The CADO Reference Frame for an Accelerating Observer

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The CADO reference frame is defined for an observer who accelerates in any manner whatsoever. Specifically, the observer's acceleration a(t), where t is any instant in the observer's life, can be whatever the accelerating observer wants it to be, without restriction.
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The CADO Frame, for the Standard Twin Paradox Scenario
Although the CADO frame is applicable to any acceleration profile, the concepts and terminology needed to describe the CADO reference frame are most quickly and easily understood if they are initially couched in the context of the standard well-known twin paradox scenario.
First, consider the even simpler scenario where two perpetually-inertial observers are moving at some fixed velocity v relative to one another, and when they momentarily are co-located, they just happen to be exactly the same age then. For example, it could just happen that they are both born at that instant of their co-location, even though their mothers could have had a relative velocity of v at that instant. Since each of those newborns is a perpetually-inertial observer, they are each entitled to use the Lorentz equations to determine, at any instant of their own life, the current age of the other. And each of them is entitled to use the well-known time-dilation result of special relativity to determine how fast or how slowly the other is currently ageing, relative to their own ageing.
In the standard twin paradox, the "home twin" is perpetually inertial by assumption, and thus is entitled to use either the Lorentz equations or the time-dilation result (or both) to determine the current age of the "traveling twin". To allow more brevity and less clutter in the writing which follows, the home-twin will always be referred to as a "she", and the traveling-twin will always be referred to as a "he".
The traveling-twin must accelerate, in order to accomplish his turnaround, so he is not a perpetually-inertial observer, and his reference frame during his trip cannot be an inertial frame. Specifically, he is not allowed, during his entire trip, to use the Lorentz equations or the time-dilation result to determine the current age of his twin.
So what is the reference frame of the traveling twin? There are five requirements that any such frame must have. It must be such that the traveler is perpetually located at its spatial origin. It must specify how the traveler, at each instant of his life, is to determine the current age and the current position of each and every object (or person) in the (assumed flat) universe. It must be internally consistent. It must not contradict special relativity. And it must be such that the traveler and the home twin agree with one another about the correspondence between their ages, when they are reunited.
More than one reference frame for an accelerating observer have been defined, and there is no consensus about which one is most appropriate. This article describes one such reference frame: the CADO frame.
The CADO frame was originally inspired by an example (Example 49) in Taylor's and Wheeler's classic book The results of their example are consistent with those obtained from the common gravitational time dilation explanation, but do not depend on the use of any fictitious gravitational fields. Their basic approach is clearly applicable to scenarios with finite accelerations, although they didn't pursue that generalization. The CADO frame accomplishes that generalization.
Even though the frame of the traveling twin, since he accelerates during some portion his trip, cannot be an inertial frame, there is, at each instant t of the traveler's life, a unique inertial frame which is momentarily stationary with respect to the traveler at that instant, with a spatial axis pointing in the same direction as the home-twin's spatial axis, and such that the traveler is located at the spatial origin of that frame at that instant. Furthermore, for uniqueness, we require that the time coordinate of that inertial frame be equal to the traveler's age, at that instant. That unique inertial frame is called the "momentarily stationary inertial reference frame, at the instant t in the traveler's life", abbreviated as the MSIRF(t). In general, MSIRF(t) will correspond to a different inertial frame from one instant in the traveler's life to the next. It is only during unaccelerated segments of the traveler's life that the MSIRF(t) will consist of the same inertial frame for the entire segment.
Given this (generally infinite) collection of inertial frames, the CADO frame is defined to be the single unique frame having the property that its conclusions about the current age and location of all objects or persons in the (assumed flat) universe, at any instant t of the traveler's life, is the same as the corresponding conclusions of the MSIRF(t). I.e., at each instant of his life, the traveler adopts the viewpoint (about the simultaneity and location of distant objects) of the inertial frame with which he is momentarily stationary at that instant. The acronym "CADO" originates from the phrase "the current age of a distant object".
The CADO Equation
Given the above definition of the CADO frame, it is possible to derive a very simple, and very useful equation, called "the CADO equation", which allows the traveler to determine, at each instant t in his life, the current age of any given distant perpetually-inertial object or person (the "home-twin" in the twin paradox scenario).
First, it is important to understand that, for any given instant t in the traveler's life, the home-twin and the traveler will generally disagree with one another about how old the home-twin is at that instant of the traveler's life. There are two quantities in the CADO equation which represent each of the twins' conclusions about the home-twin's age when the traveler's age is t. The quantity CADO_T denotes the traveler's conclusion about the home-twin's age, when the traveler's age is t, whereas the quantity CADO_H denotes the home-twin's conclusion about the home-twin's age, when the traveler's age is t.
The CADO equation can be written (most simply) as
CADO_T = CADO_H - v * L
where
v is their current relative speed, according to the home-twin, at the given instant t in
the traveler's life, with v taken as positive when the twins are moving apart,
L is the distance from the home-twin to the traveler, at the given instant t in the
traveler's life, according to the home twin,
and
the asterisk denotes multiplication.
(Strictly speaking, the quantity L(t) is the position of the traveler, relative to the home-twin, according to the home-twin, when the traveler's age is t. The distinction will be clarified later, but for now, it's simplest to just think of it as a distance.)
The above equation gives the relationship between those four quantities (CADO_T, CADO_H, v, and L), at the given instant t of the traveler's life. I.e., although it is not shown explicitly, each of the four quantities in the equation are functions of t.
In order to make the equation strictly correct, a factor of c*c dividing the last term is required, where c is the constant speed of any light pulse, as determined by any perpetually-inertial observer. If the time and spatial units are chosen so that c has unity value, the factor in that case is required only for dimensional correctness. In this article, units of years and lightyears will be used exclusively (but often abbreviated as y and ly) , and the factor of c*c will be suppressed entirely, purely for simplicity and brevity.
Idealized Instantaneous Velocity Changes
In the idealized, limiting case of the instantaneous turnaround usually assumed in the twin paradox scenario, the quantities CADO_H and L that are needed in the CADO equation are very easy to obtain, and v is given in the statement of the scenario.
For example, suppose that immediately after the twins are born, the traveling twin moves away from the home-twin at a constant relative velocity of 0.866 lightyears/year for 20 years of his life. That complicated-looking value of the velocity was chosen for this example because it produces the very nice value of 2 for the gamma factor (the time-dilation factor):
gamma = 1 / sqrt( 1 - v * v) ,
where "sqrt( )" denotes the square-root operation, and where, again for simplicity, the factor c*c that should actually be dividing the v*v term has been omitted.
The traveler then instantaneously reverses course, and spends the next 20 years of his life returning to his home-twin. The magnitude of his velocity is still 0.866 ly/y, but since he is now moving toward his twin, by convention his velocity is now negative, -0.866 ly/y. Since gamma depends only on the magnitude of the velocity, gamma is still equal to 2.
So, the traveler is 20 years old at his turnaround, and 40 years old when he is reunited with his twin. Since the home-twin is perpetually inertial, she is entitled to use the time dilation result for his entire trip. Since gamma = 2 for the entire trip, she concludes that the traveler ages half as fast as she herself does, so she concludes that she is 40 years old when he turns around, and 80 years old when they are reunited. (Of course, when they are reunited, they will each know both of their ages). So, just from the time-dilation result, we've been able to quickly determine that
CADO_H(20) = 40 years old.
Now, from the definition of the CADO frame, the MSIRF(t) for all t from 0 years up to, but not including, 20 years, is the same inertial frame ... it's the one which is moving at a velocity relative to the home-twin of 0.866 ly/y, and in which the traveler is located at the spatial origin. During that entire segment, 0 < t < 20, the traveler (by definition) agrees with that single MSIRF about the age of any distant inertial object or person, and thus he also agrees with that MSIRF about how fast or how slowly any distant person is ageing, compared to his own ageing. So, during that outbound leg (but not including the instant at t 20), the traveler is entitled to use the time-dilation result, and he concludes that the home-twin is ageing half as fast as he himself is. So he concludes that, right at the end of his constant-velocity outbound leg (but before he does his instantaneous turnaround), that the home-twin is 10 years old. Therefore we've been able to determine that
CADO_T(immediately before turnaround) = 10 years old.
The fact that the traveler is entitled to use the time-dilation result, during his entire unaccelerated outbound segment, is also true of any unaccelerated segment, of finite duration, in his life. During any unaccelerated finite segment of his life, he is a full-fledged inertial observer during that entire segment, and he is entitled to use the Lorentz equations to determine simultaneity at a distance, and he is entitled to use the time-dilation and length-contraction results that follow from the Lorentz equations.
So, for the entire outbound leg, we didn't need to use the CADO equation at all ... the time-dilation result was all that we needed. But we do need the CADO equation in order to determine what happens during the turnaround, right at the instant t = 20 years. How do we do that?
To make use of the CADO equation during the turnaround, we need to know the values of the three quantities on the right-hand-side of the CADO equation (CADO_H, v, and L), immediately before and immediately after the instantaneous turnaround. CADO_H and L are quantities that are computed in the home-twin's inertial frame, and they are always continuous ... they never change discontinuously, even when v changes discontinuously. So CADO_H and L don't change during the turnaround, but v does change.
We can denote the instant in the traveler's life, immediately before the turnaround, as t 20-, and the instant immediately after the turnaround as t 20+. So, we have
v(20-) = 0.866 ly/y,
and
v(20+) = -0.866 ly/y.
We also already know that
CADO_H(20-) = CADO_H(20+) CADO_H(20) 40 years.
So all we still need to determine is L(20). How do we do that? We know that, in the home-twin's frame, the velocity of the traveler is 0.866 ly/y during the outbound frame, and we know that that outbound leg lasts for 40 years of the home-twin's life, so she will conclude that the traveler's distance from her at the turnaround is
L = 0.866 * 40 = 34.64 ly.
Since, in the CADO equation, all of the quantities need to be specified as functions of the variable t (the traveler's age), we therefore have
L(20-) = L(20+) L(20) 34.64 ly.
So, we've got all the quantities we need, to evaluate CADO_T(20-) and CADO_T(20+) using the CADO equation. We actually were already able to determine CADO_T(20-) using only the time-dilation result for the outbound leg ... we got the value 10 years. But it is instructive to use the CADO equation for the instants immediately before and immediately after the instantaneous turnaround, just to understand why the CADO frame concludes that the home-twin's age abruptly changes during the instantaneous turnaround. Immediately before the turnaround, we get
CADO_T(20-) = CADO_H(20-) - v(20-) * L(20-) = 40 - 0.866 * 34.64,
so
CADO_T(20-) 40 - 30 10 years.
And, immediately after the turnaround, we get
CADO_T(20+) = CADO_H(20+) - v(20+) * L(20+) = 40 + 0.866 * 34.64,
so
CADO_T(20+) = 40 + 30 = 70 years.
So, the CADO equation says that, according to the traveler, the home-twin instantaneously get 60 years older during his instantaneous turnaround. And the CADO equation makes it clear why the traveler's abrupt velocity change causes (according to the traveler) the abrupt change in the home-twin's age: by definition, at any instant t of the traveler's life, he adopts as his own the conclusions of his MSIRF, at that instant, about simultaneity. The MSIRF at the instant immediately before the turnaround, MSIRF(20-), and the MSIRF at the instant immediately after the turnaround, MSIRF(20+), have very different conclusions about the current age and current position of the home-twin.
The change in the home-twin's age, before and after the instantaneous velocity change, is
delta(CADO_T) = CADO_T(20+) - CADO_T(20-),
and since nothing on the right-hand-side of the CADO equation changes during the instantaneous turnaround except the velocity, we get the very simple equation
delta(CADO_T) = - L(20) * ( v(20+) - v(20-) )
or
delta(CADO_T) = - L * delta(v).
So, getting the change in the home-twin's age during an instantaneous velocity change is very simple: you just multiply the negative of their separation by the change in the velocity.
Note that in this case (for the turnaround that occurs in the standard twin paradox scenario), the change in the velocity is negative:
delta(v) = v(20+) - v(20-) (-0.866) - (0.866) -1.732,
and so the change in the home-twin's age is
delta(CADO_T) = -34.64 * (-1.732) = 60 years.
But note that, for other scenarios, the traveler could change his velocity from (say) -0.866 ly/y to +0.866 ly/y (corresponding to an acceleration away from the home twin), and in that case, his velocity change would be positive (+1.732), and so the home-twin's age change would be -60 years .... i.e., she would suddenly get 60 years younger (according to the traveler).
The fact, that the traveler concludes that the home-twin's age changes abruptly whenever he abruptly changes his velocity, certainly has no impact on the home-twin's own perception of the progression of her own age. Lots of additional accelerating observers would generally come to very different conclusions about the way her age changes while they accelerate in various ways, and it is really of no consequence to her what they conclude. But no one's conclusions are any more correct than any one else's conclusions. They are all correct ... in special relativity, different observers generally just have to agree to disagree.
To complete our application of the CADO frame to the standard twin paradox, we've still got to
analyze the inbound leg. The analysis is essentially the same as for the outbound leg. Since the traveler is unaccelerated during the entire inbound leg, the CADO frame says that the traveler is a full-fledged inertial observer during that entire 20-year segment of his life. So he uses the time-dilation result, and concludes that the home-twin ages 10 years during the inbound leg. So, when they are reunited, she is 80 years old, and he is 40 years old. The home-twin and the traveler agree, about the correspondence between their two ages, when they are reunited (as of course they must), even though they generally disagreed about that correspondence, during the trip.
Given the above results, it is easy (and very useful) to sketch an "age-correspondence graph" ... a plot of the home-twin's age (according to the traveler) as a function of the traveler's age. I.e, we want a graph, with the home-twin's age plotted vertically, and the traveler's age plotted horizontally. What does that graph look like?
On the outbound leg, the traveler says that the home-twin's age increases half as fast as his own age. So the curve starts from the origin, and increases linearly along a straight line of slope 1/2, until his (the traveler's) age is 20, and her (the home-twin's) age is 10. At that point, the curve jumps vertically to 70 for her age (with no increase in his age). Finally, the curve increases linearly from there, along a straight line of slope 1/2, until she reaches 80 years old, and he reaches 40 years old. After that, as long as they remain together, they will age at the same rate, but she will always be 40 years older than he is.
The home-twin can do her own age-correspondence graph, again with her age plotted vertically, and his age plotted horizontally. I.e., both graphs show her age as a function of his age; the only difference is that the two graphs show the conclusions of two different observers.
Her graph will be quite different from his graph: hers will consist of a single, straight line of slope 2, because the time-dilation result tells her that, during his entire trip, he ages half as fast as she does, which means that she ages twice as fast as he does. But the two different graphs do start at the same point (the origin), and they do end at the same point (the point where she is 80, and he is 40). But in between those two points, the curves are very different.
In the standard paradox scenario (with a single instantaneous velocity change, and a reunion at the end of the trip), it is actually possible to avoid having to use the CADO equation to determine how the home-twin's age changes during the turnaround. That change can simply be inferred by determining the sum of the amount of her ageing (according to him) during the two unaccelerated segments of his life (10 + 10 = 20 years), and then using the fact that her age at the end of the trip must be 80 years. So we have to come up with an additional 60 years somewhere, and the turnaround is the only place that extra time could have occurred.
But for more complicated scenarios, where the traveler can instantaneously change his velocity multiple times during the trip (both positively and negatively), and in cases where there is never any reunion of the twins), then the CADO equation is indispensable in determining how much the distant perpetually-inertial person instantaneously ages (positively or negatively) during the traveler's instantaneous velocity changes. And even in the standard paradox scenario, the use of the CADO equation at the turnaround makes it clear why the home-twin's age (according to the traveler) instantaneously increases during the instantaneous turnaround. And the CADO equation also makes it clear why the traveler's initial instantaneous velocity change (when he begins his trip), and his final instantaneous velocity change (when they are reunited), does not cause any instantaneous change in her age (because L is zero then).
Finite Accelerations
In all of the above, the non-inertial behavior by the traveler consisted only of instantaneous velocity changes. But the CADO frame, and the CADO equation, is not restricted to these idealized, limiting cases ... the traveler can accelerate in any manner that he chooses. I.e., he can choose any function a(t) for his acceleration, for t ranging over his entire life.
For any choice of the acceleration profile a(t), the CADO equation remains exactly the same as given above. The only difference is that the quantities v(t), CADO_H(t), and L(t), on the right-hand-side of the CADO equation, are no longer quite as simple to determine. For completely general acceleration profiles a(t), all three quantities will generally require numerical integration for their determination. For the (very useful and important) cases that consist of a sequence of segments of the traveler's life in which his acceleration is constant within each segment (and possibly including segments of zero acceleration ... coasting), each of the three quantities needed for evaluation of the CADO equation can be determined analytically. But in any case, once those three quantities have been determined (for any given age of the traveler), the quantity CADO_T can be determined from the same CADO equation, with (as always) only a single multiplication and a single addition or subtraction.
The way the three quantities v, CADO_H, and L can be determined, for each instant of the traveler's life, will only be very briefly described here. Since all three quantities correspond to the conclusions of a perpetually-inertial observer (the "home-twin"), their determination is fairly widely known. For example, Taylor and Wheeler use basically the same approach in their Example 51 of how far a traveler can go, by constantly accelerating at 1g in a straight line.
The acceleration, a(t), at any given instant t of the traveler's life, is the acceleration that would be measured on an accelerometer carried by the traveler (taken as positive when directed away from the home-twin, and negative when directed toward her). This acceleration is not the acceleration that would be measured by the home-twin. At each instant t , it is the acceleration that would be measured by the MSIRF(t) , i.e., by the traveler's MSIRF at that instant. The particular MSIRF doing the measurement is generally different from one instant to the next. The entire acceleration profile, for the whole range of t corresponding to the traveler's life, is not what would be measured by any one single inertial frame.
Once we know the function a(t) for the entire trip of the traveler, we can compute the "rapidity" eta(t). The rapidity is a one-to-one nonlinear function of the velocity v, having the needed property that it is linearly additive across inertial reference frames (the velocity v itself is not linearly additive across inertial frames). Specifically, if we know what the infinitesimal changes in the rapidity is, according to each MSIRF in any finite segment of the traveler's life, we can just add up all those infinitesimal changes to get the total change in the rapidity over that whole finite segment. So, we can get the rapidity eta(t) for the entire trip simply by integrating the acceleration a(t) with respect to t, over the range of t corresponding to the entire trip. (In the above description of the calculations required, and in subsequent descriptions, there are actually some factors of c that, strictly speaking, should be present, but we will always choose our units such that c = 1, and so those factors of c are needed only for dimensional correctness. In the interest of simplicity of description, those factors will be omitted here. They can always be inserted wherever needed, if required.)
For completely general acceleration profiles a(t), the integral to get eta(t) must be calculated numerically. But in the very important (and very useful) special cases where a(t) is some sequence of segments in which the acceleration is constant (positive, negative, or zero) within each segment, the change in eta(t) over any given one of those segments is just equal to A times the duration of that segment, where A is the value of the constant acceleration in that segment. So eta(t) is very easy to determine for those cases.
Once we know eta(t), we can compute v(t), because v is the hyperbolic tangent of eta. And once we know v(t), we can compute gamma(t).
CADO_H(t) can then be computed as the integral, with respect to t, of gamma(t). In the general case, that integration will also have to be carried out numerically. But in the cases where a(t) is piecewise-constant, the change in CADO_H(t),within each segment, is just the total change in the hyperbolic sine of eta(t) within that segment, divided by the constant acceleration A in that segment.
Finally, L(t) can be computed as the integral, with respect to t, of v(t)*gamma(t), which again requires numerical integration in the general case. In the piecewise-constant cases, the evaluation is handled just like the CADO_H evaluation, except that the hyperbolic cosine is used instead of the hyperbolic sine.
The above calculations, in the case of the piecewise-constant accelerations, are very easy and quick to carry out on a computer, and can even be done (although with considerably more effort) with a good hand-calculator, if absolutely necessary.
Current Position of a Distant Perpetually-Inertial Object or Person
The foregoing descriptions have described the CADO equation, and have given a brief description of how the three required quantities on the right-hand-side of that equation can be determined, both for the idealized cases of instantaneous velocity changes, and for completely general finite accelerations, and also for the especially useful cases of piecewise-constant accelerations. Those results satisfy the requirement that a reference frame for an accelerating observer must specify how the observer is to determine, at each instant of his life, the current age of any given distant object or person. But a reference frame must also specify how the observer can determine the current position of that distant person, at each instant of his life. That turns out to be very easy, for the CADO frame.
Each of the accelerating observer's MSIRFs (one for each instant t of his life) will conclude that the current position of the distant person, at the instant t when the inertial frame of that MSIRF is momentarily stationary with respect to the accelerating observer, is
L_T(t) = -L(t) / gamma(t),
where L(t) is the position of the accelerating traveler, according to the distant inertial person, when the accelerating observer's age is t.
The minus-sign above requires some elaboration. L(t) was defined earlier as the distance to the traveler, when his age is t , according to the home-twin. That was done because that terminology makes the CADO equation more intuitive, and easier to initially understand. But that terminology isn't completely precise. The term "distance" normally is understood to be a positive quantity, whereas a "position" (in one-dimensional space) can be either positive of negative. In usual descriptions of the standard twin paradox scenario, for simplicity the issues of how spatial axes are chosen are usually not discussed, and it is just tacitly assumed that the position of the traveler can just be specified by giving a (positive) distance to the traveler. But, in more flexible scenarios, if the traveler returns, but continues to travel on past the home-twin, then his position, relative to the home-twin, will be negative. So, to be precise, the quantity L(t) in the CADO equation is actually defined as the position of the traveler when his age is t, relative to the home-twin, according to the home-twin. If the traveler's outbound velocity is positive, then his position during the trip will be positive, and thus his position is always the same as his distance from her, provided that he doesn't go on past her when he returns. For the case where L(t) is positive, then the position of the home-twin, relative to the traveler, when the traveler's age is t, will be negative (because the position of any person P, relative to any person Q, is always the negative of the position of person Q, relative to person P). That's why the minus-sign is present in the above equation for L_T(t). The term "distance" corresponds to the absolute value of some given position.
Since gamma(t) can change very quickly, for small increases in t, it's clear from the above equation that L_T(t) can also change very quickly. For idealized instantaneous velocity changes, the position of the home-twin, according to the traveler, will instantaneously change, and so he will conclude that her distance from him has instantaneously increased or decreased.
Some Additional CADO Equation Results for Instantaneous Velocity Changes
It is easy to see from the delta(CADO_T) equation that, for instantaneous velocity changes, it is possible for the current age of the distant perpetually-inertial person, in years, to instantaneously vary over a non-inclusive range (almost) equal to twice their current separation, in lightyears. For example, if their separation L at the instant t in the traveler's life when the velocity change occurs (according to the distant person) is 40 lightyears, and if the velocity immediately before and immediately after the velocity change is (almost) +1 ly/y and -1 ly/y, respectively, then
delta(CADO_T) = - L(t) * ( v(t+) - v(t-) ) = (-40) * (-2) = 80 years (almost).
The above example corresponds to the case where the traveler is moving away from the distant person at a velocity arbitrarily close to the velocity of light, and then instantaneously reverses course, and moves toward her, again at a velocity arbitrarily close to the velocity of light. In that case, the distant person instantaneously gets older by an amount arbitrarily close to 80 years.
In the opposite extreme case, where the traveler is initially moving toward the distant person at almost the velocity of light, and then instantaneously reverses course, and moves away from her, again at almost the velocity of light, the the delta(CADO_T) equation gives -80 years ... i.e., she instantaneously gets younger by (almost) 80 years.
Of course, depending on the current age of the distant person immediately before the instantaneous velocity change, the age change of +80 years might well exceed her indisputable age at death. In that case, the traveler is really determining how old she currently is, assuming that she is still alive. Similarly, the age change of -80 years might well precede her birth, which really just tells the traveler how much her mother's current age has decreased during his velocity change. Of course, the CADO equation is actually telling the traveler what the current date and time is, in the inertial frame in which the distant person (and her predecessors) are perpetually inertial. Couching the CADO equation in terms of the age of a particular distant person is just a way to make it more intuitively meaningful, and less abstract.
Some CADO Equation Results for Finite Accelerations
One might reasonably suspect that the results for the idealized cases of instantaneous velocity changes are of no value in understanding what happens for actual realizable accelerations, where velocities don't change instantaneously. But examples obtained by evaluating the CADO equation for finite accelerations show that, provided the separation is sufficiently great, the age changes of the distant perpetually-inertial person are qualitatively quite similar to the idealized results, even for perfectly reasonable 1 g accelerations. (It just happens that a 1 g acceleration is very close to an acceleration of 1 ly/y. More precisely, 1 ly/y is approximately equal to 0.970 g, and 1 g is approximately equal to 1.03 ly/y.) For 1 g accelerations, the age changes of the distant person aren't discontinuous, but her age changes (both positive and negative) can be very large, for relatively small increases in the age of the traveler.
For example, suppose that he (the traveler) and she (the home-twin) are separated by 39.97 lightyears, when he is 26 years old, and she is 47.93 years old (all according to her). His velocity then is +0.7739 ly/y (he is moving away from her), and gamma equals about 1.58.
The CADO equation says that, at that instant (when his age t is 26), that her current age (according to him) is
CADO_T(26) = CADO_H(26) - v(26) * L(26) = 47.93 - (0.7739) * 39.97 = 17.00 years old.
Then, he accelerates at -1 g for two years (of his life). I.e., he points his rocket ship toward her, and fires his rocket engine for 2 years.
During the first half of that acceleration, he is slowing down, but is still getting farther away from her. Half way through the acceleration (when he is 27), he momentarily comes to a standstill, and their separation is 40.53 lightyears then. She is then 49.12 years old (according to her). As can easily be seen from the CADO equation, they will always agree about their corresponding ages whenever their relative velocity v is zero. So he also concludes that she is 49.12 years old at that instant.
During the second half of his -1 g acceleration, he is moving back toward her, and speeding up. At the end of that acceleration, he is 28 years old, she is 50.31 years old, their separation is again 39.97 lightyears, and his velocity is -0.7739 ly/y, all according to her. The CADO equation then says that she is 81.24 years old (according to him).
So he concludes that, during that entire -1 g acceleration, she gets 64.24 years older, whereas he only got 2 years older. During the -1 g acceleration, she doesn't instantaneously get older, but she does age much faster than he does. So it is qualitatively fairly similar to what happens for the idealized instantaneous velocity-change case.
We can also use the CADO equation to determine the values of the various quantities for as many intermediate times during that -1 g acceleration as we want. Then, we can plot the age-correspondence graph ... the plot of the home-twin's age (according to the traveler) as a function of the traveler's age. I.e, we want a graph that has the home-twin's age plotted vertically, and the traveler's age plotted horizontally. We did this earlier for the case of the instantaneous turnaround in the standard twin paradox. What does the curve look like for this -1 g acceleration?
In this case, the curve starts out, when the magnitude of the velocity is fairly high (0.7739 ly/y), with a positive slope of about 17. I.e., she is ageing then about 17 times faster than he is. As the velocity v decreases, the slope of the curve gets steeper, reaching a maximum of about 41 at the instant when v is momentarily zero. I.e., at that instant, she is ageing about 41 times faster than he is. Then, as his velocity increases again (negatively, toward her) during the second half of the -1 g acceleration, the slope of the curve again decreases, until it gets to about 17 at the end of the acceleration, when his velocity reaches -0.7739. I.e., at the end, she is again ageing about 17 times faster than he is. Overall, during the entire two-year -1 g acceleration, she aged about 32 times faster than he did (64.24 / 2). So the curve has a steep, thin "S" shape.
At the end of the -1 g acceleration, suppose that he turns his spaceship around (pointing it away from her), and accelerates at +1 g for the next two years of his life. During the first half of that acceleration, he is slowing down, but is still getting closer to her. Half way through the acceleration (when he is 29), he momentarily comes to a standstill, and their separation is 39.41 lightyears then. She is then 51.49 years old (according to her). Again, they will always agree about their corresponding ages whenever their relative velocity v is zero. So, since v is zero at that instant, he agrees that she is 51.49 years old then.
During the second half of his +1 g acceleration, he is moving away from her, and speeding up. At the end of that acceleration, he is 30 years old, she is 52.68, their separation is again 39.97 lightyears, and his velocity is +0.7739 ly/y, all according to her. The CADO equation then says that she is 21.75 years old, according to him.
So he concludes that, during that entire +1 g acceleration, she gets 59.49 years younger, whereas he got 2 years older. During the +1 g acceleration, she doesn't instantaneously get younger, but her age does decrease much faster than his age increases. So it is qualitatively fairly similar to what happens for an idealized instantaneous velocity-change.
If we compute more intermediate data during that +1 g acceleration, we can continue the age-correspondence graph that we drew above for the preceding -1 g acceleration. We get a curve similar to what we got before, but this time the slopes are negative, and the curve is like a "steep thin backward S shape".
In this case, the curve starts out, when the magnitude of the velocity is fairly high (-0.7739 ly/y), with a negative slope of about -16. I.e., she is getting younger then about 16 times faster than he is getting older. As the magnitude of the velocity v decreases, the slope of the curve gets steeper, reaching a maximum of about -39 at the instant when v is momentarily zero. I.e., at that instant, she is getting younger about 39 times faster than he is getting older. Then, as his velocity increases again during the second half of the +1 g acceleration, the curve again gets less steep, until the slope gets to about -16 at the end of the acceleration, when his velocity reaches +0.7739. I.e., at the end, she is again getting younger about 16 times faster than he is getting older. Overall, during the entire +1 g acceleration, she got younger about 30 times faster than he got older (-59.49 / 2).
A useful rule of thumb, provided their separation is sufficiently great, is that for a +-1g acceleration, the maximum rate of change in the age of the distant perpetually-inertial person, relative to the traveler's rate of ageing, will be approximately numerically equal to their separation, in lightyears. When the acceleration is directed toward the distant person, she will be getting older at that relative rate. When the acceleration is directed away from the distant person, she will be getting younger at that relative rate, as the traveler gets older. And in either case, that maximum relative rate will occur when the magnitude of their relative velocity is at its minimum.
Another interesting result of the CADO frame, is that, if an observer, at some instant of his life, begins some constant acceleration that lasts for the rest of his (assumed very long) life, then the distant perpetually-inertial person's age (according to the accelerating traveler) will approach a finite limit. And if their separation, at that beginning instant, has a certain critical value, the distant person's age will not change at all, from that initial value, at all later times.
Velocities, According to the Accelerating Observer
The velocity v, that appears in the CADO equation, is the velocity of the observer, relative to the perpetually-inertial distant person, according to that distant person. Each of the traveler's MSIRFs agrees with the distant inertial person's conclusions about that relative velocity. Similarly, the velocity of any light pulse, c, is the velocity of that light pulse according to the distant inertial person, and all inertial observers agree about that.
But an accelerating observer will generally disagree with the distant person about their relative velocity. And he will generally disagree with her about the velocity of any given light pulse.
The velocity of the accelerating observer, relative to the distant inertial person, according to the accelerating observer, is
v_T = v - ( L * v * a ) / gamma,
where L is the position of the accelerating observer, relative to the distant inertial person, according to the distant person. The quantity a is the observer's acceleration (as measured on the accelerating observer's accelerometer), in ly/y. a is positive when in the direction of positive v. All of the quantities in the equation are for some arbitrary, but given, instant of the accelerating observer's life.
Since a can be arbitrarily large, and either positive or negative, it's clear that the accelerating observer can conclude that the magnitude of their relative velocity is much larger, or much smaller, than the distant inertial person (and the MSIRF) says it is. In particular, it can be larger than c. And the accelerating observer can conclude that the direction of their relative velocity is the opposite of what the distant person, and the MSIRF, say it is. Also, since a can change essentially instantaneously, v_T can change essentially instantaneously.
According to the accelerating observer, the velocity of some given light pulse is
c_T = c + a * R_T / c ,
where c is the velocity of light, according to any inertial observer, a is the acceleration in ly/yr, and R_T is the position to the light pulse, relative to the accelerating observer, according to the accelerating observer. R_T, a, c, and c_T are positive when in the accelerating observer's positive spatial direction. The quantity c , which usually denotes a positive constant, is here a signed quantity (a one-dimensional vector), positive in the distant person's positive spatial direction, but negative in her opposite spatial direction.
Note that, according to the accelerating observer, the velocity of a light pulse depends on how far away it is from him. And a light pulse, as it passes him, always has the velocity c , regardless of his acceleration (because R_T is zero then).
Note also that, since his acceleration a can be arbitrarily large in magnitude, and either positive or negative, he can conclude that the velocity of a distant light pulse is much larger or much smaller than inertial observers say it is. And he can conclude that the pulse is moving in the opposite direction than the inertial observers say it is. Also, since a can change essentially instantaneously, c_T can change essentially instantaneously.
The Non-Invertibility of the CADO Frame
In the above example of a -1 g acceleration lasting for two years of the traveler's life, followed immediately by a +1 g acceleration lasting for another two years of his life, we got an age-correspondence graph that shows how the distant perpetually-inertial person's age changes during those four years of the traveler's life. That continuous curve looks a bit like a very high, but very narrow mountain peak, rising from 17 years old for her age when he is 26, to a peak of 81.24 years old for her age when he is 28, then back down to 21.75 years old for her age when he is 30.
For each age of the traveler during that segment between when he is 26 years old and when he is 30 years old, there is some specific value for her current age then. For example, the question "How old is she, when he is t years old?", where t is some age between 26 and 30, always has an answer, and it never has more than one answer. As a specific example, when he is 26.8 years old, she is 48.92 years old.
But if you ask "How old is he, when she is 48.9 years old?", you don't get only one answer. During his two year -1 g acceleration, he was 26.8 years old when she was about 48.9 years old. But during his +1 g acceleration during his two years immediately after his -1 g acceleration, he was about 29.07 years old when she was 48.9 years old. And in between his two ages of 26.8 and 29.07, she reached an age much greater than 48.9 years old.
So during that four-year segment of his life, she was 48.9 years old twice: once when he was 26.8 years old, and once again when he was 29.07 years old. And it is possible that he could have other ages, outside that four year segment of his life, when she is 48.9 years old.
The above is a specific numerical example, but it's easy to see from the age-correspondence graph that for any given age for her, between the bottom and the summit of that mountain-like curve, there will be two ages for him, not just one. And there could be more possible ages for him, when she has that given age, for regions of the age-correspondence curve outside of the region that we chose to investigate.
So it's clear that the CADO frame isn't "invertible". I.e., it is not true that for any choice of the age of some given distant perpetually-inertial person, there is a unique corresponding age for the traveler. By contrast, the inertial frame for a perpetually-inertial observer is invertible, because in that case, the age-correspondence graph is just some one-to-one (invertible) curve.
The fact that the CADO frame isn't invertible, means that the CADO frame cannot be used as one of the possible charts that general relativity "knits together" in order to cover the entire universe. Such charts must be invertible: they must provide a one-to-one mapping between the spacetime points within the domain of coverage of the chart, and the coordinate values of the chart. But there is no need to impose that requirement in the definition of a frame for an accelerating observer. All that matters to an accelerating observer is that he be able to determine the current age and current position of any given distant perpetually-inertial object or person in the (assumed everywhere flat) universe, in a way that is internally self-consistent, and in a way that is consistent with special relativity. The fact that some distant person, merely based on her own current age, cannot uniquely determine a corresponding age for the given traveler, is certainly of no fundamental importance to the traveler. (And, likewise, the fact that some accelerating observer somewhere happens to conclude that some given perpetually-inertial person is rapidly getting younger, is certainly of no fundamental importance to that given inertial person).
The chart consisting of the Rindler coordinates is quite similar to the CADO frame. The primary difference between the two, is that the Rindler chart is restricted to a neighborhood of the accelerating observer, whereas the CADO frame applies to all of (the assumed flat) universe. That is possible for the CADO frame, because the CADO frame isn't intended to be, nor has it any need to be, a chart.
Empirical Determination of the Current Age of a Distant Perpetually-Inertial Person
If the traveler is perpetually inertial, he can determine (at each instant of his life) the current age of the distant perpetually-inertial person, by using the Lorentz equations. (Or, he can get the same answer by using the CADO equation, of course). If he uses the Lorentz equations to get the answer, it can seem like an abstract operation, without any intuitive meaning. But he can also get the same answer in a very intuitive, and meaningful way.
Suppose he arranges for the distant person to periodically broadcast a TV image of herself, holding a sign that states her current age. When the traveler receives one of those images, he knows that the age being reported on the sign is not her current age at the instant that he receives that image, because the image doesn't travel infinitely fast ... the age on the sign tells him what her age was, at the instant when she transmitted that image. She is obviously older when he receives that image.
It is possible for the traveler, by using only elementary observations and elementary calculations, to determine how much she has aged while that image was in transit, and thus to determine what her actual current age was at the instant that he received that image. If he does that correctly, he will get exactly the same result that the Lorentz equations would have given him (and the same result that the CADO equation would have given him).
The traveler who is not perpetually inertial, can in principle carry out the same type of elementary observations and elementary calculations, that the perpetually-inertial traveler carries out above. And he will find that, during any segment of his life in which his acceleration is zero, his conclusions from those elementary calculations will always agree with a co-located perpetually-inertial observer's calculations. During that entire unaccelerated interval, he has just a single MSIRF ... the co-located perpetually-inertial observer is just an observer permanently at rest at the spatial origin of that MSIRF.
The fact that the traveler's calculations, during that unaccelerated segment, always agree with the calculations of that co-located perpetually-inertial observer, remains true no matter how short that segment is. It even remains true when the traveler is stationary with respect to that inertial observer for only a single instant. So, the definition of the CADO frame (that the accelerating observer always agrees with his MSIRF, about the current age (and current position) of a distant object or person) is not just an arbitrary, abstract definition ... it defines a reference frame that is consistent with the traveler's own (potential) elementary observations and (potential) elementary calculations.
Of course, those elementary measurements, and elementary calculations, would actually require a finite amount of time to actually carry out ... for some situations, they could take a very long time. So how is the above conclusion arrived at?
The argument is basically a counter-factual/causality argument: at any instant of his life, the traveler can, if he so chooses, decide to stop accelerating for more than a single momentary instant ... for some finite segment of his life ... before resuming accelerating again. He may not choose to ever do that, but he can if he wants. If he does, he can make the same kind of observations and calculations that a perpetually-inertial observer who is (temporarily) co-located with him during that segment can make, and they will always arrive at exactly the same answer.
So, the definition of the CADO frame (that the accelerating observer always agrees with his MSIRF, about the current age (and current position) of a distant object or person) is not just an arbitrary, abstract definition ... it defines a reference frame that is consistent with the traveler's own (potential) elementary observations and (potential) elementary calculations. Those observations and calculations are characterized as being potential observations and calculations, because he may choose to stop accelerating long enough to make them, or he may not. But by causality (i.e, by fact that the future cannot affect the past), his conclusions about the distant person's current age, at some given instant of his life, cannot depend on how he may choose to accelerate in the future, after that instant. The CADO frame is a reference frame that has a tangible meaningfulness to the accelerating observer.
Graphical Interpretation of the CADO Frame
We can create a graph (a Minkowski diagram) that shows (two-dimensional) spacetime from the home-twin's perspective. The usual convention is to plot the home-twin's time coordinate T vertically, and her spatial coordinate X horizontally. However, that choice is arbitrary, and it will be more convenient here to plot T horizontally, and X vertically.
In the home-twin's inertial frame, she is always located at the spatial origin (i.e., her position is always at X = 0). And we can choose the time coordinate T of that frame to directly correspond to her age. Then, the positive horizontal axis of the diagram corresponds to her world line: as her age increases, her spacetime point moves to the right along that positive horizontal axis. Any point along that positive horizontal axis corresponds to the home-twin at some particular age. We can put "tic marks" along that axis, showing how her age progresses. If we put a tic mark for every one of her birthdays, those tic marks will be equally spaced along the positive horizontal axis.
Now, if the traveler's acceleration a(t) is given (so that a(t) gives the acceleration on his accelerometer at each instant t of his life), then his location X, according to the home-twin, can be determined at each instant T of the home-twin's life. This is just the quantity L, whose determination was given earlier, specified as a function of T. So we can plot the curve corresponding to the traveler's location, according to the home-twin, at each instant T of her life. That curve is his world line, plotted on her Minkowski diagram.
At any point on his world line, the tangent to that line has a slope that is numerically equal to his velocity v, relative to her, according to her. So the slope of that curve must always be less than +1, and greater than -1, since according to her, the magnitude of his velocity can never exceed, or even exactly reach, the speed of light. Otherwise, his world line curve can have any shape, except that it will always be continuous, even for instantaneous velocity changes. The curve can have "kinks" in it (where the slope changes instantaneously), in the idealized limiting cases with instantaneous velocity changes.
Just as we did for her world line (the horizontal axis), we can put tic marks along his world line, showing how his age progresses. If we put a tic mark for every one of his birthdays, those tic marks will not be equally spaced along his world line: their spacings will vary, but will be more widely spaced than the tic marks for the home-twin (except for finite segments of his life when their relative velocity is zero).
At any point on his world line, we can determine his "line of simultaneity" at that instant. His line of simultaneity corresponds to "now", according to him. I.e., according to him, the current time and position, for every object or person in the (flat) universe, corresponds to some unique point on that line.
The line of simultaneity, through any given point on his world line, has a slope of 1/v. This can be visualized as follows: if the angle that the tangent to his world line (whose slope is v) makes with the horizontal axis is alpha, then the angle that line of simultaneity makes with the vertical axis is also alpha. For example, if the velocity v is a small positive number (much less than 1), then alpha will be a small angle (much less than 45 degrees), and the tangent to his world line will be rotated only slightly counter-clockwise with respect to the horizontal axis. The line of simultaneity will be rotated by that same small angle, clockwise with respect to the vertical axis. For a velocity v near +1, the angle of the tangent to the world line will be just slightly less than 45 degrees CCW with respect to the horizontal axis. The line of simultaneity will be rotated by that same angle (almost 45 degrees), CW with respect to the vertical axis. If we draw a 45-degree line (slope +1) through the given point on his world line, then as v gets closer and closer to +1, the tangent line rotates more and more CCW toward that 45-degree line (from below), and the line of simultaneity rotates more and more CW toward it (from above), in such a way that the two lines are always arranged symmetrically with respect to that 45-degree line.
The above description is for the case where the position of the traveler is above the horizontal axis (X > 0), and the velocity v is positive (the twins are moving apart). When v is negative (the twins are moving toward one another), the rotations are in the opposite direction, and the 45-degree reference line is rotated 45 degrees CCW with respect to the vertical axis (its slope is -1).
So, for any given point on his world line, we can immediately draw his line of simultaneity through that point. And we can then directly see where that line of simultaneity intersects the horizontal axis. Since the horizontal axis is the world line of the home-twin, that point of intersection directly gives us her current age, according to the traveler. That result, obtained graphically, is exactly what we can get (more quickly and easily) from the CADO equation.
The CADO Frame When the Distant Person Is Also Accelerating
If, in addition to the traveler's acceleration, the home-twin also decides to accelerate, the CADO equation generally isn't applicable. In a few special cases, where their accelerations consist only of instantaneous velocity changes, and where their respective velocity changes have a wide enough temporal separation, it is possible to use the CADO equations, but the process is fairly tricky.
But there is a way to determine, at each instant t of the traveler's life, what the current age T of the distant accelerating person is, according to the traveler, for any choices whatsoever of their two acceleration profiles.
Denote his (the traveler's) acceleration as a(t), as usual, and let her (the distant person's) acceleration be b(T). We then need to choose some single arbitrary (but given) inertial reference frame, and we will then construct the Minkowski diagram for that inertial frame. For twin-type scenarios, where the traveler and the "home-twin" are initially co-located and mutually stationary before the beginning of the traveler's trip, an obvious choice is the inertial frame whose spatial origin is permanently occupied by a third person, say, the mother of the twins. Both twins will accelerate (in spite of the "home-twin's" designated name), but their mother is permanently inertial. Let the time coordinate in the mother's inertial frame be denoted by tau, and let her spatial coordinate be denoted by X.
We then construct a Minkowski diagram as before, with X as the vertical axis, and tau for the horizontal axis. We can then plot the traveler's world line, as before. And as before, we can put tic marks on that world line, which give the traveler's age t for any point on that line. For any given point on that line, we can determine the traveler's line of simultaneity through that point. But we are no longer particularly interested in where that line of simultaneity intersects the horizontal axis tau. This time, we want to know where that line of simultaneity intersects the "home-twin's" world line. We can plot the home-twin's world line on that same Minkowski diagram, using the same process that we used to plot the traveler's world line. And we can again put tic marks on her world line, which give her age T at any point on her world line. The value of T at the point where the traveler's t line of simultaneity intersects her world line gives us the answer we've been seeking: the current age T of the (accelerating) "home-twin", according to the traveler, when the traveler's age is t. And, by carrying out that process for many different ages t of the traveler, we can construct the age-correspondence graph, that shows how the "home-twin's" current age varies, according to the traveler, as the traveler's age increases.
For each age t of the traveler, the above described procedure requires an iterative numerical process, in order to "home-in" on the point of intersection. Whether it's possible to find some kind of closed-form solution, that would eliminate that need for iteration, is an open question.

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