If an entity existed such as Laplace's demon, that could know with precision the current state of the universe and - using the laws of physics - could tell you your future with certainty, would you then be able to change it?
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Possible Duplicate:             As asked in the title, can a Universe which is deterministic at its fundamental level still allow free will?
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If the theory of everything indicates a singularity-free and finite universe, will  be relevant to the universe? If so, is there any interesting physical consequence, e.g. in superstring or quantum gravity?
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I would like to know the physical meaning of the , if there is any? I've used it in thermodynamics and classical mechanics and it seemed only a change of coordinates?
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Even if determinism isn't true for everything that happens—even if   some things just happen without being determined by causes that were   there in advance—it would still be very significant if everything we   did were determined before we did it. However free you might feel when   choosing      [Thomas Nagel - What does it all mean? p.52]   What does the word "it" mean?
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I often think about problems that require an understanding of the very essence of computation and its inherent limitations. So, my questions are as followed:   Is the universe isomorphic to a universal turing machine? Is the universe isomorphic to a universal quantum computer? What evidence do we have within our universe to support an answer to such a question? What corollaries follow from a universe that is or is not isomorphic to a universal turing machine (or universal quantum turing machine)? Is this question really answerable, and what is the current literature on this subject?   I'm mostly interested in the implications and corollaries that follow from a universe that is or is not isomorphic to a turing machine (or universal quantum computer) and the insights that it would provide us with, about the nature of computation.
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Could someone judge my (stoner) hypothesis that the speed of light has changed over time -- i.e. as the universe has expanded in volume light has slowed down, perhaps going so far as back to the big bang when it was infinitely fast and there was no time because everything happened at once etc. Thinking that the speed at which information can propagate through the universe is linked to the size of it seems intuitive to me. My question -- is there an easy disproof of this? Would Einstein have to be wrong? Does it violate anything supposedly more fundamental such as quantum or string theories? Do any current experiments invalidate it? If not can you show me in any case why you think its unlikely.   Edit 8/24  I'm accepting Mark M.'s answer but will post this here because there is a character limit on comments  @Mark M thanks, good answer but as someone who has only read some popular physics, and should leave this to the experts, I'm still muddled in my personal theory. I don't see why you should need two units to measure the speed of light. The thing I have a hard time wrapping my head around is the relation of time and distance. They seem like they could fundamentally be the same thing. If you say time is measured fundamentally by the vibration of so and so quantum object in space... why can't we just measure that vibration distance as the constant... I'll repeat myself to try to be clear... there is a certain minimum distance that particles have to go to interact with each other... if it wasn't vibrating there wouldn't be time,its what creates the illusion of time...so instead of talking about speed or c as distance/time.... can't we simply talk about that distance a quantum object vibrates....I'll lead up to my point.....perhaps there can need be only one constant here, and that is the physical size of the universe. a tiny metal tuning fork doesn't appear to be vibrating at all but if you blew it up to the size of the empire state building the metal rods would move from window to window. perhaps as our universe expanded in size the length of that minimum vibration (perhaps infinite at point zero) would have expanded, and therefore created the illusion of time and the speed of light, which, as the universe expands, will continue slowing down. perhaps we are like a big balloon and we have been blown up and all the fields/particles-without-size are vibrating more and more in that space.. am i missing something obvious here?
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From what I understand, the  states that there is a fundamental natural limit to how accurately we can measure velocity and momentum at the same time. It's not a limit on equipment but just a natural phenomenon.  However, isn't this just an observational limit? There is a definite velocity and momentum, we just don't know it. As in, we can only know so much about the universe, but the universe still has definite characteristics.  Considering this, how do a wide range of quantum mechanical phenomena work? For example, quantum tunneling - its based on the fact that the position of the object is indefinite. But the position is definite, we just don't know it definitely. Or the famous light slot experiment? The creation of more light slots due to uncertainty of the photon's positions?  What I am basically asking is why is a limit on the observer, affecting the phenomenon he is observing? Isn't that equivalent to saying  because we haven't seen Star X, it doesn't exist? It's limiting the definition of the universe to the limits of our observation!
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Specifically, would a soul know that the body they're being invited into was created via Reincarnate? After all, for some souls, changing species might be a big deal, possibly of the 'rather stay dead' kind.
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I'm new here, so apologies if the question doesn't sound meaningful considering what physics is supposed to answer. I don't have a physics or mathematics background, but I did learn a few things about entropy and the different possible fates of an expanding universe from other forums.   Now, assuming the heat death "fate" is where the universe is heading towards, asymptotically reaching towards a thermal equilibrium, will it stay in a state of equilibrium forever? or the will the "arrow of time" cease pointing sharply towards the future since decrease in entropy becomes likelier when a state of near maximal entropy is achieved (I'm guessing here) ?  The wiki article on this states the number of years that would take for a random quantum fluctuation to perhaps recreate another "big bang" and restart the universe as $10^{10^{10^{56 }}}$ . I realise that's a RIDICULOUS amount of time to even comprehend, nevertheless it is still a physical possibility and $10^{10^{10^{56}} }$ years are only a tiny fraction of time when compared to an infinite amount of time.  Finally what I want to ask: Is reality a one time phase transition from a vague potential with maximal degrees of freedom (big bang) to a crisp actuality with maximal constraint (heat death) with asymmetric, unidirectional time or is it infinite cyclic with symmetric time that reverses itself (by means of highly improbable, but physically not impossible mechanisms) after a finite period of time?   I suspect this may be a borderline philosophy question, but figured might as well give it a try here before posting in more relevant forums. Thanks.
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Based on the problem of induction, nobody can assert with absolute certainty that the laws of science (i.e. physics, chemistry, etc,...) will hold all the time, in every part of the universe. Believing that that is the case is therefore a form of faith, not knowledge.  Current scientific consensus is that the universe doesn't stretch back indefinitely into the past, but came into existence a finite time ago with the big bang. The occurrence of the big bang and the subsequent evolution of the universe are both driven by the laws of physics. The laws of physics, seen this way, are the first cause of the universe and the higher power that governs it's day to day functioning.    So belief in science requires having faith, and belief in a first cause/higher power, two of the main components of theistic belief. And the laws of science then become some sort impersonal deity, akin to the deity of Deism. One could even envision a science based moral code based on neurological and environmental considerations, and we would then have all the constituents of  traditional religion: Faith, first cause, and a set of ethical laws.   Obviously, this theism is very different than the theism of the Abrahamic religions, especially since the god in question is not personal. But the way I see it, isn't it still a form of theism, with the laws of science/physics as its deity?
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Doctor Strange can teleport. There's no question of this. Thus, by teleporting - he is theoretically capable of going FTL. Now  could cause causality paradoxes - i.e. an event where the effect happens before the cause.  Could Doctor Strange theoretically use this principle to change the outcome of an event or damage the timeline (as he could with the Infinity Stone) using only his teleportation? If he can, are there any examples of him doing this?
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The possibility of randomness in physics doesnt particularly bother me, but contemplating the possibility that quarks might be made up of something even smaller, just in general, leads me to think there are likely (or perhaps certainly?) thousands of particles and forces, perhaps layers and sub layers of forces, at play that we do not know about. So this got me thinking about quantum mechanics.    I'm no physicist, but I do find it interesting to learn and explore the fundamentals of physics, so I'm wondering: Could the randomness found in radioactive decay as described in quantum mechanics be the result of forces and / or particles too weak / small for us to know about yet resulting in the false appearance of randomness?  Or rather, can that be ruled out?
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Is there a probability that an electron in an atom change its orbital by emitting a quantum of gravitational radiation instead of photon?
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I know that a perpetual motion machine (a device that produces work without an energy source) would violate several physical laws, the first that comes to mind being the . I also understand that a claim that perpetual motion is possible would probably not be congruent with the scientific method, occum's razor, etc.  What I am interested in is whether there exists some logical and intuitively meaningful (to they lay person) demonstration that perpetual motion is universally impossible, based only on axioms rooted in observable physical phenomena, rather than the obvious reasons why it is merely improbable and incompatible with some of our physical laws.  Certain violations of physical laws, such as travelling faster than the speed of light, can be shown to be unlikely as they would cause paradoxes or logical impossibilities, such as a . In that instance, the axioms needed to establish a paradox (time dilation) are empirically observable, and a counter-claim would require the existence of new paradox-resolving laws that are not established, or the permission of paradoxes which is difficult to imagine.  Conversely, the idea that some invention is impossible on the basis of its violation of a physical law alone is difficult because it requires a .  So, without relying on the assertion that a physical law is omnipresent (e.g. conservation of energy), is there some impossibility or logical paradox that would arise from the existence of a machine that could produce work without indefinitely without an energy source?  Finally, I'm very aware that perpetual motion is a bit of a dirty word and will likely touch a nerve with people, so please take the time to let me know how I can make this question less offensive and avoid the inevitable down-votes.
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I have heard several pseudoscientific explanations about the  and find them hard to believe.  As a mathematician mainly focusing on functional analysis, I have a considerable interest in this matter. Although I still have no idea about it, I aim to truly understand it one day. For now, the question is:  Can the Heisenberg Uncertainty Principle be explained so that a non-scientist can at least get a correct idea of what it says and what not?  One of the most heard explanations is the one that says naïvely that while trying to determine the position of a particle you send energy (light) so you modify its speed, therefore making it impossible to determine the speed. If I understand well this should only be a some sort of metaphor to explain it, right?
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If all distances are constantly increasing, as Hubble's law say, then lots of potential energies of form ~$\frac{1}{r}$ changes, so how is the total energy of the Universe conserved with Hubble's expansion?
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For the sake of simplicity, let's imagine that the entire universe is empty except for a single lump of (classical) matter with mass $m$. In its center of momentum frame, it is clear that the total energy is simply $E_\text{CoM}=mc^2$. However, in a frame moving relative to it with speed $v$, we have $E_\text{moving}=\sqrt{m^2c^4+p^2c^2}&gt; mc^2$ for all nonzero $v$.  Are we to infer that the energy content of a system can be different relative to the observer? Does this not violate the first law of thermodynamics?