Because it's never been observed to be variable.
How do scientists KNOW for sure?
They don't. In fact they can't know for sure anything that is accepted as a physical constant was the same before it was first measured, or that any of them will be the same tomorrow as they were today.
You could argue that no one can really KNOW that any of the accepted physical constants really are constant, and none of the calculations based on them can be assumed to be reliable - none of the calculations that are done in the course of scientific research or even in engineering and design work.
What do you think the end result of that line of reasoning would be?
For all of how many years that we've known about radioactivity?
You could argue that no one can really KNOW that any of the accepted physical constants really are constant, and none of the calculations based on them can be assumed to be reliable - none of the calculations that are done in the course of scientific research or even in engineering and design work. What do you think the end result of that line of reasoning would be?
Honesty, maybe?
Of course you can argue those points, that know one knows if what we know as constants today were always constant. Scientists say they were, as if it's written in stone. On what basis? Because of what we've observed in the last couple hundred years or so.
Talk about having faith.....