We may think that it is easy to imagine empty space. We look around us and remove every item that we can see and that leaves us with empty space. We tend to treat empty space as a cavity of some kind, some region from which all matter and radiation has been removed. And that is fine as long as matter and/or radiation exists in some other part of the universe, so that we can envisage the space between things. But what if there was no matter or radiation anywhere in the universe? Would space still exist?
Albert Einstein in a letter to Karl Schwarzschild on 9 January, 1916 said no.
“The essence of my theory is precisely that no independent properties are attributed to space on its own. It can be put jokingly this way. If I allow all things to vanish from the world, then following Newton, the Galilean inertial space remains; following my interpretation, however, nothing remains.”
The idea that something must exist for space itself to exist is hard to digest. To some, this may sound similar to the Zen-like question of whether a tree falling in a forest makes a sound if no one is there to hear it. Don’t things have an objective existence irrespective of the existence of an observer? Whatever the answer you give for sound, Einstein felt that the existence of space required matter.
The analog to the tree question is to imagine spinning around on your own axis, like ice skaters do. You will find your arms will want to get tugged outward and you will start to feel dizzy because of the way that the fluid in your ear gets sloshed around. Now imagine that you are all alone in the universe. According to Einstein, you would not be able to tell if you were spinning or not because there is no reference frame that you can use to tell the difference.
If you are finding this hard to digest, welcome to the club. Many of Einstein’s contemporaries also found it hard to accept. I have encountered this when trying to explain the Big Bang theory and the expanding universe to nonscientists. People tend to think of the Big Bang as similar to the explosions they are familiar with, where particles fly off in all directions. So they tend to think that the universe is expanding into an already existing space and naturally ask what exists beyond the boundary of that space. It is hard to grasp the idea that there is no boundary, the universe is all there is. That implies that if we run the clock backwards, space itself will shrink until we get to the Big Bang, when presumably there will be no space either.
You can read more about this here.
I thought *spinning* was one of the things you *could* tell without reference to an external frame?
What is empty space? -- MTG’s skull. (ba dum tiss : )
I use a balloon analogy. When you blow up a balloon, the space inside gets bigger and the space outside (i.e., the space in the room) gets smaller. The universe in *not* like that. It doesn’t expand *into* anything -- it just *expands*.
Yes, like xohjoh2n says, I have read articles where the exact opposite claim is made: if I were spinning in an utterly empty universe, I could know it because I would feel my arms being pulled away from my body, therefore empty space is a thing in its own right and that is how the universe can be expanding from the Big Bang.
Yeah, Einstein called it Mach’s Principle.
Attempts have been made to incorporate Mach’s Principle into general relativity, a famous example being Brans-Dicke theory. My understanding is that it has been pretty much ruled out.
xohjoh2n and moarscienceplz,
The question you raise is a deep one and the argument you give is what Newton made. I discuss this and why it is now considered to be unsatisfactory in a post about a year ago that I think you might find helpful.
If empty space requires a universe, then the universe must have an actual boundary, else the transition from space to not-space is meaningless and indeterminable.
@Matt G:
The usual ‘balloon’ analogy used for the expansion of space is that space itself is only the surface of the balloon: it can get larger without actually expanding out from any point that is actually within the space itself.
And of course any ‘boundaries’ need not be in any of the spatial dimensions we have access to. If space as a positive underlying curvature (like the balloon), it will loop back on itself and any line extended off as far as it can will never end.
That said, because of the expansion, nothing travelling at any speed less than or equal to the speed of light will never finish any sort of loop like that, because over large enough distances space expands faster than the light can keep up. Again, on the balloon, the speed of light limit only applies to things travelling along the balloon’s surface, the balloon itself can expand faster than that so that things will apparently move away faster than we can possibly catch up to them.
Yes, it’s a mess to wrap your head around it.
jenorafeuer @8:
I think that depends on the rate of expansion. If the scale factor a(t) obeys a power law, there is no limit to the distance a photon could travel (although it would get red-shifted). If the scale factor is exponential (which it seems nearly to be now), then there is a distance limit.
I’m going on memories of calculations I did years ago, for a flat but expanding spacetime. I don’t think curvature qualitatively changes those conclusions. I’ll try to dig up my notes…
Isn’t it spacetime, not just space?
Further to #9: If a(t) is proportional to t^a, where 0 < a < 1, then in a flat spacetime, there is no limit to the distance a photon can travel. That was the case until a few billion years ago.
I will start with the obvious “I am not a physicist” but, given that time (in terms of spacetime) is a measurement of the movement of matter through or across space, without matter time cannot be measured (in the same way that Berkeley and Mach viewed motion as unmeasurable so therefore non-existent in a matter less universe -- at least as far as I understand this).
If this is the case, wouldn’t matter-time be a more appropriate coupling? Is there a reason amongst physicists why this is not the case?
And doesn’t space fill matter in the same way that matter fills space (as in one does not displace the other or they fill or infuse each other)?
tuatara, without time, length cannot be measured either.
Mind you, I have hardly kept up with the recent changes to basic units.
cf. https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units#Impact_on_base_unit_definitions
tuatara @12:
It’s not either/or. Space, time and matter are all involved in motion (e.g. a spinning object).
I remember a quote, supposedly by Einstein, in which he said that without matter/energy, there is no time or space.
Come on, seriously now….. I’m as much of an Einstein fan as anybody, but it doesn’t matter in the slightest what Einstein may or may not have “felt” about it.
Did Einstein himself ever actually make that specific claim? Because it still sounds like sophistry to me.
We mere mortals (non-physicists) don’t typically define and use a reference frame in daily life to determine things like this. You can just use various senses to tell whether or not you’re spinning. If you feel the familiar pull which is noticeable when you’re spinning, that’s pretty much all you need to know.
What exactly does this have to do with “empty space” anyway? You may be alone in it, but your space definitely isn’t empty anymore if it contains a person.
I never got a substantive response to my criticisms — to be clear, just saying “Mach’s principle” isn’t a valid argument, nor is it an explanation, similar to claiming “Einstein felt that X was true.” The objections certainly weren’t original to me either, although I can’t say I know much about the full history of these arguments, which I’m sure goes back a long time.
I think the lack of substance is probably for the simple reason that that whole line of thought is just plain confused. It’s one thing to just entertain such ideas for the sake of argument, in case dismantling them turns out to be helpful or even educational, but hopefully that’s not the game we’ll always be playing.
cr:
Um, this is in the relation to the immediately preceding quotation.
That is, given that quotation, it is inferable that he felt thus.
Perhaps reconsider the last sentence of the quotation in the OP and the first sentence of the implication.
Seems very clear to me, and is the very nub of the matter — what the topic is all about. So, kinda matters in relation to this post.
Make a box well away from anything interesting. Shield the box to prevent anything from entering. Remove any stray hydrogen and helium atoms. I would say it contains empty space.
I look at space as a set of coordinates.
You can ‘look at space as a set of coordinates’ but that doesn’t mean that’s what it is.
In fact this is one of the reasons quantum gravity is difficult. With quantum electro-dynamics you can treat space as a set of coordinates and look at the particle interactions on top of it. With gravity, though, the actions of gravity distort the space they’re operating in, so there are feedback effects between the space and the objects within it that aren’t true for electromagnetism. (Or, at least, aren’t true at any appreciable level; electromagnitism is so many orders of magnitude more powerful than gravity that any distortions of space caused by the charged particles are barely a rounding error compared to the electromagnetic interactions.)
Peter B @17:
But it’s not just that. The coordinates presumably are related to distances, which can only be defined using some sort of standard ruler. Even ’empty’ space is inextricably linked to matter and energy.
Example: The two main postulates of Special Relativity are
(1) The speed of light is the same in any inertial frame
(2) The laws of physics look the same in any inertial frame
From (1), we can relate the coordinates in different inertial frames, and derive things like length contraction and time dilation. With the coordinate transformations, from (2) we can derive the generalizations of classical dynamics, and new stuff like E = mc². None of that is possible without referring to matter and energy.
There’s a tendency to think of space/spacetime as a sort of background in which stuff happens, and somehow independent of that “stuff”. But the above makes clear that spacetime is something which is defined by the stuff that happens; it has a structure which is defined by the behaviour of matter and energy..
Wouldn’t spacetime be defined by the quantum foam and matter/energy is an emergent property, including dark energy and the expansion?
ardipithecus @20: If some bright sparks ever come up with a viable theory of quantum gravity, maybe! Don’t hold your breath.
Let me rephrase that:
Isn’t the quantum foam the starting point for everything else?
Whether we know how to describe it mathematically or not?
Oh, I thought you were talking about loop quantum gravity, but apparently you mean something even more dodgy; the idea that ‘virtual particles’ are constantly popping in and out of existence.
It’s a picture some physicists thought would make quantum physics more ‘accessible’ to lay folk, but it’s complete fiction. I blame Hawking in large part, because he used this idea in his paper on Hawking radiation. In the paper, he admits it is just handwaving, and has nothing to do with the actual derivation. But in books he wrote later, he carried on using it, and other physicists have done the same. But it has absolutely no basis in quantum field theory.
Thanks Rob Grigjanis @ 14.
John Morales @ 13. Thanks also.
So, this gets me thinking (muddled-ly -- I admit) along the lines of;
given your latitude of (I guess somewhere near) 26° South compared to my latitude of near 30° South, does that mean that a metre measured by you will be slightly shorter than a metre measured by me purely due our different rotational velocities -- yours being approximately 1499kmh and mine approximately1446kmh -- causing our respective caesium clocks to be running at different speeds? And whose metre are we using to measure our rotational velocity?
tuatara, that would depend on the frame of reference. I think. 😉
Basically, I suppose Lorentz effects apply, so distance may shrink but then time expands, and it should be a net nothing. But I really don’t know.
Anyway, beyond my level of competence, but I’m pretty sure that given the current definition, it would be the same as measured either of us given the nature of the definition of a metre.
(Perhaps one of the physicists here can address that)
[um, checking reveals ‘Lorentz effects” typically apply to magnetic forces, so I should have written “Lorentz transformation”. IANAP]
tuatara @#25,
There are some subtleties involved such as how you measure the length of a moving object but your statement is broadly correct, that an object’s length gets less as its speed increases. That is what is known as the Lorentz contraction.
# 24 Rob Grigjanis
Checking this out I came across a really good article on the topic from… Forbes, of all places. 🙂
Do Virtual Particles Really Exist?
It turns out “empty space”, or the quantum vacuum, does not contain particles, but does have physical properties. These physical properties are observable in, for example, the Casimir effect.
I highly recommend reading the whole thing -- there’s even a widget on the page that will read the article to you, which says it will take 12 minutes.
Silentbob @29: Yeah, the vacuum has many interesting properties. Every distinct particle is an excitation of a corresponding field, permeating all space. Even in a region without any particles of its type, a field is present, in its ‘ground state’, and can interact with fields and particles of other types.
The ground state of the electromagnetic field (whose excitations are photons) is responsible for the Casimir effect. It is also responsible for spontaneous emission.
This is all very interesting. Thanks everyone.
John Morales. Yes, even with our different rotational velocities almost all humans are (relatively) stationary to each other so share what is essentially the same inertial frame. I guess a better example would be measuring a metre on the surface of the earth compared to measuring a metre on one of the Helios satellites or some other higher speed frame.
But if the time dilation is cancelled out by the Lorentz contraction….ah, well. I will stop here I think. My head is trying to take me to a place way above itself.
Thanks for another interesting journey Mano.
Anyway, this is all in the weeds for the actual OP.
Proposition:
Einstein, famous equation.
So, is it the case that mass-energy is necessary for space-time?
I sure don’t know.
tuatara @31:
Since presumably each person is measuring stationary objects in their respective frames, they would obtain the same results. John’s caesium clock might be running slower than yours*, and his ruler might be contracted (depending on the orientation of his ruler with respect to your relative velocity), but you’re not using his clock or ruler, you’re using your own.
Note: The largest relative velocity between two stationary observers on the Earth’s surface would be if they were both on the equator, but at opposite sides of the planet. So, about 2,000 mph. But their respective local measurements would be identical (which I think is intuitively apparent).
*In which case your clock would be running slower than John’s, from his viewpoint.
@33
So basically, you both measure your own metre to be exactly one metre, but are both convinced the other is wrong.
This is why we can’t have nice things.
(Actually, since the vacuum energy is known to be non-zero, if you “allow all things to vanish from the world” surely there’s still enough left over to keep space existing…?)
xohjoh2n, energy is not nothing, so if that does not vanish, not all things have vanished.
In context, “things” was meant to refer to material stuff that would hypothetically “vanish” or cease to exist, not the more general idea of anything whatsoever that exists (such as, for example, spacetime). Otherwise, it would just be a completely vacuous statement that if nothing exists then nothing exists, and Einstein wouldn’t have wasted his time on that.
If the only “thing” left is spacetime which does nonetheless exist (certainly at least a logical possibility), then what’s supposed to be the problem? Einstein certainly didn’t know about any QM, when talking about how he thought his theory was supposed to be understood. So what then?
cr:
Mass-energy equivalence. Energy is material stuff in this context.
It’s in the OP: “But what if there was no matter or radiation anywhere in the universe? Would space still exist?”
Again, I can’t do better than to quote the OP:
“The idea that something must exist for space itself to exist is hard to digest. To some, this may sound similar to the Zen-like question of whether a tree falling in a forest makes a sound if no one is there to hear it.”
John @38: Just a bit of clarification: matter and radiation refer to real particles (quarks, leptons, photons, etc). If you ‘remove’ these real particles, you’re left with the vacuum state. It does have energy, at least formally, but it is neither matter nor radiation. It is simply a state in which nothing happens, like Heaven.
Rob, so is it or is it not something?
I note the featured quotation dates to 1916, which I believe predates quantum field theory, and so vacuum energy was not part of Einstein’s considerations.
John, yes it is something. As you suggest, it is a quantum field theory something, not a special (or general) relativity something.
OK, so it’s something, but it’s neither matter nor radiation.
It seems to me that Einstein was getting at something like “if everything that is not space were to vanish, then space would not exist under my formulation, unlike the Newtonian formulation”. Philosophically, it’s a question of ontological dependence, that is, whether space itself is independent of other things existing.
Obviously, in light of later advances, Einstein was missing some relevant stuff — as Newton was, in relation to nuclear processes.
Rob @ 33.
Are you sure? At the equator, separated by 180 degrees of longitude, the relative velocities of the two subjects would be exactly zero. Think of sinking a tube straight down at the feet of one to meet at the feet of the other. Visually through the tube they will be stationary relative to each other, and it may seem intuitively that they are moving in opposite directions in terms of their centripetal movement, but as they are on the surface of a spheroid, they will in fact be moving in the same direction at the same velocity while via the tube will be seen to be facing in oposite directions.
Of course, relative to a fixed point in space their velocity will differ. But which fixed point can we use as our frame of reference? The sun? The centre of the milky way galaxy? A local galaxy cluster? The point from whence the singularity became the universe?
The OP asks what is empty space? And makes use of the idea that without a visual reference you would not know you were spinning (without being able to see that you are moving you are not moving). I have always found this concept problematic because vision is only one of the senses informing us of our motion. But in your example the visual reference (the view of each other via the tube) tells them that they are not moving when we know that they are.
The only fixed point of value in your example seems to me to be the centre of rotation of the Earth (the point at my rotational centre that would inform my arms if I were the only object in space and started spinning or changed the velocity of my rotation).
And space also exists inside matter, not just outside it, doesn’t it?
tuatara, no, they will be moving in opposite directions. Think of their instantaneous motion relative to the centre of Earth. While both of them are being propelled east in their frame of reference, east is facing in opposite directions on opposite side of Earth. They will only appear stationary through the tube due to it rotating with them, allowing each to stand there while remaining in line with the tube. You might also be summing their vectors, which would indeed add up to zero… because they are equal yet opposite.
tuatara @43: Holms explains it well.
Each observer is their own fixed point.
BTW, if there was a singularity, it was not at a point in space, because there was no space. The singularity would have been the birth of spacetime.
Rob @ 45 and 46. I freely admit that this is out of my wheelhouse, but no, I dont think Holms @ 44 explained it well (sorry Holms, much respect to you, and to you too Rob).
In the example, you said that their relative velocity would be about 2,000mph, but did not give a point of reference apart from the two subjects. Holms at least gave a reference point (the centre of the Earth). But if you were able to observe them both from the centre of the Earth, you would see that none of the three points (you at the centre and they at the surface) are moving relative to each other.
Only an observer not on or in the Earth would see that the surface-dwellers are moving in opposite directions relative to each other while the centre-dweller (you) would be rotating in place.
And BTW, wouldn’t the birth of spacetime be by definition a point in spacetime?
But anyway, this has moved well away from the OP now so apologies to Mano for my digression.
Perhaps this will at least help to clarify what the issue is…. The fact that the distance between the two isn’t changing does not imply that they’re not moving. This goes back to your picture of having a tube going through the Earth, with which they each could see the other, who is apparently “not moving.” But they definitely are rotating, along with the Earth (and the tube, of course).
I mean, let’s remember that you can always draw a straight line between any two points, no matter what they may be doing or how that may change over time. And the length of that line segment is your measure of (Euclidean) distance. So, if this line (or tube, e.g.) happens to be rotating, and all you’re doing is asking whether its length changes, what you’re not doing then is asking more generally about everything related to motion in that system, but instead just one potential one way in which positions of things could be changing over time (such that the length is not the same at two different times).
You are on the right track here, and it is confusing. The simplest (old) example is not really concerned with any particular type of human sensation like vision…. Suppose there is a bucket of water. You can tell whether or not the bucket is spinning because the water will curl up toward the sides of the bucket if it is being accelerated like that. If not, the water just sits there, remains level like normal and does essentially nothing special.
We could observe something similar with two masses (let’s say ones shaped like balls) tied together with a string. If they’re sitting all alone in empty space and not rotating, there’s no tension on the string. If they are rotating, then there is tension, perhaps enough to stretch the string or break it.
There’s an argument, or more like an incoherent collection of different arguments…. It’s either that there’s just no way for us to determine what would happen, if there could even be any fact of the matter to speak of, since we don’t actually live in an otherwise-empty universe with just two balls tied together. This is all “just” theoretical physics, which is not really happening in an actual experiment (and certain types of people seem to have a problem with that). So, you assert that you’re free to reject whatever that may be, no matter what implications it may have for us, and most everyone kind of gives you a funny look for being such a strange person with these elaborate positivist notions about what “science” should be like and so forth.
Or, the argument goes that somehow it’s in fact the case that we know — from Einstein? Mach? Berkeley? take your pick — that there would be no physical, observable difference between the rotating and non-rotating systems, including the shape of the water surface, the movements of the water molecules in the volume, the tension on the string, etc. if it were the case that this happened to be the only system in the whole world which had any matter. If there’s not some other matter somewhere else (and it need not have any causal influence whatsoever with our system of interest, just its mere existence elsewhere is required), then the string wouldn’t be capable of breaking because it wouldn’t be capable of rotating, and the same goes for the water in the bucket.
If either of those sound like good arguments to you, congratulations: you’re very confused. If not, you’re probably mostly alright.
Going back to a different issue…. You can of course empirically “observe” this kind of behavior in all sorts of physical systems, not just exact types of systems I was describing. It’s not at all unusual in that sense. But anyway, it’s not like there’s anything peculiar or specific about vision here. That just happens to be a sense that we human beings tend to rely on a lot, especially when it comes to things like how stuff moves around in space (i.e. physics).
Vectors.
Two points may be stationary relative to each other, yet have wildly different velocities.
—
tuatara, yes, frames of reference are a thing, which means that the verities are the things that are true in every frame of reference.
Philosophically, the distinction between an existence claim and an universal claim.
CR. Thanks for the detailed response.
Neither of those two arguments sound good to me. I guess that is a good sign.
In my defence, I never said the two subjects are not moving. I absolutely agree that they are moving in oposite directions at about 1,000mph each, or 2,000mph ‘closing speed’. What I am saying is that, to each of them, the other does not appear to move at all provided no other reference frame is visible. That is my only point. Sure the distance between them is static, but again that is the point. Without any way of telling that they are moving they will believe that they are not.
You know, like Shaquille O’Neal.
If I modify the experiment a little, imagine each is in an underground bunker at exactly the same distance from the centre of a tiny planet with a circumference of 314.16m. The floor of their cells are 5m below the surface. Between them therefore is a distance of about 90m across which they can wave to each other and even talk with each other.
Now, that planet is orbiting a star, but it is also rotating at 314.16m/s.
While the view may be odd because each is looking up at the soles of the others feet, to them as the only observers, without any external frame of reference, there is no observable relative motion between them.
We as outside observers can “see” their opposite motion (628.32m/s), but they cannot. And because of their inertia in a constant rotational speed with no acceleration either positive or negative they feel no motion either.
That is all I am saying, or at least trying to. Maybe I fucked up somewhere (it is not exactly simple stuff) but I am not so fucked up as to think there is no motion just because those two cannot see any in each other.
You know, back to Shaquille O’Neal.
Anyways, every sunrise, I marvel at the fact that I and the surface of this planet are moving toward the rising sun at about 1,446kmh (at my latitude), and the Earth is at that moment orbiting the sun roughly in the direction of straight up at about 107,000kmh, yet I feel none of that motion. With our human senses it took a lot of work to discern that we are moving at all, but we did figure it out precisely because of those external references that did not agree with our sense of motion here on the surface of the Earth.
tuatara, mmm.
A very special case indeed. What if the cells are 1m apart? They’d both be in in an underground bunker at exactly the same distance from the centre of a tiny planet and 5m deep, but they’d be only 1m apart. Heck, they could shake hands through the bars. 😉
Anyway, as you said… the moment you start using distance units you presume space.
Why not? If they’re rotating, they’re indeed accelerating.
@ 48 consciousness razor
“the argument goes that the hypothesis has been put forward”
FTFY
Rather that the concept of “rotating and non-rotating” has no meaning absent an external frame of reference.
It takes a special type of confidence to declare Einstein “confused” about… relativity. You get full marks for arrogance if nothing else. How do you intend to demonstrate rotation has meaning absent an external frame of reference thereby proving Einstein wrong? Go ahead. Wow us.
“…some region from which all matter and radiation has been removed.”
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Question: which “creates” space: matter? radiation? both (matter and radiation)?
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A thought experiment proposed by Feynman. Suppose there is only 1 excited atom in the universe. Question: will the atom emit a photon? According to field theory, the atom will emit. According to action-at-a-distance the atom will not emit for the atom has no potential atomic receiver to emit to. Which view is correct?
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Is radiation field theory predicated that there is a pre-existing infinite (or large scale) matter-free space? Can radiation field be independent of matter field? The electromagnetic zero-point fluctuations: some consider this to be the electromagnetic tails of distant matter.
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Common proverb: fish do not discover water until it is out of the water.
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We are of matter and all around we see matter. Like fish we cannot “discover” space since we will always be in the midst of matter.
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In photonic space, time is at standstill: in effect time (specifically time evolution) does not exist in this context. Time exists only with matter.
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On angular momentum. Suppose there is in the universe only 2 point-like particles in a bound state. The 2-particle universe can only be described by the inter-particle distance: a 1-dimensional space thus there is no angular momentum to consider.
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On angular momentum. Suppose there is in the universe only 1 point-like particle with moment of inertia I. Suppose this particle has angular speed ω with subsequent angular momentum Iω. But there is is no other particles in the universe with respect to which one can measure ω. In this scenario is the concept of angular motion irrelevant, un-measurable, since the particle cannot transmit its angular momentum to another particle (for there is only 1 particle in the universe)? In this scenario angular momentum, angular speed cannot be conceived.
tuatara @50: OK, now I agree that our answers were not satisfactory. Yeah, there are frames of reference in which the two observers (O1 and O2) on the equator are stationary. Imagine standing at the North Pole. From your point of view, O1 and O2 are always in the same directions (albeit well over the horizon), 180 degrees apart, and stationary*.
But a simple experiment would tell you that you are rotating (even if you ignore the ‘fixed stars’ rotating around you!); set up a pendulum, and watch as the plane of its swing rotates through 360 degrees in 24 hours.
It’s true that any frame of reference is ‘valid’ in the sense that you can describe the motions of objects within it. But the laws of motion are much more complicated in a rotating frame (Coriolis forces, centrifugal acceleration). So we attach special importance to inertial frames of reference, in which there are no fictitious forces.
In any inertial frame (ignoring special relativistic effects!), O1 and O2 have a relative velocity of 2,000 mph.
*They would actually seem stationary to anyone standing anywhere on the Earth’s surface.
Silentbob:
People making these arguments certainly act like it’s a known scientific fact, like this is just what the physics says and you’re mistaken somehow if you’re not convinced of the truth of their claims. They don’t just say shit like, “I’ve put forward a hypothesis. How about that? What do you think?”
So it doesn’t seem like you fixed it for me.
I didn’t claim Einstein was confused about relativity. Also, I could just as well throw in a big name like Newton for my “side” of this, but fallacious crap like that would not be a good argument or get us closer to understanding or agreement.
I already tried to explain that: under acceleration, there is tension on the string, for instance, or the water surface has a different shape. It’s simply incorrect that there is no observable difference between systems like that, because they’re accelerated and that is the sort of thing we see happening all the time under acceleration. There is no need to check out other external systems for this, although you could if you felt like it of course, because the behavior of the system itself provides you with sufficient information.
In contrast, if you had a body which just has some constant velocity in some reference frame of your choice, that will behave just like another one with any other constant velocity in any other reference frame of your choice. There is no observable difference in behavior in that sense, for systems at velocity X rather than velocity Y or Z or what have you. So, unlike accelerations, that’s at least not available to us empirically, as even Newton would have told you repeatedly. Here, we use reference frames to do some actual work that can’t otherwise be done, and determine a specific numerical value for the velocity of interest. That is, we need to know with respect to what it is moving at that speed and in that direction, in order to put any content into such claims. If in fact there weren’t anything else with respect to which it is moving, then it’s at least not clear what we would even be trying to say about our system.
You might imagine the second type of scenario must be extremely general and claim that we have to treat the rotating/non-rotating cases in exactly the same manner, but that does not actually follow from any of this, because they’re not that similar.
And as I said before, we could easily devise a scenario where an external system like that does exist elsewhere, yet it isn’t actually doing anything to the system in question, or at minimum nothing that is responsible for the observed behavior in it (which is, at least purportedly, either accelerated or not). I think that should make it abundantly clear that this external stuff merely existing or not existing elsewhere has no bearing whatsoever on the physics of that system. If you don’t, then you should try to explain why not.
me:
Consider this: we can actually make devices that measure acceleration, accelerometers and such with all sorts of different designs, and they do not need to work by making a calculation based on some other system located elsewhere. That is not what is happening in the real world when they’re operating normally and doing a perfectly adequate job of measuring accelerations … “e pur si muove.” So what’s going on there, do you think?
Also, we ourselves have senses with which we can feel a variety of effects due to acceleration (at least at a rudimentary level such that it is detectable, although not exactly measured very well), not only sight but also things like hearing and touch. How does any of that work?
Noticing this, it shouldn’t be hard to recognize that this “we must have an external system” business is at the very least (1) not obvious at all and (2) ought to require a fairly compelling argument, which could (3) actually make sense of all of this stuff and not just be some grand proclamation delivered by or put in the mouths of prominent scientists.