For me, too soon to tell. There is no math in that paper, and I still hold to the quaint notion that observable consequences are what counts. I also wonder what happens to the singularity in the case of "apparent horizons which persist for a period of time," after that period of time has passed.

However, I really read that paper as a screed against firewalls, one possible solution of which is not to have event horizons, and FWIW I share that inclination.

I must admit I more interested in looking at an event horizon directly, courtesy of VLBI and the EHT ; http://eventhorizontelescope.org/index.html

For me, too soon to tell. There is no math in that paper, and I still hold to the quaint notion that observable consequences are what counts. I also wonder what happens to the singularity in the case of "apparent horizons which persist for a period of time," after that period of time has passed.

However, I really read that paper as a screed against firewalls, one possible solution of which is not to have event horizons, and FWIW I share that inclination.

I must admit I more interested in looking at an event horizon directly, courtesy of VLBI and the EHT ; http://eventhorizontelescope.org/index.html

I agree with Marshall on this. In my view too much science nowadays is based on hypothetical situations and conjecture, the alternatives and different theories are basically overwhelming and it is difficult to keep up with what is real, mainstream, tested and verified. Pity we don't have mm VLBI in South Africa, otherwise we could have participated in the Eventhorizon experiment.

At least we have been funded finally for a VLBI2010 antenna so will be able to keep ourselves going. Lesotho could have a good spot for mm VLBI, some spots are ~3400 m above msl. Anyway, it is an interesting paper, by someone who has extreme insight and abilities, but I am more of an experimentalist than a conjuror of alternatives. I like the idea that black holes could be redefined as metastable bound

states of the gravitational field, sounds more safe-like.

I am still trying to do GTR tests, with what I can, and GTR seems to hold its head above water and seems to fit all VLBI, SLR and LLR measurements. Maybe VLBI, via the Eventhorizon experiment can provide some clarity.

It's a bit odd, really, and I agree with Marshall and Ludwig.

The erstwhile Hawking picture of a Black Hole with an escape velocity equal to the speed of light, with a border edge ("event horizon") where the escape velocity shifts over from just under the speed of light to exactly the speed of light, and with quantum effects at that border ('Hawking radiation') leading to the slow evaporation of the BH, is (or was) pretty convincing on both theoretical and practical grounds.

Do we have a chance of learning anything new about the (non)-existence of event horizons with the object approaching the proposed BH at the galactic center? It might be stellar dust (good to be swallowed soon) or a star, but anyway it's deforming.

My first impression is that, unlike Hawking, the current firewall conjecture is not fixated on 'evaporating black holes'. The potential evaporation rate exceeds the CMBR absorption rate only for black holes with an absorption surface (event horizon) smaller than the Moon (less massive than any stellar core collapse BH). Of course, if there is only an 'apparent' event horizon, perhaps mass-energy cannot be absorbed?

Doesn't the 'apparent' gravitational effects imparted to nearby stars by Sgr A* indicate that its condensed mass exceeds 4 million solar masses - producing a very real gravitational field whose effective escape velocity at some external point must exceed c?

I think that the observed gravitationally compact, accreting black holes - with very high energy emission events appearing to occur very near their event horizons and the EMR & particles expelled through their relativistic polar jets - provide compelling evidence that nuclear and quantum processes are occurring at their very real event horizons, as the binding mass-energy of accreted material is extracted and converted to retained gravitational energy.

IMHO, the event horizons of accreting black holes (even those that only absorb CMBR energy) are _expanding_ beyond any 'apparent' horizon, not shrinking as in Hawking's conception of evaporating black holes...

@ Michael - I distinguish between parametric and non-parameteric tests. If you go out there and measure (say) PPN gamma in a Shapiro delay test, you get an estimate for gamma - 1 = x +- y, and if |x/y| is sufficiently large, you have disproved GR. (If x < 0 you have also disproved Brans Dicke) and that is pretty much that. Arguments about parametric tests tend to be about statistics and sources of systematic error.

Observing events at a event horizon is IMO largely non-parameteric. If, say, the EHT observes vortices or other structures on the Milky Way SMBH event horizon, then a host of gravity theories are in deep trouble (not just GR), but it could be hard to quantify that trouble, and so it could be hard to be sure that the test was actually conclusive.

For myself, I want to see (with the EHT) if the event horizon behaves like it should when it "eats" a star or gas cloud. The object you are referring to is likely to disappear a little soon for this, but I am sure another will be along. One advantage is that the flux history for infalling matter can be modeled, turning a non-parametric test into a parametric one.

Michael,

As I understand, the close encounter between Sgr A* and the identified gas cloud G2 is expected very soon now - probably in June. Hopefully observers will learn a great deal of new information, depending on what type of interaction occurs. I suspect that if large amounts of material is actually accreted we will see a great deal of EMR emissions. The most exciting possible result might be indications that the event horizon has subsequently expanded! See http://www.swift-sgra.com/.