# How do we know that fundamental constants don't slowly change in time? [duplicate]

Let me get one thing straight first. I am not saying that fundamental constants like the speed of light don't have the value that we know they have today. What I am asking is whether the value of a constant could change with time. For example, we measure today that the speed of light is almost $300000000\,m/s$, but this measurement was made over the last century. But what if a long time ago (billions of years ago) that constant had a different value?

So, it's always the limit of how fast something can move, but that limit slowly changes with time.
To conclude,i am asking if there is any kind of evidence that any of the fundamental constants like the speed of light remain constant in time or if we just took that for granted.

## marked as duplicate by ACuriousMind♦, dmckee♦Jul 14 '15 at 14:25

• Just to be clear, are you asking us what evidence we have that the speed of light hasn't changed? Or are you asking us what it would mean for physics if it had changed and we just haven't noticed? – Jim Jul 14 '15 at 13:05
• I know that there is experimental evidence for the speed of light and it being a fundamental constant for velocity.I have read some interesting points from some physicists that explain some of the early processes or the universe(or some of the paradoxes in some of our measurements from the universe) and they say that the speed of light might not have always have the value that we know that it has today.So,i am asking if there is evidence that the speed of light is constant through time – TheQuantumMan Jul 14 '15 at 13:09
• i edited the question – TheQuantumMan Jul 14 '15 at 13:10
• Possible duplicates: physics.stackexchange.com/q/21721/2451 and links therein. – Qmechanic Jul 14 '15 at 13:15
• The question and title are clear to me. We have evidence that the distance between objects increases because the nature of space is changing - space is expanding. This is too small to measure in a room, but shows up for distant galaxies. You are asking if similar effects are present for fundamental constants like the speed of light. What evidence is there? – mmesser314 Jul 14 '15 at 13:16