# Is Force instantaneous?

1. Aug 28, 2009

### PhDorBust

Is force instantaneous or does it travel at the speed of light, and why?

Like is the gravitational effect felt on us by the sun immediate or does it take 8 minutes or so to be realized?

2. Aug 28, 2009

### Integral

Staff Emeritus
Assuming you are applying a force to something, then the effects of that force travel through the object at the speed of sound in that object. Much slower then the speed of light.

3. Aug 28, 2009

### rcgldr

4. Aug 28, 2009

### maverick_starstrider

Gravity's a different creature. But if you were to don a suit of magnetized iron armor and sit on the earth and send a gigantic electromagnet to the sun and then someone switched it on, then yes, it would take 8 minutes for you to feel the tug.

5. Aug 28, 2009

### creepypasta13

why would it not be instantaneous?

6. Aug 28, 2009

### Fuz

Gravity travels at exactly the speed of light. If the sun were to disappear right now, the Earth would keep revolving as if the sun were still there. After about 8 minutes the Earth would notice the absence of the suns gravity and would just float off into space.

7. Aug 28, 2009

### maverick_starstrider

Because the messenger particles that mediate the force only travel at c.

8. Aug 28, 2009

### Gear300

Mechanical forces propagate energy as mechanical waves (speed depends on the inertial and elastic properties of the the medium). Gravity, by what I've heard and read, propagates at the speed of light - meaning that if the sun suddenly disappeared, the planets that were orbiting it would take a while to go off course.

9. Aug 28, 2009

### Phrak

In this folder gravity is instantaneous. A few folders down it propagates somewhat slower--sort-of.

10. Aug 29, 2009

### Pierre007080

Gravity is instantaneous. If it were not there would be a "drag" factor on any rotating body because the force would be acting behind the radius line. Because mass absorbs space the support between two masses is removed and the effect is instantaneous. If ten matches were separating two masses and two (one from each end) were removed the shortened distance would be immediate. This is the reason for the constant acceleration of a rotating body toward the middle.

11. Aug 29, 2009

### D H

Staff Emeritus
Gravity is instantaneous in Newtonian physics. In general relativity it is not.

That is a common argument used against general relativity by those who do not understand it. If that is all there was to general relativity that argument would be correct. However, there is more (a whole lot more) to general relativity than a finite propagation time for gravity. Other terms such as frame dragging nearly cancel the effects of frame dragging. For example, for planets far from the Sun and moving at relatively slow speeds compared to light speed it looks very much like gravity does propagate instantaneously. This is not the case for Mercury. That general relativity explained a known defect with the Newtonian explanation of Mercury's orbit was one of the reasons general relativity won the day over Newtonian mechanics.

This is meaningless nonsense.

12. Aug 29, 2009

### A.T.

As DH already said, this simlistic logic is wrong. It assumes that a field can only contain information about the position of the source during emission. But a field can also contain information about the velocity of the source during emission.

The electric field is like this too: The field lines of a inertially moving charge always point towards the current position of the charge, not some old position from which the field was emitted. But this does not meant the electric field is instantaneous. It just means that the field moves at the same speed as the source was moving during emission.

13. Aug 29, 2009

### pgardn

Isn't the idea of two events happening instantaneously actually a rather bizarre idea in itself? I have read about the idea in quantum mechanics with spins of certain particles, but our basic human model of what instantaneous really is seems ambiguous and hard to define. Just my slightly philisophical take. Time is a human construct. Any ideas that would help me would be appreciated so I thank the first poster for posing the question to begin with. Human models and what reality presents can be vexing. And I really dont know what I mean by reality. anyhow... any particular way people look at events and time would be helpful for me.

14. Aug 29, 2009

### Staff: Mentor

No it isn't. It is a human word used to describe a natural phenomena. Time exists whether we are here to observe it or not.

15. Aug 29, 2009

### maverick_starstrider

The arrow of time may be a human construct.

16. Aug 29, 2009

### Gear300

We can't really be sure whether something is a human construct or not. Even reality, despite how paradoxical it is, is something we take as common sense and for granted.

17. Aug 29, 2009

### Staff: Mentor

No, the arrow of time is a consequence of thermodynamics. It is also built into the laws of the universe.

18. Aug 29, 2009

### Staff: Mentor

Yeah, we really can. You guys are arguing against the definition of science here. The whole point of science is to explain/understand natural phenomena. That is - things that exist in nature, whether we are here to observe them or not.

If we define a "year" to be something other than its current definition, that doesn't make the sun any younger or older.

19. Aug 29, 2009

### maverick_starstrider

That's not the current consensus I'm afraid. The second law of thermodynamics is not sufficient explanation. Actually Sean Carroll just wrote a book about this (Here to Eternity).

http://en.wikipedia.org/wiki/Arrow_of_time

20. Aug 29, 2009

### Staff: Mentor

??? In that wiki article there are 7 "arrows of time" listed and 6 of them are consequences/manifestations of the thermodynamic arrow.

Since that article doesn't mention Sean Carroll, it doesn't really say anything about what you are talking about...

21. Aug 29, 2009

### maverick_starstrider

Sean Carroll's kind of the Michio Kaku of GR/cosmology if you don't know who he is (http://en.wikipedia.org/wiki/Sean_M._Carroll). Anywho my point merely was that the physicality/reality of a "forward" facing, continuous "arrow of time" is by no means a proved concept in cosmology

22. Aug 29, 2009

### Gear300

Couldn't we say that we take for granted what 'natural phenomena' is?

23. Aug 29, 2009

### Staff: Mentor

We can say anything, but that doesn't mean it makes any sense.... could you explain what you mean?

To perhaps jump ahead, science requires the assumption that our senses or our God aren't screwing with us. Is that what you are getting at?

Applied to time, we measure time based on physical processes such as the oscillations of a cesium atom. Are you saying that if humans weren't here, the number of times a cesium atom oscillated since the sun formed would be different?

24. Aug 29, 2009

### maverick_starstrider

Well, if I've correctly dusted the cobwebs from my memory, all quantum systems have time symmetry and anti-particles can be treated as normal particles moving backwards in time with identical results.

25. Aug 29, 2009

### Phrak

Rocks are hard and water is wet. The concensus in discussions of physics as a science is that we are assume an independent reality. This is not a know, but an assumption, that we should be using in common.