Dear Dr B: Should 1 Report String Theory?

Strings. [image: freeimages.com]

“Greetings doctor Hossenfelder!

I am a Princeton physics major who regularly reads your wonderful blog.

I lately came across a curious passage inwards Brian Greene’s introduction to a reprint edition of Einstein's Meaning of Relativity which claims that:

“Superstring theory successfully merges full general relativity in addition to quantum mechanics [...] Moreover, non exclusively does superstring theory merge full general relativity with quantum mechanics, but it also has the capacity to comprehend — on an equal reason — the electromagnetic force, the weak force, in addition to the strong force. Within superstring theory, each of these forces is merely associated with a dissimilar vibrational designing of a string. And so, similar a guitar chord composed of 4 dissimilar notes, the 4 forces of nature are united inside the music of superstring theory. What’s more, the same goes for all of thing every bit well. The electron, the quarks, the neutrinos, in addition to all other particles are also described inwards superstring theory every bit strings undergoing dissimilar vibrational patterns. Thus, all thing in addition to all forces are brought together nether the same rubric of vibrating strings — in addition to that’s virtually every bit unified every bit a unified theory could be.”

Is all this true? Part of the argue I am scream for is that I am thinking virtually pursuing String Theory, but it has been somewhat hard wrapping my caput exactly about its electrical flow status. Does string theory attain all of the above?

Thank you!

An Anonymous Princeton Physics Major”

Dear Anonymous,

Yes, it is truthful that superstring theory merges full general relativity in addition to quantum mechanics. Is it successful? Depends on what you lot hateful yesteryear success.

Greene states really carefully that superstring theory “has the capacity to embrace” gravity every bit good every bit the other known telephone commutation forces (electromagnetic, weak, in addition to strong). What he agency is that most string theorists currently believe at that spot exists a specific model for superstring theory which gives ascension to these 4 forces. The vague phrase “has the capacity” is an aspect of this shared belief; it glosses over the fact that no i has been able to honour a model that truly does what Greene says.

Superstring theory also comes with many side-effects which all also frequently teach unnoticed. To get with, the “super” isn’t at that spot to emphasize the theory is awesome, but to betoken it’s supersymmetric. Supersymmetry, to remind you, is a symmetry that postulates all particles of the touchstone model accept a partner particle. These partner particles were non found. This doesn’t dominion out supersymmetry because the particles powerfulness exclusively last produced at energies higher than what nosotros accept tested. But it does hateful nosotros accept no testify that supersymmetry is realized inwards nature.

Worse, if you lot brand the touchstone model supersymmetric, the resulting theory conflicts with experiment. The argue is that doing then enables flavour changing neutral currents which accept non been seen. This became clear inwards the mid 1990s, sufficiently long agone then that it’s forthwith i of the “well known problems” that nobody always mentions. To salvage both supersymmetry in addition to superstrings, theorists postulated an additional symmetry, called “R-parity” that merely forbids the worrisome processes.

Another side-effect of superstrings is that they require additional dimensions of space, nine inwards total. Since nosotros haven’t seen to a greater extent than than the commons three, the other vi accept to last rolled upwardly or “compactified” every bit the terminology has it. There are many ways to practise this compactification in addition to that’s what eventually gives ascension to the “landscape” of string theory: The vast number of dissimilar theories that supposedly all be somewhere inwards the multiverse.

The problems don’t halt there. Superstring theory does comprise gravity, yes, but non the normal type of gravity. It is gravity plus a large number of additional fields, the so-called moduli fields. These fields are potentially observable, but nosotros haven’t seen them. Hence, if you lot desire to croak on believing inwards superstrings you lot accept to forbid these fields from making trouble. There are ways to practise that, in addition to that adds a farther layer of complexity.

Then there’s the number with the cosmological constant. Superstring theory plant best inwards a space-time with a cosmological constant that is negative, the so-called “Anti de Sitter spaces.” Unfortunately, nosotros don’t alive inwards such a space. For all nosotros shortly know the cosmological constant inwards our universe is positive. When astrophysicists measured the cosmological constant in addition to establish it to last positive, string theorists cooked upwardly some other laid for their theory to teach the correct sign. Even alongside string-theorists this laid isn’t popular, in addition to inwards whatever example it’s notwithstanding some other ad-hoc structure that must last added to brand the theory work.

Finally, at that spot is the query how much the requirement of mathematical consistency tin ship away maybe tell you lot virtually the existent reason to get with. Even if superstring theory is a way to unify full general relativity in addition to quantum mechanics, it’s non the exclusively way, in addition to without experimental exam nosotros won’t know which i is the correct way. Currently the best developed competing approach is asymptotically rubber gravity, which requires neither supersymmetry nor extra dimensions.

Leaving aside the query whether superstring theory is the correct way to combine the known telephone commutation forces, the approach may accept other uses. The theory of strings has many mathematical ties with the quantum champaign theories of the touchstone model, in addition to some recall that the gauge-gravity correspondence may accept applications inwards condensed thing physics. However, the dosage of string theory inwards these applications is homeopathic at best.

This is a quick overview. If you lot desire to a greater extent than details, a expert starting dot is Joseph Conlon’s mass “Why String Theory?” On a to a greater extent than full general level, I promise you lot excuse if I refer that the query what makes a theory promising is the running subject of my upcoming mass “Lost inwards Math.” In the mass I teach through the pros in addition to cons of string theory in addition to supersymmetry in addition to the multiverse, in addition to also hash out the relevance of arguments from mathematical consistency.

Thanks for an interesting question!

With best wishes for your hereafter research,

B.