system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The Double-Slit Photon Experiment

In the wise words of Bender, " Sweet photons. I don't know if you're waves or particles, but you go down smooth."

Please help me understand why the results of this experiment were so counter what was predicted, and why the results impact our view of physics?
gpt
Briefly, in the early 20th century, people like Rutherford, Planck and Einstein had competing theories as to whether light was fundamentally a particle or a wave. Thomas Young had performed the double slit experiment by showing that light that passed through two slits resulted in an interference pattern on the detector screen. This is analogous to dropping two stones in a perfectly calm lake. Waves will recede from each stone's landing spot, until the waves collide with each other. Wave crests will collide with other crests, causing supercrests, and troughs will collide with troughs, creating supertroughs (as long as the waves are in phase, which they would be in you dropped the stones at the same time). This pattern of supercrests and supertroughs is called an "interference pattern".  When Young saw an interference pattern on the detector screen, he declared that light behaved in the exact same way as water waves do, and therefore, light is fundamentally a wave.

However, Max Planck had shown that whether light was a wave or not, it existed in discrete packets called quanta. Like a case of beer is divided into 24 beer-sized quanta, you can't have a case of 24.6 beers.

So they were able to repeat the double slit experiment but this time they fired individual quanta of light through the slits, without looking to see which slit the quanta went through.  They observed little dots on the screen, representing each quanta of light.... so... particle?  Except when they kept firing quanta of light through the slits, the individual dots accumulated to form the same interference pattern that Young saw.  This was extremely counterintuitive, because it doesn't seem possible that individual quanta of light could produce such a pattern. How could it? This result suggested that the individual quanta of light were interfering with *themselves*, and therefore must pass through both slits at the same time.

So they decided to add a detector at one of the slits and see which slit the light is going through. To their amazement, when they did this, the interference pattern disappeared, and light clearly passed through one slit or the other, and just showed up on the detector as individual dots with no pattern. So... what?!?

They removed the detector and sure enough, the interference pattern returned.  In conclusion, light appeared to behave as a wave, even individual quanta of light, since it appears to pass through both slits simultaneously, which is necessary for the appearance of an interference pattern.  When you measure which slit the light when through, light appears to behave as a particle, and just flies through one slit or the other, but not both.  
**The act of observing the experiment changed the result**.  So light can be described successfully as both a particle **and** a wave.  As it turns out, all matter can be described this way, not just light. This was a tipping point for a new understanding of the universe through quantum mechanics, which is a whole different story.

**TL;DR Light is a wave, unless you look at it like a particle, then it's a particle, but also it's a wave. Simple.**