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Astrophysics signal constrains quantum gravity & string theory + What is emergence?

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Mathematical proof that rocked number theory will be published
https://www.nature.com/articles/d41586-020-00998-2



Astrophysics signal constrains quantum gravity & string theory
https://www.forbes.com/sites/startswitha...d92af92507

EXCERPT: . . . Regardless of where you're located, how fast or in which direction you're moving, or when you're performing your measurements, everyone experiences the same fundamental rules of nature. The symmetry that underlies this, Lorentz invariance, is the one symmetry that must never be violated.

However, many ideas that go beyond the Standard Model and General Relativity — such as string theory or most manifestations of quantum gravity — could break this symmetry, with consequences for what we'd observe about the Universe. A new study by the HAWC collaboration, just published on March 30, 2020, just placed the tightest constraints on Lorentz invariance violation ever, with fascinating implications for theoretical physics.

[...] The Standard Model is exactly Lorentz invariant. General Relativity is exactly Lorentz invariant. But many incarnations of quantum gravity are only approximately Lorentz invariant. Either the symmetry that mandates it is broken, or there's new physics that only appears at high-energy scales that breaks it. Although the low-energy Universe is observed to be Lorentz invariant, direct searches at particle colliders (like the LHC) are severely limited by the energies they can probe.

[...] But astrophysics gives us a laboratory to probe far beyond the limits of what the LHC, or any Earth-based physics experiment, is likely to ever provide. Individual particles, in the form of cosmic rays, have been spotted with energies in excess of 1011 GeV. ... This combination of high-energy particles traveling over astronomical distances gives us an unprecedented laboratory for testing these Lorentz invariance violating ideas that quantum gravity and string theory models motivate.

[...] One of the most sophisticated tools astronomers use to measure these high energy gamma-ray photons is HAWC: the high altitude water Cherenkov observatory. Precise measurements of these very high energy photons ... can provide the strongest searches ever for Lorentz invariance violation. In their latest publication, the HAWC collaboration announced the detection of large numbers of these high-energy photons coming from four separate sources within the Milky Way: all corresponding to pulsar wind nebulae, the remnants of supernovae that accelerate material from the surrounding matter-rich regions.

If Lorentz invariance holds, there should be a continuous spectrum of these photons coming from these pulsars, without a hard cutoff (i.e., a steep drop and falloff) in their energy spectrum. But if Lorentz invariance is violated, then above a particular threshold, the number of photons should drop: either to 0 or to 50% of their expected value, depending on the particular Lorentz invariance violation scenario. But what HAWC saw, to a precision that's almost 100 times better than any previous measurement, indicates no violation at all.

What's fascinating about this result is that it sets a limit on the energy scale at which Lorentz invariance violation is allowed to take place. Based on the latest HAWC results, we can conclude that there are no violations of this symmetry up [...] 2,000 times the Planck energy scale. This is, importantly, much higher than the energy scale at which string theory, quantum gravity, or any such exotic beyond-the-Standard Model physics scenarios that bring Lorentz invariance violation along with them. In the future, an even higher energy instrument could place even tighter constraints...

Of course, there are always theoretical contortions one can invent to still allow for the possibility of Lorentz invariance violation. [...] But the fact remains that these photon-based constraints teach us that if a quantum gravity candidate such as string theory introduces a type of Lorentz invariance violation that predicts an astrophysical signature of photon decay, as many do, they are now constrained or even ruled out by this new set of observations. The laws of physics really are the same everywhere and at all times, and any extension to the Standard Model and General Relativity must reckon with these new, robust restrictions. (MORE - details)



What is emergence? What does “emergent” mean?
https://backreaction.blogspot.com/2020/0...rgent.html

INTRO: The word “emerging” is often used colloquially to mean something like “giving rise to” or “becoming apparent”. But emerging, emergent, and emergence are also technical terms. So, today I want to explain what physicists mean by emergence, which is also the way that the expression is often, but not always, used by philosophers.

Emergent broadly speaking refers to novel types of behavior in systems with many interacting constituents. A good example is the “La ola” wave that you sometimes see in the audience of sporting events. It’s not something you can do alone. It only becomes possible because of the interaction between people and their neighbors.

Indeed, something very similar happens in many condensed-matter systems, where the interactions between atomic constituents gives rise to certain types of collective behavior. These can be waves, like with la ola. The simplest example of this are sound waves. Sound waves are really just a simple, collective description for atoms in a gas that move periodically and so create a propagating mode... (MORE)


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