Frame-Jumping: What Does it Mean?

[bcrowell]

term "frame-jumping"

I've come across internet discussions where the term "frame-jumping" is used. As far as I can tell from googling, it seems to come up a lot in discussions involving anti-relativity kooks. I looked in the indexes of some relativity books I own, and couldn't find it. Googling doesn't seem to show it being used by professional relativists. My impression, then, is that the term itself has an ill-defined, kook flavor to it, and is used by kooks (whether to criticize other kooks or to criticize standard relativity). On the other hand, it is possible that it is a meaningful term that helps to demonstrate a certain error made in kook theories. Can anyone shed any light on this for me?

 

[Naty1]

No such thing is discussed in the roughly 20 popular books on physics/relativity/blackholes/quantum gravity/holographic principle I've read over the last five years or so...nor in Penrose's THE ROAD TO REALITY.

 

[Fredrik]

I don't recall ever hearing the term before the recent thread with "frame-jumping" in the title, but it seems like an appropriate term for the common mistake that leads to the twin paradox and many other incorrect results. (To do different parts of the calculation in different coordinate systems, and not make any corrections for the errors introduced by this).

 

[jtbell]

Fredrik's guess is correct. The term "frame jumping" was and probably still is used frequently in the Usenet newsgroup sci.physics.relativity in arguments with anti-relativity cranks and crackpots.

In fact, it's even been used occasionally here on PF:

http://www.google.com/#hl=en&source=hp&q="frame+jumping"+site:physicsforums.com

 

[Frame Dragger]

The entire notion of "jumping" from one inertial frame to another is anathema to GR as far as I know. Linear and Rotational Frame DRAGGING (yes, like my nickname) almost certainly occurs. A fine example of Rotational Frame Dragging is found in the Ergoregion of a BH.

 

[bcrowell]

Thanks, all, for the information. Looking through the occurrences of the term on PF, what I seem to get is discussions about the validity of the derivation of the time dilation factor using the standard elementary argument involving a light clock and the Pythagorean theorem:

https://www.physicsforums.com/showthread.php?t=368598

http://www.physforum.com/index.php?showtopic=21780

My interpretation of these two threads is that the participants all agree that standard SR is correct, but some people don't like the light-clock derivation, and claim that the error in it is something called "frame-jumping." In neither case do the light-clock critics explain what they mean by "frame-jumping." In both cases, the light-clock critics come off to me as people with shallow, authority-based understandings of SR, who believe that the only correct way to derive the Lorentz transformation is the way Einstein did it in his 1905 paper.

Fredrik's guess is correct. The term "frame jumping" was and probably still is used frequently in the Usenet newsgroup sci.physics.relativity in arguments with anti-relativity cranks and crackpots.

In fact, it's even been used occasionally here on PF:

http://www.google.com/#hl=en&source=hp&q="frame+jumping"+site:physicsforums.com

So, jtbell, if you're saying that "frame-jumping" is a useful label for a certain type of error, do you think that there is an error in the light-clock derivation, and users Trout and sponsoredwalk are correct in the two threads above? Or if you think the light-clock derivation is correct, but Trout and sponsoredwalk are misapplying the "frame-jumping" criticism, can you point us to a case where it's applied correctly to debunk an incorrect calculation?

 

[GRDixon]

Can anyone shed any light on this for me?

My take is that it simply means accelerating out of one IRF into another (e.g., hopping from a station platform into a slowly moving train, where it's assumed that the platform and train are at rest in different IRFs). In other words, I interpret it as meaning "changing inertial rest frames".

 

[bcrowell]

My take is that it simply means accelerating out of one IRF into another (e.g., hopping from a station platform into a slowly moving train, where it's assumed that the platform and train are at rest in different IRFs). In other words, I interpret it as meaning "changing inertial rest frames".

What you've described sounds perfectly legitimate to me...? Isn't this what a Lorentz boost describes?

 

[Frame Dragger]

That does sound fine, but I don't think that "Frame Jumping" is an accurate description or a real term of art.

 

[GRDixon]

What you've described sounds perfectly legitimate to me...? Isn't this what a Lorentz boost describes?

I don't know. I had never heard the term before visiting this forum a week or two ago. (I'm an amateur and not that well read.) Elsewhere in the forum I was told that Einstein considered sequences of events from successive IRFs in order to deduce what things would "look like" to an accelerating observer. He then took the limit of infinitesimal differences in the velocities of the IRFs, as I understand it, to get an idea what the world might look like (i.e., be measured to be) to a continuously accelerating observer.

Most of my posts in relativity are SRT, as I know next to nothing about GRT. If you can recommend a good, in print introductory book for a duffer like me, I'll shop for it online.

 

[bcrowell]

Most of my posts in relativity are SRT, as I know next to nothing about GRT. If you can recommend a good, in print introductory book for a duffer like me, I'll shop for it online.

I like Relativity Simply Explained, by Gardner.

 

[GRDixon]

I like Relativity Simply Explained, by Gardner.

I'm sure I will too. I just ordered a copy from Amazon.com. Many thanks.

 

[Fredrik]

...what I seem to get is discussions about the validity of the derivation of the time dilation factor using the standard elementary argument involving a light clock and the Pythagorean theorem:

https://www.physicsforums.com/showthread.php?t=368598
...
So, jtbell, if you're saying that "frame-jumping" is a useful label for a certain type of error, do you think that there is an error in the light-clock derivation, and users Trout and sponsoredwalk are correct in the two threads above? Or if you think the light-clock derivation is correct, but Trout and sponsoredwalk are misapplying the "frame-jumping" criticism, can you point us to a case where it's applied correctly to debunk an incorrect calculation?

I realize the question wasn't for me, but I'll quote myself anyway:

The derivation isn't wrong, but it contains a hidden assumption: It's assumed that all observers will agree about the length of a straight line that's perpendicular to the direction of motion.

Kev posted an interesting argument in that thread to justify this extra assumption.

The obvious example of an incorrect calulation where it would be appropriate to use the term (if it means what I think it does) is the twin paradox: In the inertial frame that's co-moving with the rocket as the astronaut twin is moving away from Earth, the Earth twin is aging at 60% of the astronaut twin's aging rate. In the inertial frame that's co-moving with the rocket as the astronaut twin is heading back to Earth, the Earth twin is aging at 60% of the astronaut twin's aging rate. [strike]Therefore, if the astronaut has aged 10 years when they meet again, the Earth twin has only aged 6 years[/strike].

As I'm sure you know already, that conclusion is incorrect because we "jumped" from one inertial frame to another without taking into account that the two frames disagree about what event on Earth is simultaneous with the turnaround event. (In one of my posts about this, I invented a term for the correction that needs to be made because of this. I called it a "simultaneity shift". I'm guessing that "frame-jumping" is another such term that someone invented in a discussion because he felt that it was useful to have a word for it).

Some additional information, for people who are less familiar with the twin paradox than I think you are:

The part that's hard to get is that there's no way for the astronaut twin to turn his ship around without having his brother's age "jump ahead" by a large amount. You need to understand simultaneity in SR before you can understand that.

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.

I need to delete that unnecessary vertical line. It still annoys me every time I see it.