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\u201cTime flies,\u201d \u201ctime waits for no one,\u201d \u201cas time goes on\u201d: The way we speak about time tends to strongly imply that the passage of time is some sort of real process that happens out there in the world. We inhabit the present moment and move through time, even as events come and go, fading into the past.<\/p>\n
But go ahead and try to actually verbalize just what is meant by the flow or passage of time. A flow of what? Rivers flow because water is in motion. What does it mean to say that time flows?<\/p>\n
Events are more like happenings than things<\/a>, yet we talk as though they have ever-changing locations in the future, present or past. But if some events are future, and moving toward you, and some past, moving away, then where are they? The future and past don\u2019t seem to have any physical location.<\/p>\n Human beings have been thinking about time for as long as we have records of humans thinking about anything at all. The concept of time inescapably permeates every single thought you have about yourself and the world around you. That\u2019s why,\u00a0as a philosopher<\/a>, philosophical and scientific developments in our understanding of time have always seemed especially important to me.<\/p>\n Ancient philosophers were very suspicious about the whole idea of time and change. Parmenides of Elea was a Greek philosopher of the sixth to fifth centuries BCE.\u00a0Parmenides wondered<\/a>, if the future is not yet and the past is not anymore, how could events pass from future to present to past?<\/p>\n He reasoned that, if the future is real, then it is real now; and, if what is real now is only what is present, the future is not real. So, if the future is not real, then the occurrence of any present event is a case of something inexplicably coming from nothing.<\/p>\n Parmenides wasn\u2019t the only skeptic about time. Similar reasoning regarding contradictions inherent in the way we talk about time appears in\u00a0Aristotle<\/a>, in the ancient Hindu school known as the\u00a0Advaita Vedanta<\/a>\u00a0and in the work of\u00a0Augustine of Hippo<\/a>, also known as St. Augustine, just to name a few.<\/p>\n The early modern physicist Isaac Newton\u00a0had presumed<\/a>\u00a0an unperceived yet real flow of time. To Newton, time is a dynamic physical phenomenon that exists in the background, a regular, ticking universe-clock in terms of which one can objectively describe all motions and accelerations.<\/p>\n Then, Albert Einstein came along.<\/p>\n In 1905 and 1915, Einstein proposed his\u00a0special<\/a>\u00a0and\u00a0general theories of relativity<\/a>, respectively. These theories validated all those long-running suspicions about the very concept of time and change.<\/p>\n Relativity rejects Newton\u2019s notion about time as a universal physical phenomenon.<\/p>\n By Einstein\u2019s era, researchers\u00a0had shown<\/a>\u00a0that the speed of light is a constant, regardless of the velocity of the source. To take this fact seriously, he argued, is to take all object velocities to be relative.<\/p>\n Nothing is ever really at rest or really in motion; it all depends on your \u201cframe of reference<\/a>.\u201d A frame of reference determines the spatial and temporal coordinates a given observer will assign to objects and events, on the assumption that he or she is at rest relative to everything else.<\/p>\n Someone floating in space sees a spaceship going by to the right. But the universe itself is completely neutral on whether the observer is at rest and the ship is moving to the right, or if the ship is at rest with the observer moving to the left.<\/p>\n This notion affects our understanding of what clocks actually do. Because the speed of light is a constant, two observers moving relative to each other will assign different times to different events.<\/p>\n In a famous example, two equidistant lightning strikes occur simultaneously for an observer at a train station who can see both at once. An observer on the train, moving toward one lightning strike and away from the other,\u00a0will assign different times<\/a>\u00a0to the strikes. This is because one observer is moving away from the light coming from one strike and toward the light coming from the other. The other observer is stationary relative to the lightning strikes, so the respective light from each reaches him at the same time. Neither is right or wrong.Ancient philosophers on time<\/strong><\/h4>\n
Einstein and relativity<\/strong><\/h4>\n
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