8 ms·

"The more you move in space, the less you move in time" The consequences of this simple rule are rather shocking, as the recent nobel winner Roger Penrose poin

"The more you move in space, the less you move in time" The consequences of this simple rule are rather shocking, as the recent nobel winner Roger Penrose pointed out. Take the case of two people on Earth, one stationary and one travelling past in a car. They each have different events which they consider the present. Now imagine that there is an alien government in Andromeda debating a possible invasion of Earth. To the person on Earth who is moving, the debate still going on, but in the reference frame of the stationary person, it's a day or so later in Andromeda and the invasion fleet is already on it's way. If there is no absolute reference frame, and both observers reference frames are equally valid, then the outcome of the invasion debate is a foregone conclusion. This is the crux of the Andromeda Paradox and the Reitdijk-Putnam-Penrose argument. https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argume...

sudhirj 6y ago
Is this actually a problem? By the time the person travelling in the car and the stationary person synchronise their information enough to communicate won’t their frames of reference also sync up? Like if I’m moving and I see something before you, won’t you see it as well before or at the same time I can tell you I saw it?
- irthomasthomas 6y ago
  From the wiki... "Notice that neither observer can actually "see" what is happening in Andromeda, because light from Andromeda (and the hypothetical alien fleet) will take 2.5 million years to reach Earth. The argument is not about what can be "seen"; it is purely about what events different observers consider to occur in the present moment."
  - sudhirj 6y ago
    But the entire point of relativity is that there’s no such thing as a joint “present moment” between more than observer. The only meaningful construct is your own present moment, about which information radiates outward at the maximum speed of information transfer in space-time (speed of light/gravity). Information reaches other observers based on their trajectory and position in space time. No such thing as the universal now.
    - irthomasthomas 6y ago
      Which circles right back to the paradox. The moving persons reference frame has aliens still debating an invasion, and the stationary persons reference frame has the aliens already on their way. How can there be any room for free will in the deliberations of the alien government, if the timelines of both observers are equally valid? The article on relativity of simultaneity has much more detail on the subject https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity
      - 8note 6y ago
        For there to be a paradox, the observers would have to find out about it sooner than it would take light to communicate the result. Otherwise, the earliest time that the aliens can find out if they're going to invade or not is when they make the decision. Sure, the observers can find out, and try to tell each other and the aliens, but since it takes at least a round trip of light from the aliens to the observers, the observers always find out after the aliens have made up their minds. If the aliens find out that they make the decision when they make the decision, it seems to me at least that they actually made the decision rather than having it be predestined
simias 6y ago
I don't think I understand what's shocking about this. It's like saying that for somebody who watches a taping of a soccer match the result is not yet known, yet for somebody who watched it live it's a foregone conclusion. I don't really get what's the paradox here. Or maybe more to the point: if you were watching a soccer game Earth from Proxima Centauri, you could only get the result about 4 years after the game took place on earth, so for you the issue is uncertain while for somebody on Earth "at the same moment" the result has been known for years. But of course then the whole concept of "at the same moment" is pretty ambiguous and fuzzy. I don't really get what's paradoxical here.
- klodolph 6y ago
  The notion of “at the same moment” is not ambiguous and fuzzy. You use clocks and synchronize them. What relativity adds to the picture is the requirement for a common reference frame, which is surprising. It creates paradoxes because our intuitive notion of “at the same moment” does not include the reference frame, so there is a conflict between our intuitive understanding of past/future (which does not change whether you are moving) and the relativistic description of past/future (where two people can disagree about whether a particular event is int the past or the future). Because of this disagreement, if you had FTL communication and wired it up so you could make a phone call to Andromeda, your phone call would arrive before or after the debate depending on your reference frame on Earth, and you could use it to send information backwards in time.
  - TheCraiggers 6y ago
    Synchronize the clocks how? Last I knew, this was still impossible, unless you employ hand-waving. If I synchronize two clocks next to each other, and then move one away, they are no longer synchronized as one was moving. And we still don't have the speed of c one way, so you can't use that either.
    - drran 6y ago
      Move clocks at the exact same speed and acceleration in opposite directions, so they stay synchronized.
javert 6y ago
I'm fairly convinced this is intellectual masturbation. I think two situations are being confused for one another: a) If you travel close to the speed of light, causality goes more slowly (e.g. a clock ticks more slowly) than it does for someone who is stationary. So, two clocks will diverge. This is true, AFAIK. b) If you travel close to the speed of light, and somebody else is stationary, they are traveling "into the future" relative to you, whereas you are "in the past" relative to them. I believe this is nonsense. But, I believe this is required for the "Andromeda Paradox" to hold. Thus, I don't think it holds. (BTW, we know that paradoxes never hold, so there must be something wrong with it.) The solution to the Andromeda Paradox that I am proposing is to say that there is one universal ordering of events (among both observers and the Andromedans), but the two Earthling observers have different times on their clocks at the particular point in the sequence of events where the Andromedans decide to invade.
- thelastwave 6y ago
  I think it's ridiculous anonymous cowards are downvoting you instead of replying. I may disagree with you (or not) but downvoting someone for having a different interpretation of relativity/cosmology, when the world's leading physicists can't agree on it, is the height of stupidity.
- cthalupa 6y ago
  >The solution to the Andromeda Paradox that I am proposing is to say that there is one universal ordering of events Proposing this is fine, but proving it is quite a matter altogether - you have to replace special relativity. Special relativity pretty much explicitly states this isn't the case, making an exception for events that are causally connected, but there's been a slew of experiments over the past several decades that call into question even that exception at long distances, and more recently, even ones for local observers, e.g. https://advances.sciencemag.org/content/5/9/eaaw9832 https://advances.sciencemag.org/content/5/9/eaaw9832 Personally, I'm in the Rovelli and loop quantum gravity camp - time probably isn't actually a real thing on its own, and what we perceive as time is just an emergent property of thermodynamics.
  - 6y ago
amelius 6y ago
This is a nice video explaining simultaneity. https://www.youtube.com/watch?v=wteiuxyqtoM https://www.youtube.com/watch?v=wteiuxyqtoM
pdonis 6y ago
> the consequences of this simple rule The consequence you refer to is not a logical necessity based on relativity. It's just a particular interpretation, the "block universe" interpretation, which is not accepted by all physicists. See here for a refutation of the argument referred to: https://www.physicsforums.com/insights/block-universe-refuting-common-argument/ https://www.physicsforums.com/insights/block-universe-refuti...
- falseprofit 6y ago
  I don't know why it would follow from relativity, but the block universe just makes sense. To me, it follows from determinism and Occam's razor.
  - ben_w 6y ago
    Because relativity does away with the alternative, that of a universally accessible “now”.
    - pdonis 6y ago
      > relativity does away with the alternative, that of a universally accessible “now” Relativity does away with "now" period--"now" is a frame-dependent concept in relativity, and frame-dependent concepts, in relativity, have no physical meaning. As I noted in the article I linked to upthread, the physically meaningful division of spacetime, at some particular event "here and now" (like you sitting in your chair reading this), is into three parts, not two: the past light cone, the future light cone, and the spacelike separated region (what Roger Penrose, in the very same book where he presents the Andromeda scenario, calls "elsewhere"). The past light cone behaves like our intuitive concept of "past", and the future light cone behaves like our intuitive concept of "future", but the spacelike separated region does not behave like anything our intuition is familiar with. The whole "Andromeda paradox" argument is based on failing to recognize that the spacelike separated region exists in relativity and doesn't match up with our intuitions about either "past" or "future".
  - pdonis 6y ago
nojokes 6y ago
Welcome to the ancient times. The speed of information travel has always been capped but now we know that there is absolute upper limit - c. It still works on earth, now is not shared, but for most practical purposes the synchronization delay does not matter, except when it does like stock and sport betting. I think about this like this. The universe is always in exactly one state but for each inside observer the knowledge about this state depends on its location.
Uhhrrr 6y ago
Why would it be a day or so later? The Wikipedia makes that claim, but the linked source does not: https://archive.org/details/emperorsnewmindc00penr/page/302/mode/2up https://archive.org/details/emperorsnewmindc00penr/page/302/... The two people would perceive the changing of a traffic light on the same street at almost the same time - why would it be different for the Andromedans?
TheMightyLlama 6y ago
Is the person in the car moving away from or towards Andromeda? The example you gave can be interpreted two ways and my intuition tells me that the example is only true in the case that the car is moving _away_ from Andromeda. Is it also, weirdly, the case that if the car is travelling towards Andromeda that time slows down sufficiently that they perceive the outcome of the debate _after_ the person standing still?
- thelastwave 6y ago
  It is also weird that there is now a sports car that at various points in its orbit is moving towards Andromeda.
cellular 6y ago
I gave up the idea of time being a dimension long ago. It allows me to think in ways others do not. I made a video on a different way about thinking time slows near the speed of light: https://youtu.be/A-2XQQDD6QQ https://youtu.be/A-2XQQDD6QQ

sudhirj 6y ago

Is this actually a problem? By the time the person travelling in the car and the stationary person synchronise their information enough to communicate won’t their frames of reference also sync up? Like if I’m moving and I see something before you, won’t you see it as well before or at the same time I can tell you I saw it?

irthomasthomas 6y ago
From the wiki... "Notice that neither observer can actually "see" what is happening in Andromeda, because light from Andromeda (and the hypothetical alien fleet) will take 2.5 million years to reach Earth. The argument is not about what can be "seen"; it is purely about what events different observers consider to occur in the present moment."
- sudhirj 6y ago
  But the entire point of relativity is that there’s no such thing as a joint “present moment” between more than observer. The only meaningful construct is your own present moment, about which information radiates outward at the maximum speed of information transfer in space-time (speed of light/gravity). Information reaches other observers based on their trajectory and position in space time. No such thing as the universal now.
  - irthomasthomas 6y ago
    Which circles right back to the paradox. The moving persons reference frame has aliens still debating an invasion, and the stationary persons reference frame has the aliens already on their way. How can there be any room for free will in the deliberations of the alien government, if the timelines of both observers are equally valid? The article on relativity of simultaneity has much more detail on the subject https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity
    - 8note 6y ago
      For there to be a paradox, the observers would have to find out about it sooner than it would take light to communicate the result. Otherwise, the earliest time that the aliens can find out if they're going to invade or not is when they make the decision. Sure, the observers can find out, and try to tell each other and the aliens, but since it takes at least a round trip of light from the aliens to the observers, the observers always find out after the aliens have made up their minds. If the aliens find out that they make the decision when they make the decision, it seems to me at least that they actually made the decision rather than having it be predestined

simias 6y ago

I don't think I understand what's shocking about this. It's like saying that for somebody who watches a taping of a soccer match the result is not yet known, yet for somebody who watched it live it's a foregone conclusion. I don't really get what's the paradox here. Or maybe more to the point: if you were watching a soccer game Earth from Proxima Centauri, you could only get the result about 4 years after the game took place on earth, so for you the issue is uncertain while for somebody on Earth "at the same moment" the result has been known for years. But of course then the whole concept of "at the same moment" is pretty ambiguous and fuzzy. I don't really get what's paradoxical here.

klodolph 6y ago
The notion of “at the same moment” is not ambiguous and fuzzy. You use clocks and synchronize them. What relativity adds to the picture is the requirement for a common reference frame, which is surprising. It creates paradoxes because our intuitive notion of “at the same moment” does not include the reference frame, so there is a conflict between our intuitive understanding of past/future (which does not change whether you are moving) and the relativistic description of past/future (where two people can disagree about whether a particular event is int the past or the future). Because of this disagreement, if you had FTL communication and wired it up so you could make a phone call to Andromeda, your phone call would arrive before or after the debate depending on your reference frame on Earth, and you could use it to send information backwards in time.
- TheCraiggers 6y ago
  Synchronize the clocks how? Last I knew, this was still impossible, unless you employ hand-waving. If I synchronize two clocks next to each other, and then move one away, they are no longer synchronized as one was moving. And we still don't have the speed of c one way, so you can't use that either.
  - drran 6y ago
    Move clocks at the exact same speed and acceleration in opposite directions, so they stay synchronized.

javert 6y ago

I'm fairly convinced this is intellectual masturbation. I think two situations are being confused for one another: a) If you travel close to the speed of light, causality goes more slowly (e.g. a clock ticks more slowly) than it does for someone who is stationary. So, two clocks will diverge. This is true, AFAIK. b) If you travel close to the speed of light, and somebody else is stationary, they are traveling "into the future" relative to you, whereas you are "in the past" relative to them. I believe this is nonsense. But, I believe this is required for the "Andromeda Paradox" to hold. Thus, I don't think it holds. (BTW, we know that paradoxes never hold, so there must be something wrong with it.) The solution to the Andromeda Paradox that I am proposing is to say that there is one universal ordering of events (among both observers and the Andromedans), but the two Earthling observers have different times on their clocks at the particular point in the sequence of events where the Andromedans decide to invade.

thelastwave 6y ago
I think it's ridiculous anonymous cowards are downvoting you instead of replying. I may disagree with you (or not) but downvoting someone for having a different interpretation of relativity/cosmology, when the world's leading physicists can't agree on it, is the height of stupidity.
cthalupa 6y ago
>The solution to the Andromeda Paradox that I am proposing is to say that there is one universal ordering of events Proposing this is fine, but proving it is quite a matter altogether - you have to replace special relativity. Special relativity pretty much explicitly states this isn't the case, making an exception for events that are causally connected, but there's been a slew of experiments over the past several decades that call into question even that exception at long distances, and more recently, even ones for local observers, e.g. https://advances.sciencemag.org/content/5/9/eaaw9832 https://advances.sciencemag.org/content/5/9/eaaw9832 Personally, I'm in the Rovelli and loop quantum gravity camp - time probably isn't actually a real thing on its own, and what we perceive as time is just an emergent property of thermodynamics.
- 6y ago

amelius 6y ago

This is a nice video explaining simultaneity. https://www.youtube.com/watch?v=wteiuxyqtoM https://www.youtube.com/watch?v=wteiuxyqtoM

pdonis 6y ago

> the consequences of this simple rule The consequence you refer to is not a logical necessity based on relativity. It's just a particular interpretation, the "block universe" interpretation, which is not accepted by all physicists. See here for a refutation of the argument referred to: https://www.physicsforums.com/insights/block-universe-refuting-common-argument/ https://www.physicsforums.com/insights/block-universe-refuti...

falseprofit 6y ago
I don't know why it would follow from relativity, but the block universe just makes sense. To me, it follows from determinism and Occam's razor.
- ben_w 6y ago
  Because relativity does away with the alternative, that of a universally accessible “now”.
  - pdonis 6y ago
    > relativity does away with the alternative, that of a universally accessible “now” Relativity does away with "now" period--"now" is a frame-dependent concept in relativity, and frame-dependent concepts, in relativity, have no physical meaning. As I noted in the article I linked to upthread, the physically meaningful division of spacetime, at some particular event "here and now" (like you sitting in your chair reading this), is into three parts, not two: the past light cone, the future light cone, and the spacelike separated region (what Roger Penrose, in the very same book where he presents the Andromeda scenario, calls "elsewhere"). The past light cone behaves like our intuitive concept of "past", and the future light cone behaves like our intuitive concept of "future", but the spacelike separated region does not behave like anything our intuition is familiar with. The whole "Andromeda paradox" argument is based on failing to recognize that the spacelike separated region exists in relativity and doesn't match up with our intuitions about either "past" or "future".
- pdonis 6y ago

nojokes 6y ago

Welcome to the ancient times. The speed of information travel has always been capped but now we know that there is absolute upper limit - c. It still works on earth, now is not shared, but for most practical purposes the synchronization delay does not matter, except when it does like stock and sport betting. I think about this like this. The universe is always in exactly one state but for each inside observer the knowledge about this state depends on its location.

Uhhrrr 6y ago

Why would it be a day or so later? The Wikipedia makes that claim, but the linked source does not: https://archive.org/details/emperorsnewmindc00penr/page/302/mode/2up https://archive.org/details/emperorsnewmindc00penr/page/302/... The two people would perceive the changing of a traffic light on the same street at almost the same time - why would it be different for the Andromedans?

TheMightyLlama 6y ago

Is the person in the car moving away from or towards Andromeda? The example you gave can be interpreted two ways and my intuition tells me that the example is only true in the case that the car is moving _away_ from Andromeda. Is it also, weirdly, the case that if the car is travelling towards Andromeda that time slows down sufficiently that they perceive the outcome of the debate _after_ the person standing still?

thelastwave 6y ago
It is also weird that there is now a sports car that at various points in its orbit is moving towards Andromeda.

cellular 6y ago

I gave up the idea of time being a dimension long ago. It allows me to think in ways others do not. I made a video on a different way about thinking time slows near the speed of light: https://youtu.be/A-2XQQDD6QQ https://youtu.be/A-2XQQDD6QQ