PDA

View Full Version : Time, Observation & Reference Frames?



cosmocrazy
2010-Jun-14, 08:05 PM
Ok, after reading a couple of discussions on 2 other threads regarding time, observation and reference frames (This discussion as an example - http://www.bautforum.com/showthread.php?p=1746569#post1746569 ) I decided to start a new thread.

So my question is, based on relativity and current mainstream physics is it possible to observe different times in the Earth's past from different reference frames?

I am aware that the further distant the observer is then the further back in the past is what is observed.

So I guess what i'm asking is it possible to be a certain given distance away yet able to see further back in time than what that given distance should dictate?

Ken G
2010-Jun-15, 03:33 AM
Your question actually presents us with a nice example of just how limited the meaning of the idea of "seeing back in time" actually is, if we imagine it has an absolute or unique kind of meaning (it doesn't). If you have two telescopes at the same point, looking at Earth, and one is passing the other at high speed, then both telescopes will be seeing the same moment on Earth. Does that mean they are both seeing the same amount "back in time"? Well, yes and no-- they are seeing the same moment, but if they use the Einstein convention to reckon how far back in time they are seeing, they will not get the same answer for how far back in time that is. So what does the concept even mean? To avoid pursuing that question on two threads, you might turn to the Science and Technology section where I started a thread on "What is time anyway", for much the same reasons that you started this one. Apparently, the Q&A section is not meant for asking such interesting and probing questions.

tusenfem
2010-Jun-15, 07:30 AM
Apparently, the Q&A section is not meant for asking such interesting and probing questions.


Let it be stated that this is Ken G's personal opinion, and not that of BAUT.
Any and all questions are welcome in Q&A.

Jens
2010-Jun-15, 07:35 AM
So I guess what i'm asking is it possible to be a certain given distance away yet able to see further back in time than what that given distance should dictate?

If I understand what you are asking correctly, I think the answer is no. For example, two people standing next to each other on Mars could not see a different time on the earth. The obvious exception is you use a recording device of some kind. As a simple example, if I am sitting in a room, and I'm looking out the window at the moon, whereas a person next to me is looking at a photo of the moon in a book, then we are seeing the moon at different times. I think you'd probably agree that mine is the "real" one.

astromark
2010-Jun-15, 08:02 AM
So my question is, based on relativity and current mainstream physics is it possible to observe different times in the Earth's past from different reference frames?

I am aware that the further distant the observer is then the further back in the past is what is observed.

So I guess what i'm asking is it possible to be a certain given distance away yet able to see further back in time than what that given distance should dictate?

and the answer is No. Other than the fine example Jens has given you... There is no mechanism available for such a idea to work.

Ken G
2010-Jun-15, 10:19 AM
Let it be stated that this is Ken G's personal opinion, and not that of BAUT.
Any and all questions are welcome in Q&A.

Then I stand corrected-- apparently the questions are allowed to be profound and probing, but the answers are not. But we already have two posts so far that say the answer is "no", when of course it really depends on what is meant in the OP by "the certain distance." By one normal interpretation of the meaning of that phrase (the distance as normally reckoned by people on Earth), the answer is yes, as I indicated above-- two telescopes at the same distance (and indeed the same place) as reckoned from Earth will conclude, if they are applying the Einstein system for synchronizing distant clocks, that they are indeed seeing "into the Earth's past" by different amounts (because they will also reckon their distances to Earth as being different, length contraction being what it is). The two telescopes would need a substantial relative velocity, of course, and they would also be seeing the same events in Earth history.

But if by "a certain distance" the OPer means the same distance rackoned by the telescopes, such that if they had a substantial relative speed they would not be at the same place at all when they do their observation, then the answer would be "no"-- but of course the "certain distance" is determined expressly by setting the condition that they see the same amount "into the past" of Earth (though they would be seeing different events in Earth history). So in that case, the question is redundant, as it asks if two telescopes that are constrained to see the same amount "into the past" on Earth would be seeing the same amount into the past on Earth. So take your pick-- the answer is either "yes", or the question is redundant. But here we have the kind of disagreement among the answers that in recent other threads we have been asked to avoid, for fear of getting the OPer all confused. Of course, the real issue is the profoundness, complexity, and need of clarification, of the question.

To see the real problem here, it is with the whole concept of "seeing into the past" on Earth, which is explored more on the thread in S & T called "What is time anyway"?

Jens
2010-Jun-15, 10:35 AM
Yes, I actually interpreted the question as being redundant, as you suggest. Perhaps if that assumption was wrong the OP should clarify what was meant.

Ken G
2010-Jun-15, 11:28 AM
I actually think the issue of which interpretation is correct is unknowable, because the OPer probably did not recognize the potential for multiple interpretations in the first place-- which is really what their question is about here. Thus the goal in answering should be to help the OPer see why the question is either redundant as posed, or very profound and subtle. Recent clarifications by the mods seem to indicate that they want us to take all questions in Q&A at face value and just give the simplest possible answers, which here would merely create an illusion of understanding. Genuine understanding requires that the OPer see that the question is either redundant, or has an answer that is different from the simple "no" answer.

Strange
2010-Jun-15, 11:37 AM
Not wishing to get into a meta-discussion (too late!) I assume the OP wanted a deeper discussion than seemed appropriate for the original thread referenced. So don't be coy! But actually, I think you have already given pretty good examples of when and why the answer would be both yes and no. It certainly helps me (a bit) with the ideas in your "what is time" thread.

Ken G
2010-Jun-15, 11:47 AM
Not wishing to get into a meta-discussion (too late!) I assume the OP wanted a deeper discussion than seemed appropriate for the original thread referenced. So don't be coy! But actually, I think you have already given pretty good examples of when and why the answer would be both yes and no. It certainly helps me (a bit) with the ideas in your "what is time" thread.That was my goal. The "coyness" relates to something tangential, which is the recent discussions about what kinds of answers are appropriate in Q&A, where the moderators quite clearly seemed to be saying that answers like "it could be yes or no depending on what you mean by a certain distance" would be viewed as overly picky, detailed, or complicated for Q&A, whereas I'm arguing that this is what the question is actually all about. So this thread makes for an excellent "case in point" about that other discussion, which is not actually relevant to this thread itself.

01101001
2010-Jun-15, 01:06 PM
Recent clarifications by the mods seem to indicate that they want us to take all questions in Q&A at face value and just give the simplest possible answers, which here would merely create an illusion of understanding. Genuine understanding requires that the OPer see that the question is either redundant, or has an answer that is different from the simple "no" answer.

If I'm not hijacking: I read beyond the first couple of non-bookkeeping answers because I wanted more. May I pretend that everyone who's gotten to this point also wants more? Please don't limit yourself to the OP's expected expectations. Continue. Thanks.

Grey
2010-Jun-15, 04:01 PM
So I guess what i'm asking is it possible to be a certain given distance away yet able to see further back in time than what that given distance should dictate?Ken is right that the details of exactly what you mean by distance and time aren't always clear. However, I think what you're asking is this: suppose there are two observers watching the Earth with telescopes, and they're both in the Alpha Centauri system, about 4 light years away. One of them is in a nice, slow, stable orbit, so we can negelct velocity, and we know that observer is seeing the stuff that happened back in 2006. You're asking, could the other observer be moving in some way at high velocity so that he instead sees the stuff that was happening in 2003 or 2008 or something. The answer to that is no. Depending on how the two observers are moving, and what conventions they're using for distance and time, they might disagree on how far away the Earth is, or what date it is on Earth right "now" (where "now" means what each observer separately judges to be simultaneous with their own present, again using whatever convention for that they've decided to use). But regardless of that fact, they'll both be seeing the same light that left the Earth, and they'll agree on what date it was according to calendars on Earth when that light left. In other words, at the moment they pass each other, they'll be seeing the same events happening on Earth regardless of how they are moving.

I agree with Ken G's assessment that sometimes a question can bring up deeper issues than the questioner is thinking about, but I think I'm ending up with the opposite opinion. If reading the other thread leaves you wondering if maybe an observer at Alpha Centauri could suddenly see what the Earth looked like millennia ago by accelerating to the right velocity, then the more involved discussion in that thread has obscured the simpler answer.

jfribrg
2010-Jun-15, 04:18 PM
The answer is that yes you can see the Earth at two different times from the same time and place away from the Earth. Consider two observers to be looking at the Earth from far away. they are both in the same location. One is looking directly at the Earth. The other is looking at light that came from Earth, passed by a supermassive object, and said light was redirected towards the two observers. The light that was gravitationally redirected has followed a longer path and therefore shows the Earth at an earlier time than the "straight line" view of the Earth. This technique is usually associated with gravitational lensing (http://en.wikipedia.org/wiki/Gravitational_lensing) involving galaxies, but in theory it can apply any time that light makes a close encounter with a massive object.

Ken G
2010-Jun-15, 04:28 PM
I agree with Ken G's assessment that sometimes a question can bring up deeper issues than the questioner is thinking about, but I think I'm ending up with the opposite opinion.Yet note I already said (in post #2, no less) that both observers would see the same date on Earth if they were observing from the same point and time (the same "event" in distant space). But one of the observers in that case is indeed "seeing farther back in time", because that observer does not think it is 2010 on Earth, as you also pointed out. The key issue that is the same in both of what we are saying here is that time is owned by a world line (which is the path through spacetime of a particular object or observer), and the clocks on that world line. So as long as all of our references to time are referred to time on Earth clocks, we can say that both telescopes in alpha Centauri, if reckoned by Earthlings to be observing in the year 2010, will see Earth year 2006, so we on Earth might say they are seeing 4 years "back in time." But there are two immediate consequences of that answer:
1) it adopts a standard yet arbitrary coordinatization to say what it means for an alpha Centauri telescope to observe "in 2010 Earth time", and this coordinatization is not the one used for cosmological language, so we have introduced an arbitrary and unspecified dividing line between alpha Centauri and cosmological quasars, with no transition between the two, and
2) it does not apply to the way observers using the telescopes would reckon time on Earth, so does not agree with what they would say is how "far back in time" they are seeing. Since the issue of "seeing back in time" is almost always reckoned in the language of the person using the telescope, not the language of the target, it's hard to see why ignoring point #2 is really serving the needs of the OP question-- at least without further clarification. At the very least, the OPer should be educated to the need for such clarification.
If reading the other thread leaves you wondering if maybe an observer at Alpha Centauri could suddenly see what the Earth looked like millennia ago by accelerating to the right velocity, then the more involved discussion in that thread has obscured the simpler answer.It is perfectly possible for an observer in alpha Centauri to see what Earth looked like millennia ago by accelerating toward Earth, as long as they are reckoning the date on Earth "now" (their now) to be 2010 only after accelerating (prior to accelerating they would have be seeing the same events on Earth, they just wouldn't call it "millennia ago" there). So that's no kind of misconception at all, but what is happening is the "millennia ago" is coming from their new concept of what date it is on Earth "now" (their now), rather than what date on Earth they are seeing by Earth time. This is entirely the point I'm making-- the language about time gets very confusing and arbitrary when we break the simple physical rule that time applies to a clock on a world line.

Ken G
2010-Jun-15, 04:44 PM
The answer is that yes you can see the Earth at two different times from the same time and place away from the Earth. Consider two observers to be looking at the Earth from far away. they are both in the same location. One is looking directly at the Earth. The other is looking at light that came from Earth, passed by a supermassive object, and said light was redirected towards the two observers. The light that was gravitationally redirected has followed a longer path and therefore shows the Earth at an earlier time than the "straight line" view of the Earth.Yes, gravitational effects can let you see Earth at two different times from the same time and place of observing, but that brings in another layer of complexity-- gravity. This is indeed worth pointing out, but I suspect the OP was only referring to velocity differences between the telescopes (as that is something the telescope users can control). But your contribution asks us to put the complete picture together-- for a telescope to see "into the past" of some other object, it implies a comparison between the time at that object that the telescope reckons as "now", and the time at that object the telescope is actually seeing. The former can be changed by changing the telescope velocity, and the latter can be changed by gravity, so there are various ways to get different answers about the "time into the past" that is being seen. If the OP is not answered that way, then the genuine understanding about how slippery is the whole concept of "seeing into the past" cannot be understood.

I would submit that the only "clean" way to "see into the past" is to restrict to seeing the past of a single world line from that world line-- which requires seeing light that bounces off some astronomical mirror in deep space, or is turned all the way around by gravity out there. Neither of those are likely to happen, so it would just be a "thought experiment", but any other concept of "seeing into the past" is nothing but pure coordinatization-- and we say enough about coordinatizations in relativity that I think the people on this forum can be expected to function at that level of sophistication.

Grey
2010-Jun-15, 06:37 PM
Yet note I already said (in post #2, no less) that both observers would see the same date on Earth if they were observing from the same point and time (the same "event" in distant space). But one of the observers in that case is indeed "seeing farther back in time", because that observer does not think it is 2010 on Earth, as you also pointed out.Yep. But by focusing on this, it sounds like cosmocrazy (and probably others) were completely confused, and got the wrong idea entirely.


The key issue that is the same in both of what we are saying here is that time is owned by a world line (which is the path through spacetime of a particular object or observer), and the clocks on that world line. So as long as all of our references to time are referred to time on Earth clocks, we can say that both telescopes in alpha Centauri, if reckoned by Earthlings to be observing in the year 2010, will see Earth year 2006, so we on Earth might say they are seeing 4 years "back in time." But there are two immediate consequences of that answer:
1) it adopts a standard yet arbitrary coordinatization to say what it means for an alpha Centauri telescope to observe "in 2010 Earth time", and this coordinatization is not the one used for cosmological language, so we have introduced an arbitrary and unspecified dividing line between alpha Centauri and cosmological quasars, with no transition between the two, andI disagree. I didn't respond on the other thread where you were talking about this, but the "cosmological age" method of synchronizing clocks and the standard special relativistic method of synchronizing clocks will give the same result for Alpha Centauri, at least if you neglect the small relative proper motion of Alpha Centauri and the Sun.


It is perfectly possible for an observer in alpha Centauri to see what Earth looked like millennia ago by accelerating toward Earth, as long as they are reckoning the date on Earth "now" (their now) to be 2010 only after accelerating (prior to accelerating they would have be seeing the same events on Earth, they just wouldn't call it "millennia ago" there). So that's no kind of misconception at all, but what is happening is the "millennia ago" is coming from their new concept of what date it is on Earth "now" (their now), rather than what date on Earth they are seeing by Earth time. This is entirely the point I'm making-- the language about time gets very confusing and arbitrary when we break the simple physical rule that time applies to a clock on a world line.But in the case you're describing here, they'll also see the Earth at exactly the same time (that is, millennia ago) before they accelerate. That is to say, the acceleration (and resulting change of reference frame) will change their opinion of what time it is "now" on Earth (if they're using Einstein's simultaneity convention), but it will have no effect at all on what they are seeing happen on Earth.* So their relative velocity has no effect at all on what events they'll see transpiring on a distant world, it only changes what they will calculate to be the present day on that world, if you assume a particular choice of coordinatization. I know that you're aware of that distinction (indeed, I think that's much of your point), but my point is that by insisting upon going into that in detail, you've given someone with less familiarity with relativity exactly the wrong idea about what's going on. I think that's a disservice.

* Well, except that it might now be highly red or blue shifted, and you'll start seeing effects from time dilation and all of that.

Nereid
2010-Jun-15, 06:49 PM
The answer is that yes you can see the Earth at two different times from the same time and place away from the Earth. Consider two observers to be looking at the Earth from far away. they are both in the same location. One is looking directly at the Earth. The other is looking at light that came from Earth, passed by a supermassive object, and said light was redirected towards the two observers. The light that was gravitationally redirected has followed a longer path and therefore shows the Earth at an earlier time than the "straight line" view of the Earth. This technique is usually associated with gravitational lensing (http://en.wikipedia.org/wiki/Gravitational_lensing) involving galaxies, but in theory it can apply any time that light makes a close encounter with a massive object.
You can also do this with a giant mirror, located 'off to the side'; the reflected light (off the mirror) takes longer to reach the observers, so they see Earth as it was at a different time from the light that took a direct path.

This technique is used to analyse historical supernovae, from their light echoes.

cosmocrazy
2010-Jun-15, 06:51 PM
If I'm not hijacking: I read beyond the first couple of non-bookkeeping answers because I wanted more. May I pretend that everyone who's gotten to this point also wants more? Please don't limit yourself to the OP's expected expectations. Continue. Thanks.

No prob here! Yes I am fully aware and understand the standard "bookkeeping" answers given thus far. My question , although perhaps not worded too well was intended to encourage discussions on possible scenarios where it might be possible to actually view further back in our history using the examples that Ken has described.

So yes continue on all! I will look forward to reading your posts and opinions.

Oh and thanks Ken I will take a look at your thread - "What is time anyway"

01101001
2010-Jun-15, 07:23 PM
Yep. But by focusing on this, it sounds like cosmocrazy (and probably others) were completely confused, and got the wrong idea entirely.

And if it hadn't been mentioned I would have been confused because my gut told me that's what we've come to know as reality. Ya just never know who's getting confused and who's not. Glad I am for receiving details on the big picture. [Waves to far-future readers of this matter who are just then encountering it. Waves to far-past readers of this material who are just catching up to it then.] It is confusing, isn't it?

Ken G
2010-Jun-15, 07:30 PM
Yep. But by focusing on this, it sounds like cosmocrazy (and probably others) were completely confused, and got the wrong idea entirely. Then let them learn from their confusion. Often, if not always, the road to learning leads through stages of confusion.


I disagree. I didn't respond on the other thread where you were talking about this, but the "cosmological age" method of synchronizing clocks and the standard special relativistic method of synchronizing clocks will give the same result for Alpha Centauri, at least if you neglect the small relative proper motion of Alpha Centauri and the Sun.I never said they'd give different answers for alpha Centauri, I said they do not give the same answers everywhere, and what's more, there is no meaningful prescription for making the transition. That's what I meant by no "dividing line" between the coordinate choices, which is quite a standard issue with coordinate choices-- it is the flag that we are doing it, not nature.


But in the case you're describing here, they'll also see the Earth at exactly the same time (that is, millennia ago) before they accelerate. Yup, I don't think I said anything different. All the same, if you ask them, they will be seeing "farther back in Earth time" after they accelerate than before. That's why it is purely an issue of coordinatization, and there's no way to make it an issue of anything but coordinatization.


That is to say, the acceleration (and resulting change of reference frame) will change their opinion of what time it is "now" on Earth (if they're using Einstein's simultaneity convention), but it will have no effect at all on what they are seeing happen on Earth.Correct. That's what I said in my last post.


I know that you're aware of that distinction (indeed, I think that's much of your point), but my point is that by insisting upon going into that in detail, you've given someone with less familiarity with relativity exactly the wrong idea about what's going on. I think that's a disservice. Only if they can't read. I was quite clear on the distinction, especially if you also look at my response to jfribrg. But I think we've both entered our opinions and arguments-- if we go further into it, we'll be accused of turning a Q&A thread into an argument (even though I think the nature of our disagreement here is itself the crucial issue of the whole OP question).

pzkpfw
2010-Jun-15, 09:34 PM
...


...

Ken G, two of your posts contain nothing to do with the question but are instead off-topic meta-discussion about the question. That's enough please, we get it. Further non-relevant posts, making points regarding the management of Q&A, will be infracted. Take it to the feedback forum. I'd appreciate it if no one else jumps on the bandwagon.

Jens
2010-Jun-16, 04:45 AM
No prob here! Yes I am fully aware and understand the standard "bookkeeping" answers given thus far. My question , although perhaps not worded too well was intended to encourage discussions on possible scenarios where it might be possible to actually view further back in our history using the examples that Ken has described.

I think this is what you're trying to ask: there are two observers in the Alpha Centauri system, and they are standing on a planet and both see light coming from 2008 on earth. I think what you mean to ask is, is there something that one of them could do that would allow him to see light from 2004, for example? In that case I think the answer is no, that would not be possible, because the light from 2004 would already be trailing off into space somewhere, and catching up with it would require traveling faster than the speed of light.

Jeff Root
2010-Jun-16, 05:08 AM
Jens,

You can can expand the range of the question to not limit the two
observers to be on a planet or moving at the same speed. One
might be circling the Sun at close to light speed, for example.
But the answer remains the same.

-- Jeff, in Minneapolis

Ken G
2010-Jun-16, 08:05 AM
Personally, I find it rather obvious that the light that leaves Earth from 2008 is like the crest of a wave expanding out into space, so anyone at the events in spacetime where that light reaches will see Earth when it is 2008 on Earth. Is that really the question being asked? However, and this is the less obvious part, they cannot say "how far back in time" they are seeing the Earth, in 2008, unless they have some additional prescription for identifying "how far back in time" 2008 is, and that additional prescription may have essentially zero physical meaning or importance. So my comments have been about the weakness of thinking in terms of "seeing back in time", rather than seeing 2008. As has come out in this thread, there's really only physical meaning to "seeing back in time" if the light bounces off something, or is bent in a gravitational field, and even then it just means you can see light from two different times (perhaps 2008 and 2004), which you could choose to call "seeing ahead in time" just as easily, depending on how you set up the comparison. The only situation I can think of where "seeing back in time" has any physically important meaning is in cosmology, and only then because of the additional invocation of the cosmological principle. In every other situation, "seeing back in time" means nothing other than that you are seeing something that already happened so you cannot affect it-- which is always true with anything you see.

Jens
2010-Jun-16, 08:14 AM
Personally, I find it rather obvious that the light that leaves Earth from 2008 is like the crest of a wave expanding out into space, so anyone at the events in spacetime where that light reaches will see Earth when it is 2008 on Earth. Is that really the question being asked?

Yes, I do think that is the question that's being asked. I'm not absolutely certain, but I think what cosmocrazy is trying to ask is, could we sort of make a "time machine" by building a rocket that would whiz really fast around the sun, allowing us to see the earth a long long time ago when there were dinosaurs, or something along those lines. I could be misinterpreting, but I think that's the half-stated intent of the question. Hence my answer, no, I don't think so.

Strange
2010-Jun-16, 08:38 AM
Personally, I find it rather obvious that the light that leaves Earth from 2008 is like the crest of a wave expanding out into space, so anyone at the events in spacetime where that light reaches will see Earth when it is 2008 on Earth. Is that really the question being asked? However, and this is the less obvious part, they cannot say "how far back in time" they are seeing the Earth, in 2008, unless they have some additional prescription for identifying "how far back in time" 2008 is, and that additional prescription may have essentially zero physical meaning or importance.

I think turning the description around like that makes the point much clearer: the issue is less about what they are seeing (which may be what confused some people when you first brought this up) and more about "when" they think they are (in relation to Earth).

ETA: or, prehaps more accurately, "when" they think the earth is in relation to them...

Nereid
2010-Jun-16, 09:06 AM
Personally, I find it rather obvious that the light that leaves Earth from 2008 is like the crest of a wave expanding out into space, so anyone at the events in spacetime where that light reaches will see Earth when it is 2008 on Earth. Is that really the question being asked? However, and this is the less obvious part, they cannot say "how far back in time" they are seeing the Earth, in 2008, unless they have some additional prescription for identifying "how far back in time" 2008 is, and that additional prescription may have essentially zero physical meaning or importance. So my comments have been about the weakness of thinking in terms of "seeing back in time", rather than seeing 2008. As has come out in this thread, there's really only physical meaning to "seeing back in time" if the light bounces off something, or is bent in a gravitational field, and even then it just means you can see light from two different times (perhaps 2008 and 2004), which you could choose to call "seeing ahead in time" just as easily, depending on how you set up the comparison. The only situation I can think of where "seeing back in time" has any physically important meaning is in cosmology, and only then because of the additional invocation of the cosmological principle. In every other situation, "seeing back in time" means nothing other than that you are seeing something that already happened so you cannot affect it-- which is always true with anything you see.
Good point.

Perhaps cosmocrazy could clarify?

Jeff Root
2010-Jun-16, 09:08 AM
Personally, I find it rather obvious that the light that leaves Earth
from 2008 is like the crest of a wave expanding out into space,
so anyone at the events in spacetime where that light reaches
will see Earth when it is 2008 on Earth. Is that really the question
being asked?
It is obvious. And yes, it is the question cosmocrazy asked,
because it was contradicted by this statement:


A civilization 1 light year away could be studying the dinosaurs too.
Since you are normally a highly reliable source of knowledge,
cosmocrazy had to ask what the heck is going on.

-- Jeff, in Minneapolis

astromark
2010-Jun-16, 09:20 AM
Now... Without puffing my chest out and feeling all superior. After rereading this whole thread and the OP a couple of times.
The only expectable answer is. No.
It might be feasible for a light path to be so interrupted by such gravity force as to extend that light path
so as that two images from the same source could exist... It might be feasible, but highly improbable.
I have had direct experience working with and seeking out micro lensing images and opportunities. I can not support such as this idea. The answer is still. No.

tusenfem
2010-Jun-16, 10:11 AM
Okay this seems to be "one of those discussions again." The question by Cosmocrazy is quite clear actually in the OP, let me copy it for you all once more and pay attention to the bolded red and especially the green part:



So I guess what i'm asking is it possible to be [at] a certain given distance away yet able to see further back in time than what that given distance should dictate?


Seems pretty straightforward to me, i.e. "can I see what happened 2 years ago when I am 1 lightyear away from the earth?" (and do not answer "yes, when you looked one year ago")

Ken G
2010-Jun-16, 01:18 PM
So not only are times absolute now, but distances are too? (To Jeff: The reason they could see the dinosaurs 1 LY away is if the time of the observation could be left to the observer to say that it is the year 2010 on Earth "now".) If we say that the observation happens from a given event, so a given distance and time by Earth reckoning, then both telescopes see the same time on Earth (as I said in post #2), yet the two telescope operators may conclude they are seeing different times "into the past" on Earth (as literally asked in the OP) because they will reckon their own distance to Earth as being very different, and they will reckon the "now" on Earth as being very different. So I cannot see why we need to suspend all of the important issues about non-absolute time and distance just to be able to pretend the OP is so trivial as to be obvious, when in fact, including those issues actually makes the OP interesting. We really should know, when we use phrases like "seeing into the past", that by itself that phrase means essentially nothing like what we imagine it means.

And for anyone who thinks I have somehow "hijacked" this thread into territory that the OPer was not interested in or could not understand, please read the comment by 01101001 and the response to it by the OPer. I fail to see what the "problem" is in the course this thread has taken.

cosmocrazy
2010-Jun-16, 03:56 PM
It is obvious. And yes, it is the question cosmocrazy asked,
because it was contradicted by this statement:

Since you are normally a highly reliable source of knowledge,
cosmocrazy had to ask what the heck is going on.

-- Jeff, in Minneapolis

Yes I think this is basically why I asked the question, i guess I was confused by that statement by Ken.

cosmocrazy
2010-Jun-16, 04:02 PM
Personally, I find it rather obvious that the light that leaves Earth from 2008 is like the crest of a wave expanding out into space, so anyone at the events in spacetime where that light reaches will see Earth when it is 2008 on Earth. Is that really the question being asked? However, and this is the less obvious part, they cannot say "how far back in time" they are seeing the Earth, in 2008, unless they have some additional prescription for identifying "how far back in time" 2008 is, and that additional prescription may have essentially zero physical meaning or importance. So my comments have been about the weakness of thinking in terms of "seeing back in time", rather than seeing 2008. As has come out in this thread, there's really only physical meaning to "seeing back in time" if the light bounces off something, or is bent in a gravitational field, and even then it just means you can see light from two different times (perhaps 2008 and 2004), which you could choose to call "seeing ahead in time" just as easily, depending on how you set up the comparison. The only situation I can think of where "seeing back in time" has any physically important meaning is in cosmology, and only then because of the additional invocation of the cosmological principle. In every other situation, "seeing back in time" means nothing other than that you are seeing something that already happened so you cannot affect it-- which is always true with anything you see.

Thank you Ken, I understand and was already aware that time is not absolute and that it is only relative to an observers particular frame of reference. I guess my misunderstanding stems from your statement regarding the 1LY distance and observing the dinosaurs.

cosmocrazy
2010-Jun-16, 04:06 PM
So not only are times absolute now, but distances are too? (To Jeff: The reason they could see the dinosaurs 1 LY away is if the time of the observation could be left to the observer to say that it is the year 2010 on Earth "now".) If we say that the observation happens from a given event, so a given distance and time by Earth reckoning, then both telescopes see the same time on Earth (as I said in post #2), yet the two telescope operators may conclude they are seeing different times "into the past" on Earth (as literally asked in the OP) because they will reckon their own distance to Earth as being very different, and they will reckon the "now" on Earth as being very different. So I cannot see why we need to suspend all of the important issues about non-absolute time and distance just to be able to pretend the OP is so trivial as to be obvious, when in fact, including those issues actually makes the OP interesting. We really should know, when we use phrases like "seeing into the past", that by itself that phrase means essentially nothing like what we imagine it means.

And for anyone who thinks I have somehow "hijacked" this thread into territory that the OPer was not interested in or could not understand, please read the comment by 01101001 and the response to it by the OPer. I fail to see what the "problem" is in the course this thread has taken.

If its ok with the Mods feel free to include in the discussion anything you feel is relevant and that might aid in the learning process. I have a deep interest in time & relativity so any additional knowledge I can acquire is most welcome.

cosmocrazy
2010-Jun-16, 04:11 PM
I think this is what you're trying to ask: there are two observers in the Alpha Centauri system, and they are standing on a planet and both see light coming from 2008 on earth. I think what you mean to ask is, is there something that one of them could do that would allow him to see light from 2004, for example? In that case I think the answer is no, that would not be possible, because the light from 2004 would already be trailing off into space somewhere, and catching up with it would require traveling faster than the speed of light.

Not quite but yes if its relevant then let it be so. I didn't think there was some mechanism that allowed such in the scenario you described, so I would have been quite surprised if it was answered yes, and very intrigued to be shown an example how it could be possible.

Ken G
2010-Jun-16, 04:31 PM
Thank you Ken, I understand and was already aware that time is not absolute and that it is only relative to an observers particular frame of reference. I guess my misunderstanding stems from your statement regarding the 1LY distance and observing the dinosaurs.
Well I'm sort of sorry if I sowed confusion, but sort of not-- because I would say that if anyone thinks they understand time dilation and length contraction and so on, but also thinks that observers 1000 LY away really "see into Earth's past", then they still have not understood how those other concepts fit together with the prevailing message of relativity: proper time is real, but coordinate time is just a convenient fiction. But what makes it hard is that sometimes we use language that we know doesn't have the meaning it implies (like "seeing Earth's past from space"), but we're just not bothering to make that point at that moment, and other times people really believe the language, and it's often hard to tell the difference.

uncommonsense
2010-Jun-16, 08:08 PM
Just getting back from some time off, and just read thread. I understand OP to asking if it is possible for point A to get two largely different speed observations of light from point B. Relativity says no. But, I agree that more just constancy of c applies to OP question - namely, the "philosophical" ideals embedded within the whole of SR - specifically, how SR deals with "the past"?

I'm still learning so I won't offer specifics, but one must read up on SR to understand how SR treats "looking into the past".