Sorry for the confusion. I wasn’t trying to suggest that the ‘cosmic hum’ was relevant to interstellar travel - I just presented it as an example of something about the interstellar medium we have just learned recently even though we’ve barely ventured into it.
Let me be more clear: We have almost no idea of what’s in the interstellar medium in terms of space junk. And it could be a lot, or a little. We really don’t know.
For example, estimates for how many rogue planets exist ranges from two per star to over 100,000 per star. Further, looking at cratering rates on the moon, there is a cubed relationship between particle frequency and particle size. So for each rogue planet we should expect thousands of mountain-sized objects, billions of large boulders, trillions of big rocks, Quadrillions of small pebbles, etc. Maybe the cratering ratios don’t hold, but if rogue planets are the result of planetary system disruptions by passing stars, we should expect all the other junk to be disrupted as well.
We have measured the ‘bulk’ or uniform interstellar media by looking at things like starlight extinction, and we get 10^4 to 10^6 molecules per cubic meter. But that tells us nothing about rocks, boulders, or even rogue planets. We’ve only found a few Jupiter-sized rogue planets due to microlensing. Other than that, we’re blind.
In addition to that, estimates for the number of rogue black holes in the galaxy is even wider - anywhere from ‘not many’ to over 100 million.
The odds of hitting anything in any cubic meter of ISM is incredibly small. It’s way less dense that even interplanetary space in our solar system. But if you draw a cylinder the diameter of your spaceship and 4.5 light years long, how many chunks big enough to destroy you are in that cylinder? What if it’s so many that it’s nearly impossible to travel past a certain speed? There is also the problem that even coming near a large mass could deflect your course.
And this is not considering any other unknown-unknowns about the interstellar medium that might cause trouble.
Not true; we have a pretty good idea of the density of the interstellar medium, thanks to the degree of ‘extinction’ that occurs in the light travelling between distant stars and the Earth. Except for molecular clouds and dust clouds, the interstellar medium is pretty sparse. Travelling at up to one-tenth of light speed should be safe enough, unless the spaceship hits a very rare dust-mote, or decides to travel through a dust-cloud or accretion disk.
You didn’t read my whole post. I mentioned extinction, and what we measure with it is the average density of the interstellar medium - the gas and dust. That’s why I specifically said ‘space junk’ as opposed to the uniform medium.
What our measurements don’t tell us is how many rogue planets there are, how many boulders, rocks, pebbles, etc. We are using microlensing to find large rogue planets, but for the smaller stuff, we’ve got nothing.
Indirect evidence says it might be quite a lot. For example, we have had two large ‘interstellar visitors’ just in the past few years, and those are only the ones large enough to be detected. We probably have thousands, maybe millions of pieces of interstellar junk down to pebble size flowing through our solar system constantly which we can’t detect.
Thr gas and dust in the ISM will act as drag and erosion over time if your speed is high enough (relativistic), but at .1C a pebble would destroy your ship. How many pebbles are in a spaceship-sized cylinder between two stars? It only takes one.
So how long to get up to speed? And how do you guarantee they turn back on when it’s time to decelerate? The fewer lasers, the longer the acceleration. The more acceleration, more a trick (like the detachable reflective light sail for deceleration) is needed for arrival. Obviously a humongous number of lasers for a shorter time, or a lesser number for a longer time. Or a huge number for a long time because the ship is heavy, carries enough fuel to decelerate without lasers.
Light speed is a large number. This isn’t a “lasers on stun” for a few months and you’re done, unless the goal is to vaporize the ship.
Well, there should be a power-law relationship between the larger objects and the tiny objects. If so, there won’t be enough dust to worry about much. But to reduce the risk we could put a shield in front of the ship; a centimetre of tungsten should be good enough against interstellar hydrogen, but a few metres of reinforced ice would defend against the dust.
Mentioned also in my post. There’s a cube root relationship between the number of craters on the moon and their diameter, which we can possibly use as a proxy. The power law relationship is what I described in the last half of the post. The point is that for every rogue planet the power law relationship suggests there are billions to quadrillions of objects big enough to wreck your day.
Yes, we might be able to protect against gas and dust. How about protecting from a 10g pebble, or a 100g rock, or a boulder? If you are going a significant fraction of C, any of those will utterly destroy your spaceship.
We don’t know the odds of encounting any of these. Our best estimates are little more than guesses, but they range up to 100,000 rogue planets per star, 100 million rogue black holes in the galaxy, and if the power law holds, particles big enough to destroy a ship probably ranges into the quadrilions per star. Space is big, so even that may still maintain a low risk, but I’m not sure. And our estimates could be completely wrong on the low side.
We might find out that we can expect to run into 100 killer objects on any trip to another star. Or a thousand. If so, the speed limiter will come down to how fast we can go while still either avoiding these objects or armoring against them, which might require low speeds.
Drag from the interstellar medium becomes important at high fractions of C. It’s a tiny force, but it’s applied to the ship constantly for thousands of years. The minute your engines or lasers shut off you begin to slow down. The effect is probably negligible at normal interstellar speeds, but if you try to go really fast, it becomes a problem.
I’m not suggesting that any of this IS a problem. I’m providing it as an example of something that MAY be a problem, or something else we don’t know yet. And something like this seems more likely than that we are all alone in the galaxy, or that hostile aliens kill new civilizations, or whatever. Look to the mundane first.
Drag from the interstellar medium is probably useful. Accelerate the ship using laser beams (or particle beams), then use the interstellar drag to slow down.
None of this is perfectly safe, but exploration has never been safe. Look at Cook, Magellan, Scott…
Perhaps though, we may consider that the fact that nothing has seriously impacted any of our interplanetary probes is good news. i assume as residue of the accretion disk, the solar neighbourhood would be more populous than interstellar space? And of course, the amount of travel volume swept in the planetary plane is not even by orders of magnitude comparable to a large interstellar spaceship’s path. But it’s encouraging.
The speed differential between a planetary probe and any debris it encounters is probably still enough to do serious damage (measured in thousands of feet per second or more, so as good as a bullet even if not in heavy explosion territory. The gas “cloud” or dust in solar territory may not be comparable…
