11 ms·

Quantum Gravity’s Time Problem

114 points by CarolineW 10y ago

brianberns 10y ago
> "The whole state of system-plus-clock doesn’t change in time" How is this different from the classical view of the universe as deterministic? In a Newtonian universe, all future states can be determined from the current state, so the universe can be seen as a "frozen" 4-dimensional object.
- contravariant 10y ago
  Well, it's similar, except in this case the universe wouldn't have to be deterministic. As far as I can tell there's no law saying you can't have two clocks, where observers entangled with either observe a different sequence of events.
  - danbruc 10y ago
    I think Leonard Susskind touched this point in one of the theoretical minimum lectures [1] but I really can not tell which one. While we can quite easily imagine adding and removing space dimensions, nobody ever figured out what it would mean to have more than one time dimension. I think he also talked about time traveling in this context - you can go back and forth in space because there is more than one space dimension and so you can turn around and walk the other direction. Unless you instantaneously flip around that is nothing you can do with just one dimension like our one time dimension. But those things aside, the order of spatially separated events is not in general well defined if you take general relativity into account. There is a causal ordering all observers agree on but that still allows some observers to see some events in one order or another. I skimmed the papers linked in the article but they are quite a bit above what I can immediately grasp. But one thing I noticed was that they talked a lot about observable changes, so now I am wondering whether the entire system they are talking about is really static or whether we are just unable to observe the evolution of the entire system? If it is really static like the block universe, then I have not the slightest idea of how two subsystems can experience time if you think of one as a clock and the other as an observer watching the clock. In case of the block universe I can totally understand how everything is deterministic and all times exist in parallel, but I never understood what mechanism picks out the slice that is my or your now. [1] http://theoreticalminimum.com http://theoreticalminimum.com
- chrischen 10y ago
  The frozen 4 dimensional object creates a projection of the other dimensions, including time and relative time. Time is hen a subsystem within the system. Their analogy, aptly for HN, is that of computer code which is in one dimension interpreted amd executed linearly, being able to project something like a 3D game. Think something like minecraft's source code being interpreted and creating an infinite procedurally generated world that constantly evolves with an in game timer despite the source code still being static. Quantum mechanics, the mechanics governing the source code could still make it timeless and indeterministic, but output of the code (the projection), is still bound by the in game physics and timer much like time in general relativity.
  - SomeStupidPoint 10y ago
    But is their proposal that, like Minecraft, we can detect properties of the background execution through apparent non-determinism (or other mechanism)? Cause Minecraft actually has two detectable, distinct notions of time: a game tick and update ordering of the underlying computation. Minecraft can even be viewed as having quantum effects caused by a universal nonlocal variable, Bohmian mechanics styles, with the apparent non-determinism resulting from that non-local hidden variable interacting woth the update engine. (The hidden variable is the ordering of the updates in real time rather than MC time.) I always thought we should teach physics with Minecraft, but the engine sucks too much for me to seriously recommend it. Ed: Imgur album on detecting MC update ordering via redstone; pictures somehow got out of order. http://imgur.com/a/e9YKi http://imgur.com/a/e9YKi
- raattgift 10y ago
  The article's a bit scattered but its central theme seems to be the Problem of Time. There's a wikipedia article on that, of course, and it's not so bad. https://en.wikipedia.org/wiki/Problem_of_time https://en.wikipedia.org/wiki/Problem_of_time The structure of the the paragraph you quoted and the one following it is similar to the first two long paragraphs under the "Proposed solutions to the problem of time" section of the wikipedia article. There is a bit more here: https://en.wikipedia.org/wiki/Canonical_quantum_gravity#The_problem_of_time https://en.wikipedia.org/wiki/Canonical_quantum_gravity#The_... and a technical overview in the first section of https://arxiv.org/abs/1009.2157 https://arxiv.org/abs/1009.2157 I wrote "seems", because I found it hard to focus on an article whose second paragraph is a grotesque of General Relativity and in subsequent paragraphs brings in a not especially clearly related set of links to the author's previous articles on the recent Verlinde paper on emergent gravity and the amplituhedron, among others. :-(
grabcocque 10y ago
That sounds like one of those fake papers generated by a Markov chain that sometimes gets accepted into reputable physics journals when a researcher wants to prove that modern physics has gone a bit... silly.
coldcode 10y ago
Reading this made me think of the question of whether our universe is actually a simulation.
- danbruc 10y ago
  The universe as a simulation is one of those ideas that sound interesting at first but are actually pretty useless the longer you think about them. There is for example also solipsism, the idea that only you exist and the rest of the world is just in your mind, or Last Thursdayism, the idea that the world came into existence last Thursday with all our minds filled with fake memories of the time before last Thursday. Those ideas put you - if you believe in them - in a spot where you can learn nothing about the world. There is nothing that allows you, even in principle, to figure out if there was a universe before last Thursday or if there is a world outside of your mind. There is not much more that would allow you to figure out whether the universe is a simulation or not. Many people talking about that idea suggest to look for indicators of limited computing resources, rounding error or whatnot. But that is totally misguided in my opinion because it presupposes that you know what at least one of the two options, the universe or a simulation of it, looks like. Who is to say that the laws of physics of the universe do not look like they are using floating numbers? Who is to say that a simulation of a universe may have to deal with limited resources? The outside world could be so foreign to us, time, space and laws of physics may not even be a thing there. Realistically the only way to distinguish between a real universe and simulation would be if the owner of the simulation told you in a pretty obvious way like with small serial numbers printed on all the electrons. I kind of understand that people don't like the idea of not even seriously considering a substantial class of ideas that are not in conflict with what we see and that could therefore actually be correct. It also seems not very satisfying to throw some probabilities at them, last Thursday, unlikely, a simulation, totally reasonable thing to do, pretty likely. In the end, if it is not falsifiable, just ignore it and have one fewer things to worry about.
  - RobertoG 10y ago
    I don't think universe as a simulation is epistemologically equivalent to solipsism or this "Last Thursdayism" thing (love the name, by the way). Solipsism is meaningless. What it means to say that there is nothing outside of your mind? Surely you can't predict or control what is going to happen to your experience, so how can you say that everything is "inside" your mind? what that even means? And if you are a solipsist and wanted to predict what is going to happen next, what you should be doing? You should start to associate causation to patterns in your experience, and observe carefully that patterns. Those patterns behaviour are not decided by you, so, by definition, they are outside your mind. "Last Thursdayism" have the same problem that all the religions: OK, maybe it's true, but.. why do you choose Thursday, why not Monday? why not one minute ago? It's totally arbitrary. Simulation argument has serious experimental issues, but I would not discard it yet. Maybe we can think of something.
abakker 10y ago
I have a question as a non physics/math person: We assume that light speed is a constant, but we also know that speed is measured as distance/time. We know that in the presence of high gravity (or proximity to very high mass), or very high speed, we experience time dilation, and that perspective and location matters WRT actual time. So, how do we assume that light speed is constant? Does the distance moved change relative to the proximity to mass, and the time unit vary in value as well? If not, do we just assume that the lack of proximity to mass in between galaxies doesn't change the behavior of light when we say that the proximity to black holes does?
- M_Grey 10y ago
  It's constant within a given frame of reference.
  - abakker 10y ago
    So, really it isn't a universal constant, then? I guess the makes sense. Sub question - if photons are light particles, moving at the speed of light, then do they actually experience time dilation themselves? i.e. from the frame of reference of the photon, is the local time for the photon ticking away in one frame, while the outside world appears to move much much faster? Or, relative to us is time on the photon moving very slowly?
    - akeruu 10y ago
      Actually for a photon time is so dilated, it does not exists anymore. From its perspective everything "happens" at the same "time". There are quite a few answers for this : https://www.quora.com/Does-a-photon-experience-time https://www.quora.com/Does-a-photon-experience-time https://www.reddit.com/r/askscience/comments/2slx5n/how_is_it_that_photons_dont_experience_time https://www.reddit.com/r/askscience/comments/2slx5n/how_is_i... https://www.reddit.com/r/askscience/comments/2t8qr1/do_light_photons_experience_time_dilation https://www.reddit.com/r/askscience/comments/2t8qr1/do_light...
    - andrepd 10y ago
jcoffland 10y ago
It is man's fate to forever believe that the universe is a complex composite of his latest discovered technology.
cm127 10y ago
I just ran into an interesting theory for Gravity: Subquantum Kinetics It's featured in Dr. Paul LaViolette's book The Secrets of Anti-gravity Propulsion, and there's a really interesting section that challenges the Standard Model in Quantum Mechanics called the Model G in Subquantum Kinetics. The idea is that matter is born out of what's called the Ether, the origin to all energy. The positive and negative electrical energy sums a positive or negative charge with a "spin" -- which also yields positive or negative gravity. This is a video that simulates matter being created out of the Ether: https://www.youtube.com/watch?v=jPLNfgxD2ic https://www.youtube.com/watch?v=jPLNfgxD2ic Supposedly, it's been rejected by other researchers because it goes against Einstein's theory of Relativity and the Big Bang, -- however, there's been rumors the Model G is being used by underground, black-project sites like Lockheed-Martin and Boeing (e.g. Northrop Grumman's B-2 Spirit stealth bomber.) Here's a lecture by Dr. Paul LaViolette featuring content from his book: https://www.youtube.com/watch?v=ifEgGMFK-VU https://www.youtube.com/watch?v=ifEgGMFK-VU - (warning: audio isn't great)
- wfunction 10y ago
  I don't care as much for the Big Bang, but for it to go against relativity seems a bit hard to swallow.
  - cm127 10y ago
    Yeah, no kidding... because that would also challenge the "Laws" of Thermodynamics, too: Free Energy from the Ether. *I have to put that in quotes because it's not actually a law, right? You shouldn't go to jail for breaking it.
- danbruc 10y ago
  That is crackpottery, pseudoscience and conspiracy theory at 11. Forget about it if you are interested in any serious conversation about physics.
  - deleted 10y ago
    [deleted]

114 points by CarolineW 10y ago

> "The whole state of system-plus-clock doesn’t change in time" How is this different from the classical view of the universe as deterministic? In a Newtonian universe, all future states can be determined from the current state, so the universe can be seen as a "frozen" 4-dimensional object.

contravariant 10y ago
Well, it's similar, except in this case the universe wouldn't have to be deterministic. As far as I can tell there's no law saying you can't have two clocks, where observers entangled with either observe a different sequence of events.
- danbruc 10y ago
  I think Leonard Susskind touched this point in one of the theoretical minimum lectures [1] but I really can not tell which one. While we can quite easily imagine adding and removing space dimensions, nobody ever figured out what it would mean to have more than one time dimension. I think he also talked about time traveling in this context - you can go back and forth in space because there is more than one space dimension and so you can turn around and walk the other direction. Unless you instantaneously flip around that is nothing you can do with just one dimension like our one time dimension. But those things aside, the order of spatially separated events is not in general well defined if you take general relativity into account. There is a causal ordering all observers agree on but that still allows some observers to see some events in one order or another. I skimmed the papers linked in the article but they are quite a bit above what I can immediately grasp. But one thing I noticed was that they talked a lot about observable changes, so now I am wondering whether the entire system they are talking about is really static or whether we are just unable to observe the evolution of the entire system? If it is really static like the block universe, then I have not the slightest idea of how two subsystems can experience time if you think of one as a clock and the other as an observer watching the clock. In case of the block universe I can totally understand how everything is deterministic and all times exist in parallel, but I never understood what mechanism picks out the slice that is my or your now. [1] http://theoreticalminimum.com http://theoreticalminimum.com
chrischen 10y ago
The frozen 4 dimensional object creates a projection of the other dimensions, including time and relative time. Time is hen a subsystem within the system. Their analogy, aptly for HN, is that of computer code which is in one dimension interpreted amd executed linearly, being able to project something like a 3D game. Think something like minecraft's source code being interpreted and creating an infinite procedurally generated world that constantly evolves with an in game timer despite the source code still being static. Quantum mechanics, the mechanics governing the source code could still make it timeless and indeterministic, but output of the code (the projection), is still bound by the in game physics and timer much like time in general relativity.
- SomeStupidPoint 10y ago
  But is their proposal that, like Minecraft, we can detect properties of the background execution through apparent non-determinism (or other mechanism)? Cause Minecraft actually has two detectable, distinct notions of time: a game tick and update ordering of the underlying computation. Minecraft can even be viewed as having quantum effects caused by a universal nonlocal variable, Bohmian mechanics styles, with the apparent non-determinism resulting from that non-local hidden variable interacting woth the update engine. (The hidden variable is the ordering of the updates in real time rather than MC time.) I always thought we should teach physics with Minecraft, but the engine sucks too much for me to seriously recommend it. Ed: Imgur album on detecting MC update ordering via redstone; pictures somehow got out of order. http://imgur.com/a/e9YKi http://imgur.com/a/e9YKi
raattgift 10y ago
The article's a bit scattered but its central theme seems to be the Problem of Time. There's a wikipedia article on that, of course, and it's not so bad. https://en.wikipedia.org/wiki/Problem_of_time https://en.wikipedia.org/wiki/Problem_of_time The structure of the the paragraph you quoted and the one following it is similar to the first two long paragraphs under the "Proposed solutions to the problem of time" section of the wikipedia article. There is a bit more here: https://en.wikipedia.org/wiki/Canonical_quantum_gravity#The_problem_of_time https://en.wikipedia.org/wiki/Canonical_quantum_gravity#The_... and a technical overview in the first section of https://arxiv.org/abs/1009.2157 https://arxiv.org/abs/1009.2157 I wrote "seems", because I found it hard to focus on an article whose second paragraph is a grotesque of General Relativity and in subsequent paragraphs brings in a not especially clearly related set of links to the author's previous articles on the recent Verlinde paper on emergent gravity and the amplituhedron, among others. :-(

grabcocque 10y ago

That sounds like one of those fake papers generated by a Markov chain that sometimes gets accepted into reputable physics journals when a researcher wants to prove that modern physics has gone a bit... silly.

coldcode 10y ago

Reading this made me think of the question of whether our universe is actually a simulation.

danbruc 10y ago
The universe as a simulation is one of those ideas that sound interesting at first but are actually pretty useless the longer you think about them. There is for example also solipsism, the idea that only you exist and the rest of the world is just in your mind, or Last Thursdayism, the idea that the world came into existence last Thursday with all our minds filled with fake memories of the time before last Thursday. Those ideas put you - if you believe in them - in a spot where you can learn nothing about the world. There is nothing that allows you, even in principle, to figure out if there was a universe before last Thursday or if there is a world outside of your mind. There is not much more that would allow you to figure out whether the universe is a simulation or not. Many people talking about that idea suggest to look for indicators of limited computing resources, rounding error or whatnot. But that is totally misguided in my opinion because it presupposes that you know what at least one of the two options, the universe or a simulation of it, looks like. Who is to say that the laws of physics of the universe do not look like they are using floating numbers? Who is to say that a simulation of a universe may have to deal with limited resources? The outside world could be so foreign to us, time, space and laws of physics may not even be a thing there. Realistically the only way to distinguish between a real universe and simulation would be if the owner of the simulation told you in a pretty obvious way like with small serial numbers printed on all the electrons. I kind of understand that people don't like the idea of not even seriously considering a substantial class of ideas that are not in conflict with what we see and that could therefore actually be correct. It also seems not very satisfying to throw some probabilities at them, last Thursday, unlikely, a simulation, totally reasonable thing to do, pretty likely. In the end, if it is not falsifiable, just ignore it and have one fewer things to worry about.
- RobertoG 10y ago
  I don't think universe as a simulation is epistemologically equivalent to solipsism or this "Last Thursdayism" thing (love the name, by the way). Solipsism is meaningless. What it means to say that there is nothing outside of your mind? Surely you can't predict or control what is going to happen to your experience, so how can you say that everything is "inside" your mind? what that even means? And if you are a solipsist and wanted to predict what is going to happen next, what you should be doing? You should start to associate causation to patterns in your experience, and observe carefully that patterns. Those patterns behaviour are not decided by you, so, by definition, they are outside your mind. "Last Thursdayism" have the same problem that all the religions: OK, maybe it's true, but.. why do you choose Thursday, why not Monday? why not one minute ago? It's totally arbitrary. Simulation argument has serious experimental issues, but I would not discard it yet. Maybe we can think of something.

abakker 10y ago

I have a question as a non physics/math person: We assume that light speed is a constant, but we also know that speed is measured as distance/time. We know that in the presence of high gravity (or proximity to very high mass), or very high speed, we experience time dilation, and that perspective and location matters WRT actual time. So, how do we assume that light speed is constant? Does the distance moved change relative to the proximity to mass, and the time unit vary in value as well? If not, do we just assume that the lack of proximity to mass in between galaxies doesn't change the behavior of light when we say that the proximity to black holes does?

M_Grey 10y ago
It's constant within a given frame of reference.
- abakker 10y ago
  So, really it isn't a universal constant, then? I guess the makes sense. Sub question - if photons are light particles, moving at the speed of light, then do they actually experience time dilation themselves? i.e. from the frame of reference of the photon, is the local time for the photon ticking away in one frame, while the outside world appears to move much much faster? Or, relative to us is time on the photon moving very slowly?
  - akeruu 10y ago
    Actually for a photon time is so dilated, it does not exists anymore. From its perspective everything "happens" at the same "time". There are quite a few answers for this : https://www.quora.com/Does-a-photon-experience-time https://www.quora.com/Does-a-photon-experience-time https://www.reddit.com/r/askscience/comments/2slx5n/how_is_it_that_photons_dont_experience_time https://www.reddit.com/r/askscience/comments/2slx5n/how_is_i... https://www.reddit.com/r/askscience/comments/2t8qr1/do_light_photons_experience_time_dilation https://www.reddit.com/r/askscience/comments/2t8qr1/do_light...
  - andrepd 10y ago

jcoffland 10y ago

It is man's fate to forever believe that the universe is a complex composite of his latest discovered technology.

cm127 10y ago

I just ran into an interesting theory for Gravity: Subquantum Kinetics It's featured in Dr. Paul LaViolette's book The Secrets of Anti-gravity Propulsion, and there's a really interesting section that challenges the Standard Model in Quantum Mechanics called the Model G in Subquantum Kinetics. The idea is that matter is born out of what's called the Ether, the origin to all energy. The positive and negative electrical energy sums a positive or negative charge with a "spin" -- which also yields positive or negative gravity. This is a video that simulates matter being created out of the Ether: https://www.youtube.com/watch?v=jPLNfgxD2ic https://www.youtube.com/watch?v=jPLNfgxD2ic Supposedly, it's been rejected by other researchers because it goes against Einstein's theory of Relativity and the Big Bang, -- however, there's been rumors the Model G is being used by underground, black-project sites like Lockheed-Martin and Boeing (e.g. Northrop Grumman's B-2 Spirit stealth bomber.) Here's a lecture by Dr. Paul LaViolette featuring content from his book: https://www.youtube.com/watch?v=ifEgGMFK-VU https://www.youtube.com/watch?v=ifEgGMFK-VU - (warning: audio isn't great)

wfunction 10y ago
I don't care as much for the Big Bang, but for it to go against relativity seems a bit hard to swallow.
- cm127 10y ago
  Yeah, no kidding... because that would also challenge the "Laws" of Thermodynamics, too: Free Energy from the Ether. *I have to put that in quotes because it's not actually a law, right? You shouldn't go to jail for breaking it.
danbruc 10y ago
That is crackpottery, pseudoscience and conspiracy theory at 11. Forget about it if you are interested in any serious conversation about physics.
- deleted 10y ago
  [deleted]