When it comes to physics, I have an interest, but only a layman's understanding in the way it all works. My intrest mainly comes from being a science fiction fan, and my understanding from reading books by Brian Greene and Stephen Hawking.
One question I have wondered about is the relationship between Gravity and Acceleration.
Acceleration seems to mimic gravity.
1) In both a gravity well and a state of acceleration you can experience time dilation.
2) Acceleration can mimic gravity to the point that to an observer inside a ship accelerating at the right velocity it would feel just like standard Earth gravity.
If mass increases the closer you travel to c, do you get the same effect inside the gravity well of a black hole?
My question is: How are gravity and acceleration linked? What is the explanation of their similarities, is gravity a force of its own, or is it a sort of acceleration?
(Bikerman, here's your opening to put gravity into the picture 
) | Jinx wrote: |
When it comes to physics, I have an interest, but only a layman's understanding in the way it all works. My intrest mainly comes from being a science fiction fan, and my understanding from reading books by Brian Greene and Stephen Hawking.
One question I have wondered about is the relationship between Gravity and Acceleration.
Acceleration seems to mimic gravity.
1) In both a gravity well and a state of acceleration you can experience time dilation.
2) Acceleration can mimic gravity to the point that to an observer inside a ship accelerating at the right velocity it would feel just like standard Earth gravity.
If mass increases the closer you travel to c, do you get the same effect inside the gravity well of a black hole?
My question is: How are gravity and acceleration linked? What is the explanation of their similarities, is gravity a force of its own, or is it a sort of acceleration?
(Bikerman, here's your opening to put gravity into the picture ) |
^_^ What, is he waiting for a chance to drop something big?
Gravity is a force (sorta kinda, rather the product of geometry, but effectively a force), but we also (wrongly) use the term to describe acceleration due to gravity (g). (To make things even more complex, gravitation is not the same as gravity. Go figure.)
Gravity (the force) can cause acceleration (just like any force). Motion in a non-inertial (accelerating) reference frame causes apparent forces (like when you feel pushed back in the seat of an accelerating car) that can be mistaken for the force of gravity.
So gravity causes acceleration, but does acceleration cause gravity? ^_^ Sort of. ^_- But I don't want to steal Bikerman's thunder, so.... | Indi wrote: |
^_^ What, is he waiting for a chance to drop something big?
... |
LOL. Nice. 
I'm sure we will get a more technical discussion, but that is a nice explanation in layman's terms. I think Gravity is defined as the force between to bodies, but it is also used to describe the force experienced due to acceleration or curvilinear motion (which is also a type of acceleration), as in G Force. Acceleration is simply any change in velocity(speed or direction).This is just a placeholder to tell you (Jinx) that I am preparing a more detailed answer....it will take a little while longer because I'm sticking in all I know about it and whilst that is not a lot, it takes a couple of hours to get on page. It'll make me refresh some stuff as well which is good for me. My only concern is that this reply doesn't bring down a (what would be the collective noun I need here..err...ahh) a Quantum of angry physicists on my head, shouting rude words and muttering about ruddy amateur lame-brains mucking around with stuff they don't understand. I freely admit that a lot of this stuff is over my head and I just don't have the math to deal with string and M theory in anything other than a very lame and heavily metaphorical manner. As a teacher I use analogy and metaphor a lot but there is a big downside doing so in science because even the best analogies have flaws.
Anyway, give me another hour or so and I'll post my answer and sit awaiting the wrath of numerous guys with very small spanners (quantum mechanics). In my defence I'll quote the Physicists Charter 
We hold these postulates to be intuitively obvious, that all physicists are born equal, to a first approximation, and are endowed by their creator with certain discrete privileges, among them a mean rest life, n degrees of freedom, and other rights which are invariant under all linear transformations.
PS - Just read Indi's post (didn't notice it since I normally prepare anything longer than a couple of para's in Word (just in case....I've lost a few long postings in the past - although this new Firefox seems to resume from many 'situations' in a very civilised manner)...anyway, Indi probably knows much more than I on this so I may need another 30 mins to check it again lest I look a complete prat. | Jinx wrote: |
| When it comes to physics, I have an interest, but only a layman's understanding in the way it all works. My intrest mainly comes from being a science fiction fan, and my understanding from reading books by Brian Greene and Stephen Hawking. |
Not too dissimilar to myself then | Quote: |
One question I have wondered about is the relationship between Gravity and Acceleration.
Acceleration seems to mimic gravity. |
Very astute of you...
| Quote: |
1) In both a gravity well and a state of acceleration you can experience time dilation.
2) Acceleration can mimic gravity to the point that to an observer inside a ship accelerating at the right velocity it would feel just like standard Earth gravity. |
Nicely done...you have understood it well..General Relativity (GR) says that Gravity and Acceleration are the same thing for all intents and purposes. This is called 'equivalence'. | Quote: |
| If mass increases the closer you travel to c, do you get the same effect inside the gravity well of a black hole? |
OK. According to GR, gravity is the distortion in spacetime caused by mass.
You have probably seen the normal visual aid that is used to demonstrate this - a heavy ball n a sheet of stretched rubber. Here's a typical example:-
| Quote: |
| My question is: How are gravity and acceleration linked? What is the explanation of their similarities, is gravity a force of its own, or is it a sort of acceleration? |
Phew...this is a tough question - probably the toughest in physics. This question is occupying the best physics and math brains in the world at the moment. Next to characters like those, I feel like a fraud even attempting to answer this. I'll have a bash, but I can't promise to make sense. I can't even promise to be accurate since some of the math involved here is way beyond my abilities, so my normal health warning applies - I am not a physicist so this could all be baloney (any real physicists out there - please feel free to jump all over this if I'm misleading or just plain silly). Also, this question is not answerable in anything less than a great big posting, so I apologise in advance, but you DID ask!
The simple and trite answer would be that Gravity and acceleration are effectively identical. Gravity is a force in the traditional (classical) sense that it operates on mass at a distance to accelerate it (Force = mass x acceleration). Simply plug gravitational force into the left side of the equation and everything works perfectly. Therefore stop worrying - problem solved.
That won't do though.
In another sense gravity is different from other forces. This is tricky in words...let me see....
There are 4 ‘fundamental’ forces that we know of and there is no reason to suspect there are any more lurking in the observable universe. These are electromagnetism; the weak force, the strong interaction – now known as the ‘Colour’ force, and gravity.
Electromagnetism is the second strongest force and is carried or ‘mediated’ by photons. We experienced it as light, radio, TV, microwaves etc. It’s basically photons vibrating at different rates.
Weak nuclear force is well known under another guise – it produces radioactivity. It is considerably weaker than Electromagnetism.
Strong (Colour) force is what holds the quarks together as protons and neutrons in the nucleus. This is by far the strongest force being 100 times stronger than electromagnetism and it doesn’t seem to drop off with distance (which is unique amongst the 4). It is so strong that the amount of energy you would need to ‘break’ it and smash the quarks apart is so much that matter would be created in the effort according to e=mc^2.
Finally there is gravity which is incredibly weaker than the others – 1000000000000000000000000 times weaker. In the standard model it is ‘mediated’ by the graviton. Nobody has ever observed a graviton and some physicists think they never will. Here’s the thing, though. The graviton would be massless and chargeless like the photon and would therefore move at the speed of light. That would mean that gravity ‘operates’ at the speed of light. An experiment in 2002 claimed to have measured the speed of gravity
http://news.bbc.co.uk/2/hi/science/nature/2639043.stm
but not all scientists agree that the results were interpreted correctly.
Also, gravity does not vary with the thing being acted on in the same way that the other forces do. According to the GR picture, gravity is just a bend in the path that you follow, regardless of your mass. Now before anyone leaps up and shouts rubbish, yes, I know that gravity is calculated by multiplying the mass of the two objects and dividing by the distance squared. I also know that gravity works both ways - I exert a pull of 12 stones on the earth and it does the same for me. The point is that the effect of my pull on the earth is cancelled…..Lord, this is difficult in English….
To heck with it...I think it is necessary to introduce some very simple algebra - ignore it if you like but I'll feel happier if I include a simple proof of this - and I promise that it IS simple.
The famous equation which Newton gave to us is :-F = G*m1*m2/d^2 (In English that says that the force of gravity is equal to a constant - G {known as the Gravitational constant} - times the mass of object1 times the mass of object2 all divided by the distance between them squared).
Now, I've said that acceleration due to gravity is not dependant on the mass of the object being accelerated so now let's prove it. We want to obtain a term for the acceleration on object2 (I'll call it a2). We already know that Force = mass times acceleration, therefore acceleration= force/mass.
So for object2 we can say that the acceleration acting on it is the force of gravity (F) divided by its mass (m2). I hope you’re OK to that point; if not then don’t worry, just take my word on it, but let’s just finish this off (it doesn’t get any harder)
So let’s just reiterate a moment; a2=F/m2
Now we have seen from Newton's equation that the force of gravity (F) can be written as G*m1*m2/d^2 - So to get a2 we simply divide this lot by m2. Now m2 cancels out completely to give : a2=G*m1/d^2.
In other words object2 will be accelerated by a force which is completely independent of the object itself and is only dependant on the mass which is acting upon it and the distance which it acts over..
(It's soooo much easier to do 2 lines of simple algebra rather than try to put it in words and confuse the issue.If you are good at math and found that patronising then sorry 
Now, in comparison to this force of gravity, which doesn't depend on the object being acted upon, electromagnetic force is quite different. It depends on the charge, mass, spin and velocity of the particles concerned. The other forces also depend on the particles concerned for their effect. In this sense gravity can be seen as a property of spacetime itself and not of the mass inhabiting it.
OK, so where does that take us? Well, GR works fine and gives all the right answers for the force of gravity. Quantum physics works fine and gives the right answers for electromagnetism, weak nuclear and strong nuclear forces. Both of them say things about the world which the average person finds either hard to accept or deeply disturbing, but that is their problem - the theories do what they are supposed to do, they model reality very accurately and, as far as I am aware, neither of them has been found wanting in the light of later experiment or observation.
So why is there a problem? and why are physicists all trying to find a single theory of everything? Why not accept that GR works in the ‘macro’ universe and quantum physics works at an atomic level and smaller?
There are a couple of reasons..
Firstly it just seems plain wrong that there would be two completely different fundamental forces - the history of Physics shows that this is not likely. Magnetism and electricity, until fairly recently regarded as two different forces, are now known to be part of the same force - electromagnetism. Recently Quantum Electrodynamics (QED) has, I understand, managed to unite the electromagnetic and weak nuclear forces into one - called Electroweak. The pattern seems to be towards a single theory and not 2 distinct and incompatible ones. All those physicists working on the large particle accelerators have been smashing away for some time now and they are pushing towards conditions which are very similar to those which would have been present at the Big Bang (huge energies and temperatures - very extreme physics). We now have a model of mass which looks to be, if not complete, then probably not missing much. From what I understand the standard model of particles, although certainly not guaranteed to be ‘it’, is thought to have the types of matter down to the basic ‘families’ and there is very little chance that a new family exists (with the possible exception of a 4th generation quark which the model accommodates). I have to take much of this on faith since the proof is beyond my ken, but it was told to me by 2 people who I trust in this regard. The Higgs is about the only bit of the model which is a bit dodgy – the rest of the standard theory in particles has held up since Weinberg in the late 60s, and the particle physicists I have talked to seem content that they have a satisfactory and pretty minimalist model. The consensus I get is that the model may simplify down yet, but it won’t suddenly have to include a load of new particles that were not expected. If that is true then it’s quite a stunning achievement. Just a few years ago QED was having problems not unlike the problems of unifying relativity and gravity now. When physicists started to look deeply they found that it wasn’t possible to be accurate in measurement, not because of Uncertainty but because the charged world of the electron was full of fields from other charges and, to make it worse, virtual particle pairs would pop out of nowhere and influence the electron even more. The standard way of dealing with this was to add all possibilities together but when they tried they discovered, to their horror that it added up to infinity. In physics infinity pops up normally when you have made a mistake or the model has broken down. The problem was eventually solved by a technique called renormalisation - a mathematical 'trick' which essentially subtracts infinities from infinities to leave a measurable quantity. At the time is was seen as a cheat by many physicists, and some still feel that there is something a bit ‘icky’ about the technique, but it has become accepted as a valid (and vital) technique and has succeeded in making Quantum Electrodynamics arguably the most successful scientific theory we currently have. As well as combining quantum theory with the theory of special relativity, it explains the whole of chemistry, it predicts the interactions of particles with stunning accuracy and, all in all, it works a treat. (The same trick of renormalisation was tried with the GR and quantum equations but it doesn’t work.)
Then, also, we are already sure that GR breaks down in some cases. Black holes and the BB are two cases. If you want to understand what is going on in a black hole then you are going to need a quantum theory of gravity or a new theory to encompass both. The same would seem to be true for the BB.
So where does it leave us
The model of gravity you choose depends on what you need to do. For most people Newton’s model is perfectly fine. It doesn’t matter that it only describes gravity – it doesn’t say how it works – because it works and is reliable. Only when you get into extreme physics do you need to think about swapping over to the more accurate Einstein model of Relativity.
We only need a new model if we are going to even more extreme conditions – those to be found in black holes and at the BB. A new model will, I’m fairly sure, emerge within the next decade – Quantum gravity may prevail, quantum loop gravity or m-theory may be proven the better model. Will it affect us drastically? Probably not. How many people actually even vaguely understand the century old model that we are seeking to update? Is it worth the effort? Of course it is….
Regards
Chris
PS - forgot to specifically answer the question about falling into a BH.
There is a page here which describes it better than I could.
The short answer is yes you would be accelerated up to close to c RELATIVE TO the singularity. This is a good question in itself but the short answer is that your 'invariant' or rest mass stays the same as now, it is relativistic mass that increases rapidly as you approach c. This is one of the major misunderstandings of Relativity. Special relativity only applies in what are known as intertial frames of reference (frames of reference which are not accelerating with respect to one another). The measurement of relativistic mass, therefore, would only be possible from a frame of reference moving at the same speed as you as you accelerate up to c. The Lorentzian transformation (the equations which calculate mass, time and length) are probably best imagined as a translator between inertial frames of reference. The translation is only possible when you have 2 frames of reference. In your own frame your mass would be...whatever...in my case about 12 stone.
This is actually Galilean rather than Einstein relativity. Basically if you think of Newton's 1st law (Inertia) - every body remains in constant motion or rest unless acted upon by a force. Newton assumed a 'master' frame of reference where things could be related to. To two people in non-inertial frames of reference then this law doesn't work. If one is accelerating with respect to the other then the whole notion of inertia goes right out of the window.
Thus the theory of Special Relativity only holds in frames of reference where the theory of inertia works as well. That's why it's referred to as the 'Inertial frame of reference'.
What Einstein later did was to say - hey guys, there is NO master frame, every one is as good as the next. Therefore it only makes sense to talk about things like velocity and time when you fix the points of reference first.In your own frame the laws of physics work perfectly well. They work the same in every one else's frame for them too.
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Inertial/Inertial.html
RegardsMath is my weakest subject, but after reading over that bit of algebra a couple of times, I think I do see how the mass of the second object gets canceled out in the equation.
So if gravity works regardless of mass or it's physical properties, why theorize a particle (graviton) for the transmission(mediation?) of gravity? If gravity has a particle, then it seems that the properties of the matter on which it is acting would play a role, similar to elecromagnetic force, but that isn't the case. Is the graviton just introduced to add symmetry to the theories (ie: all forces have a corresponding particle [electron, photon, graviton, boson])?
If the actions of the other forces depend on the physical properties of the matter they act on, but gravity does not, why are physists trying to force it into the mold by giving it a particle which would seem to mean that it has a physical presence beyond it's effects?
I have seen the "bowling ball on a trampoline" analogy for how gravity is a function of the shape of space time, or how it affects the shape of space time, but the problem that I have always had with that analogy is that it uses gravity to explain gravity, which dosen't give one a very good explanation. (ie: the large mass causes a divot in space time, but it is gravity that makes things fall into divots.) I end up thinking in circles and never get anywhere.
Or am I missing the point somewhere?
It's like trying to visualize the proper way to fold a tesseract.
I wish I was better at math.
| Jinx wrote: |
Math is my weakest subject, but after reading over that bit of algebra a couple of times, I think I do see how the mass of the second object gets canceled out in the equation.
So if gravity works regardless of mass or it's physical properties, why theorize a particle (graviton) for the transmission(mediation?) of gravity? If gravity has a particle, then it seems that the properties of the matter on which it is acting would play a role, similar to elecromagnetic force, but that isn't the case. Is the graviton just introduced to add symmetry to the theories (ie: all forces have a corresponding particle [electron, photon, graviton, boson])? |
That is an excellent question. In a way yes, it is added to give 'symmetry' to the theories. The standard model works so well that it is logical to try and extend it to cover every force/particle and adding the graviton does this.
All models are, as the name suggests, representations of reality. Reality is what it is, our models just tell us stuff about reality. The better the model the more it tells us. The model, however, is never the same as reality. A good phrase to bear in mind is 'the map is never the territory' - that, I think, expresses the concept nicely.
The word symmetry has a particular meaning in particle physics. It describes how one particle can be transformed into another. Super-symmetry is a particular proposed model in particle physics whereby every particle of one type of matter has a corresponding particle in the other type. The two basic types are fermions and bosons. Fermions can be imagined as matter and bosons as the particles which carry force. In the super-symmetrical model a fermion (say an electron) has a partner in the bosons (in this case the 'selectron'). A theory which emerges from supersymmetry is 'super-gravity' and this is particularly relevant to this question. Super-gravity postulates that acceleration and gravity are the same and therefore replaces the concept of gravity with that of acceleration. The super gravity model is essentially identical to super-string model at the limit. IE if you propose that the strings in super-string theory are points with 0 length then super-gravity theory emerges. This is very complex stuff and at the cutting edge of physics. Unfortunately the only way to get a good handle on this stuff is mathematically since many of the concepts have no analogies in our observable world and therefore cannot be explained in terms of other things we know.
I would encourage you to do what I am currently doing - work on your maths. My starting point, and one you may find useful, was to get a copy of 'The Road To Reality' by Roger Penrose and work through it. This is an excellent book and will, if you work through it, improve and develop your math significantly.
| Quote: |
If the actions of the other forces depend on the physical properties of the matter they act on, but gravity does not, why are physists trying to force it into the mold by giving it a particle which would seem to mean that it has a physical presence beyond it's effects? |
Well, light itself has no physical presence beyond it's effects in one sense. The photon has 0 mass (I will use the word mass in the preferred current sense of non-relativistic mass which means the mass of a body at rest rather than the mass of a body which results from the relativistic effects of velocity and gravity/acceleration). The graviton also has 0 mass. It makes no sense to talk of a photon at rest and likewise a graviton, since both are defined in a way which means they simply cannot BE at rest. Again we must bear in mind that a model is just a model and also bear in mind that I am using analogy to describe what are mathematical models. The analogies will inevitably break down at some point.
| Quote: |
I have seen the "bowling ball on a trampoline" analogy for how gravity is a function of the shape of space time, or how it affects the shape of space time, but the problem that I have always had with that analogy is that it uses gravity to explain gravity, which dosen't give one a very good explanation. (ie: the large mass causes a divot in space time, but it is gravity that makes things fall into divots.) I end up thinking in circles and never get anywhere.
Or am I missing the point somewhere?
|
Sort of, yes. The 'divot' in space is all you need. Objects follow a straight line path according to Newtonian physics. The distortion in space means it follows a path which is different from a straight line. There is no need to think of a force at all once you realise this. The path depends on the structure of spacetime. If spacetime itself is distorted then the straight line is distorted and the object will follow the distorted path. No other external force is needed in this explanation and this is the classic GR model. Gravity is, therefore, not a function of spacetime - it is a property of spacetime and requires nothing other than matter behave as it normally does.
| Quote: |
I wish I was better at math. |
So do I....get yourself a copy of the book I mentioned and we can improve together
Regards
Chris | Bikerman wrote: |
| Jinx wrote: | Math is my weakest subject, but after reading over that bit of algebra a couple of times, I think I do see how the mass of the second object gets canceled out in the equation.
So if gravity works regardless of mass or it's physical properties, why theorize a particle (graviton) for the transmission(mediation?) of gravity? If gravity has a particle, then it seems that the properties of the matter on which it is acting would play a role, similar to elecromagnetic force, but that isn't the case. Is the graviton just introduced to add symmetry to the theories (ie: all forces have a corresponding particle [electron, photon, graviton, boson])? |
That is an excellent question. In a way yes, it is added to give 'symmetry' to the theories. The standard model works so well that it is logical to try and extend it to cover every force/particle and adding the graviton does this. |
Don't forget that there is also strong evidence for the existence of gravitons in the form of gravitational radiation.
It has long been known that the electromagnetic (electroweak, same thing at this level) forces causes unstable orbits - and that's why the classical model of the atom failed. A surprising related finding was that gravitational orbits are also unstable. Thus (ignoring any drag and the sun's eventual death) the Earth cannot orbit the sun forever - it's constantly emitting gravitational radiation as the orbit collapses. It's just doing that so slowly that the Universe will probably die first (i actually don't know off the top of my head what the rate of gravitational radiation between the Earth and the sun is).
The 1993 Nobel prize (or somewhere around 1993) was for inferring the existence of gravitational radiation. We can't directly detect it yet because we can't detect gravitons, but we know it's there. And if it's there, it requires a carrier particle... the graviton.Quite right Indi...thanks for picking that up....I missed it
C. Well guys, thanks for that. You gave me a better understanding than Hawkings did.
| Teezgaff wrote: |
| Well guys, thanks for that. You gave me a better understanding than Hawkings did. |
LOL...that's probably because this only scratches the surface Personally I did not think Hawking's book (Brief History of Time) was that great. I read it a couple of times and although it wasn't bad it wasn't great either. There are better to be had.
At the moment I'm working my way through 'The Road to Reality' by Roger Penrose who was a collaborator with Hawking earlier in their careers and is currently working on Quantum Consciousness amongst other things. I find his style easier to work with, although he does digress very often and very deeply when he forgets himself.....he sometimes forgets that not all of us can solve 7 dimensional matrix transformations in our heads.
They make a lot of the stuff at Cambridge/Oxford available to the public freely. Here, for example, is a Penrose lecture on the Mind and the Universe in Audio with the slides and graphics he used as well.
http://online.kitp.ucsb.edu/online/plecture/penrose/
You need Real Player.
Here is a link to the Isaac Newton seminars and lectures on-line which also have both audio and accompanying graphics and need Real Player.
http://www.newton.cam.ac.uk/webseminars/
Regards
ChrisThanks Bikerman and Indi. Sorry I didn't say that sooner. Bikerman's post in particular looks like it took a lot of work.
I haven't come across the supergravity theory before, and it sounds intriguing. It sounds like it answers they very question that I was asking, ie: are gravity and acceleration related? Apparently, yes.
I guess I need to read up on it.
As for my math skills, anything more complex that quadratic equations is a little above my level, but I did download the math texts that Bikerman suggested in another topic, and I'm trying to work through them a little at a time.
Even though this question is well answered, I feel like giving it a shot. Gravity is a force on an object essentially. The way that you measure force is by breaking it up into two components, acceleration and mass. The way science fiction describes it, its as if its some mystical force. However, gravity works the same as force, and so the thing that they are actually talking about is the acceleration due to gravity.
| Jinx wrote: |
| Thanks Bikerman and Indi. Sorry I didn't say that sooner. Bikerman's post in particular looks like it took a lot of work. |
It was useful for me....it's a good thing to test your own understanding of a concept periodically and I find that the best way for me is to try and explain it to someone else.
| Quote: |
I haven't come across the supergravity theory before, and it sounds intriguing. It sounds like it answers they very question that I was asking, ie: are gravity and acceleration related? Apparently, yes.
I guess I need to read up on it. |
Well, as I said, according to GR gravity and acceleration are indistinguishable from the point of view of the observer. Supergravity is an area of physics which simply takes the supersymmetrical model in particle physics and runs with it. | Quote: |
As for my math skills, anything more complex that quadratic equations is a little above my level, but I did download the math texts that Bikerman suggested in another topic, and I'm trying to work through them a little at a time. |
That would certainly be a good starting point. The books concerned essentially take the non-science student up to a level where they can at least understand the concepts of relativity and quantum physics in a sensible and meaningful way.
The thing to realise is that much of this discussion touches on areas of physics which are extremely mathematical and fairly cutting edge. Most people do not even have a basic understanding of the most basic physics. Relativity and quantum physics are the cornerstones of modern physics, yet most people have only the sketchiest notions of the physics which preceded this - Newtonian physics. I think that nobody who claims to be 'educated' can do so without at least a basic grounding in relativity and quantum physics. I don't mean that you should be able to quote and solve the Schroedinger equation, but you should know what it is and what it does in at least conceptual terms.
Regards
ChrisI read somewhere that according to Einstein everything, including gravity cannot travel faster than the speed of light. For example, if you were to instantly remove the sun from our solarsystem, earth would not fly out at a tangent until about 8 min after the sun was destroyed because the gravity can only travel the speed of light.
My thinking is that if you reach the speed of light and your mass effectively become infinite your gravity effectively becomes zero. If your gravity cannot propigate from you because your a moving at the same speed as it then it wont pull you anywhere.
Kev01234
| Kev01234 wrote: |
I read somewhere that according to Einstein everything, including gravity cannot travel faster than the speed of light. For example, if you were to instantly remove the sun from our solarsystem, earth would not fly out at a tangent until about 8 min after the sun was destroyed because the gravity can only travel the speed of light.
My thinking is that if you reach the speed of light and your mass effectively become infinite your gravity effectively becomes zero. If your gravity cannot propigate from you because your a moving at the same speed as it then it wont pull you anywhere.
Kev01234 |
Well.....a graviton would have no rest mass (like a photon) so it would be capable of travelling at c but no faster (according to GR). There is sometimes confusion because the phase velocity could be greater than c for gravity (as it can for light itself), but that is a different effect.
There is an explanation of how this can occur with a nice little Java app HERE
NOTE - no 'thing' is actually exceeding c. | Jinx wrote: |
So if gravity works regardless of mass or it's physical properties, why theorize a particle (graviton) for the transmission(mediation?) of gravity? ... Is the graviton just introduced to add symmetry to the theories (ie: all forces have a corresponding particle [electron, photon, graviton, boson])?
|
Yes and no.
Start from the reasonable assumption that there is a quantum theory for gravity. Now we go through a similar process as for quantum electrodynamics:
First, in "Quantum Mechanics" proper that physicists learn first, i.e. the Schrodinger equation, simple harmonic oscillators, hydrogen atoms, etc. the quantum nature of photons is not handled. In fact, particles of any kind can't be created or destroyed. We can find the energy levels of particle in a box, SHOs, hydrogen and other atoms, etc. but we always postulate some sort of external field doing the interacting. The photons only come in through the classical electromagnetic field.
In order to treat absorption and emission (destruction and creation) of photons, as well as scattering where the types of particles coming out are different than the ones going in (like beta decay), we need to make a Quantum Field Theory. In this theory we have an electron field, proton field, photon field, etc. Then some rules are made about how the fields interact with each other - won't go any further into this. However, things like the Coulomb potential (~ 1/r) are not put in explicitly; it comes out of the QFT if the interaction terms are correct.
So, really the answer is, if there is a quantum theory of gravity, it has to be a Quantum Field Theory. We need a "graviton" field to describe processes where gravitational radiation is absorbed or emitted. Once we put in the graviton, we should be able to derive the potential ~1/r.
In order for the potential to always be attractive, it turns out that the graviton has to be spin 2. The problem is that an interaction term for a spin-2 field analogous to the one in QED is not renormalizable. What that means is that there are certain calculations that come out infinite, and the infinities cannot be removed in the same way that infinities are removed in QED. So something different than QED has to be done, which is why there are many different methods of quantizing gravity being researched.
So, anyway there is a lot of complicated stuff here that muddles the picture, which I can't really explain any more without a lot of QFT background theory. But back to the original question. The answer is "yes" if we just want to make all theories quantum theories; but "no" if we say "given that gravity is a quatum theory, do we just add a graviton to be like other theories?". If we believe that gravity is a quantum theory, then there has to be a graviton.Ahh..a proper physicist....
I can stop trying to teach from 2 pages ahead in the textbook at last
Chris.
