Twin Paradox: Explaining the Relativity of Aging

[Dale]

Hi MikeLizzi, you are certainly more than welcome to join in and see if you can provide a better explanation. The best pedagogical approach is not obvious, so it is a place where you can definitely contribute.

Round trip or one way, there has to be acceleration involved.

That is simply not true. You can easily have "one way" trips that are purely inertial.

 

[Al68]

You can easily have "one way" trips that are purely inertial.

I think he was referring to my statement about one way trips that were each equivalent to half of the twins paradox, which would have to involve acceleration.

 

[JM]

To MikeLizzi: Not hostile to physics, just to this forum. It's a waste of time to post when all you get is the 'acceleration' explanation no matter what you say.

To whom 2... Please respond specifically to the following: 1. Einsteins 1905 paper is based on Kinematics, 2. Kinematics excludes external forces, accelerations and response to them, 3.Einsteins formula for the traveling clock/twin is based on the relation between two inertial coordinate frames, 4. so what are you trying to accomplish with the accelerations etc?

 

[JesseM]

To MikeLizzi: Not hostile to physics, just to this forum. It's a waste of time to post when all you get is the 'acceleration' explanation no matter what you say.

You aren't just getting the answer "acceleration", you're getting a more nuanced answer; the rate a clock is slowed down at any instant in a given frame $$\sqrt{1 - v^2/c^2}$$ is determined solely by its velocity in that frame and not by its acceleration, but this means that if you want the total time elapsed on a clock between two times $$t_0$$ and $$t_1$$ in a frame, and you know its velocity as a function of time v(t), you must do the integral $$\int_{t_0}^{t_1} \sqrt{1 - v(t)^2/c^2} \, dt$$. If you pick two paths that cross at points $$t_0$$ and $$t_1$$, one of which has a constant v(t) and the other of which has a v(t) that changes (an acceleration), the integral has the property that it will always give a larger value for the one with constant v(t) than the one with changing v(t). Are you willing to address specifically what part of this answer you disagree with or don't understand?

To whom 2... Please respond specifically to the following: 1. Einsteins 1905 paper is based on Kinematics, 2. Kinematics excludes external forces, accelerations and response to them

Kinematics excludes the cause of acceleration (forces) but it certainly does not exclude acceleration itself. Wikipedia defines kinematics as "a branch of classical mechanics which describes the motion of objects without consideration of the causes leading to the motion". Acceleration is a type of motion, namely motion where the velocity is changing as a function of time. The wikipedia article on kinematics has a section on constant acceleration, for example.

3.Einsteins formula for the traveling clock/twin is based on the relation between two inertial coordinate frames

Yes, and the integral $$\int_{t_0}^{t_1} \sqrt{1 - v(t)^2/c^2} \, dt$$ is written in terms of the coordinates of an inertial frame, not an accelerating frame. You can certainly use an inertial frame to describe the behavior of an object that is moving non-inertially, and Einstein does so in the 1905 paper, for example in section 4 where he considers the time elapsed by a clock at the equator of a rotating planet (rotation is a type of acceleration, since constant velocity implies constant speed and constant direction, while motion in a circle involves constantly changing direction).

Since you asked for a specific response, I hope you will respond specifically to these points as well.

 

[Dale]

To MikeLizzi: Not hostile to physics, just to this forum. It's a waste of time to post when all you get is the 'acceleration' explanation no matter what you say.

To whom 2... Please respond specifically to the following: 1. Einsteins 1905 paper is based on Kinematics, 2. Kinematics excludes external forces, accelerations and response to them, 3.Einsteins formula for the traveling clock/twin is based on the relation between two inertial coordinate frames, 4. so what are you trying to accomplish with the accelerations etc?

If you want a non-acceleration explanation for the twin's paradox then you are probably going to have to look past Einstein to the http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_spacetime.html" of Minkowski. That is the approach I favor for a variety of reasons including the fact that it is easily generalizable to purely inertial scenarios (GR or a series of "one-way" trips). As JesseM mentioned, kinematics includes acceleration as did all of Einstein's early explanations.

 

[yogi]

Post 75 - The Minkowski approach is nothing more than the invariance of the interval applied to the one way trip per post 72. In the usual setting, put forth by Einstein in his 1905 paper, he starts with both clocks getting synchronized in the same frame at rest - then one gets put in motion - so there is an incidental acceleration (but it was not meant to be a factor in the outcome). It was simply a necessary circumstance to get one of the clocks in motion. Einstein knew the answer he wanted which was an actual time difference on the two clocks - there is no question which clock logged the most time - everyone agrees - it was the clock that remained at rest on Earth which is the same as an Alpha Centuri clock in the same frame as the Earth - but the time difference is not the result of one clock being put in motion - you can do the experiment by having one clock already in motion as it passes by the Earth clock on its way to Alpha Centuri at a high constant speed. Sync it with the Earth clock during flyby - when it arrives at Alpha Centuri it will read less than a clock on alpha Centure that has remained in sync with the Earth clock

 

[john 8]

To Whom... I hoped for some substantive response to my posts before being sent off to read more. I have read more SR books and papers than I can remember, and my comments come from careful reading of them.

I sectioned off this quote of yours to point out that you say that you have read much on the topic of SR, and that you read them carefully. Now I do not know how old you are or to what degree of education you have had, but I am very suspect of your assertion that you have read data on SR with care. I normally do not take a tone that I am going to take with you on what you have said, but I need to point out to you that you have either not carefully read data concerning SR, or you have not been careful in what you write regarding SR.

My point being this following quote:

Thus for every result obtained with A at 'rest' and B in 'motion' there is an equal result with B at 'rest' and A in 'motion'. Thus my early post, when they reunite each twin thinks the other one is younger. I believe that this is what the 1905 paper says. How it's explained has not yet known, is it?

Do you actually read what you write? You say that when the twins reunite they think the other one is younger. In the twin paradox when the twins reunite they see each other and notice from physical changes that aging has occurred in one of the twins, what is this reunite and thinks one is younger, wow, what a misunderstanding on your part.

Next is this quote:

I believe that this is what the 1905 paper says. How it's explained has not yet known, is it?

If you read about something your intension is to gain a better understanding of that subject. Right?
So how can you say that you carefully read data on SR and then walk away from Einstein’s writing with no more than a belief. Einstein did not write something that was to be believed in, it was a scientific theory, not some belief. Maybe you did not mean to use the word believe, that is why I asked if you actually read what you write or were careful about what you write.

Sorry about being so picky, but this is a science form and you really have to be specific in what you say.

One last point.

Einstein asserts that all inertial frames are equal, or in his terms from Relativity,1952, '... every motion must be considered only as a relative motion', and '...two forms, both of which are equally justifiable: (a) The carriage is in motion relative to the embankment.(b) The embankment is in motion relative to the carriage.'

I am familiar with this example that Einstein uses to describe relative motion and frames of reference.
I only say this because I am going to point something out in this example and have you think about it.

Now I am sure that you are familiar with Newton’s three laws of motion. I want to direct your attention to the first law. For those of you reading this who are unfamiliar with Newton’s first law I will I will list it for you. Mind you this is paraphrased, you can look up all the laws on the web if you like.

The first law states: 1. A physical body will remain at rest unless an external force acts on it, a physical body will continue to move at a constant velocity in a straight path, unless an external force acts upon it.


In essence it takes the application of force or energy to cause a change in a physical body.

Now with that in mind let's look at this train and embankment example that Einstein talks about in his book.

When the train moves relative to the embankment that train is having a force applied to it in order for it to move. No force, no motion. Correct?

A passenger on this train that is moving in a straight line at a constant velocity will not notice that he is in motion, but he will see the embankment pass by as he sits on the moving train. And so the passenger could conclude that the embankment is moving and he is at rest.

Now we know that the train is moving because a force is being applied to it. Even if the passenger does not notice the motion of the train, the train is moving because it is being acted upon by a force. The motion of the train is not determined by a passengers perception of motion. The train is moving whether the passenger perceives it or not.

Now for this passenger to think that the embankment is moving and not him would be a mistake. The embankment is not moving because there is no force being applied to move the embankment. Just because the embankment appears to be moving does not make it so.

So those of you who are now reading this, chomping at the bit to say I am wrong about this whole frame of reference concept, you will have to show the force being applied to the embankment. There is none. The state of that embankment or any object that the moving train passes by will not change because a passenger mistakenly perceives it to be so.

A guy driving by a house in a car will not cause that house to move.

So all of the descriptions that Einstein made about frames of reference in his book do not change the basic laws of nature. In order to move an object you have to apply a force to it.

Anyone is free to give an example of how an object that is moving due to an application of force will in fact cause another object that is at rest due to absence of force being applied to it to move. Objects do not actually move because someone thinks it is moving. The object may appear to be moving to someone, if no force is being applied to that object then the object is not moving or being changed.

Go ahead and defend Einstein, I know that I have struck a nerve with some of you.

Alright, now let's get to the actual topic of this thread. The twin paradox and time dilation. I wrote about this in the thread entitled time dilation so I will just copy and paste it here because it is applicable.



In regards to time dilation there are a few outpoints that need to be resolved. In order for the phenomenon of time dilation to be taken seriously as an actual event we need to establish what a clock is and its function and establish if time is a physical thing or not.

First thing that needs to be established is the exact way in which a clock is motivated to move or count off numbers. Is energy being applied to it in some manner to motivate this machine called a clock? Yes/No

If No, than please explain or give a reference on how a clock can move or change without any energy being involved.

If yes, than what types of energy can be used to motivate the machine called a clock?

Can electricity be used? Yes

Can spring tension be used? Yes

Can the motion of mass (as in a water clock, an atomic clock) be used? Yes

I am sure some of you could think of other ways in which energy can be used to motive a clock, but in all of these different types of energy that can be thought of that in actuality cause a change in a clock, is time an energy that can be detected by a clock or has the ability to change the workings of this machine known as a clock.

You see the question of time dilation can only be answered when it has been established what causes a change in any clock and is time an actual physical thing that has the ability to cause change in a clock.

If you say that time is indeed is a physical thing and can actually influence the workings of a clock, then you would have to explain how this occurs. It has not been described in any writings on this planet.

In order for there to be a physical occurrence of time dilation, time would have to be a form of energy and you would need to have a physical measuring device that is capable of detecting this form of energy called time.

So. To those of you who think that time dilation is an actual physical occurrence, can you explain how this phenomenon works, or at least show a reference that explains it.

If you say that experiments on time dilation have been done to prove the occurrence. Let me remind you that two machines that go out of synch after being moved around only goes to show that machines can go out of synch, saying that this out of synch occurrence is due to some influence of a thing that physics has never defined as a thing or a form of energy is absurd.

Physics does not define or recognize time as a form of energy, yet it takes energy to change a clock. So in order to have the occurrence known as time dilation to be an actual physical phenomenon time has to be a form of energy. You cannot have it both ways.

You can argue and protest all that you like. Science does not recognize time as a form of energy. Time dilation involves the notion that this thing called time is being dilated, and the only way to measure this dilation is with a machine known as a clock. Clocks are only motivated by energy. So in order for this time thing to influence a clock this time thing has to be a form of energy.

Let the discussion begin.

 

[DrGreg]

Now we know that the train is moving because a force is being applied to it. Even if the passenger does not notice the motion of the train, the train is moving because it is being acted upon by a force. The motion of the train is not determined by a passengers perception of motion. The train is moving whether the passenger perceives it or not.

Now for this passenger to think that the embankment is moving and not him would be a mistake. The embankment is not moving because there is no force being applied to move the embankment. Just because the embankment appears to be moving does not make it so.

There isn't just a forward force acting on the train from the engine. There are lots of other forces too, such as air resistance, friction in the wheel-bearings and so on. When you add up all the forces acting on the train, the total force is exactly zero (assuming constant velocity). This isn't even relativity, it's elementary Newtonian mechanics. You said it yourself:

The first law states: 1. A physical body will remain at rest unless an external force acts on it, a physical body will continue to move at a constant velocity in a straight path, unless an external force acts upon it.

 

[matheinste]

I sectioned off this quote of yours to point out that you say that you have read much on the topic of SR, and that you read them carefully. Now I do not know how old you are or to what degree of education you have had, but I am very suspect of your assertion that you have read data on SR with care. I normally do not take a tone that I am going to take with you on what you have said, but I need to point out to you that you have either not carefully read data concerning SR, or you have not been careful in what you write regarding SR.

My point being this following quote:

Do you actually read what you write? You say that when the twins reunite they think the other one is younger. In the twin paradox when the twins reunite they see each other and notice from physical changes that aging has occurred in one of the twins, what is this reunite and thinks one is younger, wow, what a misunderstanding on your part.

Next is this quote:





If you read about something your intension is to gain a better understanding of that subject. Right?
So how can you say that you carefully read data on SR and then walk away from Einstein’s writing with no more than a belief. Einstein did not write something that was to be believed in, it was a scientific theory, not some belief. Maybe you did not mean to use the word believe, that is why I asked if you actually read what you write or were careful about what you write.

Sorry about being so picky, but this is a science form and you really have to be specific in what you say.

One last point.



I am familiar with this example that Einstein uses to describe relative motion and frames of reference.
I only say this because I am going to point something out in this example and have you think about it.

Now I am sure that you are familiar with Newton’s three laws of motion. I want to direct your attention to the first law. For those of you reading this who are unfamiliar with Newton’s first law I will I will list it for you. Mind you this is paraphrased, you can look up all the laws on the web if you like.

The first law states: 1. A physical body will remain at rest unless an external force acts on it, a physical body will continue to move at a constant velocity in a straight path, unless an external force acts upon it.


In essence it takes the application of force or energy to cause a change in a physical body.

Now with that in mind let's look at this train and embankment example that Einstein talks about in his book.

When the train moves relative to the embankment that train is having a force applied to it in order for it to move. No force, no motion. Correct?

A passenger on this train that is moving in a straight line at a constant velocity will not notice that he is in motion, but he will see the embankment pass by as he sits on the moving train. And so the passenger could conclude that the embankment is moving and he is at rest.

Now we know that the train is moving because a force is being applied to it. Even if the passenger does not notice the motion of the train, the train is moving because it is being acted upon by a force. The motion of the train is not determined by a passengers perception of motion. The train is moving whether the passenger perceives it or not.

Now for this passenger to think that the embankment is moving and not him would be a mistake. The embankment is not moving because there is no force being applied to move the embankment. Just because the embankment appears to be moving does not make it so.

So those of you who are now reading this, chomping at the bit to say I am wrong about this whole frame of reference concept, you will have to show the force being applied to the embankment. There is none. The state of that embankment or any object that the moving train passes by will not change because a passenger mistakenly perceives it to be so.

A guy driving by a house in a car will not cause that house to move.

So all of the descriptions that Einstein made about frames of reference in his book do not change the basic laws of nature. In order to move an object you have to apply a force to it.

Anyone is free to give an example of how an object that is moving due to an application of force will in fact cause another object that is at rest due to absence of force being applied to it to move. Objects do not actually move because someone thinks it is moving. The object may appear to be moving to someone, if no force is being applied to that object then the object is not moving or being changed.

Go ahead and defend Einstein, I know that I have struck a nerve with some of you.

Alright, now let's get to the actual topic of this thread. The twin paradox and time dilation. I wrote about this in the thread entitled time dilation so I will just copy and paste it here because it is applicable.



In regards to time dilation there are a few outpoints that need to be resolved. In order for the phenomenon of time dilation to be taken seriously as an actual event we need to establish what a clock is and its function and establish if time is a physical thing or not.

First thing that needs to be established is the exact way in which a clock is motivated to move or count off numbers. Is energy being applied to it in some manner to motivate this machine called a clock? Yes/No

If No, than please explain or give a reference on how a clock can move or change without any energy being involved.

If yes, than what types of energy can be used to motivate the machine called a clock?

Can electricity be used? Yes

Can spring tension be used? Yes

Can the motion of mass (as in a water clock, an atomic clock) be used? Yes

I am sure some of you could think of other ways in which energy can be used to motive a clock, but in all of these different types of energy that can be thought of that in actuality cause a change in a clock, is time an energy that can be detected by a clock or has the ability to change the workings of this machine known as a clock.

You see the question of time dilation can only be answered when it has been established what causes a change in any clock and is time an actual physical thing that has the ability to cause change in a clock.

If you say that time is indeed is a physical thing and can actually influence the workings of a clock, then you would have to explain how this occurs. It has not been described in any writings on this planet.

In order for there to be a physical occurrence of time dilation, time would have to be a form of energy and you would need to have a physical measuring device that is capable of detecting this form of energy called time.

So. To those of you who think that time dilation is an actual physical occurrence, can you explain how this phenomenon works, or at least show a reference that explains it.

If you say that experiments on time dilation have been done to prove the occurrence. Let me remind you that two machines that go out of synch after being moved around only goes to show that machines can go out of synch, saying that this out of synch occurrence is due to some influence of a thing that physics has never defined as a thing or a form of energy is absurd.

Physics does not define or recognize time as a form of energy, yet it takes energy to change a clock. So in order to have the occurrence known as time dilation to be an actual physical phenomenon time has to be a form of energy. You cannot have it both ways.

You can argue and protest all that you like. Science does not recognize time as a form of energy. Time dilation involves the notion that this thing called time is being dilated, and the only way to measure this dilation is with a machine known as a clock. Clocks are only motivated by energy. So in order for this time thing to influence a clock this time thing has to be a form of energy.

Let the discussion begin.

Any elementary textbook on Special Relativity or even Wikipedia explains these things to the stisfaction of the vast majority of people, some of who, unlike me, are very intelligent and not easily taken in. .

Matheinste.

 

[john 8]

There isn't just a forward force acting on the train from the engine. There are lots of other forces too, such as air resistance, friction in the wheel-bearings and so on. When you add up all the forces acting on the train, the total force is exactly zero (assuming constant velocity). This isn't even relativity, it's elementary Newtonian mechanics. You said it yourself:

Right, so do you think that the embankment is moving?

 

[john 8]

Any elementary textbook on Special Relativity or even Wikipedia explains these things to the stisfaction of the vast majority of people, some of who, unlike me, are very intelligent and not easily taken in. .

Matheinste.

So do these textbooks define time? Is time an energy or not? What do these textbooks say about the nature of time to your satisfaction? After reading these textbooks what is your understanding of time. You have completely ignored what I wrote. All you have to do is provide evidence from any of these textbooks or wikipedia that time is some form of energy.

What I have stated about time dilation is completely logical and follows the laws of nature. Time has to be a form of energy in order to exist and have some effect on a clock. You have ignored the question and obvious outpoint that I have brought up by saying that it is explained in textbooks and wikipedia.

Fine, show me. Show me the explanation of time motivating a clock.

I have given a complete logical explanation of how time dilation is not an actual physical phenomenon. If you disagree than give me your explanation. Your rebuttal to my explanation is lacking facts. If you disagree then explain why.

Does your understanding of subjects depend on majority rule? The more that believe it the truer it is.

Come on, use some science, that is what this form is for. Please try again.

Do you think or do these textbooks say time is a form of energy?

Anybody else up to the challenge?

 

[DrGreg]

Right, so do you think that the embankment is moving?

Relative to the train, yes.

 

[JesseM]

So do these textbooks define time? Is time an energy or not? What do these textbooks say about the nature of time to your satisfaction? After reading these textbooks what is your understanding of time. You have completely ignored what I wrote. All you have to do is provide evidence from any of these textbooks or wikipedia that time is some form of energy.

What I have stated about time dilation is completely logical and follows the laws of nature. Time has to be a form of energy in order to exist and have some effect on a clock. You have ignored the question and obvious outpoint that I have brought up by saying that it is explained in textbooks and wikipedia.

The ideas of time "having an effect on clocks" or time being "energy" don't appear to make much sense. In SR it is a clock's velocity in a particular inertial frame that corresponds to how much it slows down in this frame, and there is also no frame-independent "objective" truth about whether a clock is running slow at any moment--in a frame where the clock is in motion it is running slow, in a frame where the same clock is at rest it's ticking at the normal rate, both are equally valid perspectives. There are objective frame-independent truths about what two clocks read when they cross paths at a single location, though, so it is true that if two clocks cross paths once and then cross paths again later, and one of the two clocks moved inertially between these two crossings (constant speed and direction) while the other accelerated at some point (changed speed or direction), then the one that accelerated will have elapsed less time in total between the two meetings. This is similar to the fact that on a 2D surface, if you have two cars whose paths cross at two points, and one was moving in a straight line between the crossings while the other changed directions at some point, then the one that changed directions will have elapsed a larger amount on its odometer (which measures distance traveled on the plane rather than time) between the two crossings, since a straight line is the shortest distance between points on a plane--see my discussion of this analogy in post #64 on this thread.

 

[john 8]

Originally Posted by john 8


“Right, so do you think that the embankment is moving?”

Relative to the train, yes.

If the train has no force being applied to it, it will not move. Add enough force and the train will move.

When the train is moving, is there a force being applied to the embankment to move it?

Is the embankment having a force applied to it?


If the train is moving along the embankment and all of the windows are blacked out so no one on the train can see the embankment, is the embankment moving?

Does a moving train cause a force to be applied to the embankment?


When you take your hand and move it over a book are you causing that book to move? Is the book causing your hand to move?


Does the actual action of one object moving cause another object to move? If so, how far does this field of influence spread out from a moving object?

Right now there are cars, planes ships, stars, people, animals, planets, baseballs, fish, there are so many object moving right now in a different frame of reference to me. Which way and how fast am supposed to be moving relative to them. Are you saying that all moving objects have a physical effect on all other objects?

Do you honestly think that the train is physically moving the embankment? Yes/No

It can appear that the embankment is moving, but in reality is the embankment being forced to move?

The embankment is either physically moving in space relative to a starting point and an ending point, or it is not actually moving it just appears to be doing so. Which is it according to the laws of nature?

DrGreg, tell me if you think that all it takes to move an object is to move past it at a constant velocity in a straight line.

Is the embankment actually moving according to the laws of physics

 

[john 8]

The ideas of time "having an effect on clocks" or time being "energy" don't appear to make much sense. In SR it is a clock's velocity in a particular inertial frame that corresponds to how much it slows down in this frame, and there is also no frame-independent "objective" truth about whether a clock is running slow at any moment--in a frame where the clock is in motion it is running slow, in a frame where the same clock is at rest it's ticking at the normal rate, both are equally valid perspectives. There are objective frame-independent truths about what two clocks read when they cross paths at a single location, though, so it is true that if two clocks cross paths once and then cross paths again later, and one of the two clocks moved inertially between these two crossings (constant speed and direction) while the other accelerated at some point (changed speed or direction), then the one that accelerated will have elapsed less time in total between the two meetings. This is similar to the fact that on a 2D surface, if you have two cars whose paths cross at two points, and one was moving in a straight line between the crossings while the other changed directions at some point, then the one that changed directions will have elapsed a larger amount on its odometer (which measures distance traveled on the plane rather than time) between the two crossings, since a straight line is the shortest distance between points on a plane--see my discussion of this analogy in post #64 on this thread.

If you think that the idea of time having an effect on clocks does not appear to make much sense, then what do clocks measure and how are they motivated? If clocks do not measure this thing called time then what are clocks doing?


So is time a physical thing? Yes/No

I have been asking this question over and over and still no proof that time is a physical thing. Does anyone see this? Come on people, all I am asking for is scientific evidence. Stop with all the rhetoric and run around and provide evidence that time is a physical thing.

Let me ask you, did you in the above quote give evidence that time is a physical thing? Yes/No.

If so please point it out because I do not see any.

 

[JesseM]

If you think that the idea of time having an effect on clocks does not appear to make much sense, then what do clocks measure and how are they motivated? If clocks do not measure this thing called time then what are clocks doing?

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.

You could say something pretty analogous for "distance" on a 2D surface as an abstraction of what instruments like odometers measure when they take particular paths on this surface. There are various ways you could construct an odometer--one might be based on number of revolutions of a car's wheel, another might be keeping track of how quickly marks on the ground are passing the car at each moment, another might actually be laying down a trail of measuring-tape behind the car--but whatever increments each one is using, if you put them together in the same car the ratio of the number of increments each one measures as the car travels should be pretty close to constant (depending on how accurate they are), regardless of what path the car takes. And if we send two cars on different paths between points A and B, one car's group of odometers may measure different numbers of increments between A and B than the other car's group of identical odometers, yet the ratio of type-1-odometer-increments to type-2-odometer-increments will be the same for both cars, so we say that the two paths had different "distances" which each odometer divided up into different increments.

However, though we may say based on this that clocks "measure time" along paths through spacetime and odometers "measure distance" along paths on a 2D surface, I don't think that's quite the same thing as saying "time has an effect on clocks" or "distance has an effect on odometers...I guess it depends on what you mean by "effect" though. Certainly it wouldn't make sense to treat distance or time as things that are supplying energy to clocks or odometers though.

So is time a physical thing? Yes/No

Could you define what you mean by "physical"? The time along a path through spacetime is at least "physical" if you just mean "there's a well-defined physical procedure for determining the amount of 'time' on a path through spacetime, and this procedure gives a frame-invariant answer", but you seem to be implying something more when you suggest that time is physical and therefore must have energy. It would also help if you told me whether you think "distance" is a "physical thing" or not.

 

[DrGreg]

If the train has no force being applied to it, it will not move. Add enough force and the train will move.

There is confusion here over the meaning of the word move. When I say "the train moves relative to the embankment", I mean it continues to move at a constant velocity. I don't mean it begins to move from being at rest -- I would call that "acceleration" rather than movement, to avoid confusion.

You quoted Newtons Laws in an earlier post. Let me remind you:

1. A body persists its state of rest or of uniform motion unless acted upon by an external unbalanced force
2. The net force on an object is equal to the mass of the object multiplied by its acceleration
3. To every action there is an equal and opposite reaction

Therefore for an object to move (i.e. continue moving) no force is required. Forces cause acceleration i.e. a change of motion.

When the train is moving, is there a force being applied to the embankment to move it?

No. Neither is there a force being applied to the train. (1st law)

Is the embankment having a force applied to it?

No. Neither is the train having a force applied to it (1st law)

If the train is moving along the embankment and all of the windows are blacked out so no one on the train can see the embankment, is the embankment moving?

Relative to the train, yes.

Does a moving train cause a force to be applied to the embankment?

No. Neither does the embankment cause a force to be applied to the train. (1st law)

When you take your hand and move it over a book are you causing that book to move?

Relative to my hand, yes.

Is the book causing your hand to move?

No, my brain is.

Does the actual action of one object moving cause another object to move? If so, how far does this field of influence spread out from a moving object?

This isn't "cause", it's logic. If A moves relative to B then B moves relative to A, by definition.

Right now there are cars, planes ships, stars, people, animals, planets, baseballs, fish, there are so many object moving right now in a different frame of reference to me. Which way and how fast am supposed to be moving relative to them. Are you saying that all moving objects have a physical effect on all other objects?

No. The fact that A is moving relative to B does not imply that B (or A) caused the motion to occur.

Do you honestly think that the train is physically moving the embankment? Yes/No

I never said that. I said the embankment was moving relative to the train. I never said how that motion was initiated

It can appear that the embankment is moving, but in reality is the embankment being forced to move?

Don't understand the question

The embankment is either physically moving in space relative to a starting point and an ending point, or it is not actually moving it just appears to be doing so. Which is it according to the laws of nature?

Neither. It is moving relative to the train. No more, no less. Any motion must be relative to something.

DrGreg, tell me if you think that all it takes to move an object is to move past it at a constant velocity in a straight line.

Yes. I am talking about continuation of motion, not acceleration.

Is the embankment actually moving according to the laws of physics

Relative to the train, yes.

Everything I've said above follows from Newton's theories, never mind Einstein. Before you come here criticising relativity, you really ought to find out something about pre-relativity science first.

 

[spikenigma]

Which scenario are you talking about? The one where the rocket travels past Earth at constant velocity and the two twins are born at the moment the rocket is next to Earth, then the rocket continues onward at constant velocity forever without turning around?

In this case there is no objective truth about which twin is older, in the frame where the Earth is at rest the rocket-twin ages more slowly, in the frame where the rocket is at rest the Earth-twin ages more slowly. Are you familiar with the relativity of simultaneity?

In the Earth frame it might be true that the event of the Earth-twin's 40th birthday is simultaneous with the event of the rocket-twin's 32nd birthday, while in the rocket frame it would then be true that the event of the Earth twin's 40th birthday is simultaneous with the event of the rocket-twin's 50th birthday, so in each frame the moving twin is only aging at 0.8 the rate of the at-rest twin.

Only if you bring the twins back together to a single location in space will both frames have to agree on their respective ages at a single moment.

I think I understand what you are saying, but I need clarification. Let's say on their 40th birthday they both freeze their bodies. For the purposes of this discussion, perfect freezing

The rocket then returns to Earth, they then both unfreeze and they compare their ages

is one twin older?

On the other hand, if you're talking about the scenario in post #53 where you said "let's say that Rocket-twin travels back to Earth (very) slowly as to minimise any time dilation and meets up with his twin", in this case the rocket must have turned around at some point to travel back to Earth, so the rocket did accelerate in the rocket-twin's lifetime.

I wanted to leave any accelerational effects out of my area of this discussion, so I'll leave the 'coming back slowly' area

 

[matheinste]

So do these textbooks define time? Is time an energy or not? What do these textbooks say about the nature of time to your satisfaction? After reading these textbooks what is your understanding of time. You have completely ignored what I wrote. All you have to do is provide evidence from any of these textbooks or wikipedia that time is some form of energy.

What I have stated about time dilation is completely logical and follows the laws of nature. Time has to be a form of energy in order to exist and have some effect on a clock. You have ignored the question and obvious outpoint that I have brought up by saying that it is explained in textbooks and wikipedia.

Fine, show me. Show me the explanation of time motivating a clock.

I have given a complete logical explanation of how time dilation is not an actual physical phenomenon. If you disagree than give me your explanation. Your rebuttal to my explanation is lacking facts. If you disagree then explain why.

Does your understanding of subjects depend on majority rule? The more that believe it the truer it is.

Come on, use some science, that is what this form is for. Please try again.

Do you think or do these textbooks say time is a form of energy?

Anybody else up to the challenge?

I do not think that time is a form of energy. However, how one describes/explains time is a subject to much philosophcal speculatio and I do not have any answers. Time is of course of much use in physics and so I will continue to use it despite my lack of a fundamental understanding.

My reference to textbooks was because at the point in the thread at which I made the comment you showed a lack of understaning, whch has continued since, about such basic points of SR as that which the train and embankment thought experiment explain.

I have no interest in a discusson about time. I have read much by heavyweights such as Reichenbach, Weyl, Eddington etc. but remain unenlightened. I hope eventually that you find an answer to satisfy you.

Matheinste

 

[JesseM]

I think I understand what you are saying, but I need clarification. Let's say on their 40th birthday they both freeze their bodies. For the purposes of this discussion, perfect freezing

The rocket then returns to Earth, they then both unfreeze and they compare their ages

is one twin older?

The Earth twin is older in years he has existed according to a clock that's been next to him since birth (as compared with a clock that's been next to the rocket twin since his own birth), but obviously not older in appearance or psychological age if they were both frozen at 40. In relativity the fundamental thing physicists are interested in is time elapsed on ideal clocks that take different paths through spacetime, though, talking about "aging" is just used as a kind of shorthand.

 

[spikenigma]

The Earth twin is older in years he has existed according to a clock that's been next to him since birth (as compared with a clock that's been next to the rocket twin since his own birth), but obviously not older in appearance or psychological age if they were both frozen at 40. In relativity the fundamental thing physicists are interested in is time elapsed on ideal clocks that take different paths through spacetime, though, talking about "aging" is just used as a kind of shorthand.

makes perfect sense, thankyou

 

[yogi]

One need not contemplate long voyages - two twin pion particles in the lab will exhibit vastly different decay rates if one is moving at a relatively high speed wrt the other - clocks attached to the particles will run at different rates and log different total times for the same spacetime interval. One particle is aging less even though neither particle has left the room during the experiment - this is an example of a local one way voyage - and as per previous posts - you can double the answer to get the result if one pion turned around and zipped back to its starting point before decaying

 

[JM]

In response to post 74:
Thank you JesseM for your answers to my concerns.
I don't disagree with the first paragraph. The integral reduces to Einsteins result when v is constant and higher terms are omitted from the expansion of the radical. It's hard for me to see what variable speed will do to the result. It appears that its the addition of a new factor, variable speed, to relativity rather than a method to determine whether a frame is not inertial.
You are right, kinematics in general includes acceleration. I was thinking of a more restricted application. In any case I think the 1905 paper excludes acceleration.
Lets look at par.4 of the 1905 paper. It formulates the moving clock problem and then calculates the amount by which the moving clock is slow. The analysis starts with the time transform equation given immediately before par. 4. The position of the clock at the origin of the moving axes is identified, x =vt, this is entered into the time equation and the result simplified. The moving clock appears to be slow by t v squared/2csquared. It is then argued that a clock moving at constant ( speed ) along a closed curve will be also slow by this same amount. There is an implicit assumption here that the behavior of the moving clock is the same when moving along the closed curve as it is when moving along a straight inertial line that was used to derive the time equation. Thus this analysis excludes the external force needed to restrain the clock to move along the curved line and the resulting acceleration. This reasoning convinces me that both clocks/twins are represented as inertial by the 1905 analysis, and that there is no need to introduce rockets or acceleration to explain the results.

 

[JesseM]

In response to post 74:
Thank you JesseM for your answers to my concerns.
I don't disagree with the first paragraph. The integral reduces to Einsteins result when v is constant and higher terms are omitted from the expansion of the radical. It's hard for me to see what variable speed will do to the result. It appears that its the addition of a new factor, variable speed, to relativity rather than a method to determine whether a frame is not inertial.

It's not really a significantly new idea from the ones expressed in the 1905 paper, this sort of integral is what Einstein was alluding to in the last paragraph of section 4 when he wrote "If we assume that the result proved for a polygonal line is also valid for a continuously curved line...". To elaborate, if you have a polygonal path made up of three constant-velocity segments joined by instantaneous accelerations, with the three segments having velocities $$v_1$$, $$v_2$$, and $$v_3$$ in some inertial frame, and the lengths of time that each segment lasted for in that frame being $$\Delta t_1$$, $$\Delta t_2$$, and $$\Delta t_3$$, then to find the total time elapsed on a clock that traversed this polygonal path, do you agree that you'd just look at the sum $$(\Delta t_1 * \sqrt{1 - v_1^2 /c^2}) \, + \, (\Delta t_2 * \sqrt{1 - v_2^2 /c^2}) \, + \, (\Delta t_3 * \sqrt{1 - v_3^2 /c^2})$$? If so, then you should be able to see why if instead we have a polygonal path made up of some large number N of segments, the total time elapsed by a clock following this path would be given by the sum $$\sum_{i=1}^N (\Delta t_i * \sqrt{1 - v_i^2 /c^2})$$. For any continuously curved path, you can always approximate it using a series of straight segments, and if you choose the segments so that they all last the same time $$\Delta t$$, then the time elapsed on this approximate path is $$\sum_{i=1}^N (\Delta t * \sqrt{1 - v_i^2 /c^2})$$. In the limit as the time of each segment in the approximation approaches zero, the difference between the approximation and the original curved path also approaches zero, and since an integral is just a limit of this sort of sum but with dt taken to be infinitesimally small, this means that the time on the continuous curve must be $$\int \sqrt{1 - v(t)^2 / c^2} \, dt$$.

You are right, kinematics in general includes acceleration. I was thinking of a more restricted application. In any case I think the 1905 paper excludes acceleration.
Lets look at par.4 of the 1905 paper. It formulates the moving clock problem and then calculates the amount by which the moving clock is slow. The analysis starts with the time transform equation given immediately before par. 4. The position of the clock at the origin of the moving axes is identified, x =vt, this is entered into the time equation and the result simplified. The moving clock appears to be slow by t v squared/2csquared. It is then argued that a clock moving at constant ( speed ) along a closed curve will be also slow by this same amount.

But why do you say the analysis excludes acceleration? Do you understand that even if speed remains constant, any change in direction is a type of acceleration, so the example in section 4 of a clock moving in a circle because it's on the equator of a rotating sphere would necessarily be a problem involving acceleration?

Thus this analysis excludes the external force needed to restrain the clock to move along the curved line and the resulting acceleration.

It's true that the cause of the acceleration is not relevant, but his analysis does suggest we can use time dilation to calculate the time elapsed on clocks whose velocity is not constant.

This reasoning convinces me that both clocks/twins are represented as inertial by the 1905 analysis, and that there is no need to introduce rockets or acceleration to explain the results.

Both twins cannot be inertial, since if they are at first moving apart but later moving back towards one another, one of them must have changed direction. Of course you can treat the non-inertial twin as taking a polygonal path consisting of two inertial segments at different velocities (different directions if not different speeds) joined by an instantaneous acceleration, but the fact remains that if one twin moves at constant velocity from beginning to end while the other twin changes velocity at some point on the journey, it will always be the inertial twin whose clock has elapsed more time when they reunite. Do you disagree?

 

[JM]

JesseM. Thanks again. Sorry I can't reply now, the system claims I am not logged in and erases my reply, so I would have to retype. Do you know how to work the system when your reply is long?

 

[JM]

Hey there The-Genius, are you still listening? I have a question for you. You say you heard about A and B and think they can't be different ( my parahprase). Can you say where you got your ideas?

 

[JM]

JesseM, reply to your recent post. I agree with your first point re the integral.
Re the second point: When Einstein used the equation for inertial frames to find time, he replaced his curved line at constant v with a straight line, thus with no acceleration.
Re the third point: yes, but time dilation also occurs for two inertial frames, as shown earlier in his paper.
Re your last point: I think you are commenting on what we know about the physics of the motion, forces and accelerations are surely present. But what Einstein did was to exclude them by his choice of equations. This is ok in a kinematic analysis, in the restricted sense.

 

[JesseM]

JesseM. Thanks again. Sorry I can't reply now, the system claims I am not logged in and erases my reply, so I would have to retype. Do you know how to work the system when your reply is long?

I think when you log in you have the option to click something that says "stay logged in" and that way it shouldn't log you out. But another option is just to copy your reply before you hit "submit reply", and that way if it erases it, you can just log in again and paste in your response and submit it right away.

JesseM, reply to your recent post. I agree with your first point re the integral.
Re the second point: When Einstein used the equation for inertial frames to find time, he replaced his curved line at constant v with a straight line, thus with no acceleration.

By "find time" you mean the time dilation equation? If so I agree that you derive the time dilation equation by looking at two events on a straight line segment, although this segment can be infinitesimally short.

Re the third point: yes, but time dilation also occurs for two inertial frames, as shown earlier in his paper.

By "third point" you mean my statement "It's true that the cause of the acceleration is not relevant, but his analysis does suggest we can use time dilation to calculate the time elapsed on clocks whose velocity is not constant."? If so, yes, I wasn't denying that you can also calculate the time dilation on clocks whose velocity is constant, in fact that's the easiest case.

Re your last point: I think you are commenting on what we know about the physics of the motion, forces and accelerations are surely present. But what Einstein did was to exclude them by his choice of equations. This is ok in a kinematic analysis, in the restricted sense.

But just talking about constant-velocity motion is not sufficient if you want to analyze the twin paradox, which is the main subject of the thread. Do you agree that in order for two twins to start out at the same position, then move apart, then later reunite, at least one of them has to change velocities at some point in his journey? (assuming we are analyzing things from the perspective of an inertial frame) And if so, do you agree that if one twin maintains a constant velocity between the event of the twins departing and the event of the twins reuining, while the other twin changes velocity at some point, then these facts are enough to guarantee that the constant-velocity twin will have aged more when they reunite, regardless of the specific velocities and times involved?

 

[JM]

JesseM. Thanks again for your reply.
"...at least one of them has to change velocity..." Yes. In the path of '1905' the moving clock moves at constant speed, but changes velocity from 'moving away' to 'moving back'.
" ...constant velocity motion is not sufficient...' I read the 1905 paper to be limited to constant velocity motion, by virtue of the use of the time equation for such motion to calculate the time for the closed path. I assume you are extending that analysis.
"...facts are enough to guarantee..." I can't comment on this because I haven't seen the analysis that leads to this conclusion.

I have enjoyed our exchange of ideas given in the spirit of friendship. I would like to continue but feel it's time to step back and try to digest this lengthy thread. I plan to monitor this thread for comments from you, or others , whom we seem to have lost.

Peace, and best wishes.

 

[JesseM]

JesseM. Thanks again for your reply.
"...at least one of them has to change velocity..." Yes. In the path of '1905' the moving clock moves at constant speed, but changes velocity from 'moving away' to 'moving back'.
" ...constant velocity motion is not sufficient...' I read the 1905 paper to be limited to constant velocity motion, by virtue of the use of the time equation for such motion to calculate the time for the closed path. I assume you are extending that analysis.

But you just said above that the clock "changes velocity from 'moving away' to 'moving back'", so you seem to agree that both twins don't have a constant velocity throughout the journey. The path involves two segments which individually have constant velocity, and thus you can calculate the time elapsed on each segment using the time dilation equation and then just add the two times to get the total time elapsed, but the entire path does not involve a single constant velocity, that's all I meant by "constant velocity motion is not sufficient". If each clock moves at a single constant velocity forever, then they will just move away forever after passing each other so they can't compare their clocks on two different meetings.

"...facts are enough to guarantee..." I can't comment on this because I haven't seen the analysis that leads to this conclusion.

A simple way to prove it is to note that all frames will agree on the time elapsed on the two clocks between their meetings, so you can just analyze things from the frame where the inertial clock is at rest, and since the other clock will have a nonzero velocity for at least part of its trip in this frame, it will tick slower than the inertial clock during those parts of its trip and thus will have elapsed less total time when they reunite.

 

[john 8]

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. .

Hi JesseM. You are very thorough in your statements. I really want to be clear here. I think that you are a good terminal to talk to about many science subjects. You come across clear and concise. You use science as much as possible to explain your point of view. I like that. That is what is needed by more people who communicate on this forum.

I would like to narrow my debate, discussion between you and me. It tends to get a bit distracting trying to juggle many different conversations with different people. I am not going to ignore any other conversations or questions directed at me but I just want to have a one on one.

Having said all that, I have been asking if time is a physical thing as you know.

Let's just say that I am undecided as to the true physical reality of time. I could go either way. Now when I ask you if time is a physical thing what I am asking you is what your knowledge of time is, from what you have learned about time, do you think that time is a physical thing, that is what I am asking. So when I ask you a question regarding time I am not looking for an opinion, I am asking you to relay to me what the science of physics says about time.

As an example you said in the above quote that you would say time is an abstraction. I have seen some not so standard, simple kids dictionaries define time that way. I am not implying that what you said was childish, just that if you were to use that definition of time in a discussion on time dilation, what you would be saying is that an abstraction is dilating, now what kind of exact science is that?

You see opinions or incomplete, or simple definitions are not of much use because everyone has an opinion, and incomplete definitions raise more questions then they answer. I would like to go to the source, what does physics say that time is.

You see I have gone to many sources that contain data on time, standard dictionaries, physics books, books written by leading authorities in the field of physics, I have talked to people who teach physics, in all of this research I have not found any data that says time is a physical thing according to the definition of physical thing as defined by a physics book or standard dictionary. I have also not perceived any such force, energy, or object that is called time.

So when I pose the question is time physical, yes I get many responses from people that claim time is real, many people will say in their opinion time is real, or I think time is real and such, but when I ask these same people to tell me what physics says about time, and is time considered to be a real physical thing according to the field of physics. Well you can see by all of the gyrations people go through when asked that type of question by looking at the responses on this thread.

If you notice, there has not been one reference presented on this thread that states time is a physical thing according to physics.

Now I do not know if you believe in a god, but if you do, I am sure you have run into people who question your faith by asking you to prove your god is real, well you know that you can’t do that, your god is real to you according to what you believe and that is it.

Even if God is a real person, spirit, He is not real according to the physics definition of real. God is real according to the Christian definition of real. Those who think God is real and believe that He is real are doing so from the point of view of their religion and faith. God is in actuality real to some people, you can not deny that.

If you try to define God with the standards of physics you will run into trouble, if God is real or not has no bearing on physics at this time. Physics is what it is if God exist in this physical universe or not.

So this brings me back to my point. I am asking you if physics defines time as a real physical thing as it pertains to how it is used in physics. Physics uses the term time often, what definition of time are they using? If you think that time is a physical thing as defined by physics or any standard dictionary then please direct me to that reference.

Think of my question of time as not an attack, but just an inquiry. I have looked and found no evidence, that does not mean there is none to be found. You and others seem to disagree with my observation, so I am just asking you for the information that you have that says time is a physical thing according to the standards, teaching, laws of physics. Time is used in all physics books, what definition of the term time is being used, that’s all.

Einstein used time to describe many things and phenomenon, he gave no definition of time.

Einstein used human language to communicate his ideas and concepts to others, he picked the term time to rely an idea, if he did not feel it necessary to inform people of a specialized definition of time, then we are going to have to assume that he was satisfied with the way the world has already defined time.

If anyone today were to read the book on Relativity and they were not sure of what context Einstein was using the term time, that reader would normally turn to a good dictionary and find many different definitions of the term time, all depending on the context in which time was being used.

So that brings us back to what is time?

Could you define what you mean by "physical"? The time along a path through spacetime is at least "physical" if you just mean "there's a well-defined physical procedure for determining the amount of 'time' on a path through spacetime, and this procedure gives a frame-invariant answer", but you seem to be implying something more when you suggest that time is physical and therefore must have energy. It would also help if you told me whether you think "distance" is a "physical thing" or not.

I am defining physical the way it is defined in any standard dictionary or physics reference book. My definition of physical is the same as those.





john 8

 

[john 8]

There is confusion here over the meaning of the word move. When I say "the train moves relative to the embankment", I mean it continues to move at a constant velocity. I don't mean it begins to move from being at rest -- I would call that "acceleration" rather than movement, to avoid confusion.

You quoted Newtons Laws in an earlier post. Let me remind you:
Therefore for an object to move (i.e. continue moving) no force is required. Forces cause acceleration i.e. a change of motion.

No. Neither is there a force being applied to the train. (1st law)No. Neither is the train having a force applied to it (1st law)Relative to the train, yes.No. Neither does the embankment cause a force to be applied to the train. (1st law)Relative to my hand, yes.No, my brain is.This isn't "cause", it's logic. If A moves relative to B then B moves relative to A, by definition.No. The fact that A is moving relative to B does not imply that B (or A) caused the motion to occur.I never said that. I said the embankment was moving relative to the train. I never said how that motion was initiatedDon't understand the questionNeither. It is moving relative to the train. No more, no less. Any motion must be relative to something.Yes. I am talking about continuation of motion, not acceleration.Relative to the train, yes.

Everything I've said above follows from Newton's theories, never mind Einstein. Before you come here criticising relativity, you really ought to find out something about pre-relativity science first.

Thank you very much for your reply. you really put some effort into it. I asked you many questions and quite frequently people do not respond to all of them.

In order to keep this short I will try to ask only a few questions. First I would like to say that I have read many books on Relativity and the basic concepts just don’t jive. Maybe I have a misunderstanding, that is why I am on this thread.

I would really like to get Relativity straight if I have a misunderstanding. I would like to take one concept at a time, and start with the basic concept of frames of reference.

Here is my understanding of physics before Einstein. According to Newton’s 1st law of motion it will take energy, or a force to cause a object that is in a state of rest, or experiencing a balance of forces (as in a ball being supported by a spring), the force of gravity is pulling the ball down, the spring is pushing the ball up, there is a balance of force and the ball is not moving from point A to point B.

Correct so far?


Now in order to cause a change in an object, energy or force has to be applied. If an object is existing in one state, in order to cause a change in that object, energy has to be applied to that object.

Correct so far?

I say that Newton’s laws of motion work in all situations involving energy and objects, no exceptions.

Correct so far?


I want to ask you about frames of reference, but I want to make sure that we are in agreement on what I have stated so far.

Thank You.

John 8

 

[DrGreg]

Thank you very much for your reply. you really put some effort into it. I asked you many questions and quite frequently people do not respond to all of them.

In order to keep this short I will try to ask only a few questions. First I would like to say that I have read many books on Relativity and the basic concepts just don’t jive. Maybe I have a misunderstanding, that is why I am on this thread.

First of all I'd like to say I very much welcome what I perceive to be a change of tone in your responses, to both myself and to JesseM.

I think some people won't give direct answers to posts in which you ask many questions because they think the whole post is based on a misunderstanding, and they want to correct that misunderstanding directly instead of giving detailed answers to every question.

The impresssion you have given so far, whether you intended to or not, was that you weren't really listening to the answers, that you were simply telling us all "what science says" (and actually getting it wrong in the process). I say this to you, not out of any personal disrepect, but simply to try and explain why you've had such a bad response from most people so far.

I would really like to get Relativity straight if I have a misunderstanding. I would like to take one concept at a time, and start with the basic concept of frames of reference.

Here is my understanding of physics before Einstein. According to Newton’s 1st law of motion it will take energy, or a force to cause a object that is in a state of rest, or experiencing a balance of forces (as in a ball being supported by a spring), the force of gravity is pulling the ball down, the spring is pushing the ball up, there is a balance of force and the ball is not moving from point A to point B.

Correct so far?

Yes. In fact we say (in Newtonian physics) the upward force and downward force exactly cancel each other out, so the total force acting on the object is zero and it doesn't move. Energy is not the same thing as force. It requires energy to move a force through a distance; if you apply a force but nothing moves, no energy is used. The force of the spring does not imply any energy (unless the object moves).

Now in order to cause a change in an object, energy or force has to be applied. If an object is existing in one state, in order to cause a change in that object, energy has to be applied to that object.

Correct so far?

That is a bit too vague for me to agree to. A "change of state" could mean many things; whether that actually requires energy or force would depend on what exactly you mean. It's certainly true that a change in velocity, a change of spin, a change of temperature are all things that require a transfer of energy either into or out of the object.

I say that Newton’s laws of motion work in all situations involving energy and objects, no exceptions.

Correct so far?

Provided the laws are applied correctly (and ignoring relativistic corrections to Newton's laws), yes.

I want to ask you about frames of reference, but I want to make sure that we are in agreement on what I have stated so far.

Thank You.

John 8

I will just point out that all the answers I gave above are assuming that we choose just one inertial frame of reference and stick with it. It is possible for an object's velocity to "change" simply by measuring it in a different frame. If a train is slowly moving at 5mph, and I walk forward at 5mph relative to the train, then I am moving at 10mph relative to the ground. If someone else jumps off the train while it is moving, my velocity, relative to that person, has increased from 5mph to 10mph. But no force or energy has been applied to me.

On a general point, you have often said things like

I am defining physical the way it is defined in any standard dictionary or physics reference book. My definition of physical is the same as those.

The problem is that there are lots of different dictionaries and physics reference books. They will all use different wordings when they define things. If you want to refer to someone else's definition, you need to tell us, explicitly, word-for-word, precisely what that definition is, and precisely which book you got it from, otherwise we genuinely don't know what you are talking about and so we cannot make any sensible comment about it.

 

[JesseM]

Having said all that, I have been asking if time is a physical thing as you know.

Let's just say that I am undecided as to the true physical reality of time. I could go either way. Now when I ask you if time is a physical thing what I am asking you is what your knowledge of time is, from what you have learned about time, do you think that time is a physical thing, that is what I am asking. So when I ask you a question regarding time I am not looking for an opinion, I am asking you to relay to me what the science of physics says about time.

The problem is I still don't know what is meant by the words "physical thing". It is not a technical term that's used in physics, certainly. Perhaps you have given a definition earlier on the thread; if so, could you repost it?

As an example you said in the above quote that you would say time is an abstraction. I have seen some not so standard, simple kids dictionaries define time that way. I am not implying that what you said was childish, just that if you were to use that definition of time in a discussion on time dilation, what you would be saying is that an abstraction is dilating, now what kind of exact science is that?

But as I said, time is an abstraction from the fact that we can build physical clocks which tick at a constant rate relative to one another when they are next to each other, so when we talk about "time dilation", this is itself an abstraction from the fact that physical clocks can be measured to show fewer elapsed ticks when they are in motion relative to other clocks. If you can use the mathematics of time dilation to make accurate quantitative predictions about the behavior of clocks, isn't that an "exact science" regardless of whatever philosophical issues you may bring up with time being an "abstraction"?

I should also point out that when I refer to time being an abstraction, this isn't a unique feature of time specifically, I would say that any physical quantity which appears in the equations of physics, like distance or energy or mass or force, is also an abstraction. You determine the "mass" of a physical object by looking at readings on a scale or other mass-measuring device, just like you determine the "time" between two events by looking at readings on a clock. I could go even further and say that any of the particles that appear in particle physics is also an abstraction--the only way to detect a particle like an electron is to note that the readings of certain detectors (curling paths in a bubble chamber, for example) can be predicted in a quantitative way by mathematical models which include abstract mathematical elements called "electrons". So, to me they are all abstractions from real physical instrument readings. That doesn't mean they aren't also "real", just that since we have no way to experience them directly in a way that doesn't depend on comparing instrument readings to an abstract mathematical model (unlike, say, the planet Mars, which we can see pictures of that reveal all sorts of idiosyncratic features that are a consequence of its history and can't be predicted from physics models alone), then for us we can know them only as abstract elements of mathematical models which make accurate predictions about instrument-readings.

This is why, as I said in post #101 on the Fabric Of Spacetime thread, I think that fundamentally physics is fundamentally just about coming up with mathematical models that make accurate quantitative predictions, that any additional significance we assign to the mathematical models--how we visualize certain mathematical elements like forces or particles or time, or whether we consider them "real"--is just a matter of personal beliefs and intuitions, whether we're right or wrong about these things isn't something that can be settled by science, and so there is no scientific basis for telling anyone else they should think about the models in the same way. Did you read the long quote by Feynman I posted there, and if so do you have any thoughts? As he pointed out, it's not that it's useless to conceptualize theories of physics as something more than pure mathematics, since visualizations and such may help physicists get better intuitions and may even aid in coming up with new improved mathematical models. But all that can actually be tested experimentally is which model makes the best quantitative predictions, so if two physicists are using the same mathematical model but conceptualize it in different ways, there's no experimental evidence that can settle this. What's more, the history of 20th century physics has taught physicists to be very cautious about getting too attached to any particular way of conceptualizing a mathematical model, because often it has happened that one model has turned out to be just an approximation for an improved model whose elements are different enough that they can't really be conceptualized in the same way (for example, the forces acting instantaneously at a distance in Newtonian gravity were replaced by the idea of mass causing local curvature in spacetime in general relativity), and yet all the successful quantitative predictions of the old model can be replicated by the new model.

You see I have gone to many sources that contain data on time, standard dictionaries, physics books, books written by leading authorities in the field of physics, I have talked to people who teach physics, in all of this research I have not found any data that says time is a physical thing according to the definition of physical thing as defined by a physics book or standard dictionary. I have also not perceived any such force, energy, or object that is called time.

But do you have any sources written by physicists that use words like "physical thing" and define what they mean in any precise, experimentally-testable way? This sounds to me like a philosophical question about ontology, outside the domain of science. Again, all physics theories can do is give you mathematical models for making predictions about the readings of various physical instruments, I can't think of any conceivable way that you could determine scientifically which elements of the abstract mathematical model are "real physical things" and which are not--can you?

If you notice, there has not been one reference presented on this thread that states time is a physical thing according to physics.

That's because "physical thing" is not a term that is used in physics. Can you present any reference that states that any quantity or entity that appears in physics models is a "physical thing", whether energy or distance or electrons?

I am defining physical the way it is defined in any standard dictionary or physics reference book. My definition of physical is the same as those.

Dictionaries only define things in terms of how they are used in everyday life, and everyday usage is often fuzzy at the edges and determined more by consensus than any clear definitions (for example, precisely what range of temperatures would qualify as 'hot'?) Do you have any sources that define "physical" in a more precise sense that would allow us to tell whether something qualifies just by checking its properties against the definition? Can you find any physics textbooks or other sources written by professional physicists that give rigorous definitions for this word?

 

[JM]

JesseM' Hello again. Re your post 100. I think there is great difficulty separating the conditions observed in the twins journey from the conditions used in the analysis.
You said '... you can calculate the time elapsed on each segment using the time dilation equation and then just add the two times..."
If you use this procedure aren't you excluding any external forces or accelarations from your analysis?
The time dilation equation was derived for two coordinate systems in constant relative velocity v. If you use this equation as you said aren't you treating the two clocks as inertial? If they are both inertial aren't they indistinguishable, as The-genius suggested in his first post? If you wanted to have one clock to be non-inertial wouldn't you have to use a time dilation equation that was derived for non-inertial conditions?
So the analysis you propose ignores turn around effects and treats both clocks as inertial, just as the 1905 paper does. So what is the purpose of insisting that one clock is not inertial, again?
So I do agree that one clock changes velocity in the journey described, but I also assert that this change of velocity is not represented in the actual equations used for the analysis.
The various pieces of our discussion of this topic are scattered along this long thread, would it be useful to gather them all together?