7 ms·

>Say you woke up one day and your bed was gone. Your room, too. Gone. It's all gone. You wake up in an inky void. Not even a star. Okay, yes, it's a dumb idea,

>Say you woke up one day and your bed was gone. Your room, too. Gone. It's all gone. You wake up in an inky void. Not even a star. Okay, yes, it's a dumb idea, but just go with it. Now say you want to know if you move or not. Are you held fast in one spot? Or do you, say, list off to the left some? What I want to ask you is: Can you find out? Hell no. You can see that, sure. You don't need me to tell you. To move, you have to move to or away from ... well, from what? You'd have to say that you don't even get to use a word like "move" when you are the only body in that void. This is not as self-evident as the author believes. 1. Having taught a great many college level physics students, they have trouble grasping this. 2. More importantly, there's a reason people pursued the theory of the aether for so long -- you have to actually do the fucking experiments to show that there's no absolute reference frame you could be said to be moving in. Thinking that you can deduce physical facts about the universe a priori is the opposite of science. e: Suddenly remembered a Feynman bit from Lectures where he talks about exactly the attitude of treating this is somehow obvious: http://www.sciencechatforum.com/viewtopic.php?f=10&t=20750 http://www.sciencechatforum.com/viewtopic.php?f=10&t=207...

Dylan16807 13y ago
Not only is the point not-self-evident, the analogy is fundamentally flawed. You may not have an absolute speed but you can accelerate. If you really want a reference frame to compare to, you can use the one you started in before leaving. Edit: sorry, changed wording while being replied to
- buo 13y ago
  Without a reference frame, what experiment can you perform that would tell you that you're accelerating?
  - jkubicek 13y ago
    drop something? If it accelerates away from you, you are accelerating (or held stationary in a gravity field, but AFIAK, those are the same things).
    - buo 13y ago
      Yes, but when you drop something, you're creating a reference frame, aren't you?
      - shardling 13y ago
        Ah, you're misunderstanding the term reference frame as it's being used here. A reference frame is like a coordinate system for velocity; it's not something physical.
      - ars 13y ago
        No. When you drop something nothing whatsoever happens. And as shardling said, a reference frame isn't a physical thing that is created and destroyed - it's simply the zero point for your system of measurement - but the measurement would work just fine if you started with any number, zero is just convenient.
        buo 13y ago
        I understand a frame of reference doesn't have to be physical. However, if you're in an apparently empty and dark universe, and you're interested in knowing whether you're accelerating or not, postulating a frame of reference is not going to be of much help. I may have missed the point of dropping an object, though. I thought the idea was to create a physical point we could anchor the coordinate system to, and in that way be able to measure acceleration.
        ars 13y ago
        > However, if you're in an apparently empty and dark universe, and you're interested in knowing whether you're accelerating or not You can't. There is absolutely no test that can tell you this. You can't "drop" anything - whatever force is accelerating you also accelerates the object, so letting go of it tells you nothing.
        Dylan16807 13y ago
        Dropping an object that won't get accelerated is one way to measure acceleration, but not the only way. If you are the actor causing the acceleration on yourself, as I originally intended, then presumably you already know / measure-by-doing how much you have accelerated, and you can use dead reckoning to watch the earlier reference frame fly by. If the force is external and doesn't apply pressure then you have to drop an object. If the force affects the entire universe the same way then that's aether and can't be measured / doesn't exist. But that was not the kind of force I was using to disprove "you can't move".
  - lolcraft 13y ago
    I guess the GP understood special relativity, but not general ;) Well, to be really pedant, if you were spinning around yourself, you'd feel an acceleration.
  - kaybe 13y ago
    There will be 'gravity' -a force- in the opposite direction you are accelerating in. You should know the feeling, it happens when you accelerate in a car, or when a plane lifts off. If it's really you floating around, you will be able to feel it. (I reckon you could even guess at the amount by comparing it to the acceleration of 9.81 m/s^2 that you're used to.)
    - gizmo686 13y ago
      >There will be 'gravity' -a force- in the opposite direction you are accelerating in. Wouldn't there be a force in the direction you are accelerating in? Anyway, when you accelerate a car, you are not feeling acceleration, you are feeling the force of the car on you. Imagine being inside of a box that is in free fall. What experiment could you run to determine if you were accelerating?
      - kaybe 13y ago
        >Wouldn't there be a force in the direction you are accelerating in? Well, yes, that would be the standard way to look at it. Sorry for being confusing. >Anyway, when you accelerate a car, you are not feeling acceleration, you are feeling the force of the car on you. According to [Force = Mass * Acceleration] you can take them as equal (apart from the constant factor of your mass). In the freefalling box that is accelerating (and thus no longer in free fall) you'd also feel a force. No need for experiments if you have your normal senses. You'll be pressed against one side of the box. (And if the acceleration is 9.81 m/s^2 you couldn't distinguish it from a box on earth.) That is actually the basic premise of general relativity - the equality of 'inertial mass' that resists against acceleration and 'gravitational mass' that happens because of, well, gravity. It's like the idea of constant speed of light for special relativity. edit: Unless you're talking about gravity, which accelerates all parts of the setup equally, thus no force results.. but that would need something else in the experiment, either a great amount of matter or energy. But then you'd have a reference frame right there. (Mathematically. As for an experiment.. hm. Check for perturbances from space-time distortions that result from rotating gravity sources, maybe.. most gravity sources in this universe are rotating. There's an experiment done that checked that for Earth.[1]) [1] https://en.wikipedia.org/wiki/Gravity_Probe_B https://en.wikipedia.org/wiki/Gravity_Probe_B
- shardling 13y ago
  > If you really want a reference frame to compare to, you can use the one you started in before leaving. Nah, the analogy is fine there -- you "wake up" in a new reference frame, and weren't able to make any measurements to compare it to your old one. Whether you can measure your motion against the old reference frame starting from this situation is actually the question!
  - Dylan16807 13y ago
    Oh, that's not what I meant. I meant use the reference frame from when you first woke up. But I'll make a more detailed scenario: * Wake up in the void. Label your current reference frame R0. * Notice that there is no real way to label 'speed' vs. void. * We can however measure angular speed via tension on body parts, it seems to be 0. * Accelerate somehow * * for some magical reason it's forbidden to have more than one object, so I thought up a couple minimally-magic ways to do it that I am pretty sure don't cause physics problems by themselves. * * * You could throw something then immediately pretend the thing you threw doesn't exist, forbidding it as a reference frame. * * * You could make a punching motion and use magic to cause your arm to disappear. * * * You could swing all your limbs away from your body and use magic to reverse the direction of one of them. * Now you are in reference frame R1 because you accelerated. * Hooray you accelerated without any other objects to compare to. If you insist on comparing to a reference frame, use R0.
    - shardling 13y ago
      The point the original author is implicitly trying to make is that you can't compare to your original frame, where you went to sleep.
      - Dylan16807 13y ago
        This is the part I'm objecting to: To move, you have to move to or away from ... well, from what? You'd have to say that you don't even get to use a word like "move" when you are the only body in that void. You can move. You can have a delta v. You don't have an absolute position or velocity, but you don't need those to move. I don't see any comparison to the pre-sleep state, this is a statement about the void, and it overreaches.
jbattle 13y ago
This got me thinking. If you are floating along in a void, and float past a virtual particle - you can legitimately say you are moving (of course you could instead say the virtual particle is moving). Assume now the virtual particle winks out of existence. Have you suddenly stopped moving? Were you wrong/incorrect somehow to have chosen the particle as the rest frame? Has conservation of (your) momentum just been violated?
- Cushman 13y ago
  I'm no physicist, but my understanding is that observing a virtual particle is a contradiction in terms.
  - tekacs 13y ago
    Observing a virtual particle directly, surely is.
- boboblong 13y ago
  No, you haven't suddenly stopped moving. If you choose the virtual particle's reference frame as the rest frame, then you are moving in its reference frame. If the particle winks out of existence, you are still moving in its reference frame, you just have no way to perceive that fact.
oskarth 13y ago
I don't understand. What would the alternative be? If the answer to "can you find out" is "yes", then how?
- shardling 13y ago
  The alternative is that there's some fundamental reference frame. The obvious example is the theory of the aether I alluded to: light would propagate in different speeds in different reference frames. More generally, you can't know that the laws of physics are the same in all reference frames until you actually do the experiments. If you think otherwise, you underestimate nature's ability to confound human intuition! A good example is is parity, which you'd think would be a good symmetry of nature, but is actually violated by some interactions: http://en.wikipedia.org/wiki/Parity_violation#Parity_violation http://en.wikipedia.org/wiki/Parity_violation#Parity_violati...
Cushman 13y ago
Exactly my thought reading that. That question, "Can you find out? Hell no," is relativity in a nutshell. If you really grasp what that means, the whole deal from simultaneity to time dilation to curved spacetime all falls out more or less intuitively. (Of course it was harder to work out the first time :P) Most people implicitly believe that there must be some privileged reference frame, if not mine necessarily than at least one very close to mine, and for good reason. (I'd go so far as to say that most people believe explicitly that there is a privileged reference frame, and that it is an intelligent agent called God, but leave that alone for now.) Even if they do get past this statement with a, "Uh, okay, I guess so..." they probably have not actually adjusted their entire world view to account for that powerful fact, and if they seem to understand what you have told them it is likely that they have simply memorized the right answer. Not that there's anything wrong with trying.
- ars 13y ago
  > I'd go so far as to say that most people believe explicitly that there is a privileged reference frame, and that it is an intelligent agent called God ????? Why do you assume God is a physical being inside the universe? (Do people really think that?) God created the universe - God is outside it. Like God is the computer and the universe is the program. God isn't at rest/not at rest. The concept doesn't even apply.
  - Cushman 13y ago
    In the United States it is the majority opinion (vocally, anyway) that God is a real thing with both interest and power to intervene in both historical and day-to-day events. As to why? A lifetime of indoctrination and social pressure. The same reason anyone believes anything, I suppose.
    - ars 13y ago
      Really? The majority opinion is that God is a physical being that lives inside the universe? I know there are some religions that believe that, but they are hardly the majority. The concept of "reference frame" doesn't even exist for a non-physical being.
ambiate 13y ago
I use the void concept to explain my absurdity. I usually fall back on explaining the thought process that follows awakening in a void. The loss of identity, speech, motion, etc. Now, I wish I had taken a physics class.
breadbox 13y ago
What I was trying to express by "inky void, not even a star" was that the void didn't even include anything resembling an aether. "You are the only body in that void" is meant to include non-material bodies. All this passage is trying to establish is that in order to distinguish constant motion from absolute stillness, there has to be something else to measure against. Whether or not our universe contains something that can serve as an absolute frame of reference is not the point -- yet. So yes, it really is meant to be talking about a self-evident fact (once you think about it). But you're right; I'm not a physics teacher. I'm willing to concede that I underestimated the potential stickiness of this point, and/or that I failed to overcome my handicap in expressing this point.
- jules 13y ago
  It's only obvious because that's what you were taught. Aristotle thought that a moving body would come to a halt by itself, unless you keep pushing it. He thought that this was a law of physics. This didn't require any aether or other substance. It was almost 2000 years later that Galilei came with the idea bodies set in motion will remain in motion, and that the fact that they usually stop was solely the result of friction. But even then it is not clear that there isn't some other method by which you can find out your absolute velocity. It was only in 1887 that the Michelson-Morley experiment showed that you can't do that with light! So this is not a triviality that can be deduced by logical reasoning, it's an experimental result of physics. This makes you wonder which falsehoods we currently take for granted, that later generations can't even fathom anymore.
  - breadbox 13y ago
    I find myself disagreeing still. I would say that the idea that you can't measure your own motion without something else to measure against, is obvious to the casual person. (Whether that "something" is a physical object, the background radiation, the luminiferous aether, or whatever.) The possibility that it might not be true is actually far less intuitive, since it would require there to be some intrinstic frame of reference that is impossible to leave behind, even in a thought experiment.
tocomment 13y ago
What if there were two bodies instead of one and they were 1 light year apart. What would motion be like then?

Dylan16807 13y ago

Not only is the point not-self-evident, the analogy is fundamentally flawed. You may not have an absolute speed but you can accelerate. If you really want a reference frame to compare to, you can use the one you started in before leaving. Edit: sorry, changed wording while being replied to

buo 13y ago
Without a reference frame, what experiment can you perform that would tell you that you're accelerating?
- jkubicek 13y ago
  drop something? If it accelerates away from you, you are accelerating (or held stationary in a gravity field, but AFIAK, those are the same things).
  - buo 13y ago
    Yes, but when you drop something, you're creating a reference frame, aren't you?
    - shardling 13y ago
      Ah, you're misunderstanding the term reference frame as it's being used here. A reference frame is like a coordinate system for velocity; it's not something physical.
    - ars 13y ago
      No. When you drop something nothing whatsoever happens. And as shardling said, a reference frame isn't a physical thing that is created and destroyed - it's simply the zero point for your system of measurement - but the measurement would work just fine if you started with any number, zero is just convenient.
      - buo 13y ago
        I understand a frame of reference doesn't have to be physical. However, if you're in an apparently empty and dark universe, and you're interested in knowing whether you're accelerating or not, postulating a frame of reference is not going to be of much help. I may have missed the point of dropping an object, though. I thought the idea was to create a physical point we could anchor the coordinate system to, and in that way be able to measure acceleration.
        ars 13y ago
        > However, if you're in an apparently empty and dark universe, and you're interested in knowing whether you're accelerating or not You can't. There is absolutely no test that can tell you this. You can't "drop" anything - whatever force is accelerating you also accelerates the object, so letting go of it tells you nothing.
        Dylan16807 13y ago
        Dropping an object that won't get accelerated is one way to measure acceleration, but not the only way. If you are the actor causing the acceleration on yourself, as I originally intended, then presumably you already know / measure-by-doing how much you have accelerated, and you can use dead reckoning to watch the earlier reference frame fly by. If the force is external and doesn't apply pressure then you have to drop an object. If the force affects the entire universe the same way then that's aether and can't be measured / doesn't exist. But that was not the kind of force I was using to disprove "you can't move".
- lolcraft 13y ago
  I guess the GP understood special relativity, but not general ;) Well, to be really pedant, if you were spinning around yourself, you'd feel an acceleration.
- kaybe 13y ago
  There will be 'gravity' -a force- in the opposite direction you are accelerating in. You should know the feeling, it happens when you accelerate in a car, or when a plane lifts off. If it's really you floating around, you will be able to feel it. (I reckon you could even guess at the amount by comparing it to the acceleration of 9.81 m/s^2 that you're used to.)
  - gizmo686 13y ago
    >There will be 'gravity' -a force- in the opposite direction you are accelerating in. Wouldn't there be a force in the direction you are accelerating in? Anyway, when you accelerate a car, you are not feeling acceleration, you are feeling the force of the car on you. Imagine being inside of a box that is in free fall. What experiment could you run to determine if you were accelerating?
    - kaybe 13y ago
      >Wouldn't there be a force in the direction you are accelerating in? Well, yes, that would be the standard way to look at it. Sorry for being confusing. >Anyway, when you accelerate a car, you are not feeling acceleration, you are feeling the force of the car on you. According to [Force = Mass * Acceleration] you can take them as equal (apart from the constant factor of your mass). In the freefalling box that is accelerating (and thus no longer in free fall) you'd also feel a force. No need for experiments if you have your normal senses. You'll be pressed against one side of the box. (And if the acceleration is 9.81 m/s^2 you couldn't distinguish it from a box on earth.) That is actually the basic premise of general relativity - the equality of 'inertial mass' that resists against acceleration and 'gravitational mass' that happens because of, well, gravity. It's like the idea of constant speed of light for special relativity. edit: Unless you're talking about gravity, which accelerates all parts of the setup equally, thus no force results.. but that would need something else in the experiment, either a great amount of matter or energy. But then you'd have a reference frame right there. (Mathematically. As for an experiment.. hm. Check for perturbances from space-time distortions that result from rotating gravity sources, maybe.. most gravity sources in this universe are rotating. There's an experiment done that checked that for Earth.[1]) [1] https://en.wikipedia.org/wiki/Gravity_Probe_B https://en.wikipedia.org/wiki/Gravity_Probe_B
shardling 13y ago
> If you really want a reference frame to compare to, you can use the one you started in before leaving. Nah, the analogy is fine there -- you "wake up" in a new reference frame, and weren't able to make any measurements to compare it to your old one. Whether you can measure your motion against the old reference frame starting from this situation is actually the question!
- Dylan16807 13y ago
  Oh, that's not what I meant. I meant use the reference frame from when you first woke up. But I'll make a more detailed scenario: * Wake up in the void. Label your current reference frame R0. * Notice that there is no real way to label 'speed' vs. void. * We can however measure angular speed via tension on body parts, it seems to be 0. * Accelerate somehow * * for some magical reason it's forbidden to have more than one object, so I thought up a couple minimally-magic ways to do it that I am pretty sure don't cause physics problems by themselves. * * * You could throw something then immediately pretend the thing you threw doesn't exist, forbidding it as a reference frame. * * * You could make a punching motion and use magic to cause your arm to disappear. * * * You could swing all your limbs away from your body and use magic to reverse the direction of one of them. * Now you are in reference frame R1 because you accelerated. * Hooray you accelerated without any other objects to compare to. If you insist on comparing to a reference frame, use R0.
  - shardling 13y ago
    The point the original author is implicitly trying to make is that you can't compare to your original frame, where you went to sleep.
    - Dylan16807 13y ago
      This is the part I'm objecting to: To move, you have to move to or away from ... well, from what? You'd have to say that you don't even get to use a word like "move" when you are the only body in that void. You can move. You can have a delta v. You don't have an absolute position or velocity, but you don't need those to move. I don't see any comparison to the pre-sleep state, this is a statement about the void, and it overreaches.

jbattle 13y ago

This got me thinking. If you are floating along in a void, and float past a virtual particle - you can legitimately say you are moving (of course you could instead say the virtual particle is moving). Assume now the virtual particle winks out of existence. Have you suddenly stopped moving? Were you wrong/incorrect somehow to have chosen the particle as the rest frame? Has conservation of (your) momentum just been violated?

Cushman 13y ago
I'm no physicist, but my understanding is that observing a virtual particle is a contradiction in terms.
- tekacs 13y ago
  Observing a virtual particle directly, surely is.
boboblong 13y ago
No, you haven't suddenly stopped moving. If you choose the virtual particle's reference frame as the rest frame, then you are moving in its reference frame. If the particle winks out of existence, you are still moving in its reference frame, you just have no way to perceive that fact.

oskarth 13y ago

I don't understand. What would the alternative be? If the answer to "can you find out" is "yes", then how?

shardling 13y ago
The alternative is that there's some fundamental reference frame. The obvious example is the theory of the aether I alluded to: light would propagate in different speeds in different reference frames. More generally, you can't know that the laws of physics are the same in all reference frames until you actually do the experiments. If you think otherwise, you underestimate nature's ability to confound human intuition! A good example is is parity, which you'd think would be a good symmetry of nature, but is actually violated by some interactions: http://en.wikipedia.org/wiki/Parity_violation#Parity_violation http://en.wikipedia.org/wiki/Parity_violation#Parity_violati...

Cushman 13y ago

Exactly my thought reading that. That question, "Can you find out? Hell no," is relativity in a nutshell. If you really grasp what that means, the whole deal from simultaneity to time dilation to curved spacetime all falls out more or less intuitively. (Of course it was harder to work out the first time :P) Most people implicitly believe that there must be some privileged reference frame, if not mine necessarily than at least one very close to mine, and for good reason. (I'd go so far as to say that most people believe explicitly that there is a privileged reference frame, and that it is an intelligent agent called God, but leave that alone for now.) Even if they do get past this statement with a, "Uh, okay, I guess so..." they probably have not actually adjusted their entire world view to account for that powerful fact, and if they seem to understand what you have told them it is likely that they have simply memorized the right answer. Not that there's anything wrong with trying.

ars 13y ago
> I'd go so far as to say that most people believe explicitly that there is a privileged reference frame, and that it is an intelligent agent called God ????? Why do you assume God is a physical being inside the universe? (Do people really think that?) God created the universe - God is outside it. Like God is the computer and the universe is the program. God isn't at rest/not at rest. The concept doesn't even apply.
- Cushman 13y ago
  In the United States it is the majority opinion (vocally, anyway) that God is a real thing with both interest and power to intervene in both historical and day-to-day events. As to why? A lifetime of indoctrination and social pressure. The same reason anyone believes anything, I suppose.
  - ars 13y ago
    Really? The majority opinion is that God is a physical being that lives inside the universe? I know there are some religions that believe that, but they are hardly the majority. The concept of "reference frame" doesn't even exist for a non-physical being.

ambiate 13y ago

I use the void concept to explain my absurdity. I usually fall back on explaining the thought process that follows awakening in a void. The loss of identity, speech, motion, etc. Now, I wish I had taken a physics class.

breadbox 13y ago

What I was trying to express by "inky void, not even a star" was that the void didn't even include anything resembling an aether. "You are the only body in that void" is meant to include non-material bodies. All this passage is trying to establish is that in order to distinguish constant motion from absolute stillness, there has to be something else to measure against. Whether or not our universe contains something that can serve as an absolute frame of reference is not the point -- yet. So yes, it really is meant to be talking about a self-evident fact (once you think about it). But you're right; I'm not a physics teacher. I'm willing to concede that I underestimated the potential stickiness of this point, and/or that I failed to overcome my handicap in expressing this point.

jules 13y ago
It's only obvious because that's what you were taught. Aristotle thought that a moving body would come to a halt by itself, unless you keep pushing it. He thought that this was a law of physics. This didn't require any aether or other substance. It was almost 2000 years later that Galilei came with the idea bodies set in motion will remain in motion, and that the fact that they usually stop was solely the result of friction. But even then it is not clear that there isn't some other method by which you can find out your absolute velocity. It was only in 1887 that the Michelson-Morley experiment showed that you can't do that with light! So this is not a triviality that can be deduced by logical reasoning, it's an experimental result of physics. This makes you wonder which falsehoods we currently take for granted, that later generations can't even fathom anymore.
- breadbox 13y ago
  I find myself disagreeing still. I would say that the idea that you can't measure your own motion without something else to measure against, is obvious to the casual person. (Whether that "something" is a physical object, the background radiation, the luminiferous aether, or whatever.) The possibility that it might not be true is actually far less intuitive, since it would require there to be some intrinstic frame of reference that is impossible to leave behind, even in a thought experiment.

tocomment 13y ago

What if there were two bodies instead of one and they were 1 light year apart. What would motion be like then?