Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: annie123 on 28/04/2008 20:42:30
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How can time be calculated - 'the universe is . . .years old', etc. when 'year' is a measurement related to the earth's turn around the sun, and these bodies haven't been around that long? rather a local thing? What does 'year' mean in cosmology measurement?
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A year is the time it takes our planet to make 1 orbit around our sun. If you were at the far ends of the universe, our planet would still take a year to orbit our sun. Therefore, 10 billion years is the time it would take our planet to orbit our sun 10 billion times. It matters not whether our planet was around 10 billion years ago or not; a year would still have been the same amount of time as we measure it today.
If you wanted, you could equate a year to decay of radioactive elements. I dare say a chemist or physicist could give you a precise definition in decay terms.
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You could ask the same question about distance, temperature and electrical quantities. The units that we use are arbitrary but convenient for us.
An alien in a distant galaxy would be using different units but its conclusions about the way the Universe works would be the same (Arguable, perhaps). For instance, the speed of light would turn out the same.
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Thanks to both people, but I still don't see how the time past before the measurement of the earth's orbit measurement can be measured. How can someone tell how many earth's orbits took place relative to the time between now and the Big bang?
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No one has actually taken a ruler to measure the distance from here to the moon but we all believe that the distance is what we have calculated. We have never been to the Sun but we believe it when they tell us how far away it is.
It's the same with time. We don't have to have 'been' at a particular time to estimate timescales in the future and the past. We rely on 'extrapolation' in most of Science, one way or another and it seems to work pretty well.
We can measure the rates at which things happen around us and this can give us a really good idea of when they started and when they are likely to finish. That time is just stated in terms of the units of time that we use.
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Thanks to both people, but I still don't see how the time past before the measurement of the earth's orbit measurement can be measured. How can someone tell how many earth's orbits took place relative to the time between now and the Big bang?
Forget that a year happens to be an orbit of the Earth around the Sun. Just think of a year as being approximately 31,557,600 seconds, and a second is 9,192,631,770 cycles of a particular frequency of radiation that was emitted by a caesium atom; so at least as far back as there were caesium atoms, you could count the number of cycles of radiation emitted by it, and so calculate a fictitious year.
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Or ask JimBob coz he's that old [:D]
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What? Who woke me up, I'm in the middle of my morning nap here. Cant an old guy get some rest?
[:(!]
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OK. You can use a cesium clock, but still, how can you measure how far back things happened? Regardless of the instrument of measurement you have to have a fixed point to go back to that must be verified by some evidence of existence at that particular time which can't be verified if time didn't exist then . . .?
And then the cesium measurements have to be converted to earth orbit measurements?
And how do you know how far back cesium atoms existed?
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If you came to your car and found that the engine was warm, you would infer, from experience, that it had been used between one and three hours ago (or whatever your tables told you, M. Poirot)
You did not need to see the car being used and start a stopwatch at the time in order to have a good idea of how long ago it was used. You use knowledge of the way things behave.
Step the process up, somewhat. By knowing the basic laws of Nuclear Physics and Gravitation, cosmologist can tell, from the size, brightness and a few other things, how long ago the star was formed.
If you are worried about extrapolating a long way back in time then you have a point.
Cosmologists work on the 'Cosmologigal Principle' it assumes that Science is the same everywhere in the Universe. It doesn't absolutely have to be true but there is an awful lot of evidence to support it. Light from distant stars has the same basic spectrum as the elements have in our labs and that implies, strongly, that the same Physics was going on when the light originated. It would be truly remarkable to find an alternative set of rules which would produce light that looked as if it was produced using a different set of Science rules so we assume they are and were the same.
What we see in the sky is light from stars in all sorts of places and (because of the time the light takes to get here) all sorts of times in the past.
However, Cosmologists can only do their best . . .
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Cosmologists work on the 'Cosmologigal Principle' it assumes that Science is the same everywhere in the Universe. It doesn't absolutely have to be true but there is an awful lot of evidence to support it.
re the above - time isn't the same in every part of the universe. Even just beyond our atmosphere it goes at a different rate . . . . so how can someone talk about it millions of light years out in space or back billions of 'years'
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That doesn't imply that the relationship between space, mass time etc. in other parts of the universe isn't the same.
You can more or less rely on the evidence that c measures the same everywhere - it's been observed to be true wherever we've looked.
Don't be in a hurry to challenge these basics without a lot of research on the topic. If you don't accept such fundamental ideas, you will have to come up with a huge, well constructed, alternative model of things which is consistent with all the things that have been observed.
Have another go at grasping the accepted theory first rather than rejecting it on subjective grounds.
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I agree with sophiecentaur: "Don't be in a hurry to challenge these basics without a lot of research on the topic."
I have learned that these basics cannot be explained in one or two nice, neat sentences. I have found that it requires the read of at least one good book on the subject just to start to understand how scientists arrive at these determiniations.
It is simply detective work on their part, and each step is pretty well scrutinized by the scientific community as a whole before it is generally accepted.
Also, there seem to be multiple ways to arrive at many of the same answers regarding things like red shift, background radiation, intrinsic star brightness, etc. all which are used to arrive at a generally accepted "age of the universe", "age of our solar system", etc.
Some fundamental concepts that I have accepted:
1) The universe is expanding (need to read a bit to see how this is determined by multiple methods);
2) Therefore, it is bigger now than it was when I started typing this post;
3) It must have been smaller yesterday;
4) There must have been a point when it was very small;
5) There must have been a point when it "started to expand"...a "big bang" if you will...
6) By determining its rate of expansion (in any units we please), we can probably take a pretty good guess as to when this "big bang" happened.
-Mark