In everyday language, a theory is just an idea – something that runs in one’s mind – often unproven and untested. But that’s not the case with science.

A fatal obsession with a hypothesis has put the integrity of science at stakes! “String theory is still a hypothesis! Please correct the misunderstanding.”

My innocent-looking status update on social media sparked a debate – as expected. Most of the science-literate people (including scientists) simply love string theory, and insistently call it a theory. Others, the naïve ones, say: hey! It’s just a theory.

In everyday language, a theory is just an idea – something that runs in one’s mind – often unproven and untested. But that’s not the case with science.

Scientific Theory

When we say a theory in science, it has a very different meaning; it must fulfill certain criteria to be termed as a theory – the point I attempted to raise through my status update.

For example, Dictionary.com describes a scientific theory as “a coherent group of propositions formulated to explain a group of facts or phenomena in the natural world and repeatedly confirmed through experiment or observation.”

Wikipedia, on the other hand, offers a bit technical, detailed and in-depth definition: “A scientific theory is an explanation of an aspect of the natural world and universe that has been repeatedly tested and corroborated in accordance with the scientific method, using accepted protocols of observation, measurement, and evaluation of results. Where possible, theories are tested under controlled conditions in an experiment.”

Other definitions of a scientific theory can be found elsewhere, including the American Museum of Natural History, encyclopedia Britannica, the Field Museum, the University of Waikato, University of Hawaii, and so on. Irrespective of their length and depth, all these definitions state the same: a scientific theory must have well-tested, confirmed evidences (observational and/ or experimental) to prove the unique predictions it makes.

Now, does string theory fulfill this condition? So far, the answer is NO. Down to the scientific method, it’s at the stage of hypothesis; at the best, you can call it a strong candidate for the Theory of Everything (ToE) – a still-sought theoretical framework to unify all the fundamental forces.

Scientific Method

At the heart of every scientific investigation – related with theories and laws of science – lies a systematic procedure called the scientific method. It was originally devised and used by Alhazen (Ibn al-Haytham) 1,000 years ago and involves the following:

A question or a problem (to be researched or investigated);
A hypothesis (a probable, rational answer to the question);
Predictions (what should we see if the hypothesis is correct);
Testing (careful experiments and/ or observations about predictions of a hypothesis);
Analysis (to check if the information [data] gathered in experiments/ observations are correct and according to what the hypothesis has predicted);
Conclusion (is the hypothesis correct, partially correct, or incorrect?);
Report (let the others [experts] know about your research, so that they may evaluate, criticize – in case of disagreement – and repeat the whole procedure to check the validity of your conclusion).

Sounds like Science101? Yes, it is!
Though scientific method is integral to all the standard science curricula – including secondary-school syllabi – but seldom taught seriously. Therefore, even graduates and masters of Science know very little about it; and often use theory and hypothesis synonymously.

Furthermore, a new candidate scientific theory must predict something unique: something testable that has not predicted by the established, conventional theories of the same field, or something that such theories are unable to explain appropriately.

Quest for Theory of Everything (ToE)

Since long, scientists are trying to unify all the four fundamental forces of nature – gravity, electromagnetism, weak nuclear force and strong nuclear force – within one theory or a single theoretical framework, often referred as the Theory of Everything (ToE). To date, the Standard Model of particle physics has successfully unified three forces of nature, except gravity – leaving the quest for ToE an unfinished journey.

Uniting electromagnetism, weak nuclear force and strong nuclear force, the Standard Model is the realm of quantum mechanics (QM) – the theory of infinitesimally small objects.

On the other hand, we have General Theory of Relativity – now established as the universal theory of gravity – that deals with very large objects. In this sense, we can say that pursuit for ToE is all about unifying quantum mechanics with the general relativity.

Strings theory in brief

In 1968, a 26-years old theoretical physicist at CERN, Gabriele Veneziano, proposed “dual resonance model” based on “S-Matrix” to explain strong nuclear interactions. A couple of years later, with modifications from other theoretical physicists, it emerged as a candidate for ToE – known as the string theory since then.

In simple words, string theory seeks to unite quantum mechanics with general relativity. And, for this, it proposes that all particles are composed of extremely small, one-dimensional vibrating strings; and that the universe contains small, hidden extra dimensions.

These strings are billionth times thinner than proton, while hidden extra dimensions (6 or 7) are tightly curled within the same scale. Fundamental particles – according to string theory – are, in fact, vibrations in loops of these strings, each with its own specific frequency.

Just for introduction: Type I, Type IIA, Type IIB, and two flavors of heterotic string theory – called SO(32) and E8×E8 – are considered the most promising versions of string theory.

String theory also predicts supersymmetric counterparts for every known fundamental particle; and we should observe their telltale signs at very high energies. Ironically, despite being the most powerful particle accelerator of the world, Large Hadron Collider (LHC) at CERN has, so far, failed to detect such particles.

In face of this failure, the advocates of string theory suggest much higher energies to observe supersymmetric particles – perhaps unachievable by even more powerful accelerators technologically possible in foreseeable future.

Also, the string theory predicts 10500 different universes (10 followed by 500 zeroes) – a really, really large number! For comparison, the estimated number of atoms in our observable universe is between 1078 and 1082.

50 years old and still immature

Despite thousands upon thousands of research papers and hundreds of conferences during the last fifty years, we have no experimental/ observational evidence to favor the string theory – it is still immature.

Yet the belief of its proponents remains unshaken: with its elegance and mathematical beauty, string theory has brought physics closer to finding a ToE; and the string paradigm will, eventually, prove itself correct.

Critics, on the other hand, point out that string theory lacks testable experimental/ observational predictions. Therefore, in the absence of such predictions, it’s “not even wrong,” claims Peter Woit, an American theoretical physicist.

Lee Smolin, a former string theorist and pioneer of the Loop Quantum Gravity (LQG), goes even further in his The Trouble with Physics: It [string theory] makes no new testable predictions, it has no coherent mathematical formulation, and it has not been mathematically proved finite. “With slight variations, each [version of] string theory predicts almost infinite different results – equivalent to predicting nothing,” he says.

In response, to legitimize their belief, some string theorists cite Maxwell’s equations that unified electricity with magnetism, first published in 1865. It took them almost 30 years to be experimentally proved by Heinrich Hertz in 1890s.

To make their case for string theory even stronger, they quote another, more recent example: discovery of gravitational waves. In his general theory of relativity, Albert Einstein predicted gravitational waves in 1915. But it was one hundred years later, in 2015, when LIGO (Laser Interferometer Gravitational-Wave Observatory) first detected gravitational waves – and finally confirmed Einstein’s prediction.

The only game in town?

In view of ToE, string theory is often considered the only game in town – perhaps because of its popularity within academia and mass media. Contrary to this perception, there are other candidates, or other games in town, for ToE as well:

Loop Quantum Gravity (LQG) is a theoretical framework that describes gravity in terms of loops and networks; and unifies general relativity (GR) with quantum mechanics (QM) in a testable manner.

Causal Dynamical Triangulation (CDT) describes space-time as a network of triangles, instead of a continuum. In CDT, space-time has a discrete structure and evolves over time. This way, it attempts to unify GR with QM.

Emergent Gravity (EG) suggests that gravity isn’t a fundamental force of nature. Instead, it arises from the collective behavior of other fundamental particles and interactions.

Asymptotic Safety (AS) – yet another candidate for ToE – tries to reconcile GR and QM by describing gravity as a fundamental force that behaves in a testable and predictable way.

Geometric Unity (GU) is a new approach that describes universe in terms of “E8 manifold” – a single mathematical object with complex, multidimensional geometric structure. It claims that all the fundamental particles and forces can be derived from the geometry of E8 manifold; thus uniting GR with QM.

Though all of these approaches – including string theory – are promising, but none of them is experimentally-proven. Hence, for now, we can only call them hypotheses – and string theory is no exception.

Post-empiricism: sacrificing the scientific method

String theorists’ passion for their pet hypothesis has, seemingly, become an obsession: “put aside scientific method to accommodate sufficiently elegant, explanatory, and mathematically beautiful theories,” they suggest.

(In other words, you don’t need to experimentally test such theories.) For George Ellis and Joe Silk, two prominent theoretical physicists, this proposal is like “breaking with centuries of philosophical tradition of defining scientific knowledge as empirical.”

The scientific method serves as the foundation for modern science. Being empirical, it enabled an unprecedented progress, even revolutions, in science and technology that may have, otherwise, remained impossible.

In short, empiricism – with scientific method at its core – is a rigorous and successful approach. It’s a filter that helps us separate theories from conjectures. It’s a litmus test to check whether a hypothesis really qualifies as a theory.

So, should we disregard empiricism only for sake of some promising, yet untested, ideas with unusual explanatory power, elegance, and beauty?

If yes, then there will be an era of post-empiricism in science: “elegance will suffice” (in words of Ellis and Silk). Every hypothesis – merely having elegant mathematics and sound, rational grounds – shall qualify as a theory; no matter if it predicts nothing testable. If it really happens, no one knows how science and technology shall progress further.

A much-deeper rabbit hole

As you can see, it’s not just about whether we can call string theory a theory. Instead, to do so, you should forget almost everything that makes science empirical – including scientific method and experiments/ observations. In short, the integrity of science is at stake.

The rabbit hole of post-empiricism goes much deeper than it seems. Following the footsteps of string theorists, scientists from other disciplines may demand similar concessions for their pet hypotheses. Though we can expect the unexpected but one thing is for sure: science won’t remain the way we know it today.

To avoid the debate, some cautious theoretical physicists say that string theory is a theory of mathematics. This statement is only partially true because the advanced and complex mathematics – used in string theory – was actually developed to achieve ToE. Otherwise, we didn’t need it.

As long as the scientific method – and empiricism – prevails, calling string theory a theory shall remain a gross misunderstanding that should be corrected, indeed. Hopefully, now you can better comprehend that the status update – presented in the beginning – isn’t as innocent as it seems.