FREE GUIDED ACTIVITY · ABOUT 10 MINUTES
Two clocks.
Different amounts of time.
Predict what happens, try the calculator, then check your reasoning. Work alone or compare your answers with a friend.
You’ll need: paper for your predictions and our time-dilation calculator. No account or submission is needed. Keep this page open in a separate tab if you want to switch between the instructions and calculator.
The setup: compare ten years in a reference frame with time on a clock moving at constant speed through that frame. We leave out gravity and changes of direction. This is not a round-trip journey.
1. Make a prediction
Before opening the calculator, write your answers:
- At rest relative to the reference frame, how many years will the second clock record?
- At 80% of light speed, will it record more than, less than, or exactly ten years?
- If you increase the speed from 80% to 99%, will its recorded time increase or decrease?
2. Test four speeds
Open the calculator and move the speed slider to each setting. Record the moving clock’s time to two decimal places. Then check the answers below.
| Speed (% of light speed) | Moving clock (years) |
|---|---|
| 0% | Write your result |
| 60% | Write your result |
| 80% | Write your result |
| 99% | Write your result |
Check the calculated answers
0%: 10.00 years; 60%: 8.00 years; 80%: 6.00 years; 99%: 1.41 years (rounded).
For this setup: moving-clock time = 10 × √(1 − β²), where β is speed as a fraction of light speed. For 80%, use β = 0.8, not 80.
3. Explain what changed
Does the moving clock feel broken to someone beside it? Why doesn’t this comparison tell you the age difference after a return journey?
Compare your explanation
The clock behaves normally in its own frame. The comparison depends on relative motion and how times are assigned to distant events. A return journey changes the motion; you must account for the complete paths to compare reunited clocks.
One-sentence challenge: explain time dilation to someone who has never heard of it, without saying that time is “just an illusion.”
Try it with a group
Give everyone two minutes to predict, four minutes to test, and four minutes to explain. Ask each person which result surprised them and what assumption they changed. Encourage disagreement about predictions; use the model and its limits to resolve the numerical questions.
This is an introductory activity, not a graded assessment or a classroom-tested curriculum. You can use the same instructions on screen or use your browser’s Print option for a paper copy; expand the answer sections first if you want them included.
Keep exploring
Read the full time-dilation explanation, or compare the role of gravity in special and general relativity.
Physics references: Einstein Online: From light clocks to time dilation and Time dilation.
Original activity by Thought Observatory, prepared with AI assistance. Published September 27, 2026. See our editorial approach.