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Beards
2006-Sep-05, 10:15 AM
I was pondering this the other night:-

Why, when we look at the distant universe, why are we looking back in time?

"Because the light takes millions of years to reach us!" I hear you cry.
Well I thought that that is just from our FOR. Light travels at light speed and so reaches its destination instantaneously. Therefore, surley, as soon as the light leaves a distant galaxy it reaches us at the same instant it left?

I thought that maybe the expansion of the universe caused light to slow but then I read in another post, that light never slows down - it is always a constant speed.

Anyone shed some light (boom boom) on this for me purlease?

Why can't I seem to wrap my head round this? What's wrong with my reasoning as I'm sure you're all gonna give me an obvious answer and I'll be kicking myself again!:confused:

RussT
2006-Sep-05, 10:51 AM
[Light travels at light speed and so reaches its destination instantaneously.]

Here is the culprit.

Frames of reference certainly can be confusing in many different instances (pun intended).

Light speed is ~186,000 miles per second.

So it takes a little over a second to get from the earth to the moon and about 8 minutes to get from the sun to the earth. The nearest star is about 4.3 light years away etc.

Light speed is only instantaneous in 'its own reference frame'.

Don't kick tooooooo hard, it's all a learning experience.:)

V-GER
2006-Sep-05, 11:04 AM
So if a space craft were to travel at the speed of light to say, Andromeda,
the trip would be, instead of 2,2 million years, instantaneous for those on board? I didn't know that, interesting.


edited to add:

Is this due to photons not having a mass and therefore no time?

Ken G
2006-Sep-05, 11:50 AM
Yes, on board it's very short. Time is not absolute, so you might think it would be impossible to talk about the "age" of a galaxy in any kind of unique way. Fortunately, two factors rescue us. First of all, there is one pretty clear reference frame from which to talk about ages in the universe-- the mean frame of whatever galaxies are nearby the point in question. (This is called "proper" time, in the French sense of the word propre, meaning "own", not "correct", and when you apply it on average to a bunch of galaxies, you are using the "co-moving frame".) So that's what we use. We'd still have a hopeless muddle, however, if not for the Big Bang, which synchronizes all these proper-time clocks to the same beginning. So putting these two bits together, now it makes sense to compare the ages of the things we are looking at to the overall age of the universe. By the way, when you do this, it means that we are seeing galaxies from a younger epoch of our universe. But you said the light is old, not the galaxies, and that's correct, if we stick to the convention of always using the proper age of the local matter to discuss the elapsed time between the creation and destruction of some photon. That's kind of "universal standard time", if you will! But to a photon, note this is more like a distance scale than a timeframe, and yes, the lack of mass is the key there (no rest frame).

antoniseb
2006-Sep-05, 12:20 PM
Light travels at light speed and so reaches its destination instantaneously.
You probably already have your answer from the above, but just to look at it from a slightly different angle: You are right that the photon itself should be well preserved and seem as young and fresh as the moment it was created, though it has been stretched out along the way by space expanding around it. As Ken and others have noted, the material that created the photon are not moving as fast, and there is a duration for the movement of the photon in our frame of reference, so what we see happened a long time ago.

In a related idea, if you are driving in NYC, and the traffic light turns green, you have about 25 nanoseconds between when you see the green photons, and when they car behind you sees them. This is your window to start moving the car before he honks the horn.

Beards
2006-Sep-05, 12:35 PM
So at this early stage of my learning I should really disregard the idea of "instantaneous travel" as it's not really relevant to our FOR in the universe and only really applies to photons.
And, of course, causes confusion for me.

Therefore light travels at 186,000 mps for everyone and everything, but to the light itself it's speed is infinite? Is my understanding correct?

Ken G
2006-Sep-05, 01:29 PM
Therefore light travels at 186,000 mps for everyone and everything, but to the light itself it's speed is infinite? Is my understanding correct?

You'd think, but your are forgetting the length contraction. For a photon, not only is all time and instant, but all space is a point. The speed is still c, in the limit as you divide a small number by a small number.

Beards
2006-Sep-05, 02:22 PM
For a photon, not only is all time and instant, but all space is a point.


Mmmmmmmmm. Does that mean at light speed that time = 0 and space = 0?

Ken G
2006-Sep-05, 02:27 PM
Yes, you could say it that way, but to be meaningful you'd want to take a limit as the speed approached c. Only frames moving somewhat less than c are meaningful frames of reference.

Beards
2006-Sep-05, 02:37 PM
Only frames moving somewhat less than c are meaningful frames of reference.

Sorry Ken but I don't fully understand what you mean.
Do you mean that it's not a meaningful frame of reference as nothing (apart from photons) can travel that fast?

Ken G
2006-Sep-06, 02:59 PM
Yes, that's what I mean. A reference frame requires observers to be meaningful, and observers have mass, so far as we know.

Beards
2006-Sep-07, 12:51 PM
Thanks Ken.

I really seem to be having trouble understanding exactly what light is. I understand that it is part of the EM spectrum and is quantified by photons, but can it really be a part of our universe?
The odd properties of light just don't sit with me at all. They seem to suggest that it's outside of our universe and we are merely seeing the effects of it on our universe.
How can we never "catch up" with a light wave? Why does it appear to be travelling at c no matter the velocity of the observer? It feels very much to me that we think we know what light is, but do we really fully understand exactly what it is?
Is there a reason it appears the same at all reference frames?
How can light be redshifted?
If we can't catch up with a light wave, how can we be so sure that redshift is the stretching of it's wavelength when we can't directly observe it even has a wavelength?

Pardon the rant but my ignorance is beginning to annoy me!

mahesh
2006-Sep-07, 01:40 PM
...snip....
In a related idea, if you are driving in NYC, and the traffic light turns green, you have about 25 nanoseconds between when you see the green photons, and when they car behind you sees them. This is your window to start moving the car before he honks the horn.
:D

it's 30, here in london, Beards.....but only because the guy behind stops a little further back

Ken G
2006-Sep-07, 06:06 PM
How can we never "catch up" with a light wave? Why does it appear to be travelling at c no matter the velocity of the observer?

This question, and your other ones too, are excellent questions that may not really have a good answer. You may be trying to know your universe at a level of intimacy that is actually not available to us, our attempts to make sense of what we see are subject to certain realities that just are. Future theories may shed new "light" on the questions (string theory probably has something to say, for example, and who knows what the "final theory" will be, if there ever is such a thing). Nevertheless, relativity already gives us some insight into your questions, which I will attempt to convey as best I understand. But note that only the predictions of a theory can be tested, not the pictures we use to guide us in applying said theory.

When you are seeing the propagation speed of light, you may not actually be observing a property of light, so much as you are a property of space and time. The very "fabric" of spacetime is what is revealed when you release a massless particle like a photon. Perhaps it is somewhat like dropping a marble and a bowling ball-- they both fall together, not because of the properties of marbles and bowling balls, but because of the properties of what is happening to the space and time in which they find themselves (basically, gravity is doing something to the space and time, and the objects are "tracers" of this, which we mistakenly interpret as something happening to the objects rather than something happening to us due to the rigid surface of the Earth beneath our feet). Another way to imagine this point is that if everyone else is doing one thing, and you are doing something else, it is foolish to expect that what needs to be explained are the actions of everyone else-- it is your own action that requires explanation in that situation.

In a similar vein, I think that the reason you can't stop or slow massless particles moving through vacuum is that it isn't the particle itself that is doing anything particular, it is the very nature of your own concept of a vacuum that you are learning something about. Even if there was no such thing as light you could learn the same thing by looking at the behavior of particles with successively lower and lower rest masses.

Beards
2006-Sep-08, 11:21 AM
Aaaaaaah! So I'm asking the same question that modern science itself is asking!

Nice one Ken - I don't feel half as stupid anymore. That was a very succinct explanation considering the subject matter. It's so easy to expect all of creation to be explainable in our own worldly terms but, as I'm beginning to see, we need to change our thinking and try to come at it from a different point of view.

Your explanation of the bowling ball and the marble has made me start to think of things differently and not purely from our perspective. We don't really know the half of it do we? (At least I don't!)


the reason you can't stop or slow massless particles moving through vacuum is that it isn't the particle itself that is doing anything particular
Woah!!!!

Food for thought indeed.

Ken G
2006-Sep-08, 01:47 PM
Yes, it is certainly interesting to speculate how our understanding may increase in the future, but I think this is pretty much where we stand at the moment.