# Questions on light and gravity

So basically I'm wondering if gravity can effect the rate that light dissipates and if their is less gravity in between galaxies. If two rays of light start on a horizontal plane, one ray inside a galaxy and one outside. Both rays having no other variables in their path, dust particles, molecules, etc. Will one ray dissipate faster than the other? Could gravity be less concentrated in between galaxies?

I know this is a rather silly question but I'm only in high school, have pondered on it and can not find any answers.

## Answers and Replies

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Hi AustinJones, welcome to PhysicsForums!

1) Individual Light rays don't 'dissipate'. They can be absorbed, scattered, etc, but otherwise they stay the same (energy/momentum have to be conserved, right?)
2) A collection of light-rays can 'dissipate', in a way, by the individual 'rays' being scattered, absorbed, etc.

In the absence of obstacles or absorbing medium, the light-rays will continue indefinitely.

Let me know if I'm missing what you're wondering about.

Also, I don't understand how this relates to your question about 'could gravity be less concentrated in between galaxies'. Of course there is less gravitational potential between galaxies because there is less material there.... Maybe I misunderstand.

Drakkith
Staff Emeritus
Light is an electromagnetic wave. It WILL dissipate over distance, BUT this is not quite like your "rays" you mentioned. It turns out that EM waves only interact in little packets of energy called quanta. We use the term "photon" to represent this particle-like property of light. That being said, gravity will NOT cause the energy in an EM wave to dissipate in the way you are imagining it. It will, however, result in the redshift or blueshift of the light as it leaves or enters the gravity well, either giving the light energy or taking energy. This results in a change in the frequency of the light, and since the energy of light is dependent on the frequency, the energy changes as well.

As for gravity being "less concentrated", this doesn't really make sense. The strength of gravity from an object falls off as the distance from the object increases. This means that far away from galaxies you will be experiencing less gravity than you would be if you were very close to it.

I'm asking if gravity can have effects on light that we are unaware of. I sit nowhere to be throwing questions out like this, I'm in highschool and passing by the skin of my teeth besides the fact i've never taken a physics class. If nobody ever asked questions like these though, we would be content with the earth being the center of the universe. I understand that light being engulfed by a black hole is different than gravity having a strain on light waves, but its not even probable for gravity to effect light in adverse ways? If physics is a bunch of measurements and conversions that reveal the physical world then why does gravity have no effect on light until it reaches a certain measurement?

Your question is unclear.

The answer to my question is already apparently no. I'm trying to think of ways to challenge todays laws of physics, although i should probably get a base understanding of them before i start asking questions. I'm sorry, i just started to get interested in physics a few weeks ago. If after i learn more about physics and still think i have questions to challenge our perspective on how things work, ill come ask them here.

My questions was if gravity could act as friction against light (photons), wearing it down over time. separating miniscule amounts of photons from the wave. more or less asking if it even sounds plausible, not if it happens.

I do not yet fully understand gravity, electromagnetic light-waves, or space-time. Any info you could inform me on or websites you could refer me to?

DaveC426913
Gold Member
I'm trying to think of ways to challenge todays laws of physics, although i should probably get a base understanding of them before i start asking questions.
Your self-reflective insight shows you to be wise beyond your years. And wise beyond a large fraction of new posters here.

Drakkith
Staff Emeritus
I'm asking if gravity can have effects on light that we are unaware of.
If we are unaware on it, then how can we answer? We must be careful when claiming something is possible without having observed the actual effect.

I understand that light being engulfed by a black hole is different than gravity having a strain on light waves, but its not even probable for gravity to effect light in adverse ways? If physics is a bunch of measurements and conversions that reveal the physical world then why does gravity have no effect on light until it reaches a certain measurement?
Gravity affects light at all times. It is just that when light is far enough away from a massive object the effect is so negligible that we can ignore it for most purposes.

My questions was if gravity could act as friction against light (photons), wearing it down over time. separating miniscule amounts of photons from the wave. more or less asking if it even sounds plausible, not if it happens.
To our knowledge this does not happen, nor do we have any reason to believe it is possible.

I do not yet fully understand gravity, electromagnetic light-waves, or space-time. Any info you could inform me on or websites you could refer me to?
Wikipedia is a good source, but it's kind of dry and hard to understand sometimes. Search for Hyperphysics on google, that place is pretty good, although it's still pretty technical on most pages.

wikipedia just leaves me looking deeper into what im already trying to learn and your right its dull and dry. I will look at hyperphysics, but thankyou.