psionl0
Skeptical about skeptics
Regardless of any capitalization that you might use, you don't have a definition for reality.
Utopia and Time Travel
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alexi_drago
Graduate Poster
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This is not a depiction of spacetime or anything but is how I imagine this problem.
Draw a circle on the ground with 2 random dots in it A and B.
An observer stands somewhere on the edge of the circle, their now is a horizontal line that travels forward across the circle over time.
To the observer, A could occur before B, or the other way round or could be simultaneous. To another observer the order might be different or a different interval.
Now take away any observer, what order do events A and B occur and what is the interval?
Draw a circle on the ground with 2 random dots in it A and B.
An observer stands somewhere on the edge of the circle, their now is a horizontal line that travels forward across the circle over time.
To the observer, A could occur before B, or the other way round or could be simultaneous. To another observer the order might be different or a different interval.
Now take away any observer, what order do events A and B occur and what is the interval?
The Greater Fool
Illuminator
I had rather assumed that readers of the thread understood what reality is, but just for you:
reality
1. the state or quality of being real.
2. resemblance to what is real.
3. a real thing or fact.
4. real things, facts, or events taken as a whole; state of affairs:
the reality of the business world; vacationing to escape reality.
5. Philosophy. a.something that exists independently of ideas concerning it.
b.something that exists independently of all other things and from which all other things derive.
6. something that is real.
7. something that constitutes a real or actual thing, as distinguished from something that is merely apparent.
If A causes B, then A comes first. This is true as well for any ordered network of causation, even if one doesn't directly cause the other.
The interval is the number of events between them in a linear chain of causality. This is just a number, not a measured duration, although it might have some limit if events are discrete and finite.
So it is for me! There is a universal now! And it is the bad kind!
Sounds like you and Roboramma (and myself for that matter) are actually in agreement. The objective reality is the position of the events in spacetime.
Donn
Philosopher
Being dim, and drawing roughly on the last two pages of posts, it sounds like distance is thought real while time is not. Can one be born after one dies?
There's the trouble. It's like choosing "left" as a preferred direction and disallowing "right" because that's the way we move. With a bit of mental gymnastics you can construct a world where people die before they are born, where dying causes living which causes borning which causes parents. (I guess that last one is already the case.)
ETA: We'd probably call in un-dying or something cute.
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You will still have to send a speed of light signal in order to transport anything.
You might find this enlightening:
https://youtu.be/ozhRFi77TP8?t=20m5s
Donn
Philosopher
Relativity seems impossible to fathom! I suppose I shall eventually go to the vast grazing fields of my fellow dinosaurs, unmarred by notions of comprehension. Raaar!
We agree that reality exists. We agree that different reference frames describe the same reality in different ways. We don't agree that that reality includes a specific sequence of events. Instead it includes a spacetime geometry that can be expressed as a specific sequence of events in some particular reference frame, but the reality includes all reference frames and thus there is no correct specific sequence of events in reality.
Time and space are mixed together. They don't exist independent of each other.
But it really is simple if you look at it geometrically. Again, try thinking about an X/Y plane. There are two points A and B on the plane. You can label then with whatever coordinates you like, it won't change their relationship to each other. Does it make sense to say A is higher on the Y axis regardless of how you lay down your coordinates?
But time doesn't work exactly like space and spacetime is lorentz invariant, which is different from something made of only space dimensions. Specifically any two coordinate systems will only be rotated at 45 degrees to each other, no signal that goes from A to B in one frame will be seen as going from B to A in another. So, no, you can't be born before you die.
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Hokulele
Deleterious Slab of Damnation
OK, sorry, got caught up in football last night.
With that as a starting point, let's imagine a train that can move at any speed we want, and that train is equipped with a normal headlight. Just using intuition, would you guess that as the train's speed gets higher, including approaching the speed of light itself, would the headlight be less effective, as effective, or more effective?
A line drawn in spacetime connecting any two events that can be causally connected has a span that is the same in all reference frames. It's happens to be the time measured on a clock travelling along that spacetime path from event A to event B.
So, yeah.
The Greater Fool
Illuminator
That last bit is worth about 5 pages of posts (so far). I think I may have an idea on where we are disconnecting. We are using "sequence" differently.
Imagine a billiards table with balls arranged randomly. I think you are using sequence to identify which balls we see first. Depending on perspective, the sequence of the balls is different. I will see the near balls before the distant balls.
The question "which sequence is the correct one?" <--- [I believe] this is the question I have not answered to your satisfaction.
Now, you grab a cue, strike the cue ball sending it hurtling into one or more other balls. In turn, each collides with other balls. These events happen in a specific sequence. <---- This is the sequence of which I speak. The events happened the way they happened, in that sequence.
Good, except for the last bit. I'm imagining a Y diagram. The single cue ball comes in from the bottom and hits two other balls (the split and top of the Y). Now, the cue ball striking is first in the sequence, but those other two take different paths through space and might in time as well.
What happens next? Well, depending on where we are and how we are moving, the two balls (from the top of the Y) might hit other things in any sequence at all. We've lost the "same place, same time" element that we attach to shared events (the original "hit" by the cue ball).
All good now?
Donn
Philosopher
Robo, I appreciate that you try, even as I feel a fool for not understanding a single thing you say! The fault is all mine. I will lurk a while and divine sense from the to and fro.
The Greater Fool
Illuminator
Could be.
Describing reality, good so far...
Might? They necessarily are both moving in space and time.
Now we have a problem.
Our location and our movement do not affect the objective movements of the balls. Our location and movement will affect how we perceive these events, as SR describes.
And this part is why I think we are not talking about the same things.
The whole thing, from the initial strike to when the balls stop moving are predicated on the initial strike, and each subsequent contact. All this occurred in a sequence.
I think you are going for some sort of what happened simultaneously. Again, you seem concerned with apparent synchronicity, while I am saying these things happen in a specific sequence.
Distances and time are real, but both depend on the reference frame.
What doesn't are spacetime seperations. If you want, you can think about those as "distances" in spacetime.
Here is a nice way to visualize this. See the line in drawing 1a below, it has a certain length h. When you rotate it around on the 2d plane as seen on drawing 1b, its length always stays the same (i.e. its length is invariant to rotation on the plane). However, when you rotate it out of the 2d plane into 3d space as seen on 1c, its length on the plane changes (blue line). Imagine looking straight down on the plane in drawing 1c while you rotate the black line up. The more you do, the shorter its projection (the blue line) becomes on the plane. Finally, if you rotate it 90 degrees to the plane its length becomes 0, it's a point.
Now imagine the first plane as space and another plane (which I have drawn at 90deg in the diagram) as time. When h lies "flat" on the time plane like on diagram 2a, it's a point (red dot) in time. When you start rotating it around like on diagram 2b, it becomes shorter in space (blue line) and longer in time (red line).
In different reference frames the black line is rotated differently which leads to the counterintuitive results described in special relativity, such as time dilation and length contraction. But note that the "length" h (the spacetime separation, in fact) of the black line always stays the same, it is independent of the reference frame.
No. Events can have different sequences in different reference frames, but never events that can have causal connections. If two events can be observed in different sequences in different reference frames, it means that their separation in spacetime is "spacelike" as opposed to "timelike" or "lightlike". I think Roboramma explained that somewhere in this thread already and it's a bit more complicated. In the visualizations above (diagrams) it means that there are limits to how the black line can be rotated around, given by the finite speed of light.
Note that this also means that if you could travel faster than light, you could die before you are born. In other words, faster than light travel would allow causality to be violated. That's one of a couple of reasons why faster than light travel is considered impossible.
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It's actually much more fundamental than that.
If we had one ball and nothing else, could we agree that its state of motion is meaningless? That's the nut of it. Not that the ball isn't moving or is moving depending on how we look at it, but that motion itself in this case has no meaning at all.
The Greater Fool
Illuminator
In the context of a 'snap shot' (capturing an instant), we don't care about motion.
in the context of sequence of events (a movie), we care about motion.