# How does light lose energy as it travels over distance?

#### scijeebus

How does light "lose" energy as it travels over distance?

So the universe is said to be expanding because light's wavelength is spreading out at a greater rate per greater distance, but with the wavelength becomes longer, the energy becomes smaller, so where is that energy going? How is it actually being lost?

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#### Garry Go

Re: How does light "lose" energy as it travels over distance?

I don't think energy is being lost I think our observations of it become different. For instance when moving closer to a light source
the light becomes stronger and the frequency we observe becomes larger. The inverse happens when we move further away from
the light source. The doppler effect makes it seem like the light is changing frequency.

#### Antiphon

I don't think energy is being lost I think our observations of it become different. For instance when moving closer to a light source
the light becomes stronger and the frequency we observe becomes larger. The inverse happens when we move further away from
the light source. The doppler effect makes it seem like the light is changing frequency.
Sorry Gary. This is wrong. Intensity changes with distance. Energy of photons doesn't.

The question remains: where does (did) the energy of a cosmologically redshifted photon go?

If you get on a train moving very fast and look behind you the light is also redshifted. Where did the energy go in this easier case?

#### ThomasT

Re: How does light "lose" energy as it travels over distance?

So the universe is said to be expanding because light's wavelength is spreading out at a greater rate per greater distance, but with the wavelength becomes longer, the energy becomes smaller, so where is that energy going? How is it actually being lost?
It's just dissipated via topological irregularities and obstructions in the medium. Why do originally very localized disturbances (say, dropping a pebble into a pond of water) on the surface of a pool of water disperse as they do (ie., propagate more or less omnidirectionally), and dissipate? These are examples of fundamental dynamical physical principles or laws. The evolutions of physical systems approach equilibrium.

If our universe is a finite expanding wave in some medium of unknown structure, then it's reasonable to suppose that the total energy of our universe is decreasing -- and if so, then the behavior of waves in media at less than the universal scale is in accordance with the behavior of the universal wave.

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#### Garry Go

Re: How does light "lose" energy as it travels over distance?

Sorry Gary. This is wrong. Intensity changes with distance. Energy of photons doesn't.
Sorry my mistake I didn't mean to say that the light source itself becomes stronger (I should watch my wording more closely). Light's energy doesn't actually change.

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#### zonde

Gold Member
Re: How does light "lose" energy as it travels over distance?

So the universe is said to be expanding because light's wavelength is spreading out at a greater rate per greater distance, but with the wavelength becomes longer, the energy becomes smaller, so where is that energy going? How is it actually being lost?
I suppose that the current consensus viewpoint is that expansion is coordinate expansion not physical expansion.
I view it that way - light that is travelling toward us does not undergo any change, it's coordinate system that changes over time i.e. measurement sticks become shorter and clocks become faster.

So the question actually should be where matter gets energy instead of where light is putting it.

#### DLuckyE

Re: How does light "lose" energy as it travels over distance?

I can see how it would work with waves, since although a wave would get longer it would also come in for a longer time so the total amount would stay the same. I have no idea how this would work with photon's since you'd need to get more of them somehow.

#### spoirier

Re: How does light "lose" energy as it travels over distance?

I already answered this question in this other thread.

#### cygnet1

Re: How does light "lose" energy as it travels over distance?

Sorry my mistake I didn't mean to say that the light source itself becomes stronger (I should watch my wording more closely). Light's energy doesn't actually change.
Actually, it does. Each photon of the cosmic microwave background is far less energetic in our reference frame than it was in the frame from which it was originally emitted, shortly after the Big Bang. Where did that energy go? How did it get lost?

#### Drakkith

Staff Emeritus
Science Advisor
2018 Award
Re: How does light "lose" energy as it travels over distance?

I can see how it would work with waves, since although a wave would get longer it would also come in for a longer time so the total amount would stay the same. I have no idea how this would work with photon's since you'd need to get more of them somehow.
Why would you need to get more of them?

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