# Can we identify black holes in our path flying between the stars?

When traveling at fast speeds through the galaxy, would it be possible to identify where all the black holes were on your path? I know most black holes are identified by their affect on other nearby objects. But what would happen if it was all by its lonesome?

Would we be able to 'sense' it somehow before getting to close to be adversely affected by it? Or would we fly by, maybe getting close enough for it to deflects our course? And how much could it deflect a course (assuming you stay out of the event horizon)? Would it warping of space allow for Alcubierre drive like properties?

I guess I'd hate to get thrown into a sun because of a course alteration from a black hole, though getting caught in the event horizon wouldn't be much better. Can we detect these masses to avoid them at distance or do we need to take first trips much slower so we can scan for these anomalies and mark them for safe 'shipping lanes'?

• Not worthy as an answer in itself, but I suspect interactions with dark matter might be a futuristic method of detecting these. Dec 10 '14 at 18:21

Barring the presence of an accretion disk surrounding these black holes - unlikely, in the absence of a compansion object - our best bet is likely to exploit gravitational lensing, the bending of light from distant objects by a massive object in front of the source. Typically, the massive object is a foreground galaxy, a massive star, or - in this case - a black hole.

The angle of the light bent can be calculated as $$\theta=\frac{4GM}{rc^2}$$ where $$M$$ is the mass of the black hole and $$r$$ is the closest distance a light ray passes from the black hole. For stellar-mass black holes (on the order of a few dozen solar masses), $$\theta$$ is likely going to be small because the angle is only linearly dependent on mass. When we reach masses of $$\sim10^4M_{\odot}$$, lensing seems more easily detectable, but at that point, we're talking about intermediate-mass black holes, which should have strong effects on the kinematics of their surroundings.

We have yet to detect lensing from an isolated, low-mass black hole, but it's not out of the question. If we did see it, it might look like this:

Image credit: Wikipedia user Urbane Legend, CC BY-SA 3.0

• Formulas are good, but images explain it much better. Thanks! Yes, Black holes are easy to see. Dec 10 '14 at 17:21

Black holes are called "black" only because light does not escape from beyond the event horizon. However, they are typically not what we would call either black or invisible.

A black hole was typically once a star, and so is highly likely to still have orbiting bodies - it may have lost some mass in its supernova explosion, but not so much that it would have lost all of its satellites, though it is probable that their orbits would have been significantly altered.

Also, black holes appear to emit EM radiation as matter falling in toward them is torn apart by the gravitational tidal forces. Some black holes have very bright accretion disks.

Finally, the powerful gravitic field of a black hole would cause gravitiational lensing, that could be used to detect it, given that this effect will distort the image of the rest of the universe as the black hole passes between the observer and the rest of the universe.

• Not worth separate answer: The astrogation systems of a starship would almost certainly be sensitive enough to automatically detect the effect the gravitation of a massive object has on the ships acceleration. Unless the stardrive used makes that irrelevant, of course. Dec 10 '14 at 16:12

I think unmanned scouts could lead the main craft. Besides other hazards, the gravity would affect the course and let you map all kinds of dark bodies, including more prevalent sub-brown dwarfs. Having them not only ahead but off tomthe side gives you a long baseline for viewing your destination as well as Earth in the rear. It could be a LIGO system for detecting gravitational waves.

Another idea: Black holes shadow the cosmic microwave background. This might be used to detect them. Given that with high speeds, the microwave background in movement direction (that's the direction that's relevant if you want to avoid them) will be at higher frequencies due to the Doppler effect, I can imagine detecting them by their shadowing would be simplified.

Black holes tend to have high relative velocities, you couldn't mark out permanent safe routes regardless of detection flights. If you're heading directly towards a particular star you should be able to see the gravitational interaction between a black hole and the star light you receive from the target, this will either be in the form of occultation or gravitational lensing but if you can see these effects you're probably far enough away that the transitory mass causing them won't get near you, it's passing ahead of your course at galactic speeds. Of greater concern is near lateral detection and that's likely to be spotty because it depends on what the background around the ship is like in terms of visible object density. Black holes and other non-emitting bodies coming in at high velocities roughly perpendicular to the direction of travel are going to be the hardest to spot before they cause disruptions.