# The Naked Scientists Forum

### Author Topic: Light and density of space  (Read 2186 times)

#### mac_urban

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##### Light and density of space
« on: 18/10/2014 22:53:33 »
Hello, its my first topic here. I'm not a scientist or even a student of physic, astronomy or cosmology but i have a question. Maybe it's a stupid question but I can't find an answer in the Internet (maybe it's because of my poor English skills I can't create a proper question for google. But here it is (don't laugh  [xx(]):

As far as i know light acts differently in different densities, and interplanetary/intersolar space is not a pure vacuum so how can we exactly measure the position of objects in space, or how far they are? Or maybe the refraction rate is too small and it's not take affect for  measurements.  But if light refracts for a really tiny angle so when it comes to light-years is object real position can be light-years from what i can actually see on sky?

#### PmbPhy

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##### Re: Light and density of space
« Reply #1 on: 18/10/2014 23:30:22 »
Quote from: mac_urban
As far as i know light acts differently in different densities, and interplanetary/intersolar space is not a pure vacuum so how can we exactly measure the position of objects in space, or how far they are? Or maybe the refraction rate is too small and it's not take affect for  measurements.  But if light refracts for a really tiny angle so when it comes to light-years is object real position can be light-years from what i can actually see on sky?
Welcome to the forum. Please don't worry about asking "stupid" questions. If you're truly sincere about the question that you're asking then by definition it's not stupid. :)

The positions of objects in space can be measured by simply looking at them and recording their position in the sky and the time of the sighting. If the object is not visible then radar signals can be bounced off of them and then record the position in the sky where the return signal comes from and note the time again.

The fact that space is not a perfect vacuum can be ignored since it has very little effect on the measurements. In fact that's what the model used to make these measurements uses, i.e. that the light is traveling in a true vacuum.

Note: A model is a simplified version of reality.

#### JohnDuffield

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##### Re: Light and density of space
« Reply #2 on: 19/10/2014 13:09:02 »
...But if light refracts for a really tiny angle so when it comes to light-years is object real position can be light-years from what i can actually see on sky?
When this happens we call it gravitational lensing. See Ned Wright's deflection and delay of light. This sort of thing was detected in 1919 to test general relativity.

#### PmbPhy

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##### Re: Light and density of space
« Reply #3 on: 19/10/2014 14:18:38 »
...But if light refracts for a really tiny angle so when it comes to light-years is object real position can be light-years from what i can actually see on sky?
When this happens we call it gravitational lensing. See Ned Wright's deflection and delay of light. This sort of thing was detected in 1919 to test general relativity.

mac_urban - Please ignore this comment since it's wrong. That is only a very tiny effect which only happens to light rays which skim the edge of the sun. The further away from the sun the less the deflection. Everything you mentioned in your question is unrelated to this effect.

Please ignore the person who posted that comment too. He rarely knows what he's talking about.

#### mac_urban

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##### Re: Light and density of space
« Reply #4 on: 19/10/2014 15:48:47 »

#### chiralSPO

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##### Re: Light and density of space
« Reply #5 on: 19/10/2014 19:50:51 »
I don't know what the index of refraction is for space, but it's got to be very very close to 1. We can typically approximate air as having an index of refraction of 1, but a more accurate number is 1.000277 (at STP). Most of the matter in space is hydrogen, which has an index of refraction of 1.000132 at STP. Since the density in space is less than one trillionth (10–12 of that at STP, I think a simple approximation of the index of refraction of interstellar space as 1.000000000000000132. Even if the density of space were to vary by more than six orders of magnitude (one million times) at some sort of interface (remember the light deflection has to do with changes in refractive index, not the index itself) using Snell's law for an interface between R = 1.000000000000000132 to R = 1.000000000132, I calculate a deflection of a beam of light with incidence angle of 45° is 45.0000000076°. The error between 45° and 45.0000000076° would be about 1000 km for every lightyear that the beam has traveled to the interface that distorted it, which might sound like a large number, but bear in mind that is 0.14% of the Sun's radius so this imaginary interface would have to be 700 light years from the sun for the observer to accidentally think the Sun is somewhere that doesn't overlap with where it actually is. Also bear in mind that this kind of distortion would have very little effect on the observations of anything the size of a solar system or stellar cluster (ie each object in the system would have the same error, so the effects would cancel out when looking at distances and interactions between the objects).

#### mac_urban

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##### Re: Light and density of space
« Reply #6 on: 19/10/2014 21:23:12 »
thank you ChiralSPO for great answer!!

#### The Naked Scientists Forum

##### Re: Light and density of space
« Reply #6 on: 19/10/2014 21:23:12 »