Time and the Quantum
This article is part of a series of articles on time. This is part 4/5. Read the previous part here!
Over the last week, we have seen how time behaves, and it’s symmetry, and how an arrow of time exists, and in doing so, we have not messed with any laws that hail time as symmetrical. Now, lets turn our view into another world, where time behaves differently. Lets see how time behaves with the Quantum.
Double Slit Experiment
The experiment is simple, and is perhaps Quantum Mechanics’ most frequently used experiment. Basically, you get a laser, one which can fire one photon at a time, and can be readjusted to fire millions and billions at a time, a cardboard with two slits, and a detector screen. Turn the setting to fire multiple photons at a time. Let them pass through the cardboard obstacle and see the pattern on the screen. You won’t be amazed to see the area of those two slits light up, while the rest of the area is dark.
Now change the setting to one photon at a time. You see something strange, a different pattern. How can it be so? (This can also be done with an electron gun.)
Quantum and Probability
To many of you, it should not come out as a surprise that in the quantum world, probability reigns supreme. Since we are dealing with particles that make the size of the dot at the end of the last sentence look titanic, it should be no surprise that probability is the starting point of any quantum measurement. When a single photon is fired, it can either go through the slit on the left, or the slit on the right or get blocked by the cardboard. Thus, the probability of the photon reaching the screen is 2/3. But, this is not a great probability, since the area of the slits is smaller than the area of the cardboard. Also, the interference pattern that you see on the screen cannot be explained by thinking of some sort of wave, as there are band of light, and darkness all over the interference. But what wave?
In 1927, Max Born put forward an idea that is considered to be one of the most important ideas in Quantum mechanics, along with Schrodinger’s equation, Feynman’s proposal(which we will see next) and Bell’s theorem. He said that the wave put forward is a probability wave, and it has crests or peaks(the lighter bands) and troughs(the darker band). But why do these waves give us this pattern? This is where a brilliant mind comes up with an idea, that not only explains the interference, but also the past according to Quantum.
The past according to the Quantum
Richard Feynman, in the early 1960s, showed that probability, again comes to play. He proposed that, unlike the millions and millions of photons, a single photon can have infinitely many histories, infinite pasts. And the average of all these infinite histories is what you see on the screen. This approach, that Feynman proposed, is called the sum over histories approach. But how does this work? Lets see.
Consider a particle emitter A emitting photons which are to be detected at B, a detector. Lets say, for the sake of simplicity, that they are 10 meters apart. Then there are many ways the photon could travel from A to B. It could take the shortest distance possible, and go straight covering 10 meters from A to B. Or it could go to the right, and turn to the left by an angle to reach B, or go the left and turn by an angle to the right to reach B. It could go way past past B, make a “U-turn” and get detected. There are many ways, many histories of the single photon, as shown below. And the place where all these paths, histories meet and agree to each other, where they all average, is the place where the Photon is detected, and since in this case they meet at B, the photons are detected at B.
A similar phenomenon takes place in our double slit experiment. There are many histories that a particle can have, such as go by the top right hand corner of the right hand slit, or go by the bottom left corner on the left hand slit, and anywhere in between, and the place where all these waves of probability, where all these histories meet, is the lighter bands, and where they meet and cancel each other out, is where the darker bands are.
The past according to quantum, is what quantum always has been, a misty fog of uncertainty, and you can only check the position of the particle in the present. This foggy mist, as shown by Feynman, is the result of many probability waves and multiple histories’ intermingling.
But what according to the quantum is the future, and can you erase past? Or is the past dependent on the future? Well, find out in the final article of this series. Coming up- The Quantum Eraser and the Delayed Choice.