Help me understand light traveling through vast distances in space

[Wallsy]

Hi everyone.Ive been looking for a beginners section on this site but no joy.I am uneducated but physics and Astro physics in particular really interest me.One of the things I don't understand is the vast distance in space.We talk about things being thousands of light years away.Yet light is bent by gravity.Black holes even stop light from moving.If light is traveling such vast distances,it's bound to come close to and be affected by countless stars,planets and even black holes before finaly descending into Earth's gravitational well.So surely we are viewing the universe through a very distorted looking glass.These stars/galaxies could be much nearer/younger than we think.

My mind thinks that we need to be in a position totally unaffected by gravity.IE,well away from any gravity well such as a star or planet to be able to view the universe in its true state.Where our starting point in terms of space and time is unaffected or distorted by gravity.Even then,we would have to do all the calculations of the light coming close to other gravitational wells on its journey to us to truly perceive things as they are.

Perhaps some of you smarter guys on this site could explain this all to me in layman terms :)
Thanks in advance,
Gary.

 

[Bandersnatch]

Hi Wallsy, welcome to PF!

There is no beginner section. Just find the subforum you think your question fits best into, and mark it 'B' (like you did). This indicates your level of background knowledge, so that people can tailor their answers accordingly.

To you question.
While it's true that mass curves space-time, causing all sorts of effects, including difference in elapsed time between regions with different mass concentration - it turns out the difference from the hypothetical place far away from any mass, when calculated, is negligible for anything but somebody orbiting *very* close to a black hole.

For even a very massive person, standing on a very massive planet, orbiting a very massive star, in a very massive galaxy, in a very massive cluster of galaxies - that difference adds up to something like some thousands of years of difference in the billions of years of perceived age of the universe.

 

[fresh_42]

Unless the light doesn't pass close to a massive object, it isn't really affected by gravitation.
One of our members has made this picture of our solar system which gives you a good impression of space:
http://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html
And now imagine how the interstellar or intergalactic space looks like. And in the above, the planets are all lined up, which means, if one really would travel along a light ray, there would be effectively nothing at all.

 

[Drakkith]

.One of the things I don't understand is the vast distance in space.We talk about things being thousands of light years away.Yet light is bent by gravity.

Indeed.

Black holes even stop light from moving.

Not true. Once past the event horizon of a black hole, light still moves "locally" at c, but it has no path through spacetime leading back outside of the event horizon to take. The only possible paths through spacetime all lead towards the singularity.

.If light is traveling such vast distances,it's bound to come close to and be affected by countless stars,planets and even black holes before finaly descending into Earth's gravitational well.So surely we are viewing the universe through a very distorted looking glass.These stars/galaxies could be much nearer/younger than we think.

Not really. Most of the bending of light by gravity is canceled out by the bending of light in the other direction by another object it passes later on. And remember that light has to either get really close to a very massive, very compact object to be bent significantly, or it has to pass around a very massive, very large object such as a galaxy cluster, where gravity has a long time to act on the light as it passes by. These effects are all well understood and our understanding of the universe takes all of these effects into account (plus more that you've never even heard of).

 

[Wallsy]

Ok,thanks for replies.So basically it's only black holes in the centre of galaxies that are really strong enough to affect light.I read only today that there are two trillion galaxies!
But I have a better picture now thanks :)

 

[Drakkith]

So basically it's only black holes in the centre of galaxies that are really strong enough to affect light.

No, all black holes affect light. The magnitude of the effect depends directly on the distance the light is from the black hole and its mass. Also, the mass of galaxy clusters is dominated by matter, not by the central black hole. Even a supermassive black hole of a billion solar masses is perhaps 1% of the mass of a typical galaxy at most.

 

[Wallsy]

So..a mass of galaxies would have an even greater effect on the light passing by?

 

[Drakkith]

So..a mass of galaxies would have an even greater effect on the light passing by?

Light is bent by gravity, which is determined by mass, and almost all of the mass of a galaxy is in its matter content (normal and dark matter), not in its supermassive black hole.

 

[davenn]

So..a mass of galaxies would have an even greater effect on the light passing by?

google gravitational lensing ... there are a number of images showing galaxy clusters that their combined gravity is lensing light from other galaxies in the background

 

[phinds]

I read only today that there are two trillion galaxies!

But keep in mind that this is ONLY the Observable Universe, not the universe (in which it is believed that there are at the very least, MANY orders of magnitude more than that, and possibly and infinite number).

 

[stefan r]

My mind thinks that we need to be in a position totally unaffected by gravity.IE,well away from any gravity well such as a star or planet to be able to view the universe in its true state...

I think the opposite. Bending light with gravity allows for a lens effect similar to what happens in a refracting (Galileo type, glass not mirror) telescope. If we place a probe at 550+ astronomical units from the sun we can use the sun as a lens.

But keep in mind that this is ONLY the Observable Universe, not the universe (in which it is believed that there are at the very least, MANY orders of magnitude more than that, and possibly and infinite number).

That is speculation. Is not possible for any evidence to exist by definition of "observable". Of course I am not disagreeing. But it is "belief" not "evidence from physics".

 

[phinds]

That is speculation. Is not possible for any evidence to exist by definition of "observable". Of course I am not disagreeing. But it is "belief" not "evidence from physics".

You are of course correct that it cannot be corroborated by direct evidence, but I believe that it is much stronger than just an idle belief. For one thing, it's impossible to think that the universe just ends abruptly at the edge of our particular Observable Universe, so clearly the universe as a whole is bigger.

 

[Drakkith]

That is speculation. Is not possible for any evidence to exist by definition of "observable". Of course I am not disagreeing. But it is "belief" not "evidence from physics".

Not true. Based on measurements of the curvature of space, current models require that the universe be much, much larger than the observable universe. It is as much "evidence from physics" as anything in science else is.

 

[DaveC426913]

"Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space."
- Douglas Adams

But seriously, space is mostly empty. Light rays, for the most part, pass in pretty straight lines through it.As other have pointed out, you have to get pretty darned close (cosmologically-speaking) to a pretty big mass to have much of noticeable effect.

And they are often billions of light years away, meaning the angle of deflection to form an image of a ring where we are is vanishingly small.

Google Einstein Rings.

 

[BenAS]

Not true. Based on measurements of the curvature of space, current models require that the universe be much, much larger than the observable universe. It is as much "evidence from physics" as anything in science else is.

Is the evidence enough to suggest that the universe continues homogeneous and isotopic beyond our cosmological horizon? Clusters of galaxies etc.

 

[phinds]

Is the evidence enough to suggest that the universe continues homogeneous and isotopic beyond our cosmological horizon? Clusters of galaxies etc.

Failure to do so would imply a very weird change in physics beyond our observable universe and it is that fact more than an (non-existent) direct evidence. You would need a theory to show how/why that might be to overcome the very strong belief that the Cosmological Principle holds throughout the universe.

 

[Drakkith]

Is the evidence enough to suggest that the universe continues homogeneous and isotopic beyond our cosmological horizon? Clusters of galaxies etc.

I don't know, but I doubt it. But as phinds said, it would be a strange change in the laws of physics and/or the standard cosmological model.

 

[stefan r]

Not true. Based on measurements of the curvature of space, current models require that the universe be much, much larger than the observable universe. It is as much "evidence from physics" as anything in science else is.

Does the published literature distinguish the likelihood of "larger, "much larger", and "much much larger"? Any physical evidence that 10¹⁰ observable radii is more likely than 10¹⁰⁰ or 10¹⁰⁰⁰? More importantly, if you had evidence for 10⁴¹ observable radius what does that mean? What happens at 10⁴¹ distance that does not happen at 10⁴² distance?
If you are chained to a spot on Earth and observe you could reasonably conclude that there is probably land over the horizon. Without taking measurements from multiple locations you do not know how far the land (or surface) goes. You could try to measure/estimate the circumference of Earth because it is rotating (Eratosthenes). That could give you a statement like "x circumference ±n" the numbers would be determined by the length of your chain and the accuracy of your instruments (or uncertainty principle). Is there something analogous for the global universe and observable universe? All directions are over the horizon.

The plank mission team said the observable universe is "flat". But that meaning of "flat" includes surfaces like balloons and bagels.

 

[Drakkith]

Does the published literature distinguish the likelihood of "larger, "much larger", and "much much larger"? Any physical evidence that 10¹⁰ observable radii is more likely than 10¹⁰⁰ or 10¹⁰⁰⁰? More importantly, if you had evidence for 10⁴¹ observable radius what does that mean? What happens at 10⁴¹ distance that does not happen at 10⁴² distance?

I believe it just sets a minimum distance of at least several times the size of the observable universe. If I find a reference I'll let you know.

If you are chained to a spot on Earth and observe you could reasonably conclude that there is probably land over the horizon. Without taking measurements from multiple locations you do not know how far the land (or surface) goes. You could try to measure/estimate the circumference of Earth because it is rotating (Eratosthenes). That could give you a statement like "x circumference ±n" the numbers would be determined by the length of your chain and the accuracy of your instruments (or uncertainty principle). Is there something analogous for the global universe and observable universe? All directions are over the horizon.

I'd change your analogy to being stranded on a small island and asking how far the ocean should go beyond the horizon. Without being able to move from your island, it would be reasonable to conclude that the ocean goes on for at least some distance beyond the horizon. Depending on what measurements you can make and how accurate they are, you could put numbers to that minimum distance and possibly even determine that it should actually curve back around eventually.

I believe that observations of galaxy distributions and the CMB require that the universe exist in approximately the same general form (meaning that it should follow roughly the same natural laws and it should look roughly like the standard model of cosmology says it does) out to a distance of at least several times the size of the observable universe. Again, I don't have a reference to give, but if I find one I'll post it. I'm not sure if I read it here on PF or somewhere else.

 

[DaveC426913]

I'd change your analogy to being stranded on a small island and asking how far the ocean should go beyond the horizon.

Good analogy. (I started on one but discarded it.)

So, you might say that there is a lower constraint: the ocean goes at least as far as (and a little farther than) the horizon - since we do not see any protruding mountaintops of distant lands. But it tells us nothing at all about any upper constraint.

 

[Drakkith]

Good analogy. (I started on one but discarded it.)

So, you might say that there is a lower constraint: the ocean goes at least as far as (and a little farther than) the horizon - since we do not see any protruding mountaintops of distant lands. But it tells us nothing at all about any upper constraint.

Exactly.

 

[stefan r]

I believe it just sets a minimum distance of at least several times the size of the observable universe. If I find a reference I'll let you know.

I looked at wikipeidia's "orders of magnitude" page. They list 10^1010122Mpc and a link to this paper, and a 95% confidence of a minimum of 21 with this link. Also could be infinite. Quite a range. I cannot think of anything that I am less certain of. I could say I am confident in my uncertainty.

 

[rootone]

Me too, but anyway I know now how uncertain I am within acceptable error margins.
Light is electromagnetism, go from there.

 

[Jimjustanoldman]

Random Thoughts on the above.

I Not all of the Universe seems to run in a random pattern Afghan Quilt I.would like to bring attention to The Great Attractor".
This area seems to be a mysterious major mass abnormality.
In a infinite universe , iI there is one there are probably many.

One must ponder also on how much can/could a really massive object bend light?
Enough to completely distort the light's source location (from our perspective) from the actual correct location.?

e.g. a star is blocked from our view by a massive dust cloud but light from that star, hitting a massive object deflects enough that we see the star's light but not it's correct location? We assume the star is behind the gravity lens but in actuality it is well off in another location.

We slingshot our craft around gravity wells tor course corrections and velocity increases . on this thought , as light also is bent or deflected by a gravity well is it possible it's velocity also in some cases change?

The speed of light can not be a constant in the Real world if it can be bent or deflected from it's normal straight line course as it is traveling a greater distance.

at least not constant in the sense of how long it takes to go from A to B . Constant speed just not constant course.

 

[Nugatory]

One must ponder also on how much can/could a really massive object bend light?
Enough to completely distort the light's source location (from our perspective) from the actual correct location.?

e.g. a star is blocked from our view by a massive dust cloud but light from that star, hitting a massive object deflects enough that we see the star's light but not it's correct location? We assume the star is behind the gravity lens but in actuality it is well off in another location.

Massive objects do indeed deflect light, but the mechanism by which happens is well-understood ad we can calculate exactly how much deflection is involved. Thus, the star isn't where it appears to be just going by the direction of the approaching light, but we do where it actually is.

We slingshot our craft around gravity wells tor course corrections and velocity increases . on this thought , as light also is bent or deflected by a gravity well is it possible it's velocity also in some cases change?

The speed of light can not be a constant in the Real world if it can be bent or deflected from it's normal straight line course as it is traveling a greater distance.

It is not possible that the speed of light changes (at least not unless an enormous amount of our physics is wrong in highly implausible ways that have no experimental support). However, you do need to understand what it means to say that the seed of light is always $c$: It means that if you set up an observation station anywhere in the universe and pass a beam of light past that station, an observer on that station will find that the light is passing at speed $c$. In a curved spacetime there is no unambiguous way of defining the speed of anything at a distance from you, and therefore no meaningful way of talking about the speed of light relative to you as it moves through regions of space that are distant from you. Some of the issues here are discussed in This post and its followups.

 

[Jimjustanoldman]

"no unambiguous way of defining the speed of anything at a distance from you,"

{A touch of humor} ... and I will attempt to explain to the next police officer that pulls me over for speeding that relative to his perception yes I was over the limit however relative to my perspective i was not!

Seriously though: Not withstanding the foundations of our current knowledge . . .Mathematics aside,

I struggle with the concept that a force can action on an object and cause deflection . . .while in turn that force cannot also cause that object particle/energywave to accelerate and or or de- accelerate?

Even Newton would be scratching his head.

{remember I am old and possibly senile }

 

[DaveC426913]

{A touch of humor} ... and I will attempt to explain to the next police officer that pulls me over for speeding that relative to his perception yes I was over the limit however relative to my perspective i was not!

"Look sonny, Classical Newtonian physics is a good approximation for any legal speed on my roads. If you're invoking Einstein, I'm going to impound your car."