Silver_Swift

Well, you don't have to get into orbit to hit something in orbit, right? But yes, this is likely off topic for this thread.

100/1015 is not 0.39%, it is 0.00000000001% (that is the correct amount of zeroes, not a hyperbole). Now obviously we want to learn from our mistakes, but I don't think "Using any resource that is not literally infinite is a bad idea" is the lesson we want to learn here.

I'm hardly an expert on this, but I don't see how you get an advantage from staging if you don't drop the stage after it is spent. Isn't the main advantage of multiple stages that you don't have to carry the weight of the spent stage after it is used?

[quote name='PB666']The problem is that there are few isotopes or fission pairs that combined energy is less than iron.[/QUOTE] Aren't there literally zero of those? I thought iron-56 was the most stable isotope in the periodic table.

Well, when it comes to lottery tickets, those are drawn using some physical process that is either fully deterministic or at least obeys certain rules (probabilities), but which in any case is very well understood (how else would we know that is sufficiently random?). We don't have probability distributions for the laws of physics that could have been. There is an open question here, some unexplained phenomena. It is not necessarily something we have to or even can understand, but complexity doesn't come out of nowhere, so there has to be something. I think PB666 extended the metaphor quite nicely with the rabbit de-evolving and everything going black. Point is, I kinda want to now how the blackness works.

I think the question "what was there before the big bang?" is valid, if worded confusingly. It's all good and well to say that time started at the big bang and there is no "before" that for anything to be in, but that statement itself pre-supposes the laws of physics. Those laws have complexity, order, they can't just have poofed into existence at the start of time with no cause or explanation. If they did, there would be no reason we have this particular set of laws, rather than some arbitrary other set. Now this is a confusing question, because even if you did find a reason for why we have these specific laws of physics you could just follow the rabbit hole further down (eg. if we are living in a computer simulation, what laws is the computer running on?). There is probably always going to be some point where we are just going to have to admit that we don't know yet.

Yes, I think part of the confusion is that (like K^2 mentioned) even though you can use every kind of FTL communication to travel through time, you don't get time travel from every individual instance of FTL communication. Also worth mentioning, for purposes of science fiction stories I think you could get around the problem by throwing relativity out of the window and outright stating that FTL communication can only happen in one specific reference frame (that might make it hard to use FTL if your star system is moving at high speeds in that reference frame, but that could be a plot point). Ah, but the self-destruct message is sent at the exact same time as A1 in both reference frames. So when B1 happens, if C is not destroyed, it will/has sent the "I have seen B1" message and it will receive/has received the self destruct message so it will have/will self destruct. Verb tenses are weird when talking about time travel

What people are trying to refute is the idea that because a ship with an Alcubierre drive is somehow safe from causing CTC's because it doesn't locally go faster than C.

As an addition to my above post (to tie it back to actual time travel): if you have instant ftl travel you could groundhog-day-loop yourself by teleporting from C to D when B1 happens and teleporting from D to C when A1 happens.

I'm not 100% sure I got the scenarios you are proposing correctly so I'm going to formulate one of my own. I'm also going to assume instant FTL communications. The example generalizes to arbitrarily slow FTL communication or travel, but it is clearer with instant communications. (Note that this example is likely still way more complicated than needed) A and B are 5 light minutes apart A1 and B1 are events happening within 5 minutes of each other on A and B. D and C are spaceships travelling at speeds such that: - From C's perspective A1 happens before B1 - From D's perspective B1 happens before A1 Let's keep the events simple: A1 = A sends FTL message to C and D with "A1 happens now" B1 = B sends FTL message to C and D with "B1 happens now" Now assume that the two spaceships are unmanned drones (so no one gets hurt) that can send FTL messages to each other. On top of that C has a self destruct system. C has the following programming: - If it receives the message "Please self destruct" it will self destruct. - If C receives the message "B1 happens now" it sends the message "I have seen B1 happen" to D. D has just one programmed action: - If it receives the message "I have seen B1 happen" from C it will wait until it receives "A1 happens now" and send the message "Please self destruct" to C. So from C's perspective A1 happens before B1 and it receives the self destruct message (and self destructs) before it can send "I have seen B1 happen" to D. But D won't send the self destruct message unless it receives the "I have seen B1 happen" message, so does C get destroyed or not?

Ah ok, so basically the advantage is that a lot of the weight you have to move to Mars is already at the right speed (the second craft has enough dV to do a Mars flyby on its own, but it doesn't have the additional stuff you need to survive out there, allowing it to be much lighter)?

How would you do the transfer in the first place, if you have enough dV to rendezvous with the craft don't you also have enough dV to get to the desired speed without gravity assists?

Most important to whom? The universe is rather famously indifferent to our intuitions. All this stuff is way above my knowledge of physics, but I don't think this is as self-evident as you think it is (or, depending on your definition, it isn't self-evident that time travel breaks causality). There are certainly apparent paradoxes that can be constructed, but that is also the case for other laws of physics that we are pretty damn sure about (like the whole racecar on a train scenario with regards to lightspeed). The laws of physics as we understand them theoretically (might) allow for FTL communication and thus timetravel. The question we have to answer is whether the paradoxes we can construct can be resolved or whether they are real paradoxes, in which case there is something (that we missed) explicitly preventing FTL communication from happening regardless of loopholes used. If this question was easy, we would already have the answer. There are so many extremely smart people working on this stuff that any open problems are not going to have obvious answers.

The way it was explained to me is that at a fundamental level the order of events is not a real thing for two events that are separated enough that they cannot influence each other without FTL. So suppose you have place A and place B that are separated by 5 light minutes and event A1 and B1 that happen at those places within 5 minutes from each other. Now also suppose that from the perspective of someone at A, B1 happens before A1, then from the perspective of a spaceship C travelling at some specific (non-ftl) speed and direction A1 will happen before B1. From there, it is easy to construct a scenario where FTL communication between A and C can be used as time travel. The tricky thing to accept here is that on a fundamental level neither the order A sees not the one C sees is wrong, this is not a case of our observations not matching reality. There is no "real" order between two events that are not in each others light cones, that is simply not how the universe works. This, of course, goes completely against our intuitions about how time and distance are supposed to work, but that is because our brain was designed to not get eaten by tigers, not to think about the deeper nature of the universe. Like I said, this is just how it was explained to me, is it about correct?

Particles in the LHC are also accelerated to 99.999991% the speed of light, earth escape velocity is 0.003% of light speed, that probably reduces costs by a somewhat significant margin as well . The LHC is doing something completely different from putting payloads in orbit and should not be considered one way or another for the plausibility of circular coil guns.

I think the idea is that the payload makes multiple laps in the ring before being shot out. Thus, instead of having a 2000 km long railgun you get a 20 km long railgun that the payload travels through 100 times. It probably still isn't a very good idea, though, for the reasons you mention in the rest of your post.

Oh I don't doubt that, I was just wondering if there is a scientific reason the taste would even be different from regular earth whiskey. Cool!

From that link: Is there any reason this would be different from just storing it in a temperature controlled basement on earth? I'm cool with "here is a cool thing let's see how it works in 0g" style of experiments if that is what they are doing, but if not, I'd want to know how this would effect the aging process. Are the bacteria involved in the process known to be strongly influenced by gravity?

Define "the rest of the world". Because I very much doubt the average person is going to hear about the exact causes of the failure, all that will stick is "a rocket made by spaceX exploded". I do agree that for people involved in the industry this will move some of the blame away from SpaceX, which is probably very relevant for them.

If you don't just mean because the rest of the earth is between you and the most prominent source of light for lightyears, I'll let minutephysics explain: .

I think Rosetta is in orbit around the asteroid where Philea has landed, so orbiting around an asteroid isn't out of the question, though obviously Rosetta is a long way from a full blown space station.

I know its not exactly the biggest problem facing that suggestion, but it looks like the astronauts in the first reentry non-craft are supposed to manually aim the retrorockets. If so, that seems.... challenging when you are zipped up inside a bag without any indication of which way is retrograde and without any way of controlling your rotation other than flailing about.

Cool idea, I hadn't heard/thought of putting space elevators on the place you want to go, rather than on earth. I assume it being in a fixed location isn't that big of a deal since a project like this is basically only ever going to pay off if we have a functional permanently occupied lunar colony. In which case that is very likely where you want to go.

Do modern spacecraft contain a significant amount of iron/steel and, if so, would it be possible to make a spacecraft without any?

How about Norse mythology? Bifrost I, Jurmungandr I, Ratatoskr I... (You could drop the final r if it sounds to foreign).