Gravity and Me

[Alien 13]

Heads up BBC4 Tonight at 9.00 pm, looks interesting.

Alan

[cloudsweeper]

Promises to be good Alan.  Jim Al-Khalili is a good presenter and author!

Doug.

[saac]

I forgot about that Alan, good reminder   Agree re Jim Al-Khalili, one of my favourite presenters; he has a natural authority about him but it doesn't prevent him getting across the joy of his subject.

Jim

[Peco4321]

Too many things recording now, this is repeated at 02:00 in the morning. Loads of good astronomy stuff on at the moment. 

[JamesF]

Nicely presented programme, I thought (only watched it last night) even if much of the material was familiar to me.

I find myself strangely drawn to the idea that gravity might just be our perception of masses trying to achieve a state where time passes most slowly, as mentioned towards the end of the programme.  Though I don't like to anthropomorphise (yes, it's a word -- if it's not in the dictionary then I just invented it it strikes me as having parallels in the way that, say, electrons want to achieve a minimum energy state, or that entropy wants to increase.  I'm not saying it's "the one true explanation" of gravity, but I might have to see if there's any more "layman-digestible" discussion about the idea.

James

[Knight of Clear Skies]

I'll have to try and catch this on iPlayer. I agree Jim Al-Khalili is an excellent presenter.

[DaveS]

I watched this this the first time round and thought it excelent. I've recommended it to one of our 6th form girls who wants to read astrophysics at uni.

[Alien 13]

I have watched it again and left wondering where in the solar system would you have to be to feel zero gravity without moving, I know that although the actual gravity on the space station is not too much different to that on Earth the fact you are in orbit (falling) is the same as it being zero.

Alan

[SilverAstro]

31 minutes ago, Alien 13 said:

where in the solar system would you have to be to feel zero gravity without moving,

is the same as it being zero.

Nowhere ! Everything is "falling" round something else. The ISS is falling round the earth and experiences micro gravity, the earth is falling round the sun (and if you were orbiting the sun, again, you would experience only microgravity). Etc&etc on up to galaxy clusters orbiting round other galaxy clusters (or mass concentrations if we want to start thinking about dark matter) .

It is not quite the same as just quite similar to zero for many purposes*. It would only be the same as if there was no gravity gradient across you, or across your spacecraft.

If you attach a mass to a long rod and attach that to your spacesuit such that it is suspended below your feet then as you orbit the earth your feet will always (assuming no other perturbations) be pointing 'downwards' towards the earth. It is known as gravity gradient stabilization.

* until we start talking about special and general relativity, eeek , lets not

 

 

[saac]

52 minutes ago, Alien 13 said:

I have watched it again and left wondering where in the solar system would you have to be to feel zero gravity without moving, I know that although the actual gravity on the space station is not too much different to that on Earth the fact you are in orbit (falling) is the same as it being zero.

Alan

Alan I think due to the nature of gravity it is technically always felt (albeit weekly); nowhere in space is completely flat hence gravity has an infinite reach. Perhaps Lagrangian points are close to what you may be thinking about although this implies an object in an orbital situation. The link below is a good starting point.

Lagrangian Points

 

Jim 

[saac]

Just to add to the above, it is possible with some relatively straightforward maths to calculate L1 (Earth/Moon) the point at which the Earth's and Moon's gravitational pull on the Apollo spacecrafts was balanced.  If I remember correctly there is even a reference to it in the movie Apollo 13 where they reach the point of "zero g" - although I may have imagined that!    Again the spacecraft were obviously not stationary so really I guess this marks the point at which the orbit influences changes (might be wrong there don't quote me on it or plan a journey to the moon on the strength of  it)   I don't know how to embed YouTube videos so here's a link - ignore the pause video part just go straight to the answer - nobody's watching   

Point Of Equal Gravitation Between Earth and Moon

 

Jim    

[SilverAstro]

There is another way of looking at the "without moving" scenario that you might like : -

Suppose you were far out, like where the Voyagers etc are, and you fired your thrusters ( up/down, sideways) until you decided you were perfectly motionless (say by reference to the 'motionless' stars.) ie. did your best to cancel all orbital motion be it round the Sun or Earth or Jupiter etc. and waited , , , after a while you would find that you were moving, ever so slowly at first, towards the nearest big thing - the sun ! and soon you would be hurtling directly into the sun, provided you didnt get too close to Jupiter on the way by 

You can never get far enough away from the sun, even at infinity you would still fall into the sun, at a speed called "escape velocity" ( assuming nothing else bar you and the sun in the universe. )

 

[SilverAstro]

Oh dear !  This is the prog in which Jim Al-K corrects his great app blunder ?

He had everyone monitoring their aged time depending on how far they are 'out' (in space) on the oblate spheroid wrt gravity ?

but the whole point of E and his relativity was that time is not distinct from space, the space-time geodesic dictates that any particle will assume that place and time, that is why the earth is oblate ! and that is why any app on a phone will be subject to , , , oh well , , ,  !

 

[SilverAstro]

On 03/04/2017 at 00:11, saac said:

 

Point Of Equal Gravitation Between Earth and Moon

 

There is just so much wrong with that vid ! beginning with its title - Gravity does not 'pull' ! that is sooo newtonian !

and he has lost his suffices in his cancelled equations (two differenrt r 's ! etc ! to the extent that if you know what he meant you would have no need of him, if you didnt already have the maths/know then you would be mystified)

and anyway all those Lagrange points are in orbit and several are not stable requiring orbit corrections ( energy) to maintain. Like balancing on a knife edge on top of mount Everest. Others like the Sun-Earth Lagranges are full already with spacecraft

 

[Droogie 2001]

I watched and enjoyed the episode very much. I was not sure on the group carrying out the gravity experiments at first as I am use to seeing one presenter within an episode. Thankfully I soon appreciated their input and the fact that the episode knew when to move on rather drawing it out was pleasing.

I do like Jim Al-Kahalili a lot. The fact that he is a good physicists as well as being very good at presenting complex topics in an interesting way makes for great viewing.

The episode when he went to Sellafield is also a must see.