Biologists have an answer to that question. The egg came first, hundreds of millions of years before the chicken, but physics has a different. They both came first, and at the same time. That's because in quantum physics, cause-and-effect is not always as straightforward as one event causing another. Quantum mechanics weirdness is not something we can experience in real life. In real life you walk to you car and drive to work. Then you walk from work to your car and drive home. But in quantum mechanics means that events can happen without a set order. In a recent experiment, both events can happen first.
It's called `indefinite causal order’ and using a setup called a photonic quantum switch they were able to show that the order of events — transformations on the shape of light — depends on polarization.
“By measuring the polarisation of the photons at the output of the quantum switch, we were able to show the order of transformations on the shape of light was not set,” said Dr. Fabio Costa from the University of Queensland. “This is just a first proof of principle, but on a larger scale indefinite causal order can have real practical applications, like making computers more efficient or improving communication.”
Citation: K. Goswami, C. Giarmatzi, M. Kewming, F. Costa, C. Branciard, J. Romero, and A. G. White, Indefinite Causal Order in a Quantum Switch, Phys. Rev. Lett. 121, 090503 Published 31 August 2018 DOI: 10.1103/PhysRevLett.121.090503