michel123456 Posted December 7, 2014 Share Posted December 7, 2014 Yes, not OUR past, and for these observations, that's what matters. Aren't we supposed to come all from the same past? When we were co-located? Link to comment Share on other sites More sharing options...
swansont Posted December 7, 2014 Share Posted December 7, 2014 Aren't we supposed to come all from the same past? When we were co-located? I can't really answer that, because I don't know how you are using the term "the same past". You have to make the comparisons using the same clock (ours), which you are not doing. Compare our observation now with an observation we would have made in our past. Link to comment Share on other sites More sharing options...
DimaMazin Posted December 7, 2014 Author Share Posted December 7, 2014 There is no evidence that the big bang created the mass of the universe. Energy is observer dependent. So it depends what reference frame you measure it from. We can't define KE of the universe but it is approximately the same on any planet in any galaxy. Link to comment Share on other sites More sharing options...
Strange Posted December 7, 2014 Share Posted December 7, 2014 We can't define KE of the universe but it is approximately the same on any planet in any galaxy. It is true that it is undefined on every planet. But that is not very useful. It might help if you defined, precisely, what you mean by "KE of the universe" and show how you think it would be calculated. Link to comment Share on other sites More sharing options...
xyzt Posted December 8, 2014 Share Posted December 8, 2014 We can't define KE of the universe but it is approximately the same on any planet in any galaxy. This cannot be since KE is frame dependent. -1 Link to comment Share on other sites More sharing options...
DimaMazin Posted December 8, 2014 Author Share Posted December 8, 2014 This cannot be since KE is frame dependent. Well. What is force of space expansion and how does it work? Link to comment Share on other sites More sharing options...
Strange Posted December 8, 2014 Share Posted December 8, 2014 (edited) Well. What is force of space expansion and how does it work? There is no force involved, it is just a consequence of the geometry of space-time. As an analogy, think of two people near the north pole. They both start walking south (towards the equator) in a straight line - i.e. along (different) lines of longitude. They will initially think they are walking parallel to each other but the further they go the further apart they get. There is no force making them move apart. It is just because they are on a curved surface. Space-time is curved (by the presence of mass in the universe) which also causes things to move apart over time. A good description (and diagram) here: http://www.cfa.harvard.edu/webscope/activities/pdfs/galaxiesT.pdf Edited December 8, 2014 by Strange 2 Link to comment Share on other sites More sharing options...
DimaMazin Posted December 13, 2014 Author Share Posted December 13, 2014 There is no force involved, it is just a consequence of the geometry of space-time. As an analogy, think of two people near the north pole. They both start walking south (towards the equator) in a straight line - i.e. along (different) lines of longitude. They will initially think they are walking parallel to each other but the further they go the further apart they get. There is no force making them move apart. It is just because they are on a curved surface. Space-time is curved (by the presence of mass in the universe) which also causes things to move apart over time. A good description (and diagram) here: http://www.cfa.harvard.edu/webscope/activities/pdfs/galaxiesT.pdf Is reducing of photons energy proportional to increasing of their quantity? Link to comment Share on other sites More sharing options...
Strange Posted December 13, 2014 Share Posted December 13, 2014 Is reducing of photons energy proportional to increasing of their quantity? I'm not sure why you quoted that post, which says nothing about photons, to ask this question. But, no. A single photon will have less energy when it is received, in an expanding universe. This is just because it is received in a different frame of reference and energy is observer/frame dependent. Link to comment Share on other sites More sharing options...
DimaMazin Posted December 13, 2014 Author Share Posted December 13, 2014 (edited) I meant expansion of space divides a photon into some photons with less energy. Because I don't understand where is the lost energy going to. It could explain a cause of greater radiation of CMB from far galaxies. Edited December 13, 2014 by DimaMazin Link to comment Share on other sites More sharing options...
Strange Posted December 13, 2014 Share Posted December 13, 2014 I meant expansion of space divides a photon into some photons with less energy. Because I don't understand where is lost energy going to. It isn't going anywhere. The photon is observed in a different frame of reference so there is no reason for it to have the same energy. Link to comment Share on other sites More sharing options...
DimaMazin Posted December 13, 2014 Author Share Posted December 13, 2014 It isn't going anywhere. The photon is observed in a different frame of reference so there is no reason for it to have the same energy. Far galaxy has lost mass for emission , but the mass was a part of total energy in our frame,we have lost part of the energy. Also it didn't increase potential energy. Link to comment Share on other sites More sharing options...
Strange Posted December 13, 2014 Share Posted December 13, 2014 Far galaxy has lost mass for emission , but the mass was a part of total energy in our frame,we have lost part of the energy. Also it didn't increase potential energy. I have no idea what you are talking about. You asked about a single red-shifted photon. Its energy is observer dependent. A distant galaxy is not in our frame. What potential energy? Link to comment Share on other sites More sharing options...
Mordred Posted December 13, 2014 Share Posted December 13, 2014 A couple of things to realize conservation of energy only applies to the same inertial frame. This site has a brief run down. One thing to also realize is that an expanding universe is one that is also cooling down. http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ It is an excellent question and I'm glad you asked it. In regards to the universe and cosmological redshift the total number of particles remain roughly constant. Estimates place it at roughly 10^90 particles. As the universe expands the average density and thus temperature drops. Temperature is a measure of kinetic energy. So in turn the cosmological redshift is reflecting that loss of kinetic energy. If the universe started contracting the blueshift would reflect the gain in kinetic energy. Higher temperature gain. However we must keep in mind all forms of redshift is an Observer measured quantity. The link better explains the gravitational redshift. One common analogy is that work is performed climbing into and out of a graviational well. This is a more commonly accepted view of gravitational redshift. Hope this helps 1 Link to comment Share on other sites More sharing options...
DimaMazin Posted December 14, 2014 Author Share Posted December 14, 2014 I have no idea what you are talking about. You asked about a single red-shifted photon. Its energy is observer dependent. A distant galaxy is not in our frame. What potential energy? Only frame of distant galaxy isn't in our frame.Any galaxy is in our frame. All energy is observer dependent. Link to comment Share on other sites More sharing options...
DimaMazin Posted December 14, 2014 Author Share Posted December 14, 2014 The link better explains the gravitational redshift. One common analogy is that work is performed climbing into and out of a graviational well. This is a more commonly accepted view of gravitational redshift. Hope this helps Good point. Gravitationally increased distance of photon travel creates redshift and cosmological increasing of distance of photon travel creates redshift. Can scientists create one equation for these two reasons? Link to comment Share on other sites More sharing options...
Strange Posted December 14, 2014 Share Posted December 14, 2014 Can scientists create one equation for these two reasons? Yes, they are both described by the Einstein Field Equations of General Relativity. 1 Link to comment Share on other sites More sharing options...
DimaMazin Posted December 17, 2014 Author Share Posted December 17, 2014 Yes, they are both described by the Einstein Field Equations of General Relativity. Still I can't use this complex equation. Was earlier second slower than now second is? And was earlier meter shorter than now meter is? Link to comment Share on other sites More sharing options...
Endy0816 Posted December 17, 2014 Share Posted December 17, 2014 (edited) It isn't possible for the meter to vary because of how we have defined it. If you are talking about an arbitrary length changing, then from what I've read we wouldn't be able to observe this as occurring(ie. every distance X, is now 2x long). Not sure how clear that is. Others can probably explain better. Edited December 17, 2014 by Endy0816 Link to comment Share on other sites More sharing options...
DimaMazin Posted December 18, 2014 Author Share Posted December 18, 2014 It isn't possible for the meter to vary because of how we have defined it. If you are talking about an arbitrary length changing, then from what I've read we wouldn't be able to observe this as occurring(ie. every distance X, is now 2x long). Not sure how clear that is. Others can probably explain better. Let's consider some analogy: star gravity increases and we can see increasing redshift. You can see increasing distance,though it doesn't. Link to comment Share on other sites More sharing options...
Strange Posted December 18, 2014 Share Posted December 18, 2014 Still I can't use this complex equation. Nor can I. Luckily, there are people who can. Was earlier second slower than now second is? And was earlier meter shorter than now meter is? How would you ever know? Relativity only talks about relative differences (hence the name). You can't compare a clock now with a clock then, or a ruler now with a ruler then. If someone back then had a metre rule and a clock, they would measure metres and seconds just like we would. On the other hand, in the case of time dilation between the Earth and a GPS satellite (due to gravity and velocity) we can measure the difference. But a second on the GPS satellite is the same second it has always been, and the metre is the same as ever. Link to comment Share on other sites More sharing options...
DimaMazin Posted December 19, 2014 Author Share Posted December 19, 2014 Nor can I. Luckily, there are people who can. How would you ever know? Relativity only talks about relative differences (hence the name). You can't compare a clock now with a clock then, or a ruler now with a ruler then. If someone back then had a metre rule and a clock, they would measure metres and seconds just like we would. On the other hand, in the case of time dilation between the Earth and a GPS satellite (due to gravity and velocity) we can measure the difference. But a second on the GPS satellite is the same second it has always been, and the metre is the same as ever. Let's consider mental experiment: distant galaxy is motionless , distance between your summer position and your winter position is 3*1011 m. If your metre is reducing then you can see increase of distance till the galaxy. If your metre is increasing then you can see decrease of distance till the galaxy. Link to comment Share on other sites More sharing options...
Strange Posted December 19, 2014 Share Posted December 19, 2014 Let's consider mental experiment: distant galaxy is motionless , distance between your summer position and your winter position is 3*1011 m. If your metre is reducing then you can see increase of distance till the galaxy. If your metre is increasing then you can see decrease of distance till the galaxy. It is possible to describe the expansion of space in terms of the metre shrinking (and the speed of light changing, etc.) Although, I am told it is quite complex, counter-intuitive and has no benefit. So no one does it. But what does that have to do with comparing clocks or rulers now, with clocks and rulers in the early universe? Link to comment Share on other sites More sharing options...
DimaMazin Posted December 20, 2014 Author Share Posted December 20, 2014 But what does that have to do with comparing clocks or rulers now, with clocks and rulers in the early universe? I want to research real reason of redshift. When photon travels from gravity it is length contraction or slowing of time in gravity. We can exclude length contraction in distant galaxy as the reason when photon travels from distant galaxy. Then slowing of time was in distant galaxy and length contraction is in our galaxy now. Link to comment Share on other sites More sharing options...
Strange Posted December 20, 2014 Share Posted December 20, 2014 I want to research real reason of redshift. There are many reasons for red-shift in different contexts. Doppler effect, gravitational and expanding space. Which are you referring to? Link to comment Share on other sites More sharing options...
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