# B Measuring gravity

1. May 16, 2018

### nuby

Dumb question probably -- But is there a way to measure gravity in a particular area of space, or a "measurement" .. I.E. the gravity 10 miles above earth v.s 1000 miles above. Not force on another object, but some "unit" or measure of gravity itself.

2. May 16, 2018

### davenn

it's always going to involve the force by a mass on another mass

https://www.wikihow.com/Calculate-Force-of-Gravity

http://www.dummies.com/education/sc...e-the-force-of-gravity-on-the-earths-surface/

Dave

3. May 16, 2018

### nuby

Why can't an object's gravity be measured without comparing it to another object (edited)? It's a physical property that exists in space, no?

4. May 16, 2018

### Janus

Staff Emeritus
Well, there is the "g", which is equal to the acceleration due to gravity at the surface of the Earth. It is defined as 9.80665 m/s^2, and doesn't rely on the mass of the object being accelerated. Thus the strength of gravity at an altitude equal to the radius of the Earth would be 0.25g

5. May 16, 2018

### Staff: Mentor

Gravity is defined to be a force on another object, so if you aren't measuring the force on another object you aren't measuring gravity.

6. May 16, 2018

### nuby

How would one calculate the trajectory of a photon around a planet/star?

7. May 17, 2018

### Staff: Mentor

We can't, because a photon doesn't have a trajectory. But that's a quibble because you might as reasonably ask about a flash of light, and a flash of light does have a position and a trajectory.

To calculate the path of a flash of light around a star, you would grind through the math of Einstein's theory of general relativity, and the details of how it's done don't belong in a B-level thread (although if you want to see where to start, chapter 7 of https://www.preposterousuniverse.com/grnotes/ is online and free). But that's how you calculate the effects of gravity on paths around the star - not the measurement of gravity at a point that you asked for at the start of the thread.

When I said "gravity is defined to be a force on another object" I was using the classical definition. If we're going to go with general relativity instead, we'd have to say something like "gravity is defined to be the effect of spacetime curvature on the motion of another object or a flash of light".

Last edited: May 17, 2018
8. May 17, 2018

### davenn

no
it is only if when there is an object ( mass) to "generate" produce the gravity field

9. May 17, 2018

### lekh2003

I think you might be thinking of gravity as some kind of "gravitational field" with some "gravity measure" in just the same way that electric fields seem to work.

Gravity is a force between two objects and you can't have the force without two well defined objects.

On the other hand, you can have an acceleration due to gravity on an object since when you do the simple calculations, the mass of the second object is no longer needed.

10. May 17, 2018

### sophiecentaur

The practicalities of the basic measurement are also of interest - before we get to 'light', I think. You could drop a heavy object and plot its change in height above the ground with time. That would give you the acceleration towards the Earth (= local g). You could 'weigh' a standard mass whilst in an aeroplane but then you would need to measure / monitor the altitude of the plane to keep its altitude constant. More than 10 miles would be a difficult / inconvenient. Once you have a orbit, you are back in business.

Newton 3 requires that there is a force on the Earth, too. (And on all other masses in the Solar System / Universe). But Relativity tells us that everything is relative to 'other objects'. All the thought experiments about people in lifts (elevators) tell us that they can only tell there's any acceleration involved when there's a difference between the lift and the passenger.

11. May 17, 2018

### Staff: Mentor

I think we are making this unnecessarily complicated. There are standard methods of measuring gravity, which are used in oil and gas exploration. https://en.m.wikipedia.org/wiki/Gravimetry

12. Jun 1, 2018

### dean barry

you could work with a gravitational acceleration map

13. Jun 1, 2018

### f95toli

Not quite true. You can also measure gravity by using an (or rather two)( accurate clocks (i.e. using GR).
There have been a number of demonstration where people have done this using optical clocks and I am aware of projects where people are trying to develop this into a commercially viable method, right now the equipment is too bulky and too expensive. In reality you would probably also measure the difference between two different clocks; i.e. it would not quite be an "absolute" measurement.
See e.g.
Grotti, Jacopo, et al. "Geodesy and metrology with a transportable optical clock." Nature Physics (2018): 1.

14. Jun 1, 2018

### Janus

Staff Emeritus
But this method requires knowledge about the gravity field beyond that which can be measured by the clocks. For example, if we place our clocks in a sealed room in an unknown gravity field, the clocks can only give us the potential difference between the clocks, but not the difference in gravity experienced by the clocks. You can have the same potential difference between the clocks when gravity varies very little from the average from top to bottom of the room, as when the difference varies by more. In these two cases, the difference in tick rates for the clocks can be the same, while the difference in gravitational force between each pair of clocks is not.

15. Jun 1, 2018

### jbriggs444

Right. Dividing by the known distance between the clocks, they give you the proper acceleration of the room. i.e. the local acceleration of gravity. If one is using a pair of clocks as a gravimeter, this is the piece of information you are looking for.