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How can you tell for sure whether something is a black hole or a regular rock of the same mass, when you're too far away to resolve the size of the object in a
by tlb 7y ago
How can you tell for sure whether something is a black hole or a regular rock of the same mass, when you're too far away to resolve the size of the object in a telescope?
Do you have to send a probe close and throw something in it?
- SiempreViernes 7y agoI’d say occultation observations, ie looking closely when it passes in front of a star, would allow you to prove things one way or another given the huge difference in density and radius.
- sliken 7y agoYes, but presumably it's paste Neptune. Even if it's between the mass of earth (9mm) and jupyter (3M) we aren't going to be able to notice a direct occultation. However a lensing event seems possible, it greatly increases the effective radius of detection and is much more distinctive. Not sure what the smallest lensing event witnessed is, all the ones I'm aware of were a solar mass or more.
- verytrivial 7y agoThe first paragraph of the article is all over this lensing as a way to explain ultra-short crossing times from the OGLE experiment running in Poland. Reference [6] is titled "Constraints on Earth-mass primordial black holes from OGLE 5-year microlensing events [...]" so there's my weekend reading sorted.
- SiempreViernes 7y agoHow does that work, aren't distant stars point like? I guess the bigger problem is the shadow will be very thin and we're unlikely to correctly guess where it lands.
- sliken 7y agoRight, stars are (effectively) point sources light. But if you take images of stars like the LSST is where you can get the entire sky every few days you'll find anomalous stars that weren't on previous exposures. I don't know the physics enough to know if a small (earth to neptune mass) blackhole could redirect enough star lights to detectable by an earth (or earth orbit) telescope. With a huge database containing the time series of the sky you can start searching for unknown objects that stick out because they are a change from the previous exposures for that part of the sky. With enough data mining of those previous exposures you could find likely candidates and get telescope time to check out where you expect it to be next. This is similar to how things like Oumuamua and 2I/Borisov were found.
- Iv 7y agoFrom the abstract: > This scenario could be confirmed through annihilation signals from the dark matter microhalo around the PBH
- sliken 7y agoBlack holes are of course much smaller than anything else of the same mass. So the gravitational gradient is much higher, which causes some detectable side effects. One possibility is gravitational lensing. The abstract also mentions detecting "annihilation signals from the dark matter microhalo around the PBH". Not that I understand that completely, and I don't believe (corrections welcome) that dark matter annihilation by black holes has been confirmed experimentally. I believe high energy particles are released by black holes as they consume matter, so that might be another way to detect a small black hole instead of a similar mass rock.
- yread 7y agoWould evaporation give out any signal? Anti-particles could be a signal, no?
- vasama 7y agoA black hole of this mass is colder than the cosmic background.
- sliken 7y agoHawking radiation is a signal, one so strong it's on the order of a large nuclear bomb when a black hole finally winks out of existence. However if it's big enough to be messing with orbits around neptune it's likely pretty large. If it's large then the hawkings radiation isn't going to be visible. So if it's moon mass it's going to be 0.1mm and at 2.7 kelvin or so. Earth mass is 9mm, but even colder. Generally for the orbital changes they are seeing in a wide variety of objects it seems like the mass is even larger still. Even seeing pluto is hard, even a jupyter mass black hole is only 3 meters or so. So no I don't think we could detect it via hawking radiation. We might however see high energy particles resulting from the somewhat messy feeding that black holes are known for. If we sent a probe it could plausibly get close enough to directly observe hawkings radiation. Pretty amazing thought, maybe we will get that lucky.
- Smithalicious 7y ago
- walrus01 7y agoA rocky, very dark thing of the same mass would be much larger in diameter, and occult stars behind it. This is a somewhat common method of discovering things, through occultation.
- sliken 7y agoI suspect if you look at the smallest object detected through occultation at the distance past Neptune and it's going to be measured in km not meters (or mm).
- short_sells_poo 7y agoGenerally by it's radiation signature. Black holes don't just swallow matter without a trace. As infalling matter is accelerated to immense speeds, the tidal forces rip it into atoms and internal friction heats it up to very high temperatures. We'd basically see extremely energetic particles coming from a nearby source. Since no other known physical process can generate such high energy particles (not even fusion in main sequence stars), we'd have to conclude that there is probably some extremely dense object there.