The properties of light over time/distance

[agent83]

I have a question concerning what has been proven about the properties of light over billions of years and billions of light years.

Specifically, is it assumed/proven that light waves stay the same over billions of years as they travel through space? (Disregarding the other influences like doppler, gravitation redshift, etc.)

How would you prove that?

How can we no with absolute certainty that it's not a natural property of light to redshift over billions of years, considering all the other influences that effect light (doppler, gravitation, etc.)?

Thanks for answering to a laymen who has a "hunch".

 

[Chalnoth]

I have a question concerning what has been proven about the properties of light over billions of years and billions of light years.

Specifically, is it assumed/proven that light waves stay the same over billions of years as they travel through space? (Disregarding the other influences like doppler, gravitation redshift, etc.)

How would you prove that?

How can we no with absolute certainty that it's not a natural property of light to redshift over billions of years, considering all the other influences that effect light (doppler, gravitation, etc.)?

Thanks for answering to a laymen who has a "hunch".

Well, there are two points to make.

First of all, the properties of light are really the properties of the electromagnetic force: light is, after all, an electromagnetic wave, and proposing that light changes over time requires proposing that the electromagnetic force changes over time. And the electromagnetic force doesn't just govern light, but is the most important force to determining the shapes and behaviors of atoms and molecules. So if, when we look back in time, we see atoms and molecules behaving in the same way that they do now (e.g. through the pattern of frequencies of light they emit), then we gain confidence that the properties of the electromagnetic force haven't changed much over time. Understand that if you change how atoms behave, you don't simply redshift their emission lines, but instead change the pattern of lines in significant ways.

Secondly, this sort of consideration is precisely why it is so important to not just look at one type of data, but to combine very different sorts of data. If anyone of our assumptions were completely wrong, you would not expect different, unrelated sorts of data to show the relationships we expect based upon our current models. For example, one of the issues with the expansion is that it produces a strong relationship between the hot and cold spots we see on the cosmic microwave background (emitted some 13.7 billion years ago) and the distribution of nearby galaxies (typically up to a few billion light years out). Specifically, the hot and cold spots on the cosmic microwave background have a characteristic size, and this should, in the nearby universe, be related to the typical separation between galaxies. And it is, to an extremely high degree of accuracy.

There are many other sorts of independent observations that we look at, and so far there is precious little to go on to show where our current model may be wrong.

 

[narrator]

On the topic of light waves, I think I heard long ago that in some circumstances it's necessary to look at light as waves, and in other circumstances to look at it as photon particles. Have I got that right?

 

[Sybren]

I have a question concerning what has been proven about the properties of light over billions of years and billions of light years.

Specifically, is it assumed/proven that light waves stay the same over billions of years as they travel through space? (Disregarding the other influences like doppler, gravitation redshift, etc.)

How would you prove that?

How can we no with absolute certainty that it's not a natural property of light to redshift over billions of years, considering all the other influences that effect light (doppler, gravitation, etc.)?

Thanks for answering to a laymen who has a "hunch".

I think I know what this 'hunch' is.

 

[Chalnoth]

On the topic of light waves, I think I heard long ago that in some circumstances it's necessary to look at light as waves, and in other circumstances to look at it as photon particles. Have I got that right?

Sorta kinda. Photons, which light is made up of, are quantum-mechanical particles. Quantum-mechanical particles have a very specific mathematical description which, in some instances, looks kinda like a classical particle, in others kinda like a classical wave.

Basically, when you have a lot of photons together, in the majority of situations they collectively act like a classical wave. This is very much analogous to water: if you have a lot of water molecules together, those molecules act like a fluid. For most intents and purposes, that fluid behaves as if it had no constituent particles at all, and was just a continuous mass. Light is very much like this. But, when you start looking at the water on very small scales, the fact that it is actually made up of discrete parts, atoms, starts to become important. A similar rule is true with light: when the light source is dim enough, the fact that it's made up of individual photons becomes important.

Or, in the case of light interacting with atoms, the discrete nature of the light becomes important again. You see, when light interacts with an atom, it does so via single-photon interactions. So the amount of energy each individual photon has is extremely important. This was one of the fundamental insights of Einstein, published in the same year as Special Relativity, called the "photoelectric effect". What he found was that when you shine a light onto a metal, the brightness wasn't nearly as important as the frequency. Below a certain frequency, it was hard to knock off any electrons at all. But at higher frequencies, you could easily knock off a whole lot of photons.

This was important because it was one of the principle experiments pointing the way to quantum mechanics: it indicated that light was made up of discrete packets of energy. The brightness of the light source controls the number of these packets, but the frequency determines the energy of each one. So you could deposit the same total amount of energy in the metal either with a bright light or a high-frequency light, but it is only the high-frequency light that would free electrons.