Relativity?

How does science work? And what's all this about quantum mechanics?

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davidm
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Re: Relativity?

Post by davidm » Mon Nov 13, 2017 5:33 pm

uwot wrote:
Mon Nov 13, 2017 9:17 am
Well ken, the bits that everyone repeats are the historical claims made by Einstein, the thought experiment he proposed to demonstrate his claims, and the experimental results which show conclusively that what Einstein claimed would happen, actually happens.
But that seems to be as far as you got, because had you read the bit where it states that what is true of photons in a light clock, is true of photons involved in every exchange of electromagnetic energy on the carriage, you would not have written this:
ken wrote:
Mon Nov 13, 2017 4:47 am
A digital clock that runs off batteries will tick away at the same rate, when it is traveling at any speed, as a clock on earth does, because it was created to function that way.
Exactly! Isn't it simply amazing that Ken cannot or will not get this bloody obvious point? ALL CLOCKS IN THE MOVING FRAME WILL SLOW, FOR THE EXACT SAME REASON THAT ALL PHYSICAL PROCESSES WILL SLOW -- the invariance of the speed of light!

OuterLimits
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Re: Relativity?

Post by OuterLimits » Mon Nov 13, 2017 5:53 pm

davidm wrote:
Mon Nov 13, 2017 5:33 pm
uwot wrote:
Mon Nov 13, 2017 9:17 am
Well ken, the bits that everyone repeats are the historical claims made by Einstein, the thought experiment he proposed to demonstrate his claims, and the experimental results which show conclusively that what Einstein claimed would happen, actually happens.
But that seems to be as far as you got, because had you read the bit where it states that what is true of photons in a light clock, is true of photons involved in every exchange of electromagnetic energy on the carriage, you would not have written this:
ken wrote:
Mon Nov 13, 2017 4:47 am
A digital clock that runs off batteries will tick away at the same rate, when it is traveling at any speed, as a clock on earth does, because it was created to function that way.
Exactly! Isn't it simply amazing that Ken cannot or will not get this bloody obvious point? ALL CLOCKS IN THE MOVING FRAME WILL SLOW, FOR THE EXACT SAME REASON THAT ALL PHYSICAL PROCESSES WILL SLOW -- the invariance of the speed of light!
In a scenario in which nobody changes inertial frames, every single person is justified in claiming they are not a traveler, and that nothing in their IF ever "slows down".

davidm
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Re: Relativity?

Post by davidm » Mon Nov 13, 2017 6:33 pm

OuterLimits wrote:
Mon Nov 13, 2017 5:53 pm
davidm wrote:
Mon Nov 13, 2017 5:33 pm
uwot wrote:
Mon Nov 13, 2017 9:17 am
Well ken, the bits that everyone repeats are the historical claims made by Einstein, the thought experiment he proposed to demonstrate his claims, and the experimental results which show conclusively that what Einstein claimed would happen, actually happens.
But that seems to be as far as you got, because had you read the bit where it states that what is true of photons in a light clock, is true of photons involved in every exchange of electromagnetic energy on the carriage, you would not have written this:
Exactly! Isn't it simply amazing that Ken cannot or will not get this bloody obvious point? ALL CLOCKS IN THE MOVING FRAME WILL SLOW, FOR THE EXACT SAME REASON THAT ALL PHYSICAL PROCESSES WILL SLOW -- the invariance of the speed of light!
In a scenario in which nobody changes inertial frames, every single person is justified in claiming they are not a traveler, and that nothing in their IF ever "slows down".
Right -- of course I've actually said this several times, and so has uwot. They key is that for the travelers to actually compare their clocks, they must meet again in the same frame -- and that will require one or both breaking their IF and entering an accelerated frame, in which the symmetry of the situation is broken. This has been explained by me and uwot and is standard stuff.

OuterLimits
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Re: Relativity?

Post by OuterLimits » Mon Nov 13, 2017 6:50 pm

davidm wrote:
Mon Nov 13, 2017 6:33 pm
Right -- of course I've actually said this several times, and so has uwot. They key is that for the travelers to actually compare their clocks, they must meet again in the same frame -- and that will require one or both breaking their IF and entering an accelerated frame, in which the symmetry of the situation is broken. This has been explained by me and uwot and is standard stuff.
I just think that if one emphasizes that the two will find that one of the clocks has ticked less - rather than phrasing it that one's clock is slowed per se - that it may remove some confusion. To say that some other clock is ticking more slowly, one must have some idea of shared simultaneity, I think.

In GR, when somebody is near strong gravity, the parties can see that and agree - the one is definitely ticking slowly and the other one faster.

davidm
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Re: Relativity?

Post by davidm » Mon Nov 13, 2017 7:29 pm

OuterLimits wrote:
Mon Nov 13, 2017 6:50 pm
davidm wrote:
Mon Nov 13, 2017 6:33 pm
Right -- of course I've actually said this several times, and so has uwot. They key is that for the travelers to actually compare their clocks, they must meet again in the same frame -- and that will require one or both breaking their IF and entering an accelerated frame, in which the symmetry of the situation is broken. This has been explained by me and uwot and is standard stuff.
I just think that if one emphasizes that the two will find that one of the clocks has ticked less - rather than phrasing it that one's clock is slowed per se - that it may remove some confusion.
Well if you consider what I wrote in the post to which you replied in the context of everything else I've written, it should be clear that I'm not saying one clock ticked slower "per se." I mentioned earlier that if two observers in constant uniform motion pass each other like ships in the night, each will say that the other's clock is ticking slower; but unless they meet up again to compare clocks, the question of whose clock *REALLY* is ticking slower is meaningless.
To say that some other clock is ticking more slowly, one must have some idea of shared simultaneity, I think.
Exactly. To do that, the separated observers will have to re-enter the same frame and compare clocks, as I said.

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Noax
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Re: Relativity?

Post by Noax » Mon Nov 13, 2017 8:03 pm

uwot wrote:
Mon Nov 13, 2017 8:04 am
Noax wrote:
Sun Nov 12, 2017 2:03 pm
If the screen (and it's projector) is coming towards me at .05c, the movie will appear to run nearly 5% fast due to blue shift, and 5% slow as it recedes.
Ah. That's not how I understand the set up. True, the train is crossing your field of vision at xc, but that is at a fixed distance.
The post to which I replied didn't really specify fixed-distance.
The example is at pains to make this distance as small as possible. The point I was making is that as long as it is fixed, the distance is irrelevant.
If the distance is fixed, then yes, the movie will run at pace. My example was a different setup, and had the distance changing, as if a drive-in movie was being observed by the train passenger as he passes by.

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Noax
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Re: Relativity?

Post by Noax » Tue Nov 14, 2017 2:00 am

davidm wrote:
Mon Nov 13, 2017 6:33 pm
They key is that for the travelers to actually compare their clocks, they must meet again in the same frame -- and that will require one or both breaking their IF and entering an accelerated frame, in which the symmetry of the situation is broken.
davidm wrote:
Mon Nov 13, 2017 7:29 pm
To do that, the separated observers will have to re-enter the same frame and compare clocks, as I said.
Not to take anybody eles's side, especially ken who has no intention of getting a clue, but relativity is not about observers, it is about geometry and events. Clocks can be compared when in each other's presence and thus the comparison is one event. No matching of frames/velocities is required, and no observers are required. Clocks with matching velocities but spatially separated still cannot be compared without arbitrary frame assignment.

I can illustrate the twin paradox with no observers, no acceleration or need for non-inertial frames. Just clocks in freefall, no two of which ever are stationary relative to another.

As for a bit of terminology, a thing does not enter a frame. All (reasonably local) things are in all frames, but not necessarily stationary in it. Clocks need not match velocity to be compared. They just have to be in each other's presence.

uwot
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Re: Relativity?

Post by uwot » Wed Nov 15, 2017 9:09 am

Noax wrote:
Tue Nov 14, 2017 2:00 am
...relativity is not about observers, it is about geometry and events.
Well, the emphasis of this thread has been special relativity; one of the postulates of which is that the speed of light, in a vacuum, will be measured as the same by all observers, regardless of their motion.
Noax wrote:
Tue Nov 14, 2017 2:00 am
As for a bit of terminology, a thing does not enter a frame. All (reasonably local) things are in all frames, but not necessarily stationary in it. Clocks need not match velocity to be compared. They just have to be in each other's presence.
The effects of special relativity don't suddenly kick in at 0.6c; they are real, even at walking pace. It's splitting hairs, perhaps, because the variations are a tiny fraction of negligible; but for practical purposes, an inertial frame is just some environment in which the effects of SR can be ignored, because they are so small. You don't have to synchronise your wristwatch and the wall clock, every time you scratch your ear, for example.

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Noax
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Re: Relativity?

Post by Noax » Wed Nov 15, 2017 1:48 pm

uwot wrote:
Wed Nov 15, 2017 9:09 am
Noax wrote:
Tue Nov 14, 2017 2:00 am
As for a bit of terminology, a thing does not enter a frame. All (reasonably local) things are in all frames, but not necessarily stationary in it. Clocks need not match velocity to be compared. They just have to be in each other's presence.
The effects of special relativity don't suddenly kick in at 0.6c; they are real, even at walking pace. It's splitting hairs, perhaps, because the variations are a tiny fraction of negligible; but for practical purposes, an inertial frame is just some environment in which the effects of SR can be ignored, because they are so small. You don't have to synchronise your wristwatch and the wall clock, every time you scratch your ear, for example.
Was this comment perhaps a reply to a different comment of mine, such as one of the doppler bits? I made no mention of SR kicking in at some threshold. Me scratching my ear does indeed cause my watch to log less time, but my watch is simply not accurate enough for it to matter. I wish it did. My watch is not quartz, and tends to run fast. The watch (unlike most that are battery powered) is powered by changes in the IRF at which it is at rest and thus will not run at all without the occasional ear scratch.

My terminology comment was just about "to enter a frame", which is misleading. One is always in a given frame, just not necessarily stationary in it. To accelerate is not to exit the frame in which something was stationary, it is simply to no longer be stationary in that frame.

My primary comment was a counter to davidm's statements that two clocks needed to match velocity in order to be compared, and I said no, they need to be in each other presence, if only for a moment as they pass.

uwot
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Re: Relativity?

Post by uwot » Wed Nov 15, 2017 2:55 pm

Noax wrote:
Wed Nov 15, 2017 1:48 pm
My primary comment was a counter to davidm's statements that two clocks needed to match velocity in order to be compared, and I said no, they need to be in each other presence, if only for a moment as they pass.
Well, it's not for me to speak on behalf of someone else, but I think "the separated observers will have to re-enter the same frame and compare clocks" could be interpreted that way, especially as it is the premise on which the light clock is based. Over to davidm.
Anyway; the bit I was interested in, is your claim that relativity is not about observers. Granted that the universe will do what it does regardless of who's watching (certain interpretations of QM notwithstanding), but as I see it, relativity, the geometry and whatnot, is an epistemic instrument that is of no value without observers.

davidm
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Re: Relativity?

Post by davidm » Wed Nov 15, 2017 7:33 pm

Noax wrote:
Wed Nov 15, 2017 1:48 pm

My primary comment was a counter to davidm's statements that two clocks needed to match velocity in order to be compared, and I said no, they need to be in each other presence, if only for a moment as they pass.
Well, of course, you can compare clocks in different inertial frames. I certainly did not dispute this. What I said was that, in the case of something like the so-called twin's paradox, the twins need to rejoin a joint frame and compare their clocks to determine which *really* ticked slower.

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Noax
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Re: Relativity?

Post by Noax » Wed Nov 15, 2017 9:09 pm

davidm wrote:
Wed Nov 15, 2017 7:33 pm
Well, of course, you can compare clocks in different inertial frames. I certainly did not dispute this. What I said was that, in the case of something like the so-called twin's paradox, the twins need to rejoin a joint frame and compare their clocks to determine which *really* ticked slower.
Yes, that is what I dispute. The twins need not "rejoin a joint frame" to compare clocks. For one, no twin ever left any frame, having merely failed to remain stationary in it. This is a terminology quibble. You consider "leaving a frame" to mean not being stationary in it. I'm OK with that.
But more importantly, the comparison is made at the moment they meet at some common point with perhaps vastly different velocities, defining a frame-independent event at which the time on both clocks can be noted. These two readings can be subtracted at leisure, not requiring the observers to be present at the subtraction of the readings. It could be done by email the next day, and assuming a common velocity is not required to do any of this.

davidm
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Re: Relativity?

Post by davidm » Wed Nov 15, 2017 9:47 pm

Noax wrote:
Wed Nov 15, 2017 9:09 pm
davidm wrote:
Wed Nov 15, 2017 7:33 pm
Well, of course, you can compare clocks in different inertial frames. I certainly did not dispute this. What I said was that, in the case of something like the so-called twin's paradox, the twins need to rejoin a joint frame and compare their clocks to determine which *really* ticked slower.
Yes, that is what I dispute. The twins need not "rejoin a joint frame" to compare clocks. For one, no twin ever left any frame, having merely failed to remain stationary in it. This is a terminology quibble. You consider "leaving a frame" to mean not being stationary in it. I'm OK with that.
But more importantly, the comparison is made at the moment they meet at some common point with perhaps vastly different velocities, defining a frame-independent event at which the time on both clocks can be noted. These two readings can be subtracted at leisure, not requiring the observers to be present at the subtraction of the readings. It could be done by email the next day, and assuming a common velocity is not required to do any of this.
Yeah, I think I agree with this, if I understand it correctly. I agree we mainly have a terminological quibble. As to frames, for example, I think I said that an observer may leave an inertial frame and "enter" an accelerated frame. But you're right; he is still in the same frame. I was just trying to distinguish, perhaps inaptly (ineptly?) between an inertial and accelerated state.

davidm
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Re: Relativity?

Post by davidm » Wed Nov 15, 2017 10:40 pm

Noax wrote:
Wed Nov 15, 2017 1:48 pm
My terminology comment was just about "to enter a frame", which is misleading. One is always in a given frame, just not necessarily stationary in it. To accelerate is not to exit the frame in which something was stationary, it is simply to no longer be stationary in that frame.
I agree.

Viveka
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Re: Relativity?

Post by Viveka » Thu Nov 16, 2017 1:25 am

Noax wrote:
Wed Nov 15, 2017 9:09 pm
davidm wrote:
Wed Nov 15, 2017 7:33 pm
Well, of course, you can compare clocks in different inertial frames. I certainly did not dispute this. What I said was that, in the case of something like the so-called twin's paradox, the twins need to rejoin a joint frame and compare their clocks to determine which *really* ticked slower.
Yes, that is what I dispute. The twins need not "rejoin a joint frame" to compare clocks. For one, no twin ever left any frame, having merely failed to remain stationary in it. This is a terminology quibble. You consider "leaving a frame" to mean not being stationary in it. I'm OK with that.
But more importantly, the comparison is made at the moment they meet at some common point with perhaps vastly different velocities, defining a frame-independent event at which the time on both clocks can be noted. These two readings can be subtracted at leisure, not requiring the observers to be present at the subtraction of the readings. It could be done by email the next day, and assuming a common velocity is not required to do any of this.
Hence my argument about the shared consciousness of observers and how that ruins Einstein's thought experiment with the Train observer and Embankment observer. Instead of accepting the thought experiment of mine, they decided to discuss if communication of information can be superluminal.

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