TL;DR: quantum mechanics is freaky. In the double-slit experiment, it was shown that un-observed photons behaved like a wave, interfering with itself (top pic), while observed photons acted like particles (bottom pic). The phenomenon is known as wave function collapse.
TL;DR: quantum mechanics is freaky. In the double-slit experiment, it was shown that un-observed photons behaved like a wave, interfering with itself (top pic), while observed photons acted like particles (bottom pic). The phenomenon is known as wave function collapse.