How can the fabric of spacetime expand at a certain acceleration? Isn't speed the rate of space divided by time itself? You can't measure the speed of spacetime since calculating speed is within the paradigm of spacetime itself. Kind of like saying an eye to see. The eye can't see unless it has a pair of eyes to see itself because it's an observer obtaining and using the eye that is doing the seeing, not the eye itself. And then there's the problem of what the universe is relative to in motion.

Let's see if we can cut this gordian knot of confusing sentences.

A bit of physics 101:
speed = distance/time
velocity = speed in a direction
acceleration = velocity change/time

So:
Speed is the amount of distance covered in a certain time ie. if you travel 60 miles in one hour then you have averaged 60mph.
Velocity is speed in a given direction ie. 60mph north.
Acceleration is the change in velocity in a certain time ie. if you accelerate, linearly and in the same direction, from 0 to 100 metres per second in 10 seconds you would be accelerating at 10ms^2

Given this basis:
Scientists refer to the universe expanding at a certain rate, IIRC it is an accelerating rate. More accurately, this is the rate at which the observable universe is expanding. This is calculated by examining the redshift of stars and other cosmic phenomena of a known luminosity.

HTH

ETA:

This may be of interest.

.... that only amplifies what I'm saying.

Then please clarify.

i think what drago is trying to say is " how can you measure spacetime only using variables that are dependant on spacetime ?"

it's like trying to measure the length of a stick using only the stick.

am I on the right track Drago ?

I am not sure if I understand what you are getting at completely, but one thing I will mention is that although we cannot measure the entire universe against itself, we can measure things in the universe. We can measure the distances from our star to other stars and so on.
Hubble found that galaxies that are further away from us seem to be moving away quicker than galaxies that are closer, and those galaxies appear to be moving away from each other. Those movements can be measured and calculated.

am I heading in the right direction?

well, i think its a semantic argument over what ppl mean by "universe".... do they mean the stars and galaxies and black holes and stuff... or do they also mean the gaps in between ?

I think the gaps inbetween count aswell... so, no, i think ur off the scale.. soz..

Well, something like that, yes. Sorry, I just sometimes can't find the right words. Speed is simply a measure given by spacetime. How can spacetime expand at a certain speed?

Two non-moving objects in space have a certain amount of distance between them one day, the next day the objects have a different amount of distance between them. The change divided by the time taken is the rate of expansion of the universe.

Sorry, I just don't see the problem.

As for measuring a stick using only the stick, well that can be done: A stick is a stick's length.

Now, the problem is that saying a stick is a stick's length has no meaning as it is a variable amount. This is not analogous to the expansion of the universe, however, as we are using a fixed length (SI unit is metres) to measure the distance between objects. A better analogy would be measuring a stick using the size of its atoms.

Am I getting closer Drago?

i) but there are no stationary objects in the universe.... only objects which are stationary relative to another certain object.
ii) if spacetime is expanding, and spacetime is the stick, then u are trying to measure the length of a stick that is constantly changing.... using only the stick..
ii) u have lost the point of the discussion in saying u r now measuring the stick by another 'artificial' means, coz u cannot use an alternate method of measuring the expansion of the universe... when measuring the expansion of the universe, u are measuring acceleration, a component of which is time.... time being part of spacetime, which in turn is part of the universe... !!!

yes / no ?

Heh - you got me on that one. I meant an object at rest, ie. no physical force acting upon it.


Obviously measuring something that is expanding using something that is expanding would not work, which is why we use something that doesn't expand.


It isn't an "artificial" means whatever one of them is. You use an unchanging component of the system to measure the rest of the system. Time is constant, so long as you aren't accelerating towards the speed of light, only the space is expanding. There are some time dilation effects with regards to observing very distant galaxies but they are compensated for.

As far as anyone inside the universe is concerned, the universe is expanding. It may appear differently if you were on the outside, but I would think that would be the least amazing thing you would notice.

I feel that I'm getting to the edge of my knowledge on this subject so I would encourage you to look at this thread on physicsforums for further, clearer explanations.

But Einstein said space is time as an absolute entity of the universe called spacetime. You can't say space is expanding but not time since space is time, they're just dimensions viewed in different perspectives.

if everthing is expanding ,then you and i could be say 1000 times the size we were yesterday and we wouldnt know, because everything else would be 1000 times expanded in proportion..
so there wouldn't be a constant sized measuring stick, or would there ..

But what really is expanding? The space in and between objects or everything in reality itself? And expanding relative to what? What you said is pretty much meaning less because we know galaxies and star systems are moving away from each from the center of the universe and that is why Astronomers, Cosmologists, and physicists say the universe is expanding. So if that's true, than the space between objects is expanding faster than the matter itself, so putting that extra statement in there is useless.

no one knows these answers

but, i prefer to the yogi philosophy: that there is a rhythm to the universes( like there is a rhythm to everything) and the universe is not the big bang, but rather an 'outflowing' ( which is probably still outflowing, since we see that everthing is still moving outwards. then there is pause then an inflowing then a pause then an outflowing etc.

from what and for what reason you will have to turn to metaphysical thoughts.

What you said was meaningless. We don't know how the universe supposedly "outflows" because since there is no space outside the universe, there is nothing to flow into. And I said, there's a problem with the universe outflowing or expanding or whatever at a certain speed.

First of all, there is no "center of the universe" from which everything is expanding.

If there were, General Relativity could easily be shown to be wrong.

Secondly, space is expanding. But, the force of expansion is cumulative over long distances. This means that over small distances the force responsible for the expansion is insignificant. Hence matter itself is not expanding because the atomic and molecular forces holding matter together easily overcome the negligible force causing the expansion.

Even on a larger scale, such as a planetary, solar system or galactic scale, there is no expansion of such bodies due to the expansion of the universe. Even the relatively weak (compared to electromagnetic) force of gravity is enough to overcome the force causing expansion.

So, you see, your "stick" with which you are measuring is not expanding at all. You may feel free to measure with it without erroneous consequences.

Harte

i think too many ppl on this thread are trying to interpret too many literals into the OP question...

most scientists agree in the big bang theory of the existence of the universe, which says that the matter in the universe was thrown out from a point, distributing all matter, stars , dustclouds, dark matter etc... so there is a central point..

no, it cant. general relativity holds for the expanding universe, the oscillating universe and the stable universe. it works on macro, micro and widescale.

i agree, matter is not expanding per se, but each system is travelling away from all others.... i dont think that "redshift" shown by even the most basic telescopes can be regarded as insignificant !!

this is what i was said several posts ago. the "expanding universe" refers just to the matter in the universe, the gaps inbetween are by products of the increased spacing.

but the stick is expanding, because the stick in question is time. You cannot measure an aspect of time like acceleration when the yardstick of time is constantly changing. One of the components of acceleration is time, which itself is part of spacetime, which is part of the universe.

over to you...
Min xx

No, there is not a "central point."

If there were, all the galaxies could be observed to be traveling away from the central point, rather than away from us.

IOW, every point in the universe is the central point.


Yes, it can.
Both theories of relativity (the Special Theory is just a special case of the General Theory) are predicated on the assumption that all reference frames are equal. IOW, there is no one reference frame from which one can observe the "real" motion of the rest of the universe. If there were a central point, this center would be the reference frame that relativity claims does not exist.



Velocity (and acceleration) is (are) measured instantaneously all the time. It's the basis of Calculus.

You don't need to measure with respect to time if the measurement is done instantaneously.

Harte

You really don't have any visualization abilities do you? All galaxies would be observed traveling 360 degrees away from each galaxy if that was true, and that's not possible if there is no central point since galaxies moving away from other galaxies should be closing in on other bodies as well. So you're saying space is infinite? Even then, your point is wrong.

Actually Harte is correct. Your attitude is wrong though.

I will let you look up the theory that explains why it is that there is no "center of the universe" as you would describe it. Einstein was correct on the matter, but you can go figure that out yourself.

It is tought to try and offer help and to have people act like spoiled kids and give snarky answers in response. Its just rude.

I see the confusion. Let's use the idea of a measuring stick, however. If spacetime itself is expanding, we might naively expect our measuring stick to expand all with it, right? Now, someone mentioned above the reasons an actual stick bound together through the electromagnetic force wouldn't do so. But imagine we design a measuring stick that will expand along with spacetime if spacetime is indeed expanding. It stands to reason that if we could tell how much the stick has expanded relative to its original size (assuming we know that original size) then we'd be well on our way to measuring the expansion. Of course we look at the expansion by looking at the markers that dwell in spacetime, namely galaxies. So we need to stretch our measuring stick from here to one of those markers. But if spacetime is expanding and we extend our measuring stick to that distant point then the stick should get bigger. In a sense, we are measuring that stick against itself; more accurately the stick against what it originally was. But it does allow us to deal with the concepts of space and time even in a context where things are changing.

Luckily we do have a very good ruler that we can measure against it's original size--light.



This question could use some clarification (I'm really not sure what you mean here) but in the meantime you might enjoy this thread, which talks about a pretty interesting idea concerning the relative motion of things in the universe.

I'm wondering, did light exist immediately after the big bang happened? If it didn't, there would be no "ruler" as you said.

I think light happened after the big bang. Think about it; so much raw energy just flying in all directions, light must have been born simultaneously with the blast.

i've been itching to reply to ur post here, but i've been watching the footie, so u had to wait..
i'm gonna concede to ur points on the center of the universe and, by default only, the validity of GR and SGR... arn't i magnanimous in defeat ? lolol
but the thing i don't understand and what i can't get from the guys trying to explain to me is that if calculus uses instantaneous measurement, then how come the results gained from calculus can be valid for any given frame ? Is time expanding at the same rate as spacetime ? and if so how does calculus enable u to differentiate between a linear measurement and an abstract one, like time ? can u keep ur answer fairly simple please, i'm studying arts not sciences.. lol

Minny xx

Also, the speed of light is supposed to be relative to everything. The universe is "expanding" faster than the speed of light so the measuring sytem fails as the universe is not relative to 'c'.

Yes.



Can you clarify this? In what circumstances do you think it should be invalid?



The speed of light isn't relative at all. That's why it's the ultimate measuring stick.

i don't think it should be invalid... i just dont get how it can be valid in this discussion.... the example we were all discussing up there was is the universe expanding.
stop me where i go wrong.. ok ?
the universe is expanding
the universe is space
part of space is time... Einsteins 'spacetime'
space and time are linked, inseparable... u alter one...u affect the other..
if the universe is expanding, then time is changing also..
"Velocity (and acceleration) is (are) measured instantaneously all the time. It's the basis of Calculus."
if the time element of a calculus integration is constantly changing, now i don't mean the actual acceleration, but the unit of measurement of the acceleration...how can u get a usable result form that integration ? it's like trying to time an athlete when ur stopwatch is gaining ?

i don't know !!

Min x

The speed of light is equally relative to anything and everything in motion. It's an absolute constant. The universe is supposedly expanding faster than the speed of light. " c " can not be used as a measuring stick.

I won't say the universe itself is spacetime. Many physicists are convinced that the universe can not only be made up of spacetime in order to exist or even the fundamental part, although philosophers and cosmologists are all baffled by this conception.

You are having with the problem of finding a point of reference in the "emptiness" of space. Am I right?

You are over thinking this .... a lot. Just imagine you place a sphere made out of thousands of particles... imagine an explosion that spread these particles out.... but keep in mind the center of the original sphere. If you look from that perspective..... the particles that were originally making up a sphere are becoming further and further apart from the center. Their velocities relative from each other do not remain constant because there are different levels of gravitational attraction to other objects in space. EVERY object in space experiences gravitational attraction to another body in space. EVEN you and me have attraction to each other.... but it is very very small since we are not that massive. I am no astrophysicist....... but that's my 2 cents

You don't need a literal physical point of reference.... you just to imagine it from an imaginary point


I'd suggest that if you are really interested in an explanation that you learn some physics and some astrophysics.... seems like your putting more reliance in philosophy and semantics than science

Before I state my opinion, I would jut like to say that I have not read all of the replies in this thread--shame on me--however I will state my opinion anyway...

In my personal opinion, to understand the "expansion" of the universe one must think in "infinites." You must know that the void--the area outside of a galaxy--is infinite. Meaning that "space" itself is not expanding nor ever has.

Now just "stabbing the dark"--so-to-speak--this could possibly mean that there are several points in space--or "the void"--from which existence, as we comprehend, "Big Burst" or "Banged."

In my opinion this would explain galactic collisions. Also, at this point in our collective history, one may think this as "common sense" as the world being round. However this is probably my naiveté talking as my species seems to have difficulty comprehending the truth. It's not our fault exactly, it's just our conditioning--which makes the majority of us so easy to control.




...oh how I look forward to the arguments which will be thrown my way within the next few hours! lmao! Yippy!

I am not over thinking this and you don't seem to be the one understanding it. First of all, there is no center of the universe.

http://math.ucr.edu/home/baez/physics/Rela.../GR/centre.html

That was an analogy to help you understand what I was getting at.

But the logic is flawed. You can not think of it being an explosion through space. It's an explosion of space. Very different, and the expansion of space is supposedly not relative since it's going faster than the speed of light. And light is supposed to be an absolute constant that is equally relative to anything and everything.

the analogy is still applicable. just like the balloon analogy. if from your perspective matter is moving away from you... the distance between you is increasing.... if the distance between is larger the size of that local area will be larger... and that will add into making the universe larger (more volume). I honestly think that you should be posing these questions on a physics forum where people who actually are experts on this could help you more... not a metaphysics forum.

http://www.atlasoftheuniverse.com/bigbang.html

That doesn't follow. The constancy of the speed of light does allow it to function as a measuring stick.

Because it's moving obviously

universe expansion is not modeled with special relativity (this would only represent a static universe). it can be modeled with general relativity. also recessional velocities can exceed c with hubbles law. there is nothing complicating with the speed of light being exceeded

The speed of light is supposed to be equally relative to everything and anything. The universe is supposedly having an accelerated expansion. So the speed of light is slowing down relative the the speed of the expansion since it's moving faster than light. Therefore, the speed of light can not function as a measuring stick because it's speed is not constant relative to the expansion of the universe, but slowing down.

You've now made two separate arguments here and neither of them hold water. You originally said that expansion faster than the speed of light somehow invalidates the constancy of the speed of light. Any homogeneous expansion (the sort of expansion described by Hubble's law) of the universe results in superluminal expansion. But that has no bearing on the constancy of the speed of light. Something bigger than a measuring stick doesn't change the size of the measuring stick. Indeed, it's the existence of the measuring stick in the first place that lets you even make the statement that something is bigger than it.



If by that you mean it's constant and relative to nothing.



That's the problem with the way you're stating (and apparently conceptualizing) the fact that the speed of light is constant; it isn't "equally relative to everything," it is absolute. All other things are necessary relative to it, which is the entire point: it is the ultimate measuring stick. It doesn't go slower because something else starts going faster. Acceleration of other things doesn't somehow negate the constancy of the speed of light. I'm not sure how to get this point across to you any clearer but you're conceptualizing this incorrectly.

That's only because light is supposed to be faster than everything else. It's constant because the measure of spacetime is the same for all references at c since time stops at the speed of light. That's not the same thing for the expansion of the universe since it's going faster than the speed of light. You can't go any slower than stationary time.