Every time you use Google Maps to take the right turn while driving, or use GPS to drop your location to a friend, you invoke the two laws of relativity by Albert Einstein.<\/p>\n
This is why: GPS, or global positioning system, depends on satellites that orbit about 20,000 kilometres above Earth at high speeds. The clocks onboard these satellites do not tick at the same rate as clocks on the ground.<\/p>\n
We know why because of two landmark theories by Albert Einstein: Special Theory of Relativity (1905) and the General Theory of Relativity (1915). Here, the two work in opposite directions.<\/p>\n
General Relativity shows that due to the weaker gravity at their altitude the clocks should go faster by about 45 microseconds per day. The net result is that satellite clocks run roughly 38 microseconds faster each day.<\/p>\n
That difference may sound tiny, but without correcting for it, GPS positions would drift by several kilometres within a day. In other words, modern navigation works only because time itself behaves differently depending on motion and gravity \u2013 exactly as Einstein predicted.<\/p>\n
The speed limit of the Universe<\/p>\n
In 1905, Einstein who was then a young patent clerk in Bern, Switzerland, published a paper introducing special relativity, which would forever change how we think about space and time. At first glance, the theory seems almost absurd. It says that time can slow down, lengths can shrink, and two observers moving relative to each other may disagree about whether events happened simultaneously.<\/p>\n
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Yet more than a century later, every one of these strange predictions has been confirmed. Special relativity is not just a theory. It is an excellent descriptor of how the universe actually works. That\u2019s how everyday speeds add up. But light refuses to behave this way. Einstein took this strange experimental fact seriously and asked: what must be true about space and time if the speed of light is always the same?<\/p>\n The answer led to a radical conclusion. Space and time are not fixed backgrounds. They are flexible, and they adjust themselves so that the speed of light remains constant. Time that slows down<\/p>\n One of the most famous consequences of special relativity is time dilation, the idea that moving clocks run slower. In 1971, physicists Joseph Hafele and Richard Keating flew atomic clocks around the world on commercial airplanes. When the clocks returned, they were slightly out of sync with identical clocks that had remained on the ground, exactly as Einstein\u2019s theory predicted. The differences were tiny, on the order of a few billionths of a second, but measurable.<\/p>\n Story continues below this ad<\/p>\n Time, it turned out, really does tick differently depending on motion. This is not just a curiosity. GPS satellites, which orbit Earth at high speeds, must account for relativistic time dilation. Without these corrections, your phone\u2019s navigation system would drift by kilometers each day.<\/p>\n Energy and Mass: Two sides of the same coin<\/p>\n Special relativity also gave us one of the most famous equations in science: E = mc^2<\/p>\n This equation tells us that mass and energy are interchangeable. Even a small amount of mass contains an enormous amount of energy because it is multiplied by the square of the speed of light. This idea underlies both nuclear power and nuclear weapons. It is also what powers the Sun, where tiny amounts of mass are converted into vast amounts of energy through nuclear fusion.<\/p>\n As physicist Richard Feynman once said, \u201cIt is important to realize that in physics today, we have no knowledge of what energy is.\u201d (The Feynman Lectures on Physics, 1964). Relativity doesn\u2019t just give us formulas \u2013 it reshapes our understanding of fundamental concepts like energy itself.<\/p>\n Story continues below this ad<\/p>\n A general form of relativity<\/p>\n
The starting point of special relativity is deceptively simple: the speed of light is constant<\/a>. No matter how fast you move, light always travels at the same speed, about 300,000 kilometers per second. This idea is deeply counterintuitive. For instance, if you are on a train moving forward and you throw a ball, someone standing still will see the ball move faster than you do.<\/p>\n
As Einstein later put it, \u201cThe distinction between the past, present and future is only a stubbornly persistent illusion.\u201d (Letter to Michele Besso, 1955).<\/p>\n