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.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.We know why because of two landmark theories by Albert Einstein: Special Theory of Relativity (1905) and the General Theory of Relativity (1926). Here, the two work in opposite ways.Special Relativity dictates that at the high speed of the satellites – over 14,000 km/hour – their clocks ought to slow down about 7 microseconds per day.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.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 – exactly as Einstein predicted.The speed limit of the UniverseIn 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.Story continues below this adAlso Read | Galactic cosmic rays: Invisible rain that never stops fallingYet 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.The starting point of special relativity is deceptively simple: the speed of light is constant. 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.That’s 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?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.As Einstein later put it, “The distinction between the past, present and future is only a stubbornly persistent illusion.” (Letter to Michele Besso, 1955).Time that slows downOne 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’s theory predicted. The differences were tiny, on the order of a few billionths of a second, but measurable.Story continues below this adAlso Read | 13.8 billion years and counting: How we measured the Universe’s ageTime, 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’s navigation system would drift by kilometers each day.Energy and Mass: Two sides of the same coinSpecial relativity also gave us one of the most famous equations in science: E = mc^2This 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.As physicist Richard Feynman once said, “It is important to realize that in physics today, we have no knowledge of what energy is.” (The Feynman Lectures on Physics, 1964). Relativity doesn’t just give us formulas – it reshapes our understanding of fundamental concepts like energy itself.Story continues below this adA general form of relativityIn 1915 Einstein developed the General Theory of Relativity, which extends the idea of Special Relativity to explain gravity. Gravity is no longer seen as a force acting at a distance but is rather the curvature in spacetime caused by objects with mass. To understand what it would be like, imagine spacetime as a stretched blanket, and heavenly bodies as tennis balls placed on it.Also Reads | The universe is full of Super-Earths. What makes them specialOne of the most striking consequences of this theory is that gravity affects the passage of time. Clocks in stronger gravitational fields tick more slowly than those in weaker ones. This means that a clock on Earth runs slightly slower than a clock on a satellite orbiting high above it. The difference is extremely small, measured in microseconds, but is real and measurable.Living in a relativistic UniverseTaken together, Einstein’s two theories tell a remarkable story. Special relativity shows that motion reshapes time, while general relativity reveals that gravity does too, bending spacetime itself and subtly altering the ticking of clocks. The same physics that governs black holes and the expansion of the universe is also at work in the satellites above us, quietly adjusting their clocks so that our maps remain accurate.What began as an abstract attempt to understand light has become a framework for understanding everything from the cosmos to the most ordinary journeys we make. Every time we find our way using GPS, we are not just navigating Earth – we are, in a small but profound way, navigating Einstein’s universe.Story continues below this adShravan Hanasoge is an astrophysicist at the Tata Institute of Fundamental Research.