NowThatWeKnow Posted February 25, 2009 Posted February 25, 2009 But seriously, why did people invent ether, what good did it ever do?... Einstein considered it more then once and your friend Frank Wilczek will probably define it soon. From a layman's point of view it would solve many problems. For me it is hard to grasp the uniformity of the expansion of the universe without an ether. Or, you can only break the speed of light with the help of space (metric of nothing) expanding.
Mr Skeptic Posted February 25, 2009 Posted February 25, 2009 Mathematicians don't need mass/energy to study geometry, and geometry has properties independent of any matter/points/whatever in it. The force of gravity that keeps your ass on your chair is a property of bent spacetime (according to general relativity). Does that count as influencing time/matter? How about the quantum effects of confining a particle?
north Posted February 25, 2009 Posted February 25, 2009 Mathematicians don't need mass/energy to study geometry, and geometry has properties independent of any matter/points/whatever in it. really how so ? take away any matter/points/ whatever in it ? what then is geometry based on ? nothing but theory The force of gravity that keeps your ass on your chair is a property of bent spacetime (according to general relativity). Does that count as influencing time/matter? yet gravity is yet to be defined and yet neither space or time has any proven properties associated with them , do they ? they don't How about the quantum effects of confining a particle? and how is this confinement achieved ?
Norman Albers Posted February 26, 2009 Posted February 26, 2009 I just finished reading "Faster Than the Speed of Light" by Joao Magueijo. In the last chapter he really rocks. He mentions that 'cosmic ray' alpha particles coming to us at huge energies, magnitudes larger than E15 ev, say, see in their Lorentz-shifted frame, the photons we know as the CMB. The ones coming at them are shifted up in energy to where they start tearing away at the alpha's particle constituents. Thus there should not be such energies as we do observe. Do others see this as a challenge to relativity, or maybe better to say an avenue to understanding the vacuum and quantum gravitation?
scalbers Posted February 28, 2009 Posted February 28, 2009 Perhaps these cosmic rays are more local than we think. If they just come from within this galaxy they might be less affected by CMB photons (and also intergalactic magnetic fields)?
jamey2k9 Posted February 28, 2009 Posted February 28, 2009 I've been studying hubbles law red shift e.c.t but I still carn't see how this proves the universe is expanding
Norman Albers Posted February 28, 2009 Posted February 28, 2009 Wow, good thinking scalbers. Do we think these high energy protons or alphas come from supernovae which occur everywhere?
NowThatWeKnow Posted February 28, 2009 Posted February 28, 2009 I've been studying hubbles law red shift e.c.t but I still carn't see how this proves the universe is expanding The "red shift" (or blue shift) comes from the Doppler effect created by an object moving towards us or away from us. The Doppler concept is also used in police radar and has been proven very accurate. Supernovae are also used to determine the expansion of space. The initial bright phase of a supernova exploding close by may last two weeks. We may see a distant supernova that has an initial bright phase of three weeks because of the expansion of space while the light traveled towards us. Either one will show that space is expanding. Corrections or additions welcome.
Martin Posted February 28, 2009 Posted February 28, 2009 (edited) The "red shift" (or blue shift) comes from the Doppler effect created by an object moving towards us or away from us. The Doppler concept is also used in police radar and has been proven very accurate.... One of the first things taught in an intro cosmo course (e.g. Eric Linder's at UC berkeley, who also has a cosmo textbook) is not to think of cosmological redshift as a Doppler effect related to the recession rate (whether at the time of emission or reception). Because then you get completely screwed up. The formulas don't work out, except as approximation in the case of nearby objects. But I see what you are trying to do, which is good. A Doppler explanation, even though not quite right, might appear to be the only handle or lifepreserver you can throw to Jamey that he is prepared at this stage to grab onto. ========================= A lot of your posts are really good, and I like how fast you picked up on Ned Wright's calculator. I'll try to help you get a better grasp of redshift and figure out something to say in cases like this... It's not a trivial problem. Merged post follows: Consecutive posts mergedI've been studying hubbles law red shift e.c.t but I still carn't see how this proves the universe is expanding You already got a good response from NowThat. I'll add something. There are objects in our galaxy and even in other nearby galaxies where we can independently estimate their speeds of motion towards us and away from us and these correspond very accurately to Doppler shifts in the spectra of their light. Known wavelengths that can be measured in the lab are shifted by amounts corresponding precisely to speed. Those are Doppler shifts. Doppler shifts and cosmological redshifts are different. The redshifts described in Hubble law are called cosmological redshifts. They occur over long periods of time while the light is traveling towards us----hundreds of millions of years. The shifts can be determined very accurately because we can recognize spectral lines from chemical elements like hydrogen, sodium, iron, etc that we know the wavelengths of. Recognizable patterns of lines. The shifts actually correspond to the whole history of expansion during the light's travel. They show by what factor distances have increased in the universe while the light was in transit. =============================== The idea that distances are expanding is inherent in our theory of gravity (General Relativity) as one of two possibilities for a universe like ours with matter distributed fairly uniformly throughout. It was already predicted in 1922 or 1923, by Friedmann, before there were any redshift observations. You ask what proves expansion. What proves expansion (even before Hubble observed anything) is the tests of GR. GR predicts all kinds of gravity effects out to 5 or 6 decimal places, and better than Newtons old theory of gravity. We don't have any more accurate theory of orbits or lightbending or time differences or how GPS satellites work or any of that stuff. GR was invented in 1915 and it has been tested for over 90 years and people have been trying to come up with alternatives for most of that time and the alternatives have been mowed down. For tests on earth, in orbit, in the solar system, in our galaxy, and whereever we can measure, GR rules. And it says that there are limited possibilities for our kind of universe, it has to be contracting, or if it is not contracting then it has to be expanding. GR fits the real world out to six decimal places. That, as I see it, is at the heart. That is really what proves the universe is expanding. Hubble just found some supporting detail. Even if we hadnt invented how to measure redshift, we would still surmise that our universe was either expanding or contracting just because Einstein's model of gravity works so well. Hubble ruled out the contraction alternative, and he measured the expansion rate. But what really proves it is the 90 years of data that confirm GR. That's my perspective on it, others may want to give a different account. Different people see the idea of proof differently. Edited February 28, 2009 by Martin Consecutive posts merged.
Norman Albers Posted February 28, 2009 Posted February 28, 2009 I wonder about interpreting the term pressure which is not specifically a characteristic of galactic distributions. For there to exist much energy in 'pressure' interactions, there must be kinetic entities bouncing off one another. It seems to me that masses manifest negative pressure here, by virtue of their gravitational dance. Do we need to put this into our GR equations or is this what we have dealt with in the construction where overall mass-energy density if uniform? I realize we deal with pressure in an earlier, high-density regime, but I am trying to pin it down. I suspect this means down, like to Planck regime.
NowThatWeKnow Posted March 1, 2009 Posted March 1, 2009 I'll try to help you get a better grasp of redshift and figure out something to say in cases like this...It's not a trivial problem. I was afraid there was going to be a problem and that is why I added "Corrections or additions welcome." I am slow to learn sometimes. Moderator - Is there a reason why I can only edit a post for a short period of time after posting before the "edit button" disappears?
iNow Posted March 1, 2009 Posted March 1, 2009 Moderator - Is there a reason why I can only edit a post for a short period of time after posting before the "edit button" disappears? You have ~6hrs. It's disabled after that b/c some people had a horrible tendency of going back and changing what they wrote AFTER others had responded. Sometimes this was done to cheat and "win" an argument by pretending they'd already covered something about the response. Other times, they would flat out remove what they'd said, which made threads nearly incomprehensible. Either way, just do it within 6 hours, or make another response before someone else responds so you can made edits after the merge happens. Alternatively, PM the changes you want made to a Mod, as they can made edits indefinitely. More here: http://www.scienceforums.net/forum/showthread.php?t=35666
NowThatWeKnow Posted March 1, 2009 Posted March 1, 2009 You have ~6hrs. Thanks for the reply. I can see where it could be a problem sometimes. Maybe extended editing should allow adding text only and in a different color.
Norman Albers Posted March 1, 2009 Posted March 1, 2009 I know the math and am still working to let go of a "smithereens" picture. I also want to know what it means that we abandon differential covariance when we write the Robertson-Walker metric.
cameron marical Posted March 11, 2009 Author Posted March 11, 2009 martin- hey whats up. so, i read about this gr character. is it a person who did tests? im curios. i tried to look them up, but found nothing. what kind of tests did he/she do?
Norman Albers Posted March 11, 2009 Posted March 11, 2009 martin- hey whats up. so, i read about this gr character. is it a person who did tests? im curios. i tried to look them up, but found nothing. what kind of tests did he/she do? Einstein was only convinced he had really accomplished something when he figured the perihelion shift of the planet Mercury. It's orbit is an oval, and this slowly moves around in the large sense. Later, shifts of light sources passing behind other massive objects showed bending of light-paths. This is predicted in GR.
cameron marical Posted March 11, 2009 Author Posted March 11, 2009 oh. now i feel stupid....GR stands for General Relativity. whoops.
iNow Posted March 11, 2009 Posted March 11, 2009 It happens. After all, we each had to learn the acronym at some time, and we all didn't know what it meant at any time in our lives prior to that moment.
lakmilis Posted March 28, 2009 Posted March 28, 2009 NTWK, I'd warn against taking History channel literally! All the comment I heard about their "The Universe" program was that it was embarrassingly inaccurate. Didn't watch it myself. As a general rule it's a good idea to be mistrustful of pop media and both History and Discovery channels in particular. Not reliable authority to be quoting here at SFN. I just downloaded some THC documentaries in cluding some on space... more amateur , crap documentaries I have never seen. Stay well clear of that channel!!! (don't know how it is on archaelogy and the like though but certainly is C R A P on anything in physics).
NowThatWeKnow Posted March 28, 2009 Posted March 28, 2009 I just downloaded some THC documentaries in cluding some on space... more amateur , crap documentaries I have never seen. Stay well clear of that channel!!! (don't know how it is on archaelogy and the like though but certainly is C R A P on anything in physics). I think calling THC "C R A P" ("The Universe" program in particular) is a little unfair. It was not designed to meet the physics experts expectations for continued education. It is for the amateur and attempts to convert the math into words so normal people can understand and enjoy cosmology and relativity to some degree. Just like her in SFN, the History channel "The Universe" program presents facts and speculation and I have no trouble separating them considering their presentation. I would like you or anyone else to point out something presented as fact during "The Universe" program that was not in the ball park. I think it does well in getting people interested so they can continue their education using different avenues if they choose. You can't call kindergarten "C R A P" just because it is below your level. Accept it for what it is. Those are my thoughts.
Airbrush Posted March 28, 2009 Posted March 28, 2009 I am here because of the History Channel "The Universe". NowThat lead me here. Thank you NowThat. If there are inaccuracies then I wish people would discuss them in detail so THC will know what things were not accurate. The cgi was fantastic on "The Universe" and if the program could be corrected, and thus improved, then that would be stupendous. I don't remember hearing anything inaccurate, but I don't have much science background. My degrees are in art and accounting. Hahaha. I found inaccuracies on wikipedia and reported them here but I should have reported them to wiki.
lakmilis Posted March 29, 2009 Posted March 29, 2009 I'm sorry... it was so bad I kept falling asleep and never really could give you some detailed analysis of it... deleted it pretty fast! Didn't know this thread was based on a particular program viewing? Anyway.. Martin, I was about to start a thread when I saw your post about the cosmological expansion (or cosmological redshift etc.). (And hey, these questions are laymen , not technical as such). I am thinking.. we see light from for example andromeda.. no strike that for simplicity lets say a star in our galaxy.. 10 000 ly away. (and let us just say it is moving away from us, the principle is important, not the actual dynamics of the galaxy.. we could use a redshift galaxy as an example). Anyway.. light reaches us now, and so when we say.. hey this thing is 10 000 years away.. is that AFTER taking into account it was perhaps 7000 ly away when the light started reaching us or was it 10 000 at the time the light was emitted and so now would actuall be, lets say 11000 light years away?
NowThatWeKnow Posted March 29, 2009 Posted March 29, 2009 I'm sorry... it was so bad I kept falling asleep and never really could give you some detailed analysis of it... deleted it pretty fast! Didn't know this thread was based on a particular program viewing? I will agree that a few episodes were lame but some were pretty good. My least favorite was "Sex in space" and "Parallel universes" was pure speculation. The Light speed episode along with a few others were pretty good IMO Anyway.. Martin, I was about to start a thread when I saw your post about the cosmological expansion (or cosmological redshift etc.). (And hey, these questions are laymen , not technical as such). I am thinking.. we see light from for example andromeda.. no strike that for simplicity lets say a star in our galaxy.. 10 000 ly away. (and let us just say it is moving away from us, the principle is important, not the actual dynamics of the galaxy.. we could use a redshift galaxy as an example). Anyway.. light reaches us now, and so when we say.. hey this thing is 10 000 years away.. is that AFTER taking into account it was perhaps 7000 ly away when the light started reaching us or was it 10 000 at the time the light was emitted and so now would actuall be, lets say 11000 light years away? The Ned Wright calculator can answer your questions but it deals better with billions of light years, not thousands. http://www.astro.ucla.edu/~wright/DlttCalc.html The numbers you are talking about are still in the Milky Way and the cosmological expansion does not apply.
Martin Posted March 30, 2009 Posted March 30, 2009 I tried to say thanks with the button but it said spread some around first. So I'll just say thanks, NowThat, in traditional buttonless fashion. Lakmilis, probably the most common distance measure in cosmology is the "comoving distance" which is the same as the "proper" or actual physical distrance if it could be measured today. (By a pre-arranged sequence of observers with radar devices stretched out between here and there all measuring simultaneously according to the universal timescale.) The Friedman equation model basic to all cosmology, and the Hubble Law are both stated in terms of that distance and depend on the same criterion of rest and simultaneity. Cosmology is slightly different from GR in having those features, it is a specialized and simplified form of GR. So the usual measure is the distance to the object now. It is not the light travel time or any of the other various distrance scales, iike luminosity and angular size, that occasionally get into the act. NowThat suggested one of Ned Wright's calculators. You might also take a look at another one of his calculators (if you haven't already): http://www.astro.ucla.edu/~wright/CosmoCalc.html For some reason I would have guessed that you were thoroughly familiar with Ned Wright's cosmology tutorial website. But just in case you are not, please do check it out.
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