TRAVELLING TWINS

@Olivier5,

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There can be no such thing as a stationary twin, for a start.

This is incorrect. The word "stationary" simply means "having a velocity of zero". Why wouldn't a twin be able to have a velocity of zero?

When you solve problems like this, you pick a frame of reference. A physicist who solves this problem will refer to the "stationary" twin with the understanding that an inertial frame is implied.

The "twin paradox" is interesting because it involves non-inertial frames of reference.

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@maxdancona,

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Why wouldn't a twin be able to have a velocity of zero?

Because the universe is not empty. It's full of stuff, and these stuff tend to attract one another.

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@Olivier5,

That doesn't make any sense Olivier.

We measure velocity within a frame of reference. Generally we define that frame of reference based on an object... and by definition that object will have a zero velocity in the frame of reference.

I think the difficulty on this thread is people making the abstraction required to really understand what a frame of reference is.

But mathematically speaking, a twin can (and quite often does) have a zero velocity.

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@maxdancona,

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We measure velocity within a frame of reference

Exactly. So there is no such thing as a stationary twin, per se. It will depend on the frame you chose. One can chose a frame of reference in which one twin would appear stationary. One could also chose another frame of reference in which THE OTHER TWIN will appear stationary, so there is no such thing as a moving twin and then another non-moving twin PER SE. It all depends on the frame of reference you chose.

Quote:

A physicist who solves this problem will refer to the "stationary" twin with the understanding that an inertial frame is implied.

He will know better than assume a twin 'stationary'... At best he will say: "stationary in inertial frame of reference X".

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@Olivier5,

Hmmmm, you are missing something. Let me ask this question...

Is there any such thing as "velocity"? Can a twin have a velocity?

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@Olivier5,

By the way, we don't actually say "stationary in inertial frame of reference X"... at least not when we are talking to each other. When we talk about these things among people who have a background in physics, the frame is generally implied.

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@maxdancona,

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we don't actually say "stationary in inertial frame of reference X"... at least not when we are talking to each other.

But that is what is meant and implied.

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@maxdancona,

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Can a twin have a velocity?

In absolute sense? No. It has to be relative to something else.

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@Olivier5,

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But that is what is meant and implied.

Which is exactly my point. For me to say "the first twin is stationary" is not only perfectly appropriate and correct, it also sets a reference frame that is useful for understanding to problem.

If you take a college class in Relativity, the term "stationary observer" is pretty common.

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@Olivier5,

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In absolute sense? No. It has to be relative to something else.

Ahhhh.... A twin does have a measurable velocity. It doesn't have absolute velocity, but it has a perfectly good velocity that can be reliably and consistently measured in any given reference frame (we will leave QM out of this for the time being Wink

).

To say that something doesn't have a "velocity" doesn't make any sense in the study of physics where velocity is so important. Velocity is a real concept.

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@maxdancona,

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Generally we define that frame of reference based on an object... and by definition that object will have a zero velocity in the frame of reference.

That is quite correct insofar as SR DEFINES a frame of reference to be motionless (it has no such definition in other contexts). However, it also has nothing to do with plausibility, let alone "reality."

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@Olivier5,

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Can a twin have a velocity?

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In absolute sense? No. It has to be relative to something else.

A change in either speed or direction is a change in velocity. In SR acceleration is deemed to be "absolute." So, in SR, at least, there IS absolute velocity.

Of course I have pointed this out to you before, Ollie. Doesn't help you any, though, does it?

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@maxdancona,

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For me to say "the first twin is stationary" is not only perfectly appropriate and correct, it also sets a reference frame that is useful for understanding to problem.

It's not correct if you fail to state what your frame of reference is. There is no such thing as a stationary twin per se. It's all relative.

The twin paradox can only be solved by considering the different frames of reference involved, noticing that they are not interchangeable. If you start by stating that one twin is stationary, you de facto establish an implicit preferred frame of reference, without explaining why it is better than another for the problem at hand... It's an amateurish way to approach the issue. I prefer to be explicit and clear. None of the twins is 'stationary', but one remains in a frame of reference which can be considered (or approximated) as inertial, while the other isn't.

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@maxdancona,

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If you take a college class in Relativity, the term "stationary observer" is pretty common.

Maybe your college physics teacher was not very good?

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@Olivier5,

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It's not correct if you fail to state what your frame of reference is.

You are missing something in your understanding Olivier.

When I say "the first twin is stationary" it is absolutely correct. Any physicist would agree.... we say these things all of the time. This statement itself fixes a frame of reference. I don't need to state what the frame of reference is. It is obvious.

I have solved the "twin paradox" as part of my university work (it is a basic exercise every physics student does).

Your use of the word "amateurish" made me a chuckle a bit... I don't think you have any background in physics to be judging the people actually doing physics as "amateurish". Google the phrase "stationary observer" and see how many prominent prominent professional physicists break your rule.

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@Olivier5,

I am trying to do you a favor here Olivier, by asking you questions that may help you understand better. I have a graduate degree in Physics and have learned from a number of professors. If you are going to be silly, then this exercise is meaningless and we should both just stop.

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@Olivier5,

Quote:

If you start by stating that one twin is stationary, you de facto establish an implicit preferred frame of reference,...

This is absolutely correct (whether you "start out" or "end up" that way). The problem is that SR ostensibly does not permit the use of a "preferred frame of reference,"

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@maxdancona,

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I have solved the "twin paradox" as part of my university work (it is a basic exercise every physics student does).

How did you "solve" it, pray-tell? What is it that you were "solving?" The "paradox," or a mathematical equation?

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@maxdancona,

Don't condescend me. I've studied special relativity in some serious depth at college. You can either ignore what I say or try and understand it. Make your pick.

1) there is no such thing as a REAL STATIONARY TWIN. All the twins I know of are located on planet earth, which is evidently not stationary, as anyone opening a sink can testify (the water swirls around the sink due to the rotation of earth, aka Coriolis force).

2) Even if one would send a twin in space, his rocket will be accelerated one way or another by gravity of nearby objects, so he won't be inertial, ever. Therefore, THERE IS NO SUCH THING AS A REAL INERTIAL TWIN either. Only in a mind's experiment can one conceive of a twin in a purely inertial movement, in some totally void universe. It's a view of the mind, not a possible reality.

3) In this view of the mind, one can conceive of an 'inertial twin' i.e. a twin that is stationary (or in inertial movement) as compared to some inertial frame of reference.

To speak of a "stationary twin" without further specification and caveats is either misleading or simply, a shortcut to point 3 above: a view of the mind of a twin travelling inertially in some next-to-empty universe. You're not talking of any real twin here.

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@Olivier5,

Don't condescend you... with your post about my physics teachers Wink

That is very funny.

You have clearly crossed the line into being silly.

If you really feel this way, then google "stationary observer" and then send email to all of the real physicists who are happy to use the term. I get it that you are smarter than they are, but they have put in all the work to gain the education to get their positions... so you should cut them a little slack.

1 Reply

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