"'Time is what prevents everything happening at once.' Physicist John Wheeler’s statement is as fair a summary of what time does as any other – especially given that our hunt for the most basic ingredients of reality has left us a trifle muddled about its status. One culprit was Einstein, whose theory of general relativity merged time with space. But even before him, out understanding of the laws of physics worked the same regardless of whether you travel forwards or backwards in time. That just doesn’t tally with our experience. So what is time really? Here are five of our best ideas ..."
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