Understanding Time Dilation: How Fast Do You Have to Go?

  • #151
Sam Woole said:
I agree that moving and accelerating are not the same thing. But according to my understanding of Einstein's theory, it is the relative uniform motion that is causing time dilation. From the viewpoint of the observers on the railway station, time dilation will happen on the train; from the viewpoint of observers on the train, time dilation will happen on the railway station. Post #142 explicitly said so; either person (Bob or Jon) is considering the other aging 50 times faster. [snip] This implies that Einstein's theory gives self-contradictory results such as a>b and a<b; Bob older than Jon and also younger than Jon.
In addition to what Doc said, the simplest way to resolve the apparent contradiction is to bring the two clocks together to compare them. And doing that requires an acceleration of one or both clocks. When you do that, you have unbalanced what used to be a perfectly symmetrical situation.

Before one observer accelerates, either can be considered stationary. Afterwards, only one can, because you can feel the acceleration.
[snop]Many other people understood Einstein's time dilation in the same way as I did.
Just because a lot of people misunderstand it in the same way, doesn't make their misunderstandings correct.

Be pragmatic about it: If relativity "worked" the way you think it does, you are correct that it wouldn't work. But scientists and engineers think relativity works in a different way, and in that way, it does work. So even if Einstein meant what you think he did, you'd still be wrong because Einstein doesn't own relativity. It has grown beyond his contribution and if this was an error, it has now been corrected.
 
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  • #152
russ_watters said:
In addition to what Doc said, the simplest way to resolve the apparent contradiction is to bring the two clocks together to compare them. And doing that requires an acceleration of one or both clocks. When you do that, you have unbalanced what used to be a perfectly symmetrical situation.
Before one observer accelerates, either can be considered stationary. Afterwards, only one can, because you can feel the acceleration. .

Your words made us to think that Einstein's theory was like a child's play. Unless we sit clocks together, we would not know their difference. But DrGreg's demonstration showed quite the opposite. It showed that we became aware of the difference between the two clocks even if the clocks, Bob and Alice, were many light minutes apart, which is a huge distance.

Your argument on acceleration also contradicted DrGreg's demonstration, which stated that the motion was constant. In a constant motion, how can any observer "feel" the motion?

russ_watters said:
Just because a lot of people misunderstand it in the same way, doesn't make their misunderstandings correct.

To me, it is you who were misunderstanding, not me. Let me show you why.

In DrGreg's demonstration, there was a constant distance between Bob and Alice, say d; a constant v, and a constant c. Light needs 20 minutes to travel the d, while Alice needs 120 minutes. The time interval 120 minutes is Newtonian time, absolute time, obtainable by means of Newton's equation t = d/v. It is also the normal time we all earthlings stationary on Earth will get. Don't you agree?

My understanding of time dilation is, the moving clock will suffer time dilation, accumulating less time. Less than what? I believe it is less than the normal, absolute time that clocks stationary on Earth would get. That is, Alice's clock should accumulate less than 120 minutes. The traveling twin will become younger while the earthbound twin will age normally.

But DrGreg's demonstration did not show such a time dilation. Instead it showed that the moving observer Alice accumulated exactly normally, 120 minutes, aging normally, while her motion had caused the stationary clock to accumulate more than 120 minutes, aging faster. Don't you think that you were misunderstanding?

This is as if to say, scientists such as Hafele and Keating claimed that they could make the clocks stationary on Earth to work faster by transporting other clocks in the air. I think this was worse than misunderstanding.

As I have pointed out earlier, such a demonstration of DrGreg's led to the deduction that, any thing moving toward Earth will cause life forms there to age faster. If this deduction is false (It must be), I think time dilation is false likewise, impossible to be understood properly.
 
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  • #153
Sam Woole said:
Your argument on acceleration also contradicted DrGreg's demonstration, which stated that the motion was constant. In a constant motion, how can any observer "feel" the motion?
Half way through my demonstration Alice did a U-turn. That part wasn't constant. She would have felt a huge G-force as she decelerated and then sped off in the opposite direction. Bob would have felt no G-force at all.
Sam Woole said:
To me, it is you who were misunderstanding, not me. Let me show you why.
In DrGreg's demonstration, there was a constant distance between Bob and Alice, say d;
It wasn't constant, it was changing all the time.


Sam Woole said:
a constant v, and a constant c. Light needs 20 minutes to travel the d, while Alice needs 120 minutes. The time interval 120 minutes is Newtonian time, absolute time, obtainable by means of Newton's equation t = d/v. It is also the normal time we all earthlings stationary on Earth will get. Don't you agree?
There is no such thing as “absolute time”. Everyone has their own time.
Sam Woole said:
My understanding of time dilation is, the moving clock will suffer time dilation, accumulating less time. Less than what?
A very good question. All dilation is one clock relative to another clock.


Sam Woole said:
I believe it is less than the normal, absolute time that clocks stationary on Earth would get. That is, Alice's clock should accumulate less than 120 minutes. The traveling twin will become younger while the earthbound twin will age normally.

But DrGreg's demonstration did not show such a time dilation. Instead it showed that the moving observer Alice accumulated exactly normally, 120 minutes, aging normally, while her motion had caused the stationary clock to accumulate more than 120 minutes, aging faster. Don't you think that you were misunderstanding?
But Alice is now 10 minutes younger than Bob. Isn't this what you said in the previous paragraph?



What makes you think the Earth is “stationary” anyway? It’s moving round the sun, the sun is moving round the galaxy, the galaxy is receding from other galaxies…

Everything moves relative to something else!
 
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  • #154
Sam Woole said:
Your words made us to think that Einstein's theory was like a child's play. Unless we sit clocks together, we would not know their difference

If two objects are experiencing each other's clocks/time-system to be going slowly compared to their own, their clocks can not agree on two events being simultaneous. (Although, Post 65 uses Synchronization by being in the same place, instead of by going at the same speed, which can be valid). You can use relativity to work out what each person sees, but you can not choose a time that their clocks will both agree on to analyse, when they are separated and also in relative motion. If you want the clocks to go at the same speed and be instantly comparable at a glance, you must bring them together. Alternatively, loose yourself in the maths and definitions.
 
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  • #155
Sam Woole said:
But DrGreg's demonstration showed quite the opposite. It showed that we became aware of the difference between the two clocks even if the clocks, Bob and Alice, were many light minutes apart, which is a huge distance.

DrGreg's demonstation from post 68 was done using relativity and maths. The acceleration in the scenario also meant that there is a "right" answer not a symmetrical situation.

Sam Woole said:
Your argument on acceleration also contradicted DrGreg's demonstration, which stated that the motion was constant. In a constant motion, how can any observer "feel" the motion?

DrGreg in post 68 demonstrates the twin paradox, based on acceleration.

Sam Woole said:
In DrGreg's demonstration, there was a constant distance between Bob and Alice, say d; a constant v, and a constant c.

Which post number?

Sam Woole said:
My understanding of time dilation is, the moving clock will suffer time dilation, accumulating less time. Less than what? I believe it is less than the normal, absolute time that clocks stationary on Earth would get.

There is no absolute time in relativity. Discard this notion if you wish to understand the theory.

Sam Woole said:
But DrGreg's demonstration did not show such a time dilation. Instead it showed that the moving observer Alice accumulated exactly normally, 120 minutes, aging normally, while her motion had caused the stationary clock to accumulate more than 120 minutes, aging faster.

Motion doesn't cause stationary clocks to age faster in relativity. Instead, a different path through space-time leads to a different amount of aging.

Sam Woole said:
As I have pointed out earlier, such a demonstration of DrGreg's led to the deduction that, any thing moving toward Earth will cause life forms there to age faster. If this deduction is false (It must be), I think time dilation is false likewise, impossible to be understood properly.

You can only understand it if you do not dismiss the ideas like you have been. You also need to correct the recurring mistakes in your understanding of the theory to get a clear picture of it.
 
  • #156
DrGreg said:
Half way through my demonstration Alice did a U-turn. That part wasn't constant. She would have felt a huge G-force as she decelerated and then sped off in the opposite direction. Bob would have felt no G-force at all.

Sorry, your responses were making the dilation idea less and less credible. Here you said "she decelerated". What does it mean? When did she begin her deceleration? And "she sped off", what does it mean? Did she also speed off when she started her journey from Bob? All your words meant (to me) Alice was traveling at variable speeds, not one constant speed. If so, then Alice could not cover equal distances in equal time intervals. But your demonstration showed equal distances in all equal time intervals. Your acceleration argument violated your own design.

"She sped off" means, she begins with 0 miles per second and accelerates to 1, 2, 3,...n miles per second. Due to the very low speeds in the beginning, and the low speeds in her final stage reaching Ted, she might need to accelerate so much as to surpass the c in her mid motions, otherwise how can she finish the distance (between Bob and Ted) in one hour? I believe you were making things worse and worse for the time dilation idea.

Even if I accept that Alice experienced accelerations and decelerations, the G-force, such a situation would still work against you. This situation meant, pursuant to your demonstration, that the moving observer Alice's experience with G-force will not dilate her own time. Instead, a stationary observer Bob's time, many light minutes away from the moving observer, will be expanded. An instantaneous action took place as a result of Alice's acceleration and deceleration.

I believe you were making things worse and worse.

DrGreg said:
It wasn't constant, it was changing all the time.
This sentence of yours was given in response to my statement that there was a constant distance between Bob and Alice. I am sorry, I typed in the wrong name. I meant a constant distance between Bob and Ted. It should be understandably so because I gave 120 minutes for Alice to cover such a distance. I wish jackle would take note of my clarification.

DrGreg said:
What makes you think the Earth is “stationary” anyway? It’s moving round the sun, the sun is moving round the galaxy, the galaxy is receding from other galaxies…
Everything moves relative to something else!

This one I think was not my mistake. My words were "clocks stationary on earth".

The motion of the Earth was used by SR dissidents to attack the twin paradox. In response SR supporters came up with acceleration and G-force and so forth. But as I have pointed out above, the acceleration argument will not convince dissidents like me either.
 
  • #157
Sam Woole said:
Even if I accept that Alice experienced accelerations and decelerations, the G-force, such a situation would still work against you. This situation meant, pursuant to your demonstration, that the moving observer Alice's experience with G-force will not dilate her own time. Instead, a stationary observer Bob's time, many light minutes away from the moving observer, will be expanded. An instantaneous action took place as a result of Alice's acceleration and deceleration.
Your use of the phrase "instantaneous action" doesn't really make sense to me here. Alice's clock rate changes relative to Bob's coordinate system, but Bob doesn't actually experience any instantaneous effects where he's sitting at the moment Alice changes velocities. It might help to consider coordinate systems created physically in the way Einstein did in his paper--imagine each inertial observer has a giant grid of rulers filling space, and each observer's grid is at rest relative to himself. At each marking on an observer's grid, there is a clock attached to the ruler, and all the clocks have been "synchronized" using light-signals (for an illustration, take a look at fig. 16 at the bottom of http://www.physicspost.com/articles.php?articleId=88&page=5 ). So to identify the space and time coordinates of an event, a given observer just has to look at the markings on the rulers in his system that were right next to the event as it happened, and look at the time on the clock in his system that was right next to the event as it happened. So, for example, if the event of Alice's clock reading "80 seconds" happened as she passed the clock in Bob's system that read "100 seconds", and the event of Alice's clock reading "96 seconds" happened as she passed another clock in Bob's system that read "120 seconds", then in Bob's system her clock had only ticked 96-80 = 16 seconds when 120-100 = 20 seconds had passed in his own coordinate system, so her clock was only ticking at 0.8 the normal rate in his system. But suppose that immediately after Alice's clock read 96 seconds, she changed velocities, and then later when her clock read "102 seconds", she was passing a third clock in Bob's system that read "130 seconds". Now her clock has only ticked 102-96 = 6 seconds while 130-120 = 10 seconds have passed in Bob's coordinate system, so her clock is only ticking at 0.6 the normal rate in his system. But there's nothing "instantaneous" going on here, because we're talking about local readings on different clocks that are all located at different spots in Bob's giant ruler/clock grid, Bob won't actually know about the change until the light from these local readings has had time to reach him at the center of the grid.
 
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  • #158
Sam Woole said:
Sorry, your responses were making the dilation idea less and less credible. Here you said "she decelerated". What does it mean? When did she begin her deceleration? And "she sped off", what does it mean? Did she also speed off when she started her journey from Bob? All your words meant (to me) Alice was traveling at variable speeds, not one constant speed. If so, then Alice could not cover equal distances in equal time intervals. But your demonstration showed equal distances in all equal time intervals. Your acceleration argument violated your own design.
In such thought experiments you can make the acceleration phases of the trip as short as you want (By increasing the rate of acceleration), to the point where they are neglible when compared to the total trip time.
"She sped off" means, she begins with 0 miles per second and accelerates to 1, 2, 3,...n miles per second. Due to the very low speeds in the beginning, and the low speeds in her final stage reaching Ted, she might need to accelerate so much as to surpass the c in her mid motions, otherwise how can she finish the distance (between Bob and Ted) in one hour? I believe you were making things worse and worse for the time dilation idea.
Again, we can make this acceleration phase as short as we want. Also, as she accelerates, see will see the distance between Bob and Ted contract. It is this contracted distance that she traverses, and why her clock only records 1 hr to make the trip according to her.
Even if I accept that Alice experienced accelerations and decelerations, the G-force, such a situation would still work against you. This situation meant, pursuant to your demonstration, that the moving observer Alice's experience with G-force will not dilate her own time. Instead, a stationary observer Bob's time, many light minutes away from the moving observer, will be expanded. An instantaneous action took place as a result of Alice's acceleration and deceleration.
I believe you were making things worse and worse.
What happens is that the aceleration experienced by Alice affects her determination of how fast time is progressing for Bob during that period.
The motion of the Earth was used by SR dissidents to attack the twin paradox. In response SR supporters came up with acceleration and G-force and so forth. But as I have pointed out above, the acceleration argument will not convince dissidents like me either.

What actually happened is that the "SR dissidents", after dipping their toes into SR, came up with what they thought was an argument against it. Then others pointed out that the argument didn't take everything into account. They did not "come up" with it, it was always a part of SR. The dissidents just never delved deeply enough into the theory to learn that, and now that they've 'staked out their claim', they refuse to budge.
 
  • #159
JesseM said:
Your use of the phrase "instantaneous action" doesn't really make sense to me here. Alice's clock rate changes relative to Bob's coordinate system, but Bob doesn't actually experience any instantaneous effects where he's sitting at the moment Alice changes velocities.

My instantaneity charge referred to DrGreg's demonstration only. For example, in post #68, Alice started her return trip and applied the 3/2 rate. When Alice's clock accumulated 10 minutes (showing 13:10), the application of the 3/2 rate made Bob's clock to accumulate 15 minutes (showing 12:55). This accumulation must take place instantaneously, otherwise Bob's clock could not have produced the end result of 14:10. Namely as soon as Alice started to return, Bob's clock instantaneously started to work faster; Bob did experience an instantaneous effect pursuant to DrGreg's demonstration.
 
  • #160
Janus said:
What happens is that the aceleration experienced by Alice affects her determination of how fast time is progressing for Bob during that period.

I took your words above to mean, Alice's determination had been executed by Bob's clock. For example, at her time 13:10 (post #68), Alice determined that Bob's clock should read 12:15. As soon as she made such a determination, Bob's clock obeyed and executed 12:15. Since her determination was made when she was many light minutes away from Bob, wasn't the execution an instantaneous action? It must be, otherwise Bob's clock could not have produced the 14:10 at the end.

DrGreg's demonstration means to me, you people were using faster-than-light speeds to justify Einstein's theory.
 
  • #161
Sam Woole said:
Janus said:
What happens is that the aceleration experienced by Alice affects her determination of how fast time is progressing for Bob during that period.
I took your words above to mean, Alice's determination had been executed by Bob's clock.
I took Janus's "determination" to mean "perception", or alternatively, "calculation" or "prediction", without any implication of causation. Bob's clock of course has no way of "knowing" how Alice is moving, nor whether she is changing her state of motion or not.

Similarly, my desk has no idea of what direction I am observing it from. Nevertheless, the angle from which I view it determines the shape that I perceive, and as I walk around the desk, its apparent shape (more precisely a two-dimensional projection of its three-dimensional shape) changes.
 
  • #162
Sam Woole said:
My instantaneity charge referred to DrGreg's demonstration only. For example, in post #68, Alice started her return trip and applied the 3/2 rate. When Alice's clock accumulated 10 minutes (showing 13:10), the application of the 3/2 rate made Bob's clock to accumulate 15 minutes (showing 12:55). This accumulation must take place instantaneously, otherwise Bob's clock could not have produced the end result of 14:10. Namely as soon as Alice started to return, Bob's clock instantaneously started to work faster; Bob did experience an instantaneous effect pursuant to DrGreg's demonstration.
If you keep track of the difference between when things happen in Bob's coordinate system (which as I said in my last post, can be understood in terms of local readings on a large grid of rulers and synchronized clocks at rest relative to Bob) and when he sees them happen using light-signals, you'll see that there is no instantaneity here. In post #110 I explained when certain events would happen in Bob's coordinate system--for example, in Bob's coordinate system Alice would turn around at 13:05 (in that post I actually said 13:04.998, but that was a roundoff error), but he does not actually see her turn around until his clock reads 13:30. Since she departed at 12:00 and was moving at v=0.3846154c in his coordinate system, at t=13:05 she would have been traveling for 65 minutes at v=0.3846154c, so her distance would be 65*0.3846154=25 light-minutes. So if she turns around at 13:05 in his coordinate system, the light from this event will take 25 minutes to reach Bob, so he won't see it until 13:05 + 25 = 13:30. And not until 13:30 does he see the rate of her clock ticks change.

Another way of thinking about this is in terms of Bob's grid of rulers and synchronized clocks--when his own local clock reads 13:30, if Bob looks through his telescope at Alice, he will see her fire her rockets to turn around right when she is next to a marking on his ruler that reads "25 light-minutes", and he will see through the telescope that the clock attached to that position on the ruler reads "13:05 minutes". So he will conclude that although he's only seeing it now, the event of her turning around "really" happened 25 minutes ago in his frame (just like if you look at a supernova thousands of light-years away, you know that event really happened thousands of years ago).
 
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  • #163
jtbell said:
I took Janus's "determination" to mean "perception", or alternatively, "calculation" or "prediction", without any implication of causation. Bob's clock of course has no way of "knowing" how Alice is moving, nor whether she is changing her state of motion or not.
Similarly, my desk has no idea of what direction I am observing it from. Nevertheless, the angle from which I view it determines the shape that I perceive, and as I walk around the desk, its apparent shape (more precisely a two-dimensional projection of its three-dimensional shape) changes.

I felt your "desk" simile was supportive to my dissenting voice. If we apply this simile to DrGreg's demonstration, we saw its flaw clearly. That is, regardless how Alice perceived (or determined, or calculated), her perception cannot change the properties of Bob's clock; our perception of our desk cannot change its properties. It follows that the twin paradox, time dilation, differential aging are all false.
 
  • #164
Sam Woole said:
I felt your "desk" simile was supportive to my dissenting voice. If we apply this simile to DrGreg's demonstration, we saw its flaw clearly. That is, regardless how Alice perceived (or determined, or calculated), her perception cannot change the properties of Bob's clock; our perception of our desk cannot change its properties. It follows that the twin paradox, time dilation, differential aging are all false.
Oy! After 11 pages, that is where your understanding is?

Maybe it's time to throw some reality back into this: you do understand that the GPS system test's SR and GR's time dilation predictions, in addition to successfully dealing with the relativity of simultenaity, on a constant basis, right?

It almost seems like what you aren't able to get your arms around (or simply refuse to accept) is that when looking at someone else's clock through a telescope, it takes time for the signal to reach you, so what you see through the telescope isn't what the clock shows now. And you can run yourself in circles until smoke comes out of your ears trying to figure out when "now" is, or you can simply accept that the concept of "now" is observer dependant. Saying that I see a certain time on a clock, while calculating that the person standing next to it sees a different time does not imply that the clock is showing two different times at the same time!

IfI'mlookingthroughatelescopeataclockthenwhatIseenowisn'tnowontheclockbutwhatisnowontheclockisdifferentfromwhat
Iseesohowcantheclockhavetwodifferentreadingsonitatthesametime?
 
  • #165
Sam Woole said:
For example, at her time 13:10 (post #68), Alice determined that Bob's clock should read 12:15.

I have just gone back and looked at post #68. I can't find anything in that post that would support this assertion.

I would prefer to use the tables I used in https://www.physicsforums.com/showpost.php?p=791307&postcount=103" (instead of #68) as they are less likely to misinterpretation. Can you please reformulate your assertion in terms of these tables and then explain it?
 
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  • #166
DrGreg said:
I have just gone back and looked at post #68. I can't find anything in that post that would support this assertion.
I would prefer to use the tables I used in https://www.physicsforums.com/showpost.php?p=791307&postcount=103" (instead of #68) as they are less likely to misinterpretation. Can you please reformulate your assertion in terms of these tables and then explain it?

It was the same message regardless which table we follow. In tables in post #103, you used numbers different from post #68. These numbers were more likely to be misinterpreted by me, not less. If you don't mind, I suggest simpler numbers such as 0 minutes to begin with. At 60 minutes, Alice would have reached Ted like this:
Bob's clock-----image in Alice telescope---Alice's clock
60 minutes ------------40------------------60 minutes.

According to the supernova example pointed out by JesseM, as well as to the delay pointed by you, though Alice saw 40 in her telescope, yet she knew the true number on Bob's clock was 60, the same like her own.

Now Alice would start to return. Your table in post #103 showed that Bob's clock eventually accumulated 10 more minutes than Alice's. In order to accumulate these additional minutes, of course there must be a beginning, from 1,2,3... 10. I charged that this "beginning" was an instantaneous action resulted from Alice's return.

Let me repeat, when Alice reached Ted, both clocks had equal time, 60 minutes. Only after this point, Bob's clock started to have more minutes than Alice's as a result of the 3/2 rate.

If you refer to my post #163, I do not believe that the application of the Doppler effect or the 3/2 rate can cause Bob's clock to work faster. You may have any kind of calculations, but you cannot change the properties of Bob's clock by means of calculations.
 
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  • #167
Sam Woole said:
It was the same message regardless which table we follow. In tables in post #103, you used numbers different from post #68. These numbers were more likely to be misinterpreted by me, not less. If you don't mind, I suggest simpler numbers such as 0 minutes to begin with. At 60 minutes, Alice would have reached Ted like this:

Bob's clock-------image in Alice telescope---------Alice's clock
60 minutes ---------------40-----------------------60 minutes.

According to the supernova example pointed out by JesseM, as well as to the delay pointed by you, though Alice saw 40 in her telescope, yet she knew the true number on Bob's clock was 60, the same like her own.
NO! As I've said to you many times, the slowdown of Alice's clock as seen by Bob is not just due the delays in light-signals, it's also due to genuine time dilation, so even after he takes into account the delay, he will still conclude that Alice's clock is running slower than hers. Likewise, Alice will also conclude that Bob's clock is running slower than hers, even after she takes into account the signal delay.

As I said in my previous post, Bob will see Alice turn around at 13:30 according to his own clock, and when he takes into account the light-signal delay (based on the fact that her distance when she turned around was 25 light-minutes from him), he will conclude that this event really happened at 13:05 in his frame. Since his clock read 12:00 at the moment they departed from each other, in his frame Alice way flying away for 1 hour and 5 minutes before she turned around. But at the moment Alice turns around, Bob can see that her clock read 16:00 at that moment, and since her clock read 15:00 at the moment she departed, Alice was only flying away for 1 hour and 0 minutes according to her own clock! So you can see that even after taking into account light signal delay, Bob concludes that Alice's clock was running slow, due to time dilation.

Now look at things from Alice's point of view. From her perspective, when she turned around, if she looked through a telescope she would see Bob's clock reading 12:40 at that moment. If she has her own grid of rulers at rest relative to herself, she will see that at that moment, he was about 16.666... light-minutes away in her frame. Since her own clock read 16:00 at the moment she saw this, she would conclude that the event of his clock ticking 12:40 really happened at 16:00 - 16.666... = 15:43.333... minutes in her frame. So after taking into account signal delays, she concludes that her clock had elapsed 43.333... minutes at the moment that his clock had elapsed 40 minutes, meaning his clock was genuinely running slower than hers in her own frame.
Sam Woole said:
If you refer to my post #163, I do not believe that the application of the Doppler effect or the 3/2 rate can cause Bob's clock to work faster. You may have any kind of calculations, but you cannot change the properties of Bob's clock by means of calculations.
Do you understand that the formula for the doppler effect in relativity is different from the formula for the doppler effect in classical mechanics (say, the doppler effect with sound waves), and that this difference is because the relativistic formula was derived using the assumption that time dilation is real?
 
  • #168
Sam Woole said:
It was the same message regardless which table we follow. In tables in post #103, you used numbers different from post #68. These numbers were more likely to be misinterpreted by me, not less. If you don't mind, I suggest simpler numbers such as 0 minutes to begin with. At 60 minutes, Alice would have reached Ted like this:

Bob's clock-------image in Alice telescope---------Alice's clock
60 minutes ---------------40-----------------------60 minutes.
In the above line you are making an assumption that Alice’s clock shows 60 minutes “at the same time as” Bob’s clock shows 60 minutes (whatever that means). In other words you are assuming that Bob and Alice’s clocks remain synchronized. That is the very thing my argument is trying to disprove. If you simply assume that whatever I am trying to demonstrate is wrong, how can I demonstrate anything? You have to let go of your assumptions and ask what if clocks didn’t remain synchronized? Would it make sense? That’s what Einstein did and came to the conclusion “yes”.

Sam Woole said:
According to the supernova example pointed out by JesseM, as well as to the delay pointed by you, though Alice saw 40 in her telescope, yet she knew the true number on Bob's clock was 60, the same like her own.
Alice does not know what the true number is, unless she performs a relativistic calculation (in which case she will calculate approximately 55 minutes 23 seconds (12/13 hours).

This is the reason I prefer to use the numbers in post #103, so that you are less likely to misinterpret 13:00 on Bob’s clock as being “the same time as” 16:00 on Alice’s clock. Alice and Bob have different definitions of simultaneity and do not agree on this. Bob’s assessment is that Alice’s 16:00 occurs at his 13:05 (see JesseM’s post). Alice’s assessment, whilst traveling away from Bob, is that Bob’s 13:00 would occur at her 16:05 (if she were to keep on traveling away instead of turning round).

To give an analogy: when you measure the length of a rod you wouldn’t measure the two ends using two different rulers. So, to measure an interval of time you can’t measure the start and the end using two different clocks. You can’t measure the time it takes for light to travel from Bob to Alice by measuring the start with Bob’s clock and the end with Alice’s clock.
 
  • #169
JesseM said:
NO! As I've said to you many times, the slowdown of Alice's clock as seen by Bob is not just due the delays in light-signals, it's also due to genuine time dilation, so even after he takes into account the delay, he will still conclude that Alice's clock is running slower than hers. Likewise, Alice will also conclude that Bob's clock is running slower than hers, even after she takes into account the signal delay.
As I said in my previous post, Bob will see Alice turn around at 13:30 according to his own clock, and when he takes into account the light-signal delay (based on the fact that her distance when she turned around was 25 light-minutes from him), he will conclude that this event really happened at 13:05 in his frame. Since his clock read 12:00 at the moment they departed from each other, in his frame Alice way flying away for 1 hour and 5 minutes before she turned around. But at the moment Alice turns around, Bob can see that her clock read 16:00 at that moment, and since her clock read 15:00 at the moment she departed, Alice was only flying away for 1 hour and 0 minutes according to her own clock! So you can see that even after taking into account light signal delay, Bob concludes that Alice's clock was running slow, due to time dilation.

I did read your recent posts but I am sorry I could not understand them. Currently DrGreg was trying to prove that time dilation is genuine, while I was trying to show his proof was flawed. Flawed or not, I believe we cannot use time dilation to justify an experiment designed to prove time dilation. It was tantamount to using a flaw to justify a flaw.

Because of your use of the time dilation, you gave more and more numbers that never appeared in DrGreg's demonstration. These numberbs complicated matters more and more, leading to more difficulties and arguments. For instance you said the distance between Bob and Alice (at Ted) was 25 light-minutes.
based on the fact that her distance when she turned around was 25 light-minutes from him
. How could I understand this 25 light-minutes? Very difficult.

I wish you would not give any more numbers. Whether DrGreg's demonstration is flawed or not, I believe we must justify it according to only those numbers he has given. No more new numbers.

Not only no more new numbers, we should simplifiy them. For instance, when the two clocks came together to compare, Bob's clock read 14:10 and Alice's read 17:00. The result of the comparison is: Alice's clock had worked a lot faster than Bob's, the traveled twin had become a lot older.
 
  • #170
Sam Woole said:
I did read your recent posts but I am sorry I could not understand them.
What about them didn't you understand? Do you understand, for example, that if I see an event through a telescope that's 25 light-minutes away, then if I assume light travels at c in my frame, that must mean the event actually happened 25 minutes before I saw it? That's just the same principle as the idea that if we spot a supernova today happening 10,000 light-years away, it must have actually happened 10,000 years ago. And that's really all you need to understand my recent posts, all I was ever doing was showing the time that Alice or Bob saw a reading on the other one's clock, then subtracting the light-signal delay to figure out when the other one's clock was "really" showing that reading. I know all the numbers may look a little intimidating, but I think you can follow them if you read carefully.
Sam Woole said:
Currently DrGreg was trying to prove that time dilation is genuine, while I was trying to show his proof was flawed.
I don't think he's really trying to prove it's genuine--he's just telling you what Sam and Alice would see if you assume the relativistic doppler shift equation is correct, but the relativistic doppler shift equation already assumes that time dilation is real. It doesn't make any sense to accept the numbers in DrGreg's example and yet reject time dilation, because he only got those numbers using a formula that assumes time dilation is real. If you used the non-relativistic doppler shift equation, then the numbers would be different.

Do you understand how regular, non-relativistic doppler shifts work? It's pretty simple--suppose I'm traveling away from you at 20 meters per second, and every second I shoot my bb gun at a wall next to you. Also suppose each bb pellet travels at a constant speed relative to you, 100 meters per second. Despite the fact that I am shooting a new pellet every second, you will not see the pellets hit the wall every second--the reason is that after each second I am 20 meters further away, so each new bb pellet has 20 meters further to travel than the last one. Since the pellets travel at 100 meters per second, it will take them 0.2 seconds to travel 20 meters, so each new pellet takes 0.2 seconds longer than the last one to travel from my gun to the wall. So you will see the pellets hit the wall every 1.2 seconds rather than every 1 second. That's all the non-relativistic doppler shift is--it's just a consequence of the fact that the distance between two observers is changing, so each successive signal has a different distance to travel than the previous one.

But in relativity, this is only part of the doppler shift. The other part is that we assume in relativity that if I am traveling at high speed relative to you, my clock will genuinely slow down in your frame--that's time dilation. So if I'm sending a signal once every second according to my own clock, then not only is there a larger gap between signals than one second because I'm moving away from you, but the gap between signals is increased even more because one second on my clock lasts longer than a second according to your clocks. So the relativistic doppler effect takes into account both these factors, the changing distance and the time dilation. If you don't accept time dilation, then you shouldn't accept the relativistic doppler formula, and thus you shouldn't accept DrGreg's numbers in the first place! But it's silly to accept his numbers, which he got using the relativistic doppler equation, but then reject time dilation--it just doesn't make sense.
Same Woole said:
Flawed or not, I believe we cannot use time dilation to justify an experiment designed to prove time dilation.
Then why are you arguing as though you accept that the numbers DrGreg gave would be what Alice and Bob would see, but you just reject time dilation? Again, if you reject time dilation, it doesn't make sense to accept those numbers in the first place.

And if you think time dilation is "flawed", do you think it's logically flawed in the sense of leading to some internal contradiction, despite the fact that no mathematicians seem to agree, or do you just think that the theory isn't the correct one for describing the real world? If the first, I promise that you're wrong and that it can be proved mathematically that no contradictions arise. If the second, then we should be discussing the experimental evidence in favor of time dilation (there's a whole lot of it), not a hypothetical example which assumes time dilation is real.
Sam Woole said:
Because of your use of the time dilation, you gave more and more numbers that never appeared in DrGreg's demonstration. These numberbs complicated matters more and more, leading to more difficulties and arguments. For instance you said the distance between Bob and Alice (at Ted) was 25 light-minutes.
based on the fact that her distance when she turned around was 25 light-minutes from him
. How could I understand this 25 light-minutes? Very difficult.
Why is it hard to understand? 25 light-minutes is just the distance light travels in 25 minutes, just like a "light year" is the distance light travels in a year. I could translate this into a distance in kilometers if you'd prefer, but that'd make the calculations uglier. If the problem was that you didn't know what a "light minute" was, then you could have just asked--in general, please ask me when you don't understand a specific thing in one of my posts, otherwise this discussion isn't going to get anywhere!
Sam Woole said:
I wish you would not give any more numbers. Whether DrGreg's demonstration is flawed or not, I believe we must justify it according to only those numbers he has given. No more new numbers.
But you keep bringing up issues which demand additional numbers, like the issue of whether there is anything "instantaneous" going on. To show that no instantaneous effects happen, I had to explain the difference between the time that Bob sees certain things happen and the time he calculates they 'really' happened in his frame, just like we distinguish between the date we see the light from a distant supernova and the date it actually happened.
Sam Woole said:
Not only no more new numbers, we should simplifiy them. For instance, when the two clocks came together to compare, Bob's clock read 14:10 and Alice's read 17:00. The result of the comparison is: Alice's clock had worked a lot faster than Bob's, the traveled twin had become a lot older.
No, DrGreg specifically set up the problem so that the clocks were out-of-sync initially, so that you wouldn't make the mistake of thinking that if an event happened at 16:00 on Alice's clock, it must have happened "at the same time" that Bob's clock also read 16:00. Just look at the numbers, you can see that the two clocks were set to different times at the start--when Alice and Bob started out traveling away from each other, Bob's clock read 12:00 and Alice's read 15:00. If you like you could imagine that this difference was because Alice and Bob lived in different time zones, and when they came to meet at Alice's launch pad they forgot to reset their clocks to the time zone of the launch pad. So if Bob's clock read 14:10 when they reunited, that means it must have advanced 2 hours and 10 minutes between the time Alice left (12:00 according to his clock) and the time she returned; if Alice's clock read 17:00 when they reunited, that means it must have advanced only 2 hours between the time she left (15:00 according to her clock) and the time she returned. So if they were the same age when she departed, she must have been 10 minutes younger when they reunited.
 
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  • #171
Sam Woole said:
For instance you said the distance between Bob and Alice (at Ted) was 25 light-minutes.
JesseM said:
based on the fact that her distance when she turned around was 25 light-minutes from him
. How could I understand this 25 light-minutes? Very difficult.
The 25 light-minutes can be deduced from my tables in https://www.physicsforums.com/showpost.php?p=791307&postcount=103".

The following rows appear within those tables:
DrGreg said:
Bob...abcdefghijklm.[/color]Alice[/color]
12:40 a[/color]>>>>>>>>>>>>m 16:00[/color]
(first table)
13:30 a[/color]<<<<<<<<<<<<m 16:00[/color] (second table)
From Bob’s point of view, light leaves him (a) at 12:40, is reflected from Alice at (m), and is received by him (a) at 13:30. So this reflected light takes a total of 50 minutes to make the round trip there and back, a total distance of 50 light-minutes. The distance from (a) to (m), according to Bob, is therefore half of this, 25 light-minutes.

Also, from Bob’s point of view, he will conclude that the light reached Alice halfway between 12:40 and 13:30, that is, at 13:05. This also agrees with JesseM’s figures.

All of the above assumes that the speed of light relative to Bob is the same in both directions, Bob to Alice and Alice to Bob.
 
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  • #172
Time dilation question

JesseM said:
I don't think he's really trying to prove it's genuine--he's just telling you what Sam and Alice would see if you assume the relativistic doppler shift equation is correct, but the relativistic doppler shift equation already assumes that time dilation is real. It doesn't make any sense to accept the numbers in DrGreg's example and yet reject time dilation, because he only got those numbers using a formula that assumes time dilation is real. If you used the non-relativistic doppler shift equation, then the numbers would be different.
Now I came to the realization that some thing was fundamentally wrong, with me, or with you, depending on what perspective we look at the issue. As soon as I saw DrGreg's tables, I assumed he was trying to convince me that time dilation is genuine. Now it turned out that my assumption was wrong. Instead, DrGreg was using the genuine time dilation to show me how it works. If time dilation is genuine, of course his demonstration (those tables) is genuine likewise.

But my wrong assumption was not entirely my fault becasue, all along I have repeatedly declared my disbelief in time dilation, as can be found from my posts before DrGreg's post #68.

Hence maybe we should start all over again: Show me why time dilation is genuine. We shall leave all the other disagreements behind. All the foregoing discussion was a waste of time but I was not the only one to blame.
And if you think time dilation is "flawed", do you think it's logically flawed in the sense of leading to some internal contradiction, despite the fact that no mathematicians seem to agree, or do you just think that the theory isn't the correct one for describing the real world? If the first, I promise that you're wrong and that it can be proved mathematically that no contradictions arise. If the second, then we should be discussing the experimental evidence in favor of time dilation (there's a whole lot of it), not a hypothetical example which assumes time dilation is real.
As I have declared, I do think time dilation is flawed. I like to find that it is not. I am not able to talk about the entire SR. It is too big to me.
Why is it hard to understand? 25 light-minutes is just the distance light travels in 25 minutes, just like a "light year" is the distance light travels in a year.
DrGreg knew why I had difficulty in understanding the 25 light-minutes. The distance between Bob and Ted was designed to be a constant 20 light-minutes. A non-believer in time dilation like me would have difficulty to understand that a constant distance would suddenly lose its constancy. DrGreg explained that it became into 25 minutes because of time dilation.
 
  • #173
Sam Woole said:
DrGreg knew why I had difficulty in understanding the 25 light-minutes. The distance between Bob and Ted was designed to be a constant 20 light-minutes. A non-believer in time dilation like me would have difficulty to understand that a constant distance would suddenly lose its constancy. DrGreg explained that it became into 25 minutes because of time dilation.
It didn't "become" 25 light-minutes because of time dilation. It always was a constant 25 light-minutes (according to Bob, and Ted, but not Alice).

The figure of 20 light-minutes that you calculated was incorrect because you obtained it by subtracting a time on Bob's clock from a time on Alice's clock. As I explained in the final paragraph of post #168, that is not a valid thing to do. All time intervals must be calculated using the same clock at the start and finish.

You say you are a "non-believer in time dilation". Why do you continue to take part in this forum? If you genuinely want to understand relativity, you will eventually have to be persuaded to become a believer. You will never understand it if you persist in your non-belief as a matter of faith.
 
  • #174
Sam Woole said:
Now I came to the realization that some thing was fundamentally wrong, with me, or with you, depending on what perspective we look at the issue. As soon as I saw DrGreg's tables, I assumed he was trying to convince me that time dilation is genuine. Now it turned out that my assumption was wrong. Instead, DrGreg was using the genuine time dilation to show me how it works. If time dilation is genuine, of course his demonstration (those tables) is genuine likewise.

But my wrong assumption was not entirely my fault becasue, all along I have repeatedly declared my disbelief in time dilation, as can be found from my posts before DrGreg's post #68.

Hence maybe we should start all over again: Show me why time dilation is genuine. We shall leave all the other disagreements behind. All the foregoing discussion was a waste of time but I was not the only one to blame.
In that case, maybe we should just discuss the experimental evidence in favor of time dilation? There's plenty of it--GPS navigation systems all assume time dilation in their calculations, the decay time of particles accelerated to relativistic speeds is slowed down by just the amount predicted by the time dilation experiment, and there have even been experiments where very precise atomic clocks were placed on board the space shuttle and found to have lost a few microseconds when they returned. If you're interested in learning more about any of this, I or others here could go into more detail about it.
Sam Woole said:
DrGreg knew why I had difficulty in understanding the 25 light-minutes. The distance between Bob and Ted was designed to be a constant 20 light-minutes. A non-believer in time dilation like me would have difficulty to understand that a constant distance would suddenly lose its constancy. DrGreg explained that it became into 25 minutes because of time dilation.
I was just talking about the distance between Bob and Alice, not Ted (I haven't been reading all the posts on this thread since the beginning, so I didn't even remember that a third observer was introduced). The distance between Bob and Alice was constantly changing, because they started at 0 distance from each other, then Alice flew away at constant velocity for a while (distance increasing), then she turned around and flew back towards Bob at constant velocity (distance decreasing) until they reunited (distance 0 again).
 
  • #175
Sam Woole said:
Hence maybe we should start all over again: Show me why time dilation is genuine. We shall leave all the other disagreements behind. All the foregoing discussion was a waste of time but I was not the only one to blame.
Now we're getting somewhere. Editorially, it shouldn't have required 11 pages of discussion for you to just come out and say you assume time dilation doesn't exist. All that math and case studies are utterly useless if you just simply don't accept their validity. And I do consider that to be all you - you shouldn't have been arguing the case studies if you didn't accept the basis of them them in the first place! Your arguments made it seem like you were confused about how the cases worked, not that you simply didn't accept what they were saying. And in addition, many people have posted physical evidence and you have ignored it (such as in my previous post).

So...
Show me why time dilation is genuine.
Well, you seem to have ignored all evidence posted so far... Could you comment on some of the evidence already posted? For example, my previous post, where I mentioned the GPS system? I just asked if you are aware of how the GPS system works - we can explain it to you if you need us to.
 
  • #176
russ_watters said:
Now we're getting somewhere. Editorially, it shouldn't have required 11 pages of discussion for you to just come out and say you assume time dilation doesn't exist. All that math and case studies are utterly useless if you just simply don't accept their validity. And I do consider that to be all you - you shouldn't have been arguing the case studies if you didn't accept the basis of them them in the first place! Your arguments made it seem like you were confused about how the cases worked, not that you simply didn't accept what they were saying. And in addition, many people have posted physical evidence and you have ignored it (such as in my previous post).
So... Well, you seem to have ignored all evidence posted so far... Could you comment on some of the evidence already posted? For example, my previous post, where I mentioned the GPS system? I just asked if you are aware of how the GPS system works - we can explain it to you if you need us to.

Okay, I am really sorry for all the mess. Please forgive me as you would do some other slow students. As to the GPS, I do not know a lot. It seemed there is plenty of information available on the internet. Just tell me how time dilation is applied to GPS because I have not found any (on the internet) to this effect.

I wish to point out, my reading of other dissident scientists showed, experiments (GPS included) claimed to be verifying the time dilation idea could be interpreted in different ways. In order to silence dissent, you should be able to show, time dilation is the only mechanism to make GPS work well, no other.
 
  • #177
You say you are a "non-believer in time dilation". Why do you continue to take part in this forum? If you genuinely want to understand relativity, you will eventually have to be persuaded to become a believer. You will never understand it if you persist in your non-belief as a matter of faith.
I became a non-believer after reading a lot about SR, not as a matter of faith. The reason for my taking part here was to test my belief. So far all your arguments have not persuded me to change my stand.
DrGreg said:
It didn't "become" 25 light-minutes because of time dilation. It always was a constant 25 light-minutes (according to Bob, and Ted, but not Alice).
The figure of 20 light-minutes that you calculated was incorrect because you obtained it by subtracting a time on Bob's clock from a time on Alice's clock. As I explained in the final paragraph of post #168, that is not a valid thing to do. All time intervals must be calculated using the same clock at the start and finish.
My disbelief became stronger as I continued finding that you were contradicting yourselves. The statements above was another example of contradiction. There you said "The figure of 20 light-minutes that you calculated was incorrect because you obtained it by subtracting a time on Bob's clock from a time on Alice's clock." No, I did not do that. Intead I was just repeating your words, such as: "So Bob’s clock looks to be 5 hours 20 minutes slow from Ted’s point of view. As Bob and Ted are a constant distance apart, this delay will be constant.", post #68. The "constant delay" you described here cannot be anything else but the "5 hours 20 minutes." It works out to be 20 light-minutes, not 25. It was you who did that kind of calculation, deducting the reading on one (Bob's) clock from the reading on another (Ted's) clock. Here you assumed that the two events (12:00 on one clock and 18:00 on another) happened simultaneously.
Again, in post #91, you gave these words:"Each row of this table represents a ray of light traveling from Bob to Ted. For example in the second row, a ray of light leaves Bob, at (a), at 12:10 Bob-time, passes Alice, at (d), at 12:15 Alice-time, and arrives at Ted, at (m), at 17:30 Ted-time. If you ignore Alice’s column and just look at the rays leaving Bob and arriving at Ted, each ray arrives at Ted(m) at a Ted-time that is 5h20m later than the Bob-time that it left Bob(a)." The table you referred:
Ted’s----------Ted’s--------Ted’s
view of own--view of------view of
clock----------Alice’s-------Bob’s
----------------clock---------clock
17:20(m)---12:00(a)-----12:00(a)
17:30(m)---12:15(d)-----12:10(a)
17:40(m)---12:30(g)-----12:20(a)
17:50(m)---12:45(j)------12:30(a)
18:00(m)---13:00(m)-----12:40(a).
Hence the words and numbers you used so far explicitly told unbelievers that the distance between Bob and Ted was constantly 20 light-minutes. I never calculated in the way you alleged. As a result of Alice's experience of the G-force, Bob's clock worked faster and accumulated more minutes than Alice's; and the distance became 25 light-minutes according to Bob's clock.
The return motion of Alice, the G-force she experineced, the Doppler effect, not only have caused Bob's clock work faster, but also have added a huge distance (5 light-minutes) to the original 20, according to your latest arguments. To SR dissidents, all this was physically impossible. Many of them charged that SR was a magic, intead of science.
You said "All time intervals must be calculated using the same clock at the start and finish." In order to do this kind of calculation, we should know the actual readings on the clock. But the 14:10 reading was a fictitious number created by another kind of calculation, the 3/2 rate. It was not an actual reading. I believe when we use fictitious readings to calculate time intervals, we get fictitious results, which cannot be believed.
 
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  • #178
Sam Woole said:
...my reading of other dissident scientists showed, experiments (GPS included) claimed to be verifying the time dilation idea could be interpreted in different ways...

Do any of these "dissident scientists" have any credability at all in the scientific community? What is the reason for their status?

Sam Woole said:
In order to silence dissent, you should be able to show, time dilation is the only mechanism to make GPS work well, no other.

You can't show that there is only one explanation for anything. It could be the fairies interferring. How can you show that it isn't Father Christmas behind it all? The point is that the other explanations are not good science.

Sam Woole said:
My disbelief became stronger as I continued finding that you were contradicting yourselves...

I think you were finding contradictions that aren't there. When they are explained, you just find another. Do you think the established scientific community haven't thought about it properly?

Sam Woole said:
The statements above was another example of contradiction.

I can see the pattern here. This will be explained to you in meticulous detail and you will find something else, or refuse to accept the answer.

<edit: Wrote this when I was a bit annoyed. No need to take it personally>
 
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  • #179
Sam Woole said:
I became a non-believer after reading a lot about SR, not as a matter of faith.

An obvious follow-up question to your statement is
"What specifically have you been reading (title and author, please)?"
 
  • #180
Sam Woole said:
I became a non-believer after reading a lot about SR, not as a matter of faith. The reason for my taking part here was to test my belief. So far all your arguments have not persuded me to change my stand.
My disbelief became stronger as I continued finding that you were contradicting yourselves. The statements above was another example of contradiction. There you said "The figure of 20 light-minutes that you calculated was incorrect because you obtained it by subtracting a time on Bob's clock from a time on Alice's clock." No, I did not do that. Intead I was just repeating your words, such as: "So Bob’s clock looks to be 5 hours 20 minutes slow from Ted’s point of view. As Bob and Ted are a constant distance apart, this delay will be constant.", post #68. The "constant delay" you described here cannot be anything else but the "5 hours 20 minutes." It works out to be 20 light-minutes, not 25. It was you who did that kind of calculation, deducting the reading on one (Bob's) clock from the reading on another (Ted's) clock. Here you assumed that the two events (12:00 on one clock and 18:00 on another) happened simultaneously.
Again, in post #91, you gave these words:"Each row of this table represents a ray of light traveling from Bob to Ted. For example in the second row, a ray of light leaves Bob, at (a), at 12:10 Bob-time, passes Alice, at (d), at 12:15 Alice-time, and arrives at Ted, at (m), at 17:30 Ted-time. If you ignore Alice’s column and just look at the rays leaving Bob and arriving at Ted, each ray arrives at Ted(m) at a Ted-time that is 5h20m later than the Bob-time that it left Bob(a)." The table you referred:
Ted’s----------Ted’s--------Ted’s
view of own--view of------view of
clock----------Alice’s-------Bob’s
----------------clock---------clock
17:20(m)---12:00(a)-----12:00(a)
17:30(m)---12:15(d)-----12:10(a)
17:40(m)---12:30(g)-----12:20(a)
17:50(m)---12:45(j)------12:30(a)
18:00(m)---13:00(m)-----12:40(a).
Hence the words and numbers you used so far explicitly told unbelievers that the distance between Bob and Ted was constantly 20 light-minutes. I never calculated in the way you alleged. As a result of Alice's experience of the G-force, Bob's clock worked faster and accumulated more minutes than Alice's; and the distance became 25 light-minutes according to Bob's clock.
The return motion of Alice, the G-force she experineced, the Doppler effect, not only have caused Bob's clock work faster, but also have added a huge distance (5 light-minutes) to the original 20, according to your latest arguments. To SR dissidents, all this was physically impossible. Many of them charged that SR was a magic, intead of science.
You said "All time intervals must be calculated using the same clock at the start and finish." In order to do this kind of calculation, we should know the actual readings on the clock. But the 14:10 reading was a fictitious number created by another kind of calculation, the 3/2 rate. It was not an actual reading. I believe when we use fictitious readings to calculate time intervals, we get fictitious results, which cannot be believed.
Sam, I just looked over the post mentioning Ted and I see that you are correct that Ted should be 25 light-minutes away from Bob, since he was supposed to be at the same position where Alice turned around. But what I don't understand is why you think there is a 20-minute light delay. Remember that Ted's clock was not synchronized with Bob's in the first place. So if Ted sees Bob's clock to be 5 hours 20 minutes behind his own, and he knows the light takes 25 minutes to pass between them, then he knows Bob's clock is really only 4 hours 55 minutes behind his own in their mutual rest frame. This has nothing to do with time dilation or relativity, it's just two clocks which are running at the same speed but which are out-of-sync, just like if I came from another time zone to visit you and forgot to reset my watch, we'd see that are watches displayed different times when we held them up next to each other. And there's nothing that says Bob and Ted's clocks must be out-of-sync by an exact hour amount.

Where in DrGreg's posts do you think he indicates that the light-delay between Bob and Ted is 20 minutes? Or if he doesn't say so explicitly, what lead you to infer this?
 
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  • #181
Sam Woole said:
I became a non-believer after reading a lot about SR, not as a matter of faith. The reason for my taking part here was to test my belief. So far all your arguments have not persuded me to change my stand.
Logically there are 3 views of time dilation you can take:
  1. You can take the positive view that it is true
  2. You can take the negative view that it is false
  3. You can take the neutral view that it might be true or false
The logic of my argument in this thread has been to show that if you begin with the neutral view, the demonstration should convince you, by the end, to take the positive view.

However, if you begin by taking the negative view, then, frankly, nothing is going to change your mind. Yes, you will find contradictions, but the contradictions are not between the parts of my argument. The contradictions are between my argument and your assumptions.

If time dilation is true, it must mean that clocks that were once synchronised do not remain synchronised. Your attacks on my argument rely (whether you realize it or not) on an implicit assumption that clocks do remain synchronised.

As this argument has been going round in circles for a long time, I see no point in continuing along this road. I think it would be good for you to follow the advice in post 175 and look at this from another angle.

As for GPS, I just searched this forum for “GPS dilation” and found https://www.physicsforums.com/showthread.php?t=87010", for example, and many more. I’m sure other readers may be able to quote better references.

For a whole list of various experimental evidence look at this thread: Experimental support for SR & GR.
 
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  • #182
Potential Solution

I first want to apologize that I have not yet read all the posts throughout this thread. However, I believe that I may be able to provide a way to add a visualized approach to this subject. Until just today, I myself was having difficulty entirely grasping relativity, especially SR and how simultaneity and time dilation occured.

Being a technical artist and animator with access to advanced 3d modeling and animation programs I decided to create some visual simulations that would help illustrate how simultaneity and time dilation operate. I've had many intelligent individuals attempt to explain how SR works in the past, yet never fully understood it. Words and even mathematical expressions and equations aren't enough sometimes. A picture is worth a thousand words, and an animaiton is worth even more. Upon viewing the animations that I created the relationships jumped out at me eliminating my previous doubts about the concepts of relativity.

Anyway, if members here believed that there would be enough of a use for animations such as these then I might be willing to improve them and make them presentable in a pre-rendered avi or mov format. Unfortunately, I'm currently without webspace so I don't have anywhere where I could upload the animations, but if there is a significant demand for these type of visual tools/simulations then I'll try to make something happen.

Sometimes, seeing really is believing.
 
  • #183
Sam Woole said:
Okay, I am really sorry for all the mess. Please forgive me as you would do some other slow students. As to the GPS, I do not know a lot. It seemed there is plenty of information available on the internet. Just tell me how time dilation is applied to GPS because I have not found any (on the internet) to this effect.
Then you must not have looked, because there is tons of material out there (some already linked). The gist of it is that the clocks on GPS satellites are adjusted prior to launch to run at a different rate than identical clocks on earth. After launch, they stay in sync with clocks on the ground, to within a very high degree of precision.
I wish to point out, my reading of other dissident scientists showed, experiments (GPS included) claimed to be verifying the time dilation idea could be interpreted in different ways. In order to silence dissent, you should be able to show, time dilation is the only mechanism to make GPS work well, no other.
This absurd request highlights that you really do have a strong faith-based belief that time dilation isn't real. Unless you start taking an honest and open-minded view of the subject, you will never understand it.

Sam, try this on: using the assumption that you are wrong, prove to us that you are right! :rolleyes:
 
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  • #184
DrGreg said:
As this argument has been going round in circles for a long time, I see no point in continuing along this road. I think it would be good for you to follow the advice in post 175 and look at this from another angle.
As for GPS, I just searched this forum for “GPS dilation” and found https://www.physicsforums.com/showthread.php?t=87010", for example, and many more. I’m sure other readers may be able to quote better references.
For a whole list of various experimental evidence look at this thread: Experimental support for SR & GR.

Thank you DrGreg for the links to the GPS and other evidence material. I shall take a good look at them and see whether I can be convinced. Regardless whatever may be the outcome, my scope of view will no doubt become wider.

My thanks also go to everybody else who have taken part in this thread such as Doc Al, JesseM and russ-waters, etc.

Wishing to see you again after I have digest all the material.

Sam.
 
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  • #185
EngineeredVision said:
...if members here believed that there would be enough of a use for animations such as these then I might be willing to improve them and make them presentable in a pre-rendered avi or mov format...

I'd be very interested to view your work. Unfortunately, I don't have any free time to contribute to your project.
 
  • #186
JesseM said:
I don't know how you got that conclusion from my words. What I said was: "The two twins don't disagree about what the other twin's clock reads--the traveling twin agrees that 3.944 years have passed on the earth-twin's clock, and the earth-twin agrees that 1 year has passed on the traveling twin's clock." So if they departed at the age of n, this sentence tells you that the traveling twin would agree that the earth-twin was n+3.944, and the earth-twin would agree that the traveling twin was n+1.

I can see this post is more than an year old and probably abandoned, but if by any chance the people that were posting here managed to travel forth through time and see this reply, i was wondering:

1) why do they have to be tweens?

2) (more serious mather) can anyone explain why this phenomena is occurring? Or is it just measured in clock ticks?


I'd be greateful to anyone who answered this post. (preferable during this lifetime as i don't yet understand what makes time travel possible)
 
  • #187
How about starting a new thread with a self-contained question? This thread is too long (and too argumentative) for me to want to wade through it - at least that's my opinion.
 
  • #188
I have a problem more related to GR.

I read that if you have a stop-watch on the ground and another on a very high tower the one at the top, from the perspective of an observer on the ground, would move slower.

I can deal with that.

But then I read, I think, that if the stopwatch was brought down from the tower the two stop-watches would be in unision, reading the same time...

Now did I read this wrong? That just doesn't make sense to me... Is this example true?

I would imagine that the stop-watch on the tower, when brought back to Earth, should be BEHIND the time of the stop-watch at the bottom.

Please clarify...
 
  • #189
I read that if you have a stop-watch on the ground and another on a very high tower the one at the top, from the perspective of an observer on the ground, would move slower.
It would tick faster.
But then I read, I think, that if the stopwatch was brought down from the tower the two stop-watches would be in unision, reading the same time...

Now did I read this wrong?
You probably read this wrong.

I would imagine that the stop-watch on the tower, when brought back to Earth, should be BEHIND the time of the stop-watch at the bottom.
Yes, but it would be ahead, not behind.
 
  • #190
Ok thanks... that all makes sense to me... Now I feel like reading more =)
 
  • #191
With all the material I have read about time dialatiion, I have learned that the only effect is the illusion the observer sees and there is no physical time difference once the object traveling and the observer are brought together.

There's a nice demonstration here http://uk.youtube.com/watch?v=KHjpBjgIMVk, which explains the theory in ways that even I could understand.
 
  • #192
bydavies said:
With all the material I have read about time dialatiion, I have learned that the only effect is the illusion the observer sees and there is no physical time difference once the object traveling and the observer are brought together.
Read up on the "Twin Paradox"; the time difference when the twins reunite is quite real.

There's a nice demonstration here http://uk.youtube.com/watch?v=KHjpBjgIMVk, which explains the theory in ways that even I could understand.
That's just an illustration of time dilation. Note that the ship and the Earth observers never get to reunite and compare clocks.
 
  • #193
You see what I mean now? ByDavies brought up the exact point that made me want to never try to understand time dilation - the suggestion that the clock moving faster at the top of the tower is only an illusion and no difference would be observable when the clocks are brought together again.

How does the act of bringing the clock back to Earth suddenly make the time go back to normal, Earth time?
 
  • #194
owenhbrown said:
You see what I mean now? ByDavies brought up the exact point that made me want to never try to understand time dilation - the suggestion that the clock moving faster at the top of the tower is only an illusion and no difference would be observable when the clocks are brought together again.
But as pervect pointed out, bydavies was wrong--"the time difference when the twins reunite is quite real."
 

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