In Einstein's theory of special relativity, time is not constant, and can increase or decrease depending on your speed. Click here to learn about the Twin Paradox and how time works!
Introduction
Did you know that if your twin travels into space and returns to the Earth at the speed of light, he will be younger than you? How could that be possible? Well, his phenomenon exists because Einstein’s special theory of relativity shows that time dilates if an object moves at the speed of light. Scientists call this the “Twin Paradox”.
The “Twin Paradox”
Let's assume that two people called Jane and Joe are twins. If Jane travels to a distant star at the speed of light while Joe stays on a velocity-constant Earth, Joe will observe that his clock runs faster than Jane’s. As a result, when they reunite, Joe will be older than Jane. However, in Einstein’s theory, all moving things are relative. If we take the perspective of Jane as an observer while the rest of the universe, including Joe, moves around her, Jane will feel as if her clock ticks faster and that therefore, she will be older than Joe. Who is correct?
To analyze this problem, we need to calculate how long the journey takes for each twin. Jane will travel to the star that is 2.67 light years away from the Earth. Jane’s spaceship will travel at 66% the speed of light. So, for Joe, the journey will take (2.67 light years / 0.66 c *) = 4 years in each direction. As for Jane, the journey length is shorter by the Lorenz factor. At 66% of the speed of light, the Lorenz factor is 1.33. By taking the inverse of that factor and multiplying it by the distance of her journey, we will get 0.75 × 2.67 = 2 light years. Now, we need to divide the distance of 2 light-years with the speed at which she is traveling (0.66 c). We get 3 years on each leg of the journey.
To make it more clear, Jane and Joe decide to send a signal to the other twin every time a year has passed for them. When one twin observes this “signal”, they will know how long it takes for the other twin to pass a year.
Figure 2: A diagram of the twin paradox
It is now clear that Joe will be older than Jane when they reunite. The issue about the twin paradox is that the situations of the twins are definitely not the same. Both Jane and Joe cannot be observers. To qualify as an observer, one has to maintain a constant speed and direction relative to the rest of the universe. Joe was at rest the entire time, so his velocity was a constant zero. However, Jane changes direction during the return journey. Jane enters a different inertial frame during the return journey. Joe is in one inertial frame throughout, but not Jane.
As a result, Jane takes less time for the journey because she experiences time dilation when she accelerates.
Personal Opinion
Different scientists have different methods of explaining the twin paradox. Some methods seem to give the most accurate explanation, but they are also very confusing for people. I hope that this is the easiest way to explain the twin paradox for those who do not know about the theory of relativity!
*“c” stands for the speed of light constant
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