However, the farther apart two objects are, the more space there is between them. So they built LIGO to have tubes that are two and a half miles long. (Can you imagine that? In Manhattan, this building wouldn't even fit east to west!)
Even that far apart, the biggest gravitational waves we might see would still only change the length of the tube by less than the width of a proton (which is a tiny, tiny part of an atom). But using lasers, scientists were able to set up a system that could detect even so small a change in length.
Actually, LIGO wasn't just one observatory, it was two, one in Washington and one in Louisiana. They had to build two of these huge things because with just one, if something shakes the building (like a storm or a small earthquake or a tree falling), the detector might get a false reading. But with two, you can check to see if they both got the same readings, and know for sure that it was something from outside Earth.
On September 14, 2015, both detectors picked up a signal. Scientists rushed to examine them. Did they look like signals from two objects in space colliding, which might make a strong enough signal? Yes. Could it be an accident, like a tree falling? They did a lot of probability, and confirmed that the chance of both detectors getting the same signal at the same time was too small to be coincidence. It was real.