CHAPTER 08 · Read Cosmos
Relativity and time dilation
Are the time experienced by Earth and the traveler the same?
Explore this chapter ↗Let's begin! The experience allows you to directly interact with the scenes and observation tools.
Measure time with a traveler’s clock
In chapter 8 of the original book, contemplating the distances between stars and the speed of light, we challenge our everyday sense of space and time. For a traveler moving very quickly, the time accumulated on each clock may not be the same.
Simply knowing that increasing speed allows us to travel further is not enough. We must specify the reference frame from which time is measured and the path taken.
OBSERVE · Start with evidence
Three pieces of evidence
- The speed of light in a vacuum is the same for an inertial observer.
- The traveler's time, corresponding to a 10-year journey relative to Earth, depends on the speed.
- To fully explain a round trip, it is necessary to distinguish between acceleration and directional changes.
Whose clock will we use to traverse the universe?
Commentary following space travel and time in chapter 8 of the original book. The activity compares uniform-motion segments in special relativity; it does not simulate a complete round trip.
Looking at something far away is like looking at the past.
Since it takes time for light to reach us, when we look at something far away, we are also seeing its past. Don't confuse the observation in front of you with the event that the light originated from at the same moment.
When imagining space travel, we must distinguish not only the distance but also the time it takes for news to arrive and the time the traveler experiences. A single sentence can contain multiple clocks.
Questions to ConsiderA signal has arrived from a distant star. How do we differentiate between the two?
The traveler's clock.
During uniform motion, the traveler’s elapsed time corresponding to 10 Earth-frame years depends on speed. At 0.8c in this activity, the traveler experiences 6 years.
Do not change this into a story about a broken clock. The key is to specify the criteria and path being compared, and the amount of time accumulated on each individual's clock.
Questions to ConsiderWhat specific information must be included in the results?
The remaining part of the journey that was not included in the calculation
Actual round trips involve issues such as departure and direction changes. It is not possible to say that the calculation, which only shows the constant speed segment, explains all the conditions of the entire journey.
Imagination can lead to questions that go beyond the boundaries of the model. However, it is important not to treat observed facts, results obtained through calculation, and still-hypothetical possibilities as equally certain.
Questions to ConsiderHow to indicate remaining problems in a travel plan?
Discoveries and their limits
Neither clock is broken: different paths accumulate different amounts of proper time. Here we compare only uniform-motion segments, leaving out acceleration and gravity.
To understand "it will take 10 years for the trip" without misunderstanding, what conditions are necessary?
Explore this chapter ↗Evidence and further reading
Even stars that send light to us have their own time. Why do some stars have shorter lifespans?