Itís 3:45 as I start to write this, Iíve been in bed tossing and turning for about three hours unable to sleep so now Iím writing this. As Slash said, balance in all things.
The present is, in physics, a rather hard idea to grasp. Up until the 20th century the present was thought of as whatever occurs at a specific time. At the precise moment 1843, August 3rd, 19:54:56 the present would be a freeze frame of the universe right at that moment. Stars millions and millions of light years away were be emitting photons that wonít reach us for millions and millions of years. Those stars were be experiencing the present we were experiencing at precisely that moment, just much farther away.
However, Einsteinís theory of relativity changed that. Einstein said that time was not constant, nor space, but light was. The speed of light is the only true constant in the universe, going approximately 2.998*10^8 meters per second, it will travel that velocity regardless of anything. Meaning, if a laser shoots out a beam of light, adding velocity onto that laser (putting it in a fast car for instance) would not change the velocity of the light admitted from the laser. And more interestingly, the light would be traveling faster than the care by the exact same speed it was beating a pedestrian by, despite the fact that the car was going far faster than the pedestrian.
This of course has little to do with ďthe presentĒ, Iím just getting side-tracked. So back to the main point. Perhaps at our previous moment (1843, August 3rd, 19:54:56) a group of Martians decided to have a party. It was Billís birthday (right at that exact moment) so to celebrate they shot of fireworks at precisely that moment (those Martians are punctual!) So the universe, frozen at that time, will show us a spectacular view of the light from the fireworks exploding outwards (at 2.998*10^8 meters per second) while at the same time it will show Simon (whose for our purposes is a recently divorced British astronomer who now lives with his oddly named mother, Shirley) looking up into the skies with his telescope. Simon, at that moment, can not perceive this festive celebration, as Mars is about 4 and a half light minutes from Earth, but none the less, our freeze frame shows him existing at exactly the same time as these party loving Martians.
However, 4 and one half minutes later, our lucky astronomer Simon, views a sudden light flash from Mars (his telescope is surprisingly advanced for its time). He sees the fireworks and the celebrating Martians in the exact position they were in during our last time freeze (Bill looks a bit drunk but he seems to be enjoying himself all the same). Suddenly the event of the party enters Simonís life, minutes after itís occurred on Mars.
Whatís interesting about our rather crude party scenario is that unless Simon got an invitation early, there is no way he couldíve known about the partyís existence until 4.5 minutes after itís happened! Gravity waves (or gravitons, or whatever the theory you want to use) do not travel faster than light, nothing does. The entire effect of those fireworks going off couldíve not happened, and for those 4.5 minutes it wouldíve literally had zero effect on Simon. Not so much as an atomís difference!
This raises very interesting questions about the present. Certainly Einsteinís theory changes nothing about our original freeze frame. If indeed we did manage to stop time at one exact moment we would see Simon starring off into space and the exact moment those fireworks are going off. However, in Simonís world there is no way to instantly travel to Mars to witness this event (remember, canít break the speed of light) so there is no way of seeing these two events simultaneously, despite the fact that you can show them in the same frame.
Our universe is one thing, it can be divided into many parts, but the universe encompasses all matter. However, letís say that some where off in the distance beyond our universe (of course the use of the word ďdistanceĒ associated with some place not within our dimensional limits is rather meaningless, but letís ignore that for now) exists, but has no connection with our universe. That means no light, no gravity, no anything. Our universe would exist identical to itself now should that matter not exist. Thatís why itís not part of our universe, it doesnít have any effect.
Now letís go back to our Martian party. The photons the firecrackers are rapidly emitting exist to those Martians, theyíre busy ohhing and awwing over the fantastic show being put on. But to Simon, stuck in dreary old England, his universe is identical to the one in which those Martian firecrackers do not occur. He feels no effect from them at all, thus they do not exist to him.
To me this says something very interesting about ďthe presentĒ. Perhaps our original freeze frame idea doesnít quite encompasses it. Perhaps the elusive present changes depending on where you are. To Simon at 1843, August 3rd, 19:54:56, the present has absolutely no Martian firecrackers, at around 19:59:26 suddenly they appear. That is the first moment at which their existence impacts his, and thus that is his ďpresentĒ. So the firecrackerís existence is not admitted into Simonís present until 4.5 minutes after the Martians view them at their present.
Perhaps in our current frame of time, the light from stars millions of light years away is perfectly representative of their state right now. Because it doesnít, and can not, matter to us that at their present they have already gone supernova, that will not reach us for millions of years. To us, they are existing in the state they were in millions of years ago, what state they are in now is the future.
I didnít really sum it up, in fact I left a lot out and my explanation isnít really complete, but this is the best you are going to get at 4:20 in the morning.
This gets even worse because its possible for someone to be in one place in frame of reference A, and in a totally different place in frame of reference B.
Posts: 15770 | Registered: Dec 2001
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quote:To me this says something very interesting about ďthe presentĒ. Perhaps our original freeze frame idea doesnít quite encompasses it. Perhaps the elusive present changes depending on where you are.
In fact, that's one of the basic axioms of special relativity: there is no absolute simultaneity of events at a distance. What you see as "the present" is a function of your location and state of motion, ie which reference frame you are in. So, while Bill may think of the Martian party as happening "right now" 4.5 minutes before its light reaches him, there may be another observer who sees only 2.5 minutes passing between the beginning of the party and Bill's noticing it.
So any talk about SR that involves thinking about simultaneity is mistaken from the start. For instance, when you say:
quote:Certainly Einsteinís theory changes nothing about our original freeze frame. If indeed we did manage to stop time at one exact moment we would see Bill starring off into space and the exact moment those fireworks are going off.
Einstein's theory does change the familiar idea that we can "freeze" time at a particular moment. In any particular reference frame, this is true -- time can be frozen along a simultaneity hyperplane. So time could be frozen in such a way that everything stops at the same time in Bill's frame, when the party on Mars begins and 4.5 minutes before he sees the party. But a space traveller moving past the Earth at high speed might see the party freeze up and then see Bill freeze up some time later, because in his frame the party's beginning is not simultaneous with 4.5 minutes before Bill sees the party*. So time hasn't frozen at one particular moment; that's only true if we look at things from Bill's reference frame.
*I mean 4.5 minutes as Bill would count, since the space traveller would see time speeding up for Bill because of the velocity difference between them.
Posts: 4600 | Registered: Mar 2000
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The trouble, I think, is that science defines reality as something different than what we intuitively think it is. When we are born, the "present" reality is simply the set of sensations we experience at a given instant. As we grow older, we learn to add causation into the equation, establishing the existence of mysterious things outside our current perception that have the potential to cause sensations at some set distance in the future.
Science wishes to, instead, talk about the present reality as something objective, shared by everyone and not dependent on any given point of view. The present is not a set of experiences and sensations, but rather a set of physical objects that can be observed. Thus, different rules may apply to the scientific "present".
Posts: 8120 | Registered: Jul 2000
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I just plain like the fact that Einstein's theory (I can never remember which aspect is from special and which from general so I just lumpy them together ) says that the universe is litteraly different depending on how (or rather where) you look at it.