# A problem with the theory of relativity? **URL:** \ **Category:** Factual Questions\ **Created:** [July 21, 2000, 2:12am UTC](https://boards.straightdope.com/t/a-problem-with-the-theory-of-relativity/26310 "2000-07-21T02:12:29Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![Arjuna34](https://avatars.discourse-cdn.com/v4/letter/a/eb8c5e/32.png) [@Arjuna34](https://boards.straightdope.com/u/Arjuna34)\ **Post date:** [July 22, 2000, 3:09am UTC](https://boards.straightdope.com/t/a-problem-with-the-theory-of-relativity/26310/21 "2000-07-22T03:09:44Z")
> [@](#): > > _Originally posted by Jeff\_42 \* > \*\* Picture a person in a box in deep outer space accelerating at 1g. Now put that box on earth. The person inside will feel 1g. The person in the box has no way to look outside and see what is going on so he has no way to know whether the 1g he feels is because we’re accelerating him or have him sitting on a planet._\* Can’t that person just extend his arms and feel the difference? Lets say I’ve got hyper-sensitive arms so I can measure almost exactly the force my arms feel. Since the earth is a sphere, if I extend my arms out (to my sides) my left and right hands will both feel a force with magnitude 1g, but the directions will be slightly different, since each is attracted to the center of the earth. Of course, the difference is extremely small, since the distance from hand to hand is only a few feet, and I’m thousands of miles from the center of the earth. If I’m being accelerated to 1g with a rocket, the direction will be the same for both hands. That difference lets me figure out if I’m on a planet or being pushed. This wouldn’t work if the earth were an infinitely large flat plane, instead of a sphere. Or am I wrong?? Arjuna34 ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 22, 2000, 4:37am UTC](https://boards.straightdope.com/t/a-problem-with-the-theory-of-relativity/26310/22 "2000-07-22T04:37:21Z")
I used to wonder about that, too, Arjuna. The trick is that you can get that effect from an acceleration… as long as you’re not restricting yourself to Euclidian geometry. You can also produce a uniform gravitational field, as you mentioned, from a very large massive plane. In either case, the acceleration is indistinguishable from the gravitational effect. ---
**Author:** ![Arjuna34](https://avatars.discourse-cdn.com/v4/letter/a/eb8c5e/32.png) [@Arjuna34](https://boards.straightdope.com/u/Arjuna34)\ **Post date:** [July 22, 2000, 5:08am UTC](https://boards.straightdope.com/t/a-problem-with-the-theory-of-relativity/26310/23 "2000-07-22T05:08:41Z")
> [@](#): > > \*Originally posted by Chronos \* > \*\*I used to wonder about that, too, Arjuna. The trick is that you can get that effect from an acceleration… as long as you’re not restricting yourself to Euclidian geometry. You can also produce a uniform gravitational field, as you mentioned, from a very large massive plane. In either case, the acceleration is indistinguishable from the gravitational effect. \*\* So am I right that gravitation from a spherical planet IS distinguishable from normal acceleration? I’ve wondered about that for a while actually. Not that it disproves anything, though … just nit-picking 🙂 Arjuna34 ---
**Author:** ![JonF](https://avatars.discourse-cdn.com/v4/letter/j/cab0a1/32.png) [@JonF](https://boards.straightdope.com/u/JonF)\ **Post date:** [July 22, 2000, 12:23pm UTC](https://boards.straightdope.com/t/a-problem-with-the-theory-of-relativity/26310/24 "2000-07-22T12:23:00Z")
> [@](#): > > So am I right that gravitation from a spherical planet IS distinguishable from normal acceleration? Yes, although GR doesn’t include any concept of “normal” acceleration. ---
**Author:** ![LouisB](https://avatars.discourse-cdn.com/v4/letter/l/dfb087/32.png) [@LouisB](https://boards.straightdope.com/u/LouisB)\ **Post date:** [July 22, 2000, 12:59pm UTC](https://boards.straightdope.com/t/a-problem-with-the-theory-of-relativity/26310/25 "2000-07-22T12:59:49Z")
[http://www.ieee-uffc.org/index.asp?page=freqcontrol/fc\_links.html&Part=5#top](http://www.ieee-uffc.org/index.asp?page=freqcontrol/fc_links.html&Part=5#top) Okay, the link above will take you to an assortment of papers and tutorials on the subject of time and frequency. If memory serves, the experiment involving transporting clocks around the world is reported in one of these papers. If not, a report of the experiment is found in one of the Proceedings of the Annual Frequency Control Symposium. Since there are about fifty of such Proceedings, I would suggest searching through the abstracts. The abstracts are available at the Frequency Control Society’s site, which is part of the site referenced above. ---
**Author:** ![emarkp](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@emarkp](https://boards.straightdope.com/u/emarkp)\ **Post date:** [July 22, 2000, 4:57pm UTC](https://boards.straightdope.com/t/a-problem-with-the-theory-of-relativity/26310/26 "2000-07-22T16:57:03Z")
We had this same discussion a while back. See the thread at [http://boards.straightdope.com/sdmb/showthread.php?threadid=25449](http://boards.straightdope.com/sdmb/showthread.php?threadid=25449) or [http://boards.straightdope.com/sdmb/showthread.php?threadid=12156](http://boards.straightdope.com/sdmb/showthread.php?threadid=12156) – you’ll conclude the same thing as I did if you read some of the guy’s posts on Usenet. ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [July 23, 2000, 5:25pm UTC](https://boards.straightdope.com/t/a-problem-with-the-theory-of-relativity/26310/27 "2000-07-23T17:25:47Z")
**Gravity is indistinguishable from acceleration only on infinitesimal regions.** You can use acceleration to transform away a gravitational field only when the gravitational field is parallel at every point in your reference frame. This is never true of a real gravitational field in a reference frame of sufficient extent. In particular you can never transform away the gravitational field of a mass that is contained in your reference frame. If the earth is within your frame of reference, for example, you would have to accelerate in the southern direction at the North pole, but at the South pole, you would have to accelerate northward. There is a thought experiment where you are in an elevator, and you cannot tell whether you are accelerating at 1 g or in a gravitational field of 1 g. It is assumed that you are far enough away from the source of the field that you cannot measure that the field is converging toward the source of the field. If you are close enough to the source of the field, you can use plumb bobs. If the lines get closer together toward your feet, you are in a gravitational field. Saying that the source of the gravity is an “infinitely large flat plane” is equivalent to saying your reference fame is infinitesimal compared with the distance to the source of the gravity. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [July 24, 2000, 8:22pm UTC](https://boards.straightdope.com/t/a-problem-with-the-theory-of-relativity/26310/28 "2000-07-24T20:22:22Z")
> [@](#): > > It is assumed that you are far enough away from the source of the field that you cannot measure that the field is converging toward the source of the field. If you are close enough to the source of the field, you can use plumb bobs. If the lines get closer together toward your feet, you are in a gravitational field. While this is true, it gets much more difficult if you aren’t trying to distinguish between “only one mass” and “only acceleration”. Your plumb bobs won’t work if I have a smaller mass above the elevator and closer than the larger mass, to cancel out the transverse portion of the gravitational field. Likewise, I could “fool” you into thinking you were some distance from a large mass using a combination of a smaller mass and acceleration. I also suspect I could make an arbitrarily uniform field over a finite volume using a finite amount of mass. [Previous page](https://boards.straightdope.com/t/a-problem-with-the-theory-of-relativity/26310.md?page=1)