Question: Why do we say there are such things as gravitons, even though we can't observe them and we guess their properties?

Urslaan Chohan answered on 9 Jun 2018:

Very good question, and this crosses into the realms of what is known as the “philosophy of science”! We don’t actually guess their properties. We can see their properties, through experiments. So for example, if we say a car with fully tinted windows has four people in it, we can verify this statement in a number of ways. One way is to open the door and look inside. Another is to weigh the car, to see if it is heavy enough to have four people. So as we can see, the car with four people has different properties that we can measure through experiment. With things that are so small that we can’t see them, we search for these properties less directly that just using our usual five senses. We see the effect of something or measure other properties like its weight or energy, using apparatus. When an object has those particular properties, we then call it that thing.

In short, we measure properties and assign names to things based on those properties. Like an apple is round and red, with a stalk. Red and round are the properties, whilst apple is the name we assign to it. And this is distinct to a pear, which is green and has a stalk. We can tell the difference though they are similar through observation. Pear and apple are names of objects.