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space-race
2004-Dec-02, 05:42 PM
the distance of remote objects is measured by the time it takes for their light to reach us,but how do we know how long the light took to reach us,how do we measure time backwards?does the composition of light change in some way the longer it has been in existence?what about the influence of the space it has to travel through?if you have two equal light bursts from equidistant sources with one burst travelling through "clear" space and the other one through gas clouds and skimming event horizons,would they take the same time to reach us?

Matthew
2004-Dec-04, 02:26 AM
Ok. Light travels at a very specific speed in a vacuum. Commonly denoted as "c". As this is so specific, we can measure the distance (using trigenometry), from the we just solve for t to find out how long ago the light took. The equation we use is:

v=d/t
t=d/v

Light's composition does not change if it does not interact with matter.


what about the influence of the space it has to travel through?if you have two equal light bursts from equidistant sources with one burst travelling through "clear" space and the other one through gas clouds and skimming event horizons,would they take the same time to reach us?

Relativity states that light will take a different course if it is in a gravitational field. Such a field would mean two equidistant sources of light will vary in time taken to reach us if they were in a different gravitational field. So, if a beam of light were to pass near a black hole its course would be radically altered.

antoniseb
2004-Dec-04, 03:33 AM
Originally posted by space-race@Dec 2 2004, 05:42 PM
how do we know how long the light took to reach us, how do we measure time backwards? does the composition of light change in some way the longer it has been in existence?
We have looked at everything we can see and study in the universe and have made an educated guess that it is 13.7 billion years old. This number might get reported a little differently after the WMAP year two data comes out, but it won't be VERY different from this.

The individual photon itself does not change with age, but collectively we can measure the redshift of lines in the spectra of light from a given source. Today's cosmology assumes that there is a close correlation between red-shift and distance, and we use this idea to set an age for the light.

I think the science behind all this is pretty solid, but you can find people posting in the alternative theory section that are not so sure. Virtually all science articles mentioning the age of an object make this assumption. If you want to join the ranks of the doubters, you need merely translate what the writers are saying into your own interpretation. Usually this is simply a statement that the object is red-shifted, and no other information is confirmed.

Sphinx
2004-Dec-04, 05:32 AM
so how does one measure light in order to determine the distance of it's source? A visual discription of the steps involved would be ideal...thanks

Matthew
2004-Dec-04, 05:40 AM
Although I have no visual description here's a basic outline.

Using trigonometry we can look at an object when the Earth is at opposite ends of the sun (ie. measure things in a six month cycle), as we know this distance we have our "baseline". By measuring the angle that the object appears to move we have our angle. Dividing both the angle and baseline by two you get a nice little right hand triangle. Using the tan ratio we can find the distance.

Another option are standard candle sticks. These, while not being candles, are astronomical objects which we know to shine with the same intensity all the time. By measuring its apparent brightness, solving a few equations, you get the distance to the object. That can then be used to find the distance to other object in the region.

Both of these methods get less accurate the further from earth you get. Does anyone know which method is more accurate?

Guest
2004-Dec-06, 08:26 PM
As Antoniseb mentionned briefly, the age of a photon is non-existant. The photon does not change with time. Even if it is redshifted or blueshifted.

We can figure out the 'travel time' of a photon, which is also the age of the 'image' we are seeing. But photons themselves do not age.

antoniseb
2004-Dec-06, 08:35 PM
Originally posted by Sphinx@Dec 4 2004, 05:32 AM
so how does one measure light in order to determine the distance of it's source?
For the most distant objects, we do not get enough photons from it to provide a detailed spectrum, but it is usually assumed that the majority of the ultraviolet light emitted by the object is from Hydrogen, and that the shortest wavelength of Hydrogen light is [before red shifting] from 90 to 120 nanometers wavelength.

The object is imaged in various bands of light including a few shades of red and infrared, and they look to the see shortest wavelength light that they can still see the image in. This tells them roughly what the red-shift is. They get more detail by interpolating the brightness in the longer wavelength bands to determine a good guess as to the actual cut-off.

This degree of red-shifting [e.g. z=6.35] is plugged into the current formulas for redshift at a given age, and voila, we have the age & distance of the object. Alternative theories may give different results.

Sphinx
2004-Dec-07, 04:02 AM
so basically you don't have to quite understand all of the factors involved or even why the numbers coming into play are coming into play...just the proper equipment and the proper numbers utilized in the proper manner with the proper equation, right?

Who came up with the procedures and do they go into great detail as to why the equations are the way they are and why the factors involved are factors? Are there simple experiments one can conduct to actually measure light in their home?....very simple :D

antoniseb
2004-Dec-07, 04:23 AM
Originally posted by Sphinx@Dec 7 2004, 04:02 AM
Are there simple experiments one can conduct to actually measure light in their home?....very simple
Not to my knowledge, not if you're talking about the light from distant galaxies. This can't be detected without giant mountaintop or orbiting telescopes with very sensative detector arrays.

Concerning who came up with the procedures, several generations of observational astronomers from universities and institutions all over the world. The process evolved, and varies according to the equipment used.


just the proper equipment and the proper numbers utilized in the proper manner with the proper equation, right?
Yes, that gives you the age and distance, when you presume one of the current models is close to correct.

Sphinx
2004-Dec-07, 02:43 PM
lol, ok I think I've got it. Do the equations assume that light is always traveling at c or does it take into account the probability of it interacting with other mediums on its path to get here? This could probably be somewhat controversial and be the madness behind a few of the differing methods????

Dave Mitsky
2004-Dec-07, 03:36 PM
Originally posted by matthew@Dec 4 2004, 05:40 AM
Although I have no visual description here's a basic outline.

Using trigonometry we can look at an object when the Earth is at opposite ends of the sun (ie. measure things in a six month cycle), as we know this distance we have our "baseline". By measuring the angle that the object appears to move we have our angle. Dividing both the angle and baseline by two you get a nice little right hand triangle. Using the tan ratio we can find the distance.

Another option are standard candle sticks. These, while not being candles, are astronomical objects which we know to shine with the same intensity all the time. By measuring its apparent brightness, solving a few equations, you get the distance to the object. That can then be used to find the distance to other object in the region.

Both of these methods get less accurate the further from earth you get. Does anyone know which method is more accurate?
Trigonometric parallax is accurate to about 100 light years. Other methods of estimating even greater distances are discussed at these sites:

http://www.astro.gla.ac.uk/users/martin/ou.../distances.html (http://www.astro.gla.ac.uk/users/martin/outreach/distances.html)

http://www.talkorigins.org/faqs/astronomy/...y/distance.html (http://www.talkorigins.org/faqs/astronomy/distance.html)

http://www.bbc.co.uk/dna/h2g2/A578676

http://www-astronomy.mps.ohio-state.edu/~p...nit5/dist3.html (http://www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Unit5/dist3.html)

Dave Mitsky

antoniseb
2004-Dec-07, 03:38 PM
Originally posted by Sphinx@Dec 7 2004, 02:43 PM
Do the equations assume that light is always traveling at c or does it take into account the probability of it interacting with other mediums on its path to get here?
We know about how much material is in the column from us to distant objects. We can tell from the absorbtion spectra. We know that the light from stars in a galaxy ten billion light-years from us are probably travelling through as much gas as though it travelled through a few yards of Earths atmosphere. This slows the light down very slightly [attoseconds]. A bigger factor is the occasional curved path from a large intervening galaxy, which in some cases can make the path a light-year longer. This is rare, but even so, not in the significant digits of age or distance.

antoniseb
2004-Dec-07, 03:40 PM
Originally posted by Dave Mitsky@Dec 7 2004, 03:36 PM
Trigonometric parallax is accurate to about 100 light years.
Thanks Dave, that's a good point to add. In a decade or less the Gaia probe will increase the distance for good parallax measurements to about 5000 light years, but that is the future, and this is now.

astromark
2004-Dec-08, 02:39 AM
The original question; The age of light.
Well its as old as it has taken to reach your eye or camera. It does not seem to deteriate with the passing of time or distance traveled. Which are the same thing if my understanding of the nature of light is right. Some of it may be as old as 13 billion ly or more and its fine. Thats a clever trick.

Sphinx
2004-Dec-08, 04:55 AM
but what happens when all of the photons emitted from the source have passed us. Can it become theoretically no longer visible? Perhaps you would have to chace the light to be able to see it. <_<

Another hypothetical question. You&#39;re a space traveller travelling towards an object 100,000 light years away. We&#39;re going to assume that you can live this long and that it would only seem to take 1/2 a day or so. On this journey you are looking at the object the entire time. At the start you would be seeing it as it appeared 100,000 years ago but as you got closer the light would be getting more and more present making the object appear to speed up dramatically, hence allowing you to watch 100,000 years of a process in 1/2 a day.

Just a thought experiment I thought I&#39;d share. Pretty interesting.

antoniseb
2004-Dec-08, 05:20 AM
Originally posted by Sphinx@Dec 8 2004, 04:55 AM
what happens when all of the photons emitted from the source have passed us. Can it become theoretically no longer visible?
If you and I are outside somewhere, and I make a camera&#39;s flash-bulb go off, the photon very quickly are all passed you. It the flash bulb no longer visible? A minutes later, a friend comes outside to join us. He has no evidence that there ever was light.

zephyr46
2004-Dec-08, 05:54 AM
The detection of intersteller gasses through absorption lines (http://www.site.uottawa.ca:4321/astronomy/index.html#absorptionline) has gone into detecting and modelling the LIC (Local Interstellor Cloud) (http://www.astro.uu.se/forskarutb/LISM/LISM.html)

http://origins.colorado.edu/~sredfiel/licaas.gif
http://origins.colorado.edu/~sredfiel/

Frozen light (http://www.physics.hku.hk/~tboyce/sf/topics/lightfreeze/lightfreeze.html)