Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: playaguess on 10/11/2011 05:13:28
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In the classic example of the relativity of time, one twin traveling in a spaceship at near light speed is said to experience time at a slower rate, from the perspective of the twin remaining on earth. Conversely, the twin on earth is said to experience time at a faster rate from the perspective of the twin in the space ship.
The example implies that the twin in the spaceship has, as a consequence of this relativity of time, traveled into her/his twin‘s future.
As I have understood this example, either twin could be continuously observed by the other twin.
If my understanding is correct, the fact that either twin can observe the other twin continuously, seems to directly contradict that notion that time is relative, since at any given “time” for one twin, the other twin has a corresponding “time”.
What makes more sense to me, is that the physical processes that constitute the events that occur for each of the twins at their locations are actually occurring at different rates. If this is true, then events that we tend to think of as being governed by universal rules, for example, the rate of radioactive decay, are governed by relativistic rules instead. For the twin in the spaceship, the rate of radioactive decay appears to be the same as it would be for the twin on earth, but, in fact, it is slower. The problem the twin in the spaceship has, is that the tools he/she has to make the observations of the radioactive decay are also affected by the velocity of the spaceship.
What am I missing?
playaguess
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Hello Playaguess and welcome to the forum
For a start - the twin "paradox" is not really a paradox and is completely explained by various interlocking forms of relativity. Rather than rehearse them here is a link to the physicsfaq page on the twin paradox (http://www.desy.de/user/projects/Physics/Relativity/SR/TwinParadox/twin_paradox.html). Personally I prefer the Doppler explanation - but they are all mathematically and physically equivalent.
Whilst it is more comforting to believe that some physical processes are merely changing rate and that time is universal - this doesn't capture all the facts. the simplest and complete explanation is that time is dilated according to special relativity such that moving clocks run slowly. If we expand the idea that time is universal yet the rate of all physical processes are changed then we have problems with absolute space and reciprocity, and even if we find some way round that we have to ask; "if all physical processes are running slower then how can that be different ontologically from time being dilated?"
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heh, pick a clock, measure it against your heartbeats. Jump on that space cruiser, and get a move on :) like 90% of the speed of light. Now measure your heartbeat again relative your clock. Does your heart go slower? Then fearlessly jump down on a 'Event Horizon' of a Black Hole. Do the same experiment again and measure your heartbeat relative your clock. Does your heart go slower?
Nope, it will, relative your own clock, always show you the same local 'time rate'. And if you're going to be one hundred and thirty three years old all in all, then that is what you gonna be, no matter from where you measure your 'life time' locally.
But change place with your twin, and let him start to move, relative you on Earth instead. Now you will see that his clock is soo much slower than yours, in both cases. But it wasn't for you, was it? So, what made him so special?
Frames of reference (relative the constant 'c').
He was 'moving' relative you in both cases, in GR all mass is 'gravitationally uniformly constantly' accelerating. The reason that 'motion' will distort the 'time' you measure for someone others 'frame of reference' is motion and 'c'. Gravity is a special circumstance coupled to mass, but equivalent to 'uniform constant accelerations' according to Einstein's GR.
And 'c' is lights speed in a vacuum, the same locally wherever you go, or how 'fast', relative some defined 'frame of reference' like Earth. It never change. If it did change for you locally, with 'relative motion' and mass, the theory of relativity would be wrong.
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thanks for your responses, and for your reference imatfall.
yor_on. your observation precisely illustrates why i'm having trouble with the concept. light speed can't be constant from a particular reference point, say earth, but actually has to have multiple values.
if we assume in the twin tale that the moving twin is moving away from a third location, from which a beam of light is directed at earth, precisely timed to arrive at earth when the traveling twin has arrived back on earth and can observe its arrival with her/his twin, then both twins can agree that relative to earth the speed of light was C. however the traveling twin, would actually "know" that the light traveled to earth at a speed greater than C, since velocity (unless i have forgotten what little physics i actually know) is a measure of distant covered in a specified amount of time and according to relativity, the traveling twin has experienced less time passing from the perspective of earth. the only way that light can have a single velocity at any particular reference point in space is if the amount of "time" that passed for the traveling twin is the same as for the stationary twin.
as far as relativity being "wrong", i have no problem with that possibility (nor should any scientific thinker). i realize that relativity can be observed to be accurate over a large range of conditions, but this merely says something about about our means of making observsations, it says nothing about the absolute accuracy of the theory. if i'm not mistaken, the scientific community already accepts that relativity is "wrong" or at least not applicable, when it comes to describing events at a quantum level.
imatfall
explained to your satisfaction, not necessarily to mine. i am personally of the opinion that any theory of the processes occurring in the universe that cannot be intuitively understood by a being of reasonable intelligence without the benefit of years of study is, at the very least, incomplete. I will leave it to you to decide whether or not you think i'm reasonably intelligent.
you mentioned that the notion that time is absolute and that physical processes are relativitic doesn't capture all the facts (which i take to mean doesn't explain all our observations). i'm all ears, will you actually list one for me, since that was what i asked for. what am i missing?
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what am i missing?
Your thinking is right.But you can't define borders of different speed of light.You can't define that why time of fast objects is slower. Relativity simplifies our thinking. [;D]
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Or the time dilation is a effect of lights invariant speed :)
Then the effect is not situated to specific points in space but to the relation relative frames of reference. What you are doing there is assuming that 'distances' is something defined relative a 'objective reality', meaning that they must be the same for all involved. Thinking that way you invalidate the Lorentz contraction, even if you accepted that light was a constant 'c'. But you go one step further here and define light as a variable. I think that the problem might be how you define a 'time dilation'.
If you read what I wrote you should realise that you won't find yourself 'age slower' no matter how fast you go. And your life length will be the same as measured by you. You do not win 'time' by travelling, in fact it doesn't matter what you do. You have only one life measure, and time will to you 'flow' the same, wherever you go and whatever you do.
Let's try a personal aproach to what a 'time dilation' is, on Earth. NIST Pair of Aluminum Atomic Clocks Reveal Einstein's Relativity at a Personal Scale. (http://www.nist.gov/public_affairs/releases/aluminum-atomic-clock_092310.cfm)
That's a 'time dilation'. It's no big deal, you won't notice it on Earth, ever. Astronauts doesn't notice it either, and, it does not have to do with how 'small' the effects are. It would be the same if they travelled near lights speed.
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What makes more sense to me, is that the physical processes that constitute the events that occur for each of the twins at their locations are actually occurring at different rates.
There's the problem. A rate is a function of time, so the time really is different, and clocks prove it. You could only say they were occurring at different rates if clocks were not affected.
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What you must accept is that 'c' is a constant. That's no joke, and no theory. all experiments we have done so far point to it being a constant. There are several 'constants' existing in Physics, although not all are agreed on as being one by all. But 'c' is agreed on, at least in mainstream definitions. That people and physicists still try to ***** 'holes' in it doesn't mean that they doubt it, after all, relativity is built on it. But a theory is only as good as the experiments validating it.
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Well, your 'rate' do not change :) Only those other 'frames of reference'. And as you go there you will find the 'time rate' to be as usual for you, no faster, no slower. That may be the reason why so many find it hard to accept too. We think of time as a 'flow' that should sweep the universe with it at a constant rate. And on a purely 'local plane' it do seems to be that way, if you define it from 'locality'. Time is a local phenomena, with my 'local time/clock' defining all other 'times/clocks' relative mine. But the same goes for you, observing me, your local clock defines mine as 'slow', same or 'faster'.
But we all have a 'direction' in time, called the 'arrow of time' pointing one way.
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Playaguess,
It might be better to think of it like this: There is no such thing as time! (Hate mail is sure to follow.)
"Time" is just a term we have cooked up to explain something. There really are only events, but mysteriously, all events in a vicinity and under a particular set of circumstances are always exquisitely synchronized by natural mechanisms.
All our measurements of the thing we call "time" simply compare events, and because we know that events are always locally synchronized, if we count one set of events (like the swings of a pendulum for example), because all local events are always synchronized, we immediately know what other events to expect during that count.
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thanks again for your varied responses.
simplified
I appreciate any acknowledgement of my ability to at least struggle with the concepts under discussion, but I have failed to comprehend the full meaning of your communication.
You mention two things I can’t do, but I don’t understand the implications of either statement.
It may be true that understanding relativity simplifies your thinking and will concede that perhaps the reason it hasn’t done the same for me is that I do not. That’s what prompted me to ask my question.
Given that my knowledge of the Russian language is confined to spasiba and dosvedonya, I consider my lack of comprehension to be entirely my fault since I can’t even attempt to communicate meaningfully in Russian.
Geezer
I would agree with you if the clocks were actually measuring “time”. I am postulating that they cannot actually do that, precisely because they are affected. As you suggest in your second response the “clock” only marks an interval within which we can expect certain things to happen locally.
Yor_on
I’m not trying to define time dilation, I’m trying to understand why the relativistic effects we observe seem to lead to the conclusion that time is not a constant.
I understand that the traveling twin’s experience of an event occurring within the traveling twin’s frame of reference while he/she is traveling is exactly the same as is the stationary twin’s experience of similar event within stationary twin‘s frame of reference.
As I understand the time dilation effect, the number of similar events (the ticking of a clock each second for example) that can occur within the traveling twin’s frame of reference is actually different, from both twin’s observations, than the number of events that can occur within the stationary twin’s frame of reference.
If the twin’s agree that the ticking of the clock measures time, then both twins will agree that the traveling twin experienced less time.
If we assume that the amount of “time” that passed between the traveling twin’s departure and return was actually the same for both twins then, it seems to me, the only conclusion to be drawn is that the physical processes operating within the traveling twin’s frame of reference developed at a different rate than those operating within the stationary twin’s frame of reference.
If, if fact, my lifespan is purely a matter of passing along the time axis within space/time from point a to point b then it would be true that I would die upon arrival at point b, regardless of how I arrived at that point. I don’t believe that my lifespan consists of starting at point a in time and arriving at point b. I be surprised if you did.
For discussion purposes, let’s assume that the cause of my death will be the a bomb some evil person has injected into my body. The bomb is programmed to explode after 1000 ticks of a clock. To delay my fate, I arrange to have myself subjected to the relativistic effect demonstrated with the story of the twins. I leave earth and return, during my trip the bomb controlling clock ticks 999 times. By the time I return, 1000 ticks of a clock on earth identical to the one controlling the bomb have passed. The bomb clock ticks one more time after I return and explodes. Did I extend my life by one tick? I think the answer is yes, coordinate b on the time axis was reached when the clock on earth click for the 1000s time. I died at coordinate b + tick.
With regards to the NIST Pair of Aluminum Atomic Clocks Reveal Einstein's Relativity at a Personal Scale. article. Basically what it says is: 1. that we have observed identical clocks under different relativistic conditions ticking at different rates. 2. Einstein predicted that time would pass at different rates under different relativistic conditions. 3. Ergo we have confirmation of Einstein’s theory.
It could have said. 1. that we have observed identical clocks under different relativistic conditions ticking at different rates. 2. playaguess predicted that the operation of a clock would be affected by relativistic effects. 3. Ergo we have confirmation of playaguess’s theory.
Within a scientific framework I “mustn’t accept” an assertion, the person making the assertion must demonstrate its accuracy. It is not my intention to dispute observations that have been made repeatedly. I will stipulate that every time we have attempted to measure the speed of light the results have been the same.
I’m trying to understand why we think we know what we “know”.
It seems to me that in a universe where time runs at different rates, we have no way to measure the speed of light, since we have no way of confirming how long the light was traveling. I must admit that I have my doubts that we “know” or understand enough about what is in the universe and how it is organized to be able to say with any certainty that we “know” how much distance the light traveled. What I think we can say about the speed of light is that based on our observations it “appears” as if light travels at a constant rate “c” regardless of the conditions under which we make our observations. I can also say that what I view on my computer screen “appears” to be a single continuous picture. What I think I “know” about my computer screen tells me that the “appearance” is a fabrication.
playaguess
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As you suggest in your second response the “clock” only marks an interval within which we can expect certain things to happen locally.
Not quite how I'd put it. It doesn't really mark an interval because it doesn't really measure "time".
All local events are synchronized (by "time" if you like). A clock only counts events, but because we know that all events are locally synchronized, we know the order of other events relative to our clock's event count, and apparently, it never fails.
BTW, you do know that this is being proved "all the time"? [:)]
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Why is it important to believe time is relative?
Einstein had the great insight that for c to be constant then time has to be relative. Some people could think c is relative and time is constant. As time is a means by which we measure change which option is correct is testable. An atomic clock measures the frequency of certain atomic processes like the frequency of a caesium atom for example. This frequency is constant and unchanging in its own local time frame. Gravity, Einstein proposed dilates time. If you take two identical atomic clocks one in the gravity well of the Earth and the other high above the Earth where the strength of gravity is weaker, the clock on Earth will run slower. This has been tested and confirmed numerous times. Time is relative, c (in a vacuum) is not. Or to put it another way. The speed of light in a vacuum is a constant precisely because time adjusts in exactly the correct manner to ensure this is always so.
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The speed of light in a vacuum is a constant precisely because time adjusts in exactly the correct manner to ensure this is always so.
Mike,
"Speed" is a measure of distance in time. There is nothing special about c. Light is synchronized, just like everything else. If light had some special property that made it immune to local synchronization, we would observe variations in the value of c.
EDIT:
There seems to be an idea that c defines some sort of universal constant that is a substitute for the, now outlawed, universal constant "time". Unfortunately, c has to obey the rules, just like anything else (as Woody Allen had it.)
I think this is because the concept of time as some sort of constant is so innate to our thinking that it's really difficult to shake off. It's not so easy to come up with a definition of "time" that does not explicitly, or implicitly, include time.
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Geezer
I don't understand what you are trying to say but presumably you are disagreeing with me?
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" Can particles go backward in time?
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In the old relativistic QM (e.g., in Volume 1 of Bjorken and Drell)
antiparticles are viewed as particles traveling backward in time.
This is based on a consideration of the solutions of the Dirac equation
and the idea of a filled sea of negative-energy solutions in which
antiparticles appear as holes (though this picture only works for
fermions since it requires an exclusion principle). One can go some way
with this view, but more sophisticated stuff requires the QFT picture
(as in Volume 2 of Bjorken and Drell and most modern treatments).
In relativistic QFT, all particles (and antiparticles) travel forward
in time, corresponding to timelike or lightlike momenta.
(Only 'virtual' particles may have unrestricted momenta; but these are
unobservable artifacts of perturbation theory.)
The need for antiparticles is in QFT instead revealed by the fact that
they are necessary to construct operators with causal (anti)commutation
relations, in connection with the spin-statistic theorem. See, e.g.,
Volume 1 of Weinberg's quantum field theory book.
Thus talking about particles traveling backward in time, the Dirac sea,
and holes as positrons is outdated; it is today more misleading
than it does good." By Arnold Neumaier.
And
The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of coordinates in uniform translatory motion.
Any ray of light moves in the “stationary” system of coordinates with determined velocity c, whether the ray be emitted by a stationary or by a moving body.
—Einstein, Ann. d. Physik 17 (1905); translated by Perrett and Jeffery; reprinted in: Einstein, Lorentz, Weyl, Minkowski, The Principle of Relativity, Dover 1952.
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And that's the definition of light as a 'constant'. So we have an 'arrow of time' pointing one way. Locally the 'arrow of time' is best described by using 'c' as our clock of choice, splitting that in arbitrarily small evenly made 'sizes', like seconds.
And 'c' and 'times arrow' both share the exact same properties. They will always give you an exact same beat locally, relative your heartbeats for example, as a rather weak description but illustrating the relation between those two, 'times arrow' and 'c'.
Any other description is a conceptual one, in where you compare 'frames of reference', including 'gravitational time dilation' which also gives us an idea of how small they (Frames of reference) could be said to be, out of my description. Namely being one Planck time in one Plank length, as that it is the smallest definition we have of lights 'propagation'. This is my own definitions of course, but I think they will hold.
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The definition of one Plank length in one Plank time should best be seen as a ultimate boundary though. Frames of reference as measured by your clock relative some other, has to be above that, as one Plank length in one Plank time can be seen as 'frozen' as I think of it. And we also have HUP to consider, which introduce indeterminacy at a greater scale. Although I'm not sure where HUP 'ends' and macroscopic effects take over I still expect the idea to be correct as a general description of 'clocks', 'times arrow', and 'c'.
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So we have an 'arrow of time' pointing one way. Locally the 'arrow of time' is best described by using 'c' as our clock of choice,
You can't use a speed (like c) as a clock. Speed is a rate of change in position with respect to time, so you'd be using time to try to measure time.
You can use the distance travelled by light as a clock, but it won't tell you anything you couldn't find out just as well using lots of other types of clock. It's still only going to tell you what to expect about other events in your vicinity.
Light has to obey the rules imposed by spacetime, just like everything else.
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I can use 'c'. Atomic clocks do :)
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The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
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It doesn't really matter what you use as long at it has a 'even' duration, and you have some way of measuring its 'ticks', you can use the sun, or water, or a spring ,or oscillations, or beats on a drum. It's all a question of how accurately you can 'split' it. Anything that gives you a even rate of 'events', as defined by your measurements relative other evenly repeating occurrences will do. And you can't split 'time' further that by using 'c' in mainstream definitions. You can probably use heartbeats to define it too, then finding that different animals will have different definitions for , let's call it a 'beat' but they will all relate to 'c'.
"One Planck time is the time it would take a photon travelling at the speed of light to cross a distance equal to one Planck length. Theoretically, this is the smallest time measurement that will ever be possible,[3] roughly 10−43 seconds. Within the framework of the laws of physics as we understand them today, for times less than one Planck time apart, we can neither measure nor detect any change. As of May 2010, the smallest time interval that was directly measured was on the order of 12 attoseconds (12 × 10−18 seconds),[4] about 1024 times larger than the Planck time."
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I can use 'c'. Atomic clocks do :)
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The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
No, they don't actually. They only use the amplitude of the radiated light to adjust an oscillator. It has nothing to do with c.
There is no more connection between time and the speed of light than there is between time and the speed of bicycles.
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Once again, my thanks for your contributions to my understanding, or failing that, your attempts to help.
Based on the responses so far, I gather the most direct answer to the question I stated as the header of my original post is this: As part of Einstein’s general theory of relativity he postulated that the speed of light is a constant and time is affected by relativistic effects. All of our observations confirm that clocks run at different rates when under the influence of different relativistic effects. This is taken as a demonstration that time is affected by relativistic effects. Additionally, further insights into the nature of space/time may be adversely impacted by rejecting the notion of relative time. I have to admit that I find this unsatisfying.
Two responses didn’t feel to me like answers to the question but were certainly thought provoking. One was imatfall’s question “if all physical processes are running slower then how can that be different ontologically from time being dilated?" I have a gut feel that there is an answer to that, but have not yet been able to formulate it. The other was geezer’s suggestion that perhaps the whole concept would be more easily understood if I could accept the notion that time doesn’t exist. A fascinating notion, but one that leads me down a path I’m not ready to travel yet, if I ever will be.
That being said, I like to change my direction a bit and see if I can help you folks guide me to something that will enlighten me.
At the most basic level, I think the phenomenon we call “speed" is defined as the distance something travels during a defined interval.
If I’m wrong about that, then all the rest of anything I can suggest or that you folks can say to me will be a waste of your time.
For the moment I’m going to assume that I am correct.
Measuring the speed of light over short distances, where we could be reasonably assured of coming to a mutual agreement to the distance traveled and the interval it took, without needing to worry greatly about complications like relativistic effects, appears to be a pretty straightforward exercise. At astronomical distances through volumes of space affected to differing degrees by relativistic effects, measuring the speed of light becomes more problematic, for me at least.
So I ask two new questions:
How do we know how far the light we are measuring the speed of has traveled?
How do we know how long an interval it took to cover the distance traveled?
Really basic stuff I know, but it seems critical to me to be able to assess whether my dissatisfaction with the answer to my original question is all in my head.
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When you say "interval" you could be automatically assuming that time is a universal constant. Time is not a universal constant. The thing we call "time" only ensures that all events are properly ordered.
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playaguess
I hate to say it but the basic response to the your quest to overcome your disatisfaction, is that you read carefully an introduction to Special Relativity (and it is Special you want not General!)
You need to get your head around invariance of the speed of light in inertial frames of reference, the fitzgerald-lorentx contraction, and the absence of simultaneity. It seems unlikely but one can (with only school level maths) comprehend the gist of SR and start to understand the simplicity and audacity of Einstein's vision.
You will not figure this out based on false assumptions, gut instinct and common sense - our minds have not evolved to intuit light speed, lack of absolute space, dilated time etc (give us a pointy stick to throw and we get back into the groove); so you need to use logic, maths, and most importantly the works of the scientists of the past.
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The speed of light in a vacuum is a constant precisely because time adjusts in exactly the correct manner to ensure this is always so.
Mike,
"Speed" is a measure of distance in time. There is nothing special about c. Light is synchronized, just like everything else. If light had some special property that made it immune to local synchronization, we would observe variations in the value of c.
EDIT:
There seems to be an idea that c defines some sort of universal constant that is a substitute for the, now outlawed, universal constant "time". Unfortunately, c has to obey the rules, just like anything else (as Woody Allen had it.)
I think this is because the concept of time as some sort of constant is so innate to our thinking that it's really difficult to shake off. It's not so easy to come up with a definition of "time" that does not explicitly, or implicitly, include time.
Not sure I agree with this Geezz. There is something special about c - no object, no information, nothing can travel through space or be transmitted faster than c. If c is not a limit then we either need to lose special relativity or we need to lose causality - and both of those have stood fairly unchallenged.
And I don't understand what you mean by "light is synchronized" - synchronized with/to what. The speed of light is a universal constant and the fact that it is finite and constant regardless of frame is fundamental to our physics.
When you say "interval" you could be automatically assuming that time is a universal constant. Time is not a universal constant. The thing we call "time" only ensures that all events are properly ordered.
Time is what we measure with clocks (it's a hackneyed definition but a good one) - and within the rest frame everyone measures it the same, ie within one's own frame it is constant. SR allows us to relate the passage of time within a different frame to that of ours.
I read your post as claiming that time has only an ordering qualitative nature - this is not my understanding. Our measuring of the passing of time is a quantitative act - and within any small local rest frame that quantitative process is entirely repeatable. the use of space time metrics to characterise the geometry requires that space and time dimensions can be both included in the same metric and treated as similar measurements along 4 orthogonal dimensions.
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again, thank you for your responses.
imatfall
i will do this at my earliest opportunity, which at this point will be monday, since i don't seem to find an actual copy of einstein's work posted on the internet, but instead many attempts to describe or explain it which i have decided do not meet with the spirit of your instruction. my apologies for misnaming the critical work involved, i knew it was the earlier publication but assigned the wrong general heading to that work.
i do find myself, perhaps due to a weakness of character or intelligence, from making one argumentative point with regards to your response. i would not be injured/offended if it was deemed unworthy of response. The point: in the various recountings of the circumstances leading up to the publication of einstein's 1905 that i have been exposed to, einstein is described as having had an insight or talomg a leap beyond our then current understanding with regards to the nature of time as relative, he then proceeded to work out the implications of that insight or leap. rightly or wrongly, that strikes me as though he came to an intuitive understanding of the nature of time. it may well be that einstein's innate intellectual capacity and his educational background prepared him to be able to gain the insight or make the leap and i may never be able to cross the conceptual distance einstein originally covered, but it does appear that someone's brain was wired by birth and experience to do so. i make no claim to having einstein's educational background or intellectual capacity. i'm not hoping to be able to turn the world of physics on its ear with some equally powerful insight, i'm merely attempting, by exposing myself to some of the same influences to gain the an equally intuitive understanding. i reject the notion that, in general, the human brain is not capable of it, although mine may not be.
geezer
yes indeed i could have been, but i was not. i was merely trying to establish if my understanding of the principle by which we describe one of the fundamental attributes ("speed") we assign to objects in nature is correct. i chose the word interval specifically because, according to the dictionary, while it may generally be construed to represent "time" it need not always do so, further, i was attempting to steer the discussion, at least temporarily, away from the nature of time, as it was leading to responses that were not satisfying my attempts to get my head around an idea. That could well be because i actually advanced the wrong question in the first place, but that was how the thought originally presented itself to me. I posed two new questions to see if i could generate responses that would help me gain satisfaction. Now that i look at the questions again, i realize that they are carelessly worded. i should perhaps have used "determine" rather than "know" in each. i also have decided that it may not be obvious, but my questions were intended to refer only to making those determinations at "astronomical" distances and are intended to gain me information about the tools and methodologies we use, since astronomical distances are complicated significantly by relativistic effects.
playaguess
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oops missed a glaring typo in one of my sentences: The point: in the various recountings of the circumstances leading up to the publication of einstein's 1905 that i have been exposed to, einstein is described as having had an insight or talomg a leap beyond our then current understanding with regards to the nature of time as relative, he then proceeded to work out the implications of that insight or leap.
should read
The point: in the various recountings of the circumstances leading up to the publication of einstein's 1905 that i have been exposed to, einstein is described as having had an insight or making a leap beyond our then current understanding with regards to the nature of time as relative, he then proceeded to work out the implications of that insight or leap.
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Time is a weird subject. Almost everything in Physics becomes weird when you take it far enough. Maybe that's why I enjoy it? You wake up a sunny morning well pleased with your world, then you open a physics book, debate, whatever :) and nothing is the same any more.
Let's see what we know of time. It has a direction for us, here and now, I will die some day.. If you don't agree to this one you're in trouble.. It might be a symmetry, or as Penrose suggested, an asymmetry, or just one way, meaning that it either communicates/commute in different 'directions', equally well or unequally, or that it doesn't communicate/commute at all. All of those questions are mathematical, coming from Einstein's field equations as I understands it, not experimental.
We have some very weird stuff, that I'm hung up on, existing. Entanglements, Tunneling's, Although personally I think Entanglement is the weirdest idea I know. But it's there, for real, no joke.
But time has a direction, so how do we measure that direction? It doesn't have any tags on it telling us when one 'swamba dimba' is gone, does it? We have to find our own ways of sorting it into 'quantities'. And physics is weird.
In physics you have Planck time, and Planck size and they are related to a ' smallest propagation of 'c' ' But why?
"The Planck time is the unique combination of the gravitational constant G, the relativity constant c, and the quantum constant h, to produce a constant with units of time. For processes that occur in a time t less than one Planck time, the dimensionless quantity tP / t is greater than one. Dimensional analysis suggests that the effects of both quantum mechanics and gravity will be important under these circumstances, requiring a theory of quantum gravity. All scientific experiments and human experiences happen over billions of billions of billions of Planck times, making any events happening at the Planck scale hard to detect."
And
In 1898, Max Planck discovered that action is quantized, and published the result in a paper presented to the Prussian Academy of Sciences in May 1899.[19][20] At the end of the paper, Planck introduced, as a consequence of his discovery, the base units later named in his honor. The Planck units are based on the quantum of action, now usually known as Planck's constant. Planck called the constant b in his paper, though h is now common. Planck underlined the universality of the new unit system, writing:
...ihre Bedeutung für alle Zeiten und für alle, auch außerirdische und außermenschliche Kulturen notwendig behalten und welche daher als »natürliche Maßeinheiten« bezeichnet werden können... ...These necessarily retain their meaning for all times and for all civilizations, even extraterrestrial and non-human ones, and can therefore be designated as "natural units"...
Planck's paper also gave numerical values for the base units that were close to modern values."
And it's to me a 'constant'. Constants are the closed doors you meet in Physics. There might be a key to open each one, but then you'll have to find a new way to walk up to it. They do not apologise, and they do not care that 'common sense' demands a answer for why they are there. They just are the 'rules of the game'.
So? We have 'two' constants here 'c' and the Plank units. And times measurement is a function of your choice of 'clock' measuring them. It's quite natural to combine them. The problem is that Relativity discuss a lot of other definitions for how time comes to be, relative the observer, and that catch peoples interest. Especially those of you mathematically inclined. And the idea of a time symmetry makes people forget what they actually see, that we all die.
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There is something special about c - no object, no information, nothing can travel through space or be transmitted faster than c. If c is not a limit then we either need to lose special relativity or we need to lose causality - and both of those have stood fairly unchallenged.
And I don't understand what you mean by "light is synchronized" - synchronized with/to what. The speed of light is a universal constant and the fact that it is finite and constant regardless of frame is fundamental to our physics.
Matt - I completely agree that c is a speed limit imposed by spacetime. Now, nothing up my sleeves at all here, but notice that it's a "speed". It's a distance in time. It's always constant of course, but that just means that when we observe it, it always covers the same distance in a particular amount of time so we can use it to determine distances very accurately, or, if we know a very precise distance we can use light to tell time.
But that's not really any big deal - lots of other things are constant with respect to time too, so while c is a very special thing, there is nothing special about the relationship between light speed and time. The thing that is special is the fact that the speed is always the same.
Re. the synchronization thing - spacetime enforces rigid synchronization of all events including the distance light travels between two points! We can measure with finer and finer granularity and the events never seem to get out of order, until we get to a scale where they seem to have no order at all [:D]
The point is that when we do anything with time, we are always looking at it in terms of events, so all we can say with certainty is that, above certain scales, spacetime synchronizes events so that the relative ratios of events are always the same for a given set of conditions.
If we use anything other than events and drag time itself into the measurement, it all goes pear-shaped quickly. However, we are so conditioned to think of time as some sort of constant that it's really difficult to avoid the trap.
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The idea is simple Geezer, and it works. That's why I like it.
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/snipped
Matt - I completely agree that c is a speed limit imposed by spacetime. Now, nothing up my sleeves at all here, but notice that it's a "speed". It's a distance in time. It's always constant of course, but that just means that when we observe it, it always covers the same distance in a particular amount of time so we can use it to determine distances very accurately, or, if we know a very precise distance we can use light to tell time.
It is a speed. It is the fact that is is constant from all frames that is different
But that's not really any big deal - lots of other things are constant with respect to time too, so while c is a very special thing, there is nothing special about the relationship between light speed and time. The thing that is special is the fact that the speed is always the same.
But very few are axiomatic to current thinking in physics
Re. the synchronization thing - spacetime enforces rigid synchronization of all events including the distance light travels between two points! We can measure with finer and finer granularity and the events never seem to get out of order, until we get to a scale where they seem to have no order at all [:D]
ok - what I would call causality
The point is that when we do anything with time, we are always looking at it in terms of events, so all we can say with certainty is that, above certain scales, spacetime synchronizes events so that the relative ratios of events are always the same for a given set of conditions.
If we use anything other than events and drag time itself into the measurement, it all goes pear-shaped quickly. However, we are so conditioned to think of time as some sort of constant that it's really difficult to avoid the trap.
But that applies to all measurement - there are no absolute units of experimentation. we may find it easier to visualize abstract length dimensions - but when we conceive of a cubic metre of empty space we are in effect bounding it by a frame of metre sticks, and we struggle to do that with time; but the measurements and movement along the dimension are the same.
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again, thank you for your responses.
imatfall
i will do this at my earliest opportunity, which at this point will be monday, since i don't seem to find an actual copy of einstein's work posted on the internet, but instead many attempts to describe or explain it which i have decided do not meet with the spirit of your instruction. my apologies for misnaming the critical work involved, i knew it was the earlier publication but assigned the wrong general heading to that work.
Knock yourself out - it's a doozy. (http://www.gutenberg.org/dirs/etext04/relat10.txt) You will come to a point where you lose the plot. But the basics of SR are fairly accessible. Wikipedia and Hyperphysics and the Physics FAQ are all excellent resources.
i do find myself, perhaps due to a weakness of character or intelligence, from making one argumentative point with regards to your response. i would not be injured/offended if it was deemed unworthy of response. The point: in the various recountings of the circumstances leading up to the publication of einstein's 1905 that i have been exposed to, einstein is described as having had an insight or talomg a leap beyond our then current understanding with regards to the nature of time as relative, he then proceeded to work out the implications of that insight or leap. rightly or wrongly, that strikes me as though he came to an intuitive understanding of the nature of time. it may well be that einstein's innate intellectual capacity and his educational background prepared him to be able to gain the insight or make the leap and i may never be able to cross the conceptual distance einstein originally covered, but it does appear that someone's brain was wired by birth and experience to do so. i make no claim to having einstein's educational background or intellectual capacity. i'm not hoping to be able to turn the world of physics on its ear with some equally powerful insight, i'm merely attempting, by exposing myself to some of the same influences to gain the an equally intuitive understanding. i reject the notion that, in general, the human brain is not capable of it, although mine may not be.
/snipped
playaguess
But Playaguess - you are not giving yourself even the chance that Einstein had. We are constantly regaled by tales of Einstein the humble patent clerk, the poor mathematician, out of the academic milieu, a maverick who plucked his ideas from the depths of imagination and insight; this is a romanticisation - Einstein was well versed in maths, had a physics/maths degree (and was working on his phd), and had a support network of friends who were good scientists at the point that he started publishing his great work.
You need the basics - there is ground work to be done in all pursuits; to comprehend one of the greatest leap of imaginations in the history of our species (my opinion) you need to be up to speed, although def not superluminal. I have to say that, on this and other fora I see an argument against special relativity about once a month - and not one of them is based on anything other than a argument of personal incredulity. One point is that SR is mathematically internally consistent, completely consistent with experimental results, and has great predictive power; this means that thought experiments cannot overturn SR - a thought experiment that clashes with SR is wrongly founded, the only thing that will overturn SR is experimental results (this is why the preliminary results of OPERA/Gran Sasso are so interesting).
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Relativity is filter of guesses. [:P]
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As almost everything worthwhile :)
Like guessing your girlfriends intentions and mood.
Dangerous, but worth your while, if lucky.
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It is a speed. It is the fact that is is constant from all frames that is different
Is that any different from "it's always constant"?
But very few are axiomatic to current thinking in physics
Apparently some might disagree. The SI unit of time makes no mention of the speed of light. 'c' is defined by atomic activity.
But that applies to all measurement - there are no absolute units of experimentation. we may find it easier to visualize abstract length dimensions - but when we conceive of a cubic metre of empty space we are in effect bounding it by a frame of metre sticks, and we struggle to do that with time; but the measurements and movement along the dimension are the same.
But frames are an artifact. There are no real boundaries. And time may be an artifact too. All we know is that spacetime synchronizes events "locally" because, AFAIK, that is all we have been able to confirm by experiment.
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It is a speed. It is the fact that is is constant from all frames that is different
Is that any different from "it's always constant"?
Yes - try measuring the electronic transition between the two hyperfine states of caesium-133 as you zoom past the clock in a space ship. now try measuring the speed of light emitted by the leds on the top of the atomic clock
But very few are axiomatic to current thinking in physics
Apparently some might disagree. The SI unit of time makes no mention of the speed of light. 'c' is defined by atomic activity.
Units - even SI - are just book-keeping and accountancy. The speed of light is emergent from maxwell's equations and is axiomatic to einstein's relativity - whatever units you choose to work in.
But that applies to all measurement - there are no absolute units of experimentation. we may find it easier to visualize abstract length dimensions - but when we conceive of a cubic metre of empty space we are in effect bounding it by a frame of metre sticks, and we struggle to do that with time; but the measurements and movement along the dimension are the same.
But frames are an artifact. There are no real boundaries. And time may be an artifact too. All we know is that spacetime synchronizes events "locally" because, AFAIK, that is all we have been able to confirm by experiment.
I am not saying that time isn't an artefact - but in current physics it is no less an artefact than length
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It is a speed. It is the fact that is is constant from all frames that is different
Is that any different from "it's always constant"?
Yes - try measuring the electronic transition between the two hyperfine states of caesium-133 as you zoom past the clock in a space ship. now try measuring the speed of light emitted by the leds on the top of the atomic clock
But very few are axiomatic to current thinking in physics
Apparently some might disagree. The SI unit of time makes no mention of the speed of light. 'c' is defined by atomic activity.
Units - even SI - are just book-keeping and accountancy. The speed of light is emergent from maxwell's equations and is axiomatic to einstein's relativity - whatever units you choose to work in.
But that applies to all measurement - there are no absolute units of experimentation. we may find it easier to visualize abstract length dimensions - but when we conceive of a cubic metre of empty space we are in effect bounding it by a frame of metre sticks, and we struggle to do that with time; but the measurements and movement along the dimension are the same.
But frames are an artifact. There are no real boundaries. And time may be an artifact too. All we know is that spacetime synchronizes events "locally" because, AFAIK, that is all we have been able to confirm by experiment.
I am not saying that time isn't an artefact - but in current physics it is no less an artefact than length
Matt,
My point was that we don't "measure" time. What we do is count events, 'cos that's all clocks can do. Spacetime ensures that the ratios of the counts are always consistent. It's just another way of describing something about spacetime without having to use the term "time" itself (because that can lead to circular arguments.)
My other point is that the speed of light does not define time because the speed of light is controlled by spacetime, just like it controls everything else. (Not to say that we could not define time based on 'c' if we really wanted to, but it's not a very good method because of the uncertaintity of distance.)
While those points may not exactly conform with the axiomatic "conventional wisdom", is there anything wrong with either of them?
(BTW, the official TNS spelling is "artifact" [;D])
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Yes - try measuring the electronic transition between the two hyperfine states of caesium-133 as you zoom past the clock in a space ship.
I hope you realize that's not what atomic clocks actually do? There is no "measuring the electronic transition".
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You wanna try and phrase it in 4 words or under that is any better. [8D]
You set up a feedback loop so that a electronic oscillator becomes synchronized with the transitions between states of caesium. That's pretty much measurement - if in an indirect way; you can call it secondary observation if you are in a particularly hair-splitty mood . You then count these oscillations and every 9 million or so click off another second on your clock.
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Geez, playaguess, you picked a nice, easy one for one of your first few posts! Of course, if the CERN observations about the neutrinos travelling faster than light check out as true, Einstein could be wrong and at least a part of what we thought we knew may have to be reconsidered.
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Why is it important to believe time is relative?
Physics is not religion. You don't have to "believe". If you want to find why and when physicists use SR, you can study it, and you'll discover yourself.
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You wanna try and phrase it in 4 words or under that is any better. [8D]
You set up a feedback loop so that a electronic oscillator becomes synchronized with the transitions between states of caesium. That's pretty much measurement - if in an indirect way; you can call it secondary observation if you are in a particularly hair-splitty mood . You then count these oscillations and every 9 million or so click off another second on your clock.
I see. And what, pray tell, are the units of said "measurement"?
(As you are about to discover, this isn't anything like as hair-splitty as you seem to imagine [;D])
PS - I intentionally left out the bit about "there is no point in being precise if ........"
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Its dimensionless. And it is hair-splitty - if we are to pounce on every elision of meaning we would be here all day. So you say I am about to discover - enlighten me and stop being gnomic
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Why is it important to believe time is relative?
Physics is not religion. You don't have to "believe". If you want to find why and when physicists use SR, you can study it, and you'll discover yourself.
Yes, in some cases relativity can be wrong,but it is useful in another cases.
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Why is it important to believe time is relative?
Physics is not religion. You don't have to "believe". If you want to find why and when physicists use SR, you can study it, and you'll discover yourself.
Yes, in some cases relativity can be wrong,but it is useful in another cases.
I'm probably going to regret this, but in what cases is it wrong?
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Yes, in some cases relativity can be wrong,but it is useful in another cases.
Another example: 2 + 2 = 4 in some cases can be wrong, but it is useful in other cases.
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Yes, in some cases relativity can be wrong,but it is useful in another cases.
I'm probably going to regret this, but in what cases is it wrong?
I assume he intended it's not valid...outside its range of validity [:)]
Near big massive bodies, e.g.
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Its dimensionless. And it is hair-splitty - if we are to pounce on every elision of meaning we would be here all day. So you say I am about to discover - enlighten me and stop being gnomic
Precisely! What we are actually doing is maintaining (or trying to maintain) a ratio (in this case 1:1) of event counts, which was my point about four posts ago.
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Times arrow exist. What doesn't exist is the way we define it. That's a free decision for us in where we 'split' that arrow we observe into even durations. If I was to argue that it doesn't exist at the same time :) I know that every 'second' moves me one step closer to my death becomes rather meaningless.
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Playaguess. "So I ask two new questions:
How do we know how far the light we are measuring the speed of has traveled?
How do we know how long an interval it took to cover the distance traveled?"
I think you are asking about if all radiation is a constant 'c'? Because it's from that you will get time dilations and Lorentz contractions, as well as 'frames of references' and Lorentz transformations.
That one is validated experimentally, over and over again. We define 'c' as '299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time. In imperial units this speed is approximately 186,282 miles per second.'
The metre is our own idea, seconds, hours etc is also our own definitions. But that 'c' is a constant is not our idea, it's a experimental fact. There's been a lot of attempts trying to find loopholes there but as far as I've seen. Nobody has succeeded so far (and that's in a hundred year-span, or more, if you use Maxwell as your first definition of it.). That you can define some 'stuff' as going faster than 'c' depends on what medium it 'propagate' in. 'c' is defined in a vacuum. If we changed our definitions of metres and 'seconds', 'c' could get a new value, but it would still be a invariant constant. And that's the real beauty of it, and why I connect it to 'clocks', as most of physics do, even if they don't define it full out as me. Why I do it is because I do expect it to be a constant, and nothing else.
To me you can just as easily use it as a 'clock', which also will fit so much better with it 'not existing', except as the recoil seen in a source as it 'leaves', and in its subsequent annihilation at a 'sink', like your eye. It was never a ball 'moving', and that's the reason we use 'propagate' instead of 'motion' too.
What is the experimental basis of Special Relativity. (http://www.desy.de/user/projects/Physics/Relativity/SR/experiments.html) is rather cool to go through to see where the experiments are.
There you also can see.
"The Clock Hypothesis
The clock hypothesis states that the tick rate of a clock when measured in an inertial frame depends only upon its velocity relative to that frame, and is independent of its acceleration or higher derivatives. The experiment of Bailey et al. referenced above stored muons in a magnetic storage ring and measured their lifetime. While being stored in the ring they were subject to a proper acceleration of approximately 1018 g (1 g = 9.8 m/s2). The observed agreement between the lifetime of the stored muons with that of muons with the same energy moving inertially confirms the clock hypothesis for accelerations of that magnitude.
Sherwin, “Some Recent Experimental Tests of the 'Clock Paradox'”, Phys. Rev. 129 no. 1 (1960), pg 17.
He discusses some Mössbauer experiments that show that the rate of a clock is independent of acceleration (~1016 g) and depends only upon velocity."
Notice that it refer to 'relative motion' as the decisive action defining a time dilation and Lorentz contraction, not accelerations per se. Although you surely can argue that 'accelerations' can't be ignored as all 'motion' need something creating it, that one belongs more to philosophy than physics, which limits itself to observing and testing what we actually can see (and measure), and what hypothesises we can create explaining it, as well as test. Mach's empty universe inspired a lot of Einstein's thoughts about what 'relative motion' might be, but in the end he limited his descriptions to what we actually can measure.