Suppose a spaceship sets out for Alpha Centauri, with a human crew (these people’s great-great-great grandchildren will colonize an earthlike planet orbiting this star. Unfortunately, the ship’s radar isn’t working, and the ship plunges into the gravitational fieldof a black hole! The ship begins to accelerate,under the tremendous gravitational pull…eventually, the ship reaches a velocity of 0.9999c…and the “local time” on the ship slows dowwwwwnnnnn…as the ship begins to fall into the event horizon of the black hole, the crew members experience a local time …one second of which is millions of earth years…is this a state equivilent to immortality? At the moment that the ship vanishes, it reaches c…the velocity of light…and disappears…what happens toit? Is it destroyedin a flash of radiation? Stephen Hawking…are you there? Tell me what happens to the crew! :eek:

A) The spaceship and an entire crew would likley burn up.

B) No one can hit lightspeed. That is why it is the universal speed limit.

I am not real keen on physics, though. Perhaps you can get this moved to somewhere like GQ?

Longevity, yes… immortality, no.

True, as you fall into the black hole’s gravity well, time begins to slow down, subjectively. It’ll take you millions of years to travel a comparitively short distance. However, subjectively, that won’t BE millions of years. It’ll be enough time to crap your pants and cry for mommy before tidal forces rip you and your ship to atoms, and then tear up the atoms, long before you reach the event horizon.

The OP has a lot of incorrect concepts. You have to look at it from terms of reference frames.

From an outside (well away from the black hole) point of view, the ship is gone in a flash. It (and you) break up under tidal forces, the accelerated bits start to produce radiation, etc. But from the rest of the universe’s point of view, you’re lifespan has been tremendously reduced.

From inside the space ship, assuming it has an Amazing Tidal Force and Radiation Protection Shell ™, you’ll grow to a certain age and die, e.g., maybe at the age of 72, just like if you weren’t near a black hole. And since the ATF&RPS doesn’t exist, you’ll in fact die a whole lot sooner.

Either perspective seems to me to be the exact opposite of immortality.

See, this is what I could never grasp about physics. Time is just a unit of measure created by man. *Theoretically* time slows down? So what? It won’t FEEL like time is slowing down. You wouldn’t think to yourself “Gee, this minute seems to be lasting longer than minutes usually do.” A minute is a fucking minute, regardless of what effect speed and gravity have on the functions of a time measuring device.

So what if sending an atomic clock at extremely high speeds causes “time” to change? Big fucking deal. In reality it doesn’t alter anything, does it?

So…unless someone is willing to give me a crash course in the subject, I’m going to continue on believing that these theories are bullshit.

Time is not a manmade object. Time *is* real. Without it, nothing would happen, or everything would happen at once. Time is a very real concept. It is simply the measure of change.

What Einstein postulated was that time and space are interwoven, meaning that intense gravity can screw up with not only distances in space, but distances in time. Around a black hole, space itself is distorted, so a straight line isn’t actually straight anymore, and one meter isn’t one meter. Of course, this is just from the point of view of an outsider. If you were to watch a ruler full into a black hole, it would appear to stretch. But if you were to fall with the ruler, it would look normal.

Similiarly, if you were to look at a ruler going near the speed of light, it would appear to contract and squish together. But if you were, uh, riding the ruler, everything would appear normal.

Along with space, time is also distorted. We have measured this directly by sending particles at really high speeds and measuring their atomic decay rates, and they do indeed slow down. We’ve taken highly sensitive clocks on high speed flights around the world and there is a measureable difference from earthbound clocks (it’s a very negligible difference, but it’s there).

This is a result from the speed of light always being 300,000,000 m/s no matter how fast you’re going. If you’re on a train and you shine a flash light, the light will be moving at 300,000,000 m/s. If you’re watching someone on a train shine a flash light, the light will be moving at 300,000,000 m/s. The velocities don’t simply add like we’re used to when dealing with speeds close to light.

A helpful demo to help understand this involves moving mirrors bouncing light back and forth but I can’t seem to find anything online. Just try to imagine a clock involving two mirrors bouncing a photon back and forth. Since light is a constant speed, it’s a perfect time keeping device. Now if we move the mirrors, the light will have to move to keep up with the mirrors, and end up having a zig-zagging path. Because the diagonal paths are longer than the normal vertical paths, something has to have changed. Either light has to speed up, or time has to slow down. Because light is always the same speed, the answer is simply that time has slown down.

Just so. Let’s take a typical balck hole of. say, 5 solar masses. Such an object has an event horizon about 15 km in radius. However, even at 100 km, the tidal force on you (assuming you’re falling in head first, or feet first and you’re 2 meters tall) is 2,655,376.4 kg, which would be more than a little painful, I suspect.

It worked for Dr. Rheinheart and Maximillian in that Disney movie.

:runs away:

This sounds incredibly fascinating, but I’m not really getting it, perhaps I need an illustration or a longer explaination, either way, I’m really hoping to hear more about this particular set-up.

This is backwards. From the point of view of an **outside** observer far from the black hole, the ship never enters the black hole; so there is immortality of a sort (though a sort much more cheaply purchased by bribing historians). Such an observer will, in theory, see light from the ship (further and further redshifted), at times arbitrarily far in the future. (In practice the redshift means that the ship will become invisible after a fairly short time.) This is because light emitted outwards from very near (but outside) the event horizon can “hover” near the horizon for an arbitrarily long time before finally escaping to infinity.

The **infalling** observer–the one looking for immortality–has only a finite time (and quite a short time, for any reasonably-sized black holes) from the time he passes the event horizon (which doesn’t look any different to him than any other random bit of space) until he reaches the singularity. What happens there is not predictable using current physics, but some time before then the tidal forces will become large enough to rip his ship apart. (This may happen either before or after the event horizon is passed. For solar-mass black holes it will happen before the event horizon; for much larger black holes such as the ones which seem to be in many galactic centers, it will happen after the event horizon is crossed.)

How can there be tidal forces on a body in free-fall? Tidal forces result from unequal attraction of gravity (as seen on the earth’s oceans) However, in free-fall (in the gravity field of a balck hole) every atom of your ship is accelerating at the same rate.So how are there unequal forces?

Second,in vacuum (surrounding the black hole) the ship can accelerate to terminalvelocity …I assume this would be c. So, does the mass of the ship (and its occupants) wind up going to infinity as well?

Seriously, how willour interstellar space ships avoid such collisions? radar won’t work-the radar pulses get bent into the black hole!

This scenario was used in an interesting science fiction short story circa 1969. It was called “Kyrie”, and the idea was that a being, falling into the black hole an in telepathic contact with a being *outside* the hole, issued a screm which the one outside heard forever, because of the time dilation effect. Interesting concept.

As the ship falls toward the black hole, it passes the event horizon in a relatively short time and disappears from the sight of the outside universe (except for this magical telepathy, which seems to be immune to the laws of relativity). Once inside, my understanding is that space-time seems to keep stretching as you approach the singularity, so you never would get there. To you, however, the trip is all too brief, and it doesn’t seem infinite at all.

On the other hand, once you’re past the Event Horizon, you can’t be seen by the outside universe (lightspeed signals travelling toward the Event Horizon from the inside don’t get there, because spacetime itself is “stretching” too fast), so I don’t know that it makes any sense to say that it looks as if you’re “suspended” inside the black hole – it doesn’t look that way to observers either inside or outside.

And it’s my undersatanding that, if the Event Horizon is big enough, you can pass through it without disastrous tidal effects.

Would just like to point out that David Merman has a really good analgous way of explaining length contraction and time dilation in Chapter 12 of his book “Space and Time in Special Relativity”. In fact, the whole book is a really good explanation of Special Relativity.

No, this is incorrect. The rate of acceleration due to gravity is dependant upon the *force* of gravity. On Earth, the surface gravity accelerates an object at approximately 9.8 ms[sup]-2[/sup], but this rate falls off as the square of the distance from the center of the Earth. Since the Earth is relatively large with respect to its gravitational pull, the force of gravity doesn’t change much between the surface and, say, LEO orbits. Tidal forces are independant of any motion relative to the primary body, but depend only on the difference in instantaneous graviational force between the near side and the far side.

Can I get one of those at an ATM machine?

:running:

A body in free-fall is accelerating based on the sum of all the forces acting on it. If these forces change appreciably over the size of the body, there will be tidal forces. (Think of a very long rod falling end-on toward the Earth. Its near end sees a larger gravitational pull than its far end, so there will be a tidal force tending to stretch the rod. On the other hand, if the rod is oriented so that its ends are the same distance away from the Earth, then the forces on the ends are not pointed in quite the same direction as the force on the center, since the forces are all pointed toward the center of the Earth, so the tidal forces will tend to compress the rod. The same sort of radial stretching and azimuthal squeezing happens near a black hole.)

There are several ways to detect black holes. Radar actually will work, in theory. For a black hole there are lightlike paths that loop around the black hole and come back to you. I’m not sure what the radar cross-section is (I know it’s small) but I suspect it could be detected. If the black hole is in front of a reasonably dense stellar field, then lensing can pretty easily detect black holes. And black holes tend to form in the middle of fairly dense regions (like in the middles of supernovas); the supernova remnant ought to be its own clue, and will also probably provide enough material to make an obvious accretion disk, which can be seen even if the black hole itself is not visible.

Interesting. There’s a paper by Gottesman and Preskill which uses wormholes to do the same thing. (Basically the paper points out that if you can carry one end of a wormhole past the event horizon then you can use it to transmit information out of the interior of the black hole. This presents problems with the hypothesis that the information which appears to be lost within the black hole is in fact encoded in some way in the Hawking radiation, because quantum information cannot be cloned. (This bizarre hypothesis appears to be necessary to preserve unitarity, which is a fairly fundamental postulate in quantum mechanics.))

It actually looks like you’re suspended just **outside** the event horizon (though, as I said above, because of the gravitational redshift, a human observer looking at your ship will see it rapidly redshift into invisibility), because light can hover near the horizon for an arbitrarily long time.

Yes, this is true. IIRC, the ~solar-mass black holes that form during supernovae are not big enough, but the galactic-center black holes are big enough.

To put it simply, the force of gravity decreases with distance. The gravity above the surface of the Earth is so spread out that the force on our feet and our head are basically the same. Since black holes are so small, however, the gravity well is much deeper, so the force differences on your feet and your head are much more pronounced. At one point they’ll be so pronounced that you’ll be ripped apart rather messily.

The force that the moon and oceans feel is the same force you feel. Remember that any orbitting object is in freefall. So while you’re falling towards Earth, you’re feeling tidal forces just like the moon is. But the moon is much bigger, so the tidal force is much greater.

I’m not sure how exactly the physics of this works, but I’m pretty sure that the mass of the ship will only appear to increase from the point of view of an outside observer. From the point of view of the ship falling in, their mass isn’t increasing.