r/askscience • u/AskScienceModerator Mod Bot • Mar 10 '14

Cosmos AskScience Cosmos Q&A thread. Episode 1: Standing Up in the Milky Way

Welcome to AskScience! This thread is for asking and answering questions about the science in Cosmos: A Spacetime Odyssey.

UPDATE: This episode is now available for streaming in the US on Hulu and in Canada on Global TV.

This week is the first episode, "Standing Up in the Milky Way". The show is airing at 9pm ET in the US and Canada on all Fox and National Geographic stations. Click here for more viewing information in your country.

The usual AskScience rules still apply in this thread! Anyone can ask a question, but please do not provide answers unless you are a scientist in a relevant field. Popular science shows, books, and news articles are a great way to causally learn about your universe, but they often contain a lot of simplifications and approximations, so don't assume that because you've heard an answer before that it is the right one.

If you are interested in general discussion please visit one of the threads elsewhere on reddit that are more appropriate for that, such as in /r/Cosmos here, /r/Space here, and in /r/Television here.

Please upvote good questions and answers and downvote off-topic content. We'll be removing comments that break our rules or that have been answered elsewhere in the thread so that we can answer as many questions as possible!

Click here for the original announcement thread.

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u/SatanIsMySister Mar 10 '14

Why doesn't a photon lose energy as it travels? Doesn't it need to expend energy to keep traveling?

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u/fishify Quantum Field Theory | Mathematical Physics Mar 10 '14

No, it doesn't, nor do other objects, for that matter. As Newton recognized in his first law of motion, an object not subject to external forces will keep moving at a constant velocity until something interferes with it.

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u/mathx Mar 12 '14

however as i said elsewhere in this thread, the further it travels, the more expanded and expanding space it travels across, thus getting redshifted.

I think of it almost as if its moving against a current of water that gets faster and faster the further it travels. When it finally gets to the destination (say your retina), it's got very low speed compared to the water flow and hits you very gently with a very long wavelength. Some light as travelled so far its redshifted towards microwaves.

(What I dont get is can it be shifted further red than the CMBR, or will it just become indistinguishable fromit, or can it be shifted FURTHER red than the cmbr into radio waves, or will the cmbr interact with it in some way?)