# Measurement of gravity

1. Nov 4, 2009

If I had a hypothetical gravity measuring device that had no mass, would it be able to measure the gravity of a planet, that does have mass, as it approached it? Or would it not detect any gravity because the device has no mass?

I hope this makes sense...

2. Nov 5, 2009

### Uk_Ghost

It would have to have some form of mass to be affected by gravity. Therefore it would have no way of measuring the gravitational pull if it is unaffected by it. Unless of course it was measuring another object which did have mass being pulled towards the planet.

3. Nov 5, 2009

So the Earth, or any other planet, doesn't actually have gravity? I mean, is gravity just the attraction of two objects with mass? If an object was somehow isolated and couldn't interact with other objects, then it wouldn't really have gravity?

4. Nov 5, 2009

### Uk_Ghost

Yes, all objects with mass have gravitational pull. The ammount depends on the mass of the object. Like how the Earth has enough mass to be able to keep the Moon in orbit around it but the Moon's mass is also big enough to affect the Earth's tides. If an object was able to have no mass it would not be attracted to anything with mass.

I'm not sure what you mean by isolating something though. I'm assuming its the same concept as 'if a tree fell in the woods and no one was around to hear it would it make a sound?'

5. Nov 5, 2009

Yes, but I didn't think of it like that. So it's no really a question worth answering...

6. Nov 5, 2009

### Uk_Ghost

No worries :)

7. Nov 5, 2009

### Staff: Mentor

The previous response is incorrect. It was one of the early successful tests of general relativity that massless particles like photons are also measurably affected by gravity.

8. Nov 5, 2009

### Molydood

interesting; does that mean photons gravitationally attract other photons? or is the gravitational attraction with photons a special case and only 'one way'?

9. Nov 5, 2009

### Lsos

As has been mentioned, light is massless and yet is affected by gravity. Therefore it is possible to make an instrument that detects gravity without it itself having any mass....or at least without it relying on its mass to make the measurement. It could be an intrument that relies on light, or time, or speed instead.

In fact, we can calculate the gravity of certain cosmic objects simply by looking at how they bend light. So in effect we are measuring ther gravity without directly using any mass to do so.

Last edited: Nov 5, 2009
10. Nov 5, 2009

### Molydood

first paragraph, surely a typo on first line?
second paragraph, I am fairly sure the OP is talking about the measuring device being affected by gravity and being massless, as opposed to being massless and capable of measurement (based on the wording about approaching the object)

11. Nov 5, 2009

### Staff: Mentor

Well, that is hard to answer without a complete theory of quantum gravity, but if you change from "photon" to "brief pulse of light" then yes, they do interact. Each would be described by the Aichelburg–Sexl ultraboost solution.

12. Nov 5, 2009

### Molydood

what a great name for a theory

13. Nov 5, 2009

### Uk_Ghost

The speed of light is a constant. How could you possibly it to tell us the gravitational pull of a planet?

The only time light is 'bent' is during gravitational lensing from something huge like a black hole or a galaxy cluster which creates warped space-time which would bend everything in it. It that senario you would be able to tell the gravitational effect of warped-space on light but you wouldn't be able to use light to tell us the gravatational pull of a planet, which was the original question.

14. Nov 5, 2009

### Lsos

Gravity bends light. The more gravity the more it bends....

This means that whether it goes around a cluster of galaxies or a cluster of fat chicks, it should bend. Yeah, good luck measuring this...but nevertheless the effect is there.

15. Nov 6, 2009

### Staff: Mentor

Speed isn't the only property that light has that can be used to tell us about the gravity of a planet. Gravitational lensing has already been mentioned, and there is also Shapiro delay and gravitational redshift all of which can be observed within the solar system. The frequency change due to Earth's gravity could be detected using technology from 1959: http://en.wikipedia.org/wiki/Pound–Rebka_experiment

Last edited: Nov 6, 2009