Time Stops at the Speed of Light. What Does that Mean?
Sabine Hossenfelder・7 minutes read
Einstein's theories of special and general relativity explain that time does not pass for light, viewing it as a coordinate in four-dimensional SpaceTime, with proper time measured along an observer's world line in SpaceTime. While coordinate time continues, the proper time for light is zero, meaning everything along its path happens simultaneously, highlighting the importance of understanding the distinction between these concepts for comprehending the notion of time for light.
Insights
- Einstein's theories of special and general relativity introduce the idea of SpaceTime, where time is a dimension alongside space, and distinguish between coordinate time (a convention) and proper time (measured along an observer's world line).
- In SpaceTime, an observer moving at the speed of light experiences a proper time of zero, meaning everything happens simultaneously for light; this distinction between coordinate and proper time is essential to grasp the unique nature of time for light.
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Recent questions
How does Einstein's theory view time?
As a coordinate in SpaceTime.
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Summary
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Time for Light: Einstein's SpaceTime Theories
- The concept that time doesn't pass for light is based on Einstein's theories of special and general relativity, which view time as a coordinate in a four-dimensional SpaceTime along with three dimensions of space.
- Einstein's theories distinguish between coordinate time (a convention like latitude and longitude) and proper time (the physically meaningful time measured by the length of an observer's world line in SpaceTime).
- In SpaceTime diagrams, an observer's motion is represented by their world line, with the speed of light at a 45° angle; the length of the path in SpaceTime is calculated using the lorenzian distance, not the usual Euclidean distance.
- For an observer moving at the speed of light, the proper time is always zero, indicating that everything along their path happens simultaneously; light, being made of photons, doesn't experience time or anything else.
- While coordinate time doesn't stop, the proper time of light can be considered to stop, leading to confusion; understanding the distinction between these two notions of time is crucial in comprehending the concept of time for light.
