# Q & A: Does light travel forever?

Q:
Does light ever stop at a particular point or range...just like sound, sound has a particular range, it cant be heard from a certain distance.
- raja (age 16)
Kenya
A:

Hi Raja,

Good question. First, let's think about why sound does not travel forever. Sound cannot travel through empty space; it is carried by vibrations in a material, or medium (like air, steel, water, wood, etc). As the particles in the medium vibrate, energy is lost to heat, viscous processes, and molecular motion. So, the sound wave gets smaller and smaller until it disappears.

In contrast, light waves can travel through a vacuum, and do not require a medium. In empty space, the wave does not dissipate (grow smaller) no matter how far it travels, because the wave is not interacting with anything else. This is why light from distant stars can travel through space for billions of light-years and still reach us on earth. However, light can also travel within some materials, like glass and water. In this case, some light is absorbed and lost as heat, just like sound. So, underwater, or in our atmosphere, light will only travel some finite range (which is different depending on the properties of the material it travels through).

There is one more aspect of wave travel to consider, which applies to both sound and light waves. As a wave travels from a source, it propagates outward in all directions. Therefore, it fills a space given approximately by the surface area of a sphere. This area increases by the square of the distance R from the source; since the wave fills up all this space, its intensity decreases by R squared. This effect just means that the light/sound source will appear dimmer if we are farther away from it, since we don't collect all the light it emits. For example, light from a distant star travels outward in a giant sphere. Only one tiny patch of this sphere of light actually hits our eyes, which is why stars don't blind us!

David Schmid

(published on 01/23/2013)

## Follow-Up #1: How far does light go?

Q:
how long does light go
- Jason (age 11)
A:

Hi Jason,

Light just keeps going and going until it bumps into something.  Then it can either be reflected or absorbed.  Astronomers have detected some light that has been traveling for more that 12 billion years, close to the age of the universe.

Light has some interesting properties.   It comes in lumps called photons.  These photons carry energy and momentum in specific amounts related to the color of the light.  There is much to learned about light.   I suggest you log in to our website and type  LIGHT into the search box.   Lots of interesting stuff there.

To answer your previous question "Can light go into a black hole?" ,  the answer is yes.

LeeH

(published on 12/03/2015)

## Follow-Up #2: less than one photon?

Q:
Can light intensity reduce to a level where it's energy is less than 1 photon (probably after travelling an almost infinite distance from a point source)?
- Ajay Suri (age 40)
A:

Certainly you can run the ouput of a single-photon source through a half-silvered mirror, and get a sort of half-ghost of the photon in two places. If you put ordinary photon detectors in those places, however, each will either detect zero or one. For each source photon, you'll get at most one of the detectors to find it. How does the half-ghost at the other one know whether it's detectably there or not? The name of that mystery is "quantum entanglement". At some level we don't really know the answer.

Mike W.

(published on 02/04/2016)

## Follow-Up #3: stars too far away to see?

Q:
does there is any star that we can can't get it's light because of itis farness?......sorry with having any problems in my English gramer, my English language is not good enough.
- Hawraz (age 22)
Erbil, Kurdistan
A:

Most stars are too far for us to see them as individual stars even with our best telescopes. Still, we can get light from them, mixed with light from other stars. If our understanding of the universe is at all right, there are also stars that once were visible from here but now are outside our horizon so no light from them reaches us. It's probable that there are many more stars outside our horizon than inside, maybe infinitely more. It's hard to check, however, what's happening outside our horizon! It's even hard to define what we mean by "now" for things outside the horizon.

Mike W.

(published on 07/22/2016)

## Follow-Up #4: light going out to space

Q:
If we are reflections of light, does that reflection make it out into space and keeps traveling til its asorbed. I would think it travels into space if a satellite In space can see me, but I dont know enough to determine if that is how a satellite can see me.
- Carlos (age 28)
custer city, ok, custer
A:

Certainly ordinary light travels out to space. That's how spy cameras and such can take pictures of things here on the Earth's surface.

Mike W.

(published on 09/01/2016)

## Follow-Up #5: end of the universe?

Q:
This isn't so much a question, as it is an answer to the first question. Light does have a maximum range, which can be defined from its point of origin, until the end of the universe. Provided it doesn't hit anything along the way.
- Andy (age 52)
Virginia Beach
A:

We don't think there's any "end" in the sense of some spatial boundary. Unless something changes drastically, there also won't be an end in time. The expansion looks like it will go on forever. So that wouldn't give a maximum range.

Mike W.

(published on 03/26/2017)

## Follow-Up #6: seeing black holes

Q:
I saw a science program on TV about black holes. Would it be possible to point a laser in the direction of a hypothetical black hole and prove it's existence if the light was affected by the gravity of the purported black hole?
- Robert (age 64)
19010
A:

In principle a well-aimed beam would loop around the outside of the black hole and return to Earth. There aren't any black holes close enough to make this practical. Instead the bending of light by black holes is observed by their lensing effect on light coming from more distant objects.

The amazing gravitational wave signals observed from merging black holes provide even more direct and convincing proof that black holes exist and follow the laws of General Relativity.

Mike W.

(published on 01/29/2018)