A follow-up to “The Intergalactic Double Slit Experiment”.
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…
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 two 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.
Stephen Hawking
References:
Stephen Hawking, The Grand Design (New York: Bantam Books, 2010), pp. 82-83.
philosophy and physics 