Is energy always conserved?

Summary

In this 'Physics Girl' episode, the focus is on the universe's expansion and its effect on light, leading to cosmological redshift. Edwin Hubble's discovery of galaxies moving away from us is explored, illustrating the stretching of light wavelengths and the resulting loss of energy. The script challenges the conventional law of energy conservation by explaining that, in the context of an expanding universe, energy isn't conserved as photons lose energy without transferring it. This concept is a reminder that our understanding of physics is often based on limited, 'well' experiences, and the universe holds exceptions to what we consider universal truths.

Takeaways

• 🔭 Edwin Hubble discovered in 1929 that galaxies are moving away from us, indicating the universe is expanding.
• 🌌 The expansion of the universe causes light to get stretched out, a phenomenon known as redshift.
• 🍞 The expansion can be visualized like a loaf of raisin bread rising, with galaxies spreading apart due to the dough (spacetime) expanding.
• 🌈 Redshift refers to the stretching of light wavelengths, making them appear 'redder' as they lose energy.
• 🚫 Cosmological redshift is distinct from Doppler redshift or blueshift, which is caused by relative motion through space.
• ⬇️ Longer wavelengths of light correspond to less energy, as described by Planck's equation.
• ❓ The traditional law of conservation of energy is challenged by the energy loss observed in cosmological redshift.
• 🔍 Emmy Noether's theorem links conservation laws to symmetries in the universe, such as time invariance.
• 🌪️ General relativity and observed gravitational waves suggest that spacetime can change, impacting the conservation of energy.
• 🐸 The script concludes with a metaphor of a frog in a well, highlighting that our understanding of physics is limited to our 'well' of experience.

Q & A

What significant discovery did Edwin Hubble make in 1929?

Edwin Hubble discovered that most of the galaxies in the universe are moving away from us, which led to the realization that the universe is expanding.

What is the term for the stretching of light as it travels through an expanding universe?

The stretching of light as it travels through an expanding universe is known as cosmological redshift.

How is the expansion of the universe often illustrated?

The expansion of the universe is often illustrated by comparing it to a loaf of raisin bread rising as it bakes, with galaxies being like raisins that spread apart due to the dough's expansion.

What is the difference between cosmological redshift and Doppler redshift or blueshift?

Cosmological redshift is caused by the expansion of spacetime, whereas Doppler redshift or blueshift is caused by objects moving towards or away from the observer through space.

Why is the stretching of light waves referred to as 'redshift'?

The stretching of light waves is referred to as 'redshift' because the stretched wavelengths fall into the red part of the visible light spectrum, which has longer wavelengths.

According to Planck's equation, what happens to the energy of light as its wavelength increases?

According to Planck's equation, the energy of light decreases as its wavelength increases because energy is inversely proportional to wavelength.

How does the law of conservation of energy typically work?

The law of conservation of energy states that energy cannot be created or destroyed, only transferred from one form to another.

What does the script suggest about the conservation of energy in the context of cosmological redshift?

The script suggests that the law of conservation of energy does not hold in the context of cosmological redshift, as the energy of a photon can be lost when its wavelength is stretched.

Who is Emmy Noether and what did she prove about conservation laws?

Emmy Noether was a mathematician who proved that conservation laws can be derived from symmetries in the universe, such as time invariance leading to energy conservation.

How does the concept of time invariance relate to the conservation of energy?

Time invariance, the idea that the laws of physics remain constant over time, is a symmetry from which energy conservation can be derived according to Noether's theorem.

What does the script imply about the universality of physical laws?

The script implies that physical laws, while seemingly universal, can have exceptions in certain contexts, such as the expansion of the universe, and that our understanding of these laws is often based on limited, special cases.