Is Absolute Zero Velocity Possible?

[string querry]

Is it possible for anything in the universe to have absolute zero velocity? I'm not talking about Zero Velocity in relation to another given entity/object/particle, but rather an absoute concept, akin to the speed of light, a kind of oposite to the speed limit. In other words, if the speed of light is the "maximum speed limit", is there a "minimum speed limit", even if the limit is inifitely small? Please talk into consideration that I do not know enough about quantum physics to be aware of the existence/non-existence of massless particles, but aside from such considerations, I figure that even if avsolute zero velocity is hypothetically possible, it cannot exist the known universe because everything else is moving and would have some kind of gravitational affect. This is all assuming that there is no distance at which the force of gravity is nil, such as an object unimaginably distant from the rest of the matter in the universe, an that in and of itself assumes an universe with <curvature, if I understand the term properly.

 

[HallsofIvy]

No, there is no such thing as "absolute" speed or velocity. Strictly speaking, even the speed of light is "relative" to something- it just happens that the speed of light is the same relative to anything!

That has nothing to do with quantum physics. If you take quantum physics into account, you still are dealing with velocities relative to something (the "laboratory frame" for instance) but now, because of the uncertainty principle, if you know something is in the laboratory, it is impossible to know that it has exactly 0 velocity relative to the laboratory. That's why hydrogen and helium have such strange properties close to "absolute 0".

 

[MonstersFromTheId]

I'd like to know about this as well

For example;

Two identical clocks, one on the ground, one in a jet or in orbit. Both start out synchronized, but after one has spent time at a speed greater than the other it shows the effects of time dilation having slowed it down.

Given the demonstrable effects of time dilation on clocks, I just can not understand how it's in any way possible to claim that there isn't such a thing as absolute zero speed. There has to be a place somewhere in the universe where a clock would run faster than a clock anywhere else in the universe due to a comparative lack of the effects of time dilation, doesn't there?

What am I missing here?

 

[MonstersFromTheId]

Halls of Ivy

Apologies.
You post went up just seconds before my question. I in no way meant to infer that I didn't believe your answer.

Still however, I don't understand yet how your answer applies to time dilation. If there's a place, and I have to believe there is one, someplace, where clocks would see the least effects due to time dilation, why wouldn't that form the basis for a definition of "absolute zero velocity"?

It would seem to me, dead wrong as it may be on my part, that anyplace in the universe where every other place is moving "relatively" faster as shown by time dilation effects on clocks, could reasonably be defined as "stopped" relative to everything else in the universe.

Can you please help untangle my thinking here?

 

[ZapperZ]

Apologies.
You post went up just seconds before my question. I in no way meant to infer that I didn't believe your answer.

Still however, I don't understand yet how your answer applies to time dilation. If there's a place, and I have to believe there is one, someplace, where clocks would see the least effects due to time dilation, why wouldn't that form the basis for a definition of "absolute zero velocity"?

It would seem to me, dead wrong as it may be on my part, that anyplace in the universe where every other place is moving "relatively" faster as shown by time dilation effects on clocks, could reasonably be defined as "stopped" relative to everything else in the universe.

Can you please help untangle my thinking here?

Take a bunch of idea gas particles. Here, you have every particle is moving relative to one another. Can you pick out which one is the "absolute" particle that is this absolute reference frame that makes it more special than others?

Zz.

 

[string querry]

Keep Going!

This is exactly the questions/answers I was looking for, but please keep in mind that both realistic and purely hypothetical senarios are valid for this thread. I can't stop thinking that if, hypothetically an object, such as a sphere of pure iron, at absolute zero temperature, had no kinetic energy (I'm not talking on the subatomic scale now), it would have to have zero velocity, I realize that nothing could exist in that state in the universe as we know it, because, through gravitation effects, energy from other matter in the universe would transfer to the object in question, but imagine for a moment that no other matter in the universe existed. Wouldn't the sphere never move if it had no kinetic energy.

By the way, in regard to "even the speed of light is relative", I'm not convinced. The fact that interacts with the matter in the universe the same regardless of the velocity of any part of said matter, does by definition make it non-relative. I now there have been talks about situations where light seems to go faster or slower, for example through water as compared to air, by I really think it is the matter penetrated by the light that is causing the apparent difference, but experts would probably call me a dinosaur.

But if even the speed of light is relative, what of temperature. Does absolute zero really exist. If absolute zero can be measure, which by definition is detectable regardless of what it is compared to, isn't it hypothetically possible for an object to have no velocity?

 

[string querry]

Take a bunch of idea gas particles. Here, you have every particle is moving relative to one another. Can you pick out which one is the "absolute" particle that is this absolute reference frame that makes it more special than others?

Zz.

yes, which is why the post I wrote while you were writing this one has specified that the object is at avsolute zero temperature. My understanding is that there is no relative movement of molecules at absolute zero, by definition, no? Even if I'm wrong, take a single proton instead and make that the object moving, or rather not moving, at zero velocity.

By the way, I like the idea in the other post, the one about redefining the minimum speed by the slowest object in the universe an measureing every thing else in relation, but I don't think that could truly be called 'absolute' because measurement to anything else would depend on directional vectors, or whatever the mathematical mombojumbo wants to call direction, would cause a non constant relativity factor, even if the slowest object never changes speed.

 

[Janus]

For example;

Two identical clocks, one on the ground, one in a jet or in orbit. Both start out synchronized, but after one has spent time at a speed greater than the other it shows the effects of time dilation having slowed it down.

Given the demonstrable effects of time dilation on clocks, I just can not understand how it's in any way possible to claim that there isn't such a thing as absolute zero speed. There has to be a place somewhere in the universe where a clock would run faster than a clock anywhere else in the universe due to a comparative lack of the effects of time dilation, doesn't there?

What am I missing here?

Let's take ZZ's example of a collection of gas particles. Assign a clock and an observer to each particle (the observer moves with the particle). Every observer will determine that it is his particle's clock which is ticking the fastest, and the other clocks which are ticking slower. None of them will agree as to which clock is ticking slowest. So no, there isn't any place in the universe which everyone will agree a clock would tick slowest.

 

[ZapperZ]

yes, which is why the post I wrote while you were writing this one has specified that the object is at avsolute zero temperature. My understanding is that there is no relative movement of molecules at absolute zero, by definition, no? Even if I'm wrong, take a single proton instead and make that the object moving, or rather not moving, at zero velocity.

By the way, I like the idea in the other post, the one about redefining the minimum speed by the slowest object in the universe an measureing every thing else in relation, but I don't think that could truly be called 'absolute' because measurement to anything else would depend on directional vectors, or whatever the mathematical mombojumbo wants to call direction, would cause a non constant relativity factor, even if the slowest object never changes speed.

The problem here is that this issue has been discussed ad nauseum even here in this forum.

The issue here isn't "absolute zero". If you have a box of ping pong balls, and they are all sitting STILL, it would be absurd to define a reference frame that is moving relative to all those balls as the simplest frame. Now, do we have that in reality? NOPE! It is a meaningless point to consider such a situation because (i) we don't have such a thing and (ii) we have never observed such a thing except for some local, terrestrial, and special situations.

So what you actually have is, for example, the ideal gas sitatuation, where everything is moving with respect to everything else. Now please pick a reference frame that is more special than the others.

Zz.

 

[ZapperZ]

By the way, in regard to "even the speed of light is relative", I'm not convinced. The fact that interacts with the matter in the universe the same regardless of the velocity of any part of said matter, does by definition make it non-relative. I now there have been talks about situations where light seems to go faster or slower, for example through water as compared to air, by I really think it is the matter penetrated by the light that is causing the apparent difference, but experts would probably call me a dinosaur.

I really wish you don't make statements such as these without putting some effort into understanding the things you are using as "facts" or evidence to back up your statements.

If you want to know about light interacting with matter, especially on optical propagation through solids, I strongly suggest you read first our FAQ in the General Physics section. This would debunk some of the errors in understanding that you seem to have regarding that phenomenon.

Secondly, I hope that you are aware that you have just contradicted Special Relativity, or at least trying to. Now considering that you have depended your life on this theory, I would hope that you again, at the very least, consider the possibility that maybe you didn't understand (i) the postulates in the first place and (ii) all the possible applications and implications of those postulates that have been verified and IN USE presently. One simply cannot challenge something without addressing the fact that it has worked and one certainly cannot challenge something based on erroneous understanding of what it is.

Zz.

 

[string querry]

I really wish you don't make statements such as these without putting some effort into understanding the things you are using as "facts" or evidence to back up your statements.

I thought I made it clear that I was expressing my my lack of understanding and opening for someone to counter what I said. As for the linked information, I just joined today, so please pardon me if I don't know my way around. Thanks for the link.

 

[string querry]

The problem here is that this issue has been discussed ad nauseum even here in this forum.

So what you actually have is, for example, the ideal gas sitatuation, where everything is moving with respect to everything else. Now please pick a reference frame that is more special than the others.

Zz.

Absolutely right, but my origin question when I started this thread was intended to get someone to say if there is any kind of absolute beyond what relation to everything else. Again, this is just hypothetical, but is it possible for something to have no kinetic energy whatsoever, thus Absolutely Still? I think you're right, that this is not possible in the real world because everything interacts with everything and everything is apparently relevent--but I wish someone would answer the question about temperature: is the absolute zero measurement of temperature a misnomer, or is this different from the question of absolutely velocities that I have just been thoroughly blasted over.

 

[Janus]

Absolutely right, but my origin question when I started this thread was intended to get someone to say if there is any kind of absolute beyond what relation to everything else. Again, this is just hypothetical, but is it possible for something to have no kinetic energy whatsoever, thus Absolutely Still? I think you're right, that this is not possible in the real world because everything interacts with everything and everything is apparently relevent--but I wish someone would answer the question about temperature: is the absolute zero measurement of temperature a misnomer, or is this different from the question of absolutely velocities that I have just been thoroughly blasted over.

The thing is that kinetic energy is a relative concept itself. It is meaningless to say what an object's kinetic energy is unless you define from where you are measuring it.
Take your ball at absolute zero. From a frame at rest with the ball, it has zero kinetic energy, but from a frame moving with respect to the ball, it doesn't. And there is no way to say that one frame's measurement of the ball's kinetic energy is more correct than the other's.

 

[MonstersFromTheId]

I *Think* I've got this, but...

O.k., let's simplify this just a tad.

We've got just four particles in the 'ol box.

Particles A, B, C, and D.

At the moment, from the view point of the box (which will soon prove to be no better or worse a vantage point than anyone of the particles), they're all moving in the same direction such that...

Particle A is moving at 1V.
Particle B is moving at 2V.
Particle C is moving at 4V.
Particle D is moving at 8V.

While standing on particle A:
Particle A appears to be stationary.
Particle B appears to be moving at 1V.
Particle C appears to be moving at 2V.
Particle D appears to be moving at 4V.
Collect the clocks on Particle A and...
Clock A will show the most amount of time having passed.
Clock B will show less time having passed than clock A.
Clock C will show less time having passed than clock B.
Clock D will show less time having passed than clock C.

While standing on Particle B:
Particle B appears to be stationary.
Particle A appears to be moving at 1V in one direction.
Particle C appears to be moving at 1V in the other direction.
Particle D appears to be moving at 2V in the same direction as Particle C.
Collect the clocks on Particle B and...
Clock B will show the most amount of time having passed.
Clocks A&C will show that the same amount of time has passed, but that agreed upon amount of time having passed will be less than the amount of time having passed according to clock B.
Clock D will show less time having passed than any of the others.

While standing on Particle C:
Particle C appears to be stationary.
Particle A appears to be moving at 2V.
Particle B appears to be moving at 1V in the same direction as Particle A.
Particle D appears to be moving at 1V in the opposite direction of the other two.
Collect the clocks on Particle C and...
Clock C will show the most amount of time having passed.
Clocks B&D will show that the same amount of time has passed, but that agreed upon amount of time having passed will be less than the amount of time having passed according to clock C.
Clock A will show less time having passed than any of the others.

While standing on Particle D:
Particle D appears to be stationary.
Particle A appears to be moving at 4V.
Particle B appears to be moving at 2V.
Particle C appears to be moving at 1V.
Collect the clocks on Particle D and...
Clock D will show the most amount of time having passed.
Clock C will show less time having passed than clock D.
Clock B will show less time having passed than clock C.
Clock A will show less time having passed than clock B.

So, forget using time dilation to tell who's moving the slowest. That ain't going to work.

MY REMAINING CONFUSION---
Here's what I STILL don't get.
Remember that experiment where two "atomic" clocks were synchronized, one was left on the ground, the other flown around in a fast jet for a while, and when the two clocks were brought together, sure enough, the one that spent time on the jet showed less time having passed than the one that stayed on the ground, in exact accordance with the predictions of relativity?
O.k., suppose that the clock that had remained on the ground had instead been brought aboard the speeding jet at the end of the experiment. I.e. the speeding jet is now defined as "standing still". Once aboard the continuously speeding jet, would the clock that had remained on the ground now show less time having passed than the one that had stayed aboard the jet?

 

[MonstersFromTheId]

Never mind, I figured it out.

*If the two clocks were synchronized aboard the flying jet to begin with*, then one of 'em was left on the ground for a while, and then brought back up to the speeding jet, then yes, the one that had spent time on the ground would have shown less time having passed than the one that stayed aboard the jet.

If the two clocks are synchronized *on the ground*, then one of 'em is placed aboard the speeding jet for a while, and then brought back down to the ground, then the one that had spent time on the jet would show less time having passed than the one that stayed on the ground.

I.e., synchronize the clocks, then, the clock that undergoes accelleration will always be the one that winds up running slow.
I've got it. Makes sense.

 

[jtbell]

is it possible for something to have no kinetic energy whatsoever, thus Absolutely Still? [...] I wish someone would answer the question about temperature: is the absolute zero measurement of temperature a misnomer, or is this different from the question of absolutely velocities that I have just been thoroughly blasted over.

First, note that the concept of temperature makes sense only for a composite object or system: an solid object, liquid or gas made up of atoms, molecules or whatever. The temperature of an object or system is related to the average kinetic energy of its components when the total momentum of the system is zero, that is, when the object or system is macroscopically at rest, as a whole, even though its components may still be moving microscopically, internally.

Suppose that in some inertial reference frame, all the components of a system are at rest. Then we can say both that the system as a whole is at rest, and that its temperture is at absolute zero.

Now, observe the same system from another inertial reference frame which is moving relative to the first one. All the components of the system are now moving with the same velocity (same speed, same direction). The system as a whole has a macroscopic kinetic energy, but its temperature is still absolute zero.

In general, we can take the kinetic energies of the components of a moving system and separate it into two parts: one part associated with the macroscopic velocity of the object as a whole, and the other part associated with the internal microscopic motions of its components. We can make the macroscopic velocity (and therefore the macroscopic kinetic energy) have whatever value we like, including zero, by using a suitable reference frame. However, we cannot make the internal microscopic velocities and kinetic energies all zero at once in any reference frame.

Does this help?

 

[string querry]

thanks

First, note that the concept of temperature makes sense only for a composite object or system: an solid object, liquid or gas made up of atoms, molecules or whatever. The temperature of an object or system is related to the average kinetic energy of its components when the total momentum of the system is zero, that is, when the object or system is macroscopically at rest, as a whole, even though its components may still be moving microscopically, internally.

Suppose that in some inertial reference frame, all the components of a system are at rest. Then we can say both that the system as a whole is at rest, and that its temperture is at absolute zero.

Now, observe the same system from another inertial reference frame which is moving relative to the first one. All the components of the system are now moving with the same velocity (same speed, same direction). The system as a whole has a macroscopic kinetic energy, but its temperature is still absolute zero.

In general, we can take the kinetic energies of the components of a moving system and separate it into two parts: one part associated with the macroscopic velocity of the object as a whole, and the other part associated with the internal microscopic motions of its components. We can make the macroscopic velocity (and therefore the macroscopic kinetic energy) have whatever value we like, including zero, by using a suitable reference frame. However, we cannot make the internal microscopic velocities and kinetic energies all zero at once in any reference frame.

Does this help?

Yes that helps, firstly because you didn't kick me in the head for being less knowledgeable, but rather explained something to me that actually answers the original thread question THANKS. Second it helps, because it shows the problem of defining kinetic energy. If i understand you, your saying that, for instance, if I had a box of marbles and I slide the box across the floor but the marbles don't roll, ie: the marbles within the box are, relative to the box, stay motionless, they would appear to have no kinetic energy, but if the box hits a wall, the marbles are going to roll toward the wall, thus showing that, to the wall, the marbles were moving regardless of the box, and to the marbles, the box was not moving until it hit the wall, but the wall was always approaching, so to speak--is that essentially it?

But...

I am still confused about the previous statement that temperature is entirely relevant to the measurement system. I read in another thread that an experiment was done in which a substance was super cooled to the point where even the electrons slowed down and began to collapse toward the nucleus. Was this nonsense, or has anybody heard about it. If it is true, I fail to see how that state is relative to anything else. Whether you compare it to a supernova or an icecube, that state is going to seem to be zero. Am I missing something?

 

[string querry]

Yep!

O.k., let's simplify this just a tad.

We've got just four particles in the 'ol box.

Particles A, B, C, and D.

At the moment, from the view point of the box (which will soon prove to be no better or worse a vantage point than anyone of the particles), they're all moving in the same direction such that...

Particle A is moving at 1V.
Particle B is moving at 2V.
Particle C is moving at 4V.
Particle D is moving at 8V.

While standing on particle A:
Particle A appears to be stationary.
Particle B appears to be moving at 1V.
Particle C appears to be moving at 2V.
Particle D appears to be moving at 4V.
Collect the clocks on Particle A and...
Clock A will show the most amount of time having passed.
Clock B will show less time having passed than clock A.
Clock C will show less time having passed than clock B.
Clock D will show less time having passed than clock C.

While standing on Particle B:
Particle B appears to be stationary.
Particle A appears to be moving at 1V in one direction.
Particle C appears to be moving at 1V in the other direction.
Particle D appears to be moving at 2V in the same direction as Particle C.
Collect the clocks on Particle B and...
Clock B will show the most amount of time having passed.
Clocks A&C will show that the same amount of time has passed, but that agreed upon amount of time having passed will be less than the amount of time having passed according to clock B.
Clock D will show less time having passed than any of the others.

While standing on Particle C:
Particle C appears to be stationary.
Particle A appears to be moving at 2V.
Particle B appears to be moving at 1V in the same direction as Particle A.
Particle D appears to be moving at 1V in the opposite direction of the other two.
Collect the clocks on Particle C and...
Clock C will show the most amount of time having passed.
Clocks B&D will show that the same amount of time has passed, but that agreed upon amount of time having passed will be less than the amount of time having passed according to clock C.
Clock A will show less time having passed than any of the others.

While standing on Particle D:
Particle D appears to be stationary.
Particle A appears to be moving at 4V.
Particle B appears to be moving at 2V.
Particle C appears to be moving at 1V.
Collect the clocks on Particle D and...
Clock D will show the most amount of time having passed.
Clock C will show less time having passed than clock D.
Clock B will show less time having passed than clock C.
Clock A will show less time having passed than clock B.

So, forget using time dilation to tell who's moving the slowest. That ain't going to work.

MY REMAINING CONFUSION---
Here's what I STILL don't get.
Remember that experiment where two "atomic" clocks were synchronized, one was left on the ground, the other flown around in a fast jet for a while, and when the two clocks were brought together, sure enough, the one that spent time on the jet showed less time having passed than the one that stayed on the ground, in exact accordance with the predictions of relativity?
O.k., suppose that the clock that had remained on the ground had instead been brought aboard the speeding jet at the end of the experiment. I.e. the speeding jet is now defined as "standing still". Once aboard the continuously speeding jet, would the clock that had remained on the ground now show less time having passed than the one that had stayed aboard the jet?

I knew you'd figure it out on your own if you were able to explain the relative aspect in such detail. The thorough list of possibilies is very clarifying. Also Janus (I think) explained the definiton of kinetic energy which really helped a lot. For the most part I understand why absolute zero velocity is assumed to be impossible, though I'm still open to arguments to the contrary if anyone has any. Thanks!

 

[string querry]

okay

First, note that the concept of temperature makes sense only for a composite object or system: an solid object, liquid or gas made up of atoms, molecules or whatever. The temperature of an object or system is related to the average kinetic energy of its components when the total momentum of the system is zero, that is, when the object or system is macroscopically at rest, as a whole, even though its components may still be moving microscopically, internally.

?

Yes I understand that, but I only mentioned it for the example of a composite system for the example of the solid object (I think the example was an orb of iron), in order to try to negate the issue of microscopic movement and to focus on whether or not the object as a whole can have no velocity

?

Suppose that in some inertial reference frame, all the components of a system are at rest. Then we can say both that the system as a whole is at rest, and that its temperture is at absolute zero.

Now, observe the same system from another inertial reference frame which is moving relative to the first one. All the components of the system are now moving with the same velocity (same speed, same direction). The system as a whole has a macroscopic kinetic energy, but its temperature is still absolute zero.

In general, we can take the kinetic energies of the components of a moving system and separate it into two parts: one part associated with the macroscopic velocity of the object as a whole, and the other part associated with the internal microscopic motions of its components. We can make the macroscopic velocity (and therefore the macroscopic kinetic energy) have whatever value we like, including zero, by using a suitable reference frame. However, we cannot make the internal microscopic velocities and kinetic energies all zero at once in any reference frame.

Does this help?

Yes, I get that now. Everyone here has been helpful except in such instance as the complaints the the issue is a waste of space. If anyone feels that way, please feel free to find a more enlightened thread.

 

[G01]

String. Don't be discouraged if some members here happen to get a little flustered when you ask questions like these. They are not are ridiculing you for not understanding a topic. The reason for the harshness is because, as you will soon see if you become a regular visitor to PF, there are many people that come here with misconceptions about many areas, theories, and concepts of physics. These people make no effort to correct their errors and it becomes very frustrating trying to answer these peoples questions about physics and to help them, if, deep down they do not want to accept new information or information contrary to popular belief. This in no way means that we should have taken it out on you but, please try to understand that it can become frustrating. This is why when there is a frequently misunderstood concept, a FAQ is created and those with questions are directed to it. You are, I believe, not one of the people I described above. You seem to really want a better understanding of physics. Welcome to PF and enjoy your stay.

 

[robphy]

Is it possible for anything in the universe to have absolute zero velocity? I'm not talking about Zero Velocity in relation to another given entity/object/particle, but rather an absoute concept, akin to the speed of light, a kind of oposite to the speed limit. In other words, if the speed of light is the "maximum speed limit", is there a "minimum speed limit", even if the limit is inifitely small?

I'm not sure if this question is fully resolved... but let me chime in with this...

The whole point of *a* theory of relativity (starting with Galilean relativity) is that there is no such thing as Aristotle's idea of "Absolute Rest". From a mathematical (linear algebra) point of view [and since you're in the relativity forum]... a notion of some absolute velocity must be represented by a vector that is an eigenvector of the relativistic transformations. [It's a linear algebra problem.] In Galilean relativity, no timelike vector is left unchanged... the eigenvectors are actually the spacelike ones [which represent "infinitely fast" instantaneous propagation] . In Special Relativity, the lightlike vectors are the eigenvectors... this is the absoluteness of the speed of light.

 

[Garth]

Is it possible for anything in the universe to have absolute zero velocity? I'm not talking about Zero Velocity in relation to another given entity/object/particle, but rather an absoute concept, akin to the speed of light, a kind of oposite to the speed limit.

Just my two pennyworth...

Velocities have to be measured relative to something.

Although there is no universal 'absolute' velocity we can determine a special 'zero' velocity, which is the rest frame in which the CMB is globally isotropic.

The assumption of the cosmological principle requires such a rest frame to exist, which is that space-like slice or 'foliation' of space-time in which the universe is homogeneous and isotropic.

The Earth is traveling at about 0.1%c relative to this frame.

Garth

 

[MonstersFromTheId]

Cosmological Principle, how accepted?

How accepted is the Cosmological Principle?

According to the Wikipedia the C.P. is a "reasonable assumption or axiom", but as such, I would assume, possibly incorrectly, that it has no proof.

For example: What makes us think that the universe is "homogeneous and isotropic"?

We can only see an infinitesimally small "chunk" of the universe.

Of what we can see, the vast bulk of it isn't as it is now, but as it was millions to billions of years ago.

We can't see anything at all at distances where the light arriving in our neighborhood predates the formation of photons.

How do we know the universe isn't just the opposite of what we assume it to be? I.e., non-homogeneous, and possibly even anisotropic?

Suppose for example that the universe consists of a series of concentric "shells" or "layers" (for lack of a better term), with each "shell" or "layer" being significantly different than the others due to age, energy/mass density, temp, etc.

To take the "expanding balloon" analogy, often used to explain how galaxies are all moving away form each other, to a bit of a stretched extreme-

To a microbe, living within the thin rubber film of a balloon, it might also appear to be a "reasonable assumption" to conclude that the entire universe is "homogeneous and isotropic" and made of rubber, even though that's by no means at all the case. The bulk of such a microbe's "universe" would actually be composed of air that has vastly different properties than the rubber the microbes know about. If the microbes view was limited by the distances involved, all they'd be able to detect or know about is the rubber film they inhabit.

Not to stretch the analogy too far, but under such conditions the microbes would also see observable effects (just as we do, dark matter, dark energy, etc.) that were difficult to explain, like, the fact that the rubber seems to be expanding w/o anything to drive that expansion (after all, a "homogeneous and isotropic" ball of rubber has no air in its center to stretch the rubber film surrounding it).

I don't want to go too far with that last analogy. I don't want to give the impression that I think dark energy and/or dark matter could be explained by "air at the center of the universe" or whatever.

I'm just saying that the expansion of the universe does appear to be accelerating more quickly than anticipated, we come up with ideas like dark energy to account for it, but how do we know the problem doesn't lie in some of the axiomatic assumptions we've made, such as, "the universe is homogeneous and isotropic"?

What if it's not?

 

[Garth]

The main evidence that the observable universe is isotropic is the CMB, which, once the Earth's proper motion dipole is subtracted, is isotropic, to ~ one part in 10⁵.

It need not be homogeneous, but the simplest assumption to explain why it looks so isotropic is that it is.

Garth

 

[iox108]

I figure the only way we could define an absolute coordinate in space would be to determine the location of the big bang (if that's possible) and the universe was expanding as a perfect sphere. That's my so-called theory. So until somebody developes a "Space Anchor"...