mistermack

I seem to remember reading recently that superconductors have advanced hugely recently, and no longer need the extremes of cold that they originally required. There is even serious talk of achieving superconduction at temperatures in the region of room temperature. I can't give a link for that, but it was very recently. Just checked on Wikipedia, and they have a whole page on high temperature superconductors. "HTS have been observed with transition temperatures as high as 138 K (−135 °C), and can be cooled to superconductivity using liquid nitrogen.[2] Until 2008, only certain compounds of copper and oxygen (so-called "cuprates") were believed to have HTS properties, and the term high-temperature superconductor was used interchangeably with cuprate superconductor for compounds such as bismuth strontium calcium copper oxide (BSCCO) and yttrium barium copper oxide (YBCO). Several iron-based compounds (the iron pnictides) are now known to be superconducting at high temperatures.[5][6][7]"; The latest discovery is high pressure H2S, which can operate at about 200K. https://en.wikipedia.org/wiki/High-temperature_superconductivity So there is a real prospect of superconductors being usable on the Moon with just an effective sun shade. Especially, given the length of time that will have passed, by the time serious use is being made of the Moon. It would be surprising if no more progress had been made by then in the HTS field.

That's true, but machinery would all be electrical, not generating huge amounts of heat. They coped with the problem 45 years ago, so it shouldn't be too much of a problem now. Astronauts produce heat, but the suits coped with that, and the direct sunlight. Cooling by radiation will be much faster with no clouds or atmosphere. Even at mid Lunar Day, a radiant surface that is shaded would shed heat very fast.

Horizon is on BBC at the moment, about telescopes. The James Webb space telescope has a plastic shade, to effectively block out the Sun's rays. It's thin plastic sheeting, about the thickness of a human hair. About the size of a tennis court, and several layers, with a gap between each layer of a few inches. This is designed to keep the temperature of the telescope steady at minus 220c. so that it can pick up infra-red from the universe without swamping it with it's own infra-red. Going by that, preventing things from overheating on the Moon would be pretty easy.

The picture I have, is that when an object gets close to c, it's particles become more and more LIKE light. And to reach c, they would have to become light. Light travels in a straight line. The energy in particles is in motion in all sorts of directions. As the particle travels faster and faster, more and more of it's internal motion becomes straight-line in the direction that it's moving. To travel at c, all of it's motion would have to be in the straight line direction of travel, and it would then be light. But nothing exists that can push it to that speed, as anything that has mass is going slower than c.

I think the two moons of Mars could in the long run be more use than Mars itself. After all, what we are short of in space is raw material that doesn't have a huge price attached to it, in getting the stuff off a planet, or the Moon. And Phobos and Deimos have vast quantities of it. It may be limited in it's quality, but it's gigantic in it's quantity. There may be smaller moons going round Mars too, that could be of use. If the stuff of these moons can be used to build a space station on a truly useful scale, that can supply artificial gravity and grow food, and be effectively shielded from radiation, then people living permanently in space could become a reality. Which could actually be an insurance against natural disasters on Earth.

I think, from looking at what B sees, I can now get an idea of what happens in the instance where B's clock reading is transferred to a third twin C. If B was physically transferred with his clock reading, he would experience infinite acceleration for zero time. So his original observation of seeing A's clock whizzing round fast through about nineteen years, would happen in an instant, in this case. He would see A's clock jump 19 years. So that's where the instantaneous jump in time comes from. You are simulating an event that can't physically happen, so an instantaneous jump in time is not inconsistent with that.

Janus, I think your analysis makes sense, and the acceleration phase is where the time difference becomes actual. It's the claim that the acceleration isn't responsible for the time difference that seems illogical. If you work out what twin B sees if his telescope is trained on A for the whole trip, ( and assuming he's already in motion at the start ), then he sees A's clock running slow over both travelling legs, so the only way that it can end up faster than his over the whole trip, is if it runs extremely fast during the turn-around. If the difference in the two twins' clocks is 2yrs and 20 years at the end, then he needs to see A's clock spin through about 18 years during the turn-around, because when they are reunited, they will be able to look directly at each other's clocks, and A's will read 20 and B's will read 2. Presumably, the reason that B will see such a dramatic speed-up on A's clock, is because he's in the equivalent of a gravity well, due to the large acceleration in the direction of A, which slows down his clock, making A's appear to him to be spinning very fast.

Strange, Tim and Janus, I'm sure you're right. I posted that about the Supernovas without much thought, as I wasn't really trying to make a point about simultaneity, but about deducing things from past events. So, that post was full of crap. But I'll try to make the point about deducing things about the past, from current evidence, in a hopefully better way. Most people accept that the big bang happened. People look at the universe today, and come to that conclusion. It's the same methodology for practically all of science. You look at today's evidence, and deduce the past. And that's what I was trying to say about the twins in the twin paradox. If at the end of the trip, twin B's clock reads two years, and he is clearly a two-year-old, while twin A's clock reads twenty years, and he is clearly a twenty-year-old, then you can safely say that as some point in the process, twin A's clock was ACTUALLY going faster than that of B, and he was ACTUALLY ageing faster than B. So at that point, even though they were not undergoing acceleration, their cases were not interchangeable.

Batteries could be chipped, so that the dealer can read off how much power has been drawn from it, and charge accordingly.

It's not all bad news for engineering on the Moon though. Firstly, if you want to shade something, you can use incredibly thin and light materials, because of the lack of weather, and the low gravity. So long as your shade was erected a short distance from what it's shading, overheating would be no problem at all. The daytime temperatures max at about 125 c on the Moon. That's for a surface that's not very reflective at all. A highly reflective material should stay reasonably cool. I don't remember the US flag melting on the stick. Also, the Sun moves very slowly in the Sky on the Moon, so you wouldn't have to be constantly moving any shades that you erect.

I don't think high temperatures would be such a problem. The lack of atmosphere also means that the temperature in the shade is very cold, so if you don't want things to get hot, you can just shade them with a thin reflective material. The dust would be a problem, but not insurmountable. I started a thread the other day about new filters being made from graphene oxide, that are so precise, they can filter out salt ions from seawater. They are also looking good for all sorts of ultra-fine filtration, including separating gases by filtering out the bigger molecules. So ultra-fine dust would be manageable with modern filtration methods. But I'm sure that there will be loads of other unforeseen problems lurking. Wikipedia now has a page on colonisation of the Moon. They say that the Russians and Japanese are planning to have Moon bases by 2030, and the US is talking about something similar. I doubt that any of it is realistic, but it will come eventually. https://en.wikipedia.org/wiki/Colonization_of_the_Moon

Except that the evidence of meeting the two twins, one aged twenty, the other aged two, would prove that one's clock "really" ran faster than the other. One is holding a clock reading twenty years, the other is holding a clock reading two years. I think the difference in the two twins would also be pretty obvious. I don't know what more proof you would need than that. If the proof of clocks "really" running faster and slower than each other in the past is so strong and obvious, I can't see the problem with accepting that it can be happening in the present.

This latest episode in the Graphene story is looking very promising. I seem to remember Lockheed Martin claiming something similar a few years ago, but these people seem to have cracked the desalination bit of the puzzle, and it could make a huge difference to the energy and running costs of desalinating water : http://www.manchester.ac.uk/discover/news/graphene-sieve-turns-seawater-into-drinking-water/

Maybe it would be possible to design an installation on the moon that replicated 1g of gravity, but it would be a huge undertaking. It's the minimum diameter of 200m that's the problem. Making machines that big on the Moon is probably a thousand years away. One advantage of the cold on the Moon might be the ability to use superconductors as bearings, I only have a vague notion of how that stuff works without looking it up. But I know that they generally work much better in cold temperatures. So maybe some sort of huge 250m plus rotating station could be made and run without much maintenance or energy input.

pz and Janus, I would prefer it if someone pointed out what is actually wrong with what I said, rather than posting the diagrams etc. As far as simultaneity goes, Wikipedia says : "one must understand that in special relativity there is no concept of absolute present." I'm fully prepared to accept that there is no way to work with an absolute present, that doesn't mean that it doesn't exist. In fact, if the past doesn't exist, and the future doesn't exist, then the absolute present is all that exists. Of course special relativity doesn't work with it, but that's not the same as "it doesn't exist". If I look at two supernovas, both 1000 light years from me, in opposite directions, I can say with confidence that they were happening simultaneously 1000 years ago. Even though they couldn't see each other or interact.

The point I'm making isn't about how the clocks appear. It's about how they ran. If the moving twin returns and is one tenth as old as his stationary twin, then his clock was at some point running slower. It could be on the outward trip, the inward trip, or both. If you just focus on the interval in which his clock is running slower than his twin's, then he is moving at a steady rate relative to his twin. That's ALL that's happening. Two twins, separated by distance, moving relative to each other. And one's clock is going faster, the other is going slower. In actual terms, as proven by the different ages when they meet up.

My problem with that is that if you just take the case of the outward leg, then you have symmetry. Does the outward clock run slower? And if you just take the case of the return leg, then you have symmetry. Does the return clock run slower?

Maybe, by the time we have rotating space stations with artificial gravity, we won't need people on the Moon. The machinery will be so advanced, you will be able to run it remotely from a desk on the space station, removing the need for shuttling people back and forth almost altogether. They might just have remotely operated mining machines, that are constantly digging out what we need, and sending the raw materials into orbit via the rail gun. It might take a couple of hundred years, but it will be happening in some form. If we don't all kill each other off in the meantime.

Janus, try as I might, I can't follow your method of argument. (I'm not claiming that anything's wrong with it, I just find it hard to follow) I know that in SR, nothing can be regarded as simultaneous, unless it's in the same place at the same velocity. That doesn't mean that things AREN'T happening simultaneously. It just means we can't observe it at the time. But looking at the past, it's obvious that things WERE happening simultaneously. It's a similar case with these twins. When they get back together, and one is ten times older than the other, it's obvious that one's clock has been running slower than the other's. You can't end up younger, unless your clock was running slower. You could question on which leg of the trip that happened. But there's no question that on one, or both legs, clocks were actually running slower for the travelling twin. Not just observed to be slower, but actually slower. For the duration of that leg, neither twin was undergoing acceleration. Why are the two twins's experiences not interchangeable?

The thing with artificial gravity is that the station has to be more than 200m across, to avoid sickness due to the rotation of the balance canals in our ears. Preferably a good bit more than 200m. That would be easier to set up in zero gravity, maybe initially just two pods connected by a cable and rotating round each other. It might be an expensive thing to arrange, I agree, but still far cheaper than a return trip to Earth, to spend time in 1g. Something like a railgun might be essential to the whole enterprise. You could shoot fuel containers into orbit, eliminating the need to carry up the fuel for the descent. What would be ideal would be to divert a comet into lunar orbit. Or blast off a huge chunk of one, and send it into orbit around the Moon. You could make all the fuel you would ever need from one of those.

That was this : And this :

I think the same applies. As they are just going from the Moons surface to an orbiting space station, they would need the minimum vehicle necessary for that, just a pared down shuttle vehicle. It would be smaller and lighter than the original lunar module, so would require a lot less in the way of rocket power. It could be a lot more dangerous though, in the case of a rocket failure. There would be no chance of using a parachute. The ascent would need to be pretty much 100% reliable. But I guess that's the same with most rockets, really.

Even though I like the explanations that I linked to in the OP, from Einstein-online, I'm beginning to have problems with them, having read them a few times. Particularly, the second, The spotlight topic Twins on the road The essence of that is that the acceleration changes everything, and after the acceleration, the inertial motion of the second twin is somehow different, for the rest of the trip. (as in the case of the hiker striking off at an angle). With the hiker, you can clearly see what the difference is. He is straying into another of the spatial dimensions, now travelling in two dimensions, instead of one straight line. So what is the equivalent difference in the case of the twins paradox? What clear difference is there in the second twin, apart from the speed difference? Two non-accelerating twins with just a speed difference, we are told, have exchangeable situations. Each experiences time slowed for the other, and both are right. But according to this, if the speed difference is the result of acceleration, then all bets are off. One twin is ACTUALLY ageing faster than the other, and the two situations are no longer symmetrical. So when they meet up, one is substantially older than the other. I can't see that this is any different, from two twins who HAVEN'T undergone acceleration, but are just in motion relative to each other. For instance, twin B leaves twin A, being accelerated to speed S. What if that acceleration takes him right alongside a third twin, C, who was already travelling at a constant speed S ? You then have two twins, travelling side-by-side at the same speed. Their clocks MUST run at the same speed. So according to that, a clock that has been accelerated, runs no differently to a similar clock that is just in constant inertial motion. If you are to claim that twin B is ACTUALLY ageing slower than A, then that also applies to twin C. So the consequence is that for two twins A and C, in constant motion relative to each other, one is ACTUALLY aging faster than the other.

Yes, but even going by your own post, it's obvious that the problem is a small one. You can use a railgun to get MATERIALS into orbit, just leaving people and very delicate objects to be lifted off with very small rockets. Given the improvement in rockets and lightweight materials, since 1969, that's really not going to be a problem. Or, you could probably use a combination of the rail-gun at lower power, with rockets taking over at the end, to put people into orbit with very little rocket power needed.

A recent thread touched on the subject, but not in a satisfactory manner and the thread got locked. I think that the twin paradox goes right to the heart of SR, so should have it's own thread. I hope this one won't get hijacked with irrelevant stuff. As someone who has struggled to reconcile the case of travelling twins and it's apparent paradox with the interchangeability of inertial frames in Special Relativity, I want to share my progress with others who might want a clearer picture. I just found two pages on the net that give a brilliantly clear explanation for the outline of the paradox, and the explanation for some of it, and I recommend them to anyone: Just click on the links : The spotlight topic The case of the travelling twins. The spotlight topic Twins on the road They are pages from Einstein-online, and I recommend them as thoroughly clear and readable. What I haven't found there yet, is an analysis of the case where a third twin is introduced, apparently removing the role that acceleration is playing. I'll keep looking at that.