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*From*: John Denker <jsd@av8n.com>*Date*: Mon, 03 Nov 2014 11:43:53 -0700

On 11/02/2014 12:27 PM, Ken Caviness wrote in part:

_proper_ acceleration

In all generality: Very often there is more than one way of

looking at things.

a) The expert will be able to see it in more than one

way.

-- Sometimes the expert will be able to translate back

and forth.

-- Sometimes one viewpoint will be prohibitively inconvenient,

and the expert will have to make a choice.

b) The non-expert will be able to see it in only way.

Different non-experts will have incompatible views.

c) The beginner might not be able to see it at all.

So it is with special relativity.

++ The modern (post-1908) view emphasizes proper time,

proper length, proper acceleration, invariant mass,

four-vectors, spacetime diagrams, geometry, trigonometry,

et cetera.

-- The archaic (pre-1908) view emphasizes dilated time,

contracted length, various notions of velocity-dependent

mass, et cetera.

The archaic viewpoint is not necessarily wrong. In simple

situations, it can be used to obtain correct answers. In

more advanced situations, it can be prohibitively inconvenient.

There are pedagogical and practical reasons for preferring

the modern (post-1908) approach. It is simultaneously easier

to learn, easier to teach, easier to use, and more powerful.

It can be used to explain the archaic viewpoint easily ...

whereas the converse is not so easy.

In the modern approach, you can get along just fine without

any contracted lengths, without any dilated times, without

any notion(s) of velocity-dependent mass, et cetera.

If you rotate a ruler in ordinary Euclidean space, "the"

length of the ruler does not change. The length of the

shadow that the ruler projects on the floor might change,

but that is not "the" length of the ruler.

Similarly, if you boost a ruler in spacetime, the modern

viewpoint says "the" length does not change. That is to

say, the proper length does not change. The /projection/

of the ruler onto the lab frame might change, but that

is not "the" length of the ruler.

Let's be clear: "the" length -- the proper length --

does not undergo Fitzgerald-Lorentz contraction.

Meanwhile, if you rotate an object in such a way that the

shadow should get shorter, but it doesn't, it means the

object must have stretched.

When I wrote up my notes on relativistic acceleration of

an extended object, I used the modern (post-1908) approach

exclusively. I wasn't consciously trying to make a point;

that's just how I think about it. That's how I was trained.

The professor told us quite explicitly: "The point is not

to learn how to do contractions and dilations. The point

is to learn how to /avoid/ doing contractions and dilations."

That's the sort of pronouncement that gets your attention.

I didn't fully understand what he meant, but I could tell

it was important.

However, today I am trying to make a point: the modern

spacetime viewpoint is not the only way of looking at

things, but it has treeeemendous advantages.

I can perfectly well follow the contraction/dilation

approach, but all the time I am saying to myself, wow,

is that clumsy and archaic.

I know there are plenty of introductory-level textbooks,

web sites, and TV "science" shows based on the premise

that the history of relativity began /and ended/ in 1905,

but that is just wrong. Wildly wrong.

The point I am making today is only tangentially connected

to the "acceleration" issue; it applies to all of relativity.

OTOH since the acceleration puzzle is a tremendous magnet for

misconceptions, there are big rewards for a clear, careful

modern approach.

For the next level of detail, see

https://www.av8n.com/physics/spacetime-welcome.htm

That stands in contrast to a bunch of dirty laundry that

I would *not* inflict on introductory-level students:

https://www.av8n.com/physics/spacetime-dirty-laundry.htm

**Follow-Ups**:**Re: [Phys-L] proper acceleration, proper time, proper length, spacetime geometry and trigonometry***From:*Larry Smith <larry.smith@snow.edu>

**References**:**Re: [Phys-L] relativistic acceleration of an extended object***From:*Moses Fayngold <moshfarlan@yahoo.com>

**Re: [Phys-L] relativistic acceleration of an extended object***From:*John Denker <jsd@av8n.com>

**Re: [Phys-L] relativistic acceleration of an extended object***From:*Moses Fayngold <moshfarlan@yahoo.com>

**Re: [Phys-L] relativistic acceleration of an extended object***From:*Ken Caviness <caviness@southern.edu>

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