# Effects of Tensor + Scaler gravity

1. Mar 7, 2009

### captn

Fifty years ago (m/l), Bob Dicke and Carl Brans developed a S+T gravity model.

Concurrently, Dicke had a project to look for a small solar oblateness. He wanted the precession of Mercury caused by the scaler part of gravity to be offset by the opposite precession caused by oblateness.

In the last ten years, a scaler field has been suggested to the dark (matter? or energy?), but I have seen nothing about that field causing a part of Mercury's precesion.

Is the effect of a scaler suggested today much less than the effect caused by dicke/brans' scaler? Please help me to understand the effects of today's sacler.

Thanks, Neil

2. Mar 8, 2009

### Chalnoth

Well, it is the case that any scalar field will necessarily also cause a gravity-like force. Typically the Quintessence models of dark energy that make use of such a scalar field having one whose interactions are such that they are undetectable.

And by the way, the way to distinguish between normal gravity and gravity induced by a scalar field is by looking at the deflection of light: the two deflect differently, and so a scalar field would cause the measurement of our Sun's mass by gravitational lensing to differ from its mass as measured through the orbits of the planets. So far the two agree to within experimental precision, so any scalar field that exists must necessarily have either very weak or very short-range interactions.

3. Mar 10, 2009

### captn

Okay---Thanks,

Neil

4. Mar 10, 2009

### JinChang

Can someone provide an analogy of scalar versus tensor (fields)? I've read through the wikipedia, but it's difficult to visualize. Some examples of each that could be used to differentiate the two would be great.

5. Mar 11, 2009

### Chalnoth

Well, a scalar field is a field that is fully defined by a single number at every point in space, whereas a tensor field is one that is fully defined by a tensor at every point in space (typically a second rank tensor).

Now, an example of a scalar field would be energy density: energy density is just a single number at every point in space. Note that this isn't a quantum field in and of itself, but rather a property of other fields. A quantum scalar field would be a fundamental particle that can be identified in such a way.

An example of a tensor field would be the stress tensor. The stress tensor is composed of pressure along the diagonal components, and anisotropic shears on the off-diagonal components. This type of tensor is required to fully define the forces in an extended object. If I have a rod, for example, I can give it pressure in one direction or another by squeezing/stretching it. I can give it an anisotropic shear by twisting the rod.

Depending upon what you mean by a tensor field, you can also add in the momentum and energy density of the rod (to make a stress-energy tensor). The difference would be whether you just want the spatial components of the tensor, or also the space-time components.

6. Mar 11, 2009

### JinChang

Thanks, Chalnoth. While I need to study upon some of your explanation, it's certainly a great place to start.