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On 11th of
June 2011, I noticed my
web-logs showed a number of external links from a
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:
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
"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
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
It was replied to by user bcrowell
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
‘PF mentor’, decided to remove the link to my site, citing: Reason:
delete link to crackpot site.
The removed link was to this page:
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
So in the hopes that people might see it, I’ll put a reply here.
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."
"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
Now for his other ‘minor quibbles’.
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
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
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
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
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
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
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
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:
and no, I don't know the author, just astonished at this reprehensible
Oh and the rebuttal to this deletion can be found here:
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
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.