Attraction?

[Gareth56]

From a book:-

 

Mass differs from electric charge in that it is always positive, and that the forces between masses are therefore always attractive.

 

Electric charge can be both positive and negative, and charges of the same sign repel each other, while charges of opposite sign attract each other.

 

So if mass is always positive why do two masses attract each other and not repel as like repels like?

[ajb]

I have answered this question in this thread.

 

Basically electromagnetic forces are mediated by spin-1 bosons known as photons. What is known from relativistic field theory is that any force mediated by spin-1 bosons is repulsive. It is because we have positive and negative charges that we have attraction and repulsion in electromagnetism.

 

Now, gravity (treating general relativity as an effective theory) is mediated by a spin-2 boson know as the graviton. Again we know that forces that are mediated by a spin-2 bosons are attractive.

 

Mass (or really energy-momentum) is the "charge" for gravity. As mass appears to always be positive gravity is always attractive.

[Pete]

From a book:-

 

Mass differs from electric charge in that it is always positive, and that the forces between masses are therefore always attractive.

 

Electric charge can be both positive and negative, and charges of the same sign repel each other, while charges of opposite sign attract each other.

 

So if mass is always positive why do two masses attract each other and not repel as like repels like?

The laws are in fact different. Each law is based on observation and that is the reason for the difference, i.e. it is what we observe. I.e. for two point charges

 

Coulomb's Law - [math]F_{12} = kq_1 q_2/r^2[/math]

 

while for two masses

 

Newton's Law of Gravitation - [math]F_{12} = -Gm_1 m_2/r^2[/math]

 

Notice the negative sign in the later. It is that negative sign which gives gravitational repulsion (when both masses are postive that is).

 

Pete

[swansont]

Notice the negative sign in the later. It is that negative sign which gives gravitational repulsion (when both masses are postive that is).

 

Pete

 

Attraction?

[Pete]

Attraction?

Yes. That is correct. Thanks.

 

Pete