A TWIST IN TIME
Imagine leaving Earth for a space trip: You are travelling at 90% the speed of light toward a star 10 light-years away. When you return, you find your twin, who was the same age as you when you left, to be significantly older than you. While 22 years had passed on Earth, for you, the journey felt like less than 10 years! How is that possible? If the star was 10 light-years away, then how did the entire journey feel less than 10 years? This isn't science fiction; it is the concept of time dilation and special relativity.
In simple words, the faster an object moves through space, the slower time passes for it relative to a stationary object, and simultaneously, the distance along the direction of motion contracts. Why does this happen? We know that motion is relative. When travelling in a bus, to a person standing on the sidewalk, you would appear to be in motion, but to the other people on the bus travelling at the same speed as you, you would appear to be at rest. We also know that the speed of light is universally constant, which means that it is equal for all regardless of their own speed. It is important to note, however, that both these statements are true only when an object is either at rest or in uniform motion. But this raises a contradiction: if motion is relative, how can the speed of light remain constant regardless of the observer's motion?
Einstein solved this in his Special Theory of Relativity. Einstein showed that for the speed of light to remain constant, time and space must adjust. So if you travel at a speed near the speed of light, time would pass more slowly, and the distance to your destination would consequently contract from your perspective in order to preserve the constancy of the speed of light. These effects are known as time dilation and length contraction. This effect becomes noticeable only at speeds close to the speed of light, which is why we do not observe it in everyday life. It is quantified by the Lorentz Factor, which calculates the effects for an object moving near light speed. For your speed at 90% the speed of light, the Lorentz Factor is approximately 2.3, meaning for every 2.3 days on Earth, 1 day would have gone by for you, and similarly from your perspective the distance to the star itself contracts to about 4.3 light-years, which is why the journey felt shorter.
Yet this difference is not felt by the traveller; for you, the clock ticks as usual, even your biological processes, including aging and the perception of time itself, remain unchanged. This is because all time-related processes slow down uniformly. Hence, to you, everything seems normal. The difference only becomes apparent when you return to Earth and compare your clock with that of someone on Earth. This is why you end up younger than your twin who remained on Earth.
But that is just one perspective. As motion is relative, it would be just as fair to argue that during the journey, you would be stationary while the rest of the universe moves around you. In that case, Earth is the one moving away from you at 90% the speed of light. Time should appear to pass more slowly for people on Earth, suggesting that you would end up being the older twin. This apparent contradiction is known as the Twin Paradox. But if we look closely, it's not really a paradox, just a misconception of special relativity.
During your journey, you must turn around to return to Earth; in doing so, you must accelerate to change your velocity while changing your direction. Hence, you no longer remain in an inertial frame. This breaks the symmetry between the two situations as your twin remains in a non-accelerating frame on Earth while you experience acceleration. Therefore, your situations are not equivalent, and you experience less time due to time dilation as explained earlier. This is why you return younger, resolving the apparent paradox.
You might think time dilation matters only for sci-fi astronauts, but the truth is it also appears every time you open Google Maps! We all know apps like Google Maps are powered by GPS satellites orbiting the Earth at high speeds, so according to special relativity, their internal atomic clocks should tick about 7 microseconds slower per day than clocks on Earth. But that's not all: these satellites are also under the influence of Earth's gravitational force. General Relativity also applies, causing their clocks to tick faster by 45 microseconds every day. The net result leads to them ticking about 38 microseconds faster each day! This may seem small, but if engineers didn't preprogram satellite clocks to slow down according to Earth time, then within one day your GPS location would be off by almost 10 kilometers!
In conclusion, time dilation shows that time is not the same for everyone, but depends on how fast one is moving. While its effects are negligible in our everyday lives, they become significant at extremely high speeds, fundamentally changing our understanding of time itself. The twin paradox, though initially confusing, can be understood by recognising the role of acceleration and different frames of reference. What appears to be a contradiction is actually a powerful confirmation of special relativity, reminding us that the universe often behaves in ways that go beyond our everyday intuition.
Dhruvika Srivastava