When the distance of objects across the known universe are communicated, it is usually in lightyears. My understanding of a light year is this:
A light year is the distance light travels in one year in a vacuum.
I also understand that the speed of light changes depending on the medium that light is traveling through. For instance, in the air we breath, light is moving slower than the light speedlimit, and a particle can actually travel faster than light does in air, and appear to be travelling ‘faster than the speed of light’, even leaving a sort of visual ‘sonic boom’.
My question is this:
To get an accurate measurement of the distance of an object in terms of lightyears, then doesn’t the term assume that light is travelling through a vacuum to get to us, the observers?
What if, in space, the light travels through a lot of material that is transparent to light, but slows it down – like air on our planet?
Would that make some / many / all measurements from the far reaches of the universe incorrect?
Would there be a graduated error if such a medium is vastly preasent in the universe, such that closer objects are measured more accurately than more distant objects?
A light-year is simply a distance.
All the effects you describe above may contribute to measurement error, but they don’t change the measure of distance known as the light year.
It wouldn’t make any significant difference. Space is mostly a very, very good approximation of a vacuum, and even when light from a distant object passes through a region of denser gas, such as a planetary atmosphere, it goes right back to c when it emerges. Even if that weren’t the case, the margin of error would be on the smallish side, since the speed of light in air at STP is about 99% c.
I have heard that the term “light speed” is not a very good term. The real term should be something like the speed limit of the universe. Light happens to be something that hits the speed limit but the fact that light can slow down has no bearing on the speed limit of the universe.
Experiments have gotten it lower and lower. I believe Harvard researchers recently halted photon propagation to zero and could restart the light at will.
Perhaps there is an underlying misconception, that the term “light year” implies we measure distances by measuring how long it takes for light to travel that distance. This isn’t really the case. Most measurements of astronomical distances don’t rely on the speed of light. For example, parallax is the standard method for measuring distances to nearby stars; this is a strictly geometric method, measuring how much a given star appears to move (relative to the background of more distant stars) as the earth goes around the sun.
In fact, astronomers don’t even use light-years as a unit of distance. The parsec is far more common.
And if they actually measure the parallax, they usually just give that instead of the number of parsecs. As an example, this site has a list of the 100 nearest stellar systems (you have to click on the link) produced by a professional astronomer. Note that it has the parallax (with error bars) but not parsecs.
Light echos are sometimes seen spreading out over gas or dust clouds after a brief event like an explosion, such as a nova or supernova. It’s like watching rings of little waves spread out over the surface of a lake after a fish has jumped. Knowing the true speed of the echos or waves, and measuring the angular rate at which they expand, tell us how far away the cloud is. The local geometry generally tells us how far away the explosion is, too, though typically it’s about where the cloud is.
The most recent issue of Sky and Telescope has a nice article about this.
You’re up in the Hubbell, looking at the farthest point of light which I can also see, as I hover outside in my spacesuit. How much sooner do you see some of the photons emiited at the same time from the light source that I see outside?
This a hypthetical question. I know you can’t sit inside the HT.
(I see problems with my question but I hope you’ll overlook them with your answer.)
For those of you who don’t know, photons don’t age. A photon that was emitted 100 billion years ago is the “same age” as when it started its journey and when it hits your retina: zero. It knocks me out. And, of course, I don’t understand it.
If you’re asking whether a telescope sees photons earlier than an unaided human eye at the same distance, the answer is no. All a telescope does is gather and focus more photons into a single spot, making the source easier to see.
That seems odd to me. Given that the very purpose of measuring parallax is to determine the distance of stars, why would astronomers choose not to convert their parallax measurements to distances?
Well, no. Parallax measurements are only good to about 1,000 parsecs, or about 3,000 light years. That won’t even get you outside the Milky Way. For reference, our Local Group of galaxies is about 10 millions light years in diameter, and the red shift still isn’t relevant to the distance of the other galaxies in the local group.
