Edge of the Universe

[dangineer]

MA, I appreciate you taking the time to try to explain your position to me. I really am trying to see things the way you do, so I can understand your opinion.

Relativity doesn't allow for an absolute frame of reference, neither mathematically or conceptually, so I would have to conclude that you choose to not apply relativity to what you observe. But I don't think this is what you are are getting at.

"I choose to consider events in the absolute sense and work with them in the absolute sense, then calculate how they will appear to different relativistic frames."

When you say this, it sounds as if you are considering things in an absolute physical framework (the most common of which is Newtonian mechanics) and then converting them into a relativistic framework. To me this is really confusing, since you cannot convert between the two. Newtonian mechanics is really just a special case of relativistic mechanics. Perhaps this is not what you are saying either, though.

Could you demonstrate your approach through an example? Maybe we can start with a common one, such as the precession of Mercury's orbit.

When Newtonian physics are applied to this situation, what we observe doesn't match what we predict (apohelion of Mercury's orbit doesn't move). But then when we apply GR, the results do match. We've even measured this effect from several different frames of reference through robotic missions to the inner solar system. So I assume that there should be some absolute reference frame where Mercury's orbit doesn't precess.

 

[dangineer]

Here's another explanation of the superluminal recession paradox:

What we observe when looking at very distant galaxies is a very high redshift. SR predicts that things moving away from you will produce a redshift (doppler effect). But obviously, if nothing can travel faster than the speed of light, then there will be a maximum redshift we can observe. When astronomers were looking at distant galaxies, they noticed something disturbing. When you look at the most distant galaxies, they are redshifted beyond this maximum. This shouldn't be possible! (in a static universe, that is). In order for this to have happened, light would have had to speed up past c in order to reach us and produce the redshifts that we observe. But if space is able to expand, then through the application of GR, we can see that these high redshifts are possible.

 

[SpeedFreek]

"It is not what the observers see, it is what they calculate after taking all relativistic effects into account, that matters."

Actually, SpeedFreek, the observers in this case don't have to take into account relativistic effects. The relativistic effects are what they actually observe. Then when you apply relativity to the situation, it makes sense of the whole mess.

Yes, that is what I meant - what matters is that you can make sense of what they actually observe, only by using relativity. But you can only make sense of the whole mess if you accept that there is no sense in which simultaneity is absolute.

 

[dangineer]

Righteo, that's what I figgered you were trying to say, SF. I appologize for coming across a little matter-of-factly. I just wanted to make sure the point was clear.

 

[mental_avenger]

Yes, that is what I meant - what matters is that you can make sense of what they actually observe, only by using relativity. But you can only make sense of the whole mess if you accept that there is no sense in which simultaneity is absolute.

Speak for yourself. It all makes perfect sense to me, and as you might be able to tell, I have thought a lot about this. Perhaps it has something to do with the hardwiring in my brain. I have found that I can easily visualize some extremely complex things, and keep all the pieces in proper perspective at once. I can play the train scenario in my mind from the perspectives of Bob, Tom, and the lights all at exactly the same time. It’s sort of like watching all the rings in a three ring circus all at once. That is why I say that what the lights are doing is the absolute, and everything else is observational perspective. In this case, observation has no effect whatsoever upon the event itself. (unlike observing sub atomic interactions in which the act of observation necessarily interferes with the interactions).

 

[SpeedFreek]

That is why I say that what the lights are doing is the absolute, and everything else is observational perspective.

And I say that neither space nor time are absolute due to the constancy of c, so therefore there can be no absolute sense of simultaneity.

So, how do you deal with the constancy of c when considering the picture from both Bob and Toms frames of reference?

 

[dangineer]

"Let’s modify the train thought experiment slightly to eliminate the illusion. The train, the lights, Bob, and Tom are all still where we left them. However, instead of watching the lights come on, Bob and Tom each have an electronic comparator which measures any difference between the turning on of the lights. Each comparator is connected to the two lights with equal length wires. Of course Tom’s wires are long and flexible in order to remain attached as the train moves past. When the lights turn on, both Bob’s comparator and Tom’s comparator see the lights turn on at the same time. Amazing. Bob is in one frame of reference and Tom is in another, and yet they both measure the event as occurring simultaneously."

The only problem with modifying the paradox this way is that now Tom is getting a signal from Bob's frame of reference. So what Tom is reading on his comparator is what Bob observes. So obviously Tom is going to see the same thing Bob sees! The signal travels to the junction at the center of the train (in Bob's frame of reference) and reaches the center at the same time (because it is in Bob's frame of reference). Then the signal travels to Bob's comparator and shows that the lights turn on at the same time (obvious). Then the signal that's saying the lights are coming on at the same time (from Bob's frame of reference) comes down to Tom and tells him that the lights are coming on at the same time. Hmmmm...

 

[mental_avenger]

The only problem with modifying the paradox this way is that now Tom is getting a signal from Bob's frame of reference. So what Tom is reading on his comparator is what Bob observes. So obviously Tom is going to see the same thing Bob sees! quote]
Which proves my point. Tom is standing in a different frame of reference than the train, but the signals reaching him show him what the lights are actually doing at that exact moment (minus the delays in the wires). Tom’s brother Jim, who is standing next to him on the platform and is watching the train, sees the illusion of the lights turning on at different times created by the motion of the train. But Tom can assure Jim that it is only an illusion, because he is able to measure what is actually happening at that moment in time.

 

[SpeedFreek]

The only problem with modifying the paradox this way is that now Tom is getting a signal from Bob's frame of reference. So what Tom is reading on his comparator is what Bob observes. So obviously Tom is going to see the same thing Bob sees!

Which proves my point. Tom is standing in a different frame of reference than the train, but the signals reaching him show him what the lights are actually doing at that exact moment (minus the delays in the wires). Tom’s brother Jim, who is standing next to him on the platform and is watching the train, sees the illusion of the lights turning on at different times created by the motion of the train. But Tom can assure Jim that it is only an illusion, because he is able to measure what is actually happening at that moment in time.

Actually, I'm not sure it is as simple as dangineer put it. I don't think there is any difference between the flashes of light from the bulbs heading towards Tom at c, or signals from those bulbs propagating down a wire towards Tom at c. He would receive the signals at different times and the reason for this is that the events happened at different times, from his frame of reference.

But, for arguments sake, let's say you are correct. Let's say that by using wires Tom can find out what happened in Bobs frame of reference. How does this make the simultaneity of those bulbs absolute in any way except from within that moving train carriage?

If you acknowledge that Tom will see the lights flash at different times if he uses light as information and deducts the time the light took to reach his eyes, what difference does it make if he can calculate what happened from the frame of reference of the train? The events are only simultaneous in the frame of reference of that train.

 

[dangineer]

Well, with the setup MA described, Tom is reading a desctription of what's goint on in Bob's frame of reference. But if Tom visually watches the lights while he's looking at his comparator, they won't match. We could build a comparator that produces the exact same results as MA described without using wires by having it recieve the light signals from the train and converting the results using special relativity (this is how police speed radars work). It will produce exactly the same results by showing Tom what's going on in Bob's frame of reference, regardless of what Tom observes. All this proves is that there are ways we can see what's going on in Bob's frame of reference from Tom's frame of reference.

 

[SpeedFreek]

Anyway, Tom can easily work out that Bob would have seen the lights flash at the same time if he knew the trains speed relative to himself and did the calculation.

But that aside, like we asked before, how does Tom knowing what happened in Bobs frame of reference make the events in Bobs frame of reference absolute? Obviously, we know the events were simultaneous in that frame of reference.

The point of the relativity of simultaneity is that the events are only simultaneous in that frame of reference (or equivalent frames) but not universally. The constancy of c makes this inevitable.

 

[mental_avenger]

Actually, I'm not sure it is as simple as dangineer put it. I don't think there is any difference between the flashes of light from the bulbs heading towards Tom at c, or signals from those bulbs propagating down a wire towards Tom at c. He would receive the signals at different times and the reason for this is that the events happened at different times, from his frame of reference.

How can you say that? The speed of signal transmission down the wires would be constant, or do you think the speed of light changes somewhere along the wires between the train and the platform? If so, where?

But, for arguments sake, let's say you are correct. Let's say that by using wires Tom can find out what happened in Bobs frame of reference. How does this make the simultaneity of those bulbs absolute in any way except from within that moving train carriage?

Because, as far as the bulbs are concerned, they turn on at the same time. Everything else is an outside observation of the event, but the event does not change.

If you acknowledge that Tom will see the lights flash at different times if he uses light as information and deducts the time the light took to reach his eyes, what difference does it make if he can calculate what happened from the frame of reference of the train?

That is the point. He isn’t calculating the difference between the flashes because they remain unchanged. He is calculating the observational difference which is a result of his viewpoint.

 

[SpeedFreek]

How can you say that? The speed of signal transmission down the wires would be constant, or do you think the speed of light changes somewhere along the wires between the train and the platform? If so, where?

Well, one end of the wire is in a different frame of reference to the other end, is it not?

 

[ramparts]

How can you say that? The speed of signal transmission down the wires would be constant, or do you think the speed of light changes somewhere along the wires between the train and the platform? If so, where?

Well, one end of the wire is in a different frame of reference to the other end, is it not?

Not if the wire's moving uniformly A frame of reference is really just a coordinate system - and in the case of relativity, it usually refers to one's motion. So two things moving at the same speed (or at rest, relative to each other) are in the same "frame of reference", if you will.

 

[SpeedFreek]

How can you say that? The speed of signal transmission down the wires would be constant, or do you think the speed of light changes somewhere along the wires between the train and the platform? If so, where?

Well, one end of the wire is in a different frame of reference to the other end, is it not?

Not if the wire's moving uniformly A frame of reference is really just a coordinate system - and in the case of relativity, it usually refers to one's motion. So two things moving at the same speed (or at rest, relative to each other) are in the same "frame of reference", if you will.

How can the wire be moving uniformly if one end is on the train and the other is on the platform? He did say it was a flexible wire, after all.

 

[SpeedFreek]

Because, as far as the bulbs are concerned, they turn on at the same time. Everything else is an outside observation of the event, but the event does not change.

This is the crux of the matter. The event does not change in its own frame of reference, of course. But the difference in events that is measured by observers in other frames is not simply an observational difference, it is a real difference.

This is why your twin brother would ages less, when travelling at relativistic speed in relation to yourself. Upon his return, the difference in your ages would be real. The shift in simultaneity between you both was real to both of you.

The same applies to events on the train. Light travels at 300,000 km/s relative to Tom on the platform, so if a light turns on in the middle of a moving carriage as it passes him, that light will hit the back of the carriage before it hits the front. That is the reality of Tom's situation and the reason that simultaneity is not absolute, but completely frame dependent.

 

[ramparts]

How can the wire be moving uniformly if one end is on the train and the other is on the platform? He did say it was a flexible wire, after all.

Oh. Yes, I didn't read the above few posts too closely :lol: sorry

 

[ccart3r]

What about the theory of "membranes"

 

[dangineer]

The wires are just tools to help us conceptualize the situation. They essentially constrain the signal to follow a certain path, which is whay adding wires changes the problem. Light travels in a straight path, or at least the shortest path, at a constant speed no matter who the observer is. Signals in the wires travel at a constant speed, probably at the speed of light. But the person setting up the problem defines the length of the wire, thus defining the time it takes a certain signal to reach an observer. Say, for instance, you attach a wire to each lightbulb and attach the other end's of the wires to Tom's "comparator." You can use any arbitrary length for each wire and come up with all sorts of different results. By adding wires it essentially makes this problem anyone's game.

MA, I think I might see what you are getting at now. You say that Bob sees the lights flash simultaneously, and "as far as the bulbs are concerned, they turn on at the same time." So you are saying that the absolute reference frame is the one with Bob and the light bulbs, is this correct? Since the bulbs are the object in question, what happens to them is what is actually happening.

So in the problem with the two brothers passing by each other, one brother sees the other brother experiencing time slower, and vice versa. But what is actually happening is that each brother is experiencing time normally, and they just observe the other brother experiencing time differently. Is this what you are saying?

 

[mental_avenger]

This is the crux of the matter. The event does not change in its own frame of reference, of course. But the difference in events that is measured by observers in other frames is not simply an observational difference, it is a real difference.

Well, I can see that we disagree on this. No problem. I understand it, and I am comfortable working within my set of parameters.

This is why your twin brother would ages less, when travelling at relativistic speed in relation to yourself. Upon his return, the difference in your ages would be real. The shift in simultaneity between you both was real to both of you.

Either that or the process of accelerating to, and traveling at relativistic velocities simply makes molecular interactions slow down an amount proportional to the velocity. That would make a person (or anything else) age more slowly and it would appear that time had slowed down. Time itself does not have to slow down, the interactions can just take longer. AFAIK, there is no way to tell the difference at this time.

 

[mental_avenger]

The wires are just tools to help us conceptualize the situation. They essentially constrain the signal to follow a certain path, which is whay adding wires changes the problem.

That is essentially correct. What the wires allow Tom to do is to see what is going on in the train without the motion of the train creating what amounts to an optical illusion. I’ll give you an example of why it is an optical illusion. Take the bouncing-ball-on-the-train example for instance. Bob, on the train, sees the ball bounce straight up and down 3 feet. Tom, on the platform, watching the train go by, sees the ball bouncing in a sine wave pattern and therefore moving further in the same length of time. But George, who is in the same frame of reference as Tom, but standing on the track behind the train, sees the ball bounce up and down 3 feet. Since Tom and George are in the same inertial frame of reference, and they see the same event differently, it is clear that at least one observation is an optical illusion. But none of that changes the motion of the ball.

MA, I think I might see what you are getting at now. You say that Bob sees the lights flash simultaneously, and "as far as the bulbs are concerned, they turn on at the same time." So you are saying that the absolute reference frame is the one with Bob and the light bulbs, is this correct? Since the bulbs are the object in question, what happens to them is what is actually happening.

Bingo.

So in the problem with the two brothers passing by each other, one brother sees the other brother experiencing time slower, and vice versa. But what is actually happening is that each brother is experiencing time normally, and they just observe the other brother experiencing time differently. Is this what you are saying?

Not exactly, but close enough for now. It’s more like each brother is seeing the other brother’s situation as an equivalent optical illusion.

I think that the bottom line is that conventional wisdom treats this as an actual difference in time, and I treat it as an optical illusion. But if it is treated as an actual difference in time and compensated for, the end results will be the same as if it is treated as an optical illusion and compensated for.

 

[SpeedFreek]

I think that the bottom line is that conventional wisdom treats this as an actual difference in time, and I treat it as an optical illusion. But if it is treated as an actual difference in time and compensated for, the end results will be the same as if it is treated as an optical illusion and compensated for.

But you compensate for your optical illusion by saying:

Either that or the process of accelerating to, and traveling at relativistic velocities simply makes molecular interactions slow down an amount proportional to the velocity. That would make a person (or anything else) age more slowly and it would appear that time had slowed down. Time itself does not have to slow down, the interactions can just take longer. AFAIK, there is no way to tell the difference at this time.

What is time, if it is not the temporal distance between events?

 

[mental_avenger]

What is time, if it is not the temporal distance between events?

This thing all things devours:
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town,
And beats high mountains down.

 

[mental_avenger]

What is time, if it is not the temporal distance between events?

It is interesting that so many people believe that there is, or should be, a macro analogy to everything. IMO there is no actual property, process, or characteristic that is time. Time just is. If I said time is like a river flowing in one direction, certainly someone would say that rivers can be diverted, or that rivers can run slow or fast, or that the Amazon periodically runs backwards. Macro analogies virtually always fall short of being able to completely or accurately describe a micro event or object.

As much as we would like to believe in the possibility of time travel or different rates of time, I think that time itself may be one of the few unalterable absolutes in the Universe, sort of a background which is behind everything else that takes place.

 

[CommonMan]

I'm afraid mental_avenger may be right. Time seems to be in the here and now. Time travel would be a dream but it seems impossible at this time.