4 ms·

Depends on how exactly different. Repulsion is time reversed attraction so IIRC if antimatter is a time reversed normal matter then it should behave as a negati

Depends on how exactly different. Repulsion is time reversed attraction so IIRC if antimatter is a time reversed normal matter then it should behave as a negative gravitational mass.

p1mrx 8y ago
If you shot a video of Earth orbiting the Sun, and played it backwards, you'd still see Earth orbiting the sun, just in the opposite direction. If repulsion were time reversed attraction, then wouldn't you expect to see Earth launch into interstellar space?
- hughes 8y ago
  Yeah, this and the similar comment "anti-matter is regular matter moving backwards through time" make no sense
  - cyphar 8y ago
    The current view of antimatter (at least in quantum chromodynamics) is that it is matter moving backwards in time. This is the key thing that allows Feynman diagrams to be used to describe fermion interactions. Is it actually travelling backwards in time? Maybe, maybe not. But if we model it as travelling backwards in time we can make all sorts of accurate predictions.
    - XorNot 8y ago
      Is this accepted though? I know it's a possible interpretation, but things get super-weird once you allow regular matter to be moving in alternative time-directions because you wind up at the "nothing is real" problem - a single particle oscillating through spacetime gives rise to all matter everywhere by doing the loop in indefinite cycles. The argument against this is you should end up with equal amounts of matter and anti-matter if that's the case - though I don't think it excludes the possibility we're just in a universe where the antimatter part of the loop was extremely rarefied.
      - cyphar 8y ago
        I'm not really sure what you mean by accepted and interpretation here. Feynman diagrams encode the idea of particles traveling backwards in time very strongly -- it's one of their principle features. Feynman's 1949 paper on "The Theory of Positrons"[1] states as much in the abstract (I would recommend reading at least the abstract -- effectively he succinctly contrasts the new theory with the old one and how the model of positrons being backward-traveling electrons allows you to explain many observed effects). And given that Feynman diagrams (and many other things based on them like Feynman path integrals, QED, and QCD) are the backbone of quantum theory, I think it's more than a safe bet that this view is widely accepted. One of the many predictions using them is the existence (and method of synthesis) of the Higgs boson. The reason I said "maybe" is because it's important to have a distinction between models and reality (maybe bringing this up in this discussion was a bad idea). Physics is all about constructing models that approximate reality with strong predictive power, it's not really interested in what reality is but rather how it can be modeled so we can predict it. Obviously, the models have to be true on some level in order to be reliable, but many examples (like fields for instance) are not real objects. So while saying "the particles are traveling backward in time" (theory == reality) is the most accurate statement you can make with modern physics, I would say "at the very least the particles act like particles that are traveling backwards in time" (theory >= model). Even if newer theories are discovered that invalidate the current theories the latter is still just as true of a statement -- otherwise Feynman's models wouldn't work and we know for a fact that they do. Does that make more sense? > a single particle oscillating through spacetime gives rise to all matter everywhere by doing the loop in indefinite cycles This is the idea of the "one electron universe" (which attempted to explain why all electrons appear to be so identical that we've never detected discrepancies)[2], which was arguably one of the inspirations for Feynman's 1949 paper. To be clear, this is a non-standard interpretation of the implications of Feynman's ideas (though this was actually an idea pitched by John Weller on a phone call to Feynman very late at night). Like most interpretations of quantum mechanics, it doesn't make much of a difference to your life -- at the end of the day the maths is the same either way. [1]: https://authors.library.caltech.edu/3520/1/FEYpr49b.pdf https://authors.library.caltech.edu/3520/1/FEYpr49b.pdf [2]: https://en.wikipedia.org/wiki/One-electron_universe https://en.wikipedia.org/wiki/One-electron_universe
    - dotancohen 8y ago
      > it is matter moving backwards in time. What are the implications for cause and effect then? Stated differently: How does this affect entropy? Could a scrambled egg return to the state of an unbroken egg?
      - cyphar 8y ago
        There is an open question about what the source of "the arrow of time"[1] is, which is this apparent weirdness where at very small scales the laws of physics are symmetric in time but at larger scales there appears to be something dictating that they are asymmetric in time. It should be clear that at larger scales, most physical laws are still symmetric in time it's just that we generally see that the results are biased towards one direction (hence the "arrow" of time). Now, this is a more general and complicated thing than entropy. Entropy is a statistical effect (and has very little to do with cooking eggs -- other than its link to thermodynamics). I can give you a short primer on entropy with an example: You have 2 balls and push one so that it knocks the other one. Record it, then reverse the footage. In principle neither view of the event is more correct than the other (this is slightly untrue due to friction and the ball would appear to accelerate on the tabletop, but imagine it's a sufficiently frictionless surface for the purpose of this discussion). So motion is symmetric in time, right? As with most things in physics, the correct answer is "yes, but..." But what happens when we add more balls? Imagine you have 100 lined up in a grid and you knock one -- you end up with a completely jumbled mess. Now reverse the footage, and a jumbled mess becomes 100 perfectly lined up balls. Which is the correct direction of the footage? Obviously there is a difference and you can tell what it is without me needing to explain it -- it's almost impossible that knocking a jumbled mess would just sort it! Why is it so improbable -- after all, isn't the motion just as time-symmetric as in the 2-ball example? Yes, but the problem is the statistics. What is the probability that a given jumbled set of balls will be sorted when you knock them? It's astronomically small. In thermodynamics, these same statistics are the basis for what we consider entropy (imagine the 100 balls are countless numbers of particles). Effectively the idea is that because there are so many particles of matter involved in gas interactions, that the probability distributions are so skewed that you only ever see one macro-level result (that the ideal gas law is obeyed for instance). So while entropy may look like some fundamental force, it's a statistical effect that effectively guarantees that certain effects will happen. Entropy is the thermodynamic arrow of time (and was the first thing that spawned the idea of an "arrow of time"), but there are other arrows of time. For instance, the universe is clearly expanding and thus there is a cosmological arrow (if you reversed the motion the CMBR would be different). There is a "weak" quantum arrow of time where some reactions violate CP-symmetry and thus by CPT theorem they must violate T-symmetry (the T in CPT being time). [1]: https://en.wikipedia.org/wiki/Arrow_of_time https://en.wikipedia.org/wiki/Arrow_of_time
- coliveira 8y ago
  Earth is in fact falling towards the sun, very slowly but surely. Reversing that trajectory means that the earth would be moving away from the sun.
  - ridiculous_fish 8y ago
    If earth is falling towards the sun it is due to higher order effects, like friction or gravitational waves. It's not due to gravity itself, which is time symmetric.
  - hermitdev 8y ago
    If you reversed the trajectory, the earth would still be falling towards the sun, but reversed orbit. i.e. orbiting counterclockwise instead of clockwise from a fixed observation point.
- anonymfus 8y ago
  Negative gravitational mass is supposed to attract other negative gravitational mass and repulse positive gravitational mass. In your example time inversion makes both Sun and Earth negative mass so they still attract each other.
andrepd 8y ago
Nothing you said made any sense. I'm not saying this to be rude. I recommend (if you're interested) that you really study things more thoroughly, not just skim some material, as you plainly did. That way you avoid mixing up concepts like you are doing right now.
- coldtea 8y ago
  Notice how your comment is content-less and makes no arguments or actual corrections. Not saying this to be rude, just found your comment rude. As for what the parent said, it's quite clear. If it doesn't hold up, it's not because of lack of clarity: "Repulsion is time reversed attraction so IIRC if antimatter is a time reversed normal matter then it should behave as a negative gravitational mass". I.e. => seeing something being repulsed will be the same as seeing something between attracted played in reverse. (e.g. if we film a falling ball --attracted to Earth by gravity-- and play the video in reverse, it will look the like it's repulsed by the Earth and goes upwards). Hence, the parent argues, if antimatter is "time reversed normal matter" (something often used as a description of antimatter, at least in layman terms), then it should behave like it is mass with inverse gravitational force.
  - antidesitter 8y ago
    > seeing something being repulsed will be the same as seeing something between attracted played in reverse. (e.g. if we film a falling ball --attracted to Earth by gravity-- and play the video in reverse, it will look the like it's repulsed by the Earth and goes upwards). Here lies the problem. If you play the video in reverse, the ball won't look like it's repulsed by the Earth. It will go upwards, yes, but only because of an upwards velocity. It will still look like it's trying to fall back. That is, it will still be accelerating downward. Acceleration doesn't change under time reversal. It is velocity that switches signs: https://en.wikipedia.org/wiki/T-symmetry#Effect_of_time_reversal_on_some_variables_of_classical_physics https://en.wikipedia.org/wiki/T-symmetry#Effect_of_time_reve....
    - coldtea 8y ago
      That's a good answer -- that illuminates what's wrong with the grandparent'd idea (unlike the parent's facile dismissal).

Depends on how exactly different. Repulsion is time reversed attraction so IIRC if antimatter is a time reversed normal matter then it should behave as a negative gravitational mass.

p1mrx 8y ago

If you shot a video of Earth orbiting the Sun, and played it backwards, you'd still see Earth orbiting the sun, just in the opposite direction. If repulsion were time reversed attraction, then wouldn't you expect to see Earth launch into interstellar space?

hughes 8y ago
Yeah, this and the similar comment "anti-matter is regular matter moving backwards through time" make no sense
- cyphar 8y ago
  The current view of antimatter (at least in quantum chromodynamics) is that it is matter moving backwards in time. This is the key thing that allows Feynman diagrams to be used to describe fermion interactions. Is it actually travelling backwards in time? Maybe, maybe not. But if we model it as travelling backwards in time we can make all sorts of accurate predictions.
  - XorNot 8y ago
    Is this accepted though? I know it's a possible interpretation, but things get super-weird once you allow regular matter to be moving in alternative time-directions because you wind up at the "nothing is real" problem - a single particle oscillating through spacetime gives rise to all matter everywhere by doing the loop in indefinite cycles. The argument against this is you should end up with equal amounts of matter and anti-matter if that's the case - though I don't think it excludes the possibility we're just in a universe where the antimatter part of the loop was extremely rarefied.
    - cyphar 8y ago
      I'm not really sure what you mean by accepted and interpretation here. Feynman diagrams encode the idea of particles traveling backwards in time very strongly -- it's one of their principle features. Feynman's 1949 paper on "The Theory of Positrons"[1] states as much in the abstract (I would recommend reading at least the abstract -- effectively he succinctly contrasts the new theory with the old one and how the model of positrons being backward-traveling electrons allows you to explain many observed effects). And given that Feynman diagrams (and many other things based on them like Feynman path integrals, QED, and QCD) are the backbone of quantum theory, I think it's more than a safe bet that this view is widely accepted. One of the many predictions using them is the existence (and method of synthesis) of the Higgs boson. The reason I said "maybe" is because it's important to have a distinction between models and reality (maybe bringing this up in this discussion was a bad idea). Physics is all about constructing models that approximate reality with strong predictive power, it's not really interested in what reality is but rather how it can be modeled so we can predict it. Obviously, the models have to be true on some level in order to be reliable, but many examples (like fields for instance) are not real objects. So while saying "the particles are traveling backward in time" (theory == reality) is the most accurate statement you can make with modern physics, I would say "at the very least the particles act like particles that are traveling backwards in time" (theory >= model). Even if newer theories are discovered that invalidate the current theories the latter is still just as true of a statement -- otherwise Feynman's models wouldn't work and we know for a fact that they do. Does that make more sense? > a single particle oscillating through spacetime gives rise to all matter everywhere by doing the loop in indefinite cycles This is the idea of the "one electron universe" (which attempted to explain why all electrons appear to be so identical that we've never detected discrepancies)[2], which was arguably one of the inspirations for Feynman's 1949 paper. To be clear, this is a non-standard interpretation of the implications of Feynman's ideas (though this was actually an idea pitched by John Weller on a phone call to Feynman very late at night). Like most interpretations of quantum mechanics, it doesn't make much of a difference to your life -- at the end of the day the maths is the same either way. [1]: https://authors.library.caltech.edu/3520/1/FEYpr49b.pdf https://authors.library.caltech.edu/3520/1/FEYpr49b.pdf [2]: https://en.wikipedia.org/wiki/One-electron_universe https://en.wikipedia.org/wiki/One-electron_universe
  - dotancohen 8y ago
    > it is matter moving backwards in time. What are the implications for cause and effect then? Stated differently: How does this affect entropy? Could a scrambled egg return to the state of an unbroken egg?
    - cyphar 8y ago
      There is an open question about what the source of "the arrow of time"[1] is, which is this apparent weirdness where at very small scales the laws of physics are symmetric in time but at larger scales there appears to be something dictating that they are asymmetric in time. It should be clear that at larger scales, most physical laws are still symmetric in time it's just that we generally see that the results are biased towards one direction (hence the "arrow" of time). Now, this is a more general and complicated thing than entropy. Entropy is a statistical effect (and has very little to do with cooking eggs -- other than its link to thermodynamics). I can give you a short primer on entropy with an example: You have 2 balls and push one so that it knocks the other one. Record it, then reverse the footage. In principle neither view of the event is more correct than the other (this is slightly untrue due to friction and the ball would appear to accelerate on the tabletop, but imagine it's a sufficiently frictionless surface for the purpose of this discussion). So motion is symmetric in time, right? As with most things in physics, the correct answer is "yes, but..." But what happens when we add more balls? Imagine you have 100 lined up in a grid and you knock one -- you end up with a completely jumbled mess. Now reverse the footage, and a jumbled mess becomes 100 perfectly lined up balls. Which is the correct direction of the footage? Obviously there is a difference and you can tell what it is without me needing to explain it -- it's almost impossible that knocking a jumbled mess would just sort it! Why is it so improbable -- after all, isn't the motion just as time-symmetric as in the 2-ball example? Yes, but the problem is the statistics. What is the probability that a given jumbled set of balls will be sorted when you knock them? It's astronomically small. In thermodynamics, these same statistics are the basis for what we consider entropy (imagine the 100 balls are countless numbers of particles). Effectively the idea is that because there are so many particles of matter involved in gas interactions, that the probability distributions are so skewed that you only ever see one macro-level result (that the ideal gas law is obeyed for instance). So while entropy may look like some fundamental force, it's a statistical effect that effectively guarantees that certain effects will happen. Entropy is the thermodynamic arrow of time (and was the first thing that spawned the idea of an "arrow of time"), but there are other arrows of time. For instance, the universe is clearly expanding and thus there is a cosmological arrow (if you reversed the motion the CMBR would be different). There is a "weak" quantum arrow of time where some reactions violate CP-symmetry and thus by CPT theorem they must violate T-symmetry (the T in CPT being time). [1]: https://en.wikipedia.org/wiki/Arrow_of_time https://en.wikipedia.org/wiki/Arrow_of_time
coliveira 8y ago
Earth is in fact falling towards the sun, very slowly but surely. Reversing that trajectory means that the earth would be moving away from the sun.
- ridiculous_fish 8y ago
  If earth is falling towards the sun it is due to higher order effects, like friction or gravitational waves. It's not due to gravity itself, which is time symmetric.
- hermitdev 8y ago
  If you reversed the trajectory, the earth would still be falling towards the sun, but reversed orbit. i.e. orbiting counterclockwise instead of clockwise from a fixed observation point.
anonymfus 8y ago
Negative gravitational mass is supposed to attract other negative gravitational mass and repulse positive gravitational mass. In your example time inversion makes both Sun and Earth negative mass so they still attract each other.

andrepd 8y ago

Nothing you said made any sense. I'm not saying this to be rude. I recommend (if you're interested) that you really study things more thoroughly, not just skim some material, as you plainly did. That way you avoid mixing up concepts like you are doing right now.

coldtea 8y ago
Notice how your comment is content-less and makes no arguments or actual corrections. Not saying this to be rude, just found your comment rude. As for what the parent said, it's quite clear. If it doesn't hold up, it's not because of lack of clarity: "Repulsion is time reversed attraction so IIRC if antimatter is a time reversed normal matter then it should behave as a negative gravitational mass". I.e. => seeing something being repulsed will be the same as seeing something between attracted played in reverse. (e.g. if we film a falling ball --attracted to Earth by gravity-- and play the video in reverse, it will look the like it's repulsed by the Earth and goes upwards). Hence, the parent argues, if antimatter is "time reversed normal matter" (something often used as a description of antimatter, at least in layman terms), then it should behave like it is mass with inverse gravitational force.
- antidesitter 8y ago
  > seeing something being repulsed will be the same as seeing something between attracted played in reverse. (e.g. if we film a falling ball --attracted to Earth by gravity-- and play the video in reverse, it will look the like it's repulsed by the Earth and goes upwards). Here lies the problem. If you play the video in reverse, the ball won't look like it's repulsed by the Earth. It will go upwards, yes, but only because of an upwards velocity. It will still look like it's trying to fall back. That is, it will still be accelerating downward. Acceleration doesn't change under time reversal. It is velocity that switches signs: https://en.wikipedia.org/wiki/T-symmetry#Effect_of_time_reversal_on_some_variables_of_classical_physics https://en.wikipedia.org/wiki/T-symmetry#Effect_of_time_reve....
  - coldtea 8y ago
    That's a good answer -- that illuminates what's wrong with the grandparent'd idea (unlike the parent's facile dismissal).