Physicists have confirmed the ultimate speed limit for the packets of light called photons, which could make time travel even less likely than thought. The speed of light in a vacuum is the Universe's ultimate speed limit, but experiments in recent years suggested that single photons might beat it -and if this was proven, theory would allow for the prospect of time travel. Now, a recent paper in Physical Review Letters shows that individual photons too are limited to the vacuum speed limit. This means that photons maintain the principle of causality laid out in Einstein's theory of special relativity -that is, an event's effect cannot precede its cause by traveling faster than light.
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