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Comment by shardling | Hacker News Reader

>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=207...

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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.

> 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.

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.

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.

I can't rigorously defend something I don't think is true, but I stand behind my previous statement as it is.

Your previous statement might be true - and also has nothing to do with a "universal reference frame".

A god that intervenes has nothing to do with whether or not the God is physical.

I suspect you are reading things into what people believe without actually understanding what they believe. i.e. that it's actually you that believes God has a reference frame, not that the majority of people in the US do.

Well, I'm not that interested in the theology of an interventionist God because it's not a very exciting fairy tale to me. But as I understand it God is omnipresent and omnipotent and either those things mean a physical presence that is everywhere at once or they mean nothing at all. That is precisely the thing proscribed by relativity.

I'm sure one could define God down into some kind of good-vibes field that could logically exist in our universe, and that's all good with me. But it's not what most Americans profess to believe. In any event, discard it if it offends you; it's completely irrelevant to the topic, which is super-cool physics.

> mean a physical presence that is everywhere at once

Everywhere at once means "no privileged reference frame". (Kind of like how light, from the point of view of light itself is everywhere at once, and light also has a the special privileged reference frame of no reference frame at all.)

You were the one who claimed the most people believe in a privileged reference frame based on God. Not me. Now you are backpedaling from that claim - which is fine, because not only do most people not believe that, it doesn't even make any sense.

And I agree that this should conclude the religious digression of the main topic. Not that there is a whole lot more to talk about, unless you want to talk about (argue with?) my comment above on light.

> Everywhere at once means "no privileged reference frame".

Probably you'll never read this now, but... Nope. Everywhere at once literally means a privileged reference frame. "Over there, now" is a concept with no logical meaning in our physical universe. Being in two places at once isn't physically impossible, it's just nothing.

And that imposes very serious constraints on God. If I am on the Earth and I pray for my husband on Mars, God on Mars can not hear my prayer until it is too late. You really think that's a statement most Americans agree with?

You're also off about light. Light is not "everywhere at once from its own perspective" as you seem to think. The fact that light cannot be everywhere at once is the same fact that dictates that light cannot have a point of view, that light experiences no time and has no perspective. (Mind you, if this is the comparison you would like to draw to God, I will happily assent.) Study up and be a bit more careful with your science, and you'll be able to throw much meatier punches theologically.

????? Why do you assume »God created the universe - God is outside it.« (Do people really think that?)

Because you can not create something and be inside it (i.e. be made of it) at the same time.

My point is more that believing God created the universe is not really an obvious thing to believe. Why not just believe that the nobody created the universe and it has been around forever? Or that the universe emerged out of »nothing«?

No? That can not possibly be true? How could something be around forever? And how could something so complex and beautiful as the universe with all the fancy stuff inside emerge out of »nothing«? No way.

But wait, it is no problem for God to be around forever? It is no problem to have an entity, powerful enough to create a whole universe, without explaining where it came from? Or should we just introduce a super-god that created God? Unfortunately this just shifts the problem to the next level and we are again back at square one.

Besides that you can obviously build a house and enter through the front door after you finished it.

Wait. When did this morph into an argument of the existence of God? I thought we were talking about God being some sort of preferred relativistic reference frame.

> Besides that you can obviously build a house and enter through the front door after you finished it.

You did not enter the material house - you entered the empty space that was already there, which the material of the house surrounds. If you want to use the analogy you must actually change your physical body to be made of wood and nails.

Discussing God as a reference frame seems to naturally evoke the question if he exists. And I am definitely more comfortable discussing whether God exists or not than if he may be a (preferred) (inertial) reference frame; probably one of the strangest things I have ever thought about.

Yes, the analogy with building a house is not really strong but on the other hand the whole question if God - if he exists - exists inside or outside the universe (read spacetime) is difficult to say the least. What is outside of space? What is outside of time? Out of what is God made? And how does this affect his possibility to interact with the universe? Is he even able to interact?

Now that I got your point with being something and building the same thing, self-organizing systems come to mind - that's probably as close as you can get to being something and building the same thing. This does not address where the matter for the system come from so it is more self-shaping than self-building but ad hoc that's the best I can think of.

> Wait. When did this morph into an argument of the existence of God?

You sort of brought it up yourself when you said "God created the universe." like it's a fact, didn't you? Maybe you could explain what that's supposed to mean here?

Of course God can do so, he's God. He can do anything!1!

Most people do not think it through that way. They probably don't think God is a physical being, and they probably don't have a very specific idea about his location, but they definitely think he is viewing the universe, wherever he is.

Fine, but if he's viewing the universe he is not inside the universe - i.e. he doesn't have a reference frame.

This is your assertion, not theirs.

Most people do not think it through that way.

I suspect that not thinking it through that way is a precondition for truly believing in God for most people. That is to say, most people who start out believing in God and who attempt to think this through would give up their belief in God on the way - or, instinctively realizing that and being made uncomfortable by the thought, they would automatically shy away from the whole logic of it.

After all, explaining the world is not really what belief in God seems to be about. It seems to be more about some kind of warm fuzzy feeling that people get from it.

> 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)

You cannot deduce relativity from the knowledge that you can't find out whether you're moving. In Newtonian physics you can't find out whether you're moving either. The essential ingredient is that the speed of light is the same in all reference frames. Furthermore curved spacetime is by no means trivial; that's general relativity. It took Einstein himself 10 additional years before he got from special to general relativity, and more still to really develop it.

I sort of meant "given experimental observations it all seems quite intuitive", not "you can derive it all a priori". No offense intended to the hard work of Einstein and others :)

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.

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.

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.

You can measure acceleration without needing a special frame. More significantly, you can measure rotational motion without something to measure against.

See that Feynman quote I linked in my original post; he discusses exactly this point.

Which is exactly why that passage confined itself to talking about constant motion.

But the argument you used would seem to apply to both of those thing. That's what makes it flawed -- you claim it is obvious, when in fact this line of argument is very subtle and often misleads laymen. (Notice that in this very thread there seem to be people who think you couldn't detect acceleration.)

So you're saying that what laymen mean when they intuitively understand that you can't measure your own motion is something else than what is meant by a physicist in the context of relativity. In particular a physicist means that you can't do that even when you have access to light, or those other things. A layman understands that you can't measure whether you're moving without access to those things, but he does not understand that it's still impossible with access to those things. In that case you cannot appeal to layman intuition on the former to conclude the latter, which is exactly what's necessary for an understanding of relativity.

Also, even with this interpretation it's questionable. In Aristotle's mind you can determine whether you're moving or not, even if you are completely alone in the universe. You take an object that you have with you (e.g. your t-shirt), throw it in a random direction with a specified velocity relative to you. According to Aristotle both you and the watch will decelerate. But since the deceleration is relative to the absolute reference frame in which you will eventually come to a halt, you can deduce whether you're moving from the relative deceleration between you and your watch (although interestingly if the deceleration has a special form, then you cannot determine whether you are moving, but that is again a non trivial fact that is not obvious to a layman). To Aristotle, and the people after him until Galilei, the idea of a special reference frame was perfectly intuitively obvious! (though, of course, incorrect)

This whole thread of discussion seems irrelevant to me. The article is obviously not intended to provide a rigorous treatment of relativity. It is not meant to derive relativity from first principles. It is not even meant to be accessible to people who don't have prior knowledge of relativity. It is meant to be funny, and it succeeds at that.

I think using the word "obvious" is very misplaced here. Obvious would suggest that fully understanding the statement is something the casual person would arrive at given no help or extraneous thought into the statement. This is simply not true given what we know about the average intelligence of humans.

This still happens in cartoon/TV/movie physics: the engines of a space ship cut out, and the space ship coasts to a halt (even when the space drive is obviously Newtonian, and not magic).

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?

I'm no physicist, but my understanding is that observing a virtual particle is a contradiction in terms.

Observing a virtual particle directly, surely is.

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

Without a reference frame, what experiment can you perform that would tell you that you're accelerating?

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).

Yes, but when you drop something, you're creating a reference frame, aren't you?

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.

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.

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.

> 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.

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".

> If the force is external and doesn't apply pressure then you have to drop an object.

What kind of force is external and doesn't apply pressure, and also doesn't affect the dropped object? There is no such force, and therefor no such test.

That wouldn't work.

If you dropped something it would simply stay right next to you, even if you were accelerating - both of you are accelerating at exactly the same rate.

Why? What's making it accelerate? I wasn't trying to talk about some kind of universal/aether acceleration. I meant you and only you should accelerate.

And how do you accelerate - do you have a rocket attached to your feet?

It doesn't really matter. But there should be some mechanism. A rocket attached to your feet works quite well if you ignore the exhaust for purposes of reference.

If you want a useful thought experiment about physics then you need to give reasonable physics tools. Otherwise you get something like "Pretend you are an inanimate green blob. You can't climb a hill, can you? Therefore climbing a hill is an illusion."

If you have a rocket on your feet why do you need to drop something to figure out if you are accelerating? You can feel the pressure on your feet.

The only type of acceleration that is undetectable is gravity - and gravity will effect the dropped object exactly as much as you.

Great! You just simplified the experiment!

Dropping something was only one idea, but feeling pressure is much simpler.

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.

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.)

>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?

>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

> 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!

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.

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.

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.

This is a metaphor breakdown because you pushed it too hard, rather than a real problem with the idea. In the ideal case you are a floating point, and you haven't got discrete parts of yourself to fling away, but of course that's an abstract step too far away for the target audience of this piece.

Reflections on Relativity [1] in fact does build up to a generalize concept of relativity (not just Einsteinian relativity) starting with points in an unknown space (building up to a concept of distance, rather than assuming such a thing), but it's a mathematical treatment of the problem rather than an intuitive one. And certainly not in words of one syllable.

[1]: http://mathpages.com/rr/rrtoc.htm , the part I'm referencing starts right off at 1.1.

I think I haven't made it clear enough what my objection is. In my opinion it is the article that pushed the analogy too far. It works great to show that you can't tell what is moving. But when the analogy tries to claim that you can't move, well, it's pretty much begging the question. Of course a floating point can't move, and it says nothing about physics. That's why I assumed some mechanism for trying to cause motion: you can't see if you failed if you didn't try.

I guess I wasn't clear (no sarcasm). My point is based on the idea objecting that an analogy doesn't stand up to being pushed "too far" isn't a useful one; analogies always break down when pushed too far. If they didn't, they wouldn't be analogies, they'd be explanations. Analogies are dangerous ways of thinking, but again, this is generally targeted at people for who that is all they have available.

The analogy is useful, I agree there. The problem is that the article first uses the analogy for something it fits, then uses the analogy again for something it doesn't fit. The conclusions drawn from the second use are invalid. I don't object to the analogy, I object to that second use.

I had the same objection as the op actually - if you wake up at "rest" with just yourself and a bed, and accelerated away from the bed, you can in fact know whether "you are moving" or "the bed is moving". It is because one of the two have only been in 1 reference frame, while the other has been in 2.

Basically the same issue as the twin "paradox". I can appreciate the assumption of a constant velocity on wake though, as a way to simplify the story.

I don't think you can accelerate in an empty universe - no speed, no speed difference.

You're talking about accelerating. The original passage was just talking about simple constant motion. The point being that you can't distinguish constant motion from remaining perfectly still without something else to measure against. That's really all I was trying to say. Obviously I failed!

For what it's worth I think you did a good job overall, but flaws are pretty much inevitable when your word choice is so restricted.

>* * * You could swing all your limbs away from your body and use magic to reverse the direction of one of them.

No you can't, this leads to the twin paradox. Assume you could use magic to simply reverse the direction of an object. Take two clocks, and have one travel away from you at relativistic speeds, then turn around and come back. Because of time dilation, when the clock returns, it will have observed less time pass then you, because it was moving at relativistic speeds. However, from the clocks perspective, you were moving at relativistic speeds, so you will have observed less time pass. The resolution to this paradox involves the fact that the clock was accelerated in order to turn around.

An interesting side point is that in the hypothetical void, you actually have no way of knowing weather or not you are accelerating. For example, the space station is currently in free fall towards the earth, but from within the station you cannot detect this fact. Similarly, if you are in a plane that is descending at the rate of gravity, you will appear to be in zero-g. The mathematics behinds this is considerably more difficult than special relativity.

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.

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

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

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_violati...

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