8 ms·

I have an unrelated question to the physics nerds. Is it even possible to determine whether the universe is deterministic or not from inside the universe? Let'

I have an unrelated question to the physics nerds. Is it even possible to determine whether the universe is deterministic or not from inside the universe? Let's imagine a simplified example. I give you two sets of numbers (let's pretend they are atom coordinates). One set is purely random, another one is generated with a very simple formula (example: f(n) = SHA256(N + salt)). I will give you as many of the numbers as you want. Can you determine which set is which (if you don't know the formula ahead of time)?

Govindae 9y ago
Any randomness could in principle be predetermined. God could have rolled the dice ahead of time.
- justinpombrio 9y ago
  Amazingly, we have an experiment that shows that this is not the case, if you make some pretty reasonable assumptions. It's based on Bell's theorem. Your statement is very intuitive, and the fact that it may be wrong is very surprising. https://en.m.wikipedia.org/wiki/Bell%27s_theorem#Bell_inequalities_are_violated_by_quantum_mechanical_predictions https://en.m.wikipedia.org/wiki/Bell%27s_theorem#Bell_inequa... EDIT: tone.
  - guskel 9y ago
    https://en.wikipedia.org/wiki/Superdeterminism https://en.wikipedia.org/wiki/Superdeterminism
  - Xcelerate 9y ago
    > if you make some reasonable assumptions Not everyone agrees on what is "reasonable". I have no problem giving up locality, but "true" randomness (i.e., information generated without an algorithm) seems like a philosophical cop out. Note that something can also be globally deterministic but indeterministic from the perspective of a subsystem. In this sense, the universe would appear random as far as we're concerned, but not random to whoever is simulating the universe (this is often called superdeterminism, but I think that's a silly word—the universe is either deterministic or it isn't).
    - justinpombrio 9y ago
      The probability of an event depends on your knowledge. Whether something is deterministic is a question of probability. Hence whether something is deterministic can depend on your knowledge. If the universe is deterministic to its simulator but random to us, I would call it random. Besides this difference in terminology, I think we agree.
      - jamesrcole 9y ago
        Your knowledge of something can't change or determine whether it is deterministic or not.
        justinpombrio 9y ago
        Do you agree that the probability of an event depends on your knowledge? For example, what is the probability that the bottom card of a shuffled deck is an ace? Peek at the top card, it's an ace of spades, now what is the probability? Would you agree that something is determined iff its probability is zero or one? I'm guessing the disagreement is here - what definition would you use?
        cool_shit 9y ago
        You are right, according to quantum mechanics. If you have a pair of electrons in a product state and measure the spin of one of them, you instantly know what the "other particle's" spin is going to be (if you knew what the initial state was). When you "peek at a card", you "make a measurement" and alter the system. For anything that exists outside of a light-cone around the first electron, a measurement of the second electron will be truly random from the measurer's perspective.
        jamesrcole 9y ago
        There's two kinds of probability in question. One concerns a lack of knowledge, like in your cards example. In that case, the probability is an expression of your state of understanding of the deck, and is not a property of the deck itself. The physical details of the deck and the situation it is part of could be entirely deterministic and we could still talk about this kind of probability. In this case your knowledge is independent of whether it is probabalistic or not. The other is whether the universe contains fundamental randomness, such that you could say it is literally "probabalistic". And in this case whether that is true or not is independent of our knowledge of the probabilities.
- Houshalter 9y ago
  It can also be explained by branching. Run a simulation of a universe, and every time a random bit is required, fork into two separate processes. One where the "random" bit is 1 and another where the bit is 0. From the inside, it would seem indistinguishable from true randomness. But the system as a whole is purely deterministic.
  - hasenj 9y ago
    How would forking occur? Forking requires copying data. Does the universe get cloned infinitely many copies all the time? That's quite an extraordinary claim.
    - deleted 9y ago
      [deleted]
    - quickben 9y ago
      You have to consider his concept as an analogy. Infinity is a thing inside this universe, it may not be outside.
      - hasenj 9y ago
        The problem is not with infinity per se. If the universe gets cloned one time every second that would still be a huge claim. A million times every nanosecond is even a bigger claim. Now imagine how many states the universe can be in, and how many times a second can be meaningfully divided. If you can solve the "one clone every second" then I would be satisfied.
        virgil_disgr4ce 9y ago
        What is the problem, though? Who says the uni/multiverse has a finite memory capacity? For more in this vein: http://www.preposterousuniverse.com/blog/2014/06/30/why-the-many-worlds-formulation-of-quantum-mechanics-is-probably-correct/ http://www.preposterousuniverse.com/blog/2014/06/30/why-the-...
Rhapso 9y ago
with certainty? no. There always exists the possibility that random numbers will follow a given pattern.
quickben 9y ago
No. There is even a proof we can't know the rules outside that formed these rules.
stephengillie 9y ago
Going to piggyback with a question unrelated to yours but tangental to the topic: What about the universes in the multiverse where 'multiverse theory' is wrong, and there is no multiverse? Are they islands? Are they like multiverse 'diodes' which may allow only one-way travel into that universe, but from which return to the larger multiverse is impossible?
- jakeva 9y ago
  This sounds a lot like the question of "does the set of all sets contain itself?" which is naturally a part of Russell's paradox. I can't offer more, but I don't think the concepts are dissimilar.
- AgentME 9y ago
  Just to clarify something: the standard many-worlds interpretation (such as discussed in the original article and elsewhere in this thread) doesn't contain anything about other alternate branches having different physical laws or being fundamentally different, and in MWI the only way that branches interact with each other is that probabilities are affected when multiple branches have their physical states match up completely (after possibly taking different paths) -- there's no way for intelligent beings to interact with meaningfully different branches for example. The many-worlds interpretation isn't a baseless what-if-there-are-alternate-universes thought experiment that anything could happen in, but instead is arguably the straight-forward interpretation of what we know about quantum mechanics.
deepnotderp 9y ago
Check out Andrew Friedman's work on "Closing the Free Will Loophole". It might be interesting.
QAPereo 9y ago
Real physics nerd, non-popsci answer: everything that is obersed rather than conjectured about the universe implies that it has no "outside" and your question is a bit like asking what is North of the North Pole. It just isn't a meaningful concept, like "before the big bang" or "what did the universe expand into."
- goatlover 9y ago
  Unless there are other universes and there was a time before the big bang.
  - QAPereo 9y ago
    While that is a nice thought it's in no way supported by any evidence or observation, or really any hope of evidence for observation. MWI or some other multi universal theory could have merit, but in their current formulation they're not even falsifiable.
- raattgift 9y ago
  > everything that is obsersed rather than conjectured about the universe implies that it has no "outside" "outside" is not very rigorous. For example, we seem to be observing objects vanishing behind the comoving horizon. It seems unlikely that the individual cosmic horizons centred on each of our microscopic components is destroying these objects, and it seems unlikely that they will ever reenter the comoving horizon. The metric expansion induces other interesting horizons, too, and each has an "inside" and an "outside". But how many of these horizon-crossings are directly detectable by the objects crossing them? (Reflectively, we are each exiting the horizons of distant observers at slightly different scale factors because we aren't occupying the same point in spacetime). "More of same" for some distance outside of e.g. the cosmological event horizon is wholly reasonable. We can even put lower bounds on "some distance" depending on how we look at the homogeneity and flatness problems. They're big. IIRC Guth's original cosmic inflation work predicted that the Hubble volume is no more than 10^-26 of the total causually connected volume at the start of inflation. We can also put bounds on any sort of gradient on various apparent constants such as c and G, and the region in which they are virtually certian to have the same values we have experimentally here is also big. However we currently can't do much better than that. The bits and pieces that were close to us in the hot dense phase of the universe are mostly inaccessible to us now, but there is every probability that they have identical local physics to us. There may be bits and pieces that were insufficiently close to us in that phase that have wildly different physics; and we do not really know anything about the still denser phase of the universe. It is perfectly reasonable to consider observables generated by detailed guesses that e.g. avoid an actual singularity like Carroll & Chen (who propose one or more other universes evolving towards de Sitter space from some arbitrary shared values surface) for instance. But even there, "outside" gets tricky. Moreover, "inside" vs "outside" is not really the best way to approach the underlying question, "what is the nature of the metric expansion of space?" where the real answer should identify the cosmological frame and its preferred coordinate system in which almost all matter remains at essentially the same spatial coordinates from the big bang to the infinite future, with the scale factor relating to radar distances (and radar beam wavelength changes) between objects at distant spatial coordinates. Then we can admit that there are various ways to interpret the radar observables, with the "easiest" one being a purely local evolution of the vacuum at each point along the radar's path, which in the preferred cosmological frame can be seen as dark energy, but which with other systems of coordinates can be seen as anything from "the local creation of more space" to a Doppler effect to (somewhat less usefully) a change in the vacuum's refractive index. > what is North of the North pole Well, a change of coordinates on the Earth gets rid of that particular coordinate singularity, doesn't it? We can get rid of all sorts of oddities by swapping the coordinates we're using, which is probably the greatest strength of general covariance. What we can't do by changing coordinates (even to accelerated systems of coordinates) is eliminate the Earth's oblateness. Likewise, we can change from FLRW coordinates to other coordinates on the cosmological frame and get rid of (or introduce) all sorts of oddities. We can't, however, get rid of angle-brightness-redshift relations. It is a bit silly to introduce all sorts of additional action-at-a-distance forces to explain the Earth's oblateness in an effort to do away with the rotation-oblateness relation for objects close to hydrostatic equilibrium. Similarly, it could be a bit silly to introduce all sorts of new (and presently broken) symmetries in an effort to do away with away with causally disconnected regions (with possibly different physics) before inflation or whatever mechanisms produced the relic field anisotropies. And I think it's reasonable for physical cosmologists to argue that it's hard to foreclose on that type of multiverse without doing so. Moreover, "before the big bang" is perfectly reasonable if one starts adding in new symmetries anyway; if quantum gravitation avoids the BB singularity (as practically everyone hopes it will avoid BH singularities), why wouldn't it be meaningful to consider the universe before the classical singularity BB? It's just technically difficult as a theoretical programme and is unlikely to be guided by current data. Well, so what? It's technically difficult to predict early structures or them right after reionization, let alone say anything about or observe anything at slightly higher redshifts, and we don't even expect new physics to be relevant in those regions at all.

Govindae 9y ago

Any randomness could in principle be predetermined. God could have rolled the dice ahead of time.

justinpombrio 9y ago
Amazingly, we have an experiment that shows that this is not the case, if you make some pretty reasonable assumptions. It's based on Bell's theorem. Your statement is very intuitive, and the fact that it may be wrong is very surprising. https://en.m.wikipedia.org/wiki/Bell%27s_theorem#Bell_inequalities_are_violated_by_quantum_mechanical_predictions https://en.m.wikipedia.org/wiki/Bell%27s_theorem#Bell_inequa... EDIT: tone.
- guskel 9y ago
  https://en.wikipedia.org/wiki/Superdeterminism https://en.wikipedia.org/wiki/Superdeterminism
- Xcelerate 9y ago
  > if you make some reasonable assumptions Not everyone agrees on what is "reasonable". I have no problem giving up locality, but "true" randomness (i.e., information generated without an algorithm) seems like a philosophical cop out. Note that something can also be globally deterministic but indeterministic from the perspective of a subsystem. In this sense, the universe would appear random as far as we're concerned, but not random to whoever is simulating the universe (this is often called superdeterminism, but I think that's a silly word—the universe is either deterministic or it isn't).
  - justinpombrio 9y ago
    The probability of an event depends on your knowledge. Whether something is deterministic is a question of probability. Hence whether something is deterministic can depend on your knowledge. If the universe is deterministic to its simulator but random to us, I would call it random. Besides this difference in terminology, I think we agree.
    - jamesrcole 9y ago
      Your knowledge of something can't change or determine whether it is deterministic or not.
      - justinpombrio 9y ago
        Do you agree that the probability of an event depends on your knowledge? For example, what is the probability that the bottom card of a shuffled deck is an ace? Peek at the top card, it's an ace of spades, now what is the probability? Would you agree that something is determined iff its probability is zero or one? I'm guessing the disagreement is here - what definition would you use?
        cool_shit 9y ago
        You are right, according to quantum mechanics. If you have a pair of electrons in a product state and measure the spin of one of them, you instantly know what the "other particle's" spin is going to be (if you knew what the initial state was). When you "peek at a card", you "make a measurement" and alter the system. For anything that exists outside of a light-cone around the first electron, a measurement of the second electron will be truly random from the measurer's perspective.
        jamesrcole 9y ago
        There's two kinds of probability in question. One concerns a lack of knowledge, like in your cards example. In that case, the probability is an expression of your state of understanding of the deck, and is not a property of the deck itself. The physical details of the deck and the situation it is part of could be entirely deterministic and we could still talk about this kind of probability. In this case your knowledge is independent of whether it is probabalistic or not. The other is whether the universe contains fundamental randomness, such that you could say it is literally "probabalistic". And in this case whether that is true or not is independent of our knowledge of the probabilities.
Houshalter 9y ago
It can also be explained by branching. Run a simulation of a universe, and every time a random bit is required, fork into two separate processes. One where the "random" bit is 1 and another where the bit is 0. From the inside, it would seem indistinguishable from true randomness. But the system as a whole is purely deterministic.
- hasenj 9y ago
  How would forking occur? Forking requires copying data. Does the universe get cloned infinitely many copies all the time? That's quite an extraordinary claim.
  - deleted 9y ago
    [deleted]
  - quickben 9y ago
    You have to consider his concept as an analogy. Infinity is a thing inside this universe, it may not be outside.
    - hasenj 9y ago
      The problem is not with infinity per se. If the universe gets cloned one time every second that would still be a huge claim. A million times every nanosecond is even a bigger claim. Now imagine how many states the universe can be in, and how many times a second can be meaningfully divided. If you can solve the "one clone every second" then I would be satisfied.
      - virgil_disgr4ce 9y ago
        What is the problem, though? Who says the uni/multiverse has a finite memory capacity? For more in this vein: http://www.preposterousuniverse.com/blog/2014/06/30/why-the-many-worlds-formulation-of-quantum-mechanics-is-probably-correct/ http://www.preposterousuniverse.com/blog/2014/06/30/why-the-...

Rhapso 9y ago

with certainty? no. There always exists the possibility that random numbers will follow a given pattern.

quickben 9y ago

No. There is even a proof we can't know the rules outside that formed these rules.

stephengillie 9y ago

Going to piggyback with a question unrelated to yours but tangental to the topic: What about the universes in the multiverse where 'multiverse theory' is wrong, and there is no multiverse? Are they islands? Are they like multiverse 'diodes' which may allow only one-way travel into that universe, but from which return to the larger multiverse is impossible?

jakeva 9y ago
This sounds a lot like the question of "does the set of all sets contain itself?" which is naturally a part of Russell's paradox. I can't offer more, but I don't think the concepts are dissimilar.
AgentME 9y ago
Just to clarify something: the standard many-worlds interpretation (such as discussed in the original article and elsewhere in this thread) doesn't contain anything about other alternate branches having different physical laws or being fundamentally different, and in MWI the only way that branches interact with each other is that probabilities are affected when multiple branches have their physical states match up completely (after possibly taking different paths) -- there's no way for intelligent beings to interact with meaningfully different branches for example. The many-worlds interpretation isn't a baseless what-if-there-are-alternate-universes thought experiment that anything could happen in, but instead is arguably the straight-forward interpretation of what we know about quantum mechanics.

deepnotderp 9y ago

Check out Andrew Friedman's work on "Closing the Free Will Loophole". It might be interesting.

QAPereo 9y ago

Real physics nerd, non-popsci answer: everything that is obersed rather than conjectured about the universe implies that it has no "outside" and your question is a bit like asking what is North of the North Pole. It just isn't a meaningful concept, like "before the big bang" or "what did the universe expand into."

goatlover 9y ago
Unless there are other universes and there was a time before the big bang.
- QAPereo 9y ago
  While that is a nice thought it's in no way supported by any evidence or observation, or really any hope of evidence for observation. MWI or some other multi universal theory could have merit, but in their current formulation they're not even falsifiable.
raattgift 9y ago
> everything that is obsersed rather than conjectured about the universe implies that it has no "outside" "outside" is not very rigorous. For example, we seem to be observing objects vanishing behind the comoving horizon. It seems unlikely that the individual cosmic horizons centred on each of our microscopic components is destroying these objects, and it seems unlikely that they will ever reenter the comoving horizon. The metric expansion induces other interesting horizons, too, and each has an "inside" and an "outside". But how many of these horizon-crossings are directly detectable by the objects crossing them? (Reflectively, we are each exiting the horizons of distant observers at slightly different scale factors because we aren't occupying the same point in spacetime). "More of same" for some distance outside of e.g. the cosmological event horizon is wholly reasonable. We can even put lower bounds on "some distance" depending on how we look at the homogeneity and flatness problems. They're big. IIRC Guth's original cosmic inflation work predicted that the Hubble volume is no more than 10^-26 of the total causually connected volume at the start of inflation. We can also put bounds on any sort of gradient on various apparent constants such as c and G, and the region in which they are virtually certian to have the same values we have experimentally here is also big. However we currently can't do much better than that. The bits and pieces that were close to us in the hot dense phase of the universe are mostly inaccessible to us now, but there is every probability that they have identical local physics to us. There may be bits and pieces that were insufficiently close to us in that phase that have wildly different physics; and we do not really know anything about the still denser phase of the universe. It is perfectly reasonable to consider observables generated by detailed guesses that e.g. avoid an actual singularity like Carroll & Chen (who propose one or more other universes evolving towards de Sitter space from some arbitrary shared values surface) for instance. But even there, "outside" gets tricky. Moreover, "inside" vs "outside" is not really the best way to approach the underlying question, "what is the nature of the metric expansion of space?" where the real answer should identify the cosmological frame and its preferred coordinate system in which almost all matter remains at essentially the same spatial coordinates from the big bang to the infinite future, with the scale factor relating to radar distances (and radar beam wavelength changes) between objects at distant spatial coordinates. Then we can admit that there are various ways to interpret the radar observables, with the "easiest" one being a purely local evolution of the vacuum at each point along the radar's path, which in the preferred cosmological frame can be seen as dark energy, but which with other systems of coordinates can be seen as anything from "the local creation of more space" to a Doppler effect to (somewhat less usefully) a change in the vacuum's refractive index. > what is North of the North pole Well, a change of coordinates on the Earth gets rid of that particular coordinate singularity, doesn't it? We can get rid of all sorts of oddities by swapping the coordinates we're using, which is probably the greatest strength of general covariance. What we can't do by changing coordinates (even to accelerated systems of coordinates) is eliminate the Earth's oblateness. Likewise, we can change from FLRW coordinates to other coordinates on the cosmological frame and get rid of (or introduce) all sorts of oddities. We can't, however, get rid of angle-brightness-redshift relations. It is a bit silly to introduce all sorts of additional action-at-a-distance forces to explain the Earth's oblateness in an effort to do away with the rotation-oblateness relation for objects close to hydrostatic equilibrium. Similarly, it could be a bit silly to introduce all sorts of new (and presently broken) symmetries in an effort to do away with away with causally disconnected regions (with possibly different physics) before inflation or whatever mechanisms produced the relic field anisotropies. And I think it's reasonable for physical cosmologists to argue that it's hard to foreclose on that type of multiverse without doing so. Moreover, "before the big bang" is perfectly reasonable if one starts adding in new symmetries anyway; if quantum gravitation avoids the BB singularity (as practically everyone hopes it will avoid BH singularities), why wouldn't it be meaningful to consider the universe before the classical singularity BB? It's just technically difficult as a theoretical programme and is unlikely to be guided by current data. Well, so what? It's technically difficult to predict early structures or them right after reionization, let alone say anything about or observe anything at slightly higher redshifts, and we don't even expect new physics to be relevant in those regions at all.