Way back when, I was taught that celestial distances were measured by the amount of “red shift,” i.e., Doppler shift, of the light spectrum of the object in question. The universe is expanding, and the further away the object, the greater its red shift, so a simple proportion determines the distance.
This has a circular logic problem. How do we know how far away an object is? By its red shift. How do we know what the red shift is? By how far away the object is. What’s missing is proof that distance and red shift are related – that expansion of the universe is actually occurring and that it increases with distance in a constant and predictable way.
Has triangulation and standard candle calculation replaced red shift calculation?
I seem to recall something like that red light thing from way back when in my high school physics class (two years ago.) Only we learned that they use a black light radiation curve, or something like that.
It has to do with the fact that a objects that give off UV light (i.e. stars) have different values of some aspect of the light (wavelength, or amplitude, or frequency) at certain distances away from the star.
Since we know how far away close stars are (the sun, alpha centauri, and others) using a simplier method (like the one Cecil described) scientists can then plot what the curve of the light property over distance looks like.
The redshift of an object only directly tells you the radial velocity, not the distance. The two are related, by Hubble’s expansion law, but until recently, the calibration was rather poor: We knew that faster objects were farther away, but we didn’t know exactly how much farther away. It was only with the launch of the Hubble Space Telescope that we were able to get good data on the Supernovas 1a (in fact, that was the Hubble’s primary mission), giving us reliable distances out to the edge of the observable Universe. Up until that time, we had to calibrate the Hubble law based on closer objects, the distances to which were almost all measured by standard candle methods, using dimmer candles than the supernovas.
bouv, what you’re thinking of is the blackbody radiation curve, which is the distribution of light that is produced by a black object (i. e., one that absorbs all light that hits it) when it’s heated to a certain temperature. There’s very few perfect blackbodies known, but most stars are a pretty good approximation. In addition, stars will have dark lines in the spectra corresponding to the elements in the star’s atmosphere. Since all stars are made of about the same stuff (mostly hydrogen), we know where those lines should be, and measuring where they actualy are is how we determine the redshift.
I believe this column marks Chronos’ first Staff Report. Congratulations on this elevation in the ranks of the Straight Dope Science Advisory Board. Couldn’t happen to a nicer, or (more importantly) smarter guy.
I want to add my congratulations to those of Saltire. Good going Chronos.
But next time, if you don’t say something controversial, please include some simple error in your column for us point out and feel superior to you about. After all, we have to have something to discuss on this message board.
Let me add my welcome and congratulations to the others’ Chronos.
I do have a question, though, regarding the use of type Ia supernovae as standard candles for very remote objects - how can that method allow for the possibility of intervening, obscuring matter along the line of sight?
This “dust” could be in our galaxy, could be between our galaxy and the supernova, or could even be present in the host galaxy. I suppose the first two potential sources could conceivably be eliminated by statistical means and arguments, and by specialized knowledge of our “local” environment. But how could you ever know anything about “dust” location in the host galaxy.
Well, most dust will have a reddening effect: It absorbs light at all frequencies, but emits it primarily at low (red) frequencies. We have some idea of what the spectrum of a supernova should look like, from the nearby ones, so we can tell how much it’s been redened. On the other hand, some folks have proposed that there’s some sort of “gray dust” which might be throwing off the distances, as an alternate way to explain the somewhat uncomfortable result that the Universe appears to be accelerating. There’s a few dust models which could account for “gray” absorbtion, such as each “dust” grain being about the size of a baseball, but these are considered unlikely.
Even gray dust wouldn’t have much effect in our own galaxy, because the supernova distance scale was calibrated based on events outside our own galaxy to begin with, so any error involved will be cancelled out.
Oh, and dtilque, there’s a misspelling somewhere in this post. Feel free to find it.
I was just gearing up to become your archenemy hereabouts
(in a friendly way, bu-wa-ha-hah!) so imagine my
surprise at seeing you “promoted”. Guess I’ll just have to
work that much harder. . .
Should be:
There are a few dust models which could account for “gray” absorbtion, such as each “dust” grain being about the size of a baseball, but these are considered unlikely.
More likely colloquial. While it’s technically incorrect, it’s a common usage in everyday speech which would make it an idiom. He shouldn’t use it in his dissertation, though.
No there was a misspelling which I found but will leave as an exercise for the reader.
But misspellings were not the kind of error I was looking for. Perhaps something like: “Mars is 725 million kilopascals away from Earth.” while everyone knows that it’s really 724.99 million kilopascals away.
And just so’s nobody starts thinking that the “reddening” perpetrated by interstellar dust can throw off our red-shift measurements:
Dust-reddening is a very different effect from the red-shifting of a spectrum due to velocity or high gravity. It is very easy to tell the two apart.
Suppose this is what the spectrum of Star X looks like with NO dust-reddening or red-shifting getting in the way:
-----|—|------i–||----|–|—
The "|"s are bright lines, and the “i” is a dim line.
If this star’s spectrum were subjected to interstellar reddening via dust, it would look like this:
-----|—|------i–ii----i–i—
On the other hand, if it were red-shifted due to the star moving away from us, it would look like this:
-|—|------i–||----|–|-------
In the first case, the higher-frequency lines are dimmer. In the second case, all the lines are “shifted” to a lower frequency (longer wavelength), and none of them are any dimmer or brighter.
Dust reddening has no effect whatsoever on the amount of redshift.