Understanding Time and the Universe Through Light

Using Light to Understand Time and the Universe's Evolution

Finite Speed of Light

  • Light travels at a finite speed, approximately 3×1083 \times 10^8 meters per second.
  • In everyday experiences over short distances, light seems instantaneous.
  • Early attempts to measure the speed of light on Earth were challenging due to the high velocity of light.

Light Travel Time

  • The Sun is eight light minutes away from Earth.
    • We see photons from the Sun that are eight minutes old.
    • We never see the Sun as it currently is.
  • Measurements of Jupiter were used to calculate the speed of light.
    • Light from Jupiter takes longer to reach Earth.
    • We observe Jupiter as it was in the past.
  • Example: Video games played across continents experience delays due to the time it takes for light (information) to travel.
  • Communications with Mars rovers have delays (e.g., a 15-minute delay each way).
  • Spacecraft beyond the solar system require months for signals to return to Earth.

Look-Back Time

  • Looking further away means looking further back in time.
  • Light from distant galaxies can take billions of years to reach us.

Practical Use

  • The finite speed of light is very useful for understanding the universe.
  • We can observe the universe as it was in the past by looking at distant galaxies.
  • By measuring redshift and applying Hubble's law, we can estimate the age of a galaxy.
    • Example: A galaxy with a redshift of 2 is approximately 10 billion years old.
  • We can study the conditions of the universe at different times.
  • We can also observe how things change over time.
    • How star formation changes.
    • How the shapes of galaxies change.

Expansion of the Universe and Redshift

  • The expansion of the universe, combined with the finite speed of light, helps us understand the universe's time evolution.
  • Galaxies at different redshifts correspond to different times in the universe's history.

Thought exercise

  • Consider how far away an object is now, given the light travel time and the expansion of the universe.