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
Not as far as we know, basically it keeps going until it bumps into something like your eyes when it is then registered as light.
Imagine a small blob of paint.
Now use that same amount of paint to paint a ring.
Now again, but paint a larger ring.
Now a larger ring
Now larger again.
Each time using the same amount of paint.
You have to spread out the paint more and more to finish each ring, so the larger rings are more faded.
—————–
Now imagine a star.
It’s constantly pumping out light, but let’s focus on the light it creates this instant in time.
Now imagine that light is traveling out in all directions. The light all moves the same speed, so at any moment the light that was created in that instant forms a shell around the star.
As time marches on the light gets further from the star. The shell formed by the light grows lager and larger, so the light is more and more spread out.
When you look at a star, you see less light if the star is further away, because the light from that star is more spread out by the time it reaches you.
Light energy itself doesn’t diminish, but the ongoing expansion of the universe means that light from distant sources becomes increasingly [redshifted](https://en.wikipedia.org/wiki/Redshift) over time (basically, ‘stretched out’ because of the movement of whatever it’s coming from), and that makes it harder and harder to detect. The energy is still there, but it’s spread out over a huge distance and very difficult to get any useful information from.
Yes but in a wierd way.
The farther light travels, the greater the degree to which its frequency slowly diminishes as its wavelength correspondingly increases. We observe this phenomenon as a redshift, the tendency of visible light to drop toward the red end of the spectrum. At some point it shifts out of visible spectrum.
Redshift measurements suggest that the energy of light emitted from far distant galaxies may drop beneath visibility within a range of from 10 to 14 billion light-years from us, depending on its frequency at source.
Most of this i got from os publishers, theres a article on the subject.
Eli5 attempt: light very slowly changes frequency. Throwing a rock in a pond will produce ripples, easily visible at first. But as the circle increases in size, the waves get smaller and smaller, until they are no longer visible. Sort of..
The decay is called redshift and it happens because of the universe expanding relatively faster the farther away it is. This isn’t the reason we can’t see forever though. The reason we can’t isn’t because of redshift but because at a certain distance the universe is expanding away from us at past the speed of light so light from those stars will never reach us.
It’s called the Hubble Sphere.
When distant stars seem dimmer it’s not because their light is getting “degraded” on the way, just that less of it is reaching us because it is spread over a greater area.
Light can theoretically travel indefinitely in a vacuum. Practically it can (and does) run into stuff on the way, like interstellar dust and gas. But to answer your question no, there is no limit to how far light can travel.
It’s important to remember that light is composed of individual photons, single ‘pieces’ of light that travel through space. Distant stars appear dimmer because *fewer* photons from them are hitting your eye, but not because anything is wrong with those photons themselves, the light itself.
For any star you can see, that’s pretty much all there is to it. The light travels and unless it hits something it is the same photon it was when it left that star a few years ago.
Still, on very very large distances, the gradual expansion of the universe comes into play. This causes those photons to have longer wavelengths- imagine a very short choppy wave in the ocean, but the ocean stretches somehow and the wave suddenly has more distance between peaks. Longer wavelengths mean the light is more “red” shifted. This is actually happening at all times in space right now, but only on massive distances and time scales.
The light from the most distant galaxies in our universe is so far shifted that we can’t see it with our eyes anymore and need special cameras designed for light at those wavelengths beyond what the human eye can see. The James Webb telescope that recently launched looks at these ancient, distant galaxies to learn more about them.
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.
There is a law of physics called inertia, which means that objects in motion stay in motion and objects at rest stay at rest. The particles of light, photons, are unique in that they are born going super fast (the speed of light). Most objects have mass, meaning friction can be applied against their movement. Because photons are massless but the law of inertia still applies, photons will continue going the speed of light under almost any conditions.
The question marks are redshifts, as others have explained here, and black holes.
As far as our perception of it, think of a flashlight. If you shine it at something 6 inches away, the light is very concentrated and it’s very bright. If you shine it at a wall 20 feet away, it’s less bright because the photons are spreading out evenly towards the wall and less photons hit any one point. In the case of a star, such a small amount of the emitted light happens to hit earth. 99.9% of it misses us.
The short answer is yes: up to about 15 billion light years.
A quick explanation of why this happens:
You’ve probably heard of ‘redshift’ before when describing distance galaxies. Basically, as light travels, it gradually loses energy. Light is a form of electromagnetic radiation, and one of its properties is ‘wavelength’, or the distance between the crests and troughs of its frequency.
As light loses energy, it goes from a short wavelength to a longer wavelength. As it does, it moves deeper into the red portion of the visible light spectrum (which has the longest wavelength of all electromagnetic energy that we can perceive with our eyes, as opposed to blue light which has the shortest wavelength). Eventually, the wavelength gets too long to be in the visible light spectrum at all, becoming infrared radiation.
Another term you’ve probably heard before is the ‘cosmic microwave background’, or CMB. This was once the light of the big bang, but over billions of years it’s shifted even lower in the electromagnetic spectrum, going from infrared to microwave radiation. Eventually, it will become radio waves, which have the longest wavelength of all electromagnetic energy. And then it will disperse completely within the local electromagnetic perturbations of a cold, dark universe.
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