Is energy always conserved? | Summary and Q&A

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March 15, 2016
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Is energy always conserved?

TL;DR

The universe is expanding and as a result, light waves get stretched out, which leads to a phenomenon known as cosmological redshift.

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Key Insights

  • ❓ Edwin Hubble's discovery of the expanding universe revolutionized our understanding of the cosmos.
  • 🙂 Cosmological redshift occurs when light waves are stretched out due to the expansion of the universe.
  • 🌂 The phenomenon of cosmological redshift challenges the law of conservation of energy, as the energy of redshifted photons is lost.
  • 👮 Emmy Noether's theorem explains that conservation laws, including energy conservation, arise from symmetries in the universe.
  • 💱 Spacetime can change according to general relativity, which means the laws of physics might also change, leading to exceptions to energy conservation.
  • 🎗️ The implications of cosmological redshift remind us that our understanding of the universe is limited to our own experiences and there is much more to learn.
  • ❓ Squarespace supports "Physics Girl" in showcasing the passion and obsession for physics.

Transcript

In 1929, astronomer Edwin Hubble made an unusual discovery. He noticed that most of the galaxies in the universe are moving away from us, which led him to realize that the universe is expanding, and that expansion has strange implications. It means that everything in the universe, even light, gets stretched out. So when light gets redshifted-- that... Read More

Questions & Answers

Q: How did Edwin Hubble discover the expanding universe?

Edwin Hubble noticed that most galaxies are moving away from us, suggesting that the universe is expanding.

Q: Why does the expansion of the universe cause light waves to stretch out?

The expansion of spacetime itself causes any light waves traveling through it to also stretch out, resulting in cosmological redshift.

Q: What is the difference between cosmological redshift and Doppler redshift?

Cosmological redshift is caused by the expansion of the universe, while Doppler redshift is caused by the motion of objects toward or away from us through space.

Q: How does the law of conservation of energy challenge cosmological redshift?

The longer wavelengths of redshifted light have less energy, but according to the law of conservation of energy, energy cannot be created or destroyed. The lost energy of redshifted photons remains a mystery.

Summary & Key Takeaways

  • Astronomer Edwin Hubble discovered that galaxies in the universe are moving away from us, leading to the realization that the universe is expanding.

  • The expansion of the universe causes light waves to stretch out, resulting in cosmological redshift.

  • During cosmological redshift, the energy of redshifted photons is lost, challenging the law of conservation of energy.

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