String Theory: A Brief Overview (2023)

By Charlie Wood, Vicky Stein

last updated

String theory might be the theory of everything… or a flawed framework for theoretical physics.

String Theory: A Brief Overview (1)

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  • What is it?
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String theory is the idea in theoretical physics that reality is made up of infinitesimal vibrating strings, smaller than atoms, electrons or quarks. According to this theory, as the strings vibrate, twist and fold, they produce effects in many, tiny dimensions that humans interpret as everything from particle physics to large-scale phenomena like gravity.

String theory has been held up as a possible "theory of everything," a single framework that could unite general relativity and quantum mechanics, two theories that underlie almost all of modern physics. While quantum mechanics does very well in describing the behavior of very small things and general relativity works well to explain how very large things happen in the universe, they don't play nicely together. Some scientists think (or thought) that string theory could resolve the conundrums between the two, conquering one of the major remaining unsolved problems of physics.

But after string theory gained prominence in the late 1960s and '70s, its popularity among theoretical physicists fluctuated, according to a lecture by California Institute of Technology physicist John Schwarz, widely considered one of the founders of string theory. After countless papers, conferences and dry-erase markers, the breathtaking breakthrough many once hoped for seems further away than ever.

Nevertheless, the flurry of thought around the very idea of string theory has left a deep imprint on both physics and math. Like it or not (and some physicists certainly don't), string theory is here to stay.

What is string theory?

String theory is a framework that physicists use to describe how forces usually conceptualized on a gigantic level, like gravity, could affect tiny objects like electrons and protons.

In Albert Einstein's theory of general relativity, gravity is a force that warps space-time around massive objects. It’s one of the four forces that physicists use to describe nature. But unlike the other forces (electromagnetism, the strong force and the weak force), gravity is so weak that it can't be detected or observed on the scale of a particle. Instead, its effects are only noticeable and important on the scale of moons, planets, stars and galaxies.

Gravity seems not to exist as a particle of its own, either. Theorists can predict what a gravity particle should look like, but when they try to calculate what happens when two such "gravitons" smash together, they get an infinite amount of energy packed into a small space — a sure sign, according to astrophysicist Paul Sutter in a previous article for Space.com, that the math is missing something.

One possible solution, which theorists borrowed from nuclear physicists in the 1970s, is to get rid of the idea of problematic, point-like graviton particles. Strings, and only strings, can collide and rebound cleanly without implying physically impossible infinities.

String Theory: A Brief Overview (2)

"A one-dimensional object — that's the thing that really tames the infinities that come up in the calculations," string theory expert Marika Taylor, a theoretical physicist at the University of Southampton in England, told Space.com.

String theory turns the page on the standard description of the universe by replacing all matter and force particles with just one element: tiny vibrating strings that twist and turn in complicated ways that, from our perspective, look like particles. A string of a particular length striking a particular note might gain the properties of a photon, another string folded and vibrating with a different frequency could play the role of a quark, and so on.

In addition to taming gravity, string theory was attractive for its potential to explain so-called fundamental constants like the mass of an electron. The next step, theorists hoped, would be to find the right way to describe the folding and movement of strings, and everything else should have followed.

(Video) What is String Theory?

But that initial simplicity turned out to come at the cost of unexpected complexity — string math didn't work in our familiar four dimensions (three of space and one of time). It needed a total of 10 dimensions, with six visible only to the perspective of the little strings, much as a powerline looks like a 1D line to birds flying far overhead but becomes a 3D cylinder to an ant crawling on the wire.

How did string theory evolve?

String theory today doesn't exactly match string theory of the 1960s and '70s. Researchers disagree over whether, with modifications, it's still the best candidate for a "theory of everything" or whether theorists should abandon it in favor of other topics.

"By 1973–'74 there were many good reasons to stop working on string theory," Schwarz wrote. Physicists' attention had turned from what felt like an unfruitful exploration of tiny undetectable "soft" strings and instead taken up more compelling evidence of hadrons, subatomic particles made up of quarks whose actions couldn't be explained by strings.

"​​What had been a booming enterprise involving several hundred theorists rapidly came to a grinding halt," Schwarz wrote. "Only a few diehards continued to pursue it."

Over the next decade, a few scientists continued to pursue five different versions of string theory. Over time, the researchers began to find unexpected connections between the five ideas, which Edward Witten, a theorist at the Institute for Advanced Study in Princeton, New Jersey, gathered up and presented at a 1995 string theory conference at the University of Southern California. Witten argued that the five string theories each represented an approximation of a more fundamental, 11-dimensional theory as it behaved in a particular situation, much as how Einstein's space- and time-bending theories of relativity match Newton's description of objects moving at normal speeds. That was the beginning of one string theory spin-off: M-theory.

The "M" is likely inspired by higher-dimensional objects called membranes, Taylor said, but since the theory has no concrete mathematical equations, the "M" remains a placeholder with no official meaning. "It was really a parametrization of our ignorance," she said.

Attempts to find those general equations that would work in every possible situation made little progress, but the alleged existence of the fundamental theory gave theorists the understanding and confidence needed to develop mathematical techniques for the five versions of string theory and apply them in the context where each theory worked.

String Theory: A Brief Overview (3)

Strings are far too small to detect with any conceivable technology, but one early theoretical success was physicists' ability to describe black hole entropy in a 1996 paper published in the journal Physics Letters.

Entropy refers to the number of ways that you can arrange the parts of a system, but without being able to see into the impenetrable depths of a black hole, no one knows what type of particles might lie inside or what arrangements they can take. And yet, in the early 1970s, Stephen Hawking and others used the laws of thermodynamics and, according to an article published in 2020 in the journal Physics, quantum mechanics to calculate entropy inside a black hole, suggesting that black holes must have some sort of internal structure. What that structure might be remained a mystery.

Most attempts to describe the black hole's makeup fall short, but the configurations of hypothetical strings does the trick. "String theory has been able to give a spot-on counting," Taylor said — an actual possible explanation of black hole interiors, not just a rough idea.

The string theory framework still faces many challenges, however: It produces an impossible number of ways to fold up the extra six dimensions. Every option seems to fit the broad features of the Standard Model that governs particle physics, with little hope of distinguishing which is the right one. Moreover, wrote astrophysicist Ethan Siegel in his blog Starts With a Bang, all of those models for managing the extra dimensions rely on an equivalence between force particles and matter particles called supersymmetry. But, just like the extra dimensions that string theory requires, we don't observe supersymmetry in our world.

(Video) String Theory

On top of those objections, it's not clear that string theory — M-theory or not — can ever be compatible with our modern understanding of an expanding universe, replete with dark energy, reported Quanta Magazine in 2018.

A number of physicists, such as Peter Woit of Columbia University, view these divergences from reality as fatal flaws. "The basic problem with string theory unification research is not that progress has been slow over the past 30 years," he wrote on his blog, "but that it has been negative, with everything learned showing more clearly why the idea doesn't work."

Taylor, however, maintains that today's models are overly simplistic, and that features like cosmological expansion and a lack of supersymmetry may someday be built into future versions. Taylor expects that, while the new era of gravitational-wave astronomy may bring new tidbits of information about quantum gravity, more progress will be made by continuing to follow the math deeper into string theory.

"I have a theoretical bias," she said, "but I think the kind of breakthrough I'm describing would come from a chalkboard; from thought."

Why is string theory still important?

Regardless of whether string theory can ever be massaged into a "theory of everything," its legacy as a productive research program may be assured on mathematical merit alone.

"It can't be a dead end in the sense of what we've learned just from mathematics itself," Taylor said. "If you told me tomorrow that the universe absolutely isn't supersymmetric and doesn't have 10 [spatial] dimensions, we've still connected whole branches of mathematics" using the string theory framework.

In particular, when Witten and other researchers showed that the five string theories were shadows of a single parent theory, they highlighted connections called dualities, which have proven to be a major contribution to mathematics and physics.

Dualities let mathematicians translate from one branch of math to another, attacking problems untenable in one framework by using calculations done in the other, for example in geometry and number theory. Other dualities have helped overcome challenges in quantum computing, according to Taylor. "It's not going to make your next-generation iPhone," she said, "But it may make your iPhone for the 22nd century."

Whether string theory's ability to illuminate the dark web connecting different areas of math turns out to be a sign of its potential or just a lucky coincidence remains a subject of debate. Witten, speaking at the Institute for Advanced Study in 2019, acknowledged that while he no longer feels as confident as he once did that did that string theory will evolve into a complete physical theory, his gut tells him that the theory remains a productive field of research.

"To me, it's implausible that humans stumbled by accident on[to] such an incredible structure that sheds so much light on established physical theories, and also on so many different branches of mathematics," he told the audience. "I have confidence that the general enterprise is on the right track, but I don't claim that the argument I've given is scientifically convincing."

Additional resources

Find more in-depth history and explanations of current string theory science at WhyStringTheory.com, a website created by graduate students at the University of Oxford and the University of Cambridge for the "interested layperson." Or, if you'd rather go right to the source, check out this CERN Courier interview with string theory originator Gabriele Veneziano. If you're looking for a video explainer, watch these two videos entitled "Why string theory is right" and "Why string theory is wrong," both from PBS Digital Studios.

Bibliography

(Video) String Theory Explained – What is The True Nature of Reality?

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String Theory: A Brief Overview (4)

Charlie Wood

Space.com contributor

Charlie Wood is a freelance journalist covering physical sciences both on and off this pale blue dot. He contributes to Space.com and LiveScience, as well as Popular Science, Scientific American, Quanta Magazine, and others. These days he writes from New York but in previous lives he taught physics in Mozambique and science English in Japan. Find him on Twitter @walkingthedot.

(Video) String theory - Brian Greene

FAQs

What is a simple explanation of string theory? ›

String theory proposes that the fundamental constituents of the universe are one-dimensional “strings” rather than point-like particles. What we perceive as particles are actually vibrations in loops of string, each with its own characteristic frequency.

What does the string theory say about the universe? ›

String theory is the idea that everything in the universe, every particle of light and matter, is comprised of miniscule vibrating strings. These strings are truly tiny, many billions of times smaller than an individual proton within an atomic nucleus.

How does string theory explain the beginning of the universe? ›

After some passage of time, the string interactions cause the symmetry of the universe to spontaneously break, causing three of the nine dimensions to expand. The other six are left stunted at the Planck length. “The time when the symmetry is broken is the birth of the universe,” says Nishimura.

What is the major problem with string theory? ›

One of the challenges of string theory is that the full theory does not have a satisfactory definition in all circumstances. Another issue is that the theory is thought to describe an enormous landscape of possible universes, which has complicated efforts to develop theories of particle physics based on string theory.

What is string theory trying to solve? ›

String theory attempts to unify all four forces, and in so doing, unify general relativity and quantum mechanics. At its core is a fairly simple idea—all particles are made of tiny vibrating strands of energy. (String theory gets its name from the string-like appearance of these energy strands.)

Why do people believe string theory? ›

Many scientists believe in string theory because of its mathematical beauty. The equations of string theory are described as elegant, and its descriptions of the physical world are considered extremely satisfying.

Why is string theory not accepted? ›

Many physicists consider string theory our best hope for combining quantum physics and gravity into a unified theory of everything. Yet a contrary opinion is that the concept is practically pseudoscience, because it seems to be nearly impossible to test through experiments.

Is there any evidence for string theory? ›

Physicists have yet to produce any empirical evidence for either string theory, which was invented more than 40 years ago, loop-space theory or any other unified theory. They don't even have good ideas for obtaining evidence.

Is string theory a fact? ›

The lack of experimental evidence: Since the strings are so tiny it's nearly impossible to experiment with them and check the validity of the theory itself or the mathematics explained by it. Hence, there is no direct experimental evidence for string theory.

How many universes are there in string theory? ›

Each compactification produces a different spacetime, meaning that string theory can realistically predict a multiverse populated by 10^500 different universes.

What does string theory say about time? ›

String theory requires that space-time have 10 dimensions; LQG doesn't work in higher dimensions. String theory also implies the existence of supersymmetry, in which all known particles have yet-undiscovered partners.

How many dimensions are there according to string theory? ›

According to string theory, one of the leading physics model of the last half century, the universe operates with 10 dimensions.

What replaced string theory? ›

JB: Loop quantum gravity is an alternative to string theory, which in its earliest formulation says that particles are composed of one-dimensional lines or strings of energy. Different particles, in this theory, represent different vibrational patterns in these strings.

What disproved string theory? ›

NASA's Chandra X-ray Observatory helped power an experiment that might refute string theory. Experimentation in real life based on string theory is still pretty new, with much to discover. Scientists didn't find the particles they were seeking, which means one of a few different takeaways.

What is better than string theory? ›

New Quantum Gravity Theory Proposed As An Alternative To String Theory. Quantum mechanics and general relativity are two crucial ideas we have to explain the universe around us.

What are the 5 string theories? ›

For reference, in case you're curious, the names of the five string theories are: Type 1, Type IIA, Type IIB, SO(32) heterotic, and E8xE8 heterotic.

Who is behind string theory? ›

In the summer of 1968, while a visitor in CERN's theory division, Gabriele Veneziano wrote a paper titled “Construction of a crossing-symmetric, Regge behaved amplitude for linearly-rising trajectories”. He was trying to explain the strong interaction, but his paper wound up marking the beginning of string theory.

How likely is string theory true? ›

Polchinski explained a computation that shows that string theory is 98.5% likely to be correct, going on to claim that the probability is actually higher: “something over 3 sigma” (i.e. over 99.7%).

Does string theory explain dark matter? ›

String theory models of physics, including those from its M-theory corner, are definitely meant to include dark matter and dark energy.

What are all 11 dimensions? ›

11 Dimensions
  • A point (no dimension) We start with a point. ...
  • The first dimension – a line. ...
  • The Second Dimension – A Split. ...
  • The Third Dimension – A Fold. ...
  • The Fourth Dimension – A Line. ...
  • The Fifth Dimension – A Split. ...
  • The Sixth Dimension – A Fold. ...
  • The Seventh Dimension – A Line.

What is the 1st dimension? ›

The first dimension, as already noted, is that which gives it length (aka. the x-axis). A good description of a one-dimensional object is a straight line, which exists only in terms of length and has no other discernible qualities.

What dimension do humans live in? ›

We live in a world of three dimensions.

We move through space , either left or right, forward or backward, up or down. Everything around us, from the houses we live in to the objects we use in everyday life, has three dimensions: height, length, and width.

Does the past still exist? ›

In short, space-time would contain the entire history of reality, with each past, present or future event occupying a clearly determined place in it, from the very beginning and for ever. The past would therefore still exist, just as the future already exists, but somewhere other than where we are now present.

Is string theory the same as multiverse? ›

In physics, the concept of a multiverse is a key element of a leading area of study based on the theory of everything. It's called string theory, which is the focus of my research. In this picture, subatomic particles are just different notes on a tiny, vibrating string, which explains why we have so many of them.

What is the 5th dimension in simple terms? ›

In that case, a fifth dimension would be an extra dimension of space. Such a dimension was proposed independently by physicists Oskar Klein and Theodor Kaluza in the 1920s. They were inspired by Einstein's theory of gravity, which showed that mass warped four-dimensional space-time.

What is the most promising theory of everything? ›

String theory (or, more technically, M-theory) is often described as the leading candidate for the theory of everything in our universe.

What is the 26th dimension? ›

The 26 dimensions of Closed Unoriented Bosonic String Theory are interpreted as the 26 dimensions of the traceless Jordan algebra J3(O)o of 3x3 Octonionic matrices, with each of the 3 Octonionic dimenisons of J3(O)o having the following physical interpretation: 4-dimensional physical spacetime plus 4-dimensional ...

Is string theory math or physics? ›

String theory is the idea in theoretical physics that reality is made up of infinitesimal vibrating strings, smaller than atoms, electrons or quarks.

How does string theory explain time? ›

The symmetries of string theory suggest that time did not have a beginning and will not have an end. The universe could have begun almost empty and built up to the bang, or it might even have gone through a cycle of death and rebirth.

What is the opposite of string theory? ›

UD: What is loop quantum gravity, and how is it different from string theory? JB: Loop quantum gravity is an alternative to string theory, which in its earliest formulation says that particles are composed of one-dimensional lines or strings of energy.

Has string theory been proven? ›

Physicists have yet to produce any empirical evidence for either string theory, which was invented more than 40 years ago, loop-space theory or any other unified theory. They don't even have good ideas for obtaining evidence.

What does string theory predict about black holes? ›

In string theory, black holes are neither black nor holes. Instead, the best metaphor to explain what a fuzzball is to look at another compact-and-weird object in the universe: neutron stars. Neutron stars are what happens when an object doesn't quite have enough gravity to compress into what we call a black hole.

Is string theory related to black holes? ›

String theory is a theory being constructed to describe the quantum mechanics of spacetime. As such, the theory should explain whether black holes are consistent with quantum mechanics or not.

Videos

1. Michio Kaku Explains String Theory | Big Think
(Big Think)
2. String theory - a simple way to understand the universe | BBC Ideas
(BBC Ideas)
3. What are the Strings in String Theory?
(PBS Space Time)
4. Loose Ends: String Theory and the Quest for the Ultimate Theory
(World Science Festival)
5. Making sense of string theory | Brian Greene
(TED)
6. A (Gentle) Introduction to String Theory
(Nabil Iqbal)
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