1) but in a later section other kinds of black holes, where two types of horizons exist, will be discussed. We shall consider a spherically symmetric, static spacetime (Eq. It is interesting to note that even though the results of these two observers seem to be contradictory, nonetheless the final conclusion turns out to be self-consistent, albeit unexpected. One of these observers, located within an infalling system, would measure, by recording incoming signals from a static source located outside the horizon, the rate of change of their redshift, and relate this rate to a notional speed the other is an observer residing “at rest” inside the horizon: the counterpart of the static observer outside the horizon. We propose to consider two distinct classes of observers who may carry out a measurement on the speed of an infalling test particle, linking the outcomes of measurements inside the black hole to observations outside the horizon. The aim of this paper is to discuss speeds of radially infalling test particles as measured inside (but also on and outside) the horizon in a consistent manner. It should be noted that the problem of the speeds inside the horizon presented by Hamilton dealt with a somewhat different perspective from that considered here and test bodies crossing the horizon were not considered. Although some aspects of the problem of the interpretation of the value of the speed of a freely falling test particle outside, and on the horizon have been discussed and presented in a series of papers it appears that the particular context of this phenomenon remains worthy of greater clarification. But, as explained elsewhere in extended discussions, the speed of the test particle crossing the horizon and measured within a moving frame of reference, which also simultaneously crosses the horizon, remains smaller than c. However the critical question of how speed is measured in such circumstances arises. In this context one can ask how does the speed of the radially infalling test particle change inside the horizon? One might reasonably expect that it would further increase inside the horizon as it fell through. Outside the horizon, all of the “fish” (an analogy for both massive and massless objects), can make their way against the flow but inside the horizon everything is carried toward the ultimate fate, i.e. Recently, experimental creation of horizons were reported in a variety of systems: microcavity polaritons, water channels, atomic Bose–Einstein condensates, and others. Such a horizon would play the role of the event horizon of black holes. In pioneering work, Unruh suggested a condensed matter realization of such a strong gravitational field: in fluids there might exist acoustic horizons separating sub- and supersonic flows. The point of no return represents the horizon of the black hole: the value of the escape velocity reaches, and inside the horizon, exceeds c. As shown by Hamilton and Lisle an interpretation of a space radially flowing inward at the Newtonian escape velocity could be applied to the case of a Schwarzschild black hole. At some point, the point of no return, it exceeds a critical value of the speed, such that nothing can propagate upstream. It has been claimed that a black hole space-time resembles a river flowing toward a waterfall: the closer to the waterfall the faster it flows. Generalization of this approach has led to an interesting perspective that has inspired laboratory analogues of black holes.
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