The double slit experiment shows that matter is capable of behaving as both a particle and a wave based on how it is observed.

In the experiment, light-a stream of photons-is directed at 2 parallel slits with a photographic film behind the passage to record how the photons pass through the slits. Researchers found that when they placed photon detectors beside each slit to record them as they passed through one or the other, they behaved as particles and did just that, picked one or the other and left a parallel interference pattern on the film past the slits. However, when the photon detectors were removed and the only observed variable was the photographic film, a different pattern emerged featuring multiple vertical, parallel lines suggesting that the photons passed through the slits simultaneously as a wave rather than picking one or the other and created an intersecting wave interference pattern on the receiving end.

The experiment has been rigorously repeated since its inception in 1927 but the key takeaway remains consistent in that the photon or other form of matter involved only behaves as a particle when being observed. Over the years, the terminology for this phenomenon has been referred to as wave function collapse based on observation and the idea that matter and consciousness are interwoven has become more and more accepted in mainstream science.

One of the greatest physicists of this time, John Archibald Wheeler, proposed a thought experiment in regards to whether or not the effect of conscious observation on matter is scalable. Moreover, whether our conscious observations can not just affect matter, but time, more specifically the past. Decisions and actions we make in the present are already known to obviously have an effect on the future. Kick a rock on the ground and you’ve displaced it’s future position in space and where it will reside in the future had you not intervened.

In regards to altering the past, take a quasar - a quasar is a remote and super-massive young galaxy that emits a great deal of light among other energies. This quasar is displaced center parallel to 2 galaxies with large enough gravitational pull to draw light from the quasar towards one or the other. In the case of this thought experiment, the galaxies will act as the slits on the small scale experiment. Now a team of earth’s astronomers have decided to observe the space surrounding the thought experiment and record how light from the quasar bends around the 2 galaxies. Based on the established status quo from the double slit experiment, if scientists were able to somehow record the individual photons of light that made their way to earth to strike a photographic film on a satellite without any prior observation, the emerging pattern would show that the individual protons that made it back to be observed journeyed both paths back towards earth simultaneously to strike the astronomer’s hypothetical receiving end recording device.

Where the thought experiment gets interesting is when our hypothetical astronomers look through a telescope or other device to observe the quasar, light years away and then record the arriving photons at the receiving end showing a pattern of them choosing a path around one galaxy or the other as particles rather than simultaneously as waves. The nearest quasar to earth is IC 2497 at 730 million light years distance from earth meaning that the light and image we are observing is the quasar’s properties and image 730 million years in the past. Furthermore, the photons our hypothetical astronomers are recording as they reach earth had to travel 730 million years to reach the endpoint in the experiment, the ultimate conclusion here being that observations made towards a setting 730 million years in the past have direct correlation and effect on events recorded in the present. In other words, our present observations dictate how matter behaves 730 million years in the past in this scenario.

The premise behind this thought experiment is called retrocausality. Retrocausality doesn’t imply direct communication or “signalling” to the past rather that like many aspects of quantum mechanics, the way that researchers or more simply “observers” choose to set up an experiment affects the properties and approximations we are able to make at the quantum state. The implications of which are mind boggling enough not taking into consideration the conjecture we can assume from Wheeler’s thought experiment that we aren’t just affecting matter in the present but in the past as well.

For some time wheeler’s thought experiment on the effects of observation and measurement on matter in regards to time was just that, a thought experiment but with the University of Maryland leading the charge in 1984, there have been numerous experiments since then proving an observer’s influence on matter’s properties in the past. Take for example a recent experiment performed in 2012 by a team at the University of Bristol in which they lead a single photon through an interferometer and into a beam splitter which prompts the photon into a superposition of 2 states. A phase shifter is then elicited in order to modify the relative phase between the 2 states. Both states are finally reunified in a second beam splitter which unlike in standard experiments where it is controlled by the experimenter, it is controlled by a support photon which determines whether it is to be an active variable in the experiment at all based on the photon’s state leaving its role in the system in a superposition as well which entangles the support photon with the system in its entirety. The goal in doing this being to demonstrate genuine quantum characteristics within the frame of the experiment utilizing a quantum choice matrix rather than a human observer.

Figure 3 below from the study shows a basic diagram outlining the difference between an observer’s choice setup and a quantum choice matrix.

While there is yet to be a larger scale experiment along the lines of what’s been outlined above, mainly due to technological limitations, the base premise of an observer’s effect on properties of matter in the past has certainly been sufficiently demonstrated to warrant thought on the the large scale implications that these smaller scale findings imply.

“The universe does not exist “out there,” independent of us. We are inescapably involved in bringing about that which appears to be happening. We are not only observers. We are participators. In some strange sense, this is a participatory universe. Physics is no longer satisfied with insights only into particles, fields of force, into geometry, or even into time and space. Today we demand of physics some understanding of existence itself.” -John Archibald Wheeler

Sources:

http://discovermagazine.com/2002/jun/featuniversehttps://phys.org/news/2017-07-physicists-retrocausal-quantum-theory-future.html

http://science.sciencemag.org/content/338/6107/634