Ever since antiquity, attempts have been
made to reduce an apparently complex re-
ality to a few elementary building blocks
from which everything else is constructed.
This project – now called reductionism – has
a long history of failures. One example is the
200-year-long attempt to describe all phys-
ical processes in terms of mechanics, such
as James Clerk Maxwell’s mechanical mod-
els of the electromagnetic field. Another is
Hermann Weyl’s failed attempt to unify
electromagnetism and gravity in a single
theory shortly after Einstein had introduced
special relativity.
However, reductionism has achieved
some notable successes too, and these have
spurred on the search for unity in theoretical
physics. The Standard Model of particle
physics, which emerged in the 1970s and
describes the electromagnetic, strong and
weak forces in a single framework, is a major
triumph in this regard. But as the Standard
Model says nothing about the fourth force
of nature – gravity – many physicists believe
that the end point of unification has not yet
been reached. String theory is currently the
dominant research programme pursuing
this quest.
Although dominant, string theory is not
free from controversy. Critics, one of the
most prominent being Lee Smolin of the
Perimeter Institute in Canada, take the the-
ory to task for not having produced a single
new prediction that would allow it to be com-
pared with experiment. They claim that the
features of string theory that are at least
potentially testable, such as the existence of
supersymmetry and cosmic strings, are not
specific to string theory. In addition, those
features that are specific to string theory, first
and foremost the existence of strings, either
do not lead to precise predictions or lead to
predictions that are impossible to test with
current technology.
This, argue Smolin and other critics, is un-
acceptable because a scientific theory must
stand up to experimental scrutiny. Hence,
string theory must either undergo a funda-
mental change or else it has to be given up.
But how sound a judge of a scientific theory
is testability, and is it strong enough to rule
out string theory as a viable physical theory?
Positivist evolution
This emphasis on prediction and experi-
mentation is reminiscent of the philosophy
of science of Karl Popper and the positivists
of the Vienna Circle (who included Moritz
Schlick, Rudolf Carnap and Otto Neurath)
and the Berlin School, which grew up round
Hans Reichenbach. In the 1920s and 1930s
these philosophers, following the tradition
of Ernst Mach and other empiricists, sug-
gested that we should only accept theories
that can be compared with reality in experi-
ments. Any domain of investigation that can-
not comply with these standards should thus
be rejected as unscientific.
This view of science was widely accepted
until the 1960s, when various philosophers –
most notably Imre Lakatos, Thomas Kuhn
and Paul Feyerabend – pointed out that it
was unduly restrictive, especially when it
comes to new theories. Since new theories
are not as fully developed as long-standing
ones, they may well be sketchy in how they
deal with phenomena that are well treated
by older theories. Indeed, a new theory
may even get some of its predictions wrong.
Yet if we throw new theories out on such
grounds, or because they are not sufficiently
precise, we risk being stuck with the same
old physics, perhaps forever.
For these reasons Lakatos proposed that
the object of evaluation should not be an in-
dividual theory as viewed at one particular
time but rather the larger “research pro-
gramme” that spawns that theory. A re-
search programme is characterized by a core
set of ideas, techniques, rules and assump-
tions, and theories are expected to evolve
in accord with these. Good research pro-
grammes are those that are progressive, i.e.
those whose theories get better and better,
even if individual theories face serious dif-
ficulties at certain times.
Newton’s theory of gravity is a good exam-
ple of a progressive research programme.
When it was first proposed, the theory had
only limited empirical success and faced a
host of anomalies. But Newton and his fol-
lowers eliminated one anomaly after an-
other, with the theory then triumphing for
over 200 years before Einstein’s general
String theory under scrutiny
One of the main charges
against string theory is that it
cannot make specific predictions
that may be checked against
an experiment. But as
Nancy Cartwright and
Roman Frigg explain, other
criteria should be taken into
account too when evaluating
scientific research
Measuring up String theory can unify and explain – but how far has it progressed in other regards?
Photolibrary
String theory’s failure
to make testable
predictions leaves
us with no reason to
believe that it gives
us a true picture
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theory of relativity superseded it. For in-
stance, Newtonian gravity made incorrect
predictions for the orbit of Uranus; but
instead of regarding this as a refutation of
the theory, physicists made the auxiliary
assumption that Uranus’ motion was influ-
enced by the gravitational force of a new,
then unobserved, planet. A few years later,
this planet – Neptune – was indeed discov-
ered. Had the Newtonians adopted the strict
testability view of science, one of the most
powerful theories in physics would never
have got off the ground.
How to progress
The crucial question for string theory is thus
not whether the theory in its current form
can be tested, but whether the research pro-
gramme of string theory is progressive. A
research programme can progress in many
different ways corresponding to different
virtues that a good scientific theory is sup-
posed to have. These include the following
(which are not specific to Lakatos’ philoso-
phy): having a large range of varied empirical
applications; generating successful novel
predictions; spawning new technologies; an-
swering perplexing problems; consistency;
elegance; simplicity; explanatory power; uni-
fying power; and, last but not least, truth.
Radical string critics would then conclude
that string theory is progressive only in the
dimensions of elegance and simplicity (in the
sense that the theory contains only one class
of basic objects – strings – from which all the
basic particles and forces follow), while being
largely stagnant in the other dimensions.
However, because string theory requires the
gravitational force to exist, it represents an
important step towards a unified theory of
gravity and quantum mechanics. String the-
ory has also had some success as a tool to
study quark–gluon plasmas in energy regimes
that are difficult to address using existing
theoretical techniques (see “Stringscape” on
pages 35–47). These two achievements sug-
gest that string theory shows at least some
signs of progress in the dimensions of unify-
ing and explanatory power, respectively.
Nevertheless, a research programme that
progresses only in some dimensions, while
being by and large stagnant in the others,
surely does not count as being progressive.
Contrasting string theory with Maxwell’s
unification of electricity and magnetism, for
example, we can see that the latter was genu-
inely progressing and eventually successful
in every dimension. It used the new and
powerful concept of a field, which made the
theory simple and elegant, while at the same
time giving rise to a whole set of new phe-
nomena that led to new predictions. The
most astonishing of these was that electro-
magnetic waves were light – an unexpected
result that led to the discovery of the radio,
infrared, ultraviolet and other waves that we
now view as ubiquitous.
Sceptics might say that by taking the ana-
logy with Maxwell too seriously one imposes
values onto string theory that it need not
accept. After all, since string theory aims to
unify the basic interactions, its success in
spawning new empirical applications or
technologies is quite irrelevant. However,
renouncing the value of applicability comes
at a price, since we do not want a theory that
neither tells us how the world really is nor has
any interesting applications.
The question of how progressive string
theory is then becomes one of truth, and this
brings us back to predictions. The more nu-
merous, varied, precise and novel a theory’s
successful predictions are, the more confid-
ence we can have that the theory is true, or
at least approximately true (see box). That a
theory describes the world correctly wher-
ever we have checked provides good reason
to expect that it will describe the world cor-
rectly where we have not checked. String
theory’s failure to make testable predictions
therefore leaves us with little reason to be-
lieve that it gives us a true picture.
The appeal of simplicity
Some philosophers, and physicists alike,
argue that empirical tests are not the only
route to truth: other dimensions of progress
also have a connection with truth, albeit a
less direct one. Simplicity is one example. If
we assume that we somehow know that the
world is simple in a certain way (for example
in that it contains only one fundamental
entity such as strings), then, all other things
being equal, a theory that is not simple in this
way cannot be correct. But if such claims are
to affect what theories we judge to be true in
science, then they need to be carefully ar-
gued and justified. The question is whether
or not this is possible.
Because many physicists long for simple
and unified theories, they sometimes con-
clude that the world “just has to be” simple.
But modern science demands that claims
about the world be justified by appeal to the
phenomena in the world, not based on long-
ings. A seemingly more promising strategy
to defend simplicity is to perform a loose
induction on the history of physics: we have
accumulated a great number of hugely suc-
cessful simple theories, hence the world
must be simple.
But it is easy to cite counter cases, such as
theoretical condensed-matter physics, where
progress has not come about in this way.
Indeed, even if such counter cases could be
dismissed, it is still hard to properly articu-
late what kind of simplicity all the successful
cases share and to argue that string theory is
simple in precisely that way. In short, there
is no straightforward argument for the con-
clusion that the world is simple, which means
that claims about a theory’s truth based on
simplicity as at best inconclusive.
Although string theory has progressed
along the dimensions of unifying and ex-
planatory power, this in itself is not sufficient
to believe that it gives us a true picture of the
world. Hence, as it stands, string theory is not
yet progressive because it has made progress
only along a few of the many dimensions that
matter to a research programme’s success.
However, one of the punchlines of Laka-
tos’ methodology of scientific research pro-
grammes is that we should treat budding
programmes leniently, and string theory
therefore deserves to be pursued in the hope
that one day it will become progressive. In
practice, however, the questions of how
much to invest in this effort and what should
be sacrificed for that investment still remain.
Nancy Cartwright and Roman Frigg are in the
Department of Philosophy, Logic and Scientific
Method, London School of Economics, UK,
e-mail n.l.cartwright@lse.ac.uk
Physicists generally hold contradictory beliefs
about the role of truth in science. On the one hand
there is the rhetoric about truth: that science is all
about trying to uncover how the world really is. But
on the other hand, physicists often violently resist
foundational programmes that try to figure out
what the world would have to be like if a theory
were true. For example, rather than trying to
understand deep truths about the world that could
be lurking in issues in the foundations of quantum
mechanics, such as the measurement problem,
most physicists are eventually interested only in
deriving observational predictions – even though
their “sales talk” promises the contrary. However,
when stripped of such “truth-talk”, the focus on
prediction and application is not an illegitimate
attitude. Indeed, there is vivid controversy in the
philosophy of science over the question of whether
truth really is a justifiable aim for science, or
whether the more modest aim of empirical
adequacy would not provide a better regulatory
framework for scientific progress.
Truth also crops up in a completely different
context within the philosophy of science:
the issue of theory choice, or how best to choose
between different theories. For example, in an
ideal situation, physicists would be faced with
more than one true theory and would then have to
pick the one that has independent virtues, such as
simplicity and explanatory power. This was the
case, for instance, with different formulations of
classical mechanics. But since string theory is
currently the only contender for a unified theory
of physics, researchers are not in such a
luxurious position.
Truth in physics
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