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Does length contraction means the contraction of space time?
No. It cannot be that, because we all occupy the same spacetime yet we don't all observe the same length contraction.Does length contraction means the contraction of space time?
Looking at the twin paradox, the effect of the twin in acceleration is there that the twin on the spaceship does experience a time dilation. Right, I'll take some time to review thisNo. It cannot be that, because we all occupy the same spacetime yet we don't all observe the same length contraction.
Length contraction is a natural result of the relativity of simultaneity. The length of an object is the distance between where its ends are at the same time, so when things that are at the same time for one observer are not at the same time for another observer, they will find different lengths.
No, NOBODY ever "experiences" time dilation, It is something you see in objects that are moving relative to you but they see YOU as time dilated at the same time you see them as time dilated.Looking at the twin paradox, the effect of the twin in acceleration is there that the twin on the spaceship does experience a time dilation. Right, I'll take some time to review this
The twin paradox is an example of differential ageing, a different phenomenon than length contraction and time dilation. One way of seeing that it is different from time dilation is to consider teh time dilation that is present in the twin paradix: at every point in the journey, the travelling twin is at rest relative to himself while the stay-at-home twin is moving; therefore the stay-at-home clock is the time-dilated one as far as the traveller is concerned. However, traveller is still able to correctly calculate that he will age less than stay-at-home - even though stay-at-home's clock is dilated throughout the journey.Looking at the twin paradox, the effect of the twin in acceleration is there that the twin on the spaceship does experience a time dilation. Right, I'll take some time to review this
Hmm, the result of the twin paradox is that the person moving away from earth experience a slower time when traveling close to the speed of light. Therefore when the twin on earth ages 10 years, the twin traveling on the spaceship only ages 6 years by traveling toward a fraction of the speed of light. The same thing happens with gravitation time dilation as the person closer to earth experiences a slower time passing by then the one away from earth by a few nanosecond.The twin paradox is an example of differential ageing, a different phenomenon than length contraction and time dilation. One way of seeing that it is different from time dilation is to consider teh time dilation that is present in the twin paradix: at every point in the journey, the travelling twin is at rest relative to himself while the stay-at-home twin is moving; therefore the stay-at-home clock is the time-dilated one as far as the traveller is concerned. However, traveller is still able to correctly calculate that he will age less than stay-at-home - even though stay-at-home's clock is dilated throughout the journey.
No, the person on the spaceship does NOT experience slower time. He/she experiences time passing at one second per second just as does the stay-at-home. What happens is that the person on the spaceship takes a different path through space-time and therefore experiences fewer ticks of his one-second-per-second clock than does the stay-at-home.Hmm, the result of the twin paradox is that the person moving away from earth experience a slower time when traveling close to the speed of light. Therefore when the twin on earth ages 10 years, the twin traveling on the spaceship only ages 6 years by traveling toward a fraction of the speed of light. The same thing happens with gravitation time dilation as the person closer to earth experiences a slower time passing by then the one away from earth by a few nanosecond.
Well yes neither of the twins would experience a slower time, it feels shorter for one 6 years, and longer for the other one 10 years. I am thinking that speed causes space time to contract through length contraction matches the fact that the person on earth with a more compact space time has a time dilation. If gravity increases with a more compact space time the result would be more prominentNo, the person on the spaceship does NOT experience slower time. He/she experiences time passing at one second per second just as does the stay-at-home. What happens is that the person on the spaceship takes a different path through space-time and therefore experiences fewer ticks of his one-second-per-second clock than does the stay-at-home.
EDIT: and by the way, this is one of the most confusing things when you first start to look into special relativity so you're in good company not getting it right away.
This sentence makes it seem as though one person can travel closer to the speed of light than another. Instead there is relative motion between two people. Either one could claim that the other is the one moving closer to the speed of light, or just the opposite. Either one could claim that he is the one moving closer to the speed of light. It's a meaningless assertion either way because as you chase after a light beam you find you make no more progress in catching it than does the other person.Hmm, the result of the twin paradox is that the person moving away from earth experience a slower time when traveling close to the speed of light.
In terms of velocity and gravity:Well what Wikipedia says on time dilation is that "The laws of nature are such that time itself (i.e. spacetime) will bend due to differences in either gravity or velocity – each of which affects time in different ways." I'm still skeptical about how velocity bend time, I'm not sure if it's mass related
The sentence is just plain flat-out wrong.Well what Wikipedia says on time dilation is that "The laws of nature are such that time itself (i.e. spacetime) will bend due to differences in either gravity or velocity
I thought it was both RoS and time dilation? What do you call it when you travel to Alpha Centuari at 90% of the speed of light and measure the distance traveled to be 2 LY (if I did that math right...).Length contraction is a natural result of the relativity of simultaneity. The length of an object is the distance between where its ends are at the same time, so when things that are at the same time for one observer are not at the same time for another observer, they will find different lengths.
It's hard to separate the two - but I can say that the distance between earth and Alpha Centauri is pretty much by definition the distance between where the earth is right now and where Alpha Centauri is right now. That definition works whether I'm at rest relative to them or not, and yields the appropriately contracted length if I am not.I thought it was both RoS and time dilation?
My usual example is how do you measure the length of a beetle? You just stand it on a ruler and read off the position of its head and the position of its tail. If the beetle is walking, though, that procedure will not get you its length if you don't make the measurements at the same time. In that example, failing to measure simultaneously could just be sloppy experimentalism. But the relativity of simultaneity means that there is genuine, unresolvable, disagreement over what constitutes "at the same time" in different frames (and the beetle is moving in at least one of them), and that's where length contraction comes from. Observers at rest in the two frames use the same procedure to measure length, but because they disagree about simultaneity they get different lengths.I thought it was both RoS and time dilation? What do you call it when you travel to Alpha Centuari at 90% of the speed of light and measure the distance traveled to be 2 LY (if I did that math right...).
Isn't "an absolute rest frame" by definition the measurement of "proper length" with no distortion of observation via movement relative to the object (or distance) measured? So then the distance to Alpha Centauri or the length of a beetle is not changed by "how you look at" either in relative motion. How is the above wrong?My usual example is how do you measure the length of a beetle? You just stand it on a ruler and read off the position of its head and the position of its tail. If the beetle is walking, though, that procedure will not get you its length if you don't make the measurements at the same time. In that example, failing to measure simultaneously could just be sloppy experimentalism. But the relativity of simultaneity means that there is genuine, unresolvable, disagreement over what constitutes "at the same time" in different frames (and the beetle is moving in at least one of them), and that's where length contraction comes from. Observers at rest in the two frames use the same procedure to measure length, but because they disagree about simultaneity they get different lengths.
Time dilation isn't directly relevant to this, although you can't build a symmetric picture of the world without invoking ot as well - you end up with an absolute rest frame.
"absolute rest frame" is an ambiguous term. Do you simply mean a frame of reference in which is an object is at rest? If so, then leave out the "absolute" since it adds nothing but confusion. If on the other hand, you mean an absolute frame of reference against which anything's motion can be measured, then that is an incorrect concept as there is no such thing.Isn't "an absolute rest frame" by definition the measurement of "proper length" with no distortion of observation via movement relative to the object (or distance) measured? So then the distance to Alpha Centauri or the length of a beetle is not changed by "how you look at" either in relative motion. How is the above wrong?
That's not an "absolute" rest frame, it's the rest frame of the beetle and the ruler, which is to say a frame in which the beetle and the ruler are not moving. That's a very convenient frame to use if the beetle and the ruler are sitting on a lab bench and the lab bench is bolted to the same concrete floor upon which I am standing... But I would find it very difficult to convince an observer watching with a telescope from Mars that the beetle, ruler, lab bench, and concrete floor were not moving - they're attached to the earth, which is going around the sun at a very different speed than Mars.Isn't "an absolute rest frame" by definition the measurement of "proper length" with no distortion of observation via movement relative to the object (or distance) measured? So then the distance to Alpha Centauri or the length of a beetle is not changed by "how you look at" either in relative motion. How is the above wrong?
The phrase, "absolute rest frame" quotes Ibix above. Of course all motion is "relative to what?," so there is no absolute frame of reference for velocity. But "proper length" always refers to length as measured from at rest with the object or distance in question. That leaves the question, "Is length variable with how you measure it or do "things" and distances have objective lengths independent of "how you look at them." (Objective vs subjective, the latter meaning frame dependent.) Measurement does not change lengths or distances. They exist objectively prior to varieties of frames of reference from which they are measured. Something to consider besides repeating the rules of orthodox special relativity/ subjectivity."absolute rest frame" is an ambiguous term. Do you simply mean a frame of reference in which is an object is at rest? If so, then leave out the "absolute" since it adds nothing but confusion. If on the other hand, you mean an absolute frame of reference against which anything's motion can be measured, then that is an incorrect concept as there is no such thing.
PROPER length has an absolute value but measured/calculated length is frame dependent so yes, length is variable depending on how you measure it.The phrase, "absolute rest frame" quotes Ibix above. Of course all motion is "relative to what?," so there is no absolute frame of reference for velocity. But "proper length" always refers to length as measured from at rest with the object or distance in question. That leaves the question, "Is length variable with how you measure it or do "things" and distances have objective lengths independent of "how you look at them." (Objective vs subjective, the latter meaning frame dependent.) Measurement does not change lengths or distances. They exist objectively prior to varieties of frames of reference from which they are measured. Something to consider besides repeating the rules of orthodox special relativity/ subjectivity.
and you misunderstand, apparently, the way in which he used it, which was to say somewhat indirectly what I said specifically in post #20 which is that there IS no such thing as an absolute frame of reference.The phrase, "absolute rest frame" quotes Ibix above
You are correct that there is one frame in which the beetle is at rest, and all observers agree what that frame is and what length the beetle is in that frame. As Nugatory and phinds have said, though, that is only a special frame for that one beetle, not for beetles in general nor for the laws of physics.Isn't "an absolute rest frame" by definition the measurement of "proper length" with no distortion of observation via movement relative to the object (or distance) measured? So then the distance to Alpha Centauri or the length of a beetle is not changed by "how you look at" either in relative motion. How is the above wrong?