View previous topic | View next topic

Gravity

Page 1 of 1

UrbanSpaceman
1246697.  Fri Aug 25, 2017 5:34 pm Reply with quote

On Series N programme 13 - Naval Navigation, Sandi said a couple of times that there is no gravity in space. This mistake is made by many people, and most people are aware of the term "zero gravity". However, there is no such thing.
If there was no gravity in space then the International Space Station would fly away from the Earth, as there would be nothing to keep it in orbit. The Moon would also shoot away from the Earth, and the Earth itself, along with all the other planet, asteroids and comets, would all fly away from the Sun.
The force of gravity extends further than any other.
The reason that astronauts on the ISS - or outside it - float around is because they, along with the ISS are in a state referred to as either "weightlessness" or "free fall". They are "falling" towards the Earth, being pulled down by gravity, but this is exactly balanced by their forward speed, which tries to send them forward in a straight line - away from the Earth.
So the bottom line is "There is no such thing as zero gravity" - which is the title of my next book :)

 
Bondee
1246727.  Sat Aug 26, 2017 4:58 am Reply with quote

post 1222395

 
UrbanSpaceman
1246783.  Sat Aug 26, 2017 4:32 pm Reply with quote

That post doesn't really give any explanation, nor much information, which is why I posted my entry.

 
dstarfire
1246869.  Sun Aug 27, 2017 7:29 pm Reply with quote

Yes, but then you'd have to talk about freefall, and that means enduring that idiotic fallacy of "earth falling away".

I don't know about you, but I prefer the fiction of a gravity free space than asinine delusion that the earth is basically just a giant ball on some undetectable, permeable table.

 
gruff5
1246888.  Mon Aug 28, 2017 5:14 am Reply with quote

"Zero Gee" is a shorthand for being in free-fall, but I'd agree, that strictly speaking, the Earth's graviational attraction in low Earth orbit is very similar to that at the surface.

Though it's actually very hard, or impossible, to achieve 'zero Gee' in orbit, due to tidal effects, slight atmospheric drag and thruster adjustments, so the situation on the ISS is often called "micro gravity".

 
crissdee
1246905.  Mon Aug 28, 2017 7:54 am Reply with quote

Surely if you were far enough away from any large body, you could get to "zero" gravity? I don't claim this is possible anywhere we've been yet, but it should be theoretically possible, shouldn't it? Somewhere between galaxies?

 
PDR
1246910.  Mon Aug 28, 2017 8:36 am Reply with quote

Well in principle the gravitational field tends to zero as distance tends to infinity, so even at extreme distances (well north of the M4) it will still be present. Whether it's detectable will depend on whether you have a decent Starrett instrument or one of the cheap chinese ones from Amazon or ebay.

You could also have zero "net" gravitational field at the macro level if you were at a point in space between two or more masses where the magnitudes of their fields were precisely nulled-out. But given that we currently theorise that gravity is also a wave you would have to have identical wavelengths and identical phase for there to actually be no gravity in the literal sense. I feel intuitively that this would be difficult to achieve, but my physics stops well short of thinking about how and why it might be so, or even whether "gravity waves" must have the same wavelength (or if they don't what the resulting differences would look like). That would be more Bob's or Leith's area than mine. But iof they can come up with a principle for a gravity phase meter then I'll be sure and build one!

PDR

 
Leith
1246968.  Mon Aug 28, 2017 5:39 pm Reply with quote

Certainly. What we need is a giant laser interferometer.

Gravity waves are rather subtle things so it'll need to be quite sensitive.

Ideally we want to be able to put three references masses in a triangle, about 2.5 million kilometers to a side*, and then measure the distance between them to an accuracy of around 10 picometers.

The masses will need to be completely undisturbed, so they'll need to be in space**, preferably each 'floating' inside a hollow satellite that will protect it from being buffeted by stray atoms.

The measurement is then done by shining lasers between the masses, combining the reflected beams that come back from each arm of the triangle, and looking at resulting interference pattern (i.e. that's your phasemeter***).

Any unexpected fluctuations in the phase are potentially gravitational waves - tiny variations in the distance between the masses due to space compressing and stretching on account of something really big happening a long way off (black holes colliding or similar).

If you could knock one of those up for us on your lathe, that would be splendid.

There's a prototype up there at the moment at the Earth-Sun L1 Lagrange point (one of those points you mentioned where the gravity is 'nulled out' - between the Earth and the Sun, at least). It's a little more modest in scale, though. Rather than 2.5 million kilometers across, they went for 35 cm to start with. It can't detect waves at that scale, but seems to be proving the principle quite nicely.

The mission is called 'LISA'. You should be able to find the details here:
http://sci.esa.int/lisa/59250-europe-s-search-for-gravitational-waves/

* Originally the thinking was 5 million kilometers, but that seemed a bit excessive in retrospect

** Also they won't fit anywhere else - 'the ground' isn't big enough.

*** This is looking for waves at the macro-level of course - gravitational waves in space-time. If you want to look into the nature of gravity itself at the quantum level and see if it's made up of 'gravitons' with wave-like behaviour, then dunno. Ask Dr Bob. You'd probably need a smaller lathe, at least.

 
gruff5
1246985.  Tue Aug 29, 2017 4:15 am Reply with quote

PDR wrote:
... But given that we currently theorise that gravity is also a wave ....

Do 'we'? AFAIK Einstein's GR theory models gravitational attraction/force as a bending of spacetime with waves not required. The GR theory can then go on to predict gravitational waves generated by accelerating masses - eg big ones from merging black holes.

Are you thinking of the gravitons that have been suggested as part of a solution for quantum gravity theory? Well, then, such particles, like all subatomic particles, would have a wave associated with them. But gravitons are only a speculative idea at present and 'we' are a long way from having any evidence to show they, or the associated theory, are a reality.

 

Page 1 of 1

All times are GMT - 5 Hours


Display posts from previous:   

Search Search Forums

Powered by phpBB © 2001, 2002 phpBB Group