Is time present in a black hole?

[PAllen]

The old thought experiment about two lights flashing on a railroad car traveling at nearly c. An observer on the car sees both lights flash simultaneously but an observer standing beside the tracks sees one light flash before the other. The above argument is similar to an argument that the viewpoint of the trackside viewer must be wrong because the observer on the train sees both lights flashing simultaneously.

Not unless you reject both SR and GR. In both, simultaneity is frame or coordinate dependent, while proper time is invariant. The event of an infaller crossing the horizon is point in the spacetime manifold. It exists, period. You can choose, for example, a Mercator projection of the Earth that does not include the poles. That has no bearing whatsoever on the existence of the poles. There is no such thing in GR as events that exist or not depending on coordinates, or depending on the observer. The most you might say is that some coordinates don't include an event, or that some observer can't detect a particular event.

 

[PAllen]

Edit: Please refer to this video and note that although the lecturer refers to infalling objects slowing down asymptotically at the event horizon, he never uses the words "appears to" or "apparently". The example begins shortly after 9:00 minutes.

A couple of comments on ths:

1) While Lenny Susskind is an expert, serious physicist, this is not a serious scientific presentation or paper. It is, as its introduction notes, a completely unprepared talk for a mixed audience.

2) He does state that the 'outside view' he is describing is a consequence of coordinates chosen to describe the outside.

3) Unfortunately, at around 10:37 he makes a pure and simple mistake. I hope this is a function of the nature of the talk, not a reflection of the author in more serious work. [The mistake is the claim that, classically, all the matter forming the BH is contained in shells asymptotically approaching the horizon. This is simply false for a collapse leading to a BH. During the collapse, just before any horizon forms, most the mass is already inside where the horizon will be and cannot possibly end up outside the horizon. In fact, for a classical collapse, the horizon grows from the center out, encompassing more and more of the collapsing matter, reaching the final event horizon at precisely the same moment as all the matter is inside (as viewed from inside). From outside, a better description of the asymptotic description is that the whole collapse (with density and pressure greatest in the center) never quite reaches the point where the apparent horizon forms. The whole collapsing mass appears to go dark - once it quiesces and all nearby matter captured - just before forming a horizon. You can, from the outside, describe new matter falling in as being in thin shells around the BH.]

 

[skeptic2]

This discussion seems to have ended. If what Leonard Susskind says isn't accepted then there's zero chance anything I say will be taken seriously.

 

[PeterDonis]

This discussion seems to have ended. If what Leonard Susskind says isn't accepted then there's zero chance anything I say will be taken seriously.

We don't accept arguments from authority here. Even eminent physicists can make mistakes, or misstate things, or not translate correctly from the math into ordinary layman's language. Even Einstein made errors like these sometimes. That's why we want to talk about the actual substance of what was said, which is what PAllen's post addressed. If you have a substantive point to make, it will be considered on its merits. But if your argument is "Susskind says so", then yes, there's no point in discussion, because we don't accept anything just on someone's say-so, even if it's Susskind or Einstein.