The speed of gravity?

JackKelly

Ok, firstly, my knowledge of physics is limited to the leaving cert (in 6th year at the mo) so what i say mightn't make any sense! But anyway, obviously if you let go of an object (on earth) it will fall due to gravity,but is there not a speed in to which the "gravity" reaches the object and forces it down?Or even a minute delay from the time a person lets go of the object,and the time gravity takes over.(although i presume the force of gravity is constanly there, so there wouldn't actually be a delay, no?).

So, presuming gravity works at the speed of light -and im totally out of my depth here- and it's also true that gravity has the abillity to slightly bend light. But if the two are travelling at the speed of light, how can one have an effect on the other?

Well anyway,can anyone help me out here?

[Edit]lol, when i say, the speed of gravity, i mean, the speed of its effect i suppose, not 9.8m/s![/EDIT]

Sangre

Well, gravity doesn't have a speed afaik, it just simple is. You said that is there a delay between when you let go and gravity takes over but you must remember gravity never stops having an effect on something, its just that when you let go the force you exert on it is gone and the force of gravity brings it to earth. The earth doesnt reach up and grab it with gravity and pull it back down.
I think the best way to explain it is that gravity is everywhere, it doesnt have to move anywhere so it does not have any speed.

Of course, my knowledge is limited to an a1 in physics so this is just what i would see as a logical result.

David19

I wouldn't have as much knowledge of physics as other people here but anyway.
There's no speed that an object has to attain for gravity to affect it. Your computer is at zero speed and gravity is affecting it. Its staying on the desk, not floating off into space.
There's no delay either, gravity is there all the time.
Gravity doesn't have any speed, it doesn't go at the speed of light.

Zulu

TimAy wrote:

obviously if you let go of an object (on earth) it will fall due to gravity,but is there not a speed in to which the "gravity" reaches the object and forces it down?

No - not really. Gravity is always acting as a force on the body in question. If you are holding the object - you create a force equal but opposite to gravity until such time as you release the body. Then the only force is gravity, and the object starts to accelerate towards the ground.

TimAy wrote:

Or even a minute delay from the time a person lets go of the object,and the time gravity takes over.(although i presume the force of gravity is constanly there, so there wouldn't actually be a delay, no?).

No delay. As soon as the opposite force is removed the body starts to accelerate.

TimAy wrote:

So, presuming gravity works at the speed of light -and im totally out of my depth here- and it's also true that gravity has the abillity to slightly bend light. But if the two are travelling at the speed of light, how can one have an effect on the other?

Ok, you're getting deeper into this now. Depending on the way the forces are acting, a massive gravitational pull (say a black hole) can alter the path of a "packet" of light. The fact that they are travelling at the speed of light is irrelevant at this point. It's all about the directions. If this massive gravitational force is acting at right angles to the light, then it would pull the light in that direction.

Hope this helps...

JackKelly

that would mean, that gravity is built in to the fabric of the universe no?
which would mean that, if gravity was just there -a part of the universe- everysingle piece of matter would would be stuck together. I know that , in theory, everything in the universe IS infact attracted together.

Also,the force of gravity diminishes (sp?) with distance, so would that not have something to do with the gravity having to travel to an object, instead of just being there?

When i say the speed of gravity, i mean for example, say a planet was just created - out of the blue- in space (stupid, i know!), now what would be the time (minute) that any object around it, would take to be grasped by its gravity and pulled towards it?

im confused

daveirl

This post has been deleted.

Sangre

TimAy wrote:

Also,the force of gravity diminishes (sp?) with distance, so would that not have something to do with the gravity having to travel to an object, instead of just being there?

No, think of gravity as an elastic band between two objects, any two objects in the universe. As the distance is increase the tension of the band is reduced therefore so is the attraction between them until it is negligible

When i say the speed of gravity, i mean for example, say a planet was just created - out of the blue- in space (stupid, i know!), now what would be the time (minute) that any object around it, would take to be grasped by its gravity and pulled towards it?

Instantenous, think of it like dropping a round ball into a pond, the water would be displaced immediately and displacement would be immediate and so would the affect of gravity. Of course no one has ever done this or will ever because you can't create matter out of nothing. It can only be theory.

Crucifix

It may help to imagine gravity like a field around something with mass. And the further you get from said mass the weaker the field is. So it isn't really travelling.
As for how long it takes gravity to interact with a new particle/planet, instantaneous I suppose.

JackKelly

Zulu wrote:

No - not really. Gravity is always acting as a force on the body in question. If you are holding the object - you create a force equal but opposite to gravity until such time as you release the body. Then the only force is gravity, and the object starts to accelerate towards the ground.

Yea, i getcha. But say that, instead of considering gravity as always being there (which i know it is), if you compare gravity to the light from the sun, if the sun were to instantaniously stop giving out light,the light would take however long to reach use(because of the limit of the speed of light),giving a difference in time from when the sun actually stopped giving out light, and to when WE see the sun stopping in give out of light. So if it were the same with gravity, say the sun dissapeared!would the time it takes for the force of the sun acting on the earth to dissapear (or the gravity to stop having an effect on us) be instantaneous, or the same time as the light stops?

{edit}just thinking, wouldnt that mean, if the sun were to - in theory- disappear, we would spin out into space, and out of orbit, before we even saw the sun disappearing?{edit}

sorry, if that is hard to understand!

Sangre

I get what you're trying to say but you can't compare the scenarios.
You can make light stop so you can measure the speed.
But can not just make mass disappear or appear, we're pretty much stuck with what we've got in the universe so any affect gravity has is not going to change.

Of couse Im sure there is some quantom law contradicting this
Think there is a Physics teacher who posts on boards somewhere.

Crucifix

AFAIK, as soon as the mass is gone, the gravity is gone.

David19

TimAy wrote:

Yea, i getcha. But say that, instead of considering gravity as always being there (which i know it is), if you compare gravity to the light from the sun, if the sun were to instantaniously stop giving out light,the light would take however long to reach use(because of the limit of the speed of light),giving a difference in time from when the sun actually stopped giving out light, and to when WE see the sun stopping in give out of light. So if it were the same with gravity, say the sun dissapeared!would the time it takes for the force of the sun acting on the earth to dissapear (or the gravity to stop having an effect on us) be instantaneous, or the same time as the light stops?

sorry, if that is hard to understand!

It'd be instantaneous. Gravity doesn't travel. Its always around an object like a planet or star.
Also in your previous post, everything doesn't stick together because gravity is actually the weakest of the four main forces over a short distance.

Sangre

Ok think of it this way, if the mass could disappear, it would just be like a force being removed, it would be like your hand holding a ball let go, all the other forces start acting on it.

Say two people are pulling a ball by strings two miles long, so there is me pulling a string two miles long with a ball on the end connected to another 2 mile string being pulled by B. If B disappears we dont have to wait for his 'force' to catch up, I just suddenly fall on my ass because there is no oppossing force.

See what I mean? Its not like gravity has to catch up, it just stops being, the other forces continue as normal and the masses move accordingly.

JackKelly

i still dont quite get it, at some stage, was that mass not created from energy? (big bang), so at some stage, at the moment the mass was created - did the gravity not have to travel?

Say, then, (again, this is probably contracdicting some obvious rule) that the universe was matterless. No planets or anything. Then gravity wouldnt be there no?So gravity only rears its head inthe presence of mass?So where is it the rest of the time? Would it have to be buried in "the fabric of the universe", i dunno, like a dimension or somthing?

NotMe

TimAy wrote:

that would mean, that gravity is built in to the fabric of the universe no?
which would mean that, if gravity was just there -a part of the universe- everysingle piece of matter would would be stuck together. I know that , in theory, everything in the universe IS infact attracted together.

Yes, exactly. For every two bodies/atom/particles in the universe gravity pulls them together.

Also,the force of gravity diminishes (sp?) with distance, so would that not have something to do with the gravity having to travel to an object, instead of just being there?

No, that just means that for example there is a greater force of attraction between you and the Earth than between you and the Sun.

When i say the speed of gravity, i mean for example, say a planet was just created - out of the blue- in space (stupid, i know!), now what would be the time (minute) that any object around it, would take to be grasped by its gravity and pulled towards it?

im confused

Well that's impossible so who can say. In reality planets are created by gravity - rocks and dust etc are pulled together to form the planet.
If a planet did just appear out of nowhere then I assume it would have an immediate effect on everything around it.(everything in the universe in theory).

The most important thing to realise is that gravity is a force between two objects, not a force that one object has on another. ie. you are pulling on the Earth as much as the Earth is pulling on you. It's just that the Earth is much bigger (greater mass) so you move towards it and not vice versa.


*edit* I see there's been 9 more replies since I started typing this

JackKelly

Sangre wrote:

Ok think of it this way, if the mass could disappear, it would just be like a force being removed, it would be like your hand holding a ball let go, all the other forces start acting on it.

Say two people are pulling a ball by strings two miles long, so there is me pulling a string two miles long with a ball on the end connected to another 2 mile string being pulled by B. If B disappears we dont have to wait for his 'force' to catch up, I just suddenly fall on my ass because there is no oppossing force.

See what I mean? Its not like gravity has to catch up, it just stops being, the other forces continue as normal and the masses move accordingly.

(lol, didnt imagine this board would move so fast)

Yea, i never looked at it that way.Again though, is that not limited by the speed of light. If you could see the other person, would you not fall , before seeing him dissapear?

Sangre

TimAy wrote:

i still dont quite get it, at some stage, was that mass not created from energy? (big bang), so at some stage, at the moment the mass was created - did the gravity not have to travel?

Say, then, (again, this is probably contracdicting some obvious rule) that the universe was matterless. No planets or anything. Then gravity wouldnt be there no?So gravity only rears its head inthe presence of mass?So where is it the rest of the time? Would it have to be buried in "the fabric of the universe", i dunno, like a dimension or somthing?

No, it doesnt have to travel, ever. Its not something that can travel, it doesnt have mass to move, it is just a force. Look again at my example instead of thinking about planets.

I would assume that gravity is always there as a force but its effects are only visual once mass is put into play. Stop trying to confuse yourself with matterless universes, you might aswell start studying the physics of planets in an alternative universe where gravity is reversed or something.

NotMe

TimAy wrote:

(lol, didnt imagine this board would move so fast)

Yea, i never looked at it that way.Again though, is that not limited by the speed of light. If you could see the other person, would you not fall , before seeing him dissapear?

Yeah if the sun was to just disappear we would feel the effects of the loss of gravity before the seeing the light disappear.
(I put that really badly )

*edit* It takes ~8 minutes for the light to get from the sun to the Earth.

Sangre

TimAy wrote:

(lol, didnt imagine this board would move so fast)

Yea, i never looked at it that way.Again though, is that not limited by the speed of light. If you could see the other person, would you not fall , before seeing him dissapear?

Well I cant see 4 miles so I couldnt tell you
Yes, seeing him is irrelevant, it would be like saying I wouldn't fall because the lights are off and I cant see him. What happens is that his force stops, so the force of gravity from the earth becomes the strong force and I fall on my ass. There is no time between this.

It is not limited by the speed of light because it is not traveling, think of gravity as a spiderweb in the universe connecting everything. It doesnt move between objects.

Capt'n Midnight

Speed of Gravity eh ?
The acceleration of the earth caused by the sun's gravity is directed at where the sun is, not where it was 8 minutes ago. this means that gravity is much, much faster than light otherwise you would be able to detect the angle between the sun is and where it's gravitational effect appears to come from.

Zulu

Gravity isn't light. It is a force, like friction. It dosen't move anywhere, it just is. If there is mass - there is gravity.
A body has a mass. It dosen't take time to get that mass - it just has it.
Think of gravity as similar - it just acts on a body.

chakotha

From undergrad physics and Big Bang books I think Einstein's General Relativity explains gravity not so much as a classical force like electric field but as deformities in the structure of space and time in the vicinity of mass.

So there should be no actual speed of interaction - spacetime at the surface of the earth is already curved in the favour of the earth as the Earth is by far the body with more mass than any small loose objects above the surface - so a small light object when dropped will follow the path of least resistance and move along the curve in spacetime towards the Earth

Abdiel

As somebody already mentioned Gravity is a force that acts between two masses:

F = G*M*m/r*r

G is the universal gravitational constant, M and m are the 2 masses and r is the distance between them.

You have raised one interesting point in that if the sun instantaneously disappeared we would notice the effects gravitationally before we would actually see the sun disappear - of course all the other planets gravity would instantaneously effect us if the sun disappeared like that. Jupiter with a mass of 1000 times smaller than the sun would be the next main gravitational attractor. But we would have no heat or light so we'd all freeze anyway

Zulu

...as for the sun disappearing - it takes 7seconds (correct me here if I'm wrong) for light to travel 1 AU (distance from earth to sun).
The gravational pull would be released instantly. Earth would be slung off on a tangent (same effect as a sling-shot). We probably wouldn't notice anything for 7 seconds, but after that - it would be very dark and cold. What happens to the moon? - thats an equation I'm not bothered to work out.

As for Jupiter - it would only have a marginal effect, as it to would be slung out to space, and chances are, in an different direction.

Capt'n Midnight

Zulu wrote:

...as for the sun disappearing - it takes 7seconds (correct me here if I'm wrong) for light to travel 1 AU (distance from earth to sun).
The gravational pull would be released instantly. Earth would be slung off on a tangent (same effect as a sling-shot). We probably wouldn't notice anything for 7 seconds, but after that - it would be very dark and cold. What happens to the moon? - thats an equation I'm not bothered to work out.

As for Jupiter - it would only have a marginal effect, as it to would be slung out to space, and chances are, in an different direction.

The moon would still orbit us and we would not notice the difference since the earth has a slightly greater attraction than the sun for both. yes it would be colder (use lots of green house gases and nuclear power) and tides would be monthly rather than daily

chakotha

As somebody already mentioned Gravity is a force that acts between two masses:

F = G*M*m/r*r

G is the universal gravitational constant, M and m are the 2 masses and r is the distance between them.

Yes but that's Newton's classical equation which relativity showed is an approximation that holds true in non-extreme situations. It doesn't account for light rays being bent or black holes and the like.

Newton's equation still implies a field that is simply always there between any two objects that have mass. There is no start to the interaction. Relativity explains it in terms of spacetime curvature.

Abdiel

Indeed, glad somebody mentioned that fact i.e. gravity doesnt actually effect light as such (as it has no mass) but has an effect on light by bending the spacetime through which it travles.

chakotha wrote:

Yes but that's Newton's classical equation which relativity showed is an approximation that holds true in non-extreme situations. It doesn't account for light rays being bent or black holes and the like.

Newton's equation still implies a field that is simply always there between any two objects that have mass. There is no start to the interaction. Relativity explains it in terms of spacetime curvature.

cyberbob

Zulu wrote:

... it takes 7seconds (correct me here if I'm wrong) .

ok

google wrote:

The distance from the Earth to the Sun varies because the Earth's orbit about the Sun is elliptical.At it's closest, the distance is 91,402,000 miles and it's farthest distance it is 94,512,000 miles.This gives an average distance of 92,957,000 miles. Light travels at 186,282 miles per second. Dividing the average distance by the speed of light gives 499.01225 seconds which is 8.3168708 minutes.

Zulu

No point in getting into too much theory people - this is 6th year LC physics, we don't want to confuse the poor guy.

Suffice to say - gravity dosen't have a speed.

ecksor

My reading of physics suggested that it does have a speed and that if the sun were to suddenly disappear then we'd remain in orbit for approximately 8 minutes (not 7 seconds) before the effects of the sun's gravity disappeared. That was certainly Einstein's take upon gravity and nobody seems to have pointed out that Einstein was incorrect on this thread. Is there new evidence to contradict Einstein?

David19

ecksor wrote:

My reading of physics suggested that it does have a speed and that if the sun were to suddenly disappear then we'd remain in orbit for approximately 8 minutes (not 7 seconds) before the effects of the sun's gravity disappeared. That was certainly Einstein's take upon gravity and nobody seems to have pointed out that Einstein was incorrect on this thread. Is there new evidence to contradict Einstein?

Hmm seems like you're right.

http://www.newscientist.com/article.ns?id=dn3232

NotMe

So gravity moves at the speed of light.:eek: This is one of the reasons I loved Leaving Cert physics and then hated university physics. For the LC we were taught laws that were true all the time. Then I go to college and find out about relativity and warping spacetime and superstrings. And how none of this stuff can really be proven and advanced physics becomes more and more like philosophy. Most things that were believed to be fact in the past were proven wrong and the things that are accepted as facts now will probably be disproved in the future. Anyway, enough rambling, that's why I'm now studying computers.

David19

NotMe wrote:

So gravity moves at the speed of light.:eek: This is one of the reasons I loved Leaving Cert physics and then hated university physics. For the LC we were taught laws that were true all the time. Then I go to college and find out about relativity and warping spacetime and superstrings. And how none of this stuff can really be proven and advanced physics becomes more and more like philosophy. Most things that were believed to be fact in the past were proven wrong and the things that are accepted as facts now will probably be disproved in the future. Anyway, enough rambling, that's why I'm now studying computers.

Yeah there's certainly quite a gap between advanced physics and leaving cert physics! To the original poster, you don't need this stuff for the leaving cert. If your interested in it get a book about gravity or einstein.

JackKelly

David19 wrote:

Yeah there's certainly quite a gap between advanced physics and leaving cert physics! To the original poster, you don't need this stuff for the leaving cert. If your interested in it get a book about gravity or einstein.

ye,i know i dont need it. tbh, i hate the Leaving Cert Physics course. I far prefare the weirder physics,space time etc.Havent a clue about it though.

Zulu

TimAy wrote:

ye,i know i dont need it. tbh, i hate the Leaving Cert Physics course. I far prefare the weirder physics,space time etc.Havent a clue about it though.

Well no offence, but LC Physics is the foundation you need to understand the rest. I know the definations are a pain in the ass, but once you learn them, it makes thing so much easier.
Stick with it.

JackKelly

i wont be changing at this stage.Its grand. Just requires that you sit down and learn the stuff.Fairly boring

Son Goku

The confusion over something like this usually comes from the fact that Gravity, unlike other forces has 4 seperate ways of being viewed.

1. As the warping of Space-Time
2. As a force.
3. As a Field.
4. As a particle.

The only one that would result in gravity having an infinite speed is if it were a force.
This was abandoned in the General Relativity revolution, i.e. Einstein specifically wanted a theory of Gravity which did not imply spooky actions at a distance.

You see if Gravity acted instantaneously it would mean that information could be transmitted faster than "c" (the fastest speed for information in the universe), which is not allowed.

planck2

Son Goku wrote:

The confusion over something like this usually comes from the fact that Gravity, unlike other forces has 4 seperate ways of being viewed.

1. As the warping of Space-Time
2. As a force.
3. As a Field.
4. As a particle.

Gravity, What is gravity ?

Gravity is not a force as thought of by Newton and others until Einstein came along. Gravity is as Einstein described, a warping of the space-time by matter.

In fact the matter distribution tells the space-time how to curve and as result of this the space-time can tell matter (with little mass) how to move.

To some extent gravity is a field, but it is not a force.

Einstein's equations tell us as that matter and energy of the same magnitude couple equally strongly (have the same effect) to the space-time.

In my own opinion I think you may have something to think about when you ask ' how fast does gravity act? '

If I'm holding a ball and I let it go, how does it know to fall ?

This a question Einstein himself asked?

Particles follow what are called geodesic curves, which is the best approximation to a straight path in curved space-time.

I think particles know how to behave in a curved space-time because they are coupled to the space-time by a particle called the graviton and these gravitons tell the particles about the curvature of the surrounding space-time.

This coupling to the space-time doesn't happen immediately, but it is thought to occur extremely fast, but is bounded below by the speed of light.

cregser

Wow there planck2! You just brought this topic up a notch

I've recently developed a big interest in this despite the fact I should be studying for exams (actually it's probably jusy my brain trying to procrastinate). My only experience has been 1st Year Engineering catch-up physics (didn't do it in LC) and two books a friend lent me (from the same series: one about Einstein and another called Quantum Physics for Dummies or something like that).

Did anone see horizons on BBC last night? They are celebrating the centinery of Einsteins' theory of relativity with a series of thursday night documentaries. Last night they talked about how his theories gave rise to quantum mechanics which then challenged his beliefs in God and the universe. It cleared a few things up for me but I still stuggle to understand a few things. Like how light particles (photons) travel like particles but also like waves. And how da f*** we a supposed to get around the uncertainty principle (which basically says that you can't predict ANYTHING you can only work out the probability).

String theory and gravitons and all that is somethng I want to learn now. But it has to be put in simple terms for me. But I don't see myself imagining and understanding how reactions can occur at zero distance. (a big WTF? there!)

The thing with physics now is that we are still experimentally proving Eintiens' theories today (100years later). And Einsteins theories are great but like Newtons', they are limited (Einstein failed to make a satisfactory "Theory of Everything"). His theories work on a grand scale but fail on the tiniest scales. So we're coming up with new theories today that will take further decades to prove. I read a few months ago that they're building a new particle accelerator somewhere in Europe (France?) to observe black holes.

Next weeks' BBC Horizon will be all about e=mc^2, I think.

P.S. That speed of gravity New Scientist thing was a strange coincidence! :eek:

bcKay

First, gravity is a force of attraction felt between any two masses. So you and that neighbour who's pissing you off...are attracted to each other by gravity....but you're masses are so small that they aren't noticible in the gravitational feild of the earth.

Gravity doesn't have a speed. When we talk about a gravitational field we are talking about the area in which the force of gravity is 'felt' by an object. If you look at the equation someone mentioned earlier, F=GMm/r^2, you'll notice that the force due to gravity drops off as 1/r^2...so the further the two objects are from each other the faster the gravitational 'pull' weakens. On Earth, in our everyday lives, it's pretty constant...but if you stand on the equator and weigh yourself then do the same on a pole the reading will be different. When you go further away (r is large) the effect is noticable.

When people think of the 'speed' related to gravity they are acctually meaning how fast an object accelerates due to the force of gravity. If you assume that there are no other forces (ie, your hand holding an apple, or air resistance) then when an object falls towards the earth it will fall at 9.8 metres in the first second, 19.6 metres in the second second, 29.4 metres in the third second etc...or in other words, it's speed will increase by 9.8 m/s every second.

The reason you weigh less on the moon is because the moon has a smaller mass therefore a smaller force pulls on your mass. In space the masses are so far out that the field isn't felt and you feel 'weightless'. If you remove a mass there is instantly no gravitational field since the field isn't something that travels nor penetrates space. Gravity is an intrinsic property of mass. It's like saying if you remove the water from water, is it still wet?

When you start talking about mass coming from enery you have to be careful because you move out of 'classical physics' (essentially the physics that describes things in the macro world) and into 'quantum physics' (the physics of the micro or sub micro world)...and to understand quantum physics it helps to have some chem and a good understanding of the atom, it's parts, sub parts and how they all work. The misconceptions of E=mc^2 (Energy = mass* speed of light squared) is probably where the idea of gravity 'travelling' at the speed of light and mass coming from energy. This only happens on the atomic, or rather subatomic level.

When you talk about the bending of 'space-time' you are looking at large masses and their effect on light. Don't go here until you have a good understanding of the basics. I wrote up a little blurb to try and explain it...but it's even more 'fictional' than the physics you're doing in your classes (which isn't fictional at all, but is based on laws that can be properly explained by quantum mechanics). Classical physics is classic...good for any realistic situation. Quantum physics is more eloquent...it explains the discrepencies between the micro world and the macro world...but if you don't understand the macro, you won't understand the micro....and once you've got those down general relativity might be interesting to look into...and that's when you'll start looking at time/space and black holes in any detail. You'll probably see special realtivity in your courses...pretty great stuff...the precursor to general relativity...mind blowing stuff...

bcKay

String theory and gravitons and all that is somethng I want to learn now. But it has to be put in simple terms for me.

can't be done without the fairytales

And Einsteins theories are great but like Newtons', they are limited (Einstein failed to make a satisfactory "Theory of Everything").

careful....Einstein's theories are more than satisfactory. He didn't come up with the theory of everything partly because we still aren't sure of what gravity is. Without understanding the nature of gravity (like we understand the nature of electricity and magnetism -->electromagnets --combining of two of the forces) it's going to be a long while yet before anyone comes up with the TOE. Think of it like giving a six year old some radioactive rock and a piece of film and having him conclude that there must be electrons that jump from orbit to orbit emiting quanta (there it is )...packets of light...when the six year old has no idea that the rock is even made of atoms.
Science builds on previous knowledge. If you don't have the background you can't properly move forward.

If you want a good read, pretty involved, and you have a good understanding of the basics that you're doing in class right now, try reading "Breif History of Time'.

In the meantime...here are a couple of theories out there:

gravity is a wave with no energy

the act of looking for a new particle is what creates that particle

those are pretty old. I've fallen way behind in my physics 'current events' lol

Spear

bcKay wrote:

Gravity doesn't have a speed. When we talk about a gravitational field we are talking about the area in which the force of gravity is 'felt' by an object.

As someone already correctly stated, gravity propogates as waves moving at the speed of light, in accordance with special relativity.

If you look at the equation someone mentioned earlier, F=GMm/r^2, you'll notice that the force due to gravity drops off as 1/r^2...

This is an inaccurate simplification which is only true when the velocities involved are low are the local spacetime curvature is small.

If you assume that there are no other forces (ie, your hand holding an apple, or air resistance)

But there are always other forces, called fictitious forces, these are what Newtons theories can't account for and relativity can.

In space the masses are so far out that the field isn't felt and you feel 'weightless'.

Wrong, those forces are always there, hence it's referred to as micro gravity, not weightlessness.

If you remove a mass there is instantly no gravitational field since the field isn't something that travels nor penetrates space.

Already shown to be wrong.

The misconceptions of E=mc^2 (Energy = mass* speed of light squared) is probably where the idea of gravity 'travelling' at the speed of light and mass coming from energy. This only happens on the atomic, or rather subatomic level.

This part is purely nonsense.

When you talk about the bending of 'space-time' you are looking at large masses and their effect on light. Don't go here until you have a good understanding of the basics.

Ah, the unintended irony.

I wrote up a little blurb to try and explain it...but it's even more 'fictional' than the physics you're doing in your classes (which isn't fictional at all, but is based on laws that can be properly explained by quantum mechanics).

The stuff on the leaving cert is variously either inaccurate or simply wrong. The concepts required are simply too much for the leaving cert.

Classical physics is classic...good for any realistic situation.

Except for building all modern technology.

Quantum physics is more eloquent...it explains the discrepencies between the micro world and the macro world...but if you don't understand the macro, you won't understand the micro....and once you've got those down general relativity might be interesting to look into...and that's when you'll start looking at time/space and black holes in any detail. You'll probably see special realtivity in your courses...pretty great stuff...the precursor to general relativity...mind blowing stuff...

Again more unintended irony.

bcKay

The stuff on the leaving cert is variously either inaccurate or simply wrong. The concepts required are simply too much for the leaving cert.

Which is why my explanations are as they are. The concepts on the leaving cert are accurate for the situations that will be encountered at that level of knowledge. Newtonian physics is correct for the situations that it describes.

, gravity propogates as waves moving at the speed of light

as far as I know this is just one of many theories. It isn't conclusive and string theory offers theories that are just as plausible.

Quote:
In space the masses are so far out that the field isn't felt and you feel 'weightless'.




Wrong, those forces are always there, hence it's referred to as micro gravity, not weightlessness.

and this is why I said 'weightlessness'..it isn't in fact true..but for a discussion with someone at a level below the leaving cert this is accurate

Except for building all modern technology.

this is where quantum comes in. As a leaving cert student...understanding these concepts isn't realistic in his world. Try explaining electron tunneling, or processing at faster than the speed of light to someone without a background in physics. At that level one can believe but not understand. So then you're looking at having faith in a concept...which is what is required of junior level studies. Believe it then look to understand it.

Spear

bcKay wrote:

as far as I know this is just one of many theories. It isn't conclusive and string theory offers theories that are just as plausible.

This is an imperically observed fact, rewarded with a Nobel prize. Binary pulsar PSR B1913+16 was observed showing precession due to gravity waves.
http://www.johnstonsarchive.net/relativity/binpulsar.html

Since someone mentioned gravitons it's also best to point out that there is no evidence for these.

this is where quantum comes in. As a leaving cert student...understanding these concepts isn't realistic in his world. Try explaining electron tunneling, or processing at faster than the speed of light to someone without a background in physics. At that level one can believe but not understand. So then you're looking at having faith in a concept...which is what is required of junior level studies. Believe it then look to understand it.

Modern technology such as large television screens also requires the application of relativity. Screens above 30 inches or so require relativisitic corrections to work. And there is still no such thing as "processing at faster than the speed of light", regardless of someone's background in physics. Teaching them to have faith in a concept is always a bad idea. An important aspect of the scientific method is falsifiability. Hence string theory is not a full scientific theory as no part of it is testable. It's just a very detailed "what if?" scenario at best. As the leaving cert. content stands, it only discourages and bores the student. They see none of the interesting stuff, it's just heating things in beakers. They deserve to exposed to the more challenging and important material, not the dumbed down, if not outright lies they're fed.

bcKay

Spear wrote:

As the leaving cert. content stands, it only discourages and bores the student. They see none of the interesting stuff, it's just heating things in beakers. They deserve to exposed to the more challenging and important material, not the dumbed down, if not outright lies they're fed.

This I agree with...now understand I'm not familiar with the actual content of the Leaving Cert as I work in Canada, but I assume it's along the lines of physics courses world wide at that level. It is difficult to expose them to 'more challenging and important material' in any realistic manner...it would be like asking a student just learning algebra to use calculus...I think it's important to have qualitative exposure to different areas of physics but without a decent foundation and confidence in the 'dumbed down' physics they're more likely to get lost in the interesting stuff and end up confused and frustrated.

Thanks for that link...as I stated I'm not very well versed in current events...

Spear

bcKay wrote:

This I agree with...now understand I'm not familiar with the actual content of the Leaving Cert as I work in Canada, but I assume it's along the lines of physics courses world wide at that level. It is difficult to expose them to 'more challenging and important material' in any realistic manner...it would be like asking a student just learning algebra to use calculus...I think it's important to have qualitative exposure to different areas of physics but without a decent foundation and confidence in the 'dumbed down' physics they're more likely to get lost in the interesting stuff and end up confused and frustrated.

Thanks for that link...as I stated I'm not very well versed in current events...

I assume secondary level content doesn't vary much worldwide. I can't help but think that confidence in the dumbed down stuff will only lead them to not question it any further. How many people will leave secondary school thinking that F=ma having had it drilled into them for so long? A simple introduction to just special relativity might be enough to get across to students the idea that universe doesn't behave the way they've always believed it does.

planck2

Hey man are you applying to go to college in Galway? I've applied to go to Alberta, Vancouver and I am in the process of applying to Guelph Waterloo.

planck2

sorry, that question was for bcKay

bcKay

I assume secondary level content doesn't vary much worldwide. I can't help but think that confidence in the dumbed down stuff will only lead them to not question it any further. How many people will leave secondary school thinking that F=ma having had it drilled into them for so long? A simple introduction to just special relativity might be enough to get across to students the idea that universe doesn't behave the way they've always believed it does.

Special relativity is well within the mathamatical abilities of that age range...and it is delt with here as part of the curriculum. I'm not saying don't tell them about the world of possibilities...but don't expect to deal with these topics in any detail. The great thing about teaching Physics is that there is so much to get excited about...even the 'dumbed down' stuff can be exciting if it's placed in context of what is possible...of where it can lead.

Hey man are you applying to go to college in Galway? I've applied to go to Alberta, Vancouver and I am in the process of applying to Guelph Waterloo

Nope...I'm looking for work in Galway or Dublin. I did my undergrad at the University of Victoria and did my second degree at University of British Columbia. I lived in Calgary for a year..were you applying to UofA or UofC? If you go to UofA or Waterloo...be prepared for COOOLD I went to an undergrad conference at UofA as a presenter...it was a nice campus..but it was -70C with the windchill factor.