Excuse an amateur question, as inspired by this question: "How old is the oldest light visible from Earth?", how to determine the age of light? Does light get old? Does light have characteristic therefore we know this light "A" is different from that light "B" therefore we can track how it going over time? Like tagging animals to observe their activities? How to differentiate one light from the other?
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$\begingroup$ I'd like to point out that in some cases there might be uncertainty about the history of light we see. We don't know of any characteristic that uniquely specifies the "age" of the light, so we just assume it's traveled from as far away as we think the object that generated it is, and call the time to travel that far it's "age". $\endgroup$– Bit ChaserCommented Apr 27, 2018 at 22:02
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2 Answers
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There is no measurable difference between an "old" photon and a "new" one. A photon has a wavelength, a polarisation, and a direction of travel. And no other features.
You may ask if it is meaningful to discuss the age of light. It is certainly possible to say that photons were emitted by a certain body 100 years ago, and these photons are being detected now, and so this light is 100 years old. But it is impossible to measure the age as a property of the light.
We don't know of any characteristic that uniquely specifies the "age" of the light, so we just assume it's travelled from as far away as we think the object that generated it is, and call the time to travel that far it's "age". In the case of the Cosmic microwave background, this was (we think) released when the universe first became transparent, a few hundred thousand years after the "big bang". The light from this far back has been "stretched" by the expansion of the universe, but not changed by time.
This is normally true of many simple objects. They either exist or not. A proton doesn't age. A neutron may decay, but this is an event, not a process. It is only with complex objects made of many particles that they can change with age while maintaining their identity.
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$\begingroup$ Ok. Is that then all the lights created at once during the Big Bang? It's just distance and path of traveling plus the moment observed that gives it "age"? $\endgroup$– Mishu 米殊Commented Apr 28, 2018 at 16:32
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$\begingroup$ @Mishu米殊 yes, that's it. Remember, in the photon's reference frame, zero time has passed. $\endgroup$ Commented Apr 30, 2018 at 17:43
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According to that context, the old light means the light that travelling from far away in order to reach the observer, and vice versa.
To determine the age of light, standard method is to use spectral features (absorption/emission but we will talk about the example of emission only without the loss of generality).
Let's say that the observer detects a photon which he is very sure that it must be H$\alpha$ emission (i.e., 656.28 nm at rest frame). But, the observer detects a 700.00-nm photon instead of at that rest-rame wavelength. This is because the photon travelling through the universe and experience time dilation; in general, the longer time travelling, the bigger the observed wavelength is, relative to the rest-frame value. Since we have the relationship $\lambda_{rest} * (1 + z) = \lambda_{obs}$ where $\lambda_{rest}$ ($\lambda_{obs}$) is the rest-frame (observe-frame) wavelength, and $z$ is the redshift, we can find the redshift from the known rest-frame and observe-frame wavelengths. Then, the higher redshift means farther away, means older.
answered Apr 27, 2018 at 15:54
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$\begingroup$ So by this it meant the "age" is the distance measured between the object observed and observer. Actually not a "light" can be observed alone it must be observed by observing an existence of an object, such as, a star? $\endgroup$– Mishu 米殊Commented Apr 28, 2018 at 16:36
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$\begingroup$ The red-shift is due to the relative velocity of the emitting body (relative to us on Earth). There is no "time-dilation" effect over the time of travel which affects wavelength at all. It all happens at the moment of emission. Perhaps you're thinking of group phase velocity dispersion? $\endgroup$ Commented Apr 30, 2018 at 17:46