Looking at stars yesterday set me to thinking if there was such a thing as a maximum distance that light can travel…?
Clearly stars are less bright than our sun and this must be a function of their distance from us (and also their initial brightness). But these distances are absolutely, mind-bendingly huge: we can see stars thats are hundreds of millions of light years away. And they are still visible not only after huge distances but also massive amount of time.
So is light not subject to “decay” or some form of “resistance” to travel that would mean there is a limit to how far (length) or how long (time) it could travel before it becomes completely unseen?
I realise I am using analogies that would be more relevant to sound or waves in a liquid so it may just be my incomprehension of the “light” phenomena and how it works that means ive missed the point… 😀
In: Physics
There are a couple different limits depending on exactly what you mean by “unseen”.
First, light travels at a fixed speed and the age of the universe is finite, so even the oldest light can have traveled only so far. Light from one side of the universe hasn’t gotten to the other side (and it never will due to the expansion of the universe). If you are outside of the “bubble” that light from a star has reached, you wouldn’t be able to see it.
Second, the intensity of light is proportional to 1/r^2 where r is the distance. This is called the “inverse square law”. Basically, if you double the distance to a light source, it will appear 1/4 as bright. As you get further and further away, the intensity never gets to zero, but it will become so faint that eventually it will become practically impossible to detect.
Third, the universe is expanding making most things fly away from each other. This expansion causes light to be stretched out, lengthening the wavelength and shifting it to the red end of the spectrum. This is creatively called a “red shift”. Now, it happens that things that are farther away from us are moving away from us faster (Hubble’s Law), so the light from more distant objects is red shifted more. At some point, light would be shifted so far into the infrared that it would be more and more difficult to detect, kind of like sound that is too deep to hear.
Finally, related to the first point, the further away you look, the further back in time you are seeing because of the travel time of light. If you look far enough, you will see the time when the universe was still opaque. You can’t see anything further away/older than that because it’s like trying to see through a wall.
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