How Does the Twin Paradox Affect Aging in Relativity?

[W.RonG]

Not really challenging facts, just keeping the conclusions in check. I understand and believe in Special Relativity. In doing so, and being intellectually consistent and honest, I can only conclude that a clock moving relative to another clock appears to tick more slowly but in fact does not actually tick more slowly. Again, to conclude that it does would be to assign preference to the faster clock. This is a violation of SR.
Thanks,
Ron

 

[Jackslap]

So I have just only begun to understand the difference between GR and SR according to lectures on youtube (thanks TcheQ for the suggestion, why didn't I think of that?). Ron, you are saying then that there is still a group of people who don't believe in SR? I only ask that because you singled yourself out by saying "I believe in SR" as though some don't.

Why the debate, shouldn't special relativity just be accepted as fact? Someone is saying Einstein was wrong? What have I missed?

BTW, my wife says I'm turning into an internet physics nerd. I said "You should see what THESE guys are saying, you think I'm nerdy"...

The thing is she loves her some nerds. She has a secret crush on Conan O'brien. Yuck

 

[W.RonG]

There may be some who do not understand or believe in SR. I made that statement as a pre-emptive defense against someone who might claim that I don't. The mathematics of SR are basically inarguable. One thing that could possibly negate SR is if someone were to show that an observer measures something other than the same well-established c for the (local) speed of light. That woukl be earth-shattering. Meanwhile the Theory, if kept within its limitations, works quite well. (I have actually downloaded a copy of On the Electrodynamics of Moving Bodies in English, read it, and understand it.)
I've avoided posting any opinion yet in this thread. Since you asked, I think that some people get so buried in the fantastical specific notions implied by Relativity that they lose sight of reality. In constructing a particular Gedanken like the 'twins paradox', one must be careful to maintain the aforementioned intellectual integrity. The key is to always remember the two postulates stated by Einstein in his paper; this should help keep things grounded.
Another thing to keep in mind is that Einstein (and others) have described various physical actions but still have not explained the actions. Fodder for another thread . . .
Ron

 

[Fredrik]

This is the crux of the misunderstanding. Neither twin is 'staying still'. Unfortunately everything is moving relative to something else. That is why no clock actually runs slower than any other clock. The appearance of time dilation is not an actual slowing of anything, only a phenomenom applied to an observer's observation of a different inertial frame. Remember, there is no absolute reference frame.

There's nothing controversial in here.

Yeah I know. I actually took a long time to phrase the first post I made in this thread. I could already hear the screams of 'blasphemy' and 'heretic'! But the simple fact is that any observer is as preferred as any other; no more, no less. The actual 'paradox' is only in the phrasing of the riddle. By saying 'B' appears to 'A' to be running slower does not translate to the oft-quoted conclusion that 'B' is in reality running slower than 'A'. Just keep repeating: there is no absolute reference inertial frame, there is no absolute reference inertial frame, there is no absolute reference inertial frame . . .

...or here. It just sounds like you want people to call you a blasphemer.

Keep in mind that what I've said flies directly in the face of about a century of some very smart people saying some very wrong things. I really don't have much hope of changing minds here with my humble postings, but I could not just sit here and let the fallacious ideas continue unchallenged. Even that really really smart guy on Discover channel gets it wrong. Neither clock slows down. End.

I have no problem believing that Discovery channel gets it wrong, but it still sounds like you just believe people are saying things they aren't actually saying. I agree that the function of a clock is in no way disturbed when it's given a different velocity. It's still measuring the proper time of the curve that represents its motion, but that's now a different curve than before.

Not really challenging facts, just keeping the conclusions in check. I understand and believe in Special Relativity. In doing so, and being intellectually consistent and honest, I can only conclude that a clock moving relative to another clock appears to tick more slowly but in fact does not actually tick more slowly. Again, to conclude that it does would be to assign preference to the faster clock. This is a violation of SR.

But it does "actually" tick more slowly in the other guys reference frame. (It would however be wrong to end the sentence after "slowly"). If A and B don't have the same velocity, then A's clock is slow in B's frame, and B's clock is slow in A's frame.

 

[W.RonG]

Far be it for me to take on Mr. Michio Kaku, the really really smart guy I was referring to. But I have watched the shows he narrated about Time and remember distinctly the leap he made from stating one clock appears to another to be ticking more slowly, to the statement (after a commercial break) that one clock is really ticking more slowly than the one that is implicitly now preferred. The conclusion was unmistakable due to the implications of long distance space travel being accomplished by flying really fast (compared to what? is the question), same as the 'twins paradox'. The popular notion is that one clock, and one twin, experiences time flow more slowly, is pervasive and just plain wrong.
Hope we agree.
Ron

 

[Fredrik]

This link is helping me somewhat. http://casa.colorado.edu/~ajsh/sr/timebig.html

I understand the clocks and time dilation sections, but when I get to the Lorentz Gamma Factor and the Paradox image I get lost again.

The derivation of gamma is easy if you understand the pythagorean theorem. There are lots of proofs on the Wikipedia page. I like the one in the "algebraic" proof section. You just note that the area of the larger square is equal to the sum of the area of the smaller (light blue) square and the areas of the 4 triangles. When you write that down and simplify the expression, you get the pythagorean theorem.

The fact that the red perspective stays horizontal I get, but why does the blue need to be placed on a diagonal with a skewing of his "cone" area?

The argument can be made more rigorous than I'm making it here, but it still uses the same basic idea. Cerulean's spatial plane has to be tilted by the same amount and in the opposite direction as his time axis, because otherwise a line that's exactly half-way between Vermilion's time axis and his spatial plane wouldn't be exactly half-way between Cerulean's time axis and his spatial plane. (A line that's exactly half-way between the time axis and the spatial plane represents the motion of something that's moving at the speed of light, which is =1 in the units we're using in these diagrams, and is supposed to be the same in all frames. So if we don't tilt the spatial plane, we're either contradicting that assumption, or the assumption that no frame is fundamentally different from any other).

Is this like the Einstein example of the ball being dropped from a train? That it travels on a diagonal to a "stationary" observer at the train station?

Yes, that's sounds like an accurate description of what's going on in the time dilation picture.

 

[stevmg]

Old comic strip character from 50+ years ago - a man hires onto a cargo spcecraft which travels near the speed of light and takes a 50 year round trip, always moving really quickly. He ages a few moths. He returns to Earth and finds his fiancee is now over 70 years old.

Depressed, he goes back to that space cargo ship to work as feels isolated and alone with no one he remembers.

 

[sylas]

Far be it for me to take on Mr. Michio Kaku, the really really smart guy I was referring to. But I have watched the shows he narrated about Time and remember distinctly the leap he made from stating one clock appears to another to be ticking more slowly, to the statement (after a commercial break) that one clock is really ticking more slowly than the one that is implicitly now preferred. The conclusion was unmistakable due to the implications of long distance space travel being accomplished by flying really fast (compared to what? is the question), same as the 'twins paradox'. The popular notion is that one clock, and one twin, experiences time flow more slowly, is pervasive and just plain wrong.
Hope we agree.
Ron

No, the clock really is ticking more slowly when it flies really fast.

This can be ambiguous in some contexts, such as two clocks moving inertially. Each one is running more slowly than the other.

However, there are other cases which are not so symmetrical. Consider a clock moving in circles around another clock which is inertial. The one moving in circles really is moving more slowly, and each clock can agree on this. The distance between the clocks is unchanging, and the signals sent from one to the other come at regular intervals, which can be used to establish how fast each clock is running in the frame of the other. The clock moving in circles runs more slowly.

Similarly, a clock up a mountain runs more quickly, due to gravitational time dilations.

There's no real question about this. There's a good description of a fabulous measurement of the change in time for a clock at different altitudes in Clocks, Kids, and General Relativity on Mt Rainier. See also [post=2532205]msg #163[/post] in thread "Twins paradox and ageing"

Cheers -- sylas

 

[Fredrik]

No, the clock really is ticking more slowly when it flies really fast.

Sylas, when you say "the clock really is ticking more slowly", you really should add "in an inertial coordinate system where the clock has a non-zero velocity", "in the other clock's frame", "in the frame where it was at rest before we changed its velocity", or something like that. When you say "really is" without adding that extra information, you're making it sound like it's ticking more slowly in an absolute sense.

I think W.RonG was just saying that it's a bad idea to leave out those extra words, and I agree with him about that. This is based on his earlier posts in the thread, but I agree with you that this sounds pretty strange:

The popular notion is that one clock, and one twin, experiences time flow more slowly, is pervasive and just plain wrong.

Ron, it almost seems like you want people to misunderstand you.

 

[sylas]

Sylas, when you say "the clock really is ticking more slowly", you really should add "in an inertial coordinate system where the clock has a non-zero velocity", "in the other clock's frame", "in the frame where it was at rest before we changed its velocity", or something like that. When you say "really is" without adding that extra information, you're making it sound like it's ticking more slowly in an absolute sense.

I thought I did give that extra information, in the next line, which says that this is ambiguous; specifically in the case that there are two inertial clocks moving relative to each other. Each one really is ticking more slowly in the frame of the other.

But the circular motion case no longer has that symmetry. Each clock agrees that the one which is moving in circles is the one that is moving more slowly. (I specified that one is inertial, and the other is moving in circles around it).

Each clock perceives the other as moving in circles around it, but only one of the clocks is actually inertial. Let "A" be inertial, and "B" be non-inertial, moving in circles around "A".

In the "frame" (not inertial frame, though) of the non-inertial clock "B", there is a pseudogravitational force directed away from the (apparently) circling clock 'A", and the (apparently) circling clock "A" is running fast. In the frame of the inertial clock "A". there are no pseudogravitational forces, but the circling clock "B" is running slow. Both clocks agree that it is "B" which runs more slowly than "A".

For any other inertial observer, "A" is running slow, and "B" is running even slower than that, on average, with a curiously sinusoidal variation in the dilation factor, unless the other observer is moving perpendicular to the plane of "B"s orbit around "A".

I am probably not helping here; and could phrase the examples more clearly.

However, this example does show that a circularly moving clock "really" does run more slowly than the clock it is circling. Not because there is some absolute frame, but because "B" is slower than "A" in ALL frames, as long as you average out any cyclic variations in the dilation than can occur.

Cheers -- sylas

 

[atyy]

Ron, it almost seems like you want people to misunderstand you.

Yes, what he's saying sounds terribly standard. This sounds like another what's really real discussion. Why not get him to make an experimental prediction? As long as we agree on that, all will be happy, right?

 

[JesseM]

But the circular motion case no longer has that symmetry. Each clock agrees that the one which is moving in circles is the one that is moving more slowly. (I specified that one is inertial, and the other is moving in circles around it).

All inertial frames (and any non-inertial ones that aren't extremely contrived) should agree that over the course of an entire orbit the circling clock elapses less time, but it might be potentially misleading to say everyone agrees the circling clock "is the one that is moving more slowly", since the instantaneous rate of ticking of the circling clock at a particular moment (or its average rate of ticking over a period of time less than one orbit) can still be faster than the inertial clock in some frames.

 

[yuiop]

The conclusion was unmistakable due to the implications of long distance space travel being accomplished by flying really fast (compared to what? is the question), same as the 'twins paradox'. The popular notion is that one clock, and one twin, experiences time flow more slowly, is pervasive and just plain wrong.

Fredrick suggests you are being provocative here and actually know better and if that is the case, all you are doing is confusing the heck out of newcomers like Jackslap who genuinely seek understanding of the subject.

Can you make it clear if you are stating that in the classic 'twins paradox' you are claiming that when the traveling twin returns home and is once again united with his sibling, that they will not have aged by different amounts in a way that is not only measured by their respective clocks but also manifested physically in how they aged biologically?

If that is what you are claiming, then it contradicts your claim that you 'believe in SR", because anyone that understands SR will know the twins age differentially and that experiments with particles such as muons support that prediction.

If on the other hand you are talking about two twins with purely inertial motion relative to each other, with no turning around and no acceleration, then I would agree with you that it is impossible to define which twin is (really) ageing slower, but that wouldn't be the classic 'twins paradox' would it?

 

[sylas]

All inertial frames (and any non-inertial ones that aren't extremely contrived) should agree that over the course of an entire orbit the circling clock elapses less time, but it might be potentially misleading to say everyone agrees the circling clock "is the one that is moving more slowly", since the instantaneous rate of ticking of the circling clock at a particular moment (or its average rate of ticking over a period of time less than one orbit) can still be faster than the inertial clock in some frames.

Quite right... and I mentioned that; speaking of the average time, and indicating that there would be (for an inertial observer) a kind of sinusoidal variation in the dilation factor, unless observed with a frame moving perpendicular to the plane of the orbit.

kev has aksed the crucial question, however, in the preceding post. That should be the first point Ron should clear up, before worrying about my examples.

Cheers -- sylas

 

[Jackslap]

My mind is getting nuked here! I was really understanding what Ron was saying because I thought it was right, but now you are all saying it isn't. And even though I still don't get it fully, I'm coming closer I can feel it.

Now that I know what you guys mean by frames, inertial frames, absolute frames etc...I'm starting to grasp this whole "aging slowly" thing.

Ron had me thinking for a while that SR was some sort of exception to the rule or something, but are you all saying that it isn't?

In the twin paradox, the INERTIAL twin will actually be biologically/physically younger than the other one right? I can accept that even though I don't know it as deeply as you guys. I have come to a deeper understanding of how spacetime works and how the two are related. I've have learned that since speed is simply a measurement of the amount of distance moved in a certain time it has become clearer to me that a person who is traveling close to c is beginning to travel through time more slowly.

I watched a youtube video that said it like this : You have a little graph where the x-axis is time and the y-axis is space. If you are stationary is space, not moving at all, then you are already traveling at the maximum speed through time (x axis). But as soon as you start to move through space, in addition to time, you are covering less time as you cover more space. At the speed of light you are covering essentially only space, and no time.

I can't word it like the video shows it, but it helped to open my eyes a bit. It's hard to accept that one twin will ACTUALLY AGE LESS than the other, biologically included. I still have a tough time with that. Because to him his biological clock is normal, but to the other HIS clock is normal. There can't be two normals only one.

But I guess that is what relativity really is huh? There is no ONE normal, but infinite normals. Am I getting there yet?

 

[Fredrik]

In the twin paradox, the INERTIAL twin will actually be biologically/physically younger than the other one right?

It's actually the other way round. A clock measures the proper time of the curve that represents its motion, and of all the timelike curves from the event where your twin leaves Earth to the event where he comes back, there is no one with longer proper time than yours.

I've have learned that since speed is simply a measurement of the amount of distance moved in a certain time it has become clearer to me that a person who is traveling close to c is beginning to travel through time more slowly.

I watched a youtube video that said it like this : You have a little graph where the x-axis is time and the y-axis is space. If you are stationary is space, not moving at all, then you are already traveling at the maximum speed through time (x axis). But as soon as you start to move through space, in addition to time, you are covering less time as you cover more space. At the speed of light you are covering essentially only space, and no time.

I'm not a big fan of that way of thinking about these things, mostly because I don't find it useful. And I would say that the last sentence is wrong. But don't worry, you're in good company. What you're saying here (including the last sentence) is just what Brian Greene said in "The elegant universe".

Also note that in the "spacetime diagrams" that we keep talking about, time is usually drawn in the "up" direction (i.e, as the "y" axis).

Because to him his biological clock is normal, but to the other HIS clock is normal.
...
But I guess that is what relativity really is huh? There is no ONE normal, but infinite normals. Am I getting there yet?

I'd say that's right, and that you're getting closer.

 

[stevmg]

The atomic clocks in the GPS satellites do move slower than their "clones" on Earth. They have to be juiced up with the inverse of the gamma factor. These orbiting atomic clocks have speed and centripetal force "slowing them down." There is no reason that living creatures would behave differently.

 

[Jackslap]

So the twin in the INERTIAL frame is the twin that is not moving? I must be confused on inertia then. (Visits wiki)

O.K. I looked it up and inertia is resistance to change of motion, so that means that it IS the twin that is not moving in my opinion (hopefully everyones).

So Fredrik, when you say you are not a big fan of "that way of thinking", exactly what do you mean by that? Is there a different/better way to simplify it? The X and Y graph was the best thing I could find on google and youtube. No anger here, just wondering if there's something better. I had a hard time with that one at first too, but after watching it several times and breaking it down statement by statement it became a bit clearer. If I had to say that I'm still slightly confused about something with it, it would be that the graph has 186,000 annotated in both axes, and that the video says as long as you exist in the universe and are not moving, you are traveling at the maximum speed through time.

I guess I get it, but I have a tendency to always second guess myself until something is EMBLAZONED in my brain. For reference, here is a link to the video. The part I'm talking about is at the 2:50 mark. I watched all 4 parts of that series and it helped me with a few major points so all it all I thought it was good.

 

[Dale]

Is there a different/better way to simplify it?

The approach I prefer is the spacetime geometric approach. Do you know how to draw spacetime diagrams? If so, can you understand geometrically how the stay-at-home twin's path forms one side of a triangle and the traveling twin's path forms the other two sides?

 

[Jackslap]

Not a clue about that. But my mind is sufficiently blown for the night. Google is my friend tomorrow.

 

[Fredrik]

So the twin in the INERTIAL frame is the twin that is not moving? I must be confused on inertia then. (Visits wiki)

O.K. I looked it up and inertia is resistance to change of motion, so that means that it IS the twin that is not moving in my opinion (hopefully everyones).

"Inertial" means "not accelerating". In the twin "paradox", the twin that stays on Earth can be described as "inertial". The astronaut twin can't, because he's accelerating when he turns around to go back home. The twin on Earth will be older than the astronaut twin when they meet again.

So Fredrik, when you say you are not a big fan of "that way of thinking", exactly what do you mean by that?

I just don't see how the concept of "speed through time" can help anyone understand anything, but I do see how it can confuse people. If someone tells you that an object X with velocity v=0.8c relative to you is moving slower through time than you are, do you even know what that means?* Is it clear to you if the word "space" refers to what you consider space, or what X considers space?** Did you know that you and X won't agree about which slice of spacetime is space?

*) It means that the invariant square of the the projection of the normalized tangent vector of the curve that represents's X's motion onto the normalized tangent vector of the curve that represents your motion is greater than -1. And that's really just a complicated way of saying that it doesn't have the same velocity (through space) as you.

**) It has to be a reference to what you consider space. Any object has velocity 0 through what it considers space.

The funny thing is, when they proceed to say that a photon moving at c is only moving through space, the word "space" is now referring to what it would consider space if it made any sense to define the photon's point of view by considering speed v and taking the limit v→c. And it doesn't make sense to do that, so that statement is not only misleading in at least two different ways, it's also wrong.

This is why:

The reason why we associate a specific inertial coordinate system with the motion of an inertial observer is that there's a clock synchronization procedure that makes that the natural choice. All the statements about Lorentz contracton, time dilation, etc., are consequences of that choice. The claim that massless particles experience no time comes from applying the usual time dilation formula for speed v and taking the limit v→c, but there's no reason why we should think of the result of that procedure as "a photon's point of view". There is however a good reason not to: The clock synchronization procedure doesn't work for massless particles. See my posts in this thread (at Physics Forums) for more about this.

Is there a different/better way to simplify it? The X and Y graph was the best thing I could find on google and youtube. No anger here, just wondering if there's something better.

Nothing is better than spacetime diagrams. (OK, I can think of a few things, but none of them have anything to do with physics).

the video says as long as you exist in the universe and are not moving, you are traveling at the maximum speed through time.

Don't worry about it. It's a useless idea anyway. I would recommend that you get some practice drawing spacetime diagrams, and pay extra attention to the concept of simultaneity. A spacetime diagram has an x and a t axis, with the t axis drawn in the up direction. The most important thing to understand is that if the world line of another inertial observer has slope v in the diagram, his simultaneity lines have slope 1/v. This is the result of the synchronization procedure that I described briefly in the post I linked to in the quote above.

For reference, here is a link to the video.

You didn't actually post the link.

Not a clue about that.

The quote below links to a spacetime diagram for the twin paradox, with comments about how the twins would describe things at different points on their world lines, in terms of the coordinates of their momentarily co-moving inertial frames.

Check out http://web.comhem.se/~u87325397/Twins.PNG .

I'm calling the twin on Earth "A" and the twin in the rocket "B".
Blue lines: Events that are simultaneous in the rocket's frame when it's moving away from Earth.
Red lines: Events that are simultaneous in the rocket's frame when it's moving back towards Earth.
Cyan (light blue) lines: Events that are simultaneous in Earth's frame.
Dotted lines: World lines of light rays.
Vertical line in the upper half: The world line of the position (in Earth's frame) where the rocket turns around.
Green curves in the lower half: Curves of constant -t^2+x^2. Points on the two world lines that touch the same green curve have experienced the same time since the rocket left Earth.
Green curves in the upper half: Curves of constant -(t-20)^2+(x-16)^2. Points on the two world lines that touch the same green curve have experienced the same time since the rocket turned around.

The black vertical line serves no purpose at all. I should have deleted it a long time ago, but I've been lazy.

 

[stevmg]

R Shakar at Yale University has a series of physics lectures and he goes into that very question. In essence, the guy standing still ages while the guy moving ages more slowly. That's true for clocks and all systems that work on time.

I can't quote him, Google it and you will see. His lectures 12, 13, 14 and 15 cover simple relativity quite well.

 

[W.RonG]

R Shakar at Yale University has a series of physics lectures and he goes into that very question. In essence, the guy standing still ages while the guy moving ages more slowly. That's true for clocks and all systems that work on time.

I can't quote him, Google it and you will see. His lectures 12, 13, 14 and 15 cover simple relativity quite well.

But in Relativity, nobody is standing still. Everybody is 'the guy moving'.
Ron

 

[W.RonG]

Funny that the name Brian Greene came up. I remembered another example of the aging difference fallacy. I was perusing the physics section of a large book store about a year ago, happened on Mr. Greene's book more recent than the Elegant Universe. Not sure the title, but I opened it to the page where he makes the same mistake of confusing the appearance of a clock ticking more slowly to an observer in a different inertial frame, to make the statement that there is a 'moving' clock ticking more slowly than a 'stationary' clock. Oops.

 

[Fredrik]

Ron, I think you're just misunderstanding what other people are saying. You're right that nobody is standing still in an objective sense, but once you have picked a coordinate system, then it's possible to really stand still in that coordinate system. Is it possible that most of the people that you think are wrong are just describing things in terms of a coordinate system?

 

[sylas]

Ron, it will help clarify your position if you could answer kev, in msg #43.

Can you make it clear if you are stating that in the classic 'twins paradox' you are claiming that when the traveling twin returns home and is once again united with his sibling, that they will not have aged by different amounts in a way that is not only measured by their respective clocks but also manifested physically in how they aged biologically?

If that is what you are claiming, then it contradicts your claim that you 'believe in SR", because anyone that understands SR will know the twins age differentially and that experiments with particles such as muons support that prediction.

If on the other hand you are talking about two twins with purely inertial motion relative to each other, with no turning around and no acceleration, then I would agree with you that it is impossible to define which twin is (really) ageing slower, but that wouldn't be the classic 'twins paradox' would it?

Can you respond to this please Ron? It would help clarify things, I think.

Cheers -- sylas

 

[W.RonG]

Recall that the 'Twins Paradox' arose specifically out of Special Relativity as an effect of Time Dilation. It is quite simple; does Time Dilation cause one clock to tick more slowly than another? the consequence of this phenomenom is the paradox, that one clock will count less time than another if they meet up after 'one of them' has traveled for a while. Answer: no.
Thanks,
Ron

 

[sylas]

Recall that the 'Twins Paradox' arose specifically out of Special Relativity as an effect of Time Dilation. It is quite simple; does Time Dilation cause one clock to tick more slowly than another? the consequence of this phenomenom is the paradox, that one clock will count less time than another if they meet up after 'one of them' has traveled for a while. Answer: no.
Thanks,
Ron

That clarifies. And I mean no offense, but this means you don't believe SR... and if you think you do, then you don't understand it either.

In special relativity, the non-inertial clock will count less time, and this is confirmed when they are brought back together. The amount of time difference between the two clocks can be calculated, and it is confirmed by experiment that the calculations do give the amount of time difference experienced between the two clocks.

It was important that you answered this plainly, though, so we could understand your position more clearly. Thanks.

But you are wrong.

Cheers -- sylas

 

[atyy]

Recall that the 'Twins Paradox' arose specifically out of Special Relativity as an effect of Time Dilation. It is quite simple; does Time Dilation cause one clock to tick more slowly than another? the consequence of this phenomenom is the paradox, that one clock will count less time than another if they meet up after 'one of them' has traveled for a while. Answer: no.
Thanks,
Ron

This is wrong. It can be correct, depending on the exact definition of terms, to say that no ideal clock ever ticks more quickly or more slowly depending on its motion. However, the elapsed proper time will be different for an inertial clock and a non-inertial clock that meet up at the start and end of their spacetime trajectories.

 

[W.RonG]

To 'atyy' and 'sylas': you are assigning preference to one inertial frame over another.
Ron