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On 11^th 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:

It was replied to by user bcrowell

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

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.