# Gravity = -ve velocity ?

1. Jul 23, 2004

### Reflector

I was thinking, as speed increases, then mass also increases. So what if something has mass but no speed - will this cause it to have -ve velocity which we view as gravity corresponding to the mass of the object?

2. Jul 23, 2004

### pmb_phy

I don't know what you mean by "-ve velocity". Can you please clarify? Thanks.

Pete

3. Jul 23, 2004

### Reflector

Okay. Say your moving near lightspeed. You go from speed A to speed B. Your mass increases by 5. Now say you have mass 5 and you are stationary. Now you go from speed B to speed A... '-ve velocity'. Get it? There is a connection between mass and velocity. So if you have mass you need to have '-ve velocity'....

Also '-ve velocity' corresponds to the direction/line connecting two masses by their centre of gravity. All other 'lines' all around the mass/es becomes positive, meaning positive velocity.

I bet you weren't expecting planets to have this so-called property of '-ve velocity'.....

Last edited: Jul 23, 2004
4. Jul 23, 2004

### pmb_phy

No. First you say that the mass increases by 5. Then you say I have mass 5. Those two statements, taken together, don't make sense to me.
There is a connection between relativistic mass and speed (not velocity). There is no connection between rest mass and speed.

Note: In most cases I use the term "mass" to mean "inertial mass" (aka "relativistic mass"). However please note that some people here have a strong tenancy to become confused when you leave the "inertial"/"relativistic" qualifier out.
Please define yout terms. What is "ve"? What is "-ve"? What is "-ve velocity"? Please define these terms precisely. Otherwise they mean nothing.

Pete

Last edited: Jul 23, 2004
5. Jul 23, 2004

### Reflector

Alright, I was wrong... but it sounded cool. Gravity = negative velocity. I was just thinking the real laws of physics are based on really cool ideas and they are elusive to find.

I'll try to explain some more in case I am right. You increase in mass when you increase your velocity, meaning positive velocity. But then if you already have mass and you are at rest atleast relative to another mass, then how did you get that mass in terms of the same idea that mass increases by speed increasing? You didn't have speed to attain it. So you must pay for it by going from '0' velocity relative to another mass to something below zero, meaning you attain negative velocity. That's the only way you can pay for it, because if you get additional 'positive' velocity, your mass only increases. It doesn't explain the original rest mass. So therefore you have to have negative velocity.

But so how can 'negative velocity' not increase your mass? I don't think it ever can based on the construction of the Universe. You can't have two masses so far apart that their gravitation could cause them to increase in mass as they approach each other. Gravity is two weak, and there is not enough space to make them approach lightspeed....

Last edited: Jul 23, 2004
6. Jul 23, 2004

### pmb_phy

I've already explained to you that mass is not a function of velocity. It is a function of speed only. Speed is defined as the magnitude of velocity.
You're incorrectly assuming that m(v = 0) = 0. That is incorrect. While it is true that mass is a function of speed it does not mean that when the speed is zero then so is the mass. the height of a person under the age of 15 is a function of time difference between the current day and the day that person was born. Call that time difference the person's "age." Therefore the child's height is a function of age. That doesn't mean that when the child was born that their height was zero. Mass is related to the proper mass m0 (aka rest mass) as

$$m = \frac{m_{0}}{\sqrt{1-v^2/c^2}}$$

Part of the mass, i.e. m0, is an inherent property of a body and the rest is a result of relativity (which means that it is a result of time dilation and Lorentz contraction).

The term proper here literally means "intrinsic".

Notice that when v = 0, m (v) = m(0) = m0. And that is not zero.

Pete

Last edited: Jul 23, 2004
7. Jul 23, 2004

### Reflector

This is not the only explanation I have for gravity. I have one from before but I refrained from posting as it made too much sense to me and I can't just give it away. This one is more obscure which is why I posted it. I just thought a planet which was existing naturally without too much speed had residual mass which it paid for with a relatively weak motion towards other planets and this was sneakily classified as '-ve velocity'. I still like the idea as it is a bit specific and not too broad in its definition. It's the kind of idea you post because people generally wonder what gravity is as a thing in it of itself.

8. Jul 24, 2004

### DW

You have already been proven wrong on all this in this forum. Why are you still spamming for Planck?

9. Jul 24, 2004

### Tom Mattson

Staff Emeritus
What--exactly--is wrong with pmb's post? I know that he's not adhering to the preferred convention of defining mass as the norm of the 4-momentum. But that's all it is: a convention. Conventions aren't "proven wrong". They are simply adopted or rejected based on their usefulness or lack thereof.

10. Jul 24, 2004

### Tom Mattson

Staff Emeritus
Reflector,

I'm moving this to Theory Development. You have a lot of curiousity and are very imaginative, which is great, but the ideas you are posting are exaclty that: your ideas. They are not commensurate with relativity.

11. Jul 25, 2004

### pmb_phy

dw knows, as fact, that nothing is wrong with my post. However I'd love to see him directly answer your question. If dw does post a direct response (rather than simply repeat his claim again) please let me know. Thanks.

Reflector - Since I don't know you all that well and don't know if you're familiar with this whole debate thing on the term mass in relativity let me explain. This is one of those things that people in the relativity community have debated for many decades. That there topic is hotly debated can be seen in the physics literature such as Physics today, The American Journal of Physics (other physics journals, referances given upon request, Concepts in Mass in Contemporary Physics and Philosophy etc. The plain fact of the matter, Reflector, is that the concept is still used and to understand much of the physics literature on several points, you must know this fact.

One case that comes to mind is a book a friend of mine just sent me to read during my convalescence. The author is the author of one of the most widely used GR texts that exists today and that is used to teach GR. The name of this new (Pub. 2003) is Gravity: From the Ground Up, by Bernard Schutz, Cambridge University Press (2003). On pages 187 he lists what he calls ...the most important consequences of special relativity. They are

(1) Nothing can travel faster than light.
(2) Light cannot be made to stand still.
(3) Clocks run slower when thery move.
(4) The length of an object contracts along the direction of its motion.
(5) There is no universal definitioin of time and simultaneity.
(6) The mass of an object increases with its speed.
(7) Energy is equivalent to mass.
(8) Photons have zero rest mass.
(9) The Doppler redshift formula changes slightly.

Most of my relativity texts speak of mass in this way in some place or another in the text. And that includes texts published on or after 1994, of which there are 9. 3 of them use the concept throughoput the text. Some (Like Schutz's GR text) will say what I say, i.e. that inertial mass (aka relativistic mass) depends on speed while rest mass (an invariant) does not. The others don't use the concept at all, of which there are 4. I have 8 relativity texts published before 1994 and of them 2 do not use the mass = inertial mass (aka relativistic mass) definition.

For a complete discourse on the concept of mass in relativity please go to www.geocities.com/physics_world and click on the link labeled On the concept of mass in relativity.

Tom - m0 = Proper mass/invariant mass/rest mass is not defined as the magnitude of 4-momentum P. It is a related to 4-momentum, as you indicated, through,

$$(m_{0}c)^2 = \bold g(\bold P, \bold P) = \bold P \bullet \bold P$$

where g is the metric tensor. If you try to define proper mass in that way then you'd have a circular definition. Proper mass is an implicitly defined such that the quantity m0 P where P is 4-momentum, is a conserved quantity (for free particles which interact only through contact forces). For a precise definition see Eq. (37) in the paper I mentioned to Reflector above (I use mu for proper mass in that article since I think that's the best letter for it since I think proper quantities should be greek letters, like tau for proper time etc).

Pete

Last edited: Jul 25, 2004