecules from the last breath of a famous person
The folklore has it that each time you take a breath it contains some of the atoms/molecules from the last breath exhaled from Julius Caesar, Michelangelo, Ghengis Khan, or for that matter just about any famous person who died a long time ago. The example quoted is most usually Julius Caesar, and the tale is often known as the "Caesar's Last Breath" phenomenon.
Strange as it may seem, this is true! Admittedly some assumptions are made in the reasoning, but basically it is true. Incidentally, it isn't something specific to Julius Caesar or to any other person who has lived, but it's simply a statement that we all breathe the same air, in the same sort of way that a dozen people stuck in a lift all breathe the same air.
Although it seems extraordinary, this business about the "same atoms" is really just because the actual molecules of air are extremely small. If they were just "very" small it wouldn't be true, but they are even smaller than that, resulting in singular molecules being very well mixed.
The actual calculations: Assuming Julius Caesar's last breath was one litre of air, it would have consisted of about 1022 (exp) (1000000000000000000000) molecules. As he died a long time ago and the air has been all mixed up since then, and atoms don't go away, those molecules are assumed to have been evenly spread throughout the entire Earth's atmosphere, a total of about 5.1x1018Kg of air, a total of somewhere around 1044 (1000000000000000000000000000000000000000000) molecules. These are very approximate figures, but it's got a certain sense to it in the same sort of way that you know about how many trucks weigh the same as a ship!
So, when you divide it out, it turns out that in the average breath you take, there's a good chance of it containing a molecule or two which was once part of the last breath of Julius Caesar.
This is no reason for concern, so don't hold your breath! Having been stuck in this lift for so long, all of us, the human race, we can expect to be breathing each others' air!
A few other people who are sharing the same explainospace...
...See, it's a well-known idea!
Another thought: Why is it Julius Caesar, out of all people who have ever lived, who is the favourite for this physics explanation? My guess on this is that the origin is from Shakespeare. Not only did Shakespeare write about the death of Julius Caesar, thus making Caesar the "best-known dead person ever", but in Hamlet, quite soon after the much quoted "Alas poor Yorick!" line there is a musing on the remains of Caesar being recycled as a beer barrel bung. This may have struck the popular imagination so when asked to name someone who died long ago it's Julius Caesar who has the dubious honour of being mentioned. Or, it could be that when Enrico Fermi first thought of the "Caesar's Last Breath" idea to illustrate practical calculations he happened to choose Caesar as an example, and it's become a tradition ever since.
The last breath of Thomas Edison was captured and is in a museum. See Thomas Edison's Last Breath. Roadside America
And, if you feel that you may need to consider YOUR last breath, you can MAKE A WILL
A similar idea can be seen in the notion of "Titanic Water" which is to do with what happened to the iceberg which sunk the Titanic.
Plus there are even more astonishing cosmic levels of the consequences of Distributedness in the fact that the atoms which humans and the earth are composed from were originally made in exploding stars which shone long ago for thousands of millions of years, then exploded, and the star dust eventually settled, formed new stars, which also shone for thousands of millions of years, exploded, and eventually the starstuff settled and by luck formed the galaxy and the sun and the world as we see it now is the dust from those long ago exploded stars! Thank them if you like: Your lucky stars.
Other helpful links:
Luck - see Gambling
Stars - see Astronomy