How Everything Works
How Everything Works How Everything Works

Question 706

If time passes more slowly for someone who is moving quickly and enormous speeds are needed to explore distant space, is there any way to counteract this time/speed phenomenon so that those on earth will not die waiting for the "space travelers/explorers" to return? — BC, Ottawa, Canada
Unfortunately, no. Those of us who remained on earth would watch the explorers head off at enormous speeds toward the stars and would be old and gray before they returned. Even if the explorers could move at almost the speed of light, it would take them many years to reach nearby stars and many years to return. Since there is no way that they could travel even as fast as the speed of light, the absolute minimum time it would take for a round trip, from our perspective, would be the round trip distance to the stars divided by the speed of light.

But this brings up one of the peculiar results of special relativity. From our perspective on earth, the explorers are moving quickly as they head toward the stars and their clocks appear to be running slowly to us. But from their perspective, we are moving quickly in the other direction and our clocks appear to be running slowly to them. This apparent paradox is resolved by the fact that the explorers would not agree with us on the ordering of two events occurring at different locations—space and time appear differently to us; they are intermingled. However, when the explorers accelerate in order to turn around and headed back toward us, their perceptions of space and time undergo a radical change. They see our clocks zoom ahead while we continue to see their clocks running fairly slowly. When the explorers finally returned to earth, their clocks indicate that they had been gone only a short time. However our clocks indicate that they had been gone at least as long as the time it would take light to complete the roundtrip. This situation leads to the famous "twin paradox," in which one twin travels through space while the other remains at home. When the explorer twin returns to earth, the explorer twin is still young but the earthbound twin is very old. If near-light-speed travel were to become possible (a very remote possibility), such twin paradoxes would certainly occur.


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