Imagine looking down into a huge sports stadium, with its green playing field surrounded by empty stands and all seats available to spectators. Now add a human being standing in the middle of the field, wondering which available seat to take, since s/he is the first person there.
Your job is to predict which seat the spectator will choose.
If you imagined the Saltlake Stadium in India, you would have 120.000 possible seats to choose from, or if you’re thinking smaller, like the Estadio Azteca in Mexico, you would only have 105.000 seats to evaluate.
Still, as you can see, it can be very hard to predict exactly which seat the spectator will select. Instead, let’s consider what’s most likely or probable (in physicists terms).
For instance, it’s more likely that the spectator will select one of the longer sides instead of the shorter ones (shorter walk, better view of the whole field). Depending on the sun, it’s more likely that the spectator selects the shadowy side for a better view and less heat. I also think it’s more likely that the spectator wouldn’t walk all the way to the top, instead staying close to the field, perhaps a row or four up for a better view, etc.
All this is more likely than not, but none of it more certain than other options, since we don’t know the spectators preferences (unless you can read people’s minds, which I cannot).
Quantum physics is about probability, rather than certainty: You can calculate with great certainty where a cannonball lands depending on amount of gunpowder and angle of the cannon (classical physics), but you cannot calculate with certainty where the spectator sits down eventually (I will soon come to why this spectator example is quantum physics.)
As soon as the spectator makes a choice and starts to walk, we will immediately see in which direction, and by studying where s/he is looking, we will know something about how high up in the stands s/he ends up. This is classical physics, case closed.
Now lets enter quantum physics: Before the spectator begins to walk, we place a huge tent over the playing field, so all we can see is the tent and the stands that now surrounds the tent. We cannot see the spectator any longer, however, we still know that s/he is under the tent, ready to start walking in some direction.
The next thing that happens, is the strangest, weirdest, most bizarre phenomenon in quantum physics (if not the most bizarre thing in the universe):
As soon as the spectator starts to walk, s/he is all of a sudden visible in all the available seats in the stands! So if you chose the Saltlake Stadium in India, you now see 120.000 identical copies of the spectator filling every available seat in the stadium. They don’t even look a little transparent (like ghosts), they are all real. In fact, so much so that if the 120.000 spectator copies stood up on by one and started a wave sweeping around the stadium (they call it a probability wave in quantum physics), this wave would affect physical objects, like turbulent air affect leaves, hot-dog paper, napkins, etc.
If we suddenly tore away the covering tent, all the copies in the stands would disappear, and we would only see the original spectator walk in a certain direction, perhaps looking at a specific empty seat. Quickly add the tent again, and the 120.000 copies immediately fill the stands, creating another wave (easy with the beer, please!)
In the real world this is just silly of course, but in the world of quantum physics, this is how the photon –the light particle– works its magic. The “Stadium” story is my interpretation of the well known “double-slit” experiment quantum physicists did on light, to decide if photons are indeed particles, or if they are waves.
Part II will explain the double-slit experiment more closely (in this, hopefully, understandable way), but let me emphasize again: at the quantum level of reality, nature is spooky, real objects can be at many places at the same time, and all the identical copies can play with ‘itself’… It is said that if quantum physics doesn’t shake you up, you haven’t understood it!
Stay tuned, Part II will shake you up!