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Ask Dr. SETI ®

Chapter 1: Astrophysics

Black Holes and FTL

Dear Dr. SETI:
In your response to an interested party's question on the attainability/feasibility of the speed of light, you mentioned "not one shred of verifiable evidence" of anything traveling faster than the speed of light. What about a black hole? Would not anything that crosses the Event Horizon be doing exactly that (traveling faster than light)? Could a gravity-assist maneuver propel a spacecraft to such a speed?

Portland Reader

The Doctor Responds:
An interesting question, dear reader. First off, I must stipulate that black holes are a little outside of my personal experience. I expect the same is true for you (even in Portland).

That said, it is my understanding (which is admittedly limited) that the immense gravity well represented by a black hole warps spacetime. Such distortions will have effects upon electromagnetic radiation which can only be described as wierd. It may appear that the 'c' limit is violated. On the other hand, if it is space that is distorted, rather than velocity, it's hard to say whether an apparent acceleration is genuine or illusory. And if it is time that is distorted, it becomes equally difficult to discern velocity when viewing the event from within an inertial reference plane. So, no, I would have to say that, at least from the perspective of viewers outside the event horizon, the 'c' limit is preserved.

But, what about what happens inside the event horizon? Could not a particle sucked into a black hole attain superluminal velocity? I am willing to concede the possibility, although how we will ever verify or refute that hypothesis without being sucked into the black hole ourselves is beyond me.

As I read it, the thrust (that sounds more dynamic than 'gist,' doesn't it?) of your argument deals with accelerating spacecraft (a practical application involving harnessing the black hole's gravity gradient in some way). I would speculate that, as long as the event horizon is not breached, the universal speed limit would be preserved for any gravity assist maneuver. Once inside the event horizon, any object which is accelerated beyond c will cease to exist, from our vantage point. I find it hard to imagine how we could harness this effect in any practical way.

But, what do I know? Not only have I never actually seen a black hole, I'm not even a real doctor...

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