Overview
This lecture explains where electromagnetic waves come from, how they travel through space, and why observing distant objects means looking into the past.
Origin and Travel of Electromagnetic Waves
- Most electromagnetic waves on Earth originate from the sun.
- Electromagnetic waves (including light) can travel through the vacuum of space without a medium.
- Light travels at 300,000 km/s (3 × 10⁸ m/s).
Light Travel Time and Observation Delay
- It takes about eight minutes for sunlight to reach Earth due to the vast distance.
- We see the sun as it was eight minutes ago, not as it is now.
- Observing any object by its light means seeing it as it was in the past, with the delay increasing with distance.
Observing Distant Stars and Galaxies
- Electromagnetic waves from distant stars and galaxies can reach us, but take much longer due to vast distances.
- Light from Proxima Centauri, the closest star after the sun, takes over four years to reach Earth.
- Light from extremely distant galaxies hasn't had time to reach us if they're farther away than light could travel in the universe's lifetime.
Limitations of Observation
- Sound and mechanical waves cannot transmit through space, so only electromagnetic waves provide information about celestial bodies.
- The farther an object is, the dimmer it appears because its light spreads out as it travels.
- We always see stars and galaxies as they were in the past, never as they are at this exact moment.
Key Terms & Definitions
- Electromagnetic wave — Energy wave (like light) that can travel through a vacuum.
- Light speed — The speed at which light travels: ~300,000 km/s or 3 × 10⁸ m/s.
- Proxima Centauri — The nearest star to the sun, about 4.2 light-years away.
- Light-year — The distance light travels in one year.
Action Items / Next Steps
- Review the concepts of electromagnetic wave propagation and light travel time.
- Prepare for questions about how and why we see celestial objects as they were in the past.