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.

Outlines

00:00

🌌 The Expansion of the Universe and Redshift

In this paragraph, Edwin Hubble's groundbreaking discovery in 1929 is discussed, where he observed that galaxies are moving away from us, indicating the universe is expanding. This expansion causes light to be stretched out, resulting in a redshift where the wavelength of light increases, causing it to appear cooler in color. The analogy of a raisin bread expanding, with galaxies as raisins, is used to explain how spacetime expansion leads to cosmological redshift. The paragraph also touches on the difference between cosmological redshift and Doppler redshift, and raises the question of energy conservation in the context of redshift. It explains that as light's wavelength increases, its energy decreases according to Planck's equation, but the energy isn't transferred or dissipated as it would be in a medium like water. The video concludes this section by suggesting that the law of conservation of energy may not hold in the context of cosmological redshift.

05:01

🔍 Questioning the Universality of Energy Conservation

The second paragraph delves into the philosophical and scientific implications of the law of conservation of energy in the context of the expanding universe. It starts by challenging the conventional understanding of energy conservation by referencing Emmy Noether's theorem, which links conservation laws to symmetries in the universe. The paragraph then contrasts this with Einstein's general relativity, which shows that spacetime can change, implying that the laws of physics, and thus energy conservation, may not be absolute. The conclusion drawn is that during cosmological redshift, a photon's energy is not conserved but is instead lost, highlighting exceptions to what is often considered a universal truth. The paragraph ends with a metaphor about a frog in a well, suggesting that our understanding of physics is limited to our 'well' of experience and that there is more to discover beyond it.

Keywords

💡Edwin Hubble

Edwin Hubble was an American astronomer who made groundbreaking observations in the 1920s. He is known for discovering that galaxies are moving away from us, which led to the understanding that the universe is expanding. In the video, Hubble's discovery is central to explaining the concept of the expanding universe and its implications for the behavior of light and energy.

💡Universe Expansion

The universe expansion refers to the observation that the universe is continually growing in size. This concept is crucial in the video as it sets the stage for discussing how galaxies move away from each other and how this affects light and energy. The script uses the analogy of raisin bread to illustrate how objects in space, like raisins, move apart as the universe expands.

💡Redshift

Redshift is a phenomenon where the wavelength of light from distant galaxies is stretched, causing it to shift towards the red end of the spectrum. The video explains that as light travels through the expanding universe, its wavelength gets stretched, and it appears redder. This concept is essential for understanding how the universe's expansion affects the light we observe from distant galaxies.

💡Cosmological Redshift

Cosmological redshift is the stretching of light's wavelength due to the expansion of the universe. It is different from Doppler redshift, which is caused by relative motion. In the video, this concept is used to explain why distant galaxies appear redder, as their light is stretched as it travels through the expanding universe.

💡Wavelength

Wavelength is the physical length of a single wave, measured from one peak to the next. The video discusses how the wavelength of light gets stretched as it travels through the expanding universe, leading to the redshift effect. This is a key concept in understanding the changes in light's energy as it moves through space.

💡Energy Conservation

Energy conservation is a fundamental principle in physics stating that energy cannot be created or destroyed, only transformed from one form to another. The video challenges this principle in the context of the expanding universe, suggesting that energy can be lost during cosmological redshift, which is a significant departure from classical physics.

💡Planck's Equation

Planck's equation relates the energy of a photon to its frequency (or inversely, its wavelength). In the video, it is mentioned to explain that as the wavelength of light increases (redshift), its energy decreases. This equation is crucial for understanding the energy changes in photons as they experience cosmological redshift.

💡Noether's Theorem

Noether's theorem is a fundamental result in theoretical physics that connects symmetries in physical systems to conservation laws. The video explains that Emmy Noether proved that energy conservation is a consequence of time invariance, which means the laws of physics do not change over time. However, the video also points out that if the universe's structure (spacetime) is changing, then energy conservation may not hold.

💡General Relativity

General relativity is Einstein's theory of gravity, which describes how massive objects cause spacetime to curve. The video mentions general relativity to explain that spacetime can change, which has implications for the conservation of energy. It suggests that if spacetime is dynamic, then the laws of physics, including energy conservation, may not be constant.

💡Gravitational Waves

Gravitational waves are ripples in spacetime caused by the acceleration of massive objects. The video briefly mentions the first observed gravitational waves as an example of spacetime changing, which challenges the notion of energy conservation. This concept is used to illustrate that the universe's dynamics can be more complex than what classical physics predicts.

💡Time Invariance

Time invariance is the idea that the laws of physics remain constant over time. The video discusses how Noether's theorem connects time invariance to energy conservation. However, it also points out that if the universe's expansion means that spacetime is not static, then time invariance and, by extension, energy conservation may not universally apply.

Highlights

Edwin Hubble discovered in 1929 that galaxies are moving away from us, indicating an expanding universe.

The expansion of the universe causes light to get stretched out, leading to redshift.

Redshift occurs as the universe expands, similar to how raisins spread apart in rising bread.

Cosmological redshift is different from Doppler redshift, which is caused by relative motion.

Longer wavelengths of light have less energy, as per Planck's equation.

The law of conservation of energy is challenged by the redshift phenomenon.

Light waves do not dissipate energy in space due to the absence of a medium like air.

Emmy Noether's theorem links conservation laws to symmetries in the universe.

Einstein's general relativity showed that spacetime can change, implying laws of physics may also change.

Cosmological redshift results in the loss of photon energy, contradicting the conservation of energy.

The conservation of energy is a special case, not universally applicable.

The folk tale of the frog in the well illustrates the limits of our understanding of the universe.

The law of conservation of energy is accurate for most daily human experiences but not for cosmological phenomena.

The video encourages viewers to question universal truths and consider exceptions.

The video is supported by Squarespace, which is mentioned for its tools and templates for showcasing passion.

A survey is being conducted by PBS Digital Studios to learn more about the viewers.

Transcripts

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 is its wavelength

gets stretched out and it becomes a cooler color--

is energy conserved?

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Let's first look at how this red shift happens.

Imagine the expansion of the universe like a loaf of raisin

bread, rising as it bakes.

Galaxies, like the reasons in the loaf,

spread apart, not because they're

moving through the loaf, but because the dough, itself,

is expanding, just like spacetime does.

As spacetime gets stretched out, so

do any of the light waves that travel through it.

This is known as cosmological redshift.

Imagine a galaxy far away emits some yellow light at you.

We would measure that light as more stretched out,

so maybe we would see it as more red.

That's why we call this redshift, because the longest

wavelengths of visible light are in the red part

of the spectrum.

Note that this cosmological red shift

is different from Doppler redshift or blueshift, which

is caused by things moving toward or away

from us through space.

Now, longer wavelengths of light have less energy,

or at least Planck's equation tells us that.

Energy equals Planck's constant times the speed of light

over the wavelength, so as light's wavelength gets longer,

its energy decreases.

But wait.

You learn in intro physics about the law of conservation

of energy, that energy can neither be created

nor destroyed.

It's just transferred from one form into another.

So where does the redshifted photon's energy go?

One could guess that, well, maybe light waves die out

the same way a ripple in water dissipates,

but the water molecules are bumping into each other,

stealing kinetic energy through friction

away from the wave's energy until the ripple finally

dies out.

Total energy is conserved, though.

In contrast, light from a galaxy is not

traveling through a medium.

Most of space is near emptiness.

There is no air resistance or friction,

so there is nowhere to pass the energy.

So if the light wave isn't giving up energy,

then where is the energy going?

Well, the short answer is, actually, energy

isn't conserved.

The photon's energy is just lost.

Seems freaky, right?

I just told you that the law of conservation of energy

doesn't always hold, but there is

nothing to worry about if we think back to how we first

came up with this law.

Physicists first developed the law of conservation of energy

by doing experiments, like when they saw boiling steam lift

a pot lid, indicating that heat could be transformed

into mechanical energy.

And burning wax transforms chemical energy

in the molecular bonds into heat energy, and so on.

But in 1915, Emmy Noether proved a logical reason

for why energy is conserved.

Noether's theorem stated conservation laws

can be derived from symmetries in the universe.

For example, she proved using math

that energy conservation is a consequence of when you assume

that the laws of physics don't change

over time, what physicists call time invariance.

If you threw a ball in the air today,

time invariance says that it will rise and fall

exactly the same way tomorrow or a billion years from now,

but are the laws of physics actually unchanging in time?

In that same year that Noether published her theorem,

Einstein showed using general relativity

that spacetime can change.

It can warp and ripple and expand.

In fact, the first observed gravitational waves,

a couple weeks ago, are an example of spacetime changing.

If spacetime, itself, is changing,

then that means that the laws of physics must also be changing,

which means, by Noether's theorem,

energy isn't conserved anymore.

So during cosmological redshift, when a photon's wavelength

gets stretched out, its energy is simply lost.

So it turns out that something we usually

talk about as a universal truth has cosmological exceptions.

What do those exceptions mean?

For me, they are a reminder that everything

we know and intuitively understand

is far from universal.

It's a special case.

There is an old folk tale about a frog that

lived his entire life down a well,

and everything there made sense to him.

He knew where the bugs hid.

He knew where the comfortable rocks were, until one day

a turtle showed up and told him about the outside world, about

mountains and trees and oceans.

And the frog just couldn't wrap its head around it.

He couldn't imagine anything that

was bigger or better or different from his own well.

The law of conservation of energy

applies down our own well.

For almost everything we experience as humans

on a daily basis, sure, it's accurate enough

to say that the laws of physics don't change over time,

and so energy is conserved.

But when you zoom out, the physics starts to look foreign.

There is more to the story.

Thank you so much for watching, and happy physicsing.

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