7 ms·

The crisis inside the physics of time

49 points by dnetesn 8y ago

chopin 8y ago
If time is an emergent property shouldn't space be as well?
- TheOtherHobbes 8y ago
  Yes. https://en.wikipedia.org/wiki/Loop_quantum_gravity https://en.wikipedia.org/wiki/Loop_quantum_gravity
- lostmsu 8y ago
  What is an "emergent property"?
  - cobbzilla 8y ago
    A particle can have intrinsic properties like charge and spin that are directly measurable attributes of the particle itself. Other properties like temperature and even size are called emergent because they’re not truly fundamental but rather “emerge” when we observe the particle in some system of other particles. We can measure emergent properties but they’re more akin to properties of the system as a whole than some specific attribute of one or more particle. I hope I made sense, it can be confusing.
    - doitLP 8y ago
      Thanks for the simple answer. Wouldn’t temperature still be intrinsic? Assuming there’s only one particle in the universe it would still have a temperature right? Yes it’s not in relation to anything but then neither would charge be in our one particle example. I am the merest amateur so bear with me.
      - Pica_soO 8y ago
        You observe the particles motion- aka its position- the jiggly motion speed and distance is the temperature. https://en.wikipedia.org/wiki/Uncertainty_principle https://en.wikipedia.org/wiki/Uncertainty_principle But measuring it we can only be certain of position or momentum, never of both. Thus we can not reliably measure its temperature. We can measure the interaction with other particles though- but that only gives us a rough estimation on the whole measurement system.
      - jessedhillon 8y ago
        When you say "the gas in this container is 100'C," that is a statement about the vibration and motion of the molecules which constitute that gas. To say something is hot or cold is to say that the average molecule in that substance has or lacks kinetic energy. Therefore, temperature is a description of the kinetic motion in a system, and so it emerges from position and velocity. Source: also an amateur
      - fjdndidjd 8y ago
        No, it would only have a state. In classical physics the state would be its Kinetic energy. In QE it would be.... well it’s state. T is only defined in an ensemble of other particles, i.e. when you can no longer keep track the particles and it’s necessary to use averages. Then the T is the average kinetic energy. Furthermore T is only technically defined in thermodynamic equilibrium, so nothing happens at all (otherwise you’re not in equilibrium). Finally, you can have “negative temperature” I.e T < absolute zero, if you have population inversion (I.e. a lasing cavity) which is really an abuse of nomenclature since population inversion cannot occur in equilibrium (and therefore T is not defined). However, the Boltzmann (?) equations have pop inv. only when the the T parameter is negative (even though pop inv. is the “hotter” than infinitely hot). All this to say that 1. T is not defined for a single particle 2. T is not well defined for very many systems at all!
      - cygx 8y ago
        Assuming there’s only one particle in the universe it would still have a temperature right? Not really. One (not quite universal) way to think of temperature is as a measure of average energy per degree of freedom (linear motion, rotational motion, vibration, ...). This assumes a system where random interactions spread the energy throughout, exciting all these different energy states. In contrast, when you take your single particle and add some energy, there's no spreading of anything and no excitation of other degrees of freedom - the system doesn't 'thermalize', and no average emerges. Note that there are other ways to think about this, eg in terms of inverse temperature (thermodynamic beta), which is essentially a measure of phase space growth under addition of energy. Your single particle system will only exist in a single state no matter how much energy you add, so it's not really useful to take a thermodynamic approach...
nyc111 8y ago
> We don’t see, hear, smell, touch, or taste time. And yet we somehow measure it. I’m not sure we measure time. What we measure is distance. What is called a clock is always an oscillator. This oscillator is used as unit distance. Then this unit distance is counted. The distances are converted with appropriate choice of units for counting. I don’t see how comparing two distances can be called “meausuring time”. Maybe I’m missing something here, I don’t know. For instance, we measure the period of a pendulum. This is distance not time. Can anyone clarify how “time” enter in mesurement of distances? Thanks.
- GW150914 8y ago
  To be fair, you keep using words like “measure” and “compare” which implies an interval or something more than space right there. Without time how can you take a measurement or make a comparison? If I throw a ball to you and we both nite a change in distance occurring, over what is that change occurring? It’s baked right into something as basic as the formula for velocity = d/t. Time is difficult to define without reference to space, and measurements of the evolution of a system, which may be the problem I guess? What would time be in a purely static, closed system? What would time be after the complete heat death of the universe? The article seems to suggest But as the balloon blows up, the curvature of its surface grows shallower and shallower. “The changing geometry,” explains Kucha, “allows you to see that you are at one instant of time rather than another.” In other words, it can function as a clock. Then goes on to point out that any given clock has limitations when applied to different regimes. I’m a little concerned though, because the author seems to be under the somewhat common misapprehension that the Planck scale is some kind of limit on quantization, so the whole article might be... well... not fantastic. It is theorized thst below that you might get a spacetime foam, but the truth is that may or may not be the case, and spacetime may not even be quantized. As in the case of so much regarding the base nature of spacetime, and the union of QM and GTR, we just don’t know.
  - nyc111 8y ago
    > velocity = d/t. Thanks for the example. Here d/t is a proportionality. Or at least half of a proportionality. But the terms of a proportianality must be of the same type. And here they both must be distance. Because we compare what is measured (distance d) to the unit distance, here denoted with the letter t. I'm not denying the existence of time. I'm saying that we are not measuring time but comparing two distances. You are saying that if time did not exist the oscillator would not oscillate. Maybe. But we are not measuring time but comparing two distances. We are counting how many unit distance there are in the distance to be measured. > Time is difficult to define without reference to space, and measurements of the evolution of a system, which may be the problem I guess? Same with the concept of space. What you call "space" is the distance between two points. > What would time be in a purely static, closed system? The system maybe static but this does not mean time is not passing. Time is still passing but there is no oscillator. Without an oscillator to define the unit distance you cannot count the distance to be measured. > What would time be after the complete heat death of the universe? This is the philosophic time and it is independent of our ability to measure it. So I'm not talking about the existence of time but about the measurement of time. Because I observe that what we do is compare two distances. I don't understand how comparing two distances is interpreted as "measuring time." I also observe that no one else have any doubts that what is measured is time. So the problem must with me:)
jeromebaek 8y ago
Isn't information theory a new way of thinking about time? Time, as change, is entropy, which, under certain formulations, is equal to uncomputability. In this way we could think of the flow of time as the march of some arbitrary computation which cannot be computed in advance, that is, ignoring time.
- adrianN 8y ago
  What formulations equate entropy with uncomputability? That doesn't seem to make much sense to me. Do you have a reference that I could read?
  - jeromebaek 8y ago
    It is somewhat of a leap. However there are sensible formulations where it is pointed out that entropy is information, and "amount of uncomputability" is information in the sense that to know the output of an arbitrary Turing machine (which outputs either 0 or 1) one needs exactly one bit of information.
kaycebasques 8y ago
Side note: I love Nautilus’s design, and their overall mission.
- byron_fast 8y ago
  I really wish they would ship me the magazines I paid for, though.
  - bcraven 8y ago
    They're desperately short of money, did you see that email from them a few weeks ago?
    - byron_fast 8y ago
      All my emails from them have been about money.
lostmsu 8y ago
"the subatomic rules of quantum mechanics, which continue to work within a universal, Newtonian time frame" Wait, what? QM is based on special relativity. It's problem with general relativity compatibility only remotely touches time, if at all.
- thiagotomei 8y ago
  Quantum mechanics is not based on special relativity at all. In fact, you can show that you can write Hamilton-Jacobi's equation (purely Classical Mechanics) as a limiting case of Schrodinger's equation. It is true that QM came about initially as a problem with the quantization of the electromagnetic field, but that was a bit of an accident.
- tobmlt 8y ago
  In vanilla QM, time is Newtonian - taken as a given, independent variable. You can "tack on time dependence" as it were. (Special) relativistic QM becomes quantum field theory (well, first Schrodinger's eq becomes the Klein Gordon equation, then Dirac stares into the fire and anti matter pops out... etc...). I will put this here for others who may not know about these distinctions.
- T-A 8y ago
  > QM is based on special relativity Not at all. QM makes no assumption about spacetime; you can formulate it without even mentioning space and time. As Scott Aarons puts it [1]: Basically, quantum mechanics is the operating system that other physical theories run on as application software (with the exception of general relativity, which hasn't yet been successfully ported to this particular OS). Historically, QM was developed in the context of Newtonian space + time; then came relativistic quantum mechanics, which is essentially about a fixed number of particles in a special-relativistic spacetime; and then came quantum field theory, where particles can be created and destroyed freely (because they are just excitations of quantum fields). [1] https://www.scottaaronson.com/democritus/lec9.html https://www.scottaaronson.com/democritus/lec9.html
  - nonbel 8y ago
carapace 8y ago
> This is a picture so different from the world of classical physics that even Einstein railed against its indeterminacy. He declared that he could never believe that God would play dice with the world. This is a common misconception. Einstein kept pointing out that Quantum non-locality and the Relativistic limit of velocity at c are incompatible. Bohr ignored or misunderstood him and led everyone on a merry jaunt into irrelevancies. Now that non-locality is established we will hopefully see some new physics in this century.
- enkid 8y ago
  Non-locality has nothing to do with randomness. How does the quote apply to that argument?
lisper 8y ago
I find that the quantum-information-theoretical interpretation of QM provides a very satisfying (to me) account of what time is. It's not a fundamental physical phenomenon, it's an emergent property (or perhaps it might be better called a side-effect) of decoherence, which leads to the emergence of classical reality from the quantum wave function: http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
eafkuor 8y ago
> More promising as a quantum clock is the geometry of space itself: monitoring spacetime’s changing curvature as the infant universe expands or a black hole forms. Kucha surmises that such a property might still be measurable in the extreme conditions of quantum gravity. The expanding cosmos offers the simplest example of this scheme. Imagine the tiny infant universe as an inflating balloon. Initially, its surface bends sharply around. But as the balloon blows up, the curvature of its surface grows shallower and shallower. “The changing geometry,” explains Kucha, “allows you to see that you are at one instant of time rather than another.” In other words, it can function as a clock. Brilliantly said, wow!

49 points by dnetesn 8y ago

chopin 8y ago

If time is an emergent property shouldn't space be as well?

TheOtherHobbes 8y ago
Yes. https://en.wikipedia.org/wiki/Loop_quantum_gravity https://en.wikipedia.org/wiki/Loop_quantum_gravity
lostmsu 8y ago
What is an "emergent property"?
- cobbzilla 8y ago
  A particle can have intrinsic properties like charge and spin that are directly measurable attributes of the particle itself. Other properties like temperature and even size are called emergent because they’re not truly fundamental but rather “emerge” when we observe the particle in some system of other particles. We can measure emergent properties but they’re more akin to properties of the system as a whole than some specific attribute of one or more particle. I hope I made sense, it can be confusing.
  - doitLP 8y ago
    Thanks for the simple answer. Wouldn’t temperature still be intrinsic? Assuming there’s only one particle in the universe it would still have a temperature right? Yes it’s not in relation to anything but then neither would charge be in our one particle example. I am the merest amateur so bear with me.
    - Pica_soO 8y ago
      You observe the particles motion- aka its position- the jiggly motion speed and distance is the temperature. https://en.wikipedia.org/wiki/Uncertainty_principle https://en.wikipedia.org/wiki/Uncertainty_principle But measuring it we can only be certain of position or momentum, never of both. Thus we can not reliably measure its temperature. We can measure the interaction with other particles though- but that only gives us a rough estimation on the whole measurement system.
    - jessedhillon 8y ago
      When you say "the gas in this container is 100'C," that is a statement about the vibration and motion of the molecules which constitute that gas. To say something is hot or cold is to say that the average molecule in that substance has or lacks kinetic energy. Therefore, temperature is a description of the kinetic motion in a system, and so it emerges from position and velocity. Source: also an amateur
    - fjdndidjd 8y ago
      No, it would only have a state. In classical physics the state would be its Kinetic energy. In QE it would be.... well it’s state. T is only defined in an ensemble of other particles, i.e. when you can no longer keep track the particles and it’s necessary to use averages. Then the T is the average kinetic energy. Furthermore T is only technically defined in thermodynamic equilibrium, so nothing happens at all (otherwise you’re not in equilibrium). Finally, you can have “negative temperature” I.e T < absolute zero, if you have population inversion (I.e. a lasing cavity) which is really an abuse of nomenclature since population inversion cannot occur in equilibrium (and therefore T is not defined). However, the Boltzmann (?) equations have pop inv. only when the the T parameter is negative (even though pop inv. is the “hotter” than infinitely hot). All this to say that 1. T is not defined for a single particle 2. T is not well defined for very many systems at all!
    - cygx 8y ago
      Assuming there’s only one particle in the universe it would still have a temperature right? Not really. One (not quite universal) way to think of temperature is as a measure of average energy per degree of freedom (linear motion, rotational motion, vibration, ...). This assumes a system where random interactions spread the energy throughout, exciting all these different energy states. In contrast, when you take your single particle and add some energy, there's no spreading of anything and no excitation of other degrees of freedom - the system doesn't 'thermalize', and no average emerges. Note that there are other ways to think about this, eg in terms of inverse temperature (thermodynamic beta), which is essentially a measure of phase space growth under addition of energy. Your single particle system will only exist in a single state no matter how much energy you add, so it's not really useful to take a thermodynamic approach...

nyc111 8y ago

> We don’t see, hear, smell, touch, or taste time. And yet we somehow measure it. I’m not sure we measure time. What we measure is distance. What is called a clock is always an oscillator. This oscillator is used as unit distance. Then this unit distance is counted. The distances are converted with appropriate choice of units for counting. I don’t see how comparing two distances can be called “meausuring time”. Maybe I’m missing something here, I don’t know. For instance, we measure the period of a pendulum. This is distance not time. Can anyone clarify how “time” enter in mesurement of distances? Thanks.

GW150914 8y ago
To be fair, you keep using words like “measure” and “compare” which implies an interval or something more than space right there. Without time how can you take a measurement or make a comparison? If I throw a ball to you and we both nite a change in distance occurring, over what is that change occurring? It’s baked right into something as basic as the formula for velocity = d/t. Time is difficult to define without reference to space, and measurements of the evolution of a system, which may be the problem I guess? What would time be in a purely static, closed system? What would time be after the complete heat death of the universe? The article seems to suggest But as the balloon blows up, the curvature of its surface grows shallower and shallower. “The changing geometry,” explains Kucha, “allows you to see that you are at one instant of time rather than another.” In other words, it can function as a clock. Then goes on to point out that any given clock has limitations when applied to different regimes. I’m a little concerned though, because the author seems to be under the somewhat common misapprehension that the Planck scale is some kind of limit on quantization, so the whole article might be... well... not fantastic. It is theorized thst below that you might get a spacetime foam, but the truth is that may or may not be the case, and spacetime may not even be quantized. As in the case of so much regarding the base nature of spacetime, and the union of QM and GTR, we just don’t know.
- nyc111 8y ago
  > velocity = d/t. Thanks for the example. Here d/t is a proportionality. Or at least half of a proportionality. But the terms of a proportianality must be of the same type. And here they both must be distance. Because we compare what is measured (distance d) to the unit distance, here denoted with the letter t. I'm not denying the existence of time. I'm saying that we are not measuring time but comparing two distances. You are saying that if time did not exist the oscillator would not oscillate. Maybe. But we are not measuring time but comparing two distances. We are counting how many unit distance there are in the distance to be measured. > Time is difficult to define without reference to space, and measurements of the evolution of a system, which may be the problem I guess? Same with the concept of space. What you call "space" is the distance between two points. > What would time be in a purely static, closed system? The system maybe static but this does not mean time is not passing. Time is still passing but there is no oscillator. Without an oscillator to define the unit distance you cannot count the distance to be measured. > What would time be after the complete heat death of the universe? This is the philosophic time and it is independent of our ability to measure it. So I'm not talking about the existence of time but about the measurement of time. Because I observe that what we do is compare two distances. I don't understand how comparing two distances is interpreted as "measuring time." I also observe that no one else have any doubts that what is measured is time. So the problem must with me:)

jeromebaek 8y ago

Isn't information theory a new way of thinking about time? Time, as change, is entropy, which, under certain formulations, is equal to uncomputability. In this way we could think of the flow of time as the march of some arbitrary computation which cannot be computed in advance, that is, ignoring time.

adrianN 8y ago
What formulations equate entropy with uncomputability? That doesn't seem to make much sense to me. Do you have a reference that I could read?
- jeromebaek 8y ago
  It is somewhat of a leap. However there are sensible formulations where it is pointed out that entropy is information, and "amount of uncomputability" is information in the sense that to know the output of an arbitrary Turing machine (which outputs either 0 or 1) one needs exactly one bit of information.

kaycebasques 8y ago

Side note: I love Nautilus’s design, and their overall mission.

byron_fast 8y ago
I really wish they would ship me the magazines I paid for, though.
- bcraven 8y ago
  They're desperately short of money, did you see that email from them a few weeks ago?
  - byron_fast 8y ago
    All my emails from them have been about money.

lostmsu 8y ago

"the subatomic rules of quantum mechanics, which continue to work within a universal, Newtonian time frame" Wait, what? QM is based on special relativity. It's problem with general relativity compatibility only remotely touches time, if at all.

thiagotomei 8y ago
Quantum mechanics is not based on special relativity at all. In fact, you can show that you can write Hamilton-Jacobi's equation (purely Classical Mechanics) as a limiting case of Schrodinger's equation. It is true that QM came about initially as a problem with the quantization of the electromagnetic field, but that was a bit of an accident.
tobmlt 8y ago
In vanilla QM, time is Newtonian - taken as a given, independent variable. You can "tack on time dependence" as it were. (Special) relativistic QM becomes quantum field theory (well, first Schrodinger's eq becomes the Klein Gordon equation, then Dirac stares into the fire and anti matter pops out... etc...). I will put this here for others who may not know about these distinctions.
T-A 8y ago
> QM is based on special relativity Not at all. QM makes no assumption about spacetime; you can formulate it without even mentioning space and time. As Scott Aarons puts it [1]: Basically, quantum mechanics is the operating system that other physical theories run on as application software (with the exception of general relativity, which hasn't yet been successfully ported to this particular OS). Historically, QM was developed in the context of Newtonian space + time; then came relativistic quantum mechanics, which is essentially about a fixed number of particles in a special-relativistic spacetime; and then came quantum field theory, where particles can be created and destroyed freely (because they are just excitations of quantum fields). [1] https://www.scottaaronson.com/democritus/lec9.html https://www.scottaaronson.com/democritus/lec9.html
- nonbel 8y ago

carapace 8y ago

> This is a picture so different from the world of classical physics that even Einstein railed against its indeterminacy. He declared that he could never believe that God would play dice with the world. This is a common misconception. Einstein kept pointing out that Quantum non-locality and the Relativistic limit of velocity at c are incompatible. Bohr ignored or misunderstood him and led everyone on a merry jaunt into irrelevancies. Now that non-locality is established we will hopefully see some new physics in this century.

enkid 8y ago
Non-locality has nothing to do with randomness. How does the quote apply to that argument?

lisper 8y ago

I find that the quantum-information-theoretical interpretation of QM provides a very satisfying (to me) account of what time is. It's not a fundamental physical phenomenon, it's an emergent property (or perhaps it might be better called a side-effect) of decoherence, which leads to the emergence of classical reality from the quantum wave function: http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...

eafkuor 8y ago

> More promising as a quantum clock is the geometry of space itself: monitoring spacetime’s changing curvature as the infant universe expands or a black hole forms. Kucha surmises that such a property might still be measurable in the extreme conditions of quantum gravity. The expanding cosmos offers the simplest example of this scheme. Imagine the tiny infant universe as an inflating balloon. Initially, its surface bends sharply around. But as the balloon blows up, the curvature of its surface grows shallower and shallower. “The changing geometry,” explains Kucha, “allows you to see that you are at one instant of time rather than another.” In other words, it can function as a clock. Brilliantly said, wow!