When things move, they create waves.
For example, if you shake a stick back and forth in water: water waves.
Vibrate a piece of metal back and forth really fast: air pressure waves.
Shake some electrons back and forth really fast: radio waves.
And yes, shake a planet or star back and forth really fast: gravitational waves.
Gravitational waves happen because the effects of gravity don't travel outwards at infinite
speed – so if the sun were to suddenly jump a few hundred thousand kilometers to the side,
the changed gravitational field would take time to pulse outwards.
And if the sun shook back and forth and back and forth, instead of a single pulse, you'd
get continuous gravitational waves.
So what's doing the "waving"?
In the case of water, the height of the water increases and decreases at any particular
location as the waves travel past.
In the case of sound, the pressure of the air increases and decreases at any particular
location as the waves travel past.
In the case of radio or cell phone signals or any other electromagnetic waves, the electric
and magnetic fields get stronger and weaker at any particular location as the waves travel
past.
And in the case of gravitational waves, the gravitational field gets slightly stronger
or weaker as the waves travel past.
You can tell a wave has passed by looking at how nearby particles behave – a bobber
on the water rises up and down, the electrons in a radio antenna move back and forth because
of the changing electric field, and free-floating people or planets or cats move back and forth
because of the changing gravitational field – though in this last case, the peculiarities
of gravity mean that the free-floating things actually experiencing the gravitational wave
don't feel like they're moving.
But if you measure the space between them by sending a pulse of laser light and measuring
the time it takes for it to come back, you'll find that the distance between them increases
and decreases.
In practice, physicists don't actually measure gravitational waves with free-floating cats
– they use very very fancy expensive mirrors which are effectively free-floating because
they're hung on pendulums suspended on isolation tables suspended on isolation tables, or which
are _actually_ free-floating because they're attached to satellites floating in space – though
this hasn't been done yet.
The reason physicists need fancy floating mirrors to detect gravitational waves is that
the waves are very, very weak.
For example, the electrons that vibrate back and forth in a radio antenna to make electromagnetic
waves ALSO make gravitational waves; electrons _are_ matter moving back and forth after all.
But the waves they make are super weak: a 200 watt radio transmitter gives off something
like a quadrillionth of a quintillionth of a quintillionth of a quintillionth of that
power as gravitational radiation.
And that's why here on earth we can only detect the biggest baddest astronomical events – like
superfast spinning neutron stars or merging black holes or the big bang.
Though so far, we've only detected black hole collisions.