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Why does time change when you travel close to the speed of light? – Timothy, age 11, Shoreview, Minnesota
Imagine driving across the country in a car and looking at the scenery. A distant tree moves closer to your car, passes close to you, and then recedes into the distance behind you.
Of course you know that tree doesn’t actually get up and walk towards you or away from you. It’s you in the car going to the tree. The tree only moves in comparison to you, or relatively, that’s what we physicists phone call relativity. If you had a friend standing by the tree, they would see you approaching them at the same speed as you see them moving towards you.
In his 1632 book “Dialogue about the two main world systems”, the astronomer Galileo Galilei first described the relativity principle – the idea that the universe should behave the same way at all times, even if two people perceive an event differently because one is moving relative to the other.
If you are in a car and throw a ball into the air, the physical laws acting on it, such as gravity, must be the same as those on an observer watching from the side of the road. However, while you see the ball moving up and down, someone on the side of the road will see it moving both up and down.
Special relativity and the speed of light
Albert Einstein much later came up with the idea of what is now known as special relativity to explain some confusing observations that had no intuitive explanation at the time. Einstein used the work of many physicists and astronomers in the late 19th century to put together his theory in 1905, starting with two key ingredients: the principle of relativity and the strange observation that the speed of light is the same for every observer and that nothing can move faster. Anyone measuring the speed of light will get the same result no matter where they are or how fast they are moving.
Let’s say you are in the car going 60 miles per hour and your friend is standing by the tree. When they throw a ball at you at a speed of what they perceive as 60 miles per hour, you might logically think that you would see your friend and the tree moving towards you at 60 miles per hour and the ball towards you would move at 120 miles per hour. While that’s very close to the correct value, it’s actually a bit wrong.
This discrepancy between what you would expect by adding the two numbers together and the true answer widens as one or both of you get closer to the speed of light. If you were traveling in a rocket moving at 75% of the speed of light and your friend threw the ball at the same speed, you wouldn’t see the ball moving towards you at 150% of the speed of light. This is because nothing can move faster than light – the ball still appears to be moving towards you at less than the speed of light. Although this all seems very strange, it is much experimental evidence to support these observations.
Time dilation and the twin paradox
Speed isn’t the only factor that changes relative to who’s making the observation. Another consequence of relativity is the concept of time dilationwhere people measure different amounts of time depending on how fast they move relative to each other.
Each person experiences time normally in relation to themselves. But the person who moves faster experiences less time passing for them than the person who moves more slowly. It’s only when they reconnect and compare their watches that they realize that one watch says less time has passed while the other says more.
This leads to one of the strangest results of relativity: the twin paradox, who says that if one of the twins takes a trip to space on a high-speed rocket, they will return to Earth only to find that their twin has aged faster than them. It’s important to note that time behaves “normally” as perceived by each twin (exactly as you now experience time), even if their measurements disagree.
You might be wondering, if each twin sees themselves as standing still and the other as moving towards them, wouldn’t they measure each other as aging faster? The answer is no, because they can’t both be older than the other twin.
The twin on the spaceship not only moves at a certain speed keeping the frame of reference the same, but also accelerates compared to the twin on Earth. Unlike velocities which are relative to the observer, accelerations are absolute. When you step on a scale, the weight you measure is actually your acceleration due to gravity. This measurement remains the same regardless of the speed at which the Earth moves through the solar system, whether the solar system moves through the galaxy, or the galaxy moves through the universe.
Neither twin experiences anything strange with their watches as either of them gets closer to the speed of light – they both experience time as normally as you or I do. Only when they meet and compare their observations will they see a difference – a difference perfectly defined by the mathematics of relativity.
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