Have you ever wondered whether events in the past still exist? There are two schools of thought. One point of view, made possible by Einstein’s famous theory of relativity, is that time is just another coordinate mathematically, no different from the familiar three of regular space. The universe is like a giant block of bread. Every slice of the block is a moment, a "now." But the whole block, not just the slice, conceivably though not necessarily exists.
The present then is like a thread weaving through the stack of slices. But the other slices (the past and future) don’t perforce stop existing —they’re just inaccessible to us for some reason, possibly because going back in time would retroactively allow someone from the future to alter the past in such a way that it changes the future, making it impossible to change the past to begin with. This contradiction causes what is called the Grandfather Paradox.
The other point of view, which seems consistent with everyday experience, is what’s been called “presentism,” the idea that the present is all that exists. The Buddha is supposed to have said, “The past is already gone, the future is not yet here. There's only one moment for you to live, and that is the present moment." That moment moves through the ever-advancing now and immediately flows into the past, which has no more reality than memory. This was, for many, the intuitive view and consistent with Newtonian physics.
Then, Einstein came along, and presentism came under attack. From the perspective of relativity, the present is not supremely privileged. Relativity treats time as part of a four-dimensional spacetime. The events from earlier in time don’t vanish when you’re further on in the movie; that’s just not where the play head is. The theory is that if you could find the right technology, somehow travel faster than light, or manipulate spacetime, you might access those past events.
But that’s in theory, right? Or in science fiction. Could it be true in the real world? Could it have significance outside of laboratory settings? According to the theory of relativity, the concept of a "simultaneous now" is not absolute; it depends on the observer's frame of reference, meaning that what one person perceives as happening at the same time as another event might not be simultaneous for someone moving relative to them—this is known as the "relativity of simultaneity."
Could there be many nows, not just one universal one? One engineer who had to know for sure was Bradford Parkinson, the chief architect for GPS, who led the original advocacy for the system in 1973 as an Air Force Colonel. GPS would rely on a network of satellites orbiting Earth 20,200 km (12,550 miles) up. Each satellite has an atomic clock ticking away, broadcasting precise time signals. GPS receivers would pick up these signals from at least four satellites, calculate how long they took to arrive, and use that data to triangulate its position. Since light travels at 299,792 km/s, even a tiny timing error—like a microsecond—translates to hundreds of meters off position on the ground.
The satellites would zip around Earth at about 14,000 km/h (3.9 km/s). According to Einstein’s special relativity, a clock moving relative to an observer ticks slower—a phenomenon called time dilation. Compared to a clock on Earth’s surface, the satellite clocks would "lose" time because of their speed by about 7 microseconds per day. On the other hand, general relativity held that clocks in weaker gravitational fields (farther from Earth’s mass) tick faster than those in stronger fields (like at sea level). The satellites high up where gravity’s pull is weaker would see their clocks run faster than clocks on the ground by 45 microseconds per day.
Satellite clocks would, in the net, gain about 38 microseconds per day compared to Earth clocks if their nows were different. To settle the issue experimentally, Gravity Probe B, of which Parkinson was co-chief principal investigator, was launched in April 2004 ‘to test two previously-unverified predictions of general relativity: the geodetic effect and frame-dragging. This was to be accomplished by measuring, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth-orbiting satellite at 650 km (400 mi) of altitude, crossing directly over the poles.”
The experiment showed Einstein’s predictions were correct. To fix the error, GPS satellite clocks are set to run slower than terrestrial ones. Albert Einstein once remarked, “The Universe is stranger than we imagine!” To which Heisenberg added, “Not only is the Universe stranger than we think, it is stranger than we can think.” One might think that such questions are only of academic interest. Yet your GPS works because it accounts for the fact that there is no single now.
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