I have a question. In string theory, what is the nature of time? Is it universal and absolute, like QM, or dynamic like relativity, or yet something else?
Mark Srednicki's Quantum Field Theory book has a very nice and often quoted passage in its introduction regarding two approaches to incorporating time in a relativistic theory. The problem is that non-relativistic QM treats space (position) as an operator while time as a sort of label. But relativity demands that space and time be on equal footing. So we can either make space a label (as is done in QFT), or time an operator (leading to string theory).
Actually, time is relativistic in quantum field theory (QFT), the framework upon which the standard model is built. It has special relativity baked in, but not general relativity (which describes how gravity works).
Unifying QFT and general relativity is a holy grail of physics.
R.e. String theory, I would think time is relativistic also, it is a direct consequence of the speed of light being constant in all frames of reference, which is a fact consistently proven by many experiments over the last 100+ years.
Normal QM is what you get when you do quantization of Newtonian Mechanics, that's why it has an absolute time.
If you quantize General Relativity instead, you'll get a variant of QM with relative times. But it's much more complex, and useless for most daily applications¹, so people don't use it often.
1 - X-ray measurements and nuclear physics are two important exceptions.
TL;DR “I think we now understand that space-time really is just a geometrical representation of the entanglement structure of these underlying quantum systems,” said Mark Van Raamsdonk
I'm a bit disappointed since this article really doesn't get to much new or much substance. The emergent time/gravity idea is interesting, but this is a rehash of other recently discussed results in theoretical universes much different from ours, that are believed by a small group of physicists to be relevant... but haven't been shown to be.
The problem with all these quantum gravity theories is they cannot be proven, or falsified, by experiment.
Not the theorists fault, we'd need an LHC the circumference of the galaxy, or build a black hole in a lab, to test quantum gravity. But it means we will never know if the theory in question is actually how nature works.
I am not aware of any mechanism that definitively places experiments testing theories of quantum gravity outside of our reach. We may not be able to probe Planck scale effects in the laboratory for a very long time or possibly even never, but it seems to depend a lot on the specific theory one wants to test where and at which scales deviations from current theories occur. I don't see any reasons why deviations could not occur only an order of magnitude beyond our current experimental limits and therefore be experimentally accessible in a lifetime. And even if experiments in the laboratory were currently impossible, there is always the possibility that we can gather indirect evidence by observing distant galaxies, black holes, the cosmic microwave background or something along that line.
The Planck energy is where we can be 100% certain to observe quantum gravity. There are 16 orders of magnitude between the LHC collision energies and the Planck scale. That's a humongous energy range in which to look for new physics. You could build a collider that's 10x more powerful than the LHC, but it'd be a stab in the dark, as the new physics may occur at 100x the LHC energy, or much much more.
To date there is no indirect evidence (e.g. Particle decay rates that deviate from the standard model predictions) that points to quantum gravity effects occuring below the Planck scale.
This indirect evidence is required to make a case for building a super expensive particle collider. For example, the LHC was built because we knew through indirect evidence, that it would either find the higgs boson, or new physics if the higgs wasn't found.
Without indirect evidence your experiment is a stab in the dark, which is fine, but for particle physics an insanely expensive stab in the dark.
Regarding astronomical evidence, it can give hints on quantum phenomena, but can't shed much light on their cause. For instance, we know dark matter exists via astronomy, but it can't tell us what the stuff actually is, we need laboratory evidence for that, I.e. Produce it in a collider, measure its mass, spin, other quantum properties, and how it interacts with other particles.
Certainly not within our lifetimes, within our species lifetimes, who knows.
We need to be able to probe the planck energy scale to observe quantum gravity effects. That is around 10^16 TeV, to put that into perspective, the LHC collision energy is 14 TeV.
Below the planck scale, gravity is too weak to be measured at the quantum level.
The worldsheet is time-symmetric, otherwise, if you pulled a single point of the worldsheet, the point would remain, and no elastic resistance would occur. If we presume that elastic resistance occurs, then we assume any pulled point will restore to elastic equilibrium with the aggregate elastic potential of the entire worldsheet, and thus, time symmetry IS the zero aggregate elastic equilibrium of a worldsheet.
We are not talking about zero UNIFORM elastic equilibrium, in which each point is at zero elasticity between all other points in a worldsheet. We are talking about zero aggergate elastic equilibrium between all other points in a worldsheet, thus, we are presuming the elastic resistance between all points on a worldsheet is always in flux, but eventually trends towards zero.
Eventually, this natural jiggling of the elasticity of points forms elastic folding within the worldsheet itself. Most of these folds revert back to zero aggregate elasticity, but sometimes, these structures hold because while they have odd topography, they achieve elastic equilibrium with the rest of the worldsheet because of their odd topography. This is because of the distribution of tension within the topology. Because it is at elastic equilibrium, the topology remains. Eventually, more topologies appear and eventually, some even interlock with each other like worldsheet velcro.
If the elastic equilibrium of these topological structures is pulled beyond its internal tension balance, each of these "tension contracts" will be pulled apart in favor of the dominant tension. Despite these topologies being made of time symmetric worldsheets, the topologies themselves are time asymmetric and cannot be made again by simply reserving the elasticity as the worldsheet still remains at zero aggregate elasticity, and thus, preserves its time symmetry AS A WHOLE. This is how arrows of time can arise from time symmetric universes.
From here, these tension contracts combine with each other, forming either a velcro binding or a tension binding, where one topology correctly scales the flow of tension to its neighbors, increasing its tensile strength, and thus, extending its influence upon the rest of the worldsheet. This, I believe, is the foundation behind Higgs fields.
The key take away is that every particle in the entire universe is nothing more than complicated folds of a two-dimensional conformal field theory coupled to symmetric time. Nothing rests on the worldsheet or exists beyond it. All energy and matter is made up of a complex combination of elastic equilibrium. Gravity, then, is the resolution of elastic equilibrium between two tension contracts. To prove this, we simply have to envision a blackhole as being anchored upon the universe at the very edge of the event horizon because that is where the rest of the worldsheet has achieved elastic equilibrium with the blackhole.
This is why negative energy does not appear in the wild. EVERYTHING is already made up of "negative" energy.
