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On 11th of June 2011, I noticed my web-logs showed a number of external links from a Physics Forums discussion:  http://physicsforums.com/showthread.php?t=513491
(a screen capture here, in case it’s deleted)

Naturally being interested to know what was being discussed, I hopped onto their site to find the following:


Is this GR explanation legit?

mishima  wrote:

From an online book where the author tries to discredit GR, there's a part where he talks about forgetting about the rubber sheet analogy and just remembering:

"A body moves along the path that makes time dilation a minimum."

Then he presents a diagram and a little argument that is really clear. I'm wondering if at least that part is ok or if the whole book is garbage.

Link: [...link deleted by bcrowell...]

I'm curious because I've been struggling for a way to conduct a lesson on GR for 9th graders and this is the simplest explanation I've come across so far (if correct at all). Thanks and sorry if this is taboo to post. I just don't honestly know enough about GR to criticize this properly.

It was replied to by user bcrowell

Hi, mishima,

Actually it is a no-no to link to crackpot sites, and Burchell's Alternative Physics site is definitely crackpot. I've edited the link out of your post.

But yes, the quote is more or less right.

The one important mistake in the quote is that it's not time dilation that is minimized, it's simply the time measured on a clock that moves along with the body from the initial event E1 to the final event E2. This clock time is usually referred to as the proper time ("proper" meaning "its own").

More minor quibbles: (1) The body has to have a small mass. (2) The proper time doesn't actually have to be minimized, just extremized. That is, it could be a local maximum rather than a local minimum. (3) The extremum is local, not global. That is, the time is only at an extreme compared to other paths that differ from it infinitesimally.

A book that presents a significant amount of GR without any math is Geroch, General relativity from A to B. Gardner's Relativity simply explained is lots of fun, although it's pretty cartoonish. A book that uses a little more math, but that I like better than Geroch, is Taylor and Wheeler, Exploring black holes.


Within 30 minutes of the original message, bcrowell (Benjamin Crowell?), a ‘PF mentor’, decided to remove the link to my site, citing:  Reason: delete link to crackpot site.

The removed link was to this page:
www.alternativephysics.org/book/GeneralRelativity2.htm

Since PhysicsForums forbids any discussion that runs contrary to the mainstream view it’s not possible for me to reply there.  Not only that, the forum administrators have an open policy of deleting posts that don’t support their view.  So even if I did reply they would remove the evidence.  Does this seem reasonable to anyone?

So in the hopes that people might see it, I’ll put a reply here.


>>The one important mistake in the quote is that it's not time dilation that is minimized, it's simply the time measured on a clock that moves along with the body from the initial event E1 to the final event E2. This clock time is usually referred to as the proper time ("proper" meaning "its own").

There’s no ‘important mistake’ here, just a quibbling over wording.  Basically, instead of this:

"A body moves along the path that makes time dilation a minimum."

He prefers:

"A body moves along the path that makes the time measured on a clock that is moving along with the body from an initial event E1 to a final event E2, a minimum."

OK, well if you want to be a purist for semantic accuracy and confuse people in the process then go with the second sentence.  But I’m sure most readers would prefer the first.  Besides, the diagram and discussion that follows makes it clear what is meant, as mishima agreed.

Now for his other ‘minor quibbles’.

>>(1) The body has to have a small mass.

Actually for GR it doesn’t.  We know this because the ‘small mass’ doesn’t appear in the GR equations.  This is as opposed to Newton’s gravity force formula which includes both masses.  In fact the most used GR equation, the Schwarzschild Metric, specifically refers to massless particles.  Also GR describes the impact of gravity on photons, which have no mass; or at least no rest mass.  Whereas Newtonian gravity is effectively undecided on how photons should be affected; since it calculates a zero force acting on a zero mass.

Now on a practical level all physical bodies will be made of subatomic particles and these of course have mass.  But that’s not to say GR requires it.

>>(2) The proper time doesn't actually have to be minimized, just extremized. That is, it could be a local maximum rather than a local minimum. (3) The extremum is local, not global. That is, the time is only at an extreme compared to other paths that differ from it infinitesimally.

Mathematically you could argue this but in reality it isn’t going to apply both ways.  Solving the field equations involves taking a derivative of a function for total elapsed local time and setting it to zero.  In this situation the solutions would contain both maxima and minima but, unless I’ve missed some special case, in reality there won’t be any local maxima, just minima.  So my original statement stands.

------

All in all it’s good to know bcrowell can’t find faults with the web page in question, which indicates its correctness.  It’s also reassuring to know that PF admins can’t provide counter arguments without resorting to censorship of the material in question or denying their opponents the right of reply.  It seems bcrowell was just looking for something to pick on for the sake of discrediting my site in general but alas came away empty handed.  Better luck next time.

Now if he wants something ‘crackpot’ I could recommend that book he mentioned, “Exploring black holes”, since he should know that GR actually forbids them.  Alternatively, I could recommend Physics Forums.  Or a mirror.



Update – Total censorship this time

On 14th September 2013, someone added to the discussion.  The posting was:

Buckethead wrote:

I'm astonished that this forum would censor links to crackpot or otherwise sites. Last I checked free speech was preferred over censorship. This is a link fer cryin' out loud, not a thread! Do the powers that be prefer to determine what its members should or should not read thinking that maybe they may think too much for themselves and get uh....ideas?

Shame on you!

This post will no doubt be removed, but in case anyone catches it before it does and is curious about perhaps what non-conformists and even crackpots might be thinking, the link that was deleted is:

http://www.alternativephysics.org/bo...elativity2.htm

and no, I don't know the author, just astonished at this reprehensible behavior.

Oh and the rebuttal to this deletion can be found here:

http://alternativephysics.org/feedba...sEncounter.htm

Again, catch it while you can and copy it, because this all will surely be deleted in very short order.

Geez, one of these days I'm movin' to America....Oh wait, I am in America!!!!!


Within a few hours the comment was deleted.  Not just deleted with a “Last edited by” tag but completely erased, along with several follow-up comments.  Fortunately I was able to screen-capture them before they disappeared.  The full set of comments can be viewed here (minus the ads).

The forum administrators must really be desperate to resort to such measures.  If there was no validity to my rebuttal they should have no problem in putting a link to it there.  In fact, they should be especially pleased to include it since it would only bolster their case, as they could use it to help their readers see examples of what is wrong.

According to one of the commenters (Drakkith), the reason for deleting contrary information is so that people don’t get confused instead of learning actual science.  Given that General Relativity is a subject studied by adults at a graduate level, is he saying these students are so incapable of thinking that they can’t cope with anything that challenges their textbooks?

Incidentally the original commenter (Buckethead) later contacted me to say his comment earned him 3 infraction points.  Apparently once you get 8 you are banned for 10 days.  Nice to know the free exchange of ideas is alive and well.