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Cuddjnu
2003-May-26, 10:11 AM
Ok, i'm new to the board and have a nagging questiong that has been bothering me for a while....
If all the universes are flying apart from each other at more than the speed of light(I can't remember where i heard this, please tell me if this is this is the false part of my question), how can we see the light from these universes? I mean, if you are traveling at 60 MPH in a car and throw an object in the opposite direction, the object will go at 60 MPH minus the strength of the throw (or something like that) in a perfect vacuum. So if light is being 'thrown' from all these galaxies and stars, shouldn't the speed/velocity make it go the other direction from us? therefore making it so that we will never see the light from the galaxies?

Sorry if this is a dumb question, but I get headaches thinking about it when I am out to sea(the blackest sky you have ever seen, to bad the ship rocks to much to use a scope)

Thanks,
Shawn

ocasey3
2003-May-26, 01:10 PM
Welcome to the board!

I think when you say "universes" you mean galaxies, no? With my limited knowledge I can say that not all galaxies are moving away from each other. In fact, some are moving toward each other and some have even sort of "crashed" into each other. I remember reading that our galaxy, the Milky Way, and Andromeda are moving closer together.

Hubble image of two galaxies colliding:
http://astrowww.phys.uvic.ca/~patton/openhouse/antennaehst.jpg

Page about Andromeda and the Milky Way:
http://www.cita.utoronto.ca/~dubinski/tflops/

Light is not "thrown" but emitted and the light from galaxies are emitted in all directions. The light we are seeing is basically millions of years old and we are actually seeing these galaxies as they appeared then.

Hope this helps.

ToSeek
2003-May-26, 02:32 PM
Galaxies are generally not separating from each other at speeds greater than that of light. The current best estimate for the separation speed set by the expansion of space (the Hubble Constant) is 71 kilometers per second per megaparsec. For example, a galaxy one megaparsec (3.26 million light years) away would be heading away from us at 71 km/second. With the speed of light at 300,000 km/sec, galaxies have to be very, very far away from one another to be separating at the speed of light. You are correct that if there are any galaxies that far away, we would not be able to see them. In fact, someday (since the universe is expanding), all of the other galaxies may be that far away. Lonely times....

Argos
2003-May-26, 02:49 PM
Ok, i'm new to the board and have a nagging questiong that has been bothering me for a while....
If all the universes are flying apart from each other at more than the speed of light(I can't remember where i heard this, please tell me if this is this is the false part of my question), how can we see the light from these universes? I mean, if you are traveling at 60 MPH in a car and throw an object in the opposite direction, the object will go at 60 MPH minus the strength of the throw (or something like that) in a perfect vacuum. So if light is being 'thrown' from all these galaxies and stars, shouldn't the speed/velocity make it go the other direction from us? therefore making it so that we will never see the light from the galaxies?


Speed of light is constant; in fact it is one of the constants of nature. The speed of the source doe not affect the speed of light. The only effect of the speed of the source on a ray of light is to change the frequency (the wavelegth) of the emitted ray.

Light propagates in all directions. If the source of light is in motion, the wavelength of the light pulse will be shortened in the direction of motion, making it shift to the blue regions of the spectrum. The opposite direction will exhibit a shift to the red regions of the spectrum. Because light propagates in all directions we can see other objects in the universe, provided that the light is intense enough to reach us. Because of the shifts (what is called "Doppler Effect"), the light of those objects can be detected, but shifted to the red when they get farther and shifted to blue when they get closer. In the large scale, the component parts of the universe (groups of galaxies and supergroups of galaxies) are getting apart, in an expansion. This makes us see the light of such objects as shifted to the red. But we can see blue shifted light from approaching objects like Andromeda galaxy (a member, along with our own galaxy, of what is called "Local Group" of galaxies), which is now coming close to our galaxy.

Allow me to suggest some key expressions for a web search: LOcal Group; Doppler Effect; Hubble constant; Red shift; expansion of the universe; Relativity.

Also, wait for contributions of people more skilled than me.

And there´s no such thing as a dumb question. :)

Hale_Bopp
2003-May-26, 04:19 PM
Well, good question actually. We can only see galaxies that are withing about 13.7 billion light years of us. Why? The unvierse is 13.7 billion years old (according to the recent WMAP results). There are probably galaxies farther away that we can't see because the light has not had time to reach us.

Okay, now the fun part. Since the discovery that the rate of expansion of the universe is accelerating, the picture changes. As time goes on, more and more of the universe will be outside our "light cone" (the section of the universe whose distance in light years is less than the age of the universe so we can actually see it). Therefore, in the future, we will be able to see fewer and fewer distant galaxies as they get farther away from us at ever increasing rates.

Fun, huh?

Rob

DStahl
2003-May-27, 08:17 PM
It is indeed an excellent question. If I may expand on Hale_Bopp's answer...

Some quasars are calculated to be moving away from Earth faster than the speed of light (well, if one simply says that the distance between Earth and the quasar is increasing at more than 300,000 kilometers per second). So how in the world can we see light from something that is retreating from us faster than the speed of light?

Well, just as Hale_Bopp wrote, the light we see now left the quasar 13.7 billion years ago. At that time the quasar was not nearly so far away, and its relative velocity was not nearly so great. The light we see today simply didn't start out from the quasar when it was retreating from Earth so fast.

Eh, but since the light we see now is 13.7 billion years old, how do we know that the quasar is currently moving away from us so fast? How can we tell anything about the quasar now from light that left it so long ago?

Well, the light has been crossing an ever-widening expanse of space for 13.7 billion years. In that process the wavelength of the light itself has been stretched a bit; the group of spectral lines that are the "fingerprint" of hydrogen, for instance, occur farther toward the red end of the spectrum than they do when we look at light from our Sun or at light from a laboratory source. So the light itself is telling us, "I have crossed space which has stretched by a factor of 4 since I started my journey." That in turn tells us how much the space between us and the quasar has probably stretched, and THAT tells us roughly how fast the quasar must be retreating from us due to this stretching.

It turns out that the stretching of space allows light to reach us from objects that are calculated to have a recession velocity several times the speed of light, while we will never be able to see any light from other objects that were just a little farther away from us to begin with.

beskeptical
2003-May-28, 07:09 AM
It is indeed an excellent question. If I may expand on Hale_Bopp's answer...

..........

Boy Dstahl, I'm going to have to think about that for days before it makes sense in my Universe contemplation sessions. :o

DStahl
2003-May-28, 08:32 AM
Aw, geez, I was really trying to be clear...

I once wrote a little program that let one specify how fast the linear space between two dots on a line expanded, and how far the dots were apart to start with. Then the program launched a light beam (represented by a yellow line) from one dot toward the other across expanding space. It worked OK, though it crashed if one let it get into really huge numbers--bad programming.

Anyway, I think the graphical representation made it a lot clearer for me. Maybe tomorrow I'll post some screenshots from the prog.

DStahl
2003-May-28, 08:59 PM
OK, here's a screenshot from the prog I mentioned. I stacked three clips from the animation on top of each other to show how the thing runs over time. In the animation window the dark blue line represents space as a 1-dimensional string; the green dot on the right represents the source of a light pulse, and the green dot on the left represents the receiver some distance away. The yellow dot represents the light pulse itself, and the yellow line behind it represents the distance the light has travelled "under its own power"--ie, just the time it's been travelling multiplied by the speed of light.

So in the top animation window the light pulse has left the source and started on its journey toward the reciever. But the blue line is expanding, carrying the green dots apart and stretching everything between them as well.

In the middle animation window you can see that the dot which was the source of the pulse has moved quite a long way to the right of its original position, and there's a space now behind the light pulse which is "new space" that the light pulse never had to cross. But the space which the light pulse DID cross is also stretching (in this model, all the space stretches equally, all the time) and so the prog has added a red line to represent the strretching of space that the light pulse has already crossed.

Another way to look at it is that the red line also represents the amount by which the light pulse has been redshifted by the expansion of the very space it is crossing!

In the bottom animation window the light pulse has reached the receiver. Note that the source is now a LONG way to the right of its original position, and there's a lot of empty "new space" which the light pulse never had to cross because it was "created" after the light had gone by. Also notice that the light got a substantial "boost" in distance it travelled due to the expansion of space--the red line accounts for roughly 25% of the total distance the light travelled, so by the time the light reached the receiver it had "travelled" 25% further than it could have by simply crossing space at the speed of light!

----

In this run the two light source and the receiver started out 7,500,000 light years apart, and space expanded at a rate of .00001% per year. There was no acceleration or deceleration of the expansion. At the end of the run, when the light pulse reached the receiver, the recessional velocity of the source relative to the receiver was over 1.5 times the speed of light, and the light had been travelling for roughly 13 million years. These numbers aren't really related to the real Hubble expansion rate or anything, they just gave a nice graphic run... ;)

Sorry the graphic is so wide:

http://frogshop.hypermart.net/images/stringspace1.gif

BigJim
2003-May-28, 09:08 PM
Also , in relation to what DStahl said, many quasars also appear to eject gas jets that travel in our direction at up to twelve times the speed of light. How is this possible? Consider a gas cloud moving at 95% of the speed of light towards us, and it emits a light beam. 35 years later, the light has traveled 35 light-years, but the cloud has traveled 33 light years. So when it emits light from there, it will appear that the light that was emitted 33 years after the first one was only emitted 2 years after, as it will reach us merely two years after the first light ray.
This special edition of Scientific American is a MUST (http://www.sciam.com/special/index.cfm?issueid=6&lsource=bookstore&sc=bookstore) for this topic.

Argos
2003-May-29, 12:12 PM
OK, here's a screenshot from the prog I mentioned.

Congratulations! Very nice GUI. What language did you use to develop it?
It seems to be a VBA project.

newt
2003-May-30, 12:32 AM
Great thread. When I first read about the "light cone" that Hale_Bopp mentioned, it was a trip. The discussions of the limits of 'C' notwithstanding,
"Happily we have one check. When we look at the universe on a cosmological scale, we do not see the universe as it is at present, but as it was in the past. The reason is not difficult to understand. It depends upon the finite speed of light. To see the universe, we need light, but the light only goes at a certain speed, and no faster. On a cosmological scale, it may take light millions or even billions of years to journey to us from distant galaxies - so what you see is not what the galaxies are like at present, but what they were like in the past. Even the nearest of major galaxies, the Great Galaxy in Andromeda is seen as it was two million years ago. The further we look out into space, the further we see back into the past. Indeed, we can see almost the entire history of the universe."

What does that mean? Damned if I can explain it, except that it may modify the apparent discrepancies in the cumulative relative accelerations (also referred to by Cuddjnu ) of the galaxies (see comment by ocasey3). For a discussion of large-scale structures and the use of accelerations as units of distance
, see http://web.uct.ac.za/general/inaug/fairall.
Cheers. Newt.

tracer
2003-May-30, 01:09 AM
Congratulations! Very nice GUI.
Except I'd've put a little bit of space between each of those three "Go" buttons in the upper-left, and it's got a narrow band of junk above the second graphical panel that needs to be repainted.

Cuddjnu
2003-Jun-12, 11:05 AM
Wow, thanks everyone, now i'm going to get a headache thinking about all these new revelations you guys opened my mind to.
:)