Space is a "vacuum"?

What is meant by the term "space is a vacuum"? In a vacuum, objects travel toward a certain direction; but how does this apply to space? Is there one particular direction that everything is traveling in?

Basically, why do people say space is a vacuum?

Originally posted by vivify
What is meant by the term "space is a vacuum"? In a vacuum, objects travel toward a certain direction; but how does this apply to space? Is there one particular direction that everything is traveling in?

Basically, why do people say space is a vacuum?

A vacuum is simply an absence of objects. It may however contain energy in the form of electromagnetic radiation (actually I am not entirely sure about whether electromagnetic radiation counts when it comes to vacuums - are photons matter?).
This has nothing to do with the law of conservation of momentum. The fact that a moving object continues to move unless acted on by a force has nothing to do with vacuums or space. It is a general rule for all objects. A vacuum is usually mentioned because without a vacuum, there are the forces of other objects to take into account. Nevertheless, even in a vacuum there may be forces (gravity, magnetism, electromagnetic radiation).
An objects movement is relative to other objects, so no, there is no particular direction that all objects are moving and space itself does not have movement, nor do objects move relative to space, but rather relative to other objects.

Originally posted by vivify
What is meant by the term "space is a vacuum"? In a vacuum, objects travel toward a certain direction; but how does this apply to space? Is there one particular direction that everything is traveling in?

Basically, why do people say space is a vacuum?

A vacuum (in this context) is a volume devoid of air/gasses. (matter)

Space is a near perfect vacuum as the density of gas molecules is incredibly low.


It's basically as simple as saying their is no air in space.

Originally posted by vivify
What is meant by the term "space is a vacuum"? In a vacuum, objects travel toward a certain direction; but how does this apply to space? Is there one particular direction that everything is traveling in?

Basically, why do people say space is a vacuum?

As pointed out vacuum is "empty space". And if we come to a struct definition it doesn't exist, since there is a phenomenon called "vacuum energy" or "quantum fluctuation" which is probably difficuklt to explain below 1000 words.

If we come to your definition of "unobstructed travel" we come to a diffferent definition. You can readily calculate the particle density of space via kinetic theory of gaes under which collision are less likely than any arbitrary value.

If we come to telluric vacuum technology you can achieve this quite easily if your condition is set that a collision with wall is more likely than a collison between moleuees. This is called the Knudsen region.

And for your last question i fact we talk about "vacuum" somwhat sloppily if in a given vessel the pressure is below the surrounding pressure. The is a somewhat qualitative scale for vacuum:


low vacuum (3 to 100 kPa)
medium vaccum (100mPa to 3 kPa)
high vaccum (100nPa to 100 mPa)
ultra high vacuum (100 pPa to100 nPa)

A vacuum cleaner is not really proving something considered "vacuum" by any physical standards. In space the pressure is much lower than achieveable on earth and such for all practial physical operations it is "vacuum"

Roughly speaking, vacuum is what you get when you have some piece of space, and it cannot have anything less. Note that this is not the same as emptiness, as the universe is expanding it is creating "new space".

Disclaimer: the above is greatly oversimplified.

Also, even in the absence of matter, space is not a void. It is an entity in of and by itself, responding to matter by curving and responding to gravity or is gravity depending on the theory, Einstein says gravity is the curvature of space. You can tell the curvature of space by the way a light beam travels through it. That makes it an entity much more than a simple absence of matter.

Vacuum, which is just 3D volume of space with no matter within it, does not consist of 'nothing' but rather is intrinsically just as much as being 'something' as solid objects. It does not have to have 'something within' it for it to be 'something'.
This is something even many scientists (including, I noticed, shamefully, some of the top physicists) and philosophers alike misunderstand and get confused with;
'nothing' itself does not exist but 'empty' space is not 'nothing'!

Originally posted by vivify
What is meant by the term "space is a vacuum"? In a vacuum, objects travel toward a certain direction; but how does this apply to space? Is there one particular direction that everything is traveling in?

Basically, why do people say space is a vacuum?

Vacuum The final frontier.

Originally posted by joe beyser
Vacuum The final frontier.

Even vacuum may be quantized.

Originally posted by sonhouse
Even vacuum may be quantized.

virtual particles poppin in then out again everywhere.

Originally posted by sonhouse
Even vacuum may be quantized.

And should result in quantum foam: http://en.wikipedia.org/wiki/Quantum_foam

If I remember correctly, there once was a piece of evidence that appeared to show the existence of this quantum foam.
This 'evidence' was supposed to be in the form of the apparent observation that, when we observe gamma ray bursts from billions of light years away, there is a measurable time delay between the arrival of the shorter wavelength gamma rays and the longer wavelength gamma rays and the only credible known way to account for that delay is to assume that quantum foam is slowing down the shorter wavelengths more than the longer wavelengths.
However, more resent evidence appears to completely invalidate and contradict that earlier evidence:
http://www.newscientist.com/article/dn18068-universes-quantum-speed-bumps-no-obstacle-for-light.html
so I guess the jury is still out on the existence of quantum foam?

Originally posted by vivify
In a vacuum, objects travel toward a certain direction; but how does this apply to space?

I don't understand this part of your question at all. What do you mean by "objects travel toward a certain direction"?

All of the other answers in this thread are good ones, but they don't really address this portion of the question.

Why did i find 2 smarties in my packet of mini chocolate eggs?!

Originally posted by vivify
What is meant by the term "space is a vacuum"? In a vacuum, objects travel toward a certain direction; but how does this apply to space? Is there one particular direction that everything is traveling in?

Basically, why do people say space is a vacuum?

Can anyone comment on the possibility that the fabric of space-time may in fact be dark energy? Einstein postulated that an ether must exist. With so pervasive a "medium", it's remarkable that we've had so little success predicting it. It's remarkable that the finest minds in history still regard dark energy as unobservable.

Originally posted by sasquatch672
Can anyone comment on the possibility that the fabric of space-time may in fact be dark energy? Einstein postulated that an ether must exist. With so pervasive a "medium", it's remarkable that we've had so little success predicting it. It's remarkable that the finest minds in history still regard dark energy as unobservable.

Einstein postulated that an ether must exist.

http://www.helical-structures.org/einstein_about_ether.htm

“...In his articles in 1905 Einstein did not fully rejected the existence of Ether. He only suggested a mathematical treatment of some relativistic problems. In 1920, after he developed the General relativity, he arrived to the conclusion that the ether should exist. This fact is of enormous importance, when citing the contributions of Albert Einstein. The Einstein statement from 1920 is missing in the scholar physics textbooks, where only his articles from 1905 are mentioned.
The overthrowing of the Ether concept by the contemporary Physics relies on a very weak argument - the data interpretation of the Michelson-Morley experiment. In the past decades more accurate experiments have been provided and recently the Michelson-Morley original data have been reanalyzed. All they show that this experiment has been inconclusive. In a few words, the wrong interpretation of Michelson-Morly and other similar experiments is a result of effect found and recognized by R. R. Hatch (a pioneer in GPS system): the effect of Doppler shift is canceled by the relativistic dependence of the clock rate. In order to avoid this effect, the experiment for detection of the velocity dependence of the speed of light must include a light chopping.  A laboratory experiment arranged in a such way is able to detect the Earth motion through space. First sets of such experiments is provided by Stefan Marinov in 1973, 1976 and 1984. The measured velocity is the velocity of the Earth and solar system rotation around the centre of the Milky Way. These experiments defy the postulate of relativity formulated by Einstein, but opens the door for deeper understanding the physical vacuum and what is behind the space-time. ...”