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#360 by Stormbringer on 19 Aug, 2014 01:38
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shamelessly stolen and stored for future use elsewhere. 
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#361 by frobnicat on 19 Aug, 2014 02:07
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A frame of reference is relative. Just a comparison to another system. It is not a physical thing. It is a physical system, be it thermodynamic, relativistic, quantum, non/inertial, whatever. A lot of folks just assume that when frames of reference are talked about, that it must be inertial.
Not necessarily inertial, but beware that inertial or not can be told locally without any outside reference, a simple accelerometer would suffice. So inertial frames are special. In an absolute sense.Quote.../... Thankfully because acceleration is relative, an observer outside of your frame of reference will see your virtual particles become real. That doesn't help us either, but the methodology is key. As you know, frames of reference are relative.
Inertial frames are relative, as proper time/distance transforms can translate observables of one frame into observables in the other. Well, I guess taking acceleration into account that is also possible for accelerated frames (introducing Coriolis "forces" for instance).QuoteSo I flipped it. How about this: We live in an accelerating universe. Our universe is accelerating at rates which we cannot ever hope to achieve. Furthermore, the rate of expansion is a curve, meaning galaxies further away are flying apart faster than the ones flying apart closer to us. Neat. So from the point of view of the universe, which is the accelerating frame of reference, our emdrive is accelerating slowly, compared to say the CMB frame of reference.
It's not clear what time derivative you are talking about : the rate of expansion (Hubble "constant") that goes as ~2.3e-12(m/s)/m (hey, it has dimension of a frequency !), or the change of this rate, which right now seems to be decelerating ( for a given distance you will see slower receding objects crossing this distance in the near future ) in spite of the mysterious acceleration of expansion (expansion not slowing as fast as it should, but still slowing from usual mass content gravity) ?
More confusing : the universe isn't locally accelerating, makes no sense, its local content has a local average speed that can be used as a local inertial frame of reference, but it is not accelerating in any absolute sense, there is nowhere special toward where it could accelerate ! The CMB is part of the content, take or give a few 100s km/s it's the same local inertial frame as the stars and gas averaged on a decently sized local patch, it goes with the flow, and this flow is 0 relative to receding neighbouring patches all around. Local universe is not accelerating if seen at coarse graining bigger than anisotropies. What particular direction of the sky the CMB is supposed to be accelerating (I'm not speaking of our galaxy relative velocity with CMB) ? And even if it were, it would just be free falling with us (say, with the local cluster) at the same acceleration toward a big lump in the neighborhood. Anyway it's not accelerating relative to us and relativity makes no difference between free-falling and inertial velocity in nothingness.QuoteSo lets imagine ourselves on a spaceship in Earth orbit and you are equipped with our brand new emdrive technology. From your frame of reference you are barely moving compared to the accelerating speed of universal expansion happening around you.
I'm barely moving relative to the speed of a specific cluster 1Bly away but in the walls of my ship, or even when considering the whole galaxy, the Universe could be static rather than expending wouldn't change a thing for me, how could a phenomenon occurring over cosmological times/distances could make any difference locally ?QuoteSo imagine yourself inside your space ship and you're looking intently at your prototype emdrive looking for real particles to appear out of the qv fluctuations, you don't see them. Now imagine the universe is the observer. The universe observing your emdrive would see some virtual particles from the qv becoming real. This only works if the universe is expanding.
What part of the universe is observing, my local patch average ? A co-orbiting inertial frame ? Earth ? Sun ? Galaxy ? None of those scales make any care of the fact that universe is expending, or that the rate of expansion varies a bit in a long while (sorry, can't find precise numbers here, but next to impossible to measure inside a lab, so making no difference for a ship).
QuoteSo with this mechanism, you get some arbitrary flux of real particles from the qv frame of reference (I'm assuming the qv and its randomly produced particle pairs are not inertially related to our inertial frame of reference) popping into your local frame of reference (as observed from the point of view of the universe). So I'm picturing the pair production happening randomly, becoming real, and immediately flying away at a rate matching the speed and direction of the expansion of the universe minus the influence of gravity, isotropically. My head hurts.
I believe so. Trying hard and seriously to get your point but "matching the speed and direction" in one hand and "isotropically" the other seems like an oxymoron. Looks like your are longing for the inflation epoch when
a field of accelerated expansion (second order) was so strong it could everywhere rip apart virtual particles pairs to make them real. Would be compatible with the isotropy. Thing is, when such a field is so strong, reality tends to be not very stable and quickly evolves. We are in a stable epoch, stability of time prevents energy conservation breaking (as would be the case when virtual particles become real, note that the accelerating field is actually decaying, it loses some punch in the process). And stability of space (isotropy) prevents momentum conservation breaking. As per Noether's theorem.Quote.../... Is gravity keeping the expansion of the universe in check at small scales? .../...
Yes. Expansion observed now is just an inertial collective movement from initial expanding conditions. Gravitationally bound objects are not obliged to abide to Hubble law. The dark energy or lambda parameter on the other hand is supposed to act on metrics by "fabricating" more space between objects (while a relative inertial receding velocity don't really make more space per se), so if its pressure increases it could conceivably first alter then rip apart gravitationally bound objects. Local effects (at galaxy scale) are not expected to occur soon.
QuoteI think the logic above would have us all awash in real particles all the time. Maybe that's where matter comes from, lol.
Some think this is exactly what happened but not all the time, just at the end of the inflation when the inflaton field decayed into usual particles, and thus stopping inflation, see, this process can't last long. It's not stable.
Can't give other opinion for the rest as the Unruh effect is above my head, also I would tend to agree that resonance seems pointless (except maybe for efficiency of the coupling to the RF generator as the system must be analysed as a whole).
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#362 by su27k on 19 Aug, 2014 05:24
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Jeff Foust has half an article on this in Space Review: http://www.thespacereview.com/article/2580/1, including a brief interview with White after his talk in Mars Society conference
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#363 by su27k on 19 Aug, 2014 05:31
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We also under QM fully, too
I think Feynman may disagree:QuoteI think I can safely say that nobody understands quantum mechanics.
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#364 by Star One on 19 Aug, 2014 06:25
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Has there been any new news on the EM/Cannae drive?
http://xkcd.com/1404/
Blimey people don't half like quoting that cartoon that's several times in this thread alone.
Anyway from what was in the report I don't think we'll be hearing any more too either much later this year but more likely next.Quote from: su27kJeff Foust has half an article on this in Space Review: http://www.thespacereview.com/article/2580/1, including a brief interview with White after his talk in Mars Society conference
Thanks for that puts it in a better context. Funnily the reporting of this seemed more accurate at the start but as it became reporting of reporting it seemed to get more outlandish. -
#365 by Mulletron on 19 Aug, 2014 15:03
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Quote
It's not clear what time derivative you are talking about : the rate of expansion (Hubble "constant") that goes as ~2.3e-12(m/s)/m (hey, it has dimension of a frequency !), or the change of this rate, which right now seems to be decelerating ( for a given distance you will see slower receding objects crossing this distance in the near future ) in spite of the mysterious acceleration of expansion (expansion not slowing as fast as it should, but still slowing from usual mass content gravity) ?
More confusing : the universe isn't locally accelerating, makes no sense, its local content has a local average speed that can be used as a local inertial frame of reference, but it is not accelerating in any absolute sense, there is nowhere special toward where it could accelerate ! The CMB is part of the content, take or give a few 100s km/s it's the same local inertial frame as the stars and gas averaged on a decently sized local patch, it goes with the flow, and this flow is 0 relative to receding neighbouring patches all around. Local universe is not accelerating if seen at coarse graining bigger than anisotropies. What particular direction of the sky the CMB is supposed to be accelerating (I'm not speaking of our galaxy relative velocity with CMB) ? And even if it were, it would just be free falling with us (say, with the local cluster) at the same acceleration toward a big lump in the neighborhood. Anyway it's not accelerating relative to us and relativity makes no difference between free-falling and inertial velocity in nothingness.
Hubble's Constant: H0 = 67.15 ± 1.2 (km/s)/Mpc. For every million parsecs of distance from the observer, the rate of expansion increases by about 67 kilometers per second.
The further away from the observer, you'll observe the velocity of galaxies increasing. -
#366 by frobnicat on 19 Aug, 2014 16:20
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Hubble's Constant: H0 = 67.15 ± 1.2 (km/s)/Mpc. For every million parsecs of distance from the observer, the rate of expansion increases by about 67 kilometers per second.
The further away from the observer, you'll observe the velocity of galaxies increasing.
Yes, for every added meter distance you will see an added receding speed of about 2 picometers/second, it would be around 1m/s between Earth and Jupiter, but this is not an intrinsic law of the vacuum or space-time, just an average of inertial expansion from initial conditions and gravitationally bound "objects" like a galaxy don't see this effect within. Dark energy or accelerated expansion on the other hand appears to be intrinsic to space-time, but even if majority of energy content, its effect is very very weak on local scales, now and for the foreseeable future, so weak it is only slowing the slowing of expansion rate due to gravity, not yet accelerating it. Its only strength is it don't dilute like ordinary mass-energy content so it might win at the end. Until then its effect couldn't be detected locally inside a lab, even less used for any purpose.
The further away from the observer, you'll observe the receding velocity of galaxies increasing. Alright. Then taking the mean velocity of the galaxies all around at a given distance, you would see a residual relative velocity due to the velocity of our galaxy relative to average local content (say, CMB dipole, A few 100s km/s) but there is really no reason this velocity in the "neighbourhood frame as a whole" would accelerate. The neighbourhood is expanding but we (as any other content anywhere) are always like at the centre of this expansion, with no acceleration in any particular direction. And even if it did accelerate in a particular direction, what gives ? This would be like a free fall. A collection of free falling objects (not around a very compact object) don't make a distinction with just floating in nothingness.
And what is the relation of those far away galaxies and a ship orbiting Earth ? If we could build tethers out to 100s Mpc to grab onto receding neighbouring galaxies around, then we would surely have a good source of energy, and if dark energy holds that could even be a physically sound unlimited "free energy" source, albeit a little cumbersome. Short of that, what is our physical relation to those receding galaxies beyond a bunch of redshifted photons ?
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#367 by Stormbringer on 19 Aug, 2014 20:26
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according to Wheeler and Feynman and Ernst Mach we are tethered to everything all the way to the ends of the universe
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#368 by JohnFornaro on 20 Aug, 2014 14:42
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Just wanna add that we understand quantum mechanics "quite well" too. Nobody claims complete knowledge of QM or magnetism.
No, we understand exactly what's going on in magnetism. We also under QM fully, too, it's just very counter-intuitive. Just because you don't understand it and just because it takes a lot of effort to understand it doesn't mean it isn't understood.
You confuse general or even detailed understanding with complete understanding. -
#369 by JohnFornaro on 20 Aug, 2014 14:59
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My objection continues to be the careless, unsubstantiated use of the word "likely".
...Some fusion propulsion is likely near term both because there are a couple of fusion projects that lend themselves to propulsion ...
EDIT I need to fix it again XD.
There is one fusion project blatantly stating they are working with NASA on a fusion engine. ...
Unfortunately, you continue to misuse the term "likely". Because "there are a couple of fusion projects that lend themselves to propulsion", and even "one fusion project blatantly stating they are working with NASA on a fusion engine" does not prove anything at all about the liklihood of these efforts succeeding in their experimental work.
I will make this prediction: It is likely that there will be more fusion propulsion research projects in the future, and some of these might get substantial funding. -
#370 by JohnFornaro on 20 Aug, 2014 15:02
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What we do know from innumerable experiments is that in a broad range of contexts QM is phenomenally accurate. ...
This is a true statement. It does not go beyond what has been proven, nor does it suggest implausible warp drives.QuoteSo while we don't understand all QM at the frontiers of its validity and beyond, we do know quite well experimentally where the frontiers are...
Beyond those frontiers be dragons.Quote...indulging themselves in "what if" scenarios of missions at the end of the paper does not serve them in terms of credibility...
This is always done as a precursor to the never ending request for additional funding. The request is not new, innovative, different, illegal, nor at all non-predictable.
I like your post. -
#371 by ChrisWilson68 on 21 Aug, 2014 05:53
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I also want to point out that inanimate interpretations of the universe and evolution are as faith based as any other interpretations.
You obviously don't understand the difference between "faith based" and "evidence based".
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#372 by JohnFornaro on 21 Aug, 2014 13:18
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I also want to point out that inanimate interpretations of the beginnings of the universe and evolution are as faith based as any other interpretations.
[Edit: Inadvertently forgot to include the important word "beginnings" of the universe, as was pointed out below.]
You obviously don't understand the difference between "faith based" and "evidence based".
Thanks for pointing out my inadvertent grammatical sloppiness, accompanied with the the bonus personal aspersion.
CW brought up a larger point, lost to Chris Wilson by my grammatical mistake:I think one has to be careful when calling found physical principles 'laws'. They are not laws in the absolute sense, as in given by 'God', or being the final answer. The only thing we can say about the principles that we found and verified by peer-reviewed experiments up to any given point in time is: To the best of our current knowledge, this is what happens. A very important point to make.
Quote from: jfJust want to add the observations of C.S. Peirce, who has suggested quite some time ago, that the "laws" of physics might be "habits", and that they change over time.
The gravitational constant, it turns out, is not exactly constant, but appears to vary.
The physical principles discovered so far, fairly well understood by a small group of specialists, (not including Feynman) while accurately explaining some events, are not "laws" in some absolute sense.
As CW puts it, To the best of our current knowledge, this is what happens. CW mentioned a common TLA, and I just riffed on that, pointing out that alternate faith based origination myths are not "absolute" either.
I'm guessing that the universe might very well be more Godelian than some might admit, but that's a personal guess of mine.
Every so often I take a goog on CS Pierce's idea that the universe's "laws" may only be "habits", but I haven't found any recent research on the idea.
There's also the either deliberate or poorly informed confusion between complete knowledge, and a sound working knowledge, mentioned above, again, in a n unnecessairly scathing personal tone.
In 1986, at least, it was not known about the relationship of QM and inertia. From the abstract:
http://iopscience.iop.org/0264-9381/3/3/009QuoteA new point of view towards the problem of the relationship between gravitational and quantum phenomena is proposed which is inspired by the fact that the distinction between quantum fluctuations and real statistical fluctuations in the state of a system seems not to be maintained in a variety of phenomena in which quantum and gravitational effects are both important. One solution to this dilemma is that quantum fluctuations are in fact real statistical fluctuations, due to some unknown, but universal, phenomena. At the same time quantum fluctuations have certain special properties which distinguish them from other types of fluctuation phenomena. The two most important of these are that the action of quantum fluctuations is non-dissipative for the special case of systems undergoing inertial motion in the absence of gravitational fields, and that the dispersion constant for quantum fluctuations for a particle is inversely proportional to the inertial mass of the particle. These properties are summarised in a set of principles which, it is proposed, govern the relationship between quantum phenomena, gravitation and inertia.
The author, Lee Smolin, coinvented "double special relativity". (soon to be a flavor at the Baskin-Robbins Institute)
Per the oracle:
http://en.wikipedia.org/wiki/Lee_Smolin#Experimental_tests_of_quantum_gravityQuoteSmolin is among those theorists who have proposed that the effects of quantum gravity can be experimentally probed by searching for modifications in special relativity detected in observations of high energy astrophysical phenomena. These include very high energy cosmic rays and photons and neutrinos from gamma ray bursts. Among Smolins contributions are the coinvention of doubly special relativity (with Joćo Magueijo, independently of work by Giovanni Amelino-Camelia) and of relative locality (with Amelino-Camelia, Laurent Freidel and Jerzy Kowalski-Glikman).
Thinking for a minute again about the notion of "complete" scientific understanding, Smolin is also interested in and has been working on, the foundations of QM.Quote from: the oracleSmolin has worked since the early 1980s on a series of proposals for hidden variables theories, which would be non-local deterministic theories which would give a precise description of individual quantum phenomena. In recent years, he has pioneered two new approaches to the interpretation of quantum mechanics suggested by his work on the reality of time, called the real ensemble formulation and the principle of precedence.
In fact, in 1992, he wrote a book about his idea of "cosmological natural selection", which sounds pretty interesting. See the oracle for more info on that.
Susskind: "I'm not sure why Smolin's idea didn't attract much attention. I actually think it deserved far more than it got."
I feel his pain. A prophet is never appreciated in his own time.
This wiki page is a great education, I think. Know it alls, won't think so, I guess.QuoteSince 2006, he has collaborated with the Brazilian philosopher and Harvard Law School professor Roberto Mangabeira Unger on the issues of the reality of time and the evolution of laws. ...
A book length exposition of Smolin's philosophical views appeared in April 2013...
Sounds like Smolin knows QM "quite well" but not "fully". Sheesh.
I get tired of arguing with amateurs. -
#373 by Mulletron on 21 Aug, 2014 14:30
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I'm seeing a lot about knowing qm quite well vs fully. This really is moot. It is a distraction from the subject at hand which is the emdrive or derivatives. It is the height of hubris to state you fully understand anything. If we fully understood qm or its offshoots, the subject would be closed, further study wouldn't be warranted and, we'd all be Doc Brown.
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#374 by SteveKelsey on 22 Aug, 2014 06:50
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I like the notion, also expounded by Smolin I beleive, that physics is good at describing what happens, but not good at explaining what happens. The great precision with which QM describes events can be mistaken for accuracy in explaining what is going on.
Agreed this is getting away from Emdrive. -
#375 by JohnFornaro on 22 Aug, 2014 13:34
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I'm seeing a lot about knowing qm quite well vs fully. This really is moot. It is a distraction from the subject at hand which is the emdrive or derivatives. It is the height of hubris to state you fully understand anything. If we fully understood qm or its offshoots, the subject would be closed, further study wouldn't be warranted and, we'd all be Doc Brown.
Absolutely. I just get tired of reading the same assertions over and over. EM drive doesn't work as they have explained the theory so far. -
#376 by JohnFornaro on 22 Aug, 2014 13:35
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I like the notion, also expounded by Smolin I beleive, that physics is good at describing what happens, but not good at explaining what happens. The great precision with which QM describes events can be mistaken for accuracy in explaining what is going on.
Agreed this is getting away from Emdrive.
That Smolin page is most excellent. I had heard of him, but yesterday was the first time I read about his work. I would enjoy meeting him, if anybody here cares to set up the meeting. -
#377 by aceshigh on 22 Aug, 2014 18:46
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Hubble's Constant: H0 = 67.15 ± 1.2 (km/s)/Mpc. For every million parsecs of distance from the observer, the rate of expansion increases by about 67 kilometers per second.
The further away from the observer, you'll observe the velocity of galaxies increasing.
a parsec = 3.2 ly
observable universe 12-13 billion light years... let“s consider it 13... 13 billion / 3.2 = 4 billion parsecs.
4 billion / 1 million = 4000
4000 * 67 = 268 thousand kilometers per second.
So at the edge of the universe, it is expanding almost at light speed away from us, right?
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#378 by MP99 on 22 Aug, 2014 23:04
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Hubble's Constant: H0 = 67.15 ± 1.2 (km/s)/Mpc. For every million parsecs of distance from the observer, the rate of expansion increases by about 67 kilometers per second.
The further away from the observer, you'll observe the velocity of galaxies increasing.
a parsec = 3.2 ly
observable universe 12-13 billion light years... let“s consider it 13... 13 billion / 3.2 = 4 billion parsecs.
4 billion / 1 million = 4000
4000 * 67 = 268 thousand kilometers per second.
So at the edge of the universe, it is expanding almost at light speed away from us, right?
Not "almost".
As we look at objects receding from us faster, they have a greater redshift. (This is an over simplification, but it's essentially correct.)
Once you reach the distance where Hubble expansion gives you recession at light speed, any light from there would undergo an infinite redshift, so it never reaches us. This is the definition of the edge of the "observable universe".
So, well spotted.
That edge is basically an event horizon, just like the boundary of a black hole.
BTW, the assumption is that the "observable" part of the universe may be a miniscule fraction of what was created from the big bang, in the same way that the universe outside a black hole is much bigger than the hole itself.
One big difference, though - an observer 10 billion light years away in our universe should also see a similarly-sized visible universe, but in a sphere centred on them. We share observability of lots of space, but they can see things that are beyond our horizon, and they can't see stuff that's distant from us in the opposite direction.
Every observer sees a different universe centred on them.
Cheers, Martin -
#379 by aero on 23 Aug, 2014 02:37
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Oh - So my kid is right. He is the center of the universe. Oh well, I guess we are all the center of our own little, and our own big universe.
shamelessly stolen and stored for future use elsewhere. 