Philosophy Of The Cosmos - Twin Paradox
The twin Paradox
At the last lecture and tutorial, the issue of the twin paradox kept some questions open. So, I was wondering if someone could clarify something for me.
I was wondering, like in Dayal's example, someone would travel near to the speed of light around the universe. He would then experience less time compared to the person on earth, which makes time something private. Now, if this person in this spaceship would be able to see what happens on earth, would he see everything happen quicker? And the person on earth would he see the person in the spaceship everything happen slower (slow motion)? Or would they both see everything happen the same when seeing each other, with the only difference that through the time line being curved time would have passed differently? And how is that actually possible?
Also, can therefore other Galaxies experience time differently to us, because they are expanding at a higher speed, or is the difference so minimal that it does not effect them as such?
And if they should experience time differently, can we then actually ever talk about a public time?
Also, hypothetically speaking, if we would travel faster then the speed of light (which is not possible according to modern physics, but lets assume it for the sake of the argument), would you be going back in time and therefore would actually see yourself before you started to travel?
Thanks for your help.
In answer to your first question, both observers would see the time of the other as running slow, as velocity is relative. The difference is that one of them had to accelerate relative to the other to reach that velocity. So if the person travelling around the universe were somehow able to see the earth, time (on earth) would appear to be running slow and the same would be true for someone looking at the spaceship. However, everytime the person in the spaceship accelerates (or decelerates) the symmetry between the two frames of reference is broken and it is at this point that time would appear to speed up on earth. I'm not entirely sure if this is right, but its what i gathered from the lectures i had in physics last year.
Reyner - u 4674572
Leonidas, good issues. Reyner, what you say is entirely correct. You're earning good participation marks here!
To expand on what Reyner says about the twin paradox, it might help to remember that in relativity theory VELOCITY is completely relative to a frame of reference and therefore symmetrical but ACCELERATION is not.
As for the question about distant galaxies experiencing time very differently from us, yes, they do.
And as for going faster than the speed of light, there's a choice of how to fit that into the laws of physics. ONE way is to consider things that go faster than the speed of light to be travelling backwards in time. We'll have an example in a later lecture of a physical theory (the Transactional Theory of quantum mechanics) which depends on things going backwards in time … although it turns out that putting that together with relativity theory is easy and therefore not particularly enlightening, in that particular case, in general this sort of thing is a wide open area of research.
Having considered similar scenarios, I had considered replying earlier with the note of waiting for clarification on whether such scenarios were accurately possible in terms of the currently accepted models of physics, and since some helpful replies have since been posted, I will continue ' hopefully they will be philosophically useful, though clarification would still be welcome if they are incongruent with the actual physics under discussion.
Would it be the case that, to go faster than the speed of light allows one to nominally move backwards in time, because times prior to what we consider to be our current time would have travelled further, that is, beyond the current limits of the emissions of light as from our time? Thus one would be moving through past light emissions which had travelled further than the current light emissions, in moving faster than the current speed of light. I say nominally, because those spaces would not necessarily be interactive worlds (they would be more like animations of past events), so there is no sense in which you go to another past world which is running parallel to this one that can be interacted with as one interacts with in this world (it would all be after the event). So it would strictly be the past of our world, but a present animation to the traveller, and the traveller may only be able recognise it as past if they came from this world.
Of course, one could then ask whether this moving backwards in time could exclusively be in moving faster than light, as one might shift into a mode of interactive events which also are just too fast to meet with the approximately symmetrical frames of reference in our world, and such that they would interrupt any of the epiphenomenal images of our past world ' so you would be travelling both through 'the past' of our world but also into a different present of another world. So the basic supposition here is that there is only a present and that any past is merely an after effect, thus there is no future that one can manipulate, apart from being beyond the current scope of our approximately symmetrical frames of reference (say in the speed one is travelling), so that people at a slower speed will eventually meet with your past after-effect. As to the past one can travel into then, perhaps this past also would affect the futures of another present elsewhere, since they would travel to them before our present does, they would experience that as a present into past (even as it is past to present to us), and so this also raises interesting questions as to the cognitions of beings which travel at different speeds. This would basically mean that the past is the future, for everything past meets with a present and makes for that present the future. An issue which seems to become very convoluted when dealing with time as a metaphysical concept, but in fact becomes quite simple if we are treating of time here as a physical concept ' the results of how we perceive time is the metaphysical issue, since what is really a passed event physically is what is a future event in our perceptions, and what is passed for us is simply a matter of what we (or those in approximate symmetry to our frame of reference) have already experienced.
Likely explained this with too much convolution, but hopefully the essential points can be extracted.

Kyle, u 4686832.
I'm not sure I understood all of that, but im still trying to come to grips with the whole concept of the relationship between time and the speed of light. One thing i found interesting from the previous posts was that going at the speed of light is equivalent to time standing still, and going faster than the speed of light is equivalent to going backwards in time. This ofcourse is an extension of the logic that we (while moving at speeds much slower than the speed of light) are moving forward in time. but what speed are we exactly talking abt here? are we all moving at exactly the same speed or is there some variation. and if there is some variation, do we as individuals experience time going at different rates (however small the variation might be)? also continuing from this idea, is there any theoretical way to go into future time?? it seems to me that if going faster than light takes u bak in time, and going slower than light keeps u in the 'present', you must have to somehow go at negative speeds to go into the future. but im thinking of this purely mathematically, and dont know what i actually mean by negative speed :S any ideas? this is getting very sci-fi ish!!
hope ur all enjoying ur hols!
~sharmila u 4848798
Maybe if time is bounded and circular, then going at negative speeds (so going backwards in a geometrically defined direction) would result in you coming into contact with events which are always being directed at the "present", so you could experience an animation of the future events as they would or could occur, so either deterministically or probabilistically. Fascinating.

Kyle, u 4686832.
Hey all,
Well, the idea that was related to Special Relativity might be one example where something similar is possible.
Let me explain. If now twins were born and one would travel near to the speed of light through the universe and would return to earth after ten years, then her/his twin who stayed on earth would then have experienced 50 years (example from the lectures). Now it is obviously a matter of perspective. Because the person in the space ship would see the future, she would be 40 years out of her normal time and the twin would think similarly that she travel 'forward' in time to be at this moment 40 years younger. Whereas she actually would have travel back in time in terms of length when comparing their two experiences. So, you would be 'quicker' in the future when you would travel at high speed and experience time less. Obviously, this is bound to the idea that this person would enter a place that would travel at a considerably slower rate then the person siting in the spaceship. Therefore, it becomes something rather psychological when looking at it like this.
Hope that made any sense as well as be of any help.
I like these points.
I wonder to what extent we might mean different things by "psychological".
Interesting. Well, I guess it can be seen in two ways.
One is out of the perspective of perception, the way we personally perceive the situation and draw conclusions upon our experiences. Therefore, one can have a different understanding and experience of time than the other and still neither are incorrect nor correct. Because it becomes a personal experience.
The other may be the Arrow of Time out of a Psychological perspective. The fact that we see time pass only in one direction (compared to a mathematical perspective of time, which seems to allow such phenomenas). Because we base our understand of time on the past. That is what we remember, and can build our conclusion on. We don't know the future and we live and arguably experience the present. Thus, seeing time as something passing forward into the future, that all humans experience out of the same perspective.
That are the once I can think of when talking about psychological. One that is bound to our personal experience and the other towards our physical limitations.
I wonder, if someone can only experience time through the fact that he knows the past. Then what happens if someone could physically not remember anything that just happen (literally no short or long term memory), would this person actually notice what we call 'time'? If not, would then as a whole the psychological aspect matter?