Ask Ethan: When A Photon Gets Redshifted, Where Does The Energy Go? (Synopsis)

" every kind of chemical change no loss of matter occurs [...] in all the varied modes of physical change, no loss of energy takes place." -Sir Henry Enfield Roscoe
When it comes to the physical laws of the Universe, perhaps the most unbreakable of all seems to be the law of conservation of energy. In every mechanical, chemical, or even nuclear reaction, the total amount of energy, when all sources are considered, appear to be conserved.
Image credit: NASA / SDO. Image credit: NASA / SDO.

Yet in General Relativity, no strict definition of energy exists. So when the Universe expands, the photons within it get redshifted, which means they lose energy. Yet there's no discernible location which gains a commensurate amount of energy. Is it truly lost? Is energy not conserved? The answer, as best as we can determine it, lies in the physics of the expanding Universe.

Image credit: E. Siegel, from the book Beyond The Galaxy. Image credit: E. Siegel, from the book Beyond The Galaxy.

Find out the story on this edition of Ask Ethan!


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My son had asked me this very question a week ago. My stumbling answer was pretty much the same as yours. So its
not really determined. Do we have a measure of the energy represented by the spacetime itself? I imagine the actual energy being lost by the photon component is not large enough to have any observable influence on the expansion, is that correct? Otherwise it we somehow caused a lot of mass energy to energy conversion, would that increase the expansion rate?

By Omega Centauri (not verified) on 19 Dec 2015 #permalink

The energy is moved to potential energy that allows the photon to be that much higher in the potential energy field it is progressing out of.

It's no longer in the photon.

The photon lost energy.

i really don't know how, on any suitable level for a child or layman, it could be explained any better.

And the tensor that defines the metric of spacetime includes the energy contained therein. not merely the photon, but the random motions of the constituents, the potential of the atoms when split or fused, and all forms added within.

Not just the inertial mass.

That, however, isn't really ever promulgated to the layman by any, even "advanced educational" programme on television or in magazines,. so not knowing isn't really a problem with you per se, but with the education system.

Mind you, in this day and age, if you're smart enough to formulate the question, you should be smart enough to search for the answer yourself, rather than rely on being told.

And it's harder to remember things you were just told than things you at least TRIED to find out.

Remember how many times you had to repeat your ABC's and your times tables?

What makes you think that being told "The metric includes the energy of the photons and other forms of energy within it" just once or twice will make the fact stick?

Not to mention, the evaluation of that metric and the proof of it is 100% outside what I can do here, especially so with the fact I'm not going to put that much effort into trying to bypass the limits of ASCII character set and a small textbox to provide a better answer.

Sorry, Ethan... I'm quitting "Starts with a Bang" since Forbes banned AdBlock.

By João Carlos (not verified) on 19 Dec 2015 #permalink

There's still this site.

It contains very little other than poster content, but it's more than nothing.

And if forbes find they aren't getting "value" from SWAB, they won't let it take up "valuable" space.

I don't understand your point, Wow. Are you suggesting that Ethan pare his "Ask Ethan" series to a single sentence: "Look it up!" ??

By Patrick Dennis (not verified) on 19 Dec 2015 #permalink

I also see the graphic and understand i was thinking of gravitational shift.

Linear shift? Throw a ball from a moving bus at someone standing by, and the ball hits them harder when moving toward them than moving away.

The extra energy is taken from the moving bus, the lost energy goes into a faster bus.

Unless you're a superhero and the ball very heavy, the difference won't be really visible, though.

Pat, no.

But if you don't even attempt, you won't find an answer stays with you, and then you'll just be back and looking again.

Here's another problem with just going to Ethan "Tell me THE ANSWER!": how would you tell whether he's right?

It's far too easy for lazy people to just want everything landed at their feet because learning is just so damn haaaaarrrrd, and trying (especially failing) is felt for some (too many) as if it is some form of the universe telling you you don't DESERVE the answer.

Go here to see how to ask questions:

If I understand Ethan correctly, then if everything in the universe magically stopped emitting photos somehow, then the expansion of the universe would slow down?

By Alan doak (not verified) on 19 Dec 2015 #permalink

No, you clearly don't understand correctly, since your lightbulb emits photons today, long after the event you claim photons stopped being emitted "magically".

Give it another read.

@Carlos #3

I run firefox with ad blocker and it opend forbes fine... yes you have to click "continue" twice.. because forbes says you still have adblocker still enabled.. but after that it opens the page without fault.

By Sinisa Lazarek (not verified) on 20 Dec 2015 #permalink

I run firefox with ad blocker and it opend forbes fine… yes you have to click “continue” twice.. because forbes says you still have adblocker still enabled.. but after that it opens the page without fault.

I haven't had any success with FF + ABP, but I haven't looked closely at how Forbes is doing the detection. They don't seem to be able to see iOS ad blockers, and OS X Safari + Ghostery works fine for me.

Their commenting interface is ghastly, but I'm starting to think that it may well be a better option than having the posts in one place and then returning to the vestigial SB presence.

Noscript may help. Though to keep the functionality you want to retain, you may need to use a blacklist.

Perhaps I didn't pose my thought experiment clearly: Ethan's article seems to suggest that if there were somehow fewer/no photons in the universe, then one component of outward pressure on the expanding universe would be reduced/eliminated, resulting in a slower expanding universe.

By Alan Doak (not verified) on 20 Dec 2015 #permalink

OK, Alan.

The thing is that the "mass" of the universe for calculating how it is shaped includes the energy of the photons. If the photons aren't there, then the gravitational well of the universe is lower. Which means that the expansion of the universe from the same dark energy expansion is increased.

So you can't really do anything about concluding it unless you do the maths, and find out what the net effect is.

Also refrain from using terms like "magical". Unless you're in a Harry Potter movie, you don't need to use that term.

self-aggrandizing little creature aren’t you

You've long since entered "pot, kettle, black" territory. I thought comment spamming was one of Ethan's nominal red lines, but whatever – it's not my name on the joint.

I will note with mild amusement that the iOS ad blocker I'm testing out is nullrouting some SWAB pages here. It's really of no consequence at this point, though; the SB presence is merely a marketing redirect.

NAds, you really don't have any room to talk of pots, kettles et al, nor does your past, and especially current, activity indicate that you are any reliable indicator of reality.

I would be far more deserved of laughing at you morons for calling pots black.

When you decide that you would prefer to actually provide rational evidence and discourse, do so, but until that point in time, please take your ridiculous insulting personnae to some other place where your childishness and arrogance isn't so laughably described.

Not to mention that that "rebuttal" (who would have guessed, a moron like yourself using a fallacious argument as "rebuttal"?!!?) is merely accepting the claim.

Further evidence of your lack of intellect.

@João Carlos, Sinisa, Narad, et al. I just found a workaround for ABP getting into the Forbes site: Turn it off at the "splash page", turn it back on once the article page is loaded, then refresh the page.

Having said that, and while running ABP myself, I realize that I am being a perpetrator of the "tragedy of the commons." The revenue from advertising is what provides sites like Forbes with the incentive (and ultimately, the ability) to make content available at all. By blocking those ads, I'm marginally reducing the "eyeball count" for them, and hence reducing the revenue gained from them. If we all do that, advertisers won't pay to put up ads, sites won't earn money from ads, and those sites will eventually shut down.

By Michael Kelsey (not verified) on 21 Dec 2015 #permalink

"The revenue from advertising is what provides sites like Forbes with the incentive"

And they used to be able to GET that revenue.

Then they made ads the bigger part of the content, served the ads up without a care for the site, or the customers, then served up malware. They track and infest your computer, then sell that information. The ads got more intrusive, less relevant, more important than the site itself (how many times has a site waited for doubleclick to finish answering before sending ANYTHING on the page?).

The commons were a tragedy, and it wasn't the customers and viewers that did it.

The revenue from advertising is what provides sites like Forbes with the incentive (and ultimately, the ability) to make content available at all.

I'm inclined to disagree. This assumes that Forbes is a "content provider" rather than a platform that attempts to monetize sometimes* cheap labor in exchange for author exposure (see, e.g., here).

I don't think blocking ads at Forbes is anywhere near the "tragedy of the commons." After all, it's not a commons to start with. Moreover, can they even afford to close up shop?

From my perspective, it's a lousy UX and I'm unlikely to miss anyone who wouldn't be blogging on their own with or without Forbes.

* h[]tps://…

@Wow #23 and Narad #25: You both make decent points. I've been running ABP for a while, so I didn't ever experience the full horror which is the Ad Light Forbes :-) If you're right, Wow, in terms of the extremely low content/ad ratio, then I agree with both of you that my interpretation was a bit too idealistic.

I think my argument does stand in general, however, even if the F*rbes case isn't a good exemplar. Providing content costs the provider money (whether actual salaried authors, or even just paying for hosting and bandwidth), and if none of us are willing to pay for subscriptions (tried and failed, for the most part), then advertising is essentially the only way to recoup those costs.

By Michael Kelsey (not verified) on 23 Dec 2015 #permalink

Oh, aye, in general you're right. But each case is based on the value perceived and received, and these are personal assessments.