For decades, most physicists have agreed that string theory is the missing
link between Einstein's theory of general relativity, describing the laws of
nature at the largest scale, and quantum mechanics, describing them at the
smallest scale. However, an international collaboration headed by Radboud
physicists has now provided compelling evidence that string theory is not the
only theory that could form the link. They demonstrated that it is possible to
construct a theory of quantum gravity that obeys all fundamental laws of
physics, without strings. They described their findings in Physical Review
Letters last week.
When we observe gravity at work in our universe, such as the motion of
planets or light passing close to a black hole, everything seems to follow
the laws written down by Einstein in his theory of general relativity. On
the other hand, quantum mechanics is a theory that describes the physical
properties of nature at the smallest scale of atoms and subatomic particles.
Though these two theories have allowed us to explain every fundamental
physical phenomenon observed, they also contradict each other. As of today,
physicists have severe difficulties to reconcile the two theories to explain
gravity on both the largest and smallest scale.
No strings attached
In the 1970s, physicists proposed a new set of physics principles to address
this problem, extending the laws proposed by the general theory of
relativity. According to this so-called "string theory," everything around
us is formed not by point particles, but by strings: one dimensional objects
that vibrate. Since its introduction, string theory has been the most
widespread theoretical framework that is thought to complete Einstein's
general theory of relativity to a theory of quantum gravity.
However, a new demonstration by theoretical physicists at Radboud University
now shows that string theory is not the only way to do this. "We show that
it is still possible to explain gravity using quantum mechanics without
using the laws of string theory at all," says theoretical physicist Frank
Saueressig. "We demonstrate that the idea that everything consists of point
particles could still fit with quantum gravity, without including strings.
This particle physics framework is also verified experimentally, for
example, at the Large Hadron Collider (LHC) at CERN."
Seen in experiments
"For scientists, this alternate theory is attractive to use because it has
been extremely difficult to connect string theory to experiments. Our idea
uses the physical principles that are already tested experimentally. In
other words: nobody ever observed strings in experiments, but particles are
things that people definitely see at LHC experiments. This lets us bridge
the gap between theoretical predictions and experiments more easily."
Only one set of laws
After having demonstrated that their ideas are capable of resolving
long-standing problems in particle physics, the consortium is currently
exploring the resulting implications of their new laws at the level of black
holes. "After all, there is only one set of laws of nature and this set
should be able to apply to all kinds of questions including what happens
when we collide particles at fantastically high energies or what happens
when particles fall into a black hole. It would be fantastic to demonstrate
that there is actually a link between these seemingly disconnected questions
which allows to resolve the puzzles appearing at both sides."
More information:
Tom Draper et al. Finite Quantum Gravity Amplitudes: No Strings Attached,
Physical Review Letters (2020). DOI:
10.1103/PhysRevLett.125.181301
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