Who Truly Experiences Slower Time in the Paradox of Time Dilation?

[JesseM]

A thought experiment - There are four clocks. Two are conventional clocks that are self-powered and two that are powered by pulses of light from a distant source.

When you say "powered", do you mean something like the idea that the clock will tick once each time it receives a light pulse from the source, and the source is sending out pulses once a second in its own rest frame?

All clocks are synchronised. A conventional and a pulse powered clock travel away from the other two along a curved path that keeps all clocks always the same distance from the source of the light pulse. When the “travelling” clocks return to the others, how would all clocks compare? (time to take some more medication).

If my understanding above is right, then the "powered" clock will naturally have ticked forward by the number of pulses the source has sent out since it left, which is the same as the number of ticks or a normal clock that's been sitting next to the source the whole time (since the source sent out one pulse each time the normal clock next to it ticked). So, the only clock that will show a different time is the normal clock that took the curved path away from and back to the other two that are next to the source, whereas the clock powered by light that took the curved path will read exactly the same as the other two when it returns to them.

 

[swerdna]

When you say "powered", do you mean something like the idea that the clock will tick once each time it receives a light pulse from the source, and the source is sending out pulses once a second in its own rest frame?

Yes. Sorry I wasn’t more specific.

If my understanding above is right, then the "powered" clock will naturally have ticked forward by the number of pulses the source has sent out since it left, which is the same as the number of ticks or a normal clock that's been sitting next to the source the whole time (since the source sent out one pulse each time the normal clock next to it ticked). So, the only clock that will show a different time is the normal clock that took the curved path away from and back to the other two that are next to the source, whereas the clock powered by light that took the curved path will read exactly the same as the other two when it returns to them.

If one clock orbits another is there any time dilation of the orbiting clock compared to the orbited clock? If so shouldn’t the frequency of the pulses of light effectively have sped up for the traveling pulse clock as it would have been time dilated compared to the pulse source?

 

[JesseM]

If one clock orbits another is there any time dilation of the orbiting clock compared to the orbited clock? If so shouldn’t the pulses of light effectively have sped up for the traveling pulse clock as it would have been time dilated compared to the pulse source?

They would be "sped up" relative to the normal clock next to it, but obviously the externally powered clock would say they were coming in at one pulse per second, since it's designed to tick forward by one second every time it receives a pulse.

 

[swerdna]

They would be "sped up" relative to the normal clock next to it, but obviously the externally powered clock would say they were coming in at one pulse per second, since it's designed to tick forward by one second every time it receives a pulse.

Isn’t the pulse emitter essentially a fifth clock being observed from a different frame by the traveling pulse clock? Wouldn’t that mean that it runs faster (hope that's correct) compared to the traveling pulse clock? In other words, how can clocks in different frames run at the same speed if one is time dilated compared to the other?

 

[JesseM]

Isn’t the pulse emitter essentially a fifth clock being observed from a different frame by the traveling pulse clock?

But the traveling pulse clock is receiving its pulses from the same emitter, right? If so, in what sense is it "observing" it? It has no independent basis for judging how fast the pulses are coming in, since again it's designed so that by definition the pulses come in at one tick per second.

Wouldn’t that mean that it runs faster (hope that's correct) compared to the traveling pulse clock? In other words, how can clocks in different frames run at the same speed if one is time dilated compared to the other?

The time dilation formula is designed to work for clocks that tick at the "correct" rate in their own rest frame, it doesn't tell you anything about incorrectly designed clocks like a clock that ticks based on external pulses. I could design a clock that went forward at 1 tick ever 3 seconds on Mondays, 1 tick every 12 seconds on Tuesdays, 1 tick ever 0.0001 seconds on Wednesdays, etc., presumably you see why it's obvious the time dilation formula isn't meant to apply to the ticks of this clock, the same is true for the "externally powered" clock.

 

[swerdna]

But the traveling pulse clock is receiving its pulses from the same emitter, right? If so, in what sense is it "observing" it? It has no independent basis for judging how fast the pulses are coming in, since again it's designed so that by definition the pulses come in at one tick per second.

The time dilation formula is designed to work for clocks that tick at the "correct" rate in their own rest frame, it doesn't tell you anything about incorrectly designed clocks like a clock that ticks based on external pulses. I could design a clock that went forward at 1 tick ever 3 seconds on Mondays, 1 tick every 12 seconds on Tuesdays, 1 tick ever 0.0001 seconds on Wednesdays, etc., presumably you see why it's obvious the time dilation formula isn't meant to apply to the ticks of this clock, the same is true for the "externally powered" clock.

Can’t quite follow your logic so will need to give it some thought - Thanks.

ETA - But a “correct” second for the traveling clocks is not the same as a “correct” second for the emitter and stationary clocks. Wouldn’t this mean that the frequency between pulses would effectively speed up for the traveling clocks? (or is that slow down?) Didn't know that it was possible to incorrectly design a clock that works.

 

[sylas]

I agree that my statement (1) of post #36 is incorrect and as diazona (and others) correctly pointed out “Something can be assumed to be moving in an absolute sense if it is accelerating.” I already knew this and am puzzled and disappointed that I wrote what I did.

No problem... thanks for clarifying, and glad to be back on the same page.

As I understand it acceleration itself doesn’t cause time dilation but apparently is somehow important because it establishes that a thing has changed its direction or speed and therefore experiences different “frames“. Unless things retain some form of memory of acceleration I can‘t see that which thing accelerates to cause it to move relative to something else is important.

The first sentence is, IMHO, an excellent description of the role of acceleration in special relativity. It's not the acceleration, per se, but the change in perspective.

There's no need of "memory". When you move frames, everything changes. If you "remember" what you were observing in the past, then you'll be able to infer that you've shifted into a new frame. But if you don't remember, then you can still just use current observations to infer your current circumstance, and draw the appropriate conclusions.

Specifically. In the case of traveling twins. Suppose that twin A remains at home, while twin B moves at 60% light speed for 8 years (by their own ship-clock) and then reverses to come back at 60% the speed of light.

Observations of twin A

Twin A infers that twin B has a clock running 80% slow. Hence, the 8 years ship time will be 10 years elapsed time by A's own clock, and the turn around occurs at a distance of 6 light years, 10 years after B left. Twin A actually sees this turn around 6 years after it occurs, because the light takes that long to get back from the turn around point. Hence, 16 years after B left, A will observe that B has turned around. The angular size of B in the sky indicates that the turn around occurred at 6 light years distance.

Everything makes sense to A, and they infer that B traveled for 10 years outbound, and 10 years back, or 20 years in total. With the 80% time dilation, twin A infers, correctly, that twin B will be 16 years older on return.


Observations of twin B

Twin B also observes twin A receding at first, at 60% light speed.

Just before the turn around at 8 years into the trip, twin B can see twin A in the far distance. They can tell, by red shift and the Doppler effect, that twin A is still receding at 60% light speed. They can tell, by the angular size of A in the sky, or the luminosity, that the light they are receiving is coming from 3 light years distance, and hence represents where A was three years previously.

Just after the turn around, twin B can see twin A in the far distance. They can tell, by blue shift and the Doppler effect, that twin A is approaching at 60% light speed. They can tell, by the angular size of A in the sky, or the luminosity, that the light they are receiving is coming from 12 light years distance, and hence represents where A was twelve years previously.

This is not a contradiction; it is simply a different frame of view. If twin B actually remembers what they had been observing just previously, then they could infer that they must have moved into a new reference frame -- even if this shift occurred with no acceleration, as if by a strange warp in space that simply turns the ship to a new direction without altering the time or place.

But suppose twin B merely notes down the expected age of A at the point where A apparently started to approach again.

Twin B can figure out that just before the shift in perspective 8 years into their journey, that they are seeing twin A as they were after 5 years. Hence, from time dilation, twin A will have aged 4 years.

On the second 8 years ship-time, twin B sees twin A approaching from a distance of 12 light years. The elapsed time of the trip for A, at 60% light speed, is 20 years, but with time dilation A is expected to age 80% of 20, or 16 years.

Total expected age of A, based on the observations of B, is 20 years. And that is just what they see when the twins are reunited.

I can’t see how relative movement is anything but symmetrical regardless of which thing accelerates. I guess what I find hard to accept about relativity is that it seems to consider things from abstract partial views (frames) and doesn’t consider a universal or omnipresent view.

You are not alone in finding it hard to understand.

However, it is definitely the case that the situations of the two twins are not symmetric at all. They observe very different things in what they actually see by looking at the other twin. Furthermore, a third observer will in general have no trouble seeing which twin was the one that reversed their direction of travel.

The whole point of relativity is that there ISN'T a universal view. There is no such thing as absolute motion: a velocity is always with respect to some observer.

Once you actually get this with all its logical implications, all the paradox evaporates. The whole situation is consistent, and both twins can calculate correctly the amount that the other one is expected to have aged, based on their own observations of the other twin during the trip.

Cheers -- sylas

 

[JesseM]

ETA - But a “correct” second for the traveling clocks is not the same as a “correct” second for the emitter and stationary clocks.

By "correct" I just mean it ticks at the same rate as any other correct clock that's right next to it and at rest relative to it throughout its trip. For example, one type of correct clock would be an atomic clock, which ticks based on the regular oscillations of certain types of cesium atoms. The pulse-powered clock isn't ticking at the same rate as the (presumably correct) self-powered clock, so the pulse-powered clock's time doesn't correctly match what relativity would say about the proper time along its worldline.

Wouldn’t this mean that the frequency between pulses would effectively speed up for the traveling clocks? (or is that slow down?)

Speed up or slow down relative to what? And when you say "travelling clocks", are you still talking about the clock whose rate of ticking is based on external light pulses, or are you talking about normal clocks?

 

[swerdna]

The whole point of relativity is that there ISN'T a universal view. There is no such thing as absolute motion: a velocity is always with respect to some observer.

I would say that there is potentially or theoretically a universal view but it not possible to achieve it by anything other than perhaps the universe itself. A limitation of observation isn’t a limitation of existence. I would also say that a velocity is always with respect to some other thing rather than observer. I‘ve never been able to see any significance in whether a thing is observed or not. Things exist and do things regardless of whether they are observed or not. A tree that falls in a forest makes a noise regardless of whether there is a listener or not. If A has a velocity compared to B then B has the exact same velocity compared to A. I guess I’m saying that I believe velocity should be attributed to the relative movement of things and not the actual things.

Here’s mud in your eye (aka cheers ;-)

 

[swerdna]

By "correct" I just mean it ticks at the same rate as any other correct clock that's right next to it and at rest relative to it throughout its trip. For example, one type of correct clock would be an atomic clock, which ticks based on the regular oscillations of certain types of cesium atoms. The pulse-powered clock isn't ticking at the same rate as the (presumably correct) self-powered clock, so the pulse-powered clock's time doesn't correctly match what relativity would say about the proper time along its worldline.

Speed up or slow down relative to what? And when you say "travelling clocks", are you still talking about the clock whose rate of ticking is based on external light pulses, or are you talking about normal clocks?

“Travelling” clocks are the two (one conventional and one pulse) that accelerated to a different frame from the other two (one conventional and one pulse) and the pulse emitter that are all “stationary“. Sorry but I have no more time to continue right now.

 

[swerdna]

Speed up or slow down relative to what?

Relative to a second in the frame of the traveling clocks which is dilated relative to a second in the frame of the light emitter and the stationary clocks. Each frame has different “proper” times. I realize that relativity says that light travels at c regardless of which frame it travels through but I’m talking about the periods between the light pulses. Essentially the period between the pulses is what time is isn’t it? Rather than faster or slower perhaps dilated would be more correct.

 

[JesseM]

Relative to a second in the frame of the traveling clocks which is dilated relative to a second in the frame of the light emitter and the stationary clocks.

But do you understand that the clock powered by external pulses will also be dilated relative to coordinate time in its own rest frame? (and relative to the normal clock traveling along with it, which is running at the same rate as coordinate time in their rest frame) The notion of coordinate time in a "frame" is assumed to match that of what I have called "correct" clocks that are at rest in that frame.

Each frame has different “proper” times.

"Proper time" is not frame-dependent, the term refers to the time along a particular worldline (between one specified point on the worldline and another), as measured by a (correct) clock moving along that worldline. All frames agree in their predictions about the proper time along a worldline.

I realize that relativity says that light travels at c regardless of which frame it travels through but I’m talking about the periods between the light pulses. Essentially the period between the pulses is what time is isn’t it?

No, time is what is measured by a correct clock that agrees with other correct clocks. You can build a "light clock" based on the period of light, but this is based on having the clock tick each time the light travels a certain distance along the clock itself, not on ticking every time it receives a pulse from some external source. You can also build clocks whose time isn't based on light at all, like atomic clocks or clocks whose ticking is based on springs. I'll repost I said about the notion of time in an older thread:

Well, I'd say time is an abstraction based on the fact that we see various physical objects which exhibit regular cycles (like the atomic oscillations that atomic clocks are based on) such that when the objects are next to each other the ratio of their cycles remains constant. For example, if I have an atomic clock based on oscillations of cesium 133 atoms, and a spring clock which ticks in the units we label as "seconds", then if you place them next to each other on Earth you'll find the atomic clock always registers around 9,193 billion ticks between each tick of the spring clock (it will depend on how good the spring clock is of course, nowadays a second is supposed to correspond to exactly 9192631770 oscillations of such a cesium 133 clock). If you take a second atomic clock/spring clock pair which is physically identical to the first and take them on a relativistic journey through space and then return them to Earth, the pair that took the journey will have registered less ticks than the pair that remained on Earth, but the ratio between the number of ticks registered on the atomic clock that took the journey and the number of ticks registered on the spring clock that took the journey should still be about 9,193:1, assuming both clocks were next to each other as they traveled so their velocity at each moment (in whatever frame we choose) would have been the same. From this you can abstract that all paths through spacetime have a certain "proper time" along them, different clocks will divide the proper time into different increments but the ratio between ticks of different clocks should stay the same as long as they take the same path through spacetime.

 

[swerdna]

Another check to see if I understand things correctly (once again in layman-speak)

(1) A thing that is not accelerating can’t correctly be defined as being either moving or stationary.

(2) A thing that is accelerating can be correctly defined to be moving.

(3) After a thing has been through a period of acceleration (and is no longer accelerating) it can’t correctly be defined as being either moving or stationary.

 

[swerdna]

But do you understand that the clock powered by external pulses will also be dilated relative to coordinate time in its own rest frame? (and relative to the normal clock traveling along with it, which is running at the same rate as coordinate time in their rest frame) The notion of coordinate time in a "frame" is assumed to match that of what I have called "correct" clocks that are at rest in that frame.

"Proper time" is not frame-dependent, the term refers to the time along a particular worldline (between one specified point on the worldline and another), as measured by a (correct) clock moving along that worldline. All frames agree in their predictions about the proper time along a worldline.

No, time is what is measured by a correct clock that agrees with other correct clocks. You can build a "light clock" based on the period of light, but this is based on having the clock tick each time the light travels a certain distance along the clock itself, not on ticking every time it receives a pulse from some external source. You can also build clocks whose time isn't based on light at all, like atomic clocks or clocks whose ticking is based on springs. I'll repost I said about the notion of time in an older thread:

Thanks - That will take some time to read and digest.

 

[JesseM]

Another check to see if I understand things correctly (once again in layman-speak)

(1) If a thing is not accelerating it can’t correctly be defined as being either moving or stationary.

Not in any absolute sense, no, although it can be defined as moving or stationary relative to a particular choice of inertial reference frame.

(2) A thing that is accelerating can be correctly defined to be moving.

You can't say it's moving at any given instant even if you stick to inertial frames, since at any instant there will be some inertial frame where it's instantaneously at rest. You can say that its velocity is changing in every inertial frame though. It's also possible to use non-inertial coordinate systems in which even an accelerating object (in the sense of an object experiencing G-forces) is at rest, but the laws of physics don't take the same form in such a coordinate system that they take in inertial frames (for example, the speed of light is not necessarily constant in non-inertial frames like it is in inertial frames).

(3) After a thing has been through a period of acceleration (and is no longer accelerating) it can’t correctly be defined as being either moving or stationary.

Again, not in any absolute, frame-independent sense.

 

[JM]

To p.tryon and swerdna: Keep questioning, you have much correct.
To all: The discussion on the thread 'In twin paradox, please help' in relavent here, the twins were diiscussed at length there.

 

[JM]

To Sylas: I like your idea of treating each twin separately.
Re twin B: Toward the end you say '...A is expected to age 80% of 20 years, or 16 years.' Doesn't this mean that twin B calculates A to be younger when they reunite? If so then each thinks the other is younger when they reunite.

 

[JM]

Again re post #57: The view of twin B can be found as follows. On his outbound segment he is inertial, and so is entitled to consider himself to be at rest and to use the usual formula to calculate the time dilation of A's clock. The time on the clocks is not affected by the turnaround if it is quick enough. After turnaroound B is inertial again and can calculate A's dilation as before. B's view of A's motions is the same as A's view of B's motions, so each will calculate the same dilation, and each will calculate the other to be younger when they reunite.

 

[sylas]

I am the author of message [post=2185614]msg #57[/post] to which JM refers.

Again re post #57: The view of twin B can be found as follows. On his outbound segment he is inertial, and so is entitled to consider himself to be at rest and to use the usual formula to calculate the time dilation of A's clock. The time on the clocks is not affected by the turnaround if it is quick enough. After turnaroound B is inertial again and can calculate A's dilation as before. B's view of A's motions is the same as A's view of B's motions, so each will calculate the same dilation, and each will calculate the other to be younger when they reunite.

That is incorrect.

The fundamental error is this statement: The time on the clocks is not affected by the turnaround if it is quick enough.

That's incorrect, because in fact, time and distance all depend on an observer. When you turn around, there is a shift of the observer into a new inertial frame, in which everything is different. That's a bedrock fact about physics that you have failed to take into account.

Note that the twins are not together to compare their clocks directly at the turn around point. All they can actually see is light that left the other clock a long time ago. Conclusions about what is happening at that "same time" (from their perspective) are inferences, not observations.

I described a case in which twin A stays at home, while twin B sets out at 60% of the speed of light, for 8 years according to their own spaceship clock.

In this case, A observes B for 16 years with a redshift, and 4 years with a blue shift. At 60% light speed, the clock is viewed running slow, or fast, by a factor of 2. This factor includes both the time dilation and also the effects of approach or recession on the light travel time. A thus calculates B ages 16/2 + 4*2 = 16 years.

On the other hand B observes A for 8 years with a redshift, and 8 years with a blue shift. B thus calculates A has aged 8/2 + 8*2 = 20 years.

Each twin makes different observations, and is able to correctly calculate the elapsed age of the other. The twin who turned around aged 16 years. The one who stayed home aged 20 years.

If you calculate anything different, you aren't using relativity, and you are calculating incorrectly.

Cheers -- sylas

 

[JesseM]

Again re post #57: The view of twin B can be found as follows. On his outbound segment he is inertial, and so is entitled to consider himself to be at rest and to use the usual formula to calculate the time dilation of A's clock. The time on the clocks is not affected by the turnaround if it is quick enough.

You are ignoring the relativity of simultaneity! Even if the turnaround is instantaneously brief, the time on twin A's clock at the moment of the turnaround in the inertial frame where B was at rest during the outbound phase of the journey is very different from the time on A's clock at the moment of the turnaround in the inertial frame where B was at rest during the inbound phase of the journey, you can't combine results from two frames that way without considering simultaneity issues. Did you read my post #112 in response to you on this thread? I gave a numerical example there which illustrates this point. You might also look at my discussion with otg about the issue of simultaneity and the twin paradox in this thread.

 

[swerdna]

Not in any absolute sense, no, although it can be defined as moving or stationary relative to a particular choice of inertial reference frame.

You can't say it's moving at any given instant even if you stick to inertial frames, since at any instant there will be some inertial frame where it's instantaneously at rest. You can say that its velocity is changing in every inertial frame though. It's also possible to use non-inertial coordinate systems in which even an accelerating object (in the sense of an object experiencing G-forces) is at rest, but the laws of physics don't take the same form in such a coordinate system that they take in inertial frames (for example, the speed of light is not necessarily constant in non-inertial frames like it is in inertial frames).

Again, not in any absolute, frame-independent sense.

The use of the word “absolute” suggests that something less then absolute is okay to use to correctly define the reality of the universe. I don’t agree with this. Rather than “absolute” I think words like “real“, “actual“ or “correct“ would be more scientifically valid. If it CAN’T be established that anything is actually stationary, and that anything thing that is not accelerating is actually stationary or moving, then it CAN’T - PERIOD! Please explain why it is considered scientifically valid to take an incomplete, abstract perspective from a single frame to represent the actual reality of the universe. This is what I really don’t understand and have difficulty accepting.

 

[diazona]

The use of the word “absolute” suggests that something less then absolute is okay to use to define the reality of the universe. I don’t agree with this.

In a sense, I think that's your problem. As long as you don't believe that statement, you cannot correctly understand relativity.

Rather than “absolute” I think words like “real“, “actual“ or “correct“ would be more scientifically valid. If it CAN’T be established that anything is actually stationary, and that anything thing that is not accelerating is actually stationary or moving, then it CAN’T - PERIOD! Please explain why it is considered scientifically valid to take an incomplete, abstract perspective from a single frame to represent the actual reality of the universe. This is what I really don’t understand and have difficulty accepting.

It is not scientifically valid to take a single frame, and no other frame, to represent the "actual reality of the universe" - that is a fundamental assumption of relativity, that there is no single, unique a.k.a. absolute "actual reality of the universe." None of the people who are trying to explain this to you have taken a single reference frame to represent some "actual reality."

It is simply the case that the laws of physics are valid in any inertial reference frame. The perspective from a single reference frame is not "incomplete" or "abstract" (I am slightly curious as to why you make that statement). No one reference frame has any more right to be called "absolute" than any other. For any given problem/physical situation, you just pick the reference frame that is most convenient for your purposes.

 

[JesseM]

The use of the word “absolute” suggests that something less then absolute is okay to use to correctly define the reality of the universe. I don’t agree with this. Rather than “absolute” I think words like “real“, “actual“ or “correct“ would be more scientifically valid. If it CAN’T be established that anything is actually stationary, and that anything thing that is not accelerating is actually stationary or moving, then it CAN’T - PERIOD! Please explain why it is considered scientifically valid to take an incomplete, abstract perspective from a single frame to represent the actual reality of the universe. This is what I really don’t understand and have difficulty accepting.

I don't follow, why do you say I am taking the a single frame's perspective to "represent the actual reality of the universe"? The whole point is that there is no "actual reality" about whether something is moving or stationary, just like there is no "actual reality" about which direction in space is "up" and which is "down"--these questions are intrinsically dependent on human choices about how to assign coordinates to points in spacetime, they have no objective reality.

 

[sylas]

Reminder: the notion of "real" or "actual" is not the same as the notion of "absolute".

For a completely trivial example... the velocity of a particle with respect to an observer is REAL. It's an ACTUAL unambiguous value. It's just not "absolute" in the sense that it is the same value for every observer.

Lots of things depend on velocity. Energy, and momentum, and time dilation, for example. All of these things are not absolute either -- but they ARE real.

One of the things that lies behind almost all the confusion in these threads is that simultaneity and distance is not absolute, but depend on velocity. Two observers at precisely the same time and place will have different real values for the distance to another unambiguously identified time and place, and for the time at which the remote event occurred.

This does NOT mean it isn't "real". Only that it isn't absolute. This is counter intuitive at first, but it is not illogical or paradoxical or an indication of being "unreal".

For example. Suppose we have two twins, who synchronize their watches to zero. One twin then moves off for 8 years by their clock, turns around, and comes back for eight years, at 60% light speed all the way both directions.

The stay at home twin sets off a nuclear bomb 4 years after the other left, by their own watch. This is carefully chosen so that the light from the explosion reaches the other twin at the moment of turn around. This bomb defines an unambiguous event, in space and time.

However, particular time and location of the event will depend on the observer's perspective. The time and location of the event shifts when the traveling twin turns around.

For the traveling twin, outbound, the bomb went off at a location 3 light years away, and was seen at time 8. Hence it occurred 3 years previously, at time 5. They will see the explosion redshifted (because it is receding) and the distance of 3 light years can be obtained from its angular size in the sky.

Then they turn around, still seeing the same explosion event in the distance. The explosion is suddenly blueshifted, because now in this new perspective it is approaching, not receding. The angular size of the explosion in the sky suddenly reduces, corresponding precisely to the new perspective of the traveler. The explosion is now identified -- correctly -- as occurring at a distance of 12 light years, and hence 12 years previously.

That sounds weird, but it's still real. It is just a change in perspective. It's analogous to something being at a different direction in the sky when you turn around. The direction is real, and it is relative, not absolute. Directions, distances, times, etc are all real, but they are relative to an observer. Not absolute.

 

[JM]

Dear Sylas,
Re Post #57:
My source for the twin paradox is Einsteins 1905 paper ' On the Electrodynamics of Moving Bodies", Part I, paragraph 4. There he explains his 'peculiar consequence' ( later called the clock paradox, then the twin paradox) completely and (relatively) clearly without using light signals, red/blue shifts, Doppler, or visual size observations. Why do you include these factors?

 

[JM]

Dear JesseM,
Re Post #70: You seem to be saying that each time the moving clock changes direction the time on the stationary clock jumps to a new value. I recall you saying that the time dilation for a polygonic path can be calculated by summing the dilations for each segment of the path without any change of the clocks due to the change of direction ( if the change is quick enough.) There are no such jumps in Einsteins calculations, that I could find. So what's going on?

 

[sylas]

Dear Sylas,
Re Post #57:
My source for the twin paradox is Einsteins 1905 paper ' On the Electrodynamics of Moving Bodies", Part I, paragraph 4. There he explains his 'peculiar consequence' ( later called the clock paradox, then the twin paradox) completely and (relatively) clearly without using light signals, red/blue shifts, Doppler, or visual size observations. Why do you include these factors?

You can explain relativity in many different ways. They are all mutually consistent with each other. For some people it helps to look at the real concrete physical details of what the twins actually observe. The observations all follow directly from the theory of the 1905 paper, but if people don't understand that then they are going to need a different explanation. Different people will find different ways of looking at it useful.

You don't understand the 1905 paper correctly as yet, as far as I can tell. People will continue to try and explain it to you. Ich put it very simply in [post=2172259]msg #108[/post] of thread "In twin paradox, Please Help", and he's right.

Einstein's own explanations are fine, but many people still get it wrong, and make assertions about the traveling twins example that are false. Such assertions are inconsistent with Einstein's 1905 paper, but the errors need to be explained by someone else, and with specific explicit reference to the later errors being made by someone who may need a bit of help getting the details correct.

I am not sure, but I think you are reasoning as follows. Please tell me if I have this incorrect. I am using the case where all relative movement is 60% light speed, and in which the traveling twin moves 6 light years in 10 years according to the stay at home twin, and then returns. In this case the traveling twin ages 16 years; and they experience a shift in perspective 8 years into their journey, according to their own clock.

Some people think of it like this:

• Twin B is traveling. During the first 8 years, they consider that A will have aged 6.4 years. (80% dilation).
• During the next 8 years, they consider that A will have aged another 6.4 years, for 12.8 in total.

The error in the above method of analysis is in treating simultaneity as absolute. Let X be the event of the traveling twins clock marking 8 years. This is the turn around event.

How old is the other twin at this time? Even posing the question makes the error. There is no unique "at this time". In the instant that the traveling twin turns around, all assumptions about what is simultaneous changes. By adding together the 6.4 + 6.4 you are effectively assuming that there is a uniquely identified moment for the stay-at-home twin that can be uniquely identified with a particular moment for the traveling twin. That's false, and the 1905 paper explains that it is false – although it does not pick apart your specific mistake. You need someone else to explain it for you. If my explanations don't help, then you may benefit from someone else's work. But for you, so far, that 1905 paper has not been enough because you don't understand it all yet. No offense intended.

One way to get the correct answer is to single out the event of the stay-at-home twin sending out a light speed message which reaches the traveler at precisely the turn around instant. That's what I tried to explain previously, and then again using a bomb set off to identify the event precisely.

THIS event IS uniquely identified. THIS, after all, is when the other twin actually observes a change in velocity of the stay-at-home twin!

I also am attempting to correct a common confusion about acceleration.

A lot of people suggest that it is acceleration that changes things. That's true, but not because the acceleration causes a dilation of its own. The relevance of acceleration is that it marks transition into a new reference frame. It is certainly misleading to say that at a certain time all the dilation takes effect. The effects of time dilation and aging and so on depend on whole spacetime trajectory.

What matters about turning around is that there is a change in perspective, or a new inertial frame. We can imagine a space-warp for the traveling twin, in which there is a twist in space time that reverse the direction of the ship with no experience of acceleration at all. As long as the world line is continuous, each twin can correctly calculate the age experienced by the other, using nothing more than the theory of the 1905 paper.

If my own explanations don't help everyone that's fine, and perfectly normal. Some people don't need any additional explanations or ways of looking at it beyond Einstein's own seminal paper in 1905. But you are going to need a bit of extra help. That is completely normal. I was the same.

Cheers -- sylas

 

[JesseM]

Dear JesseM,
Re Post #70: You seem to be saying that each time the moving clock changes direction the time on the stationary clock jumps to a new value.

Not in any single inertial frame, no. But if you consider two segments A and B of the non-inertial clock's worldline which are joined by an instantaneous acceleration, then look at frame A where the non-inertial clock was at rest during segment A and frame B where the non-inertial clock was at rest during segment B, then the time on the inertial (stationary) clock at the moment of acceleration in frame A can be totally different than the time on the inertial clock at the moment of acceleration in frame B (though both frames agree about the time on the non-inertial clock at the moment it accelerated).

I recall you saying that the time dilation for a polygonic path can be calculated by summing the dilations for each segment of the path without any change of the clocks due to the change of direction ( if the change is quick enough.)

Sure, but you do this by picking a single inertial frame and calculating the time elapsed on each segment by taking the coordinate time in that frame between the beginning and end of that segment and multiplying by the time dilation factor sqrt(1 - v^2/c^2), using the velocity v the clock had in that segment relative to your choice of frame.

There are no such jumps in Einsteins calculations, that I could find. So what's going on?

Einstein's calculations didn't involve switching between frames when adding different segments.

 

[swerdna]

I have no academic qualifications and left a very poor education system aged 14. I can only explain and learn therefore in relative layman-speak.

The following doesn’t consider the effects of circular motion or gravity . . .

I understand that nothing can be said to be actual stationary and in fact I don’t see that it can be known or even assumed that a stationary state actually exists. That a thing can’t move relative to itself and that some things don’t move relative to other things doesn’t mean that anything is ever actually stationary. Acceleration is a change of a thing’s speed and/or direction but it can’t be correctly define that the change is either an actual increase or decrease in speed. The only lasting effect of acceleration is how a thing moves relative to other things. When a thing isn’t accelerating it can’t be correctly defined as being either stationary or moving.

As I understand Relativity it says that non-accelerating things that don’t move relative to each other are in a particular inertial frame. Other non-accelerating things that do move relative to the things in this particular inertial frame are in different particular inertial frames. Acceleration takes a thing from one frame to another and a thing can only be in one particular frame at a particular time. If a thing accelerates from one frame to another then back to the original it will be time dilated compared to all things that remained in that frame. It seems that it doesn’t matter what particular frame the accelerating thing started in or how many different frames it experiences it will always be time dilated when it returns to the original frame and never time increased. Relativity seems to give preferred importance to the original frame and I can’t understand any reason why it’s justified in doing this.

The silhouette of a rod that has a length equal to it’s diameter will be observed from an end-on observational frame as a circle and from a side-on observational as a square. Each observational frame on only allows an incomplete, abstract observation and neither observational frame correctly represents the actual overall shape of the rod. As I see it observations from inertial frames are similar incomplete, abstract observation.

A and B share the same fame then B accelerates to another frame and A and B continuously move apart. B accelerates again in the opposite direction and returns to the same frame and position as A. As it can’t be correctly defined that A and B were actually stationary to being with it also can’t be correctly defined that either periods of acceleration in either direction were an increase or decrease in the actual speed of B. Neither can it be correctly defined that the B turned around as the accelerations may have merely caused B to travel faster then slower or slower then faster relative to the direction that A and the original frame may have been moving to begin with. Rather than consider the events from a particular frame I think all frames should be considered concurrently and no frame should be given any preference over any other.

If the turn around is so important in the twins paradox (that's not a paradox) what would happen in the following scenario that doesn’t have a turn around?

Clock A and clock B share the same inertial frame (frame 1) but are a great distance apart. At the exact mid-point between the clocks (also in frame 1) there is a thing that emit’s a light pulse every second. The clocks tick every time they receive a light pulse so both clocks are synchronised and always show the same time. Also in frame 1 with clock A there is Clock C that ticks every second by some independent onboard means. All clocks are synchronised and show the same time. Clock C accelerates way from clock A and toward clock B in frame 2 until it reaches clock B. It accelerates again to be positioned with clock B in frame 1. Clock C went from frame 1 to frame 2 then back to frame1 without turning around. Clock C then compares it’s time with clock B which always shows the same time as clock A. Essentially clock C is also comparing it’s time with clock A regardless that it's a great distance away. According to Relativity wouldn’t clock C be time dilated compared to the other two clocks without having to have turned around?

 

[sylas]

I understand that nothing can be said to be actual stationary and in fact I don’t see that it can be known or even assumed that a stationary state actually exists. That a thing can’t move relative to itself and that some things don’t move relative to other things doesn’t mean that anything is ever actually stationary. Acceleration is a change of a thing’s speed and/or direction but it can’t be correctly define that the change is either an actual increase or decrease in speed. The only lasting effect of acceleration is how a thing moves relative to other things. When a thing isn’t accelerating it can’t be correctly defined as being either stationary or moving.

I think you are still mixing up the notions of "actual" motion with "absolute" motion.

Objects most certainly CAN be stationary, or moving. It is simply that movement is always quantified in relation to some chosen reference. If you pick a different reference point, you get different velocities, but they are no less real for that.

You seem to think that because the measure of velocity depends on the frame of reference, that it all becomes entirely arbitrary and unreal. That's misleading. Any particle has a trajectory through spacetime, and this defines unambiguously a definite and real velocity for a given frame of reference.

As I understand Relativity it says that non-accelerating things that don’t move relative to each other are in a particular inertial frame. […]

You can omit "that don't move relative to each other".

All non-accelerating particles have an associated inertial frame within which they are stationary. Such particles DO move relative to other particles, with an unambiguous real relative velocity. The "relative velocity" of one particle to another means the velocity of the first particle in the reference frame of the second particle. It's real, and well defined.

[…] Other non-accelerating things that do move relative to the things in this particular inertial frame are in different particular inertial frames. Acceleration takes a thing from one frame to another and a thing can only be in one particular frame at a particular time.[…]

Yes. Exactly so.

[…] If a thing accelerates from one frame to another then back to the original it will be time dilated compared to all things that remained in that frame. […]

Misleading. You don't need accelerations at all. Any particle moving in relation to another particle is time dilated with respect to that particle. Even with unaccelerated motions. But the time dilation, like the velocity, is relative to a particular observer.

The issue of acceleration, or change of reference frame, only arises because some people find it paradoxical that A is time dilated with respect to B, and that B is also time dilated with respect to A. They want to know which one is "really" time dilated.

The answer is that time dilation is not absolute, but relative. The two particles are really moving relative to each other, and each particle is really stationary in its own reference frame. Similarly, each particle is undilated in its own reference frame, and each particle runs more slowly in the reference from of the other.

So people try to get a paradox, by having one particle turn around and come back so that they can compare clocks. Problem is, you can't do that without a third reference frame.

(Addendum. Once two particles are together again at the same place and time, there is a definite and unambiguous elapsed time experienced by each particle since they were previously synchronized. This is called the "proper time" of the particle, and it can be calculated from their respective trajectories through spacetime. This "experienced time" of a particle is not dependent on an observer. It's real. It can be calculated and has only one possible answer.)

It seems that it doesn’t matter what particular frame the accelerating thing started in or how many different frames it experiences it will always be time dilated when it returns to the original frame and never time increased. Relativity seems to give preferred importance to the original frame and I can’t understand any reason why it’s justified in doing this.

This is not a preference for one specific reference frame, but a general rule about moving in a straight line.

The shortest distance between two points is a straight line. If you have two particles together at time t₀, and then back together at time t₁, AND if particle A has remained inertial the whole time, then when you look at their motions in ANY reference frame you like, particle A has moved in a straight line, and particle B… hasn't.

Therefore, particle B has traveled a greater distance than A, considered in any frame you like. Therefore B has a greater average velocity, when considered in any inertial reference frame you like. Therefore B will be older when the two twins are together again.

Unambiguously. The age difference is a real age difference, with a definite value, that can be calculated in any reference frame at all, using special relativity.

I'm skipping a bit here…

If the turn around is so important in the twins paradox (that's not a paradox) what would happen in the following scenario that doesn’t have a turn around?

Clock A and clock B share the same inertial frame (frame 1) but are a great distance apart. At the exact mid-point between the clocks (also in frame 1) there is a thing that emit’s a light pulse every second. The clocks tick every time they receive a light pulse so both clocks are synchronised and always show the same time. Also in frame 1 with clock A there is Clock C that ticks every second by some independent onboard means. All clocks are synchronised and show the same time. Clock C accelerates way from clock A and toward clock B in frame 2 until it reaches clock B. It accelerates again to be positioned with clock B in frame 1. Clock C went from frame 1 to frame 2 then back to frame1 without turning around. Clock C then compares it’s time with clock B which always shows the same time as clock A. Essentially clock C is also comparing it’s time with clock A regardless that it's a great distance away. According to Relativity wouldn’t clock C be time dilated compared to the other two clocks without having to have turned around?

If A and B share the same reference frame, then they have no relative motion with each other.

As soon as C is moving relative A and B it becomes time dilated with respect to A and B and the central clock, but the rate at which it receives ticks from the central clock is not a measure of its own time dilation at all. When directly approaching the clock, it gets the ticks more rapidly. When receding at the same speed, it gets them less rapidly. The time dilation, however, is the same in each case. But this much is definitely true. If C starts out from the middle, and moves around for a while at high speed and then comes back to the middle again, then C will have received pulses, on average, faster than the stationary clock was emitting them.

Cheers -- sylas

 

[swerdna]

If A and B share the same reference frame, then they have no relative motion with each other.

As soon as C is moving relative A and B it becomes time dilated with respect to A and B and the central clock, but the rate at which it receives ticks from the central clock is not a measure of its own time dilation at all. When directly approaching the clock, it gets the ticks more rapidly. When receding at the same speed, it gets them less rapidly. The time dilation, however, is the same in each case. But this much is definitely true. If C starts out from the middle, and moves around for a while at high speed and then comes back to the middle again, then C will have received pulses, on average, faster than the stationary clock was emitting them.

Sorry but I obviously didn’t make it clear enough that the ticking of clock C isn’t governed by the pulses of light and that it ticks in a conventional self-contained manner. When all clocks are in frame 1 they all tick at the same rate. As I understand Relativity clock C will time dilate compared to clocks A and B by the same amount but it didn’t need to turn around in the process. My understanding of earlier posts in this thread was that turning around was important.

 

[sylas]

Sorry but I obviously didn’t make it clear enough that the ticking of clock C isn’t governed by the pulses of light and that it ticks in a conventional self-contained manner. When all clocks are in frame 1 they all tick at the same rate. As I understand Relativity clock C will time dilate compared to clocks A and B by the same amount but it didn’t need to turn around in the process. My understanding of earlier posts in this thread was that turning around was important.

When C is moving, its clock is running more slowly than A and B, when considered from the perspective of A and B. From the perspective of C (in constant motion), the clock of A and B is running more slowly.

Both perspectives are equally correct.

The time dilation is the same, regardless of the direction of motion. What happens when you change direction is that the notion of simultaneity changes, and this means that a particle which has turned around cannot calculate the elapsed time of another particle just by adding up two times, before and after the turn around moment. The reason is that there is no unique moment that is the "same time" as the turn around instant for the other particle.

The relevance of C counting the pulses of the other clock is that this confirms a real difference in elapsed time. C can count the rate at which pulses are received, measuring time by their own local self-contained clock. When C gets back to the central inertial clock, C will have experienced less elapsed time by their own self-contained clock than is given with the inertial clock. Hence C also measures a greater average rate for receiving pulses from the central clock, than A or B.

 

[JesseM]

A and B share the same fame then B accelerates to another frame and A and B continuously move apart. B accelerates again in the opposite direction and returns to the same frame and position as A. As it can’t be correctly defined that A and B were actually stationary to being with it also can’t be correctly defined that either periods of acceleration in either direction were an increase or decrease in the actual speed of B. Neither can it be correctly defined that the B turned around as the accelerations may have merely caused B to travel faster then slower or slower then faster relative to the direction that A and the original frame may have been moving to begin with. Rather than consider the events from a particular frame I think all frames should be considered concurrently and no frame should be given any preference over any other.

All inertial frames are considered equal in relativity. An inertial frame is defined in terms of a hypothetical network of rulers and clocks that are moving inertially for all eternity--they never accelerate (and changes in speed or direction qualify as accelerations, both cause an observer to experience G-forces as measured by an accelerometer moving along with them). And I think you're misunderstanding the point about the importance of a "turn around" in the twin paradox--you're quite right that there could be an inertial frame where the twin who changes velocity never actually changes direction, but the mere fact that he changed velocity midway through the trip while the other twin had a constant velocity ensures that when they reunite, the twin that changed velocity will be younger.

Clock A and clock B share the same inertial frame (frame 1) but are a great distance apart. At the exact mid-point between the clocks (also in frame 1) there is a thing that emit’s a light pulse every second. The clocks tick every time they receive a light pulse so both clocks are synchronised and always show the same time.

They'll only be synchronized in their inertial rest frame, of course. Because of the relativity of simultaneity, they'll be out-of-sync in other inertial frames.

Also in frame 1 with clock A there is Clock C that ticks every second by some independent onboard means. All clocks are synchronised and show the same time. Clock C accelerates way from clock A and toward clock B in frame 2 until it reaches clock B. It accelerates again to be positioned with clock B in frame 1. Clock C went from frame 1 to frame 2 then back to frame1 without turning around. Clock C then compares it’s time with clock B which always shows the same time as clock A. Essentially clock C is also comparing it’s time with clock A regardless that it's a great distance away. According to Relativity wouldn’t clock C be time dilated compared to the other two clocks without having to have turned around?

As I said, you're misunderstanding about the significance of a "turnaround"--in the twin paradox the issue is just which twin accelerated so the distance between them would start decreasing again after it had been increasing for a while, it doesn't matter if this acceleration actually involved a change in direction in whatever frame you're using. But in your scenario above, you could remove the issue of accelerations altogether by having clock C just moving inertially past clock A and B, with clock C synchronizing with A at the moment they pass one another, and then later clock C comparing its time with clock B's at the moment they pass. But here the question of which clock elapsed less time is frame dependent. In inertial frame 2 where clock C was at rest during the journey, clock B's time was actually significantly behind clock A's time because of the relativity of simultaneity, so even though clock B ticked slower than clock C throughout the trip in frame 2, frame 2 still makes the prediction that B's time will be ahead of clock C's time when they meet, because it had that "head start".

 

[swerdna]

When I tell a theist that I don’t accept their claim that a god exists they invariably start quoting from the bible. This is totally pointless as the bible is only valid if a god exists. When I tell people that I don’t accept Relativity because I can’t accept some of the basic building blocks it’s constructed on then it’s equally pointless to use Relativity to validate the building blocks.

One such building block I don’t accept is the way Relativity uses frames. A frame and the things in it only represent an abstract part of existence and a frame can’t and doesn’t exist independently from a possible infinite number of other frames and things they contain that are of equal importance. When considering two things moving relative to each other I don’t see how it’s valid to arbitrarily give either frame any preference or quality that is different to the other. I don’t see that any number of periods of acceleration attribute any actual (absolute) definition of movement to a thing other than how it moves relative to something else.

I guess I’m suggesting using some form of common frame that treats each thing equally. When things are merely moving relative to each other I can’t see that there is any actual (absolute) evidence to define that their movements aren‘t equal and opposite. Perhaps a concentric frame that is always equidistant to both moving things. This would mean of course that relative movement is always viewed symmetrically equal and opposite and there would be no imbalance to cause time dilation. I don’t expect anyone to accept or agree with this but I hope you can at least give it some unbiased consideration before you roll about the floor in laughter.

If this type of talk is inappropriate and unacceptable to this thread or forum then please just let me know rather than locking the thread (which isn’t mine) and I will cease and desist such unsavoury practice.

 

[sylas]

When I tell a theist that I don’t accept their claim that a god exists they invariably start quoting from the bible. This is totally pointless as the bible is only valid if a god exists. When I tell people that I don’t accept Relativity because I can’t accept some of the basic building blocks it’s constructed on then it’s equally pointless to use Relativity to validate the building blocks.

If you don't accept relativity because you reject even more fundamental aspects of the scientific world view, that is your prerogative, I guess.

Your fundamental problem is that the actual predictions of relativity are real. It isn't just assumption. The time differences from time dilation exist and are measured. You are trying to build up some alternative picture to fit your own personal intuitions. The fundamental basis for science is to build up a picture that fits with observations and measurements.

Relativity does that.

If this type of talk is inappropriate and unacceptable to this thread or forum then please just let me know rather than locking the thread (which isn’t mine) and I will cease and desist such unsavoury practice.

You are welcome to try and understand the models used in science better.

But if you are not interested in doing that, then you are in the wrong place. It's not that it is "unsavoury" (naive is a better word, IMO) but simply that the role of physicsforums is to explain the models used by scientists, not to give a platform for other models.

It is pretty clear that your personal intuitions are running into headlong conflict with what we really observe in the world. This suggests that there's something wrong with your intuitions; not that that the world needs a new basis for science.

Cheers -- sylas

 

[Al68]

I don’t see that any number of periods of acceleration attribute any actual (absolute) definition of movement to a thing other than how it moves relative to something else.

If A and B are in relative inertial motion, and A accelerates, the relative velocity between A and B changes. But A's acceleration does not change B's velocity relative to any object other than A.

However A's acceleration results in a change in relative velocity between A and every other object in the universe.

I'd call that a significant difference.

When things are merely moving relative to each other I can’t see that there is any actual (absolute) evidence to define that their movements aren‘t equal and opposite.

Just try hitting your brakes real hard to change the relative speed between you and another car while drinking a cup of coffee. Then see if the other driver also has a lap full of hot coffee. If so, then it would be evidence that the motions of the two cars were "equal and opposite".

 

[swerdna]

If A and B are in relative inertial motion, and A accelerates, the relative velocity between A and B changes. But A's acceleration does not change B's velocity relative to any object other than A.

However A's acceleration results in a change in relative velocity between A and every other object in the universe.

I'd call that a significant difference.Just try hitting your brakes real hard to change the relative speed between you and another car while drinking a cup of coffee. Then see if the other driver also has a lap full of hot coffee. If so, then it would be evidence that the motions of the two cars were "equal and opposite".

I said that the relative motion of non-accelerating things is equal and opposite. Not that acceleration is equal to non-acceleration.

 

[swerdna]

If you don't accept relativity because you reject even more fundamental aspects of the scientific world view, that is your prerogative, I guess.

Your fundamental problem is that the actual predictions of relativity are real. It isn't just assumption. The time differences from time dilation exist and are measured. You are trying to build up some alternative picture to fit your own personal intuitions. The fundamental basis for science is to build up a picture that fits with observations and measurements.

Relativity does that.



You are welcome to try and understand the models used in science better.

But if you are not interested in doing that, then you are in the wrong place. It's not that it is "unsavoury" (naive is a better word, IMO) but simply that the role of physicsforums is to explain the models used by scientists, not to give a platform for other models.

It is pretty clear that your personal intuitions are running into headlong conflict with what we really observe in the world. This suggests that there's something wrong with your intuitions; not that that the world needs a new basis for science.

Cheers -- sylas

I will cease and desist and only use this forum to learn the currently acceptable models used by current mainstream scientists.

 

[JesseM]

I said that the relative motion of non-accelerating things is equal and opposite. Not that acceleration is equal to non-acceleration.

What do you mean by "equal and opposite"? It's true that if two objects A and B are in relative (non-accelerated) motion, the the velocity of B in A's rest frame is equal in magnitude and opposite in direction to the velocity of A in B's rest frame, and that both frames are considered equally valid in relativity. And you could also find a third frame where both A and B were in motion with equal speed and opposite directions.

 

[swerdna]

What do you mean by "equal and opposite"? It's true that if two objects A and B are in relative (non-accelerated) motion, the the velocity of B in A's rest frame is equal in magnitude and opposite in direction to the velocity of A in B's rest frame, and that both frames are considered equally valid in relativity. And you could also find a third frame where both A and B were in motion with equal speed and opposite directions.

By equal and opposite mean that the relative movement of non-accelerating things can’t be correctly attributed to one thing and not the other, regardless that one or both may have previously accelerated. The movement is between the things not of the things. In my opinion this is the actual or absolute reality of the relative movement of non-accelerating things and I don’t understand why Relativity only considers only an abstract part off this reality.

 

[sylas]

In my opinion this is the actual or absolute reality of the relative movement of non-accelerating things and I don’t understand why Relativity only considers only an abstract part off this reality.

It's not abstract; it's real.

When something is accelerated, the acceleration is felt. You are pushed back in your seat. The cutlery rattles. The pencil rolls across the table. Things fall over.

A theory of science, like relativity, is not some abstract thought experiment, but a description of the real world, which has definite consequences, and can be falsified if wrong.

Unaccelerated motion is in a straight line at constant velocity. It turns out that the laws of physics really don't take any account of different straight line motions. It really and truly makes no difference to the laws of physics whether you treat A as moving and B at rest, or B as moving but A at rest. This is a discovery about the world... one that was initially very surprising.

When it was found that the laws of physics -- Maxwell's equations, in fact -- implied a particular velocity for the speed of light, it was a natural reaction to think that here was something that would allow you identify absolute movement. Someone would be absolutely at rest if the speed of light had this value relative to them. Surprisingly, this is not true. It really and truly is the case that two individuals who are moving relative to each other still measure the same velocity for the same ray of light.

Some people can't accept that, but it is true. Any real description of the real world has to deal with this.

The solution is relativity.

Relativity is not an abstract philosophical notion that "everything is relative". It is a definite, concrete account of precisely WHAT is relative and how. Velocity is relative. Time is relative. Position is relative. Distance is relative. Acceleration is not relative. Rotation is not relative.

You can use relativity to calculate how various things relate to each other, like the measurements of a clock. The reason relativity is used for this is because it is a concrete, well tested, accurate account of what happens to clocks in reality.

You are continuously suggesting that things "ought" to be different. They aren't.

Now in fact, there are cases where you need more general physics. You need general relativity to deal with gravity. You need quantum mechanics to deal with very small scale phenomena and wave effects. There will be other changes as well to deal with extremes we are still unclear about -- like conditions in the very early universe or approaching the singularity of a black hole. All of those factors are going to clash EVEN WORSE with your own intuitions.

It remains the case that special relativity is a concrete, accurate, well tested and measured mathematical account of what really happens with clocks and motions.

Acceleration is not relative. The description is not "equal and opposite" for two particles... if one particle accelerates, and the other doesn't, there's no ambiguity. One particle really is the one that is accelerating, not the other.

Cheers -- sylas

 

[JesseM]

By equal and opposite mean that the relative movement of non-accelerating things can’t be correctly attributed to one thing and not the other, regardless that one or both may have previously accelerated.

If I'm understanding you right, that's already true in relativity. Motion is always relative, there is no objective truth about which of the two objects is moving, regardless of whether either one accelerated previously.

The movement is between the things not of the things. In my opinion this is the actual or absolute reality of the relative movement of non-accelerating things and I don’t understand why Relativity only considers only an abstract part off this reality.

I don't understand what you mean by "Relativity only considers an abstract part of this reality".

 

[atyy]

When I tell a theist that I don’t accept their claim that a god exists they invariably start quoting from the bible. This is totally pointless as the bible is only valid if a god exists. When I tell people that I don’t accept Relativity because I can’t accept some of the basic building blocks it’s constructed on then it’s equally pointless to use Relativity to validate the building blocks.

One such building block I don’t accept is the way Relativity uses frames. A frame and the things in it only represent an abstract part of existence and a frame can’t and doesn’t exist independently from a possible infinite number of other frames and things they contain that are of equal importance. When considering two things moving relative to each other I don’t see how it’s valid to arbitrarily give either frame any preference or quality that is different to the other. I don’t see that any number of periods of acceleration attribute any actual (absolute) definition of movement to a thing other than how it moves relative to something else.

You draw a straight line on a piece of paper. The straight line is real. Is it horizontal or vertical? The choice of whether to call it horizontal or vertical is like a free choice of inertial frame in flat spacetime. You can even choose coordinates in which the straight line is "wavy" - that is making your life hard - like choosing a non-inertial frame in flat spacetime. In flat spacetime, the assertion that inertial frames exist is just the idea that there are coordinates which make straight lines "straight", and which make your life easy. Why should it be possible to make life easy? - that you have to ask God - and in fact maybe it can't always be done, since gravity exists

 

[matheinste]

Hello swerdna.

Picking a frame is not just picking a part of "reality". By picking different frames you just measure things differently. The results in all frames are equally valid. Every frame contains all the objects in the universe but each object is at rest in only one frame.

Matheinste

 

[swerdna]

Written in layman-speak . . .

An inertial frame is defined by non-accelerating things that don’t move relative to each other.

No inertial frame can be stationary relative to any other inertial frame. It follows therefore that all inertial fames move relative to all other inertial frames.

There is absolutely no evidence that anything can ever be absolutely stationary. It follows therefore that no inertial frame can be absolutely stationary.

In other words inertial frames are always relatively and absolutely in motion.

Anything wrong with any of this?

 

[sylas]

An inertial frame is defined by non-accelerating things that don’t move relative to each other.

omit "that don't move relative to each other". Non-accelerating things can still move relative to each other just fine.

No inertial frame can be stationary relative to any other inertial frame. It follows therefore that all inertial fames move relative to all other inertial frames.

False... two inertial reference frames can differ only by the location of the origin. Neither frame moves with respect to the other.

There is absolutely no evidence that anything can ever be absolutely stationary. It follows therefore that no inertial frame can be absolutely stationary.

It's not that there's no "evidence". Say rather that physics provides no basis for singling out anyone inertial frame as an "absolute" frame. All inertial frames have the same standing, in physics. Physics does not use a concept of "absolute" motion.

Cheers -- sylas

 

[swerdna]

omit "that don't move relative to each other". Non-accelerating things can still move relative to each other just fine.

Are you saying that accelerating things can be in inertial frames?

False... two inertial reference frames can differ only by the location of the origin. Neither frame moves with respect to the other.

So things in one inertial frame don’t move relative to each other, and things in another inertial frame don’t move relative to each other, and these two frames don’t move relative to each other? Doesn’t this mean that the things in one frame don’t move relative to things in the other frame, and doesn’t this make the two frames the same frame?

It's not that there's no "evidence". Say rather that physics provides no basis for singling out anyone inertial frame as an "absolute" frame. All inertial frames have the same standing, in physics. Physics does not use a concept of "absolute" motion.

Cheers -- sylas

Is that physics per se or just theoretical psychics referring to Relativity? I think it’s a self-evident fact of reality that nothing can ever be absolutely stationary so it automatically follows that everything is absolutely moving. If there is any credible proof that this is incorrect I would love to see it (non-mathematical).

 

[sylas]

omit "that don't move relative to each other". Non-accelerating things can still move relative to each other just fine.

Are you saying that accelerating things can be in inertial frames?

I am saying that non-accelerating things can move with respect to each other. It's written right there, you just quoted it!

"Being in" a reference frame is awkward phrasing. In SR, everything is in every reference frame. A "thing" can define a reference frame by tracing out the time axis... we make a reference frame by making this thing always at location zero. If the thing is accelerating, then the associated reference frame is not inertial. That is... accelerating things do not define inertial reference frames.

So things in one inertial frame don’t move relative to each other, and things in another inertial frame don’t move relative to each other, and these two frames don’t move relative to each other? Doesn’t this mean that the things in one frame don’t move relative to things in the other frame, and doesn’t this make the two frames the same frame?

If by frame you mean a particular set of co-ordinates, and by inertial you mean inertial, then no, it doesn't. Two inertial frames, or co-ordinate systems, with no relevative motion between them, may still have a different origin.

Is that physics per se or just theoretical psychics referring to Relativity? I think it’s a self-evident fact of reality that nothing can ever be absolutely stationary so it automatically follows that everything is absolutely moving. If there is any credible proof that this is incorrect I would love to see it (non-mathematical).

No, it does not "automatically" follow that everything is absolutely moving. There is no absolute, either for absolute movement or for absolute stationary.

You CAN be at rest with respect to something else. REALLY at rest with respect to something else. And your view is just as good as anyone else's... therefore things are not "absolutely moving" either.

In my view, the lack of any absolute frame of reference is a genuine discovery. There's nothing self-evident about it. The ancients considered it "self-evident" that the Earth provided a fixed absolute reference frame. Turns out that this is an arbitrary choice, as far as physics is concerned, which was highly counter intuitive at first.

Much of this is convention. There's nothing to stop someone from defining for some reason a particular reference frame as the absolute basis for deciding what is and is not stationary. What physics says... and this is a discovery; not a self-evident truth ... is that this choice is arbitrary in the sense that the laws of physics can't be used to single out a frame.

Cheers -- sylas

 

[JesseM]

An inertial frame is defined by non-accelerating things that don’t move relative to each other.

omit "that don't move relative to each other". Non-accelerating things can still move relative to each other just fine.

But swerdna was talking about how an inertial frame is defined, i.e. a single one. A single frame is indeed defined in terms of a set of non-accelerating rulers and clocks which don't move relative to each other.

There is absolutely no evidence that anything can ever be absolutely stationary. It follows therefore that no inertial frame can be absolutely stationary.

It's not that there's no "evidence". Say rather that physics provides no basis for singling out anyone inertial frame as an "absolute" frame. All inertial frames have the same standing, in physics. Physics does not use a concept of "absolute" motion.

Well, saying there's "no basis for singling out anyone inertial frame" is the same as saying there's no evidence for a violation of Lorentz-symmetry which would be the only conceivable basis for defining a preferred frame. To put it another way, all the evidence supports the idea that the fundamental laws of physics are locally Lorentz-symmetric, meaning that their equations will remain unchanged when you transform from one frame in SR to another using the Lorentz transformation.

 

[sylas]

But swerdna was talking about how an inertial frame is defined, i.e. a single one. A single frame is indeed defined in terms of a set of non-accelerating rulers and clocks which don't move relative to each other.

Ah! Thanks JesseM, and my apologies to you, swerdna!

I made a very silly reading of your text. By "An inertial frame is defined by non-accelerating things that don’t move relative to each other", I had taken you to mean that different non-accelerating things with different frames don't move in relation to each other. My fault for a dumb reading.

Yes, I agree. An inertial frame is defined by a collection of things that are not accelerating and not moving in relation to each other.

Well, saying there's "no basis for singling out anyone inertial frame" is the same as saying there's no evidence for a violation of Lorentz-symmetry which would be the only conceivable basis for defining a preferred frame. To put it another way, all the evidence supports the idea that the fundamental laws of physics are locally Lorentz-symmetric, meaning that their equations will remain unchanged when you transform from one frame in SR to another using the Lorentz transformation.

Yes. Nicely expressed.

Cheers -- sylas