NowThatWeKnow

But I think they both could have a master clock (or call it what you want) that is synchronized with each other. The clocks on the GPS satellites are a good example of clocks adjusted for both GR and SR so they are synchronized with earth clocks. Future technology will allow the clocks to maintain synchronization during acceleration. At this point I can use my mind to imagine any technology necessary to help me understand relativity.

I see universal time as mans attempt to synchronize clocks in different frames, such as the GPS satellite clocks. Not to be confused with absolute time that does not exist. I am starting to see why swansont prefers "master time".

If it is seen as simultaneous from different frames, it is most likely not simultaneous in universal time.

Interesting but... It sounds like they are talking about singularity like it happened. Isn't that what we are trying to discourage. And it didn't explain the expanding universe, something else we are promoting. Why do I not see this contributing to the purpose of this thread?

For some reason I am not clear on what you are saying above. Let me put it a different way and you let me know if we agree. To keep it simple, both twins start off on Earth and both have master clocks that stay synchronized to each other. Example #1: The master clocks are synchronized to Earth time and the twin blast off. Earths master time and proper time remain synchronized. The traveling twins proper time starts slowing down relative to master time. The Earth twin starts aging faster then the traveling twin. Example #2: the master clocks are synchronized to the traveling twins proper time and the twin blast off. Now the traveling twins proper time stays synchronized to the master clocks and the Earths proper time starts speeding up relative to the master clock. Again we can see the earth twin aging faster. Do we agree? It seems that acceleration is the key. Starting off with both twins moving makes my head hurt. Maybe a good exercise, but not good for me, the beginner, trying to grasp the basics.

I saw confusion in post #3: "What would Prime observe if he were to constantly look at Unprime?" "So he would at first observe Unprime to be aging more slowly as well - right?" "So at the distant station, Prime would observe Unprime to be aging just as fast as he was (but at a younger age?)." "What would he observe as he decelerates to the distant station? As he accelerates away from the distant station? As he decelerates at home?" "There would have to be some point at which Prime observes Unprime leaping ahead in years to catch up to and surpass Prime in age. At what point is that?" I saw confusion in post #5 "Is that at all points of accerelation (including deceleration) or just one in particular (like when he first leaves Unprime or when he begins his return trip)?" That is when I chimed in. I have seen almost everyone (including myself) have trouble with the twin paradox. I finally figured out that you could look at it in steps and it was easier to understand. You must learn to add and subtract before moving onto algebra. My simple version is NOT complete when it comes SR, but it lets you understand addition before moving on. Again, I need the basics, one step at a time, to even understand some of relativity. Then I will move onward. I basically understand relativity of simultaneity but that does not make my story invalid. While the twins speeds are not uniform to each other there is an 'actual' and continuous difference occurring in there proper times. This is why having a master clock set to Earth time shows when and where the traveling twins 'actual' changes take place. When you understand this, the second part makes much much much more sense (at least to me). I consider myself a beginner and I need simple to grasp things that are not intuitive. I watched both videos and thought the second one was especially well done. I have watched similar videos in the past trying to grasp relativity at a basic level. We all crawl before walking. I am far from an expert and do not want to get into a P!$$!ng match with anyone. I just try to add my input when I think it may be food for thought. From reading your post I see you are ahead of me in many areas and I am trying to catch up. I did see one area where I could add a view from a different angle to help grasp the difficult concepts of SR. I did not mean to offend anyone and hope we can all be friends and learn from each other.

I am fairly new here but have already noticed many new people asking similar questions and they rarely prepare themselves with the basics. I often point them to Einstein Online or the stickies on cosmology basics. I would hate to run good people of requiring a prerequisite but we can suggest certain reading. The "Balloon Sermon" worked well for me for understanding expanding space/increasing distances rather then movement through space. It helped when I visualized the balloon itself as a (most likely non existent) ether or fabric of space holding the galaxies somewhat in place. When you get a good visual in your head it sticks. Instead of Wigglers called photons I kept seeing ants on the balloon. Not sure why. One problem with singularity and the big bang is that google searches still come up with this concept since it was common mainstream not that long ago. Next

It has been determined that the temperature,in different directions, of the cosmic microwave background radiation http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation tells us we are at rest and it is the universe that is expanding. Check out Hubble's law for info on the expansion of space. Most believe that space is just a metric and nothing else. The link in my signature may be a good source of basic information.

Galaxies are basically at rest in space relative to the cosmic microwave background radiation. There is little movement relative to the speed of light but local gravitational forces do cause some local movement. Hubble's law talks about space expanding. Every light year in open space is getting bigger causing galaxies to spread out. At the edge of the observable universe, galaxies are separating from us at near the speed of light. Galaxies much further away, because of the expansion of space, exceeds light speed and we can not see them. Nothing is moving that fast, it is all because of expansion. There are a couple of other threads next to this one talking about the same stuff. You may want to join in there.

You can put in a large enough z # to take you back to the big bang but that doesn't mean there was anything to actually see then. Seems like Martin did that once just to show what the #'s would be. Another thing to point out in this thread is that if you were at the edge of the observable universe, you would probably still see the universe in the same way we see it from the Milky Way in all directions. It could be really big. If it is infinite and the big bang was infinite, what size is it after expanding?

You were asking questions that indicated the combination of information presented in the twin paradox was confusing you. I tried to break it down twice in a way for you grasp where and when the time differences actually took place and not where and when it appeared to take place from different frames. If my simple version did not make sense to you then I am not sure how else to put it. It seems the powers that be had no problem with my version. Good luck on a complete understanding as it is not intuitive. "travelling at uniform velocity". Uniform relative to what? Each other? The CMB? The Earth? You lost me.

Correction accepted, synchronized was the wrong word. It would be running at the same speed but behind the master clock when they shared the same inertial frame mid way. Thanks for keeping an eye on me.

It sounds like you are complicating things combining what is observed and what is really happening and worrying about simultaneous events when it isn't necessary. That is what I was saying is a problem to me with most twin paradox examples. Sort of like mixing frames and getting all screwed up. You mentioned "when they are travelling at uniform velocity" which I take as they are at rest with each other and in the same frame as far as SR goes. I think post #7 put it well (swansont approved) but this is what I understand. If the Earth twin and the traveling twin both had a universal clock synchronized at rest with the CMB with them, the Earth twin would see a consistent minute difference between his local and universal clock. This is because Earth is not totally at rest with the CMB. The traveling twin would notice during acceleration out of Earth's frame that his proper time clock would start running slower and slower then the universal clock. When the engines were shut off and he becomes an inertial frame, the proper time clock would run consistently slower then the universal clock. When the traveling twin turned around and accelerated (decelerate) towards Earth's frame his proper time clock would eventually become synchronized with the universal clock. As he accelerates towards Earth the whole thing starts over. Martin, swansont, do you see any flaws? I very much consider myself a layman and get corrected often.

"Irrespective of the observer"?? All observers will see light at c irrespective of the source. Or am I brain dead tonight?

Local gravitational forces in a galaxy OR in a cluster of galaxies overcomes the expansion occurring in open space. I calculated the percent of expansion needed for this rate and it is actually very small. I do not recall at the moment the exact #'s but local gravitational forces overcome it. That is what the people that have a clue tell me anyway.

The Ned Wright calculator gives some really neat information but may seem a little complicated at first. http://www.astro.ucla.edu/~wright/CosmoCalc.html If light took 13.665 billion light years to get here, the age of the universe, it was emitted when it was only 0.045541 billion light years away and is now 45.587 billion light years away. The galaxies are basically at rest in the universe and the change in distance is because of the expansion of space itself. The light coming from that same galaxy now is so far away we will never see it because space is expanding faster then the speed of light at that distance. This shows that the universe was more dense long ago but the distance we see is likely not all there is.

I never understood why they confused the twin paradox with "the way it appears" to each twin. At least until you go through it in real time using local clocks. The reality of the relativity is the stay at home twins clock (due to no acceleration) is going much faster then the traveling twin. That is apparent when you compare the ages when they reunite. The way it appears to each twin because of relativistic speeds should be covered in part two IMO.

I am not sure if what I prefer the word "simultaneous" to mean is important. I would prefer that both events we observe as simultaneous also agreed with universal time but it does not usually do that in the big scale of the cosmos. I will not go any further as I do not want to influence any one's choice.

It said I already gave you a prize and wouldn't let me give you another one. I did try a new avatar for 24 hours but I missed my ant. The poll shows that the ant is liked by most so the masses have spoke.

Relativity of simultaneity examples use relativistic speeds to show agreeing on absolute simultaneous events unlikely. Some of the layman here are using the standard definition of "simultaneous" and saying .0000000000001 sec is close enough. Who you are determines the "nonstandard definition" of simultaneous.

I have to agree.

If you can have synchronized clocks in different frames, why can't two events be simultaneous? Is it the size of the time slice? In actuality our head and feet are in different frames. Does that mean simultaneous events do not exist? My understanding from reading Martin's post is that simultaneous events do occur in different frames. Where am I going wrong?

Your thanks button doesn't work either so thanks.

I decided to give the ant a break. Thanks for contributions to this thread.

So, does Martin's universal clock satisfy you as far as the present and "now"?