3 ms·

Time is weird though. With e.g. mass, you can define the kilogram as the mass of a certain number of atoms, which makes sense. Time though is defined as some am

Time is weird though. With e.g. mass, you can define the kilogram as the mass of a certain number of atoms, which makes sense. Time though is defined as some amount of caesium oscillations. Also, time (and therefore length, or vice versa if you prefer) changes in relativistic scenarios, only the speed of light is constant. Even as a physicist, I've often thought that time is more of a useful mental construct, but maybe it's time for something else. And then there's the question of why time doesn't flow backwards (aka the arrow of time), which is genuinely interesting. Only a few fields have answers for this, e.g. thermodynamics with entropy. I hope answering this question will give us a much better framework to define time. I'd love it if it was shown in my lifetime that time is quantised.

cLeEOGPw 10y ago
You just said yourself that only speed of light is constant, mass is also relativistic like time. And how do you know that time doesn't flow backwards? On quantum scale, how do you tell past from future? Gravity is one directional too, but somehow everyone are mystified by time. I think it's because it's far too easy to let the imagination run wild on the possibilities of reversing time, but much less intuitive to imagine "reverse: gravitation or reverse entropy.
- guitarbill 10y ago
  > And how do you know that time doesn't flow backwards? On quantum scale, how do you tell past from future? E.g. you don't observe a glass un-smashing. But as you probably also know, the question if/how a quantum arrow of time relates thermodynamics is currently unsolved and probably Nobel Prize territory. Not sure how much progress we'll make on Hacker News. > Gravity is one directional too I assume you mean it's purely attractive? It isn't too difficult to imagine a sphere with negative mass repelling another (like magnets), even if you've never seen negative mass :) How do you picture reversing time? If you imagine it like reversing a video, it's obvious how this breaks down, again from the glass un-smashing. Entropy can be very visceral.
  - cLeEOGPw 10y ago
    Glass unsmashing is an emergent behaviour. Even if you were an operator of a simulation of that glass being smashed, you could never change the rules so that the glass would come back together like it was. Time is not some parameter you can adjust. Time is just a word we have to express amount of quantum interactions that happen to an object. That's why I don't imagine reversing time. It makes no sense. You can't reverse effects of interactions even if you reversed the laws of nature. And you can't record reality and play it backwards too.

cLeEOGPw 10y ago

You just said yourself that only speed of light is constant, mass is also relativistic like time. And how do you know that time doesn't flow backwards? On quantum scale, how do you tell past from future? Gravity is one directional too, but somehow everyone are mystified by time. I think it's because it's far too easy to let the imagination run wild on the possibilities of reversing time, but much less intuitive to imagine "reverse: gravitation or reverse entropy.

guitarbill 10y ago
> And how do you know that time doesn't flow backwards? On quantum scale, how do you tell past from future? E.g. you don't observe a glass un-smashing. But as you probably also know, the question if/how a quantum arrow of time relates thermodynamics is currently unsolved and probably Nobel Prize territory. Not sure how much progress we'll make on Hacker News. > Gravity is one directional too I assume you mean it's purely attractive? It isn't too difficult to imagine a sphere with negative mass repelling another (like magnets), even if you've never seen negative mass :) How do you picture reversing time? If you imagine it like reversing a video, it's obvious how this breaks down, again from the glass un-smashing. Entropy can be very visceral.
- cLeEOGPw 10y ago
  Glass unsmashing is an emergent behaviour. Even if you were an operator of a simulation of that glass being smashed, you could never change the rules so that the glass would come back together like it was. Time is not some parameter you can adjust. Time is just a word we have to express amount of quantum interactions that happen to an object. That's why I don't imagine reversing time. It makes no sense. You can't reverse effects of interactions even if you reversed the laws of nature. And you can't record reality and play it backwards too.