Eelpie

ok another way of putting the same question a different way.... if we had two objects where the expansion of the universe offset the extent to which they were gravitationally attracted to each other how would they behave? would tiny bits of space be created but then be pushed away by gravity so that the distance was unchanged? many thanks!

Thanks very much. I always thought that where two objects were in "equilbrium" wrt to each other gravity was typically predominantly offset by angular momentum? Or is it a combination It seems odd that it should exactly offset inside superclusters.... If we took two points in an "empty" bit of space ie between superclusters and these were 1m apart, then according to your formula and ignoring gravity (I am assuming points not particles with mass) after a billion billionth of a second space should have expanded but by less than a planck length. How is that possible? Or do we have to think in terms of quantum mechanics when thinking about small units of time? Is a billion billionth small?

sorry may have used terms like "local" wrongly...I know they have specific meaning in physics but I am using them as a layman would...also is relativity the right area for this post...I always assumed that general relativity was intricately related to theories about the expansion of the universe...

a naive question probably, obviously the expansion of the universe now in proportional terms is very very slow (would be interested to know how slow, how much further would an object exactly a billion light years away be after a year), but presumably even objects much closer together are moving away from each other and we could in principle observe the expansion of space? if the theoretical expansion between two points is below a planck unit how would it occur exactly?

how do you measure to 20 cm precision? has any one checked the speed of light recently? maybe its changed?

am i being too Bayesian if I think that this is more likely to be an experimental error than relativity is fundamentally wrong?

Apologies for replying twice last time (terrible internet connection and thought my previous post hadn't worked). The Swiss system involves excavating tunnels rather than building say overground plastic tubes. Clearly the former is sensible for a mountainous country like Switzerland but wouldn't tubes laid overground be cheaper in the UK? Are there cheap materials which would withstand the necessary pressures? "If I'm travelling in a tunnel at ~500 km/hr with a very complex system of equipment and infrastructure ensuring my safety, I won't be comforted by the knowledge that my mode of transportation was implemented using "...the cheapest way possible..."" Fair enough!

Thanks everyone for the responses. The C14 experiments I have found only look at DNA...does that mean the non-DNA parts of neurons are constantly being replaced? Could it be argued that although the majority of material in a brain cell is replaced after x numbers of years?

Using current technology probably yes but my challenge was to focus on the cheapest way possible of implementing a solution. Would it theoretically be possible to get the per km cost down to WCM levels or are the materials required inherently so expensive that it will always cost a multiple? New infrastructure can often be cheaper. The WCM was expensive partly because it wasn't new (it was also expensive because of bungling bureaucracy). The TGV line to Metz cost around half as much on a per km basis even though it was completely new. It became the fastest line in France (despite already having the fastest lines in Europe) and ended up nearly twice as fast as the WCM.

I think the authors control for that, also "natural" C-14 is tiny relative to anthropomorphic C-14 of the subjects tested.

Cheers just reading a slightly earlier paper by Spalding et al which makes broadly the same argument using carbon-14 analysis. It seems odd that the 7 year claim has become so ubiquitous when there is strong evidence for it not being true....

I found this: http://askanaturalist.com/do-we-replace-our-cells-every-7-or-10-years/ This is very different to what was said on "start the week" on BBC's Radio 4 this morning by a philosopher (I think) called Julian Baggini who claimed all the body was replaced... Would be interested in comments...

I agree that it probably would be a lot more expensive than the West Coast Mainline but saying it will cost many times does not really tell us much....from a materials perspective I could conceive it costing less...hence the under £30mn per km challenge. With creative thinking I am sure we can develop lower cost solutions, hence why we should look at partial vacuums and lower speeds. I wonder if an X prize would be a sensible way to stimulate research ie a prize for a team that was able to develop a system that would cost below £30 mn per km....we all know that projects led by large organisations particularly governments suffer from cost inflation hence the focus on would be the lowest cost system possible. It is not true to say that because it is a new infrastructure it will necessarily be more expensive. The West Coast Mainline was in part incredibly expensive because it was an upgrade of existing infrastructure. Even at the massively over budget level it did not deliver the orginal specification. £13bn delivered a marginal increase in speeds. By contrast a NEW line to Metz cost below £2.5bn at the exchange rates of the time (so a bit more than half the cost per km of the WCM upgrade) yet has top and average speeds at nearly twice that of the upgraded WCM. If Vacuum-Maglev cost say 6x the price per km of the Metz TGV (a complete guess) then we would be looking at a project that cost 3x the WCM but would be giving far greater value. If it is likely to cost 60x as much then clearly it is a non-starter.

Apologies if I have put this in the wrong forum, couldnt see anywhere in biology where this was quite right... I often hear various permutations of the statement that every atom in your body is different after a period of x years. Is there an authoritative source for this? Secondly, surely there has to be a positive probability that some of the atoms in your body are the same after x years even if it is in acompletely different place...and a very small chance. Thirdly I am sure I read that after a certain age the body does produce any new brain cells, so presumably this means that for the 7 year rule to be true it must be somehow rebuilding the same cell? Thank you!

If the earth was to rotate over say a period of 25 hours what would the impact be on night time and daytime temperatures? Would the relationship be linear, eg a 2 hour increase in rotation time would increase peak day time temperature on average by twice as much as a 1 hour increase?

Ok...I had assumed that you would still get big efficiency improvements you were sub-sonic and it would probably cost a lot lot lot less....and get from the centre of London to Glasgow in less than an hour......if we were really cost focused and innovative, plastic tubes, elevated stations etc, I wonder if it could be done for less than £30mn per mile and hence get it done below the £13bn for the West Coast mainline upgrade?

The "we have an infinite number of multiverses arising from bubble universes" claim was also made by Marcus Chown in the "never ending days of being dead" he also said that this was a COROLLARY of the standard theory. This seems highly dubious to me but would be interested to know if anyone thinks that the argument is sound.

Interesting...would it really have to be below a millibar to be more efficient than aircraft? Few planes fly at an alttiude of 50km? How do you work out the maths exactly?

Thanks very much. I thought that it had to be a over simplification. Googling had turned this up on: ehow.com which still seems to focus on the number of different ways a system can be ordered (to me a mixed mixture can be ordered in far more ways than a separated mixture): Energy and Entropy Mixing oil and water would break hydrogen bonds, which takes up energy and is therefore unfavorable. Moreover, mixing oil and water results in decreased entropy. (Entropy basically measures the number of different ways you can arrange the components of a system to get the same state). As we know from the laws of thermodynamics, nature always tends toward lower energy and higher entropy; the only way we can reverse this trend is by doing work, putting energy into a system--as when you shake a bottle of dressing to mix the oil and vinegar. Ultimately, however, given time the system will slowly revert to the higher-entropy, lower-energy state.Still don't think I fully understand entropy but nice to know that's my previous understanding, which was closer to Zarnaxus' description, is a better "approximation" than the ordering example. Great disappointment, massive budget BBC/Discovery channel production which is long on panoramic shots and uplifting music but doesn't really teach anything to anyone who has read a couple of pop science books on cosmology.

i assume that a full vacuum is prohibitive but has anyone done cost estimates on maglev/partial vac tubes, presumably a bit of gravity assistance for accelerating/breakingwould be helpful and stations could all be elevated?

Not sure I follow.....I got this from wikipedia Is the point that having the oil and vinegar seperated out is much more likely than them being mixed and hence the entropy still increases even though it appears more ordered? Cheers.

Thanks swansont....I see that there is more potential energy in a sand castle than a cone of sand although I doubt very much that a sand castle could collapse into a smooth pile that looks like a cone either, although I guess that wind etc could then erode it into a smooth structure. I can accept that there are probably marginally more ways to order the cone than the castle but it didn't seem like a great example.... the cynic in me wondered if the choice of the example was driven more by wanting to go to a tropical beach than whether it was the best way of demonstrating the concept...anyway my main problem is that it doesn't seem to describe other increases in entropy like salad dressing separating out? Is the best description of entropy in physics really the number of ways that a system can be ordered?

In "wonders of the universe" I was slightly confused by the description of entropy. Prof Cox used the example of a low energy sand castle and a high entropy pile of sand in a cone. Can the grains of sand in the sand castle really only be arranged in that order to create a sand castle? Surely there are many ways the grains could be ordered to create a sand castle; and is this number really massively less than the cone of sand which he claimed was high entropy? I also wondered if Prof Cox's hairstyle was low or high entropy? At first sight it looks like a high entropy system but I would imagine over the course of the day it flattens out....so presumably it is actually intially low entropy? I am also confused by salad dressing again on Cox's ordering example mixed salad dressing is high entropy yet this would imply that once the vinegar and the oil seperated out it would have lower entropy. Looking for a better description please!

Hi, I'm a scientifically illiterate guy (coordinated science GCSE was all that my school offered) who wants to learn more.....