Say you were a spy, and you think there’s a laser guarding the largest diamond in the world.

There’s no way to detect if the laser is there without putting some form of matter in the way of the laser. Be it a hand, or spray bottle.

Now at the museum, a mist won’t set off the alarm, but you’re still reflecting some of the photons out of the beam and into your eyes. Otherwise you wouldn’t see the beam…

But for a quantum experiment, where we care about each individual photon, spraying a fine mist will affect the experiment. Sticking anything in that laser beam path will affect the measurement.

So how do you make a measurement without interacting?