## [SOLVED] Penrose's critique of string theory

> Roger Penrose offered a lecture on string theory title "Fashion,
> Faith, and Fantasy in Theoretical Physics".

Let me first say that Roger Penrose has done a large number of
contributions to physics and thinking and many of them are being intensely
used and studied especially by string theorists - for example twistors;
causal diagrams; pp-wave limits of geometries; cosmic censorship, and so
on.

We all admire his talent and his precious insights. I am pretty sure that
if he decided to study current physics seriously - instead of thinking
(like an overly speculative fan of physics) about gravitational collapses
of the wavefunction in the brain and instead of giving shallow lectures
inspired by science like this one, he might become a tough competitor for
some of the current leaders of theoretical physics, including Edward
Witten.

> [RP] apparently is critical of string theory, and prefers loop quantum
> gravity.

Well, Roger Penrose anticipated the emergence of spin networks in quantum
gravity, which eventually occurred in loop quantum gravity. On the other
hand, although his insights are valuable for string theorists, none of
them is truly string-theoretical.

> polemic aside, what are his criticisms of string theory, how has
> string theorists responded, including edward witten and lubos motl,

Honestly, I find it inappropriate to appear in the same sentence as Edward
Witten who has done more than me for science by several orders of
magnitude, but let me assume that your point was different. It would be
great if Edward Witten responded, too.

> and why does he think LQG is preferable?

It may be a good idea to ask him, and of course, he will be always welcome
if he appears on this newsgroup. ;-) If you ask me and you want to know
what I really think, the real reason behind this preference may be that he
might be viewed as a grandfather of loop quantum gravity, but he has not
contributed much to string theory.

> the reason i am posting is that roger penrose is one of the world's
> most preeminent physcist, easily on par in prestige with edward witten
> and stephen hawking, and i am curious as to whether penrose offers new
> or original criticisms from string theory and whether there are new or
> original counter-arguments from string theorists.

As far as the contents of the criticism goes, I've heard about a few
lectures by Penrose about these issues. One lecture, two years ago or so
(Hawkingfest?), was dedicated to singularities. He pointed out that the
moduli spaces of manifolds (such as the Calabi-Yau manifolds) have
singularities in them, and therefore $- he$ argued - string theory was in
trouble. Such a statement showed that, sadly, Penrose had no idea what was
really going on in string theory. String theory has marvelously resolved a
large class of timelike singularities (e.g. of the conifold) and all
potential problems arising from such singularities were shown to be
regular due to branes and their associated physics. See "The Elegant
Universe" (chapters 11,13, for example) to learn why it is exactly string
theory that wins near the singularities where all other theories lose.

Penrose's criticism $- at$ least this aspect of $it -$ is therefore rather
original and different from other criticisms - but it does not seem to be
well-informed.

> String theory has had a kind of tulip-mania fad or craze and now seems
> to be in decline.

Well, we've heard such things since the middle 1980s at least. It is
certainly a longer-lasting fad than any other fad in the history of
humankind. ;-) String theory has had its revolutions and its declines
(and it may be more fair to talk about a decline today, at least if we
compare the present with the middle 1990s or middle 1980s), but it remains
the only real contender for a conceptual framework that goes beyond
quantum field theory and classical GR. Most string theorists are also too
nice - perhaps except for $me -$ and therefore they won't comment on
declines of Roger Penrose although there would certainly be a lot of stuff
to discuss.

> But it is also a kind of religious faith for some of
> those who have invested a lot of effort in understanding it.

That's right. So was relativity and other important breakthroughs in
science.

> so there are some diehard believers who will give you elaborate
> specious arguments why it is impossible that any of the newer
> approaches to quantizing gravity can work

It would be more interesting scientifically if Roger Penrose could take,
for example, my 25 kilobyte long (devastating) review of Rovelli's new
book (to appear) which is also an analysis of the whole field of LQG and
if he showed which arguments against this "newer approach" may have a
loophole. Instead, Penrose seems to believe that he can influence the
direction where physics goes without any arguments. I am not sure whether
the overlap between science and religion is *that* far-reaching.

> and it lives in a kind of fantasy realm, making no testable
> predictions and ungoverned by experimental evidence, so the
> researchers indulge in untrammelled mathematical inventiveness

On the other hand, this statement is not original at all. String theory is
the most conservative extension of the successfully tested principles of
modern physics, and all its features are $- at$ least qualitatively - very
physical and realistic. It is a rich theory but all of its different
phenomena will remain to be highly interesting and important mathematical
subjects to study.

> finally such an embarrassing richness of possibilities has emerged
> that the distinct variations of the theory have been estimated by its
> insiders (Susskind, Douglas) at ten-to-the-100 different base states
> and things like the Anthropic Principle, a latterday Hand of God, are
> being invoked in a desperate effort to find the right one.
> So it has gotten bogged down in its own fecundity.

We have discussed these questions a lot on this board. If the number of
possibilities to create a Universe - including working cosmology - in the
correct theory *is* that huge, we will have to live with this fact. String
theorists don't agree yet whether the usage of the Anthropic Reasoning
will be necessary. Many of us hate it. But it is a logical possibility. At
any rate, as long as theoretical physics exists as a field, the scholars
in it will study something. Because they have no new experiments, they
must study more or less pure theory. String theory remains the most
promising game in town, perhaps the only game in town. This might
hypothetically change - but only if someone found something equally (or
more) interesting. It cannot change by political speaches without
scientific content, even if the speaker is as famous as Roger Penrose.

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Roger Penrose offered a lecture on string theory title "Fashion, Faith, and Fantasy in Theoretical Physics". I have not heard his lecture, nor am i acquainted with his latest work, Road to Reality: A Complete Guide to the Physical Universe by Roger Penrose, which apparently is not yet available in the US, but he apparently is critical of string theory, and prefers loop quantum gravity. polemic aside, what are his criticisms of string theory, how has string theorists responded, including edward witten and lubos motl, and why does he think LQG is preferable? the reason i am posting is that roger penrose is one of the world's most preeminent physcist, easily on par in prestige with edward witten and stephen hawking, and i am curious as to whether penrose offers new or original criticisms from string theory and whether there are new or original counter-arguments from string theorists. FASHION, FAITH AND FANTASY IN THEORETICAL PHYSICS and it is at the Dublin concert hall at 8PM on Friday 23 July String theory has had a kind of tulip-mania fad or craze and now seems to be in decline. But it is also a kind of religious faith for some of those who have invested a lot of effort in understanding it. so there are some diehard believers who will give you elaborate specious arguments why it is impossible that any of the newer approaches to quantizing gravity can work and it lives in a kind of fantasy realm, making no testable predictions and ungoverned by experimental evidence, so the researchers indulge in untrammelled mathematical inventiveness finally such an embarrassing richness of possibilities has emerged that the distinct variations of the theory have been estimated by its insiders (Susskind, Douglas) at ten-to-the-100 different base states and things like the Anthropic Principle, a latterday Hand of God, are being invoked in a desperate effort to find the right one. So it has gotten bogged down in its own fecundity.



Daniel: >Roger Penrose.. is critical of string theory I think he mentioned in a newspaper article the other day that it would be nice to have some experimental evidence for string theory.I'm sure even string theorists would agree with this! Stephen Hawking says that it is the existence of the dualities in string theory that make him believe string theorists could be on to something.

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