[Closed] Gravity - Is there a Physicist in the house?

Newtonian: Gravity is an attractive force exerted between any two objects.

General Relativity: Mass warps spacetime such that objects travelling in a straight line will seem to fall toward each other to those experiencing the universe as a euclidian geometry. (ie: us.)

(Note that it's not small objects pulled towards big objects, they're each pulled toward the other.)

(I went to the pub at lunch and I'm not a physicist.)

Gravity or Quantum Gravity?

Note that it's not small objects pulled towards big objects, they're each pulled toward the other

Magnets innit?

Note that it's not small objects pulled towards big objects, they're each pulled toward the other
Indeed everything is pulled towards everything else.

Yep got that. I did my graduate training with a guy who had a PHD in Physics from Cambridge but he couldnt explain this to me.....he was a bit odd though.

Someone give it a stab?

Indeed everything is pulled towards everything else.

.. which is why we notice gravity at all, it's incredibly weak compared with electromagnetic and nuclear forces eg. you can pick up a pin with a fridge magnet, even though the [b]whole of the earth[/b] is pulling down on it.

The other forces have attractive/repulsive components which tend to average out for large collections of matter, but gravity just adds up.

[img] ?v=4bb742c1[/img]

You might want to skip to 5:30 where she is fisting the couch.

Question for you all. Can someone explain to me what gravity really is, and how it works?

o had a PHD in Physics from Cambridge but he couldnt explain this to me.

Which pretty much answers it I think - is it possible that it's something we can observe and measure but no-one has a real understanding of how it works? There's a few odd theories about suggesting it could be some form of radiation or particles, but they're just unproven theory.

Its just the way it is. The GR view is usually summed up as "mass tells spacetime how to bend... space tells mass how to move".

The more mass you have, the more spacetime gets bent nearby.

In fact to be precisely correct its [i]mass-energy[/i] that warps spacetime, which is one of the reasons why making quantum theories of gravity turns out to be so hard... gravitational disturbances themselves carry energy (!) so they gravitate themselves, and interact with each other in horribly complicated non-linear ways 😯

Can someone explain to me what gravity really is, and how it works?

Can you also fill in the scientific community whilst you're there, please?

The whole general relativity bit is just a way of describing what we see. It doesn't explain WHY things are attracted to each other/why mass warps spacetime.. at least I don't think it does.

I know that it is the route to madness to try and think too hard about concepts like gravity, it's better just to understand how the physics pioneers predicted certain outcomes perfectly without leaving their desks. I will tell anyone who asks that I am an athiest, that I can't accept religion because I have to test things for myself.....but science can become so complex, you just have to trust that the experts are peer-reviewing everything and arent part of some global conspiracy to undermine the faithful.

I just love this

Neptune was mathematically predicted before it was directly observed. With a prediction by Urbain Le Verrier, telescopic observations confirming the existence of a major planet were made on the night of September 23, 1846, and into the early morning of the 24th,[1] at the Berlin Observatory, by astronomer Johann Gottfried Galle (assisted by Heinrich Louis d'Arrest), working from Le Verrier's calculations. It was a sensational moment of 19th century science and dramatic confirmation of Newtonian gravitational theory. In François Arago's apt phrase, Le Verrier had discovered a planet "with the point of his pen."

In retrospect, after it was discovered it turned out it had been observed many times before, but not recognized, and there were others who made various calculations about its location, which did not lead to its observation. By 1846, the planet Uranus had completed nearly one full orbit since its discovery by William Herschel in 1781, and astronomers had detected a series of irregularities in its path which could not be entirely explained by Newton's law of gravitation. These irregularities could, however, be resolved if the gravity of a farther, unknown planet were disturbing its path around the Sun. In 1845, astronomers Urbain Le Verrier in Paris and John Couch Adams in Cambridge separately began calculations to determine the nature and position of such a planet.

Le Verrier looking through a telescope 160yrs ago sees Uranus "wobble" and so predicts the existence and position of a completely different planet, Neptune. Aren't humans incredible.

Aren't humans incredible

A tiny minority of us, yes. The rest of us are bumbling around in the dark in comparison.

Is space-time anything like air-time? or bed-time even?

Eddies in the space-time continuum.

Don't make the mistake of trying to understand these weird sciency things in the context of the real world. The 'real' world is just a sub-set of reality in which we have evolved. Common sense and a physical grasp of things only apply to this world.

You can do many thought experiments on a real world level by imagining objects and how they'd react, based on our own experiences. Engineers, craftsmen and people working manually do this all the time. However our experiences don't extend to the relativistic or quantum scales, so there's no point in trying to apply your previous practical experience to it.

So particles can be waves at the same time - ok, create a new world in your mind based around that idea 🙂

Le Verrier looking through a telescope 160yrs ago sees Uranus "wobble" and so predicts the existence and position of a completely different planet, Neptune. Aren't humans incredible.

Yup, though it's also possible to get the right answer for totally wrong reasons. Jonathan Swift got the number, size and orbits of the moons of Mars pretty much spot on, but through faulty logic. Venus had no moon (Mercury hadn't been discovered yet), the Earth had one and Jupiter had 4, so logically Mars must have two, and since he couldn't see them they must be small and close to the planet.

Eddies in the space-time continuum

Le Verrier looking through a telescope 160yrs ago sees Uranus "wobble" and so predicts the existence and position of a completely different planet, Neptune. Aren't humans incredible.

It doesn't always work though. When a similar "wobble" was noticed with Mercury another planet was predicted (called Vuldan I believe) but of course no-one ever found it.

Le Verrier looking through a telescope 160yrs ago sees Uranus "wobble" and so predicts the existence and position of a completely different planet, Neptune. Aren't humans incredible.

It doesn't always work though. When a similar "wobble" was noticed with Mercury another planet was predicted (called Vuldan I believe) but of course no-one ever found it.

So rather than being clever, he made an educated guess that turned out to be correct, and therefore we all look back thinking 'wow, clever' rather than 'what an idiot' ...

PRetty much all science is educated guesses until it's proven. That's the way it works. Propose an idea, develop the kit to test the idea, test it, find out you're wrong or right, move on to next idea. Taking completely random guesses would leave you looking a bit silly most of the time. Testing something you already knew for sure would also leave you looking silly. 🙂

Brian Cox was the drummer in D:ream. I think.

Things can only get better.

All you have to remember is that when you fall off your bike, you don't actually fall, you merely travel in a straight line through timespace that has been distorted by the Earths mass.

or something. 😕

So particles can be waves at the same time - ok, create a new world in your mind based around that idea

Was there not a recent experiment shedding light on this quandary ???

EDIT: everything else you said was spot on and I am not as daft as try to argue with a degree level physicts with my O level and passing interest, its a genuine question.

bigthunder - Member

Brian Cox was the drummer in D:ream. I think.

That prooves drummers are clever than guitarists..if only Keith Mooon had lived we'd all have a nuclear fusion generator in the garage and go around on monkey powered hover shuttles

So particles can be waves at the same time .....
.....I thought they behaved as each depending on how we observed them rather than they could necessarily be both at the same time.

Something like that isn't it ?

I tend to think along the lines of there being two models, one being 'particle' and one being 'wave', both of which under certain circumstances may describe the properties of 'things' that can only be properly represented by mathematical equations.

When 'things' fall outside the scale range of direct human experience our language cannot accurately describe them, which is where we rely on the scientists to give us representations we can try to grasp in common terms.

I love the Sixty Symbols videos; only discovered them after someone posted them on here. the fact that the guys in the video can't remember constants and stuff makes me feel much better about my bumbling through those things.

Junkyard.. sort of... They are neither and both. This tells us that the nature of matter is stranger than we realise. Being two things at once doesn't make sense at our scale, but is perfectly reasonable at others. Determinism is so.. limiting, dude.. free your mind..

The way they describe in the Cox book is that an electron moving from A to B does so along every possible path simultaneously, so it acts like a wave. And then I saw a TV programme on it which showed that if you actually set up a sensor to observe the electron (or photon) when it does this....it stops doing it and acts like a single particle again.

ie it knows it is being watched. Mindwarp.

One (of many) 'weird physics' questions I've not got my head round is how massless photons can be affected by gravity.

And following on from that: given light [i]has[/i] been observed to be subject to gravity, and objects subject to a force have their velocity altered, how is the speed of light constant ?

how is the speed of light constant?

Velocity and speed aren't the same thing. The velocity has a direction while speed is just the magnitude of the velocity.

Plus, light often travels at speeds slower than 'the speed of light' - that's a maximum when in a vacuum. It travels slower through glass and water than in air, which is why we get refraction and rainbows.

miketually - Member
how is the speed of light constant?
Velocity and speed aren't the same thing. The velocity has a direction while speed is just the magnitude of the velocity.

aaah, so it's a vector thing ???

Plus, light often travels at speeds slower than 'the speed of light' - that's a maximum when in a vacuum. It travels slower through glass and water than in air, which is why we get refraction and rainbows

Yes, I meant in vacuum (well space, if that [i]is[/i] a vacuum) - specifically I was thinking of gravitational lensing...

...still don't 'get' how massless particles are subject to gravity though, but I'm sure there's some complex equations for it 😕

You've REALLY got a degree in politics & history?!? 🙂
You do know your side lost the last big one?

Jesus H Corbett , we are all doomed to hell 😀

The speed of light can be calculated from the electromagnetic properties of the medium through which it is travelling - permittivity and permiability. Funnily enough empty space has finite values for these two constants, so you get a finite speed. A material with different values gets a different speed.

still don't 'get' how massless particles are subject to gravity

Why? They have energy, and energy and mass are interchangeable after all 🙂

Cheers molgrips others for the replies

Why? They have energy, and energy and mass are interchangeable after all

yeah, but something doesn't 'click' for me - didn't think E=MC^2 applied in the same way to zero mass 'particles' - but I get confused on the differences between resting mass and quantum mass....

...good job I'm a biologist and only have to think down to the size of molecules because subatomicals are far too quirky for me, and as for cosmologicals 😕

I am not sure about general relativity and photons and stuff but I was making the point that there's a lot of strangeness.

That's why the language of Physics is Maths. If you knew all the equations it'd make perfect sense.

is it ok to not really understand this stuff but to be completely fascinated by it? cos I am.

molgrips - Member
.....I was making the point that there's a lot of strangeness.

Yeah, great isn't it - reality, but not as we know it 🙂

wish I was brainy enough to do proper mathemetictacs 😳

Yes. But remember that it's really just waffle unless you do the maths, so to speak.

molgrips - Member
Yes. But remember that it's really just waffle unless you do the maths

I'm happy enough for brainy folk to do the maths and just 'look at the pictures' 🙂

It might help by starting from some fundamentals. All matter interacts through one of four forces via a field. These forces are:
1) strong force - couples quarks inside nuclear particles. gets stronger as particles move apart.
2) weak force - responsible for changes in these nuclear particles (radioactivity). Acts over a short range
3) electromagnetic force - charge dependent and gets weaker as particles move apart.
4) gravity - always attractive and gets weaker as particles move apart.

Now forces work through coupling to a "force field". They do this by the exchange of particles. If the particle has mass, then the distance the force can work over is finite. The strong and weak forces work this way. Electromagnetic force uses massless photons, and gravity uses particles callex gravitons. I like the analogy of two rugby players passing to keep in contact as they run up the pitch.

The basic principle of forces coupling particles is however the same. As energy increases, the forces begin to work the same way. This is called "unification". The first unification was by Maxwell who showed electricty and magnetism were really the same force. Next came the weak force, which is propagated by heavy particles called W and Z bosons. This was in the 1980s. Going higher in energy, the next unification would be the strong force. The Higgs boson is the particle that would carry this force, if we see it.

Now back to gravity. Although fantastically weak, it is the bad boy of the four because it is universally attractive. Further unification of tbis very uncoupled force via the graviton would require much much higher energy (we won't be making that in any millenia soon). Superstring theory is one attempt to unify gravity with the other four and account for it's odd behaviour. Another approach is the quantum gravity, that uses an approach more akin to general relativity and the warping of the field carried by gravitons.

I don't play the drums, but before discovering Biology, I was a Professional Theoretical Physicist...

At last someone who knows what he is on about.
Maybe I think 😕
Probably be the end of the thread now! And I was so enjoying not having a clue about WTF anyone was going on about! 😀

djaustin makes a good post but it still doesn't explain WHY any of it happens, just how...

Well, the "why" gets a bit metaphysical, or even religious if you're that way inclined. Really, it's because the universe is built that way. Then it's personal preference whether you believe in the weak or strong anthropic principle. The weak anthropic principle says, simply, that the universe is the way it is because if it were otherwise then we wouldn't be around to see it. The strong says that the universe was made for us to observe it.

Here's a anology for gravity

1. Imagine a square rubber tile - thats pinned in all four corners. This is Space and Time. Meaning that were you to be driving a little toy car across the rubber tile, it's takes 'Time' to go from one corner to another. By virtue of the joutney, you also move in Space too. 

2. Without over-complicating it, your position on the tile (in your toy car) is always relative to other positions. Ie your view from your part of the rubber tile is currently different to that of another part of the tile. This is important to remember when the Theory of Relativity is discussed.

3. Now imagine a big iron ball dropped into the middle of the rubber tile. You can imagine the tile bending quite severely can't you? This is gravity. The effect of large mass in the space-time continuum.

4. To test this theory, now imagine driving your toy car past the iron ball. Clearly towards the outside of the rubber tile, the effect of the bend or 'gravity' is quite minimal. 

5. But the closer you drive toward the iron ball, the greater the distortion of the tile and the 'greater the gravitational pull'

6. Now imagine that we dont have a single tile but we have an infinite amount - making a 3D space and time continuum.

7. The analogy of a large planet in space makes sense - the bigger the mass the bigger the gravitational field

8. Now imagine a super dense star that's collapsed in upon itself. It's so heavy it's become a incredible gravitational pull

9. Now imagine that the super dense star is sooooo heavy that our 'rubber' tile is so distorted that the corners have folded in towards each other - and may even touch! 

10. Now imagine that our iron ball was soooo heavy that it ripped the rubber tile and fell through? Does this suggest alternative dimensions on the other side of conventional space?

YOU'RE ALL WRONG!

Brian Cox played [i]keyboard [/i]in D:ream.

*Leaves thread, having made a positive contribution*

Now if you could effect our Relativity - by bending space/time with energy (e=mc2) you might be able to not only move huge vast interstellar distances in no time at all, but to perhaps move in Time too

Remember - if every position in the space time continuum is 'relative' in both space and time, it figures that portals or tears might be created by incredible explosions - like the ones being developed in CERN when particles are being driven together at incredible speeds

This is why there were scientists who genuinely felt that a blackhole might have been unleashed under Switzerland last year 😉

Sorry mol. Why isn't really the realm of Physics. The observation that the universe seems to conform to some ordered laws, albeit sometimes hard to understand laws, is really the realm of metaphysics. You might like to look up anthropomorphic principle, an unsatisfying circular argument about the role of the observer (it's that way because we are here to see it that way), or the many worlds interpretation of quantum mechanics, which I like, but does assume uniformity of laws across universes.

I know about the anthropic principle, and it is a way of answering some questions like how come our planet is exactly like the right distance from the sun etc. I do however think why is a question for Physics. However when you get very fundamental that is rather hard to think about.