5 ms·

Probably biased by the physicist I mostly listen to because they have a lot of lectures and talks online, Leonard Susskind and Nima Arkani-Hamed. The idea that

by danbruc 11y ago

Probably biased by the physicist I mostly listen to because they have a lot of lectures and talks online, Leonard Susskind and Nima Arkani-Hamed. The idea that space is quantized is pretty unlikely to be true because of relativity, i.e. because of length contraction the size of those space quanta depends on the motion of the observer. So if you find those space quanta have a specific size in your reference frame I can just go to a reference frame moving relative to yours and I will see length contracted space quanta and we will therefore disagree on their size. It is actually believed that neither space nor time are fundamental but that they emerge from something more fundamental because spacetime, relativity and quantum physics taken together are not really compatible, see for example "Space-time is doomed. What replaces it?" [1]. Susskind explores the idea that entanglement is what holds space together under the name "ER = EPR" [2]. [1] http://www.cornell.edu/video/nima-arkani-hamed-spacetime-is-doomed http://www.cornell.edu/video/nima-arkani-hamed-spacetime-is-... [2] https://youtube.com/watch?v=OBPpRqxY8Uw https://youtube.com/watch?v=OBPpRqxY8Uw

mstade 11y ago
Great links, thank you very much!
- danbruc 11y ago
  You may also enjoy "The theoretical minimum" [1], a set of courses by Susskind covering physics from classical physics over relativity and quantum physics to string theory and cosmology. It's quite a bit beyond popular science and comes with math included but is still quite accessible if one is interested in some serious physics. [1] http://theoreticalminimum.com/ http://theoreticalminimum.com/
qrendel 11y ago
I'd be curious to know why the Planck length wouldn't be reference frame independent, though... (which may be explained in your links, but it will take some time to go through them).
- danbruc 11y ago
  Planck length, mass, time and so on are built from the most fundamental physical constants - the speed of light, the Planck constant and the gravitational constant - all of which are independent of the reference frame. But I don't think that is actually important. It has more to do with units, i.e. the concrete value of the speed of light is determined by the way we defined the meter and the second and the same holds for the other constants. But that has no physical content, the value 299,792,458 m/s is a historical accident and we could as well have ended up with 42 foo/bar. So what physicist do is just set those constants to one and get rid of those arbitrary numbers by choosing appropriatei units, e.g. the speed of light becomes one lightyear per year. Because nothing moves faster than the speed you can then also go further and measure velocities with a unitless value as a fraction of the speed of light, i.e. a number between zero and one, as it it appears in the equations of special relativity. So if you throw those constants together in relatively simple expressions like hG/c³ you again get one as the numerical value and here for example something with units of area, the Planck area. But there is also no real physical content in the Planck units beyond that. Because the laws of physics usually contain all the constants and values in small powers and with small factor like a Pi here and one half there those units define scales of length, time and so on where the quantities are all about one, nothing is ten to the twenty or minus thirty. So at that scales the laws of physics may show interesting behavior like (1 - v²/c²) becoming zero as v approaches c but that are also more like heuristics than hard truths. So to make a long story short, Planck units are frame independent because they are built from constants and they are nice to think in because many things are of order one - a black hole of one Planck mass has probably a horizon of one Planck area or four Pi Planck areas or something small like that - but they are not that special.
  - qrendel 11y ago
    But you seem to be claiming that spacetime is unlikely to be quantized because the Planck length or Planck time, i.e. the quanta of length in spacetime, would be contracted in different reference frames. Therefore Planck units of length/time wouldn't be frame invariant - they'd depend upon your inertial reference frame. So claiming Planck units are frame independent violates the idea that spacetime can't be quantized (at least for the reason you stated), since the quanta would experience relativistic contraction and dilation. The explanation seems to go against your original comment. Edit: I may have given the wrong impression - when I said "Planck units" I meant quanta of space, time, area, etc. Not "natural units" where all relevant physical constants are taken to equal one. It appears the term "Planck units" refers to one of multiple sets of natural units, of which I was not aware: https://en.wikipedia.org/wiki/Natural_units#Planck_units https://en.wikipedia.org/wiki/Natural_units#Planck_units
vanderZwan 11y ago
I always wondered if Boulet's pixel-dream is an accurate depiction of the issues you run into if space is quantized: http://www.bouletcorp.com/hidden/quantum-pixel/ http://www.bouletcorp.com/hidden/quantum-pixel/
- xlm1717 11y ago
  So the lengths would seem contracted because you are actually traversing fewer pixels. Your information is "compressed" into fewer pixels, and pixels being the unit of length, your length contracts. Hmm
contravariant 11y ago
That just means it can't be quantized in a lattice-like structure, it can still be 'foam'-like with some characteristic length. Also I'm not entirely convinced that space-time and quantum field theory are completely incompatible. General relativity and Yang-Mills (in the classical limit) fit beautifully together, it would be extremely odd if this becomes impossible when you add quantum physics.
- danbruc 11y ago
  I don't see how the exact structure could be important. No matter what it exactly looks like you would probably introduce a preferred reference frame, the frame in which the space quanta are at rest. Or looking at it the other way round, you would observe different lengths when you measure those space quanta from different reference frames no matter what the exact structure is but that contradicts the assumption that they are fundamental and have observer independent properties. Relativity does not allow for fundamental lengths because you can mix space and time by going to a different reference frame. You could maybe say that you have quanta of spacetime with some proper length, quanta that look like separation in space to some observers and like separation in time to others, but no idea if that makes any sense and it probably has not less problems than quanta of space. Quantum physics and special relativity fit nicely together, the standard model and its parts are relativistic quantum field theories. The problems arise when one tries to combine general relativity and quantum physics. When someone says quantum physics and relativity are incompatible he pretty surly means general relativity not special relativity.
  - contravariant 11y ago
    >I don't see how the exact structure could be important. No matter what it exactly looks like you would probably introduce a preferred reference frame, the frame in which the space quanta are at rest. Or looking at it the other way round, you would observe different lengths when you measure those space quanta from different reference frames no matter what the exact structure is but that contradicts the assumption that they are fundamental and have observer independent properties. Well, you can make a Lorentz invariant theory of an ideal gas, this gives you what's called a relativistic fluid. This can be formulated in a completely Lorentz invariant way, meaning that things like the temperature and the average speed of the particles will be the same for every observer. This also implies that the average distance between particles will be the same for all observers. Which combined with the average speed, will give you a characteristic time. So supposing you simply freeze a relativistic fluid in place you'll get an amorphous solid with a characteristic length scale and characteristic time scale. Which are both Lorentz invariant. >Quantum physics and special relativity fit nicely together, the standard model and its parts are relativistic quantum field theories. The problems arise when one tries to combine general relativity and quantum physics. When someone says quantum physics and relativity are incompatible he pretty surly means general relativity not special relativity. I was talking about general relativity; quantum field theories almost always use special relativity, it's trivial to combine those two. But it is even possible to extend (classical) Yang-Mills field theory from special to general relativity in a very natural way. But for some reason it's very difficult to 'quantize' the resulting field theory.

by danbruc 11y ago

mstade 11y ago

Great links, thank you very much!

danbruc 11y ago
You may also enjoy "The theoretical minimum" [1], a set of courses by Susskind covering physics from classical physics over relativity and quantum physics to string theory and cosmology. It's quite a bit beyond popular science and comes with math included but is still quite accessible if one is interested in some serious physics. [1] http://theoreticalminimum.com/ http://theoreticalminimum.com/

qrendel 11y ago

I'd be curious to know why the Planck length wouldn't be reference frame independent, though... (which may be explained in your links, but it will take some time to go through them).

danbruc 11y ago
Planck length, mass, time and so on are built from the most fundamental physical constants - the speed of light, the Planck constant and the gravitational constant - all of which are independent of the reference frame. But I don't think that is actually important. It has more to do with units, i.e. the concrete value of the speed of light is determined by the way we defined the meter and the second and the same holds for the other constants. But that has no physical content, the value 299,792,458 m/s is a historical accident and we could as well have ended up with 42 foo/bar. So what physicist do is just set those constants to one and get rid of those arbitrary numbers by choosing appropriatei units, e.g. the speed of light becomes one lightyear per year. Because nothing moves faster than the speed you can then also go further and measure velocities with a unitless value as a fraction of the speed of light, i.e. a number between zero and one, as it it appears in the equations of special relativity. So if you throw those constants together in relatively simple expressions like hG/c³ you again get one as the numerical value and here for example something with units of area, the Planck area. But there is also no real physical content in the Planck units beyond that. Because the laws of physics usually contain all the constants and values in small powers and with small factor like a Pi here and one half there those units define scales of length, time and so on where the quantities are all about one, nothing is ten to the twenty or minus thirty. So at that scales the laws of physics may show interesting behavior like (1 - v²/c²) becoming zero as v approaches c but that are also more like heuristics than hard truths. So to make a long story short, Planck units are frame independent because they are built from constants and they are nice to think in because many things are of order one - a black hole of one Planck mass has probably a horizon of one Planck area or four Pi Planck areas or something small like that - but they are not that special.
- qrendel 11y ago
  But you seem to be claiming that spacetime is unlikely to be quantized because the Planck length or Planck time, i.e. the quanta of length in spacetime, would be contracted in different reference frames. Therefore Planck units of length/time wouldn't be frame invariant - they'd depend upon your inertial reference frame. So claiming Planck units are frame independent violates the idea that spacetime can't be quantized (at least for the reason you stated), since the quanta would experience relativistic contraction and dilation. The explanation seems to go against your original comment. Edit: I may have given the wrong impression - when I said "Planck units" I meant quanta of space, time, area, etc. Not "natural units" where all relevant physical constants are taken to equal one. It appears the term "Planck units" refers to one of multiple sets of natural units, of which I was not aware: https://en.wikipedia.org/wiki/Natural_units#Planck_units https://en.wikipedia.org/wiki/Natural_units#Planck_units

vanderZwan 11y ago

I always wondered if Boulet's pixel-dream is an accurate depiction of the issues you run into if space is quantized: http://www.bouletcorp.com/hidden/quantum-pixel/ http://www.bouletcorp.com/hidden/quantum-pixel/

xlm1717 11y ago
So the lengths would seem contracted because you are actually traversing fewer pixels. Your information is "compressed" into fewer pixels, and pixels being the unit of length, your length contracts. Hmm

contravariant 11y ago

That just means it can't be quantized in a lattice-like structure, it can still be 'foam'-like with some characteristic length. Also I'm not entirely convinced that space-time and quantum field theory are completely incompatible. General relativity and Yang-Mills (in the classical limit) fit beautifully together, it would be extremely odd if this becomes impossible when you add quantum physics.

danbruc 11y ago
I don't see how the exact structure could be important. No matter what it exactly looks like you would probably introduce a preferred reference frame, the frame in which the space quanta are at rest. Or looking at it the other way round, you would observe different lengths when you measure those space quanta from different reference frames no matter what the exact structure is but that contradicts the assumption that they are fundamental and have observer independent properties. Relativity does not allow for fundamental lengths because you can mix space and time by going to a different reference frame. You could maybe say that you have quanta of spacetime with some proper length, quanta that look like separation in space to some observers and like separation in time to others, but no idea if that makes any sense and it probably has not less problems than quanta of space. Quantum physics and special relativity fit nicely together, the standard model and its parts are relativistic quantum field theories. The problems arise when one tries to combine general relativity and quantum physics. When someone says quantum physics and relativity are incompatible he pretty surly means general relativity not special relativity.
- contravariant 11y ago
  >I don't see how the exact structure could be important. No matter what it exactly looks like you would probably introduce a preferred reference frame, the frame in which the space quanta are at rest. Or looking at it the other way round, you would observe different lengths when you measure those space quanta from different reference frames no matter what the exact structure is but that contradicts the assumption that they are fundamental and have observer independent properties. Well, you can make a Lorentz invariant theory of an ideal gas, this gives you what's called a relativistic fluid. This can be formulated in a completely Lorentz invariant way, meaning that things like the temperature and the average speed of the particles will be the same for every observer. This also implies that the average distance between particles will be the same for all observers. Which combined with the average speed, will give you a characteristic time. So supposing you simply freeze a relativistic fluid in place you'll get an amorphous solid with a characteristic length scale and characteristic time scale. Which are both Lorentz invariant. >Quantum physics and special relativity fit nicely together, the standard model and its parts are relativistic quantum field theories. The problems arise when one tries to combine general relativity and quantum physics. When someone says quantum physics and relativity are incompatible he pretty surly means general relativity not special relativity. I was talking about general relativity; quantum field theories almost always use special relativity, it's trivial to combine those two. But it is even possible to extend (classical) Yang-Mills field theory from special to general relativity in a very natural way. But for some reason it's very difficult to 'quantize' the resulting field theory.