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General relativity actually tells us nothing about whether causality is absolute. You can easily make up spacetime configurations that violate causality and sti

General relativity actually tells us nothing about whether causality is absolute. You can easily make up spacetime configurations that violate causality and still conform to general relativity. It's just that most people believe that causality should be protected, because... well it would be really weird if it wasn't. But it's just a conjecture and not like a theorem or anything.

deleted 5y ago
[deleted]
wyager 5y ago
It doesn't guarantee that causality is protected, but it certainly seems pretty suggestive that GR creates a big infeasibility region for positive-mass objects (outside of the future-past light cone). There are ways you could still violate causality despite this restriction, but it makes it a lot harder. There's no particular reason we couldn't have a ++++ spacetime, which wouldn't have this form of causal censorship, so I count this as one more piece of evidence towards "maybe causality violation is impossible".
nine_k 5y ago
AFAICT, GR is symmetric in relation of the direction of time, as are most other important physical theories. What may add the arrow of time is the second law of thermodynamics: a statistical evolution of a system is not symmetric. I wonder now how a violation of causality could either circumvent of contradict the second law.
- jchrisa 5y ago
  This is the content I visit the orange site for. Thanks. To add something substantive, my question to the thread is: anyone here have thoughts on how these questions relate to McTaggart's unreality of time? https://en.wikipedia.org/wiki/A_series_and_B_series https://en.wikipedia.org/wiki/A_series_and_B_series
- dleslie 5y ago
  It's not creating new energy, right? It reads to me like causality violating configurations simply move energy around time as we'd move mass through space.
  - saagarjha 5y ago
    The second law, like all physical laws, does not violate conservation of energy. However, reversing causality can cause the reversal of entropy, which is what the second law deals with.
    - zaarn 5y ago
      It's not necessarily about reversing causality, it can simply be breaking it (ie, the effect happens before the cause without being able to influence the cause).
    - T-A 5y ago
      https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_...
    - dleslie 5y ago
      I don't believe it reverses entropy, if we consider the entire system to include all of time. Moving energy around through time may serve to increase entropy overall.
- potatoman22 5y ago
  I don't think we know that time's arrow is caused by the second law. Isn't that just a theory?
  - T-A 5y ago
    https://www.scientificamerican.com/article/just-a-theory-7-misused-science-words/ https://www.scientificamerican.com/article/just-a-theory-7-m...
    - tripzilch 5y ago
      8. Bonus misuse: The term "science words" is not really used in academic literature You can't be a scientific american if you keep using the science words bad. It's like cargo culting science: Use these words correctly if you want to seem to know what you're talking about! But anyone who actually knows what they are talking about would use these words correctly, or at least not be mistaken about their meaning. So this article is teaching people that have no idea what they're talking about, how to sound like they do. Maybe it's time we start using another language for science. Clearly it's confusing the Scientific Americans.
- gradschool 5y ago
  I would envision two chambers A and B connected by two vents X and Y, like Maxwell's demon but with two vents instead of one and both vents permanently open. Whereas X is an ordinary vent, Y has the property that any particle passing from A to B through it reaches B shortly before it leaves A. Any particle passing from B to A through Y disappears for a short time before reappearing in A. It seems to me that this setup would induce a current of air blowing from A to B through Y and from B to A through X, but I'm only an amateur and could be full of hot air myself.
  - jjk166 5y ago
    Let's look at the two possible timelines here: At some point a particle pops up into existence in B, increasing B's pressure. Some time later, a particle from A disappears, decreasing A's pressure. This pressure differential would cause flow from B to A through X. Alternatively, a particle randomly disappears from B, decreasing B's pressure. Some time later, a particle appears in A, increasing A's pressure. This would cause flow from A to B through X. The more particles that cross from A to B via Y, increasing B's pressure, the more likely it is for particles to pass from B to A via Y, keeping these two effects equal. These two pressure differentials would cancel out for large numbers of particles. You've essentially just hooked up the system to itself, but at a future point in time. Unless the overall pressure of the system is changing, there is nothing to induce the flow. If you are increasing the pressure, valve Y is essentially just a standard one-way valve.
AnimalMuppet 5y ago
That conjecture is one of the two foundational principles of relativity - that the laws of physics are the same in all valid frames of reference. "Violating causality" means that in some valid frame of reference A, event X caused event Y, and in some other valid frame of reference B, event Y caused event X. The laws of physics cannot be the same in A and B for both of those to be true.
- gopher_space 5y ago
  I'm watching two friends go through a nasty breakup that "violates causality" if you take both points of view into account, and am realizing that it's impossible to do any work with pop-sci physics because I'm relying on an extended metaphor to grasp the topic. It's clear that your point of view matters in physics, but it's also clear that I have no idea what a "point of view" means in this context.
  - salawat 5y ago
    Point of view means coordinate system, or reference frame. If I am stationary and measuring you flying past me in a relativistic train, I would measure you in a coordinate grid say, centered on myself. You may be watching me from your train and doing the same, but from a reference grid stationary with respect to you but moving with respect to me. Even if we both agreed on identical measures ahead of time, your measurement of me from your reference frame and mine of you from my reference will be off from a measurement we do ahead of time in the same reference frame as a calibration. However, that difference comes out in the wash through the magic of the Lorentz Transformation. Using the Lorentz Transformation, I should be able to negotiate transforming a measurement I do of you in my own reference frame to what it would look like from yours and you should be able to transform yours to mine, through which we'd both come to the same numbersassuming we're in spacetime that shares the same general degree of curvature (same gravity). This can even be generalized to arbitrary degrees of spacetime curvature. I.e. if your relativistic train is racing past a black hole with a near tangent intersect, and I'm stationary with reference to the black hole centric reference frame but further out in the system, as you blast through the high curvature part of your trip your clock may seem to de-sync from mine. This too, however, can be mitigated through copious amounts of math I don't get for more than half an hour after looking away from a text book. I just know if I ever have a problem to solve that involves hitting that book again, I have either made some terrible or awesome decisions in my life.
phendrenad2 5y ago
So we're traveling away from one another at 0.9c. I send a paper airplane in your direction at 500x the speed of light. You receive it and send it back (at 500c). Does it not arrive before I sent it, from my perspective?
- ben_w 5y ago
  I’m still studying this, but IIUC that’s only a problem when you and the other party both launch the paper plane at 500c relative to your own reference frames, and (separate thing I might have misunderstood) “no preferred frame” is because Occam’s razor says GR doesn’t need one rather than because such a frame would be incompatible with observations. (Again I may have misunderstood, but the frame of the CMB sometimes gets talked about as if it might be a preferred frame).
- prox 5y ago
  Does this reverse time though? 0.9c is 90% of lightspeed, and for the sake of your example, 500c is just instantaneous. If light is just information, wouldn’t it just be a matter of seemingly getting something very fast?
  - cowboysauce 5y ago
    If you're traveling at 0.9c then there will be significant time dilation. Since velocity is relative, both parties will see each other as severely time dilated which can lead to somebody receiving a reply to a message before they send it.
    - prox 5y ago
      But how do you know that you need to send a message or what if you don’t send a message? How can you get a reply? Everytime I think I get it, I don’t!
- grishka 5y ago
  See, the formula for velocity is v=s/t. We know the distance the object has traveled, so it's the time it took to get there that gets adjusted so as the resulting velocity never exceeds the speed of light. At least that's my understanding of it. I might be entirely wrong.
circlefavshape 5y ago
I've never managed to see how causality is a problem for FTL travel If we perceived the world through sonar then causality would be violated by anything supersonic. If you were shot then you'd feel the bullet first and then "see" it moving back into the gun. AFAICS if information can travel at >c, then physical objects traveling at >c doesn't violate causality
- harpersealtako 5y ago
  Here's a good explanation of why FTL causes causality issues. http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... The sonar example works only if everything is traveling in roughly the same frame of reference. You're not accounting for relativity and its effects on space and time in your analogy.
  - circlefavshape 5y ago
    I still don't get it. That analogy assumes that light is the only way causality information can be transferred - i.e. if I see the phone call being placed before its sent then it seems like causality is violated. But it I look at what's happening via the FTL phone then I can see that it's not
- sxp 5y ago
  Comparing to sonar isn't valid since objects at subsonic and supersonic speeds still obey the rule that 1 + 1 = 2. For high velocities, this rule is no longer true and you need to resort to using a spacetime diagram to compute velocities. If you draw a tachyon on a spacetime diagram going faster than the speed of light, you can create situations where the tachyon travels back in time for certain observers. > AFAICS if information can travel at >c, then physical objects traveling at >c doesn't violate causality If information can travel faster than c, then you can create situations where it's also violating causality.

deleted 5y ago

[deleted]

wyager 5y ago

It doesn't guarantee that causality is protected, but it certainly seems pretty suggestive that GR creates a big infeasibility region for positive-mass objects (outside of the future-past light cone). There are ways you could still violate causality despite this restriction, but it makes it a lot harder. There's no particular reason we couldn't have a ++++ spacetime, which wouldn't have this form of causal censorship, so I count this as one more piece of evidence towards "maybe causality violation is impossible".

nine_k 5y ago

AFAICT, GR is symmetric in relation of the direction of time, as are most other important physical theories. What may add the arrow of time is the second law of thermodynamics: a statistical evolution of a system is not symmetric. I wonder now how a violation of causality could either circumvent of contradict the second law.

jchrisa 5y ago
This is the content I visit the orange site for. Thanks. To add something substantive, my question to the thread is: anyone here have thoughts on how these questions relate to McTaggart's unreality of time? https://en.wikipedia.org/wiki/A_series_and_B_series https://en.wikipedia.org/wiki/A_series_and_B_series
dleslie 5y ago
It's not creating new energy, right? It reads to me like causality violating configurations simply move energy around time as we'd move mass through space.
- saagarjha 5y ago
  The second law, like all physical laws, does not violate conservation of energy. However, reversing causality can cause the reversal of entropy, which is what the second law deals with.
  - zaarn 5y ago
    It's not necessarily about reversing causality, it can simply be breaking it (ie, the effect happens before the cause without being able to influence the cause).
  - T-A 5y ago
    https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_...
  - dleslie 5y ago
    I don't believe it reverses entropy, if we consider the entire system to include all of time. Moving energy around through time may serve to increase entropy overall.
potatoman22 5y ago
I don't think we know that time's arrow is caused by the second law. Isn't that just a theory?
- T-A 5y ago
  https://www.scientificamerican.com/article/just-a-theory-7-misused-science-words/ https://www.scientificamerican.com/article/just-a-theory-7-m...
  - tripzilch 5y ago
    8. Bonus misuse: The term "science words" is not really used in academic literature You can't be a scientific american if you keep using the science words bad. It's like cargo culting science: Use these words correctly if you want to seem to know what you're talking about! But anyone who actually knows what they are talking about would use these words correctly, or at least not be mistaken about their meaning. So this article is teaching people that have no idea what they're talking about, how to sound like they do. Maybe it's time we start using another language for science. Clearly it's confusing the Scientific Americans.
gradschool 5y ago
I would envision two chambers A and B connected by two vents X and Y, like Maxwell's demon but with two vents instead of one and both vents permanently open. Whereas X is an ordinary vent, Y has the property that any particle passing from A to B through it reaches B shortly before it leaves A. Any particle passing from B to A through Y disappears for a short time before reappearing in A. It seems to me that this setup would induce a current of air blowing from A to B through Y and from B to A through X, but I'm only an amateur and could be full of hot air myself.
- jjk166 5y ago
  Let's look at the two possible timelines here: At some point a particle pops up into existence in B, increasing B's pressure. Some time later, a particle from A disappears, decreasing A's pressure. This pressure differential would cause flow from B to A through X. Alternatively, a particle randomly disappears from B, decreasing B's pressure. Some time later, a particle appears in A, increasing A's pressure. This would cause flow from A to B through X. The more particles that cross from A to B via Y, increasing B's pressure, the more likely it is for particles to pass from B to A via Y, keeping these two effects equal. These two pressure differentials would cancel out for large numbers of particles. You've essentially just hooked up the system to itself, but at a future point in time. Unless the overall pressure of the system is changing, there is nothing to induce the flow. If you are increasing the pressure, valve Y is essentially just a standard one-way valve.

AnimalMuppet 5y ago

That conjecture is one of the two foundational principles of relativity - that the laws of physics are the same in all valid frames of reference. "Violating causality" means that in some valid frame of reference A, event X caused event Y, and in some other valid frame of reference B, event Y caused event X. The laws of physics cannot be the same in A and B for both of those to be true.

gopher_space 5y ago
I'm watching two friends go through a nasty breakup that "violates causality" if you take both points of view into account, and am realizing that it's impossible to do any work with pop-sci physics because I'm relying on an extended metaphor to grasp the topic. It's clear that your point of view matters in physics, but it's also clear that I have no idea what a "point of view" means in this context.
- salawat 5y ago
  Point of view means coordinate system, or reference frame. If I am stationary and measuring you flying past me in a relativistic train, I would measure you in a coordinate grid say, centered on myself. You may be watching me from your train and doing the same, but from a reference grid stationary with respect to you but moving with respect to me. Even if we both agreed on identical measures ahead of time, your measurement of me from your reference frame and mine of you from my reference will be off from a measurement we do ahead of time in the same reference frame as a calibration. However, that difference comes out in the wash through the magic of the Lorentz Transformation. Using the Lorentz Transformation, I should be able to negotiate transforming a measurement I do of you in my own reference frame to what it would look like from yours and you should be able to transform yours to mine, through which we'd both come to the same numbersassuming we're in spacetime that shares the same general degree of curvature (same gravity). This can even be generalized to arbitrary degrees of spacetime curvature. I.e. if your relativistic train is racing past a black hole with a near tangent intersect, and I'm stationary with reference to the black hole centric reference frame but further out in the system, as you blast through the high curvature part of your trip your clock may seem to de-sync from mine. This too, however, can be mitigated through copious amounts of math I don't get for more than half an hour after looking away from a text book. I just know if I ever have a problem to solve that involves hitting that book again, I have either made some terrible or awesome decisions in my life.

phendrenad2 5y ago

So we're traveling away from one another at 0.9c. I send a paper airplane in your direction at 500x the speed of light. You receive it and send it back (at 500c). Does it not arrive before I sent it, from my perspective?

ben_w 5y ago
I’m still studying this, but IIUC that’s only a problem when you and the other party both launch the paper plane at 500c relative to your own reference frames, and (separate thing I might have misunderstood) “no preferred frame” is because Occam’s razor says GR doesn’t need one rather than because such a frame would be incompatible with observations. (Again I may have misunderstood, but the frame of the CMB sometimes gets talked about as if it might be a preferred frame).
prox 5y ago
Does this reverse time though? 0.9c is 90% of lightspeed, and for the sake of your example, 500c is just instantaneous. If light is just information, wouldn’t it just be a matter of seemingly getting something very fast?
- cowboysauce 5y ago
  If you're traveling at 0.9c then there will be significant time dilation. Since velocity is relative, both parties will see each other as severely time dilated which can lead to somebody receiving a reply to a message before they send it.
  - prox 5y ago
    But how do you know that you need to send a message or what if you don’t send a message? How can you get a reply? Everytime I think I get it, I don’t!
grishka 5y ago
See, the formula for velocity is v=s/t. We know the distance the object has traveled, so it's the time it took to get there that gets adjusted so as the resulting velocity never exceeds the speed of light. At least that's my understanding of it. I might be entirely wrong.

circlefavshape 5y ago

I've never managed to see how causality is a problem for FTL travel If we perceived the world through sonar then causality would be violated by anything supersonic. If you were shot then you'd feel the bullet first and then "see" it moving back into the gun. AFAICS if information can travel at >c, then physical objects traveling at >c doesn't violate causality

harpersealtako 5y ago
Here's a good explanation of why FTL causes causality issues. http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... The sonar example works only if everything is traveling in roughly the same frame of reference. You're not accounting for relativity and its effects on space and time in your analogy.
- circlefavshape 5y ago
  I still don't get it. That analogy assumes that light is the only way causality information can be transferred - i.e. if I see the phone call being placed before its sent then it seems like causality is violated. But it I look at what's happening via the FTL phone then I can see that it's not
sxp 5y ago
Comparing to sonar isn't valid since objects at subsonic and supersonic speeds still obey the rule that 1 + 1 = 2. For high velocities, this rule is no longer true and you need to resort to using a spacetime diagram to compute velocities. If you draw a tachyon on a spacetime diagram going faster than the speed of light, you can create situations where the tachyon travels back in time for certain observers. > AFAICS if information can travel at >c, then physical objects traveling at >c doesn't violate causality If information can travel faster than c, then you can create situations where it's also violating causality.