Will the cup break if I drop it?
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If you drop a glass, it breaks. If you flip a light switch, a light bulb lights up. Effect follows cause. This is an absolute law of the universe, but perhaps not at a fundamental level, because causation seems much murkier when you’re dealing with the electrons that power a light switch, or the atoms in a lightbulb that convert electrical energy into light.
In 2017, a team of researchers from the University of Vienna in Austria published an experiment showing that in the quantum world of atoms and particles, it’s impossible to say which observations are effects and which are causes. In the words of the researchers who carried out the experiment, it was “the first conclusive demonstration of a process with indeterminate causation.”
Still, the research community at large didn’t drop its coffee cup. On the contrary, it was welcome news to at least some of those trying to figure out where space-time comes from. For them, a quantum theory of gravity, which holds that space-time is an emergent property of the universe’s more fundamental building blocks, might necessarily lack the clear one-way causality of everyday life.
Space-time, as explained in Albert Einstein’s theory of relativity, is already fraught with ambiguity in defining the order of events. People who move through space and time in different ways have different “frames of reference”, and people who move in different ways are always…
