I'm struggling with the understanding of the perception of the speed of light, the theory of relativity and how we determine if stars are moving towards earth or away from earth.

I understand that light has a fixed speed in any given medium. In a vacuum it is 3x10^8 mps (meter per second). And I understand that if I am moving away from a fixed point at 1 mps, that the relative speed of light perceived by that point is (3x10^8 mps) - (1 mps).

Given that, my question is: How do we know how fast an object is traveling on an astronomical scale? And how do we know if it is traveling towards or away from earth when the data we are collecting is anywhere from hundreds of years old to hundreds of millions of years old?

3/29/2011 2:37:27 AM

short answer: light is a self propagating particle wave, like any wave form it experiences a apparent shift in wavelength/freqency due to the motion of it's emission source.

exactly like doppler effect, or when a siren approaches, it sounds higher pitched and louder, this is wave compression, which when expressed in light = blue shift. it results when a object is approaching you.
when a siren passes you and begins to recede it sounds lower pitched and gets quieter, this is wave elongation, (or stretching), which when expressed in light = red shift. it results when a object is receding from you.

light, like sound still travels at it's predetermined speed regardless of the velocity of it's emission source. two objects one approaching and one receding when they pass each other at that exact moment the light gets to you in exactly the same amount of time from BOTH sources, but to do this the light from the object approaching you gets compressed, and the light from the object receding from you gets elongated. but both still cross the finish line at exactly the same time.

we can determine relative red or blue shift in stars and other objects by examining the emission spectra of them, that is certain shadow lines in the rainbow (yes the sun has them as do all stars, it's how they 'express' their color) we observe the shifting of these lines into the red or the blue end of the spectrum. we use this rather than just star color.

hope that explains it, go to wikipedia for more explanation and pictures of anything you don't understand. http://en.wikipedia.org/wiki/Redshift

[Edited on March 29, 2011 at 3:29 AM. Reason : link]

3/29/2011 3:25:48 AM

This is the powerpoint slide i show to the high school freshmen i teach...

Light moves at the same speed in space regardless of if the source is moving towards or away from the observer.

if the object is moving towards you, the waves are "squeezed" and therefore have a shorter wavelength (higher frequency... more "blue")

if the object is moving away from you, the waves are "stretched" and therefore have a longer wavelength (lower frequency... more "red")

[Edited on March 29, 2011 at 7:51 AM. Reason : ]

3/29/2011 7:48:58 AM

For some reason I thought you were a geologist.

3/29/2011 9:20:05 AM

even if he is, it's not like this isn't pretty elementary to a scientist type.

3/29/2011 11:14:13 AM

^^^ i hope that you give them a printout of that power point, thats a pretty terrible slide

3/29/2011 11:32:06 AM

Lrn2powerpoint

3/29/2011 11:39:09 AM

^^^ Oh absolutely. It's definitely at worst, HS freshman level. Probably more middle school.

I was just thinking he was a geologist for a living.

3/29/2011 11:42:34 AM

yeah it's not the best powerpoint slide... it's a reference i put up, but i actually teach it using old fashioned white board and markers.

i got my undergrad in geology, masters in science education... teach earth science and astronomy



[Edited on March 29, 2011 at 12:25 PM. Reason : ]

3/29/2011 12:24:54 PM

what no slate and block of chalk?

3/29/2011 2:56:15 PM

Ill repost sober, and when I'm not merging theories

[Edited on March 30, 2011 at 1:46 AM. Reason : .]

3/30/2011 1:18:34 AM

^^we are living in the dark ages here, but i've heard tell they have come out with that new fangled stuff in some places.

3/30/2011 8:10:02 AM

Quote :

"And I understand that if I am moving away from a fixed point at 1 mps, that the relative speed of light perceived by that point is (3x10^8 mps) - (1 mps). "

nobody talked about this ...

the speed of light is perceived to be 3x10^8 mps for ALL observers, no matter how fast you're moving or in what direction. So if you were moving at 1 mps, you would still observe the speed of light to be 3x10^8 mps. wacky, huh

3/30/2011 8:51:40 AM

^Absolutely, good call. With special and general relativity it's better not to rely on intuition: light doesn't behave in the same way as particles with mass.

Also, in terms of expansion of the universe, galaxies are accelerating away from us in all directions... one way to think of this is as a consequence of a change in the space-time metric (mathematical object used to measure distance). Think of our galaxy as a dot drawn with marker on the surface of a balloon, and other galaxies as dots surrounding us. As the balloon is blown up the dots move away from each other, not so much in the sense that they are moving along the surface of the balloon, but rather because the underlying structure of the balloon itself is expanding.

Quote :

" How do we know how fast an object is traveling on an astronomical scale? "

Stars are observable within our own galaxy. Distance and relative velocity can be measured using observations such as stellar parallax and luminosity: http://en.wikipedia.org/wiki/Luminosity#In_astronomy.

We can measure the distance to and motion of galaxies by comparing their observed-red shift and other observations to that predicted by the Hubble Constant: http://en.wikipedia.org/wiki/Cosmic_distance_ladder

Some good articles on the motion of local galaxies:
http://en.wikipedia.org/wiki/The_Great_Attractor
http://en.wikipedia.org/wiki/Shapley_Supercluster.

Also, the Cosmic Microwave Background Radiation pops up a lot when talking about astronomical distance scales. This is because it is a relatively uniform, observable consequence of a universe-wide event in the early stages of the formation of the universe and hence serves as a good comparison for other measurements.

3/30/2011 10:09:40 AM

^^ I did in my drunken rambling that I deleted

3/30/2011 3:27:27 PM

m/s = meters per second.

3/30/2011 3:40:00 PM

Quote :

"the speed of light is perceived to be 3x10^8 mps for ALL observers, no matter how fast you're moving or in what direction. So if you were moving at 1 mps, you would still observe the speed of light to be 3x10^8 mps. wacky, huh"

my brain just collapsed

[Edited on March 30, 2011 at 4:01 PM. Reason : ]

3/30/2011 4:01:14 PM

honestly, I thought this was common knowledge (for college educated folks). It's not true that the speed of light is constant in all mediums or that it's always 3x10^8 m/s but for a given medium it is constant for all observers.

[Edited on March 30, 2011 at 4:48 PM. Reason : clarification, most people are dumb as rocks.]

3/30/2011 4:47:50 PM

Ok, I'm really trying to read about time dilation and such (as I have tried many times before) and I'm still confused as hell. Let me try to condense my confusion and maybe someone can set me straight.

Suppose I travel 1ly away from earth at the speed of light (perhaps this throws the whole thing apart already, not sure... maybe .999c would suffice). Intuitively, I would guess that time would appear to stop on Earth and what I would see of Earth would appear the same as when I had left it.

If instead light is travelling at c relative to me, then it would have to be travelling 2c relative to Earth, no? And what I would see is Earth 6 months ago. Is that what we actually expect? Goddamn, I am lost.

[Edited on March 31, 2011 at 11:55 AM. Reason : asdfsd]

3/31/2011 11:54:11 AM

Quote :

"Suppose I travel 1ly away from earth at the speed of light (perhaps this throws the whole thing apart already, not sure... maybe .999c would suffice). Intuitively, I would guess that time would appear to stop on Earth and what I would see of Earth would appear the same as when I had left it. If instead light is travelling at c relative to me, then it would have to be travelling 2c relative to Earth, no? And what I would see is Earth 6 months ago. Is that what we actually expect? Goddamn, I am lost."

I tried to cross things out in order to make your question into a coherent question.

If you are traveling at 0.999c away from earth, and you've gone 1 light-year away from Earth in Earth's reference frame, then I can clearly tell you that you will see an image of Earth 1/1000th of a year later than when you left - that is, about 8.7 hours later.

Relativity is tricky because what you see is not what they talk about in the conceptual examples. If one sees an image in space, they can back calculate to draw conclusions. Most examples assume that you already do this. Statements about the behavior of something "in xx reference frame" do not refer to things will appear to that point, it refers to how an observer would infer things are happening. This is very difficult and one of the most hairy things about teaching this subject.

Now. light travels c relative to both you and Earth. According to you, light you emitted in the direction opposite to Earth is moving away at c, and Earth would claim it moves at c relative to it. The definition of time and distance adjust themselves to make this true.

3/31/2011 12:06:08 PM