# Time of flashlight crossing the event horizon as seen from an external observer

This is a follow-up to my previous question: Can something (again) ever fall through the event horizon?

Consider the following thought experiment: I am again on my rocket at height $$h$$ over a black-hole event horizon. I am stationary with respect to the black-hole because my thrust perfectly counters the gravity.

I have a special flashlight: its light is polarized, and every second it flips the light polarization by 90°. I also have a telescope capable of detecting the polarization of light with arbitrarily long wavelength.

I turn the flashlight on and drop it from the rocket into the black-hole, which is big enough so the flashlight won't be spaghettified as it approaches the event horizon. I keep counting how many times the polarization flips as it falls. For a mass $$M$$ black-hole, with a drop of height $$h$$, how many times will the polarization flips before reaching the event horizon?

It must be a finite number, right? Otherwise the flashlight would never cross the event horizon from its own frame of reference. If so, it possible for the external observer to see the time the flashlight crossed the event horizon? Shouldn't the flashlight fall forever, never crossing the event horizon for the external observer?

• "Also the falling observer never crosses the EH in any frame of reference." Except the one in which he is at rest :) – timm May 14 '18 at 8:09

This is also a note on Ben Crowell's answer (it started as a comment but I think it makes a point which isn't in the other two answers).

As Ben says, the total number of flips you observe is finite, say $n$. But the intervals you observe between the flips are not constant. In particular, if you adjust the timing mechanism on the torch suitably before dropping it, you can arrange things so the time you have to wait (your proper time) before you see the $n$th (last) flip is arbitrarily large.

As A.V.S. also points out, the total amount of energy emitted by the torch before it crosses the horizon is finite. A consequence of this is that it becomes increasingly hard to observe the flips, and in particular the last flip: the light you see becomes very faint and very red-shifted. Especially if you assume that the torch emits a stream of photons, you quite quickly (quickly in your proper time) reach the point where the probability of you being able to observe any more photons from the torch is vanishingly low. At that point there can be no real observational difference whether the torch has crossed the horizon or not: there is always a chance that you might detect another photon, but that chance is vanishingly small.

In fact see A.V.S.'s comment below: the photons pretty rapidly drop below the background Hawking radiation from the BH in fact, so it really is the case that the infalling object becomes undetectable, even in theory.

• there is always a chance that you might detect another photon I kind of disagree with this. The closer the photon emission time to the time of the horizon crossing the more redshifted it would be once outside. As I explained in my answer, observer should have a threshold frequency $\omega_0$ below which it cannot detect photons at all. Even assuming absolutely ideal observer, if the wavelength of redshited photon becomes larger than $r_s$ it would be impossible to distinguish it from a quantum of Hawking radiation, and this redshift is achieved in a finite time. – A.V.S. May 14 '18 at 14:40
• @A.V.S. That's a good point:I was assuming a completely classical BH (so no background radiation from it) but quantized emission, and that's a bogus assumption. – tfb May 15 '18 at 11:23

If we model the flashlight and the observer somewhat realistically, than flashlight would have a finite power that it radiates and observer would have a finite threshold for the detection of such radiation. As the flashlight crosses horizon in a finite time by its own clock it would radiate only a finite amount of energy. Observer outside at finite radius would see that the frequency of light and its power start to drop exponentially. While the intensity of the signal from the flashlight as absorbed by an observer may remain positive for all times, quite soon it will drop below any theoretically feasible threshold of detection.

So in summary, observer will see the flashlight for a finite amount of time, the number of polarization flips (and total number of photons emitted by flashlight) would remain finite.

• "As the flashlight crosses horizon in a finite time by its own clock it would radiate only a finite amount of energy." Here "finite time" includes a photon emitted radially - at $r > 2 M$ to the exterior will escape - at $r = 2 M$ will "stay" there - at $r < 2 M$ will be pulled to $r = 0$. So the observer will see the photon corresponding to the first case. – timm May 14 '18 at 8:00
• That is why I have a "telescope capable of detecting the polarization of light with arbitrarily long wavelength". I know that doesn't exists, but this question is really about the difference in the frames of reference, not about actually observing something falling into a black hole. – lvella Jan 30 at 19:45

You would see flip $n$, which was the last to occur outside the horizon, and you would never see flip $n+1$.

I guess my own answer to the question would be: for any frequency of flipping, there will be one last flip before it can never be observed again (because the next flip would be beyond the event horizon on flashlight's proper time). But the higher the frequency, the more it will take for you to observe the last flip. If there is no limit on how high that frequency can be, there is no upper limit on how long the last visible flip can take to happen.

Please correct me if I am wrong.

• This is correct, but there also is another point. The time when you see the last flip can be arbitrary large at any frequency of flipping. In one case you may see it fairly soon, in another - in a trillion years (assuming a magic ability to detect photons redshifted to a nearly zero energy). It depends on how close to the horizon the last flipping occurs. In any case, from your perspective, the flashlight never crosses the horizon whether or not you see its light. – safesphere Aug 30 at 8:08

## protected by Qmechanic♦May 14 '18 at 9:54

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