The speed of gravity?

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.