Mm-hmm, and over and over and over in that link it clarifies -or defines- “causality” as “a causal trigger that gets from point A to point B by traveling conventionally through space at a speed not greater than the speed of light.”
By that definition of causality all FTL travel by definition would break causality. Duh. Yawn. Wait, why am I yawning boredly? Because if you do your FTL travel by not traveling conventionally through space then that definition clearly doesn’t apply to you. (It also doesn’t apply to you if you’re literally traveling FTL, but literally traveling FTL seems more overtly against the laws of physics than just stepping briefly out to another dimension - and while stepping briefly out may or may not be quiet, burning superlight rubber would definitely be loud and probably do lots of damage when you decelerated at your destination.)
I comprehend the light cone. It’s how come after I tesseract next to you, I can look through the telescope next to you and describe what I’m ‘going to do’ before it happens - because in actual fact I already did it, and you’re merely slow in getting the memo because the memo (that is, light) has to take the long way to get to you while I took the shortcut.
But while it looks like time travel through the telescope, it’s not time travel, and if I instantaneously tesseracted back I wouldn’t instantly appear in your vision - I’d appear there later, when the light showing my return finally got back to you.
Firstly, I think you are missing the main point. When it comes to space travel, we’re talking about problems that the human race has only recently become aware of: indeed within living memory.
It’s absurd to me to declare that such problems must be insurmountable even to species with a technological history thousands of times longer than ours.
And let me be clear: I would never assume that loopholes will be found in any particular area of engineering, but I think it’s a safe bet that in general many things that seem impossible to us would be possible to them.
And IMO, a particularly ludicrous example of an “impossibility” is the recent popular factoid / meme that hiding a ship would be impossible. As I pointed out in previous threads on this, if a ship is flying directly at you at close to lightspeed, good luck detecting it.
So that’s a non-scientist, living in a civilization that only left their atmosphere for the first time a few decades ago, finding an exception to this supposedly incontrovertible truth, off the top of his head no less. But sure, I’m sure a civilization that has been in space for millions of years would find no other methods :dubious:
This is true, but of limited utility. Earlier in the thread we discussed at length just how difficult it is to accelerate anything to a speed which is ‘close to lightspeed’. If you want to use this as a method of concealing your ship you will need to start accelerating a very long way away, a very long time ago, and use a fabulous amount of energy to do so. If you tried this manoeuvre anywhere within the confines of our solar system we’d see you during the acceleration phase.
Just to expand a bit on what eburacum45 is saying, you aren’t going to instantly go from a full stop to 99% the speed of light…or vice versa. Even with some sort of magic future tech, which we are assuming just talking about this, it’s going to take you time to build up to that. Next, you are going to be coming from a long way away. So, let’s say your future star ship is traveling at 99% the speed of light directly at us. Where is it coming from? At a minimum, several light years away, but most likely several dozen or several hundred. Do the math. If you are going 99% the speed of light and ramming into gas and dust at those speeds you are going to be producing a hell of a lot of radiation and released energy…and that stuff is going to be moving away from you and your bow shock wave AT the speed of light. Even if it’s only moving away from you at 1% (or even half a percent) difference we are going to see it years or even decades before you get here…maybe centuries. And it’s going to be pretty much continuous. Then there is the flip side, so to speak. As these future aliens plan to, you know, actually stop here and steal our cows and probe our butts they have to STOP here…that means they have to slow down. Again, from a long way out as slowing from near the sped of light isn’t something you do quickly. Which means they have to fire up their fancy future drive to go from 99% the speed of light or whatever we are talking about to a speed that will allow them to actually enter the solar system and go into orbit, and that’s going to leave a mark, so to speak…and it’s one where they are going to be continuously slowing, meaning they will go from 99% the speed of light to 90%, 80%, 70%…etc…for months, or years…down to whatever fraction of that speed they need to enter the solar system and eventually achieve orbit with the Earth. If they decide, screw those braking maneuvers, well, they are almost certainly going to slam into something larger than dust and gas entering the solar system at those speeds. Plus, they won’t be able to actually come here to harass us, just do a fly by, waving to us at near light speed as they go through the solar system.
That’s the thing. Yeah, we don’t have a lot of space faring experience, certainly not at what we are talking about here. But we DO have a bit of experience with observing the universe. Unless they can break the laws of physics as we know them there is no way to move something as big as a ship carrying mass at those speeds (full stop, but we’ll go on) without leaving a signature of some kind that we could detect. If they are traveling in real space at those speeds we’d see them. If they were using magic wormholes we’d be able to detect that because of the massive energy released. If they were bending space to travel faster than the speed of light wrt our observations we’d see that as well as it would take star levels of energy. Unless they can magically wish themselves from their star system to ours instantaneously using some sort of dark energy power system we can’t detect we’ll see them coming.
Thanks for the support, but I’m not entirely sure that we’d be able to detect wormholes using our current technology. We can’t detect stellar-mass black holes either unless they are in a binary system (there are lots of candidates for stellar black holes, but these are all in close binaries so far.) An article about detecting wormholes here, with more links if you are interested.
At the end of the day, I think we would be wasting out time looking for wormholes, since they are mostly the product of wishful thinking. But it would be jolly nice to find one.
From the futurist speculation threads I’ve seen on creating a worm hole (something we obviously can’t do and won’t be able to for a hell of a long time, if ever) it would take a huge amount of energy to create one (plus stuff like negative energy to keep it open and goes know what else), so I’m pretty confident that if one opened up in the solar system, we’d see it. Granted, we probably wouldn’t detect the far end if it was more than a few light years away (would take years, decades, centuries or…well, a hell of a long time) for that to get to us, but this end would be pretty spectacular, assuming the speculations are correct. Kind of hard to believe something that energetic would go unnoticed on this end though.
The point you’re missing is the profound implications that arise from the fact that the speed of light in a vacuum always looks the same to everyone, regardless of their reference frame. This revolutionized our understanding of spacetime and the nature of simultaneity in points separated in space. Light propagates in a vacuum at the speed it does not because of any particular properties of the medium, the way sound propagates through air, but because of the nature of spacetime. The outer edges of the light cone are defined by c; they’re called “null lines” and define the outer limits of causality. Any events outside the past light cone can never be seen and can never affect the present, while any events outside the future light cone can never be affected by events in the present. Outrunning a beam of light is thus not like outrunning sound, and any kind of FTL travel or signaling can be shown to violate causality.
Again, no. See the second link above. FTL travel or just FTL signaling can be set up to clearly violate causality by making effects precede the events that caused them.
A.
To declare that the limit of human technology right now is the limit of what any technological species could achieve would be absurd. We are very new to space travel (we started less than one human lifetime ago), in a universe where there could potentially be species millions or even billions of years ahead of us.
And often in science as we learn more about a phenomenon we find blanket “impossible” statements to have been wrong.
B.
Nevertheless we can be fairly confident ruling out certain technology because it runs contrary to very fundamental properties of the universe or logic itself: so no perpetual motion machines, P and !P etc.
However, “hiding a ship” clearly belongs to paragraph A and not paragraph B, because I was able to think, trivially off the top of my head, of an exception to the idea that it’s impossible.
It irrelevant whether the exception is of little or no utility. The fact that it belongs to A and not B implies there may well be other exceptions, just ones that you and I cannot conceive of right now.
You appear to be addressing a straw man; the rule you are arguing against is that ‘stealth in space is impractical’, not impossible. A spacecraft can be stealthy if it does nothing, and emits no waste heat. This rule doesn’t rule out kinetic missiles launched from a long way away, and proximity mines disguised as asteroids, and so on. But neither of these methods allow you to ‘cloak’ your spacecraft during a firefight.
Your example of ‘trivial stealth’ turns out to be fantastically non-trivial. Rather than a way of making a ship invisible, it requires years or decades of planning and preparation, requires extravagant amounts of energy, and can only be used once during a very limited time window. By the time the relativistic kinetic missile arrives, your enemy might have already settled for peace, making the arrival of the missile a diplomatic disaster. In no way does it negate the rule that stealth in space is impractical.
Quite a lot of hedging going on here. Are you saying it’s impossible or not?
If you’re saying cloaking ships *is *impossible then I stand by the argument in my previous post. “Impossible, except for all these exceptions we can think of” is pretty weak sauce when we’re talking about something that will supposedly be insurmountable to aliens millions of years ahead of us.
If you’re saying that what you meant was it is merely impractical i.e. very difficult and costly to engineer, then OK…so what?
Every aspect of space travel is difficult and expensive from the point of view of 21st century humans.
The “rule” that it’s impractical? What are you talking about?
Yes; it is a particularly intractable rule, too. You can’t hide an active, heat-emitting spaceship in space except by being a long way away, in time and in space. I’ve devised quite a few ways round this rule myself, but they all make space battles really difficult to fight in real-time; having to plan every attack years in advance makes space warfare useless as an extension of politics.
Out of curiosity, how do you stealth something that is going a large fraction of the speed of light? Even if you get it moving a long way out (say, 100 light years), and are able to get it up to the 99% speed of light we were using earlier within a few years and, presumably, it’s coasting along towards us, how do you hide it impacting interstellar dust and gas? How do you slow it down in a stealthy way?
I don’t think this has to do with humans being millions or billions of years behind the aliens, I think some things are just not possible. It’s possible at those sorts of time scales, assuming a contiguous civilization, that they could work out how to move large masses at high fractions of the speed of light. But if it’s actually moving through real space it’s going to be impacted by that medium.
Of course, if these aliens are using magic or some form of science that isn’t limited to the universe as we know it, then all bets are off. But you might as well say ‘god did it’ at that point because ‘god can’ and leave it at that. I get what Mijin is saying, but this seems to me no different than the god can do anything argument. If we are going to have a discussion at all that doesn’t rely on technologies that are flat out impossible to our understanding of the universe then some things just aren’t likely.
That said, if you didn’t need to travel at those sorts of speeds and you could essentially have a cold ship coasting through interstellar space for millions or billions of years, it’s plausible you could have some sort of stealth tech that could hide the difference in temperatures between the background and what little the ship would need so that it wasn’t a dead hulk when it arrived and, perhaps, able to maneuver very subtly into orbit in the outer solar system (maybe in the Oort cloud or Kuiper belt), then send in some small craft to investigate. THAT is possible. Of course, the reason for doing so, wrt the OP would be obscure, and the ability to hide so well except when some dude with a camera phone is around seems improbable.
I’m guessing that Mijin is assuming that the ship concerned does not slow down. A ship approaching at 99%c would be following close behind the light emitted by friction with the interplanetary or interstellar medium, so would be very difficult to spot in time. But you don’t need to slow down to fight - if a ship travelling at that speed simply threw a bucket of rice out of the window, each grain of rice would carry enough energy to explode with the force of a small nuke. About 5 kilotons, if I’ve not dropped the decimal point somewhere.
Sure…at those speed even grains of sand would be nasty. But it doesn’t really go with the theme of the thread, which is UFOs coming here in semi-stealth to steal our cows and probe our butts though. We’d certainly notice stuff hitting us at some high percentage of the speed of light, but yeah, if you were wanting to attack a planet then I can’t think of a more devastating weapon than tossing mass at it at those sorts of speed. :eek:
You didn’t get the objection.
You cannot have a “rule” that something is impractical any more than we can have a *Physical Law of Gee, washing up is a much more annoying chore than mopping the floor, amirite? *
Please answer the following question.
You and I, two non-scientists (I presume), can think of several exceptions to the idea that ships travelling in space will always be detectable. Do you think it is likely that a species thousands or even millions of years ahead of humanity technologically will be able to think of additional ideas or technologies that we cannot conceive of right now?
Using a spaceship travelling as an arbitrarily high fraction of c as an ‘exception’ is really a bullshit exception. Even a spaceship travelling at 99% of c is preceded by the light emitted from its friction with the interstellar medium, so it is sensu strictu, not ‘undetectable’. That seems to eliminate your original exception fairly comprehensively. I’ve tried to write fictional scenarios incorporating these sort of ‘exceptions’ to the stealth rule; they always break down at close range.
Yes, but I can not say for certain that those technologies will include either reliable ‘cloaking’ technology or any kind of faster-than-light technology. Both of these concepts are just wishful thinking. Another one we haven’t discussed yet is a reliable force-field; more wish-fulfilment that has no plausible real-life counterpart.
But (getting back to the OP) the observed characteristics of UFOs (as reported) seem to suggest that the aliens have access to most, or all, of these technologies. Either they do have these capabilities, or the observations are wrong. I’m fairly sure that the observations are in error.
Some things are basically impossible…at least as far as our current or even speculative physics is concerned. Moving through real space with mass at those types of speeds is right on the verge of being impossible. Moving at those sort of speeds through real space without leaving a signature is impossible. We know that even in interstellar ‘empty’ space there is stuff at those sorts of speeds…and that means you are going to either have to move it out of the way or impact it (and somehow survive), both of which are going to make a hell of a lot of fireworks. I get what you are saying…if they are millions or billions of years advanced they will have tech we can’t even grasp. But there are limits to what even the most advanced could do, at least in terms of what we know about the universe. There could be other ways they could come here (slow boat to China method should work), but if they are moving in the physical universe they have to obey the basic laws. Or they are gods that don’t have to, but then we can’t really have any sort of discussion about this stuff that’s different than talking to a religious person who can always just pull the God Can do Anything card, which is really no fun.
I wasn’t trying to be unfair, and basically I’m probably one of the few posters on this board willing to get all Sci-Fi speculative here…I LOVE this stuff…but there are limits. Just trying to point that out.
Not really because that relies on several implicit assumptions about what kind of vessel we’re talking about and how it moves through the medium.
And it’s irrelevant anyway since you have agreed that there are a number of exceptions to the supposed “rule” that ships must always be detectable.
You could phrase it as “wishful thinking” if you like.
I would put it more like this:
When I say aliens “millions of years” ahead of us, many people don’t appreciate the scale of what we’re talking about.
After all, we talk in geological time scales all the time. Evolution and plate techtonics, for example, happen over those time scales but not spectacular changes if we’re just talking a couple million years.
Soo…aliens millions of years ahead of us must just mean somewhat fast ships with Giger-esque interior design and green mood-lighting, right?
But from our one sample of sentient life, millions of years would be a rather big deal. For humans the stone age only finished a few thousand years ago, and our progress is accelerating all the time.
What a sentient species could do with millions of years…it’s unfathomable to us. I think a conservative estimate for Dyson spheres would be 10,000 years in humans’ future…and then what after that?
That’s why it’s important to make the distinction between “Very clearly and explicitly impossible according to fundamental laws of physics” and “Utterly impractical; would require stupendous amounts of energy and solutions to a plethora of massively difficult engineering problems”.
The former problems are insurmountable (although I expect in at least some cases a fuller understanding of physics may show a few black-and-white situations are actually not quite so).
But many of the latter kind will become absolutely trivial to such a species.
They will have moved on to solving problems that are unknown-unknowns to us a long time ago.
The “wishful thinking” part is whether a species will ever get to this point.