Superphysics
The Delayed-choice Experiment
5 minutes • 995 words
A key quantum principle is that observing a system alters its course.
According to quantum physics, you cannot “just” observe something.
To make an observation, you must interact with the object that you are observing.
For instance, to see an object, we shine a light on it.
Shining a light on a pumpkin will have little effect on it. But shining even a dim light on a tiny quantum particle has an effect.
It changes the results of an experiment in just the way that quantum physics describes.
We send a stream of particles toward the barrier in the double-slit experiment and collect data on the first million particles to get through.
When we plot the number of particles landing at various detection points, the data will form the interference pattern pictured.
When we add the phases associated with all the possible paths from a particle’s starting point A to its detection point B, the probability we calculate of landing at various points agrees with that data.
We repeat the experiment, this time shining lights on the slits so that we know an intermediate point, C, through which the particle passed. (C is the position of either one of the slits or the other.)
This is called “which-path” information because it tells us whether each particle went from A to slit 1 to B, or from A to slit 2 to B.
We now know through which slit each particle passed.
The paths in our sum for that particle will now include only:
- paths that travel through slit 1, or
- only paths that travel through slit 2.
It will never include both:
- the paths that go through slit 1 and
- the paths that pass through slit 2.
This is because Feynman explained the interference pattern by saying that paths that go through one slit interfere with paths that go through the other.
If you turn on a light to determine which slit the particles pass through, thereby eliminating the other option, you will make the interference pattern disappear.
When the experiment was performed, turning on a light changed the results from the interference pattern, to a pattern like that!
Moreover, we can vary the experiment by employing very faint light so that not all of the particles interact with the light.
In that case we are able to obtain which-path information for only some subset of the particles.
If we then divide the data on particle arrivals according to whether or not we obtained which-path information, we find that:
- data pertaining to the subset for which we have no which path information will form an interference pattern
- the subset of data pertaining to the particles for which we do have which-path information will not show interference.
This idea has important implications for our concept of “the past.”*
Superphysics Note
In Newtonian theory, the past is assumed to exist as a definite series of events.
In fact, given complete data about the present, Newton’s laws allow one to calculate a complete picture of the past.
This is consistent with our intuitive understanding that, whether painful or joyful, the world has a definite past.
There may have been no one watching, but the past exists as surely as if you had taken a series of snapshots of it.
But a quantum buckyball cannot be said to have taken a definite path from source to screen.
We might pin down a buckyball’s location by observing it, but in between our observations, it takes all paths. Quantum physics tells us that no matter how thorough our observation of the present, the (unobserved) past, like the future, is indefinite and exists only as a spectrum of possibilities.
The universe, according to quantum physics, has no single past, or history.
The fact that the past takes no definite form means that observations you make on a system in the present affect its past.
That is underlined rather dramatically by a type of experiment thought up by physicist John Wheeler, called a delayed-choice experiment.
Schematically, a delayed-choice experiment is like the double-slit experiment.
In it, you can observe the path that the particle takes, but you postpone your decision about whether or not to observe the path until just before the particle hits the detection screen.
Delayed-choice experiments result in data identical to those we get when we choose to observe (or not observe) the which-path information by watching the slits themselves.
But in this case the path each particle takes—that is, its past—is determined long after it passed through the slits and presumably had to “decide” whether to travel through just one slit, which does not produce interference, or both slits, which does.
Wheeler even considered a cosmic version of the experiment, in which the particles involved are photons emitted by powerful quasars billions of light-years away. Such light could be split into 2 paths and refocused toward earth by the gravitational lensing of an intervening galaxy.
Though the experiment is beyond the reach of current technology, if we could collect enough photons from this light, they ought to form an interference pattern. Yet if we place a device to measure which-path information shortly before detection, that pattern should disappear.
The choice whether to take one or both paths in this case would have been made billions of years ago, before the earth or perhaps even our sun was formed, and yet with our observation in the laboratory we will be affecting that choice.
Like a particle, the universe doesn’t have just a single history, but every possible history. Each history has its own probability.
Our observations of its current state affect its past and determine the different histories of the universe, just as the observations of the particles in the double-slit experiment affect the particles’ past.
