First of all, it is important to note that I'm not very savvy in neither general relativity nor any other area of expertise that answering this question may require. Therefore, I mean for the question to be answered in layman's terms.  Okay, now to the question itself: as far as I understand, for a given energy total in a system, its contents can theoretically be put into any other configuration of energies with the same total sum (if that makes sense). Therefore, if that sum is zero, the contents can be configured into, basically, 'nothing', which also has an energy total of zero.  However, with the universe, the only sources of negative energy are the fundamental forces. I think it may be reasonable to put all forces other than gravity aside, as only gravity is generated (sorry for funky wording) by something which itself is another source of energy (mass), and all the other forces eventually cancel out. If we also ignore the dark energy, we are left with a collection of masses that will eventually crunch together. Here is the problem I have with this: where will the mass go? Even if the potential energy of every point in the universe is zero (i.e. all the mass in the universe has now been smushed into a point), there is still mass to deal with, right?   I know I am probably wrong in many, many places in this question, but can you explain the general, underlying flaw behind my reasoning?
122465
Does the concept of energy always exist for any physical system?  If yes is there a proof?  If no, any counterexamples?
51567
I am assuming there are two constituents that obliterate our current model of physics;   that it's infinitely dense that it's infinitely small   Please correct me if I am wrong.
79338
I often hear people say that general relativity predicted its own demise because of the singularities it predicts. If I'm not mistaken, this is also a problem in QFT. I wonder why singularities garner so much hate. To me, it seems like negative or complex numbers: we used to hate these things but now they are more generally accepted. Why can't it be that nature just has singularities? We might not get it, we might not like it, but it may be so.
117676
I have trouble imagining how two point-particles can have different properties.  And how can finite mass, and finite information (ie spin, electric charge etc.) be stored in 0 volume?  Not only that, but it can also detect all fields without having any structure. Maybe it can check curvature of spacetime to account for gravity, but how can a point contain the information of what the other fields-vectors are? This seems to mean that also the information/volume in space is infinite.  Mathematically, a point cant have any intrinsic structure, so how does physics which is a mathematical theory explain this?
63178
asks what constraints there are on the global topology of spacetime from the Einstein equations. It seems to me the quotient of any global solution can in turn be a global solution. In particular, there should be non-orientable solutions.  But does quantum physics place any constraint? Because it seems to me that if space by itself is non-orientable then what happens to, say, neutral kaon interactions along two different paths that come back to the same spot with opposite orientations?  So then, is there any reason why time cannot be non-orientable? For example my mental picture (two space dimensions suppressed) is of a disc. The big bang is the centre, time is the radial direction, space is the circumferential direction. A timelike geodesic that avoids black or white holes will start on the big bang, go out to the edge of the disc, continue on the opposite edge with time and orientation (and presumably matter/anti-matter) reversed, and return to the big bang (which is also therefore the same as the big crunch). The "reflection time" of the universe would be large enough that thermodynamic violations are not observed.
54089
Due to the mass-energy equivalence, both matter and EM radiation bend spacetime, and both are capable of forming singularities (black hole, white hole/kugelblitz). In light of this, why do photons traveling from the most distant reaches of the observable universe not lose energy due to the gravitational radiation they must emit? Furthermore, mustn't cosmic rays (e.g. protons) slow down or stop as they lose energy through the same mechanism?
33873
Given the idea that gravity is always a factor (even though greater distances are excessively tiny), would everything eventually end up forming a single black hole?  My thinking is that, with infinite time, all mass will eventually be pulled together.
17487
The simplistic undergrad explanation aside, I've never really understood what energy really is. I've been told that it's something when converted from one kind of something to another kind, or does some "work", as defined by us, but what is that something?  Moreover, if the total amount of energy in the universe is finite and we cannot create energy. Then, where did it come from? I've learned from thermodynamics where it goes, but where does it come from?  I know this sounds like something trivially simple, but there is so much going on in the physical universe and I just can't grasp what it is. Maybe it is because I lack the mathematical understanding that I can't grasp the subtle things the universe is doing. Still, I want to understand what it is doing.  How do I get to the point of understanding what it's doing?  (Note: What prompted me to ask this was . I'm afraid that it just puzzled me further and I sat there staring at the screen for a good 10 minutes.)
20305
Let me start by saying that I have no scientific background whatsoever. I am very interested in science though and I'm currently enjoying Brian Greene's The Fabric of the Cosmos. I'm at chapter 7 and so far I understand most of general ideas he has talked about. Or at least, I think I understand them :-)  There is however one part, at the end of chapter 6 that I can't grasp.  It is about entropy and the state of the universe a few minutes after the big bang.  On page 171 he says:     Our most refined theories of the origin of the universe -our most refined cosmological theories- tell us that by the time the universe was a couple of minutes old, it was filled with a nearly uniform hot gas composed of roughly 75 percent hydrogen, 23 percent helium, and small amounts of deuterium and lithium. The essential point is that this gas filling the universe had extraordinarily low entropy.   And on page 173-174:     We have now come to the place where the buck finally stops. The ultimate source of order, of low entropy, must be the big bang itself. In its earliest moments, rather than being filled with gargantuan containers of entropy such as black holes, as we would expect from probabilistic considerations, for some reason the nascent universe was filled with a hot,   uniform, gaseous mixture of hydrogen and helium. Although this configuration has high entropy when densities are so low that we can ignore gravity, the situation is otherwise when gravity can't be ignored; then, such a uniform gas has extremely low entropy. In comparison with black holes, the diffuse, nearly uniform gas was in an extraordinarily low-entropy state.   In the first part of the chapter Brian Greene explains the concept of entropy with tossing the 693 pages of War and Peace in the air.  At first, the pages are ordered. The specific order they are in make sense and are required to recognize the pages as a readable book called War and Peace. This is low entropy. It is very highly ordered and there is no chaos.  Now, when you throw the pages in the air, let them fall and then pick them up one by one and put them on top of each other, the chances you get the exact same order as the initial state are extremely small. The chance you get another order (no matter what order, just not the one from the beginning) is extremely big. When the pages are in the wrong order, there is high entropy and a high amount of chaos. The pages are not ordered and when they are not ordered you would not notice the difference between one unordered state and another one.  However, should you swap two pages in the ordered, low entropy version, you would notice the difference.  So I understand low entropy as a highly ordered state with low chaos in which a reordering of the elements would be noticeable.  I hope I'm still correct here :-)  Now, what I don't understand is how a uniform mixture of hydrogen and helium can by highly ordered? I'd say you wouldn't notice it if some particles traded places. I'd say that a uniform mixture is actually in a state of high entropy because you wouldn't notice it if you swapped some hydrogen atoms.  Brian Greene explains this would indeed be the case when gravity plays no important role, but that things change when gravity does play a role; and in the universe right after the big bang, gravity plays a big role.  Is that because a reordering of the particles would change the effects of gravity? Or is there something else that I'm missing here?
66015
Sorry if this is really naive, but we learned in Newtonian physics that the total energy of a system is only defined up to an additive constant, since you can always add a constant to the potential energy function without changing the equation of motion (since force is negative the gradient of the potential energy).   Then in Quantum Mechanics we showed how the ground state of a system with potential energy $V(x) = \frac{1}{2} m \omega^{2} x^{2}$ has an energy $E_{0}=\frac{1}{2} \hbar \omega $.   But if we add a constant to $V(x)$ won't that just shift the ground state energy by the same constant? So in what sense can we actually say that the ground state energy has an absolute value (as opposed to just a relative value)? Is there some way to measure it?   I ask this in part because I have heard that Dark Energy might be the ground state energy of quantum fields, but if this energy is only defined up to a constant, how can we say what it's value is?
122988
I'm making several assumptions, not sure if any are correct:   there is a black hole at the center of a galaxy the black hole is eating the galaxy   Eventually the galaxy will be gone, right?  Has this been observed? Do we know what happens afterwards?  Posting here since astronomy got merged into physics
38709
What is the exact shape of the universe? I know of the balloon analogy, and the bread with raisins in it. These clarify some points, like how the universe can have no centre, and how it can expand equally everywhere in all directions. But they also raise some questions, like if you are on the surface of a balloon and travel in 1 direction, you will eventually return to your starting point, is it possible our universe have this feature?  If it has, or had, would this be a symmetry of sorts ($\psi(x)=\psi(x+R)$), and as such have a conserved quantity associated with it (by Noether)? Assuming "small curled up dimensions" wouldn't these dimensions have this type of symmetry, what are the associated conserved quantities? Is it known exactly what the geometrical shape of the universe is? (on a large scale) (I am not talking about only the observable universe). How does one define the "size" of a dimension, is this scale only applicable to curled up ones? Is it possible to describe to a layman the shape of the universe without resorting to inept analogies?
90880
By looking at this picture:    The earth is near the center of the universe. I've read that the universe look the same no matter where the observer is located. It is the same distance everywhere.  So I understand that for general relativity the universe need to be homogeneous and isotropic, so it will look the same no matter where I am.  But what if I'm on one of the planets near the right or left of the image, then if I draw the same picture of the universe, but from my perspective, then I would be also located in the center? If that's not the case (I'm actually near an edge), then part of my sky would be completely dark, and all the sky that way won't be isotropic?
58253
Added: 5 times down vote for now! Down voter is this religion or physics, please try to explain your decision.  I'm confused about this.  In physics we know for a , but I think that there is a contradiction in this term. The  are the phenomena that  contradicts the vacuum existence because, according to them, the vacuum isn't the empty space.  In vacuum the creation of particle-antiparticle pairs is allowed for small times and this is also proven in practice.  From the other side, the relativistic theory says about space-time that interactions like gravitation bend the vacuum (empty space). There seems to be a contradiction to me. If vacuum is an empty space then we can't bend it, because we can't bend something nonexistent. In other words: we can't bend 'nothing'.  Can empty space really exist in physics?  EDIT1: quote: Luboš Motl     "By definition, the space without   energy is the space whose total value   of energy is equal to 0."   But this space is nonexistent, so it is abstract and should exist only in our mind... Why is so? My second statement of this post says that in physics we cant bend nothing because such a bending is only thinking and not physics!  Another possibility is that space is unknown kind of energy, but this is contradiction in modern physics!  EDIT2:  Can any physics believe that nothing exists? By mathematical logic no! And mathematic is elementary tool in physics. Anything other than that is religion!  Nothing (empty space without energy) is only a logical state!  EDIT3: quote: Roy Simpson:     The General Relativity Vacuum is a   space-time model region without   matter.   and Luboš Motl says: "By definition, the space without energy is the space whose total value of energy is equal to 0."  Agree...   But this is only mathematical Euclidean space + time so this is only a mathematic and not physics! With other words: this is only a method of mathematical mapping. But, in real (not theoretic) physics we can't mapping empty things. Empty is only a logical state!  EDIT4:   Roy Simpsons argumentation seems to me acceptable.   quote: Roy Simpson:     Einstein struggled with this too, and   the problem has come to be known as   the "Hole argument" within GR. You   have to decide whether you are just   interested in GR's vacuum (empty   space) or the full physical vacuum   which includes quantum aspects as   well.   Thanks
47024
I know that potential energy exists for a system of objects. But why? Is there no potential energy for a single object system? I assume that there is.  Consider the earth-object system. Suppose an external force lifts the object above the earth. Then the work done by it gets stored in the earth-object system as potential energy.Right?  Now, my question is that is the potential energy always zero for a single object system.   MY REASONING  If we consider the object only as the system and lift it above the Earth, we are doing positive work on it and filling it with energy and the gravitational field of the Earth is doing negative work on it and draining it of energy. Thus whenever the object is moving it has kinetic energy which is fine. But when it is not moving,i.e. when its weight is equal to the force applied by us, the net work done is zero and so the object should not have any kinetic energy. Does it have potential energy?
105508
I've read a number of the helpful Q&amp;As on photons that mention the mass/mass-less issue. Do I understand correctly that the idea of mass-less (a rest mass of 0) may be just a convention to make the equations work?  From a layperson's view, it's difficult to understand how a particle of light (photon) can be mass-less. A physical object (everyday world-large or quantum-small) must have a mass. Yet, if my understanding is correct, the mass of a moving object/particle increases in proportion to its speed/velocity...so that at the speed of light, its mass would be infinite. A photon travels at the speed of light, but it obviously doesn't have infinite mass, right? Can someone formulate a practical explanation that can be understood by middle-school to high school kids? Much thanks for the help.    Wow--your answers to my original Q below clear up much of my confusion. I now have the daunting task of going over these nuggets and working up an equation-less (hopefully) explanation of the mass-less photon for non-physicist types.  Yes, from a layperson's view, it does seem remarkable that an existing piece of matter-- which has to be made of physical substance--could have zero mass at rest (though a photon is never at rest). It would be almost understandable if a piece of matter made of nothing had zero mass, but that  seems to be an oxymoron, and "nothing" would equate to nonexistent, right?  In case you might find it interesting: I'm working on a writing project that posits we inhabit a universe that consists of matter (physical stuff) only, and that the NON-physical (aka supernatural) does not (and cannot) exist. For instance, if a purported supernatural phenomenon is found to actually exist, then by definition, its existence is proof that it is mundane/natural. All it would take to disprove this premise is reliable proof that ONE supernatural event has occurred. Despite thousands of such claims, that's never yet happened.  Who else better than physicists to confirm my premise? However, I do wish the TV physicists would explain the terms they throw about, some of which mislead/confuse their lay viewers. Case in point: "The universe is made up of matter and energy" (without properly defining the term "energy" as a property of matter).  The result is that laypersons are left with the impression that energy must therefore be something apart from or independent of matter (ie, nonphysical). Their use of the term "pure energy" without specifying exactly what that means adds to the confusion. (Thanks to your replies on this forum, I now understand that "pure energy" refers to photon particles.) However, "psychics" and other charlatans take advantage of such confusion by hijacking terms like energy (as in "psychic energy"), frequencies, vibrations, etc to give perceived scientific legitimacy to their claims that a supernatural spirit world, etc., exists. As you may realize, the majority of people in the US (per 2009 Harris Poll) and around the world believe in the existence of nonphysical/supernatural stuff such as ghosts and spirits.  My purpose is to give laypersons the information they need to distinguish what's real from what's not.  Thanks so much for help...And, PLEASE, add any further comments you think might be helpful/insightful to better inform laypersons.
14537
Dark energy is suggested to be a repulsive force in the universe causing an accelerated expansion. If the amount of mass outside our observable universe is greater than inside (higher mass density), would it not cause an accelerated expansion from our viewpoint?