MigL

When most people talk of an expanding universe they imagine the whole and think of the edges moving apart. They should instead consider the expansion as the constituents of the universe moving apart from each other. Whether it is infinite or finite is not important, the galaxies are still moving apart, so it is expanding. But to get back to a previous point, My understanding of Airbrush's 'coconut shell model ' may have been incorrect, but I still think simple connectivity is essential to a model of the universe. I assumed the 'coconut shell model' had a hole in the middle. A torus certainly does and is not simply connected. Other than the loop test failiure, a torus, or any other manifold which is not simply connected, also has preferred directions. In effect movement along the 'orbit' of the donut id different from radial movement. Now of course I cannot provide rigorous proof ( I'll leave the heavy lifting to DrR who has an understanding of differential geometry ). Now, I may be wrong, but preferred directions imply a violation of longitudinal symmetry, which would then imply a violation of the momentum conservation law. This would then mean quantum theory as well as Newtonian laws of motion ( and gravity ) become obsolete ( not sue abour GR ). I don't think we're ready to, or can, take that step.

Haven't you been following? There is no moving edge. Your premise of an edge implies an outside. There is and, cannot be, either ( and even if there was, it could never affect us or be detected, so it's inconsequential ). The universe ( means everything ) is all that we know and can ever know, and it is expanding by the average distance between objects of galactic scale getting larger.

Airbrush, assume we lived in a version of your thick-skinned balloon universe, or 'coconut shell mode'. We now take a string and make a giant loop. We then grab the ends of the loop and pull it tight. In an isotropic universe where all things happen the same throughout the universe, we should be able to pull the loop right down to zero dimension. In your model, however, if the loop goes around the innermost shell ( or inner sphere ) we can only pull the loop down to the dimension of the inner shell. Incidentally this also holds true for a torus and higher order topologies such that the 3-sphere is the only topology which seems to work. And unfortunately I forget what this property, exibited by the loop contraction, is called. Help me out here DrR, is it called simple connectedness, or am I thinking of something else ( math and especially topology aren't my strong suits )

How does one arrive at the predicted vacuum energy which is supposed to be 10^120 times greater than observation ?? And is it the result of the infinities that arise in QFT ?? Damn that P.A.M. Dirac for the mess he made ( Just joking )

Why would you expect anti-matter to behave any differently with respect to electromagnetic phenomena. An atom would consist of anti-protons and anti -neutrons surrounded by a cloud of positrons with eqivalent ( to regular matter ) energy levels. Anti-matter has charge and intrinsic spin reversal and since EM force is not 'handed' ( ie doesn't violate parity ), there is no difference. Anti-matter can also be handled mathematically as regular matter moving backwards through time, such that emission and absorption of a photon, although reversed in sequence, happen exactly the same.

Interesting topic which shows the application of theories to specific subjects. Yes, gravity gravitates, ie gravity will act on gravitational energy. But GR in of itself does not predict gravitons. They would come out of a quantum gravity theory ( in the future ). Gravitons have never been observed, or even their by-products, and may never be. They are postulated. Its unfortunate that we have very differing theories of the fundamental forces, geometric deterministic for gravity nd quantum probabilistic for the others, but it means you have to have some understanding of the 'physical' problem before applying the approriate theory.

I believe your assumption that the two masses gravitate towards their 'previous' locations, and not their 'present' locations is inaccurate or misleading at best. There is no 'present' location, the only true position is the 'previous' position. It isn't just gravity which acts at the speed of light, all information is constrained to travel at this speed, i.e. where you see it ( after the light has travelled to your eyes ) is where it is. You will not see it at a 'present' location which is farther ahead than the 'previous' location to which it gravitates.

I find it funny that in this thread regarding time travel you insist on proof and empirical verification. I.e. no visitors from the future, so time travel is impossible. Yet on the other thread regarding the topology of space/time ( the locked one in relativity forum ) you find it convenient to disregard all the many verifications of curved space/time and GR that have been made since 1916. In that one you base your arguments on ontology, the philosophical study of reality, instead of the proven science. Myself, I have a 'gut' feeling that time travel is impossible, although I cannot prove or disprove it. My only objections to time travel are the possibility of violation of causality, i.e. you cannot have something happen that hasn't been caused yet, and the fact that the dimension of time is not like the other three spatial dimensions in our GR description of it. Maybe GR isn't a complete picture of it yet, since in some circumstances it allows for CTL. Incidentally the term closed timelike loops was coined by Kip Thorne when he started investigating the possibility of travel through wormholes, which when arranged in a certain way do permit time travel. The media thought this very sensational so he changed the term from travelling through time, to closed timelike loops. Notice that the equations permit it, as to wether its actually possible, who knows. ( Relativistic QM equations also permit Tachyons which always travel faster than c and move backwards in time, but do they exist ??? )

I haven't done the math myself, but I seem to remember that if you consider the bending of light around a massive object as due to the rest mass of the photon, you end up with bending which is half of that predicted by GR and curved space/time. Which happens to be half of what was actually observed by Eddington during the eclipse of 1919. So GR is accurate, newtonian gravitational attraction of the photon's rest mass is not !!!

Wasn't calling you names, just having some fun. But again, these GR models of space/time, with 3 spatial dimensions and 1 of time, do make a lot of verifiable predictions, like the bending of light around massive objects, and are certainly more accurate than Newton's model of ( flat , Euclidian ) gravity, which cannot explain the orbit of Mercury. So whether 4-dimensional space exists or not, is academic, but as a model of physical reality , it is pretty darn accurate. And you'd be a fool to argue with that.

Tried to play chess with my cat once. Other than the crap all over the board, same result. Knocked all the pieces over and continuously licked my fingers when I tied to make a move.

The only reason you can call it negative energy is because it has to be paid back to the universe, according to conservation laws, at the end of e period of time dictated by the Heisenberg Uncertainty principle. You are using concepts you don't understand and drawing the wrong conclusions. Energy, wether negative or positive, has a mass ( remember E=MC^2 ??? ). Maybe you would like to give your definition of negative energy and how it differs from positive energy ??? Forget Susskind and re-read ( if you ever did ) Hawking on Hawking radiation. Maybe read it several times to be sure you understand it. Then come back and argue. Incidentally, if you are questioning the very existence of black holes, maybe you can tell us what magical device will keep a 4 solar mass stellar core from collapsing no farther than neutron star density. I know it can't be neutron degeneracy, so pray tell, what exactly stops the collapse ???

You keep picking and choosing statements from people's replies without reading their whole post. I have previously stated that Hawking radiation is due to a black hole's absorption of one half of a virtual particle pair, wether you choose to assign negative energy or positive energy to it. If you accept that Hawking radiation occurrs, you cannot maintain that black holes cannot injest mass because they will evaporate before that mass can cross the event horizon. EVAPORATION IS DUE TO MASS CROSSING THE EVENT HORIZON!!!

You're now spouting garbage as metre long waves have low enough energy to be localised at a much maller dimension according to Heisenberg's uncertainty principle. Do the math, then you can make such statements. A photon's wavelength is not linearly proportional to its size . By the way , the definition of frequency is repititions per second. An infinite wave cannot by definition repeat.

My point, slinkey, is that frequency and wavelength are inversely related , ie a zero frequency is equivalent to an infinite wavelength. When a photon's frqequency becomes zero at the event horizon, its wavelength becomes infinite, and it ceases to exist. Oh, and yes, a wavelength can be greater than its containment, it happens all the time at quantum mechanical levels and leads to what we call tunneling ( Please don't ask me if the infinite wavelength photon can then 'tunnel' out of our universe, because at infinite wavelength THERE IS NO MORE PHOTON ! ). Just a thought, do you think a photon's wavelength is related in any way to its size ( since you say an infinite wavelength means it is wider than the universe and so cannot cross the event horizon ) ???? If so, you really need to check your basic physics.

Check your basic physics, slinkey, a photon's energy is inversely proportional to its wavelength, so when the wavelength becomes infinite, its energy becomes undefined. Alternately, if infinities make you unconfortable, a photon's energy is proportional to its frequency, ie when it becomes zero, the photon has no enrgy. Negative and positive energy are just conventions which make the math more manageable. Read more advanced Hawking, rather than "A Brief History of Time" for a more complete explanation. Always keep the physics of the situation in mind, what exactly would be the properties of 'negative' energy ?? I'm at work, will try to get back later for more discussion.

Alan2here, there are numerous books that describe black hole formation due to stellar collapse. Radiation pressure constantly fights gravitational collapse in large objects like stars. When nuclear reactions no longer supply the outward radiation pressure because the stellar core has become iron, the star, or more specifically the core will collapse. If the star/core is smallish it will collapse until electron degeneracy ( Pauli exclusion principle for occupation of Quantum States ) starts pushing back and halts the collapse. These are known as white dwarf stars. Larger stars won't be stopped by electron degeneracy, but will actually force electrons to combine with protons to form neutrons, until the whole core is mostly neutronium and neutron degeneracy stops the collapse. these are known as neutron stars or pulsars. Even larger stars' cores won't stop at solid neutrons, as a matter of fact we know of no force which can stop the collapse if the core is of a certain size. Even if they blow away most of the mass in a supernova explosion. These are known as black holes.

It is not a question of wether an egg can be unsrambled, That is an entropy question, and yes entropy can decrease locally. In effect there is a small possibility that the egg will unsramble. It is very small because of the statistical mature of macroscpic objects like eggs, but it can be large for non-statistical, sinle microscopic object. Atomic interactions will look perfectly normal wether the film runs foreward or backwards. Causality violation is more subtle. You cannot scramble an egg without first breaking it, beating it, and cooking it, ie the breaking/beating/cooking are the causes that enable the scrambling effect to happen. Causality violation would permit the egg to scramble BEFORE breaking/beating/cooking. Just like in my previous example where the writing appears in the book BEFORE having gone into the past to write it.

Alan2here, your argument/assumptions have more holes than swiss cheese. Slinkey, your argument ultimately reduces to "black holes cannot exist", since they cannot even ingest the original mass/energy that creates them. If you would like to argue THAT point, I've gotta warn you, I have GR on my side, and helping me with my argument will be Einstein, Swartzchild, Oppenheimer, Wheeler, Kerr, Penrose, Thorne and Hawking. Who's on your side ??

Why is it reasonable to asume that space is an empty volume ??? Maybe 'ontology' cannot explain virtual pair creation and all the effects attributed to this phenonenon, but GR/QM can. There is an energy associated with 'empty' space/time. This energy accounts for various effects ( such as the above pair creation and Casimir effect ) and predictions which are still being worked on ( possibly even the origin of the universe and its expansion ). So yes space/time is an entity, and it has many measurable quantities. Because philosophy and ontology do not bother with measurable quantities and experiment doesn't mean space/time is not an 'entity'.

Don't understand your objection to the fact that an infinitely red shifted photon has zero energy, and since energy cannot be destroyed it must end up as added energy ( mass ) to the black hole. Maybe I wasn't too clear in my explanation, or maybe you mis-read. Don't follow your reasoning that "a photon reaches an infinitely long wavelength then can it be said to actually exist?" What does that mean? An existing photon is red-shifted to infinity and CEASES to exist, so the energy/mass must go to the black hole! Also I only used a photon to make the argument more manageable, but feel free to use an astronaut or a spaceship, they will both 'loose' their mass/energy at the event horizon, and it will have to go to the black hole since a black hole is the ultimate information shredder. Incidentally thesame but reverse argument is responsible for evaporation ( Hawking radiation ). If mass/energy 'appears' next to an event horizon due to virtual particles, then even though the black hole absorbs one of the pair, it must repay the debt to the universe and LOOSES mass/energy ( instead of gaining ).

Oh, good greif, owl. I didn't realise there was an ontological interpretation of time as well as space. Just kidding. Most physicists will agree with you. It is always now, everywhere. But will add, thogh not necessarily the same now. Say you have a time machne and a notebook. Next week you will travel two weeks into the past, at which time you will write in the notebook "it worked!". If you open the notebook now, are the words "it worked!" written in it or not ??? What I'm getting at, is could there be a law which we haven't formally expressed yet which forbids this ? Hawking has put foreward a law which forbids time travel ( I forget its proper mame ). It seems that if time travel were possible in the case presented above, causality would be violated, ie. something would happen which hasn't been caused yet. Could there be a law that forbids causality violation ?? HOWEVER! Causality seems to be violated often in Quantum Mechanics. Take an electron beam passing through a single slit. It will give the classic single slit diffraction pattern. Now take the same beam and pass it through two slits and you get the classic two slit diffraction pattern. Now if you reduce the beam to a single electron and pass it through the two slits, obviously it can only pass through one of the slits, yet the same two slit diffraction pattern results. How does the electron know that another slit is open, unless it actually passes through both, then goes back and chooses the path of least action ?

I will try this again. Let's not consider an infalling astronaut, into a black hole. Let's consider a photon of visible light, with a given amount of energy, approaching the event horizon. Now we all agree that as it approaches the event horizon, it approaches a deep gravity well, and so time slows down. Time will eventually slow to zero at the event horizon so some members assume that since time is at a standstill, the photon cannot actually cross the event horizon. This logic is flawed. We don't actually 'see' the photon slow down, what we see is a lengthening of the photon's wavelength, which tends to infinity at the event horizon. Now one important property of a photon with infinite wavelength is zero energy, ie the photon ceases to exist, since the photon, before it began its journey towards the black hole had an energy inversely proportional to its wavelength. According to the law of conservation of mass/energy, the energy of the photon cannot simply disappear. It has to go somewhere, so the black hole gains the equivalent mass of the photon's energy. In effect THE PHOTON HAS CROSSED THE EVENT HORIZON INTO THE BLACK HOLE. I don't know how some people got the idea that things falling into a black hole stop at the event horizon, but that is not the case. If it were true, it would be impossible for a black hole to form ( think about it ) and this whole argument would be moot. To recap. Things do fall into black holes. Primordial microscopic black holes ( left-overs from the big bang ) do evaporate and probably did so a long time ago or else we would see extreme flashes of gamma rays when their mass becomes less than critical and they explode. Massive black holes, formed from collapsing stars do evaporate, but they continuously ingest more mass than the evaporated amount and so gain mass, not loose it, so in effect they will never evaporate enough to explode and re-enter our space/time ( that may even violate the second law of thermodynamics as entropy would decrease on a very large scale ).

My apologies, owl, I assumed a spelling mistake where there was none. I was ( always seem to be ) in a bit of a hurry. My only point was that extra dimensions are a useful tool, whether real or imagined. If they have predictive effects then they may as well be real. There are a lot of things which cannot be seen or proved directly, only by their effects ( black holes, dark matter, sub-atomic particles, radiation, even air ). Yet I'm sure you have no problem accepting them.

I'll repeat, the distant observer sees the light coming out of the gravitational well more and more red-dhifted, until it finally disappears. But the infalling astronaut falls at the acceleration due to gravity just like anywhere else. He may see the 'end' of time when looking back out of the event horizon but the ONLY thing in his future is the ( possible ) singularity.