Supersymmetry. What would be the implications for string theory if supersymmetry is not detected at the LHC?=

In that way, supersymmetry can no longer help solving the hierarchy problem.

Negative result would shift the focus of string theorists from model building to AdS/CFT. This is because one could study quantum gravitational theories in the bulk using their dual CFT description on the boundary.

Is string theory unique? Even with a unique microscopic string theory, there are still many possible backgrounds. There might be other theories of quantum gravity which are not string theory.

One nice way of studying theories of quantum gravity is via numerical simulations of the holographic CFT duals. People can put matrix models on computer, and recover thermodynamics of black holes in the dual gravitational description. We should be happy that we have a toy model of quantum gravity via AdS/CFT. It remarkably incorporates the holographic principle, where physics in a volume is encoded in the surface.

Problem with zero modes in one-dimensional periodic cavities.

We might no have to take for granted the connection between standard thermodynamic entropy, and gravitational entropy of black holes. But it is not something we were looking for. It fell out!

We should be looking for experiments directly probing strongly gravitating regimes.

Perhaps measurements of properties of highly-massive black holes. Also, measurements of the precessions of planetary orbits around highly massive stars.

We should be looking for more efficient ways of probing properties of elementary particles. Making the accelerators bigger and bigger is not likely to work for much longer.

Are there black holes? We certainly want to get rid of the singularity, but if there is no singularity, why do we need the event horizon in first place. As opposed to the singularity, the event horizon is not a physical object since it cannot be detected locally.

Which of these proposals are actually realizable in near future?

Planck results are already telling us a lot about inflationary models. They recently ruled out a large class of phi cube potentials.

Pulsar measurements can probe gravitational waves originating in early universe.

Cosmology should be emphasized much more. We will hardly ever get a lab on our planet probing the energies at the cosmological scales, which can be as high as 6-7 orders of magnitude below the Planck scale.

Measuring information transfer in the brain.

The discovery of extra-dimensions of space.

A quantum computer.

Given our answers for question 1, it is not surprising that the discussion quickly veered towards experiments that demonstrate a failure of quantum mechanics. People proposed understanding the decay to the ground state of an oscillator that emits gravitational waves, a Schrodinger cat state superposed in two gravitational potential wells, cold atom gas excited by gravitational waves, and evidence of quantum gravitational waves in the cosmic microwave backgrounds.

Consider an oscillator. If the gravitational radiation causes the oscillator to go to lower energy... is this allowed? Theory says it should be allowed. But not clear that an experiment is possible.

There is a prediction by a Perimeter alumni: take a cold atom gas. If a gravitational wave passes through it, would it excite the gas? Calculations by this alumni predict it should be detectable, but very hard to detect for experiments like LIGO.

A major challenge is that the Planck mass is so large. “When I first heard about Planck mass... I was surprised it so big. A tiny speck of dust, you can think it might have the Planck mass.”

The typical size of an atom is substantially larger than Compton wavelength. But an excited proton has a size the same as Compton wavelength. Would it emit a gravitational wave?

Take a particle with a very very small mass, maybe Planck mass. Put it into a double gravitational potential well. If the wells, A and B, have different depths, say h1 and h2, then there will be different amounts of gravitational time dilation in each well. This means that there are two notions of time. Going back to our previous discussion, this means there's no absolute time and we have an apparent contradiction. Is a gravitational wave emitted here? This is similar to Penrose's proposal where an observer experiences a superposition of gravitational fields.

Experimentally, people have already created quantum superpositions of very large objects, like molecules and Bucky balls. And they've created double-slit experiments with these superpositions. Can we perhaps push this to the Planck mass?

People would like to see experimental evidence for the failure of quantum mechanics... perhaps because some evidence for nonlinear quantum mechanics will emerge. This might be possible with large many-body systems. If QM fails for one atom, then a many body system should amplify the effect. Perhaps a subtle signal would appear in 1,000,000 atoms if the system is coherent. But it's hard to experimentally observe these systems.

“I bet we will have evidence that quantum mechanics fails in the next 10 years.”

“That's too fast. I predict 20 years.”

“My worldview would change if LIGO failed to detect gravitational waves.”

If gravitational waves appear in the CMB, as we thought they might have with BICEP2, is this good evidence that gravity is quantized because we are seeing quantum fluctuations in the vacuum? Maybe. Unfortunately, the mechanism that freezes out the quantum modes as they become superhorizon destroys the correlation between the modes so that the spectrum appears to be a classical probability distribution.

Gravitational wave detection: Could lead to the opening of gravitational wave astronomy

Gravitational Wave non-detection: Implies the need to fundamentally change the structure of general relativity

Detection of dark matter

Discovery of tensor modes in the CMB

Non-detection of SUSY

Would be interesting to see the movement away from the energy sector, i.e. higher and higher energy detectors, to rare event detection such as neutrino detectors

Could see some new effects here, even at low energy

Spatial or temporal variation of fundamental constants

More radically, the detection of some paradigm shifting effect such as the violation of scale decoupling in which one observes some correlation between large and small scales

• Signal that effective field theory was doomed

• Only current example of such an effect is the cosmological constant

• Along this line, could our experiments already be biased by our theories in that we are not actively seeking effects which would violate the structure of our theories.

Discovering non-trivial topology for the universe

Discovering what dark matter is, especially if it's something fundamental

Disocvering non-local correlations stronger than the maximum that quantum allows.

It's fun to consider crazy facts which could be true (though presumably aren't) that would change our worldview. For instance, imagine that by applying some sipmle algorithm to a number like Pi, a machine could produce some particular ancient sacred text.

In the same vein, imagine a Bell-type experiment where the only way to violate Bell's inequalities is to use humans to choose measurement settings, rather than computers.

Is there an experiment (like Bell's experiment) for which a certain outcome would indicate that we must abandon some premises, one of which is definite causal structure in quantum mechanics? (That is, in the same way that Bell's experiment indicates we must drop either Bell-style locality or Bell-style reality.) The great thing about Bell's' experiment, of course, is that it may be formulated in general language, i.e. it's model-independent, or almost so -- we are still using much of our standard language/intuition about the world, but we're not constraining ourselves to quantum mechanics, for instance.

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