Quantum Double Slit Experiment
The double slit experiment is one of the most well known in modern physics. It supports wave-particle duality, which is a concept in quantum mechanics that every particle is also described partly in terms of waves.
So if a wave passes through a parallel double slit, whether it’s water, sound or light, an interference pattern will be observed. A modified version of the double slit experiment is set up to fire a single photon through a double slit such that the scientists don’t know which slit it travelled through. When the effect against the backdrop is measured, an interference pattern can be observed suggesting the photon travelled through both slits as a wave. However, as soon as they place a detector to determine which slit the photon travels through, the interference pattern disappears, and a splatter pattern can be seen against the backdrop. This shows that the photon travelled through one of the slits as a particle, and not as a waveform. So somehow by the act of observing a particle, we change it.
A modified version of the double slit experiment called the delayed choice experiment, has results that just beg more questions. At each slit they place a crystal which splits incoming photons into identical pairs. One photon from this pair will form a standard interference interference pattern and the other one will travel to a detector. Even if the photon that hits the detector is measured after the first photon hits the screen, it still changes whether or not an interference pattern was observed. So not only can we influence particles just by observing them, but those observations can alter what happened in the past.
This experiment has been repeated with molecules as large as Buckminsterfullerenes (60 carbon atoms) with the same results, and there are plans to attempt the same experiment with viruses.