# Is the universe bounded?

## Main Question or Discussion Point

If the universe is bounded then there will be an absolute frame of reference.

This could be found by summing all the frames of reference up to the unltimate one ie your in a car travelling on road thats on the earh which is spinning on it axis and simutaneously moving about the sun . The sun is moving within the galaxy. The galaxy is moving within some larger structure and so on and so forth up to the bounded universe.

what are the arguements for a bounded universe ?

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JesseM
In physics when people talk about an "absolute frame of reference" they mean that the equations of the laws of physics are different in one inertial frame than another, it doesn't have anything to do with contingent facts about how matter happens to be arranged. In any case, there is no evidence that the universe has a physical boundary.

Conversly there is no evidence its unbounded either. I guess if it is unbounded then it must contain an infinite amount of energy and I have an aversion for infinity in reality.

if the universe is bounded there are profound implications for physics such as there will be an absolute frame of refernce,there will be absolute time, absolute distnace, absolute velocity, absolute acceleration and on and on.

At the moment without knowing if there is an absolute frame of reference not a single physical measurement is accurate. Every measurement takes place inside a moving frame of ref in which the direction of travelling and velocity is unknown. For all we know the absolute frame of ref may be travelling a close to the speed of light and all our mesaurement are horribly inaccurate.

For instance when you measure your hieght using a tape measure that measurement takes place inside a moving frame of ref. The measurement is inaccurate as it is contracted by some unkonwn factor due to the velocity of the frame of reference. If you knew the absolute frame of reference then you could calculate your exact height by allowing for the contraction due to the absolute frame of ref.

So i thought it maight have some importance

Several problems here. First, it is quite possible that the universe could be unbounded, but still of finite extent - something like the surface of a sphere. (Whether or not this is the case is, or course, entirely unknown.) Second, the universe could be infinite in extent but only contain a finite amount of energy, which just happens to be concentrated in a certain region. This would very much violate the idea that the universe is, at large scales, homogeneous and isotropic, but it fails to be impossible. Finally, the simple fact that we can directly observe that the principle of relativity works (i.e. that physical laws operate in the same manner in different inertial frames of reference) tells us that, whatever it looks like, the large scale structure of the universe does not pick out an "absolute frame."

As it happens, however, there is a sort of preferred frame picked out cosmologically. This is the frame of reference at rest with respect to the cosmic microwave background radiation (which can be identified by the lack of a dipole anisotropy in measurements of the CMB). This, however, is not an absolute frame of reference as relates to physical laws; but, is simply, in a sense, the rest frame of the matter that makes up the universe (or, at least the observable part of it).

JesseM
Conversly there is no evidence its unbounded either. I guess if it is unbounded then it must contain an infinite amount of energy and I have an aversion for infinity in reality.
As Parlyne said, it could be finite but unbounded--if you can imagine a 2-dimensional universe, like the famous book "Flatland", which is actually curved into the surface of a sphere, then by analogy you can imagine that our 3-dimensional space might be curved like the surface of a 4-dimensional "hypersphere", and thus have a finite volume without boundary.
rab99 said:
if the universe is bounded there are profound implications for physics such as there will be an absolute frame of refernce,there will be absolute time, absolute distnace, absolute velocity, absolute acceleration and on and on.
No! Did you read what I wrote above? Even if the universe was finite and bounded, so that there was a single frame in which the matter of the universe was on average at rest, this would not be what physicists mean by a "preferred frame", because the basic laws of physics would be no different in this frame than any other. The distribution of matter alone cannot make one frame "preferred" in this sense.

I think it might be relevant to speak of the universe being "closed", as in a closed surface. That would be the case where it was finite but unbounded, as JesseM and Parlyne have said.

And just to reinforce what they have already said, the fact that there is a convenient frame of reference, which we may choose to identify as a standard one by convention, does not mean that it is absolute.

Mentz114
Gold Member
rab99:
At the moment without knowing if there is an absolute frame of reference not a single physical measurement is accurate. Every measurement takes place inside a moving frame of ref in which the direction of travelling and velocity is unknown. For all we know the absolute frame of ref may be travelling a close to the speed of light and all our mesaurement are horribly inaccurate.

For instance when you measure your hieght using a tape measure that measurement takes place inside a moving frame of ref. The measurement is inaccurate as it is contracted by some unkonwn factor due to the velocity of the frame of reference. If you knew the absolute frame of reference then you could calculate your exact height by allowing for the contraction due to the absolute frame of ref.
No, this is wrong. Measurements in your own frame are not affected by how you may be moving relative to other frames. They are not 'horribly inaccurate'. The effects of time dilation and length contraction are only apparent to an observer moving inertially wrt to your own frame.

actually what I said was wrong the question shouldnt be "is the universe bounded" the question i wanted to aks is, is there a stationary frame of ref out there somewhere? a ref point for all other frames of ref. It is irrelevant if the universe is bounded. It may only be that god knows of this frame

And stationary to wrt what?....I guess if there is no ether then space itself may be stationary...or i suppose if light has an absolute velocity in all frames ie its velocity is unaffected by the myriad of different frames of ref, maybe the abs frame can be derived from a property of light.

also

If you do not have an absolute frame then how do you measure if an observer is moving inertially wrt to your own frame?

say you have a peron in a plane travelling in a straight line east at 100 kph and you have a person stationary hovering in a helicopter looking at the plane. The person in the copter will apparently see the length of the plane contract and time slow down in the plane. But if the milky way is travelling west at 100 kph then actually the plane is staionary and the guy in the copter is moving at 100 kph. An observer stationary to the milky way will see the copter contract and time slow down in the copter. The plane will remain unaffected. But if our galaxy is travelling north at 300 kph then... and so on and so forth

2 frames of ref are always contianed within another frame of ref or are they

Mentz114
Gold Member
If you do not have an absolute frame then how do you measure if an observer is moving inertially wrt to your own frame?
1. use your eyes. You can tell if someone on a bicycle is moving wrt to you.
2. use radar, like a speed-trap.
3. Use the doppler shift of light in the case of galaxies.
4 etc, etc

<snip> ... is there a stationary frame of ref out there somewhere? a ref point for all other frames of ref. ... It may only be that god knows of this frame

And stationary to wrt what?....I guess if there is no ether then space itself may be stationary...or i suppose if light has an absolute velocity in all frames ie its velocity is unaffected by the myriad of different frames of ref, maybe the abs frame can be derived from a property of light.
This is a good question ... or it was, especially in the first quarter of the 20th century! I think it's fair to say that before the days of Michleson and Morley, and the resulting success of SR, most scientists would have said quite naturally that there was such a thing as an absolute stationary frame of reference - it just seems intuitively likely, and I think most people simply assumed that it existed.

What M&M and then SR force us to accept however, is that there is no such thing. Any frame you identify as the "official" frame, in reference to which all others may be absolutely identified as moving or stationary, will behave exactly like any other inertial frame, in terms of physical observations. There is no way to define such a frame in a physically unique way, meaning that other frames would behave in a physically distinguishable way. This is really at the heart of the principle of relativity. If it were otherwise, then we could detect the difference. (I guess there are experimental limits on the accuracy of this, but they are very, very good at this point.)

We can, as has been pointed out already, always pick some convenient frame and agree to refer all other frame to it as a reference, but that's just a convention and is not physically significant.

But I can distinguish one frame of ref from another at home. I have a remote controlled car and a block of cement one moves the other doesnt?

If i sit in the car and see the dirt moving under me and the block of cement stationary wrt the dirt and the dirt is immovable and connects me and the block of cement I know Im moving in the car.

If I sit in the block of cement and see the dirt stationary under me and see the car whizz by....hey that cars moving.

The ref point is the dirt.

How can you say that its impossible to distinguish one frame of ref from another when i can do that at home?

if the physical equivalent of the dirt ( which I beleive is a photon as a photon has constant velocity irrespective of the frames of ref) then it should be possible to find the ultimate frame of ref

I note there are some compelling arguements that MM was flawed.

there is serious scientific endevour into proving the existance of the ether (probably dark matter)

and the jury is out on SR

A theorey is only true until it is proven false just ask Newton

JesseM
But I can distinguish one frame of ref from another at home. I have a remote controlled car and a block of cement one moves the other doesnt?
No, you are free to use a coordinate system where the car is at rest while the cement is moving, and the laws of physics will work exactly the same in this coordinate system.
rab99 said:
If i sit in the car and see the dirt moving under me and the block of cement stationary wrt the dirt and the dirt is immovable and connects me and the block of cement I know Im moving in the car.

If I sit in the block of cement and see the dirt stationary under me and see the car whizz by....hey that cars moving.

The ref point is the dirt.
Your car is moving relative to the dirt. But why do you think it would be "wrong" to pick a coordinate system where the dirt is moving and your car is at rest? The laws of physics would not be any different in this system, only the specific details of which physical objects are moving and which aren't.
rab99 said:
How can you say that its impossible to distinguish one frame of ref from another when i can do that at home?
You can't distinguish them in terms of the laws of physics--if you're in a windowless train car and you do some experiment in the train car, which doesn't involve anything external, then your experiments will come out the same regardless of whether the train car is moving inertially relative to the ground, or at rest relative to the ground (assuming the ground is also inertial). Of course if you open a window you can visually distinguish whether you're at rest relative to the ground or moving relative to it, but all the laws of physics are the same regardless.

Mentz114
Gold Member
But I can distinguish one frame of ref from another at home. I have a remote controlled car and a block of cement one moves the other doesnt?
The whole point is that you can choose any one to be your 'rest frame' and it makes no difference to the laws of physics. Coordinates are man-made and mean nothing in the end.

I note there are some compelling arguements that MM was flawed.

there is serious scientific endevour into proving the existance of the ether (probably dark matter)

and the jury is out on SR
Oh yeah ? I've heard all this before from every kind of cracked-pot.

Conversly many have heard irrefutable from many a cracked pot before as well ... irrefutable is an obsession of certainty ...... before Einstein everyone claimed Newton was irrefutable. Einstein died in 1955 ... short memory