So the ligo detector detects ripples in spacetime. If a gravitational wave passes the detector, one of it's arms is being slightly 'stretched out', wich causes a beam of light to be detected a very tiny bit later than would normally be the case.

So my question is: if speed is the distance crossed in a certain amount of time, and space itself is being stretched out by these ripples....why does the beam of light arrive later?
I mean, if space itself is being stretched, wouldn't that mean that the amount of kilometers per second remains unchanged? I mean, if the amount of space that contains a kilometer of planet earth, is being stretched, shouldn't that mean that a kilometer is still a kilometer?

OK, i realise i'm being a bit vague.
So...people Always explain the curving and bending of spacetime by a big object like a planet, by comparing it with a biljartball on a sheet. The weight of the ball stretches the sheet...you've probably seen it being explained that way before.
So, if the sheet is being marked so a certain amount of it would represent a meter, and that sheet is being stretched....wouldn't that mean that the meter would ALSO being stretched? And if that's so, wouldn't that mean that, say 1 centimeter per second on that sheet, would mean that this stretched centimeter still would be crossed in one second?

imagine going from point A to B in a straight line and then going from A to a non collinear point C then to B, the second path is definitely longer... same with gwaves, the light at first was going in a straight space-time line from A to B(which is the shortest path and it's the only one), now space-time is bent which definitely means the new path is longer so light will take more time to arrive. any other path than the straight line is longer. light speed is constant in all references so yes it would take more time for light to travel a stretched or bent spacetime centimeter than a straight/regular one.

i get what you are trying to say, if they are using a ruler for example to measure distance and the spacetime was stretched the ruler will stretch with it and nothing will be noticed.. if light speed is constant then if it takes more time for light to arrive when a gwave passes then it must mean that the traveled distance was longer, buuut i don't know how they are measuring time, wouldn't time be stretched too when spacetime is stretched? maybe there's a trick with references or something like that to measure time correctly.

nothing scientific really, just my thoughts

Its not really that simple.
They have two laser in a big L shape that have no interference. But the ripples they create interference by moving the mirror in 1 arm slightly away and then towards.
Time is not present in the measurement thats whats important.

So we actually dont measure distance that we were "distorted for" we measure the effect or the "distortion" of the gravitational waves on the entire thing. Thats why the tubes in the LIGO are VEERY long

Why shits wierd explained ^

aaahhhh the interferometer! thanks Ulim

Majestically simple idea applied with incredible precision and engineering for profound connotations.

Wow...that's clever.

I love the part where this guy, i believe his name is rana adhakari, says that 20 years ago, he would have said that what he's doing now is just not possible.