Relativity Question
I have a few questions on the speed of light and how that relates to the age of the universe. I will preface these remarks with the disclaimer that while I have read a bit on relativity from time to time in an attempt to understand it, I unfortunately have found that most of it seems to be beyond my ability to grasp. I was pondering recently while reading an article about the Big Bang and the age of the universe. It was probably one of numerous recent articles about Einstein's life. From my understanding, Einstein determined that because the speed of light is constant, there must be variability in time to make up for this. This explains why time moves slower or faster for two relative observers. So here is the meat of my question. I read in books and magazine articles about the age of the universe. Since time is relative, what does it mean that the universe is so many billions of years old? Don't we need to know billions of years in relation to something? Does that just mean that given the current size of the universe (as we have measured it) is so many light-years across? Or does our age-estimate have more to do with the rate of expansion extrapolated over time? If two beams of light are leaving the Big Bang at the same time traveling in opposite directions, what is their time relative to each other? If they travel for one year in their time are they two light years apart? Or is it less because if they are two light years apart, they would be traveling at two times the speed of light relative to each other? Thanks in advance for any light that can be shed on this. :) Aaron
Aaron: I don't pretend to be able to wrap my mind around these issues - usually I just take Mr. Einstein's word that his concepts are correct. However, I think that some of your reasoning is flawed, based on our propensity to picture a multi-dimensional universe with our three-dimensional human perspective. (Ok, for some it may be only two- or one-dimensional.) First, I think that the age of the universe is fairly well established, at least within the usual cosmological constraints, by several means, only one of which is by extrapolating the observed expansion rate. Second, the problem with looking at two photons traveling in opposite directions is that the universe itself (each photon's frame of reference) is expanding at the same time. It's not the same as two cars leaving from the same point at 50 mph and driving in opposite directions, with a speed relative to each other of 100 mph, but it's more like two drivers leaving from the same point on a highway that also gets longer the more you drive...or something like that. (Sorry, that's the best analogy that my one-dimensional mind can think of.) So, I have my own question: Is it true that two photons can only be traveling at twice the speed of light relative to each other in relatively small reference frames, such as in the "jets" observed to be leaving some stars and galaxies, and from our perspective, but not in a universal reference frame? Or, in other words, two photons may appear to us to be traveling in opposite directions at twice the speed of light, but from each photon's perspective, the other is only traveling at the speed of light? Is my reasoning even close to Einstein's? (I failed high school physics, by the way.) If a tree falls in the forest and I'm not there to hear it, does the tree exist? ----- Original Message ----- From: "Lambert, Aaron" <Aaron.Lambert@Williams.com> To: "Utah Astronomy" <utah-astronomy@mailman.xmission.com> Sent: Wednesday, July 06, 2005 9:49 AM Subject: [Utah-astronomy] Relativity Question |I have a few questions on the speed of light and | how that relates to the age of the universe. | | I will preface these remarks with the disclaimer | that while I have read a bit on relativity from | time to time in an attempt to understand it, I | unfortunately have found that most of it seems | to be beyond my ability to grasp. | | I was pondering recently while reading an | article about the Big Bang and the age of the | universe. It was probably one of numerous recent | articles about Einstein's life. From my | understanding, Einstein determined that because | the speed of light is constant, there must be | variability in time to make up for this. This | explains why time moves slower or faster for | two relative observers. | | So here is the meat of my question. I read in | books and magazine articles about the age of the | universe. Since time is relative, what does it | mean that the universe is so many billions of | years old? Don't we need to know billions of | years in relation to something? Does that just | mean that given the current size of the universe | (as we have measured it) is so many light-years | across? Or does our age-estimate have more to | do with the rate of expansion extrapolated over | time? | | If two beams of light are leaving the Big Bang | at the same time traveling in opposite | directions, what is their time relative to each | other? If they travel for one year in their time | are they two light years apart? Or is it less | because if they are two light years apart, they | would be traveling at two times the speed of | light relative to each other? | | Thanks in advance for any light that can be shed | on this. :) | | Aaron | | _______________________________________________ | Utah-Astronomy mailing list | Utah-Astronomy@mailman.xmission.com | http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy | Visit the Photo Gallery: http://www.utahastronomy.com | | ______________________________________________________________________ | This e-mail has been scanned by Cut.Net Managed Email Content Service, using Skeptic(tm) technology powered by MessageLabs. For more information on Cut.Nets Content Service, visit http://www.cut.net | ______________________________________________________________________ | |
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Kim Hyatt -
Lambert, Aaron