Valenzuela is a theoretical physicist at the Institute for Theoretical Physics at the University of Madrid. One of the questions is if string theory is the unique theory of quantum gravity. This goes along the lines of string theory.
The way the calculation was done was pointed out by other commentators as too bold a leap.
Corrected Einstein.
The minimum possible value of alpha is calculated by the number that Vieira, Guerrieri and Penedones calculated. The size of the first and largest mathematical term that you have to add to Albert Einstein's gravity equations is roughly Alpha.
gravity is depicted as a curve in the space-time continuum created by matter and energy. It describes large-scale behavior such as a planet. When matter is packed into small spaces, general relativity doesn't work. Simon Caron-Huot, a theoretical physicist, said that Einsteinian gravity needs to be corrected.
Physicists can organize their lack of knowledge of gravity using a scheme devised in the 1960s by Kenneth Wilson and Steven Weinberg: They simply add a series of possible "corrections" to general relativity that might become important at short distances. Predicting the chance of two graviton interacting in a certain way is what you want to do. As distances get smaller, you can add new terms using any and all relevant variables as building blocks. These terms are fronted by numbers that set their sizes. There are different theories of quantum gravity that will lead to different corrections. These corrections are the first way to tell apart possibilities.
alpha has only been calculated in string theory for highly symmetric 10-dimensional universes. In the 1990s, the English string theorist Michael Green and colleagues determined that alpha must be at least 0.1389. In a stringy universe, how much higher depends on the string's propensity to spontaneously split into two. All versions of string theory unite in a master framework called M-theory, where the stringcoupling constants correspond to different positions in an extra 11th dimensions.
Alternative quantum gravity ideas can't make predictions. Physicists haven't been able to directly measure alpha as a way of investigating and testing quantum gravity theories because they can't detect gravitons.
A few years ago, Penedones, Vieira and Guerrieri started talking about using the bootstrap method to constrain what can happen during particle interactions. The approach was applied to particles called pions. We said, okay, here it's working very well, so why not go for gravity? "Guirrieri said that."
The Bound is being Bootstrapped.
The trick of using accepted truths to constrain unknown possibilities was devised by particle physicists in the 1960s, then forgotten, then revived to fantastic effect over the past decade by researchers with supercomputers, which can solve formidable formulas that bootstrapping tends to produce.
The three men are trying to find out what alpha has to be in order to satisfy two conditions. Unitarity states that the probabilities of different outcomes must add up to 100%. The same laws of physics must be held from all vantage points according to the second.
The range of values allowed by those two principles was considered by the trio. The calculation is simple enough to pull off in that setting, but it also allowed them to compare their predictions to string theory.
When the gravitons approach and scatter off each other, they might fly apart as two gravitons, or become three gravitons or any number of other. Unitarity demands that the chance of the two gravitons emerging from the encounter never surpass 100%, as you crank up the energies of the approaching gravitons. The form of the equations cannot be restricted by how an observer is moving relative to the gravitons. The rules yield a complicated expression. The computer clusters were programmed to solve for values that make the two-graviton interactions unitary.
The computer spits out its lower bound for alpha: 0.14 which is an extremely close and potentially exact match with string theory's lower bound of 0.1389. In the 10D place where the researchers checked, string theory seems to span the whole space of allowed alpha values. It was a huge surprise.
There is a 10-Dimensional Coincidence.
What might the coincidence mean? Simmons-Duffin, whose work helped drive the bootstrap's resurgence a few years ago, said that they are trying to tackle a fundamental and important question. Which is the extent to which string theory covers the space of all possible theories of quantum gravity?
The 1960s saw the emergence of string theory as a picture of the gluey glue that bonds mesons. A different description ended up prevailing for that purpose, but years later people realized that string theory could set its sights higher if strings are small. The same fundamental string would bestrummed in different ways. The theory gives a description of gravity by showing that a graviton is a closed string with equal waves travelling clockwise and counterclockwise around it. This feature underlies the properties of gravity.
It takes some fiddling to match the theory to reality. string theory needs a property called supersymmetry to double the number of string vibration modes to get rid of negative energies. The force particle must come with another mode for the matter particle to be vibrated in. The strings have to have 10 space-time dimensions to wiggle around in. Our universe is 4D with three dimensions of space and one of time, but we haven't found any supersymmetric partner particles.
Supersymmetry must be broken if string theory describes our world. If the partner particles exist, they must be much heavier than the known set of particles. If there are 10 dimensions, six must be curled up so small that they are invisible to us. In a 4D-looking universe, these dimensions could have many possible arrangements, all affecting strings and numbers differently.
Many quantum gravity researchers prefer non-stringy ideas because of the broken supersymmetry and invisible dimensions. The rival approaches have struggled to come up with the kind of calculations that string theory can produce.
Physicists hope that string theory will win hearts and minds by being the only description of gravity that is logically consistent. We will have no choice but to believe in hidden dimensions and an inaudible orchestra of strings if researchers can prove string universality.
To string theory sympathizers, the new bootstrap calculation opens a route to eventually proving string universality, and it gets the journey off to a rip-roaring start.
Some researchers disagree with those implications. A theoretical physicist at the University of SouthernDenmark and the University of Heidelberg who specializes in a non-stringy approach called asymptotically safe quantum gravity, told me that he would consider the relevant setting to collect evidence for or against a given quantum theory of gravity.
There might be descriptions of gravitons in 4D that don't make sense in 10D. She said that setting one might have ruled out alternative quantum gravity approaches.
When you go to 4D, there are many theories, but there is only one string theory. He said, "I doubt it."
Even if string theory saturates the range of allowed alpha values in the 10-dimensional setting the researchers probed, that doesn't stop other theories from lying in the permitted range. Andrew Tolley of Imperial College London doesn't think that string theory is the only answer.
Just the beginning.
If bootstrappers can generalize and extend similar results to more settings, it will be easier to assess the meaning of the coincidence. Alexander Zhiboedov is a theoretical physicist at Europe's particle physics laboratory.
They have already completed a dual bootstrap calculation, which bounds alpha from below by ruling out solutions less than the minimum rather than solving for viable alpha values above the bound. The calculation shows that their computer clusters didn't simply miss smaller allowed alpha values, which correspond to additional viable quantum gravity theories outside string theory's range.
They plan to bootstrap the lower bound for worlds with nine large dimensions, where string theory calculations are still under control, to look for more evidence of a correlation. bootstrappers aim to calculate alpha and gamma, the allowed sizes of the second- and third-biggest quantum gravity corrections, and they have ideas for how to approach harder calculations about worlds where supersymmetry is broken or notexistent. They will try to carve out the space of allowed quantum gravity theories and test string universality in the process.
A theorist at Imperial College said bootstrap principles are useful for exploring more ideas than just string theory. Positively, the rule that probabilities are always positive, she and Tolley have used to constrain a theory called massive gravity, which may or may not be a realization of a string of theory. If certain exotic particles exist, massive gravity only satisfies positive vibes. De Rham sees bootstrap principles and positivity bounds as one of the most exciting research developments at the moment.
No one has done the job of taking everything we know and putting it together. He said that theorists have work to do at a very basic level.
The magazine is funded by the Simons Foundation, which supports a research program called the Nonperturbative Bootstrap. The funding decisions of the Simons Foundation have no influence on our coverage.
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