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S16k. What is real?
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All physics is just a handy way of thinking about certain phenomena.
This - a handy way of thinking - is what it means that something 
- the concept we find useful - exists. 

We say that people exist, because they are a handy way to describe 
certain blobs of matter like ourselves. We say that electrons exist, 
because they are a handy way to describe ionization phenomena.
We say that photons exist because they are a handy way to describe 
quantum optics phenomena.

Photons are objectively real because they are needed in the only
comprehensive coherent theory of microscopic interactions that we 
know of. 

On the other hand, 'photon' is merely a word that physicists use on 
paper and in conversation. But in precisely the same sense that
entropy, energy, or the electromagnetic field are merely words that 
physicists use on paper and in conversation. 
 
Even our best concepts are 'merely' words.

If we give up concepts, only an undifferentiated happening in 
space-time remains, and even talking about this becomes impossible.