Perfect Motion

[5614]

for those who dont know, perfect motion is when an object which starts moving never stops......

i had always thought this was impossible..... due to air resistance.....

how dumb was i??? what happens in space?

but then perfect motion is apparently impossible.....

 

why doesnt it work in space???

is it due to the gravity?

or just, is it possible?

and if it was 'invented', what uses does it have

[JaKiri]

Because of particle interactions with the quantum fuzz.

 

No space is 'empty', nothing near.

[5614]

so your saying that a vacume is an empty space without 'air' not without any atoms?

[Cap'n Refsmmat]

Space is not really a vacuum. There's few atoms out there, just very, very, very few. Enough to cause drag.

 

There is no such thing as a "perfect" vacuum.

[Martin]

for those who dont know' date=' perfect motion is when an object which starts moving never stops......

i had always thought this was impossible..... due to air resistance.....

how dumb was i??? what happens in space?

[/quote']

 

hello JaKiri and 56!

this question has an interesting connection to special relativity

 

if there is no preferred frame, then "stops" has no meaning

an object that looks to you like it is in uniform motion is already 'stopped' in its own rest frame

 

so special rel would have to allow perfect motion---physics must be the same for two different observers in relative motion and something can only come to a stop for one of them

 

 

but special rel (the 1905 theory) does not quite match nature

and general rel (1915) is a slightly more realistic fit

 

gen rel allows a preferred frame and it allows recession speeds faster than light, and it would allow us to say that everything eventually will come to rest with respect to the frame of the universe's expansion

 

the expansion of the U picks out a rest frame for us---Hubble even figured this out----the frame in which the redshifting looks the same in all directions

 

when people found the CMB (microwave background) they confirmed this rest frame and determined it (with the COBE satellite) very accurately

 

it is the frame in which there is no big doppler hotspot in one direction balanced by a coldspot in the opposite direction----no microwave "dipole"

 

if something is moving with respect to the microwave Background then it the microwave photons will be hitting it harder on its front face than they do on its back

 

light exerts pressure, delivers momentum,

 

so after trillions of years, if nothing else happend, a thing that is coasting thru space will come to rest with respect to the CMB

(it will no longer see a doppler hotspot ahead or a coldspot behind, and it will know it has stopped)

 

this is even without any impurities in the vacuum no dust to bump into, no stray molecules to slow you down, no gas, no gravity wells to confuse things, just sheer nothingness----plus the microwave background

 

BTW the earth and sun's motion with respect to CMB was first measured by Lawrence Berkeley Lab in IIRC 1960s using a high-altitude U2 aircraft and it was discovered that our motion is in the direction of the constellation Leo and it is about 360 kilometers a second

 

there is a website that the U2 experiment made with a star map showing the hot and cold spots in the sky

 

the COBE satellite CMB dipole measurement, published in 1990s confirmed the earlier result and provided more accuracy

 

In Quantum Field Theory the vacuum has no preferred frame, so I do not think that virtual particles would slow uniform motion even theoretically----experiencing drag from them would constitute a kind of measurement and violate virtuality----and also show existence of a preferred frame which is a no-no. Particle theorists call having a preferred frame "Lorentz violation"----strangely, cosmologists have the CMB frame (the one stationary with respect to U expansion) and work with it but the particle people eschew such things.

 

Anyway virtual particles in the QFT vacuum will not produce drag and slow you down

 

But microwave background photons (and other cosmology stuff) will.

[superstorm]

Space is not really a vacuum. There's few atoms out there' date=' just very, very, very few. Enough to cause drag.

 

There is no such thing as a "perfect" vacuum.[/quote']

 

This is true. In every matchbox size of vacuum there are at least twelve hydrogen atoms/

[JaKiri]

Yeah, I forgot about De Broglie; I haven't done any physics (other than answering questions) for about a year or two, so I keep on forgetting when things interlink.

[[Tycho?]]

How does EM radiation exert pressure on something?

[Martin]

That U2 microwave sky map is so nice I went and dug up the link

 

"...a star map with the temperature of the Background as an overlay, showing the hotspot. So you can see the stars around Leo and a kindof contour map of temp:

 

http://aether.lbl.gov/www/projects/u2/

 

...."

in case someone wants more precision here is the 1996 report

 

http://arxiv.org/astro-ph/9601151

 

"The Dipole Observed in the COBE DMR Four-Year Data"

-----------------------------

 

IIRC the Background is just about 3.4 millikelvin hotter in the Leo direction

 

when Leo is visible in the sky you can point your finger in the direction the sun and earth are traveling, in absolute terms, in space

 

this is a combination of our motion within the MilkyWay framework and the overall motion of the MilkyWay itself, relative to background

[swansont]

'']How does EM radiation exert pressure on something?

 

Photons have momentum of p=E/c. Whan an atom absorbs a photon, it gets a momentum "kick." When it releases a photon, there is another "kick." But the directions don't have to coincide, so there can be a force on the atom. Even though E/c is small, atoms aren't very massive and an atom can absorb and emit millions of times a second.

 

The 1997 Nobel prize in Physics was awarded for laser cooling, which uses the radiation pressure concept. (Chu, Phillips and Cohen-Tannoudji)

[[Tycho?]]

Photons have momentum of p=E/c. Whan an atom absorbs a photon' date=' it gets a momentum "kick." When it releases a photon, there is another "kick." But the directions don't have to coincide, so there can be a force on the atom. Even though E/c is small, atoms aren't very massive and an atom can absorb and emit millions of times a second.

 

The 1997 Nobel prize in Physics was awarded for laser cooling, which uses the radiation pressure concept. (Chu, Phillips and Cohen-Tannoudji)[/quote']

 

 

Huh, I didn't know that. I knew they were able to for something like a light sail to work. But boy, thats pretty neat.

[5614]

so perfect motion really is impossible?