# Thread: exact distance where gravity of earth ends?

1. kindly tell me this and also tell me the fromula for this

2.

3. It doesn't end. It goes on for ever. It just gets weaker and weaker and weaker...

4. Originally Posted by Strange
It doesn't end. It goes on for ever. It just gets weaker and weaker and weaker...
whre it becomes zero

5. It follows an inverse square law. The strength of gravity is proportional to 1/r2 (where r is the distance).

Whatever number you put in for r, the result will never be zero.

6. (Doesn't gravity propagate at he speed of light? If thats the case shouldn't it stop x billion of light years away (where x is the formation of earth) except that much much before this distance the earth's specific gravity should be blurred into irrelevancy by all the mass from that distance away.?)

the distance is extremely far but the gravity of earth specifically at great ranges is extremely small(insignificant).

7. Originally Posted by icewendigo
(Doesn't gravity propagate at he speed of light? If thats the case shouldn't it stop x billion of light years away (where x is the formation of earth) except that much much before this distance the earth's specific gravity should be blurred into irrelevancy by all the mass from that distance away.?)

the distance is extremely far but the gravity of earth specifically at great ranges is extremely small(insignificant).
sir explain in some detail.
thanx

8. Since the universe is 13.7 billion years old, earth's gravity will not be felt at distances > 13.7 billion light years.

9. Originally Posted by mathman
Since the universe is 13.7 billion years old, earth's gravity will not be felt at distances > 13.7 billion light years.
a bit logical, but a far away from my understanding.

10. Listen carefully. It never reaches zero. The gravity of every object in the Universe continues on throughout the Universe. At some distance away, other objects have stronger gravuitational influence, but earth is still affecting everything else in the Universe. As are you. And a pea sitting on your plate.

11. Originally Posted by mathman
Since the universe is 13.7 billion years old, earth's gravity will not be felt at distances > 13.7 billion light years.
Good point mathman!

12. Okay, well, err... the above is complicated by at least two factors.

The first factor that needs consideration is the effect the expansion of the universe has on distance, according to current mainstream theory.

We currently detect light that was emitted ~13.7 billion years ago from a place that is now ~46 billion light-years away. The light I am referring to is the Cosmic Microwave Background radiation, and the place is known as the surface of last scattering or the particle horizon, and it marks the edge of our observable universe in every direction from here - a conceptual sphere around us where all the CMB radiation we currently detect was released long ago, when the universe was a lot smaller and was expanding really quickly. It was expanding so quickly that radiation that was emitted only a few tens of millions of light-years away (yes, the original distance is really that small!) took 13.7 billion years to reach us, as the distance "in front of it" was increasing faster than the speed of light at that time. Wow!

We also see light from galaxies that were ~3.5 billion light-years away, 13 billion years ago, and those galaxies (or whatever has become of them!) will now be around 30 billion light-years away due to the expansion of the universe.

If gravity propagates at the speed of light, presumably we would also feel the almost infinitesimally small gravitational influence of those galaxies as their light reaches us, no?

But the second, even more subtle issue is that of the nature of the propagation of gravity. It is a little known fact, but according to General Relativity, in the case of most celestial objects, it is as if the gravitational influence of an object acts instantaneously. The Earth is actually "falling towards" the Suns current instantaneous position, rather than the light-retarded position we see it in the sky!

Now before you all start protesting about hearing that gravity propagates at the speed of light, you should read this other thread to see how that propagation applies. But the upshot of it is

Originally Posted by SpeedFreek
In weak fields, space-time conspires to "telegraph" ahead the instantaneous position of a gravitational source. Unless we suddenly accelerate, we can consider the curvature of space-time to be telling us the instantaneous position of the source, rather than the retarded light-delayed position, and it is towards the instantaneous position that we accelerate. Gravity points us towards the place things are "now", not where we see them to be.
*We can think of gravity as the curvature of space-time, and that curvature itself does not need to propagate. What propagates across that curvature are gravitational waves, caused when an object changes its acceleration. It is these waves that propagate at c.

So the answer to the question in the OP is a little more complicated than it might seem at first, but the answer is probably infinity! (It depends on how far you want to take it - I mean, the Earth hasn't been around for the whole history of the universe, but the stuff that makes up the Earth has).

*(Ok, it is not quite as simple as that, as you will find out if you read that other thread, or research this yourself)

13. Originally Posted by precious
Originally Posted by Strange
It doesn't end. It goes on for ever. It just gets weaker and weaker and weaker...
whre it becomes zero
That would be at infinity.

In theory, it never reaches zero. If I double my distance, I quarter the gravitational force. I can double/quarter as many times as I wish, and I still have a real number.

14. why is gravity weaker than the other three forces ?

15. Originally Posted by MeteorWayne
Originally Posted by mathman
Since the universe is 13.7 billion years old, earth's gravity will not be felt at distances > 13.7 billion light years.
Good point mathman!
What if you consider that the matter/energy the earth is made up of was formed during the big bang? As I have it, neither inflation, nor expansion would erase the force. All that it does, is it would stop the information of a change in the said matter/energy's momentum from ever reaching beyond the observable universe. Matter > 13.7 billion light years away would certainly still "feel" the pull of the earth's gravity though, however small that might be.

Edit: Also read SpeedFreek's post. (missed it for some reason)
Edit 2: Though I wonder: if light beyond the observable universe is "infinitely red-shifted", perhaps something similar would have happened to the gravity well of the earth's matter as well, meaning you would not feel earth's gravity. Sure sure about this...

16. Originally Posted by Rohith
why is gravity weaker than the other three forces ?
That's a separate topic, and deserves it's own thread. And the answer is no one knows for sure. Some aspects of string theory provide plausible explanations.

17. Originally Posted by KALSTER
Edit 2: Though I wonder: if light beyond the observable universe is "infinitely red-shifted", perhaps something similar would have happened to the gravity well of the earth's matter as well, meaning you would not feel earth's gravity. Sure sure about this...
Light from beyond the observable universe is not infinitely redshifted, it just hasn't reached us yet!

Next we need to discuss the nature of causal connections - our "past light cone", and what that might seem to imply for gravity. If we have never received a particle from an object, due to it always having been too far away, will we feel its influence, or not?

Talk about muddying the waters, eh?

18. Light from beyond the observable universe is not infinitely redshifted, it just hasn't reached us yet!
But the further something is, the more it is red shifted is. So, at some point light would be shifted into oblivion, no?

19. Originally Posted by KALSTER
Light from beyond the observable universe is not infinitely redshifted, it just hasn't reached us yet!
But the further something is, the more it is red shifted is. So, at some point light would be shifted into oblivion, no?
Perhaps, but the CMB "light" that reaches us from the very edge of the observable universe is only redshifted into microwaves ("stretched" by a factor of ~1100). We assume that, as the CMB was released throughout the universe, that we will continue to detect CMB far into the future - and all that CMB was originally released from a distance currently outside of our observable universe. When we detect CMB far in the future it will have a higher redshift than the CMB we detect today. It will be a long time indeed before the CMB would be redshifted beyond detectability, which would mark the limit of our observable universe in the far future.

Theoretically, only at the end of time would the CMB be redshifted to infinity. Practically, it is hard to estimate when the wavelength becomes undetectable, but I think theoretically it becomes undetectable once it is larger than the observable universe!

20. I was/am seriously confused about this. Someone on another forum worked out that z goes to infinity for matter currently at 46 BLY. HERE. Even if his math is wrong, it implies that z can reach infinity. Is that right? Perhaps we need a separate thread for this?

21. That's a separate topic, and deserves it's own thread. And the answer is no one knows for sure. Some aspects of string theory provide plausible explanations. thanks sir

22. Originally Posted by KALSTER
I was/am seriously confused about this. Someone on another forum worked out that z goes to infinity for matter currently at 46 BLY. HERE. Even if his math is wrong, it implies that z can reach infinity. Is that right? Perhaps we need a separate thread for this?
I can see why you are confused after reading that post. It is a very confused and misleading description, the last part of which is completely wrong, to my understanding!

Originally Posted by Marcus
I take that to be the STANDARD MODEL with the usual parameters. You need some definite values of the main parameters in order to calculate distance from redshift. No big deal. Say .27, .73, 71 for the matter fraction, cosmo constant, and current Hubble rate.

In that case z goes to infinity for material that is currently at around 46 billion lightyears. The relationship is not linear.

From a practical standpoint what Phyzguy says is right. Beyond z = 1100 you cant see with light. Maybe neutrinos but not light. There is more to talk about but I have to go.

So beyond a current distance of about 45-some billion LY (the current dist. to the surface of last scattering) you can't see because the universe before that time was opaque. the hot ionized gas would scatter any light.

BUT IF YOU COULD see back thru the fog, you could only see to about 46 because z is blowing up and we already said that z going to infinity imposes another sort of limit.
Now then, z actually represents the scale factor of the universe, in the form 1+z. So when we look at a galaxy with a redshift of z=7, we are looking at the universe when it was 8 times smaller than it is today. If we look at the CMB (which has a redshift of z=1089), we are seeing radiation that was emitted when the universe was 1090 times smaller than today (46 billion / 1090 = ~ 41 million).

So, to my understanding, z only goes to infinity at t=0, when the universe was infinitely smaller than it is today!

Beyond z=1100, if we were able to see what was going on, we would be looking at a volume smaller than 40 million light-years in radius, but unless we can look all the way back to t=0 we are never looking at an infinitely small volume, so we will never find z to be infinity.

Where t is postive, z is always finite.

The above author even states that "beyond a current distance of about 45-some billion LY (the current dist. to the surface of last scattering) you can't see because the universe before that time was opaque. the hot ionized gas would scatter any light." So why would a theoretical z factor suddenly blow up to infinity there?

Perhaps someone should ask this question at BAUT?

23. Makes perfect sense the way you explain it. Thanks!

24. I reworded this post because I think it could have introduced more confusion than it helped.

Having re-read that post from Marcus a few times, I think he is confusing the particle horizon (the edge of our observable universe, defined by the light that reaches us that has been travelling for the longest time), with the cosmological event horizon (the distance beyond which we will not be able to see, due to the accelerating expansion of the universe). That event horizon is actually our theoretical past light cone, at the end of time!

I would recommend the following paper once again:[astro-ph/0310808] Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe

Especially the space-time diagrams shown on pages 3 and 11. These really help link all these concepts together - its how I do it!

25. Getting back to the original question:

While gravity does extend to "infinity", there are practical limits to the Earth's gravitational reach. This is where the Sun's gravitational influence begins to overwhelm that of the Earth's.

Most notable is the Hill sphere, found by:

~

where
a is the Earth-Sun distance.
ME is the mass of the Earth
MS is the mass of the Sun.

And works out to be ~1.5 million km.

This is maximum distance at which the Earth could hold an object in orbit without the Sun pulling it away into a Solar orbit. (assuming a circular orbit)

That is not to say that the Earth has no influence on objects further away than this, just that it can't permanently hold them in orbit.

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