# How does light from distant galaxies reach us?

Why doesn’t a ray of light which is emitted by a distant galaxy, say about a thousand light years away, die down in between?

I mean, how do the light rays from different galaxies so far away reach us in this day and age, when they were emitted so long ago? Why aren’t they dampened and lost altogether? What drives the light rays to travel humongous distances WITHOUT being dampened or being lost altogether?

Now, how to explain this if constant momentum and energy are what drives a photon through space (according to https://physics.stackexchange.com/a/667784/311056)?

• FYI, $1000$ light years away is still inside our own galaxy. The nearest large galaxy is the Andromeda galaxy, about $2,500,000$ light years away. Space is big! Sep 23 at 21:23
• Are you asking why light does not disappear in empty space/vacuum ? Sep 23 at 21:54
• Conservation of energy. What would cause it to be dampened or lost? Sep 24 at 0:23
• This question might arise from the day-to-day phenomenon of light seeming to "decay" the further you get from the light source. What's actually happening is that the photons are spreading out and scattering, resulting in less photons reaching you. But the photons themselves don't lose energy on the way. Sep 24 at 16:20
• @DrakeP In day-to-day experiences they do get absorbed as well. Sep 24 at 18:08

Intergalactic space is estimated to have a mean density of about $$1$$ molecule per cubic meter.

Air has a density of about $$3 \times 10^{25}$$ molecules per cubic meter.

1 Light Year is about $$9 \times 10^{15}$$ meters.

A crude bit of multiplication would thus suggest that a photon passing through 13.5 billion light years of intergalactic space has about as many encounters with molecules as a photon passing through 4 meters of air.

Nothing drives light. An object with momentum does not lose its momentum unless it has an interaction in which it transfers momentum to something else. As to why that is the case, no-one knows - it's just the way the universe is. A photon is an object with momentum, so it keeps going forever unless it has an interaction with something else. Joseph H's linked answer covers the interaction with an expanding universe, known as cosmological redshift, which dims$$^1$$ and cools$$^2$$ distant light, but does not blot it out or change its direction. To change its direction, light needs to be scattered, which only happens in interactions with matter or gravity, not empty space. To cease to exist, light needs to be absorbed, which only happens in interactions with matter.

1: Light is dimmer which has lower number of photons per second per unit area.

2: Light is cooler which has lower energy per unit photon. In the visible spectrum, red is the lowest-energy color, which is why we call this phenomenon redshift.

A reality-check edit: An equivalent ratio of smaller numbers, like 1 billion light years of intergalactic space to about 30 centimeters of air, would be more in line with real traveled distances through space that really has that density, since if we go too far back in time we have to unwind both gravity concentrating matter in galaxies and cosmological expansion spreading out whatever's left.

• Comments are not for extended discussion; this conversation has been moved to chat. Sep 24 at 11:50
• But the photons do lose their energy during red shift due to change in their frequencies don’t they? Their frequency changes so how is their momentum conserved?
– A.M.
Oct 6 at 21:20

The passage of a photon is only "dampened" if it interacts with something

You assume that objects tire when travelling. This is wrong even for heavy objects as we have known since Newton (or earlier).

It was once widely believed that objects with mass would need to be pushed to continue moving. But Newton's first law of motion says that things in motion continue moving in a straight line until acted on by a force. The intuition that things stop (which is what we usually observe on earth) is wrong when we take into account all the forces (gravity, air resistance etc.) But you seem to hold to the pre-Newtonian idea and have extrapolated it to the more complicated case of photons (which are massless and always travel at the same speed in a vacuum whatever happens).

But the assumption you make that photons must get tired from all that motion is the same sort of error. In reality the only thing that can "dampen" a photon's motion is its interaction with something. And interstellar space is spectacularly empty of things for any photon to interact with.

Why is its motion not dampened? There is not enough matter for it to interact with to dampen it (or, more precisely, for lots of photons there is not enough matter to block them all). What drives the light rays to keep traveling? Nothing. Like massy objects in Newtonian gravity they keep travelling unless something stops them.

• Newton’s classical mechanical laws i.e. law of inertia was never meant for massless objects which behave on a totally different scale then the objects of Newtonian mechanics! If an object doesn’t have mass. And it’s position is not known to us whenever it’s velocity is known, how can we possibly say that it will follow ‘this’ very path and will never die down until the end of time? For law of inertia to apply we should have the coordinates as well as velocity so we could be sure that neither the path nor the velocity is changing! How can we say that for photons?
– A.M.
Sep 24 at 15:27
• @A.M. I didn't imply that Newton's laws do apply to photons, just that the misunderstandings of how photons behave are analogous to the misunderstandings of how massy objects behave pre-Newton. A quantum treatment of photons also doesn't have any "decay" over time if they don't interact with anything or travel a long distance. Sep 24 at 17:18
• AM - physicists are not that gullible/naive. There are many many sophisticated ways that have tested if the properties of matter and energy that we know on earth, appear to also hold true in the distant past, or over cosmic times and distances. So far, it seems that they do. It doesnt actually mean a thing if Newtons laws were "never meant" to apply to massless particles. Within the limits of classical vs. quantum physics, the fact is, the underlying principles of those laws, do apply: Momentum is conserved. Particles in motion stay in motion unless they experience a force or interaction Sep 25 at 4:40
• @A.M. You are wrong and also somewhat incoherent. Photons have energy and momentum, "depends" has no meaning here. Redshift doesn't violate conservation laws (but requires a long explanation). Photons don't "just" travel at the speed of light arbitrarily: it is a fundamental consequence of the structure of spacetime. All conservation laws apply. Sep 25 at 8:55
• @A.M. No. Photons all travel at the same speed unless they interact with something. Sep 25 at 9:22

It is dampened and lost on its way here by clouds of dust, and by interacting with certain gaseous chemical compounds in deep space (see lyman-alpha forest). The galaxies we can see from far, far away are those where we can get a dust-free view. Note also that it's possible for the gases in space to knock out certain characteristic wavelengths of light but still pass enough visible light for the galaxies, etc. to be seen with telescopes.

In the absence of dust and gas, light can travel for billions of light-years through empty space because empty space is transparent to photons i.e., it contains no dissipative mechanisms to get rid of the photon energies.

• But the observable universe is huge and matter is present everywhere, this makes it practically impossible for light to not interact with matter on its way!? Besides this doesn’t answer my question which clearly states why isn’t it LOST on it’s way?
– A.M.
Sep 23 at 21:25
• The universe is huge, but generally speaking, matter (gas) in space is very sparsely spread out. There is the Oort cloud, however, which should dampen the view I suppose… Sep 23 at 21:33
• @A.M. matter is not present everywhere in the observable universe
– Tim
Sep 24 at 18:42
• @A.M. While there is a lot of matter in the universe, and it should be fairly evenly distributed, only about 5% of the matter is visible, and most of it should be concentrated in galaxies, leaving most of the rest of space empty of stuff that would block light. Sep 24 at 19:16

I suppose that the background of your question is that in our daily experience waves do die down when they travel far. We can overhear a conversation only from so far, waves in a pond have limited range, and we don't feel an earthquake that happens far away even though it happens on the same planet Earth.

There are two reasons we can see light from far away.

1. The earthquake example gives us a hint, as does the one about the conversation. Because, in fact, we can "feel" the earthquake. We just need a specialized instrument — a seismometer — that amplifies the faint motion that still reaches us. The same is true for conversations. Given the right equipment to amplify the faint signal reaching us we can overhear conversations over astonishing distances. The same is true for distant celestial objects. We can "see" them because here, too, we use special equipment to amplify the faint signal that reaches us.

2. The everyday examples all have in common that the signal moves through a material medium like the Earth, water, or air. This movement is not free of friction — no macroscopic process (a process involving many microscopic particles) is. By contrast, light moves through vacuum which is not a medium in the sense of water or air. Rather, it is (at least until we have quantum gravity) the stage, the scaffolding, the lattice on which things happen. On the particle level, there is no friction: Friction is an emergent phenomenon resulting from the interaction of aggregates of many particles.

While there is no friction on the particle level, you can have scattering. When light passes through a medium like air, water, or glass the photons are sometimes deflected and robbed of some of their energy which leads to some atom excitation, typically heating the medium. On the macroscopic level this appears — emerges — as scattering, leading to "noise" like a blue sky or heat and dampening the original signal. That's why you can look into the sun at sunset without immediately burning your retina out. (I'm not liable for anything if you actually do that.)

But for scattering you need matter. There is next to no matter in intergalactic space, so there is next to no scattering, and hence next to no dampening. It is not by chance that Hubble is in space: The last 80km of air would compromise the image of a galaxy 13.4 billion light years away more than the 1,698,817,160,999,999,999,999,920km of vacuum before that, because during this last millisecond of its billion-year travel a photon from GN-z11 encounters thousands of times more atoms than during its entire trip through the universe before that. It's the matter that matters.

There still is, however, the general principle that the intensity of radiation emanating from a source decreases with the square of the distance. You are probably well aware of that. It has nothing to do with scattering or interaction of any sort but simply reflects that the surface area of imaginary spheres through which the same "flux" of radiation passes successively grows quadratically: The same photons are distributed over increasingly large areas. That's the main reason we need equivalently large mirrors to observe distant objects: To collect as many photons as we can from the so very spread-out flux reaching us.

Your basic problem is that you're thinking of light as rays. Instead, think of it as individual particles - photons. Photons don't "degrade", they either exist, or they don't. Any photon thus travels at the speed of light until it strikes something - a particle of interstellar/intergalactic dust, the mirror of the Hubble telescope, or your retina.

Now a quick search finds that a fairly typical star like the sun emits $$10^{45}$$ photons every second. There are around $$10^{11}$$ stars in a typical galaxy, so that's $$10^{55}$$ photons per second.

Now some simple geometry* gives the number of photons emitted by a galaxy -say Andromeda, since it's close, and you can see it without a telescope in good conditions - that hit your retina. If my math is correct (always debatable :-)), about $$3.6 *10^6$$ photons/second, which is not a lot considering that ordinary daylight is billions/second.

So how do we see distant galaxies at all? Three ways, in combination.

1. Lenses or mirrors collect the photons from a larger area.

2. Look at the object for a longer time, and collect the photons.

3. Use a detector - originally photographic film, now various electronic gizmos - that's more sensitive than the eye.

To give a concrete example, the Hubble telescope has a 2.5 m diameter mirror. To create the Extreme Deep Field image, it collected photons for 2 million seconds: https://www.nasa.gov/mission_pages/hubble/science/xdf.html

*A quick back-of-the-envelope calculation - math corrections welcome: Andromeda is about $$2.5*10^6$$ light years away. One light year = $$1.9*10^{15}$$ meters. So the photons that Andromeda emits are spread out over the surface of a sphere with radius $$4.75*10^{21}$$ meters. Plugging that into the formula for the area of a sphere $$4\pi r^2$$ gives $$2.8 * 10^{44} m^2$$ So about $$3.6 * 10^{10}$$ photons strike each square meter at Earth's distance.

1 square meter is 1 million $$mm^2$$. The retina has an area of about 100 $$mm^2$$. Thus about $$3.6 *10^6$$ photons from Andromeda reach your eye.

As other answers tried to explain, it's not about what mechanism would prevent photons from getting lost but finding a mechanism that would remove sufficient photons so we couldn't observe them any longer.

according to our understanding of fundamental physics:

### mass less objects travel with the "speed of light"

According to relativity, everything with zero mass always travels with the (local) speed of light. There is nothing "driving" these objects. It is a fundamental property of spacetime. Mass is "resistance to acceleration", without mass, there is no resistance and objects move with the "maximum allowed velocity".

### energy is conserved

Once you have created a photon, you need to find a mechanism to get rid of it again. Photons do not decay, so you need other entities to interact with and dissipate / absorb them, cf. e.g. the argument concerning dust and other molecules. If you cannot find an explanation why we wouldn't see light from billions of light years away then that is your explanation why we do.

• This contains a misleading statement. Mass is not "resistance to movement". It is resistance to acceleration, at best, which is very much not the same thing. Sep 25 at 4:49
• fair enough. It was meant in the sense of "resisting to get moving", which would be acceleration. I will edit it. Sep 25 at 6:44
• Still not quite right, although thank you! Try something like this?: "..... without mass, a particle must travel at the speed of light, unless and until it interacts in a way that it gains mass or is transformed into something else (eg is absorbed by an atom)" Sep 25 at 7:38
• I wanted to make it easily understandable for somebody that seems to struggle with the concept in the first place. For that I was willing to sacrifice physical rigor... "… without mass, a particle must travel at the speed of light …" I think I conveyed that in the first sentence of the paragraph? But it still lacks a reasoning as to why it must; hence the "mass is resistance" part. Sep 25 at 8:17
• Theres a difference between simplifying and misinforming. Fundamentally mass ISNT resistance. Thats the problem with explanations of the Higgs field - people pushing through a crowd, or moving through syrup. Mass just isn't resistance, in any sense that we can use when discussing both massive and massless entities, however much the reader lacks knowledge. Sep 25 at 8:43

I am adding this answer because I feel like the other answers do not focus on your question about "What drives the light rays to travel humongous distances"? The other answers nicely explain why we have an unobstructed view of certain objects billions of light years away, explaining the vastness of empty space and the fact that matter is distributed really sparsely. I would like to mention two interesting things:

1. Why light does not need a driving force to travel (theoretically infinitely) and keep its speed c in vacuum

On the other hand, it is very important to understand that light, and photons that build it up, are massless particles and always travel at speed c when measured locally, in vacuum. Emphasis on vacuum. Photons do not need an extra driving force to travel these vast distances. Unless the photons interact (and get absorbed or get scattered in different directions or gravitational field changes their direction) with something (matter), they will continue their way unobstructed to us at the same speed c.

1. Why not only interaction with matter can affect the photons, but the gravitational field itself can do that too

I would like to mention certain ways the gravitational fields along the travel affect the photons:

1. gravitational redshift, as the universe expands, the photons traveling in expanding space will be stretched, their wavelength will increase, thus they lose energy, but this cannot change their speed (they always travel at speed c when measured locally)

2. on the vast distances the photons travel along until they reach us, they constantly climb in and out of gravitational wells, that is, gravitational fields of giant objects like galaxies, this reduces and increases their energy, but cannot change their speed, cannot slow them down

3. gravitational lensing, can change the photons original direction, and create lensing effect

4. certain objects, like black holes, can trap light for good (photon sphere)

So bottom line, not only interaction with matter counts as obstruction, but the gravitational fields on the way can alter the photons' direction or energy (or even trap them).

As previous answers have discussed, the density of intergalactic space is so low that light can travel across the universe largely unimpeded.

The questioner may be trying to find out why photons can travel indefinitely through empty space. All available evidence suggests that these particles are massless. Therefore, according to special relativity, photons won't experience any passage of time. Thus they won't appear to have a finite lifetime to an outside observer.

Alternatively, there is not thought to be a set of particles that a lone photon could decay into based on conservation of energy, momentum and charge.

• This answer is getting at what the OP is asking. But there are problems with saying photons do not experience the passage of time because they travel at the speed of light. See How does a photon experience space and time?. Also the Earth has traveled around the Sun for billions of years without coming to a stop. Sep 24 at 1:02
• I’m not even referring to time here! What I’m really asking is what drives a photon to travel this long without being dampened or die down altogether? There’s no concept of time you say, okay, but how is it able to travel these long distances? Light years themselves are units of distances and not time!
– A.M.
Sep 24 at 1:55
• @mmesser314 it is accurate to say the proper time interval between two events on the light ray is zero. Therefore, it is not possible for a photon travelling through empty space between two points to decay into other particles or disappear (even if we allow processes that break conservation of energy-momentum).
– FTT
Sep 24 at 3:05
• @A.M. I suspect you have a deeper misunderstanding here. Particles such as photons do not simply disappear (though massive particles may decay into other particles). They don't require constant external input to continue to exist, their momentum and energy are conserved in the absence of interactions with their surrounds.
– FTT
Sep 24 at 3:13

Think for a moment about biological evolution and how it works through the selection process. The same process is applicable to your question. The light we see is not an accurate representation of all the light that travels from distant galaxies. We are seeing only the light that has been selected by the circumstances of its path. Light that doesn't encounter enough matter to block it arrives here. Light that encounter enough "stuff" never arrives here. The answer to your question is answered by the light we dont see. There is no mystery here. Matter will attentuate light. Light does die down on its journey. When we look at distant galaxies some are brighter, some are faint, some can only be seen in wavelengths that are not in the visable spectrum and some are not seen at all. These obsevations tell us that light from distant galaxies does "die down." The answer to your question is the journey does take its toll on light.

According to electromagnetic theory, the rest mass of photon in free space is zero and also photon has non-zero rest mass, as well as wavelength-dependent. The very recent experiment revealed its non-zero value as $$10^{-54}$$kg (5.610 ×10-25MeV$$c^{-2}$$) [1]:https://doi.org/10.1016/j.rinp.2019.102866. One consequence of that would be that photons could then decay into lighter elementary particles, However its debatable. The idea here is that Light is an electromagnetic wave which consists of electric and magnetic vector perpendicular to each other. The important notion here is that oscillating electric field produces an oscillating magnetic field even when no charges are present and no physical current flows. This leads to a electromagnetic wave propagation to infinity through a perfect vacuum without losing energy in space ,In space there is no matter and the pressure is so low that any particles there do not affect any processes being carried out. Hence, Light can travel long distances in Vacuum

The electric field vector and magnetic field vector fall off as $$1/r$$ when you do the actual calculations, which is consistent with energy being proportional to $$E^2$$ as energy then falls off as $$1/r^2$$, which is what you'd intuitively expect.