# The Naked Scientists Forum

### Author Topic: Can I prove that I'm moving/accelerating?  (Read 4295 times)

#### krool1969

• Full Member
• Posts: 53
##### Can I prove that I'm moving/accelerating?
« on: 22/10/2012 03:49:11 »
Okay according to Relativity, it's not possible for me to perform an experiment that would prove that I'm moving, or if I'm feeling gravity to prove that I'm standing on a massive object (like Earth) or accelerating.

But I think I might have thought up a way to do just that. If I build my spaceship in the shape of a right triangle with side "A" being parallel to my distraction of travel and side "B", with the engines mounted to it, perpendicular to my line of travel. If when the ship is built side "A" and side "B" are the same length then the angles will be equal

I fire up the engines and accelerate at 1 G. As I go faster and faster my entire ship will get shorter, meaning Side A will shorten but side B (the back of the ship) will get thinner, but stay the same length. This means that by measuring the angles and finding something other than 45 degrees I'd know I was moving or if the angles change over time I'd know I was accelerating.

Would this work in the real world? Was I lied to when told no experiment could show I'm moving?

#### Phractality

• Hero Member
• Posts: 523
• Thanked: 1 times
##### Re: Can I prove that I'm moving/accelerating?
« Reply #1 on: 22/10/2012 04:38:16 »
You meter stick would shorten just as much as the ship does.

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #2 on: 22/10/2012 11:12:26 »
It depends on what you mean by moving. You can easily prove yourself to be moving relative anything you formerly was at rest with, as your bed :) You can also refer to moving as a intrinsic property of your body's senses, as when you accelerate, because a acceleration should give you the same experience intrinsically anywhere inside our universe. Or you can define moving as two phenomena, one relative some arbitrarily made reference point, as your bed, and the other way of 'moving' being the intrinsic change found in a acceleration.

Einstein didn't think of it as geometries, even though it makes a lot of sense describing it that way to me. In his view the calculations and equations he found was more real than whatever we think us able to describe using our normal perceptions of nature, as I read him. And I think he is right there, we have a lot of presumptions, dimensions, motion, etc, that we use, but I'm not sure any of them are perfect descriptors of our universe.

#### lean bean

• Guest
##### Re: Can I prove that I'm moving/accelerating?
« Reply #3 on: 22/10/2012 19:21:59 »
Okay according to Relativity, it's not possible for me to perform an experiment that would prove that I'm moving, or if I'm feeling gravity to prove that I'm standing on a massive object (like Earth) or accelerating.

Hope this is not a pedantic point diverting the thread.
Two test masses freefalling to earth's surface are really falling to the centre of mass/earth, and so, would be seen to be drawing towards each other as they fell towards the point at earth’s centre. Of course, assuming a symmetrical uniform density earth.
On the accelerating rocket this would not happen, the masses would keep the same distance from each other as the ‘floor’ came ‘up’ to them.
« Last Edit: 22/10/2012 19:23:57 by lean bean »

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #4 on: 22/10/2012 19:57:20 »
As long as they rest on the rockets floor you should be correct but assuming two masses in parallel geodesics, in a otherwise gravitationally 'empty space', they will attract each other. But maybe I'm missing something here? A accelerating rocket and geodesics is somewhat of a 'collider' in terms :)

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #5 on: 22/10/2012 20:00:47 »
What you can differ should be the Coriolis force for example, that we find on earth. But that's due to Earths spin, and assuming no spin I find it impossible to differ a uniform constant acceleration at one G from being on a non spinning planet at one G. Assuming both to be a black box scenarios naturally.

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Re: Can I prove that I'm moving/accelerating?
« Reply #6 on: 22/10/2012 23:00:38 »
If you're on a rocket moving at a constant speed, then you'll always measure the right angles to be right angles.

When you're accelerating, you can measure deviations from right angles, but you have no way of knowing whether this deviation is caused by a real acceleration or by your ship sitting still in a gravitational field.

#### krool1969

• Full Member
• Posts: 53
##### Re: Can I prove that I'm moving/accelerating?
« Reply #7 on: 23/10/2012 00:04:17 »
You meter stick would shorten just as much as the ship does.

I'm not measuring the LENGTH of my ship. I'm measuring the angle between side "A" and the hypotenuses of my triangle shaped ship. As I approach the speed of light the ship and as you say, the length measuring tool would shorten, but only in the direction of travel. The ship would not become any less wide, so if I measure the angles they would no longer be the same as when I started.
« Last Edit: 23/10/2012 00:09:45 by krool1969 »

#### David Cooper

• Neilep Level Member
• Posts: 1505
##### Re: Can I prove that I'm moving/accelerating?
« Reply #8 on: 23/10/2012 00:15:39 »
I'm not measuring the LENGTH of my ship. I'm measuring the angle between side "A" and the hypotenuses of my triangle shaped ship. As I approach the speed of light the ship and as you say, the length measuring tool would shorten, but only in the direction of travel. The ship would not become any less wide, so if I measure the angles they would no longer be the same as when I started.

What are you going to measure the angles with? Any tool you have in mind will be length-contracted in the same way as the ship, so the angle you're hoping to see vary will always be measured as the same.

An observer who isn't travelling with the ship will see the length shorten and the angle change though, but that still won't give you any way to pin down a frame of reference in which things are stationary in absolute terms.

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #9 on: 23/10/2012 10:05:19 »
I strongly suspect David and Phractality to be right there. But it does give me a headache considering myself using that meterstick, turning in a arc inside the ship, fitting a pencil to its end, to draw the curve (inside the ship in air:). Because what it imply is that we then should have 'degrees' of contraction as we deviate from the motion straight ahead, if you see my drift here. I expect this one to be simpler, making more sense, using algebra than when geometry.

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #10 on: 23/10/2012 10:08:54 »
This is assuming a constant uniform acceleration at one G. Not a non uniform acceleration.

#### evan_au

• Neilep Level Member
• Posts: 4130
• Thanked: 249 times
##### Re: Can I prove that I'm moving/accelerating?
« Reply #11 on: 23/10/2012 12:21:06 »
Quote
according to Relativity, it's not possible for me to perform an experiment that would prove that I'm moving, or if I'm feeling gravity to prove that I'm standing on a massive object (like Earth) or accelerating.

Normally this "thought experiment" is qualified by "if performed in a small room". If you had a larger room (or more sensitive equipment), you can detect effects like:
• Coriolis Effect (mentioned above), although a spinning spaceship may produce similar effects
• Tidal effects as Earth's gravity gets weaker the further you are away from the surface of the Earth
• There are some accurate clocks and optical devices which can detect the relativistic effects of Earth's gravity over height differences as small as 3 meters.

#### lean bean

• Guest
##### Re: Can I prove that I'm moving/accelerating?
« Reply #12 on: 23/10/2012 12:23:33 »
As long as they rest on the rockets floor you should be correct but assuming two masses in parallel geodesics, in a otherwise gravitationally 'empty space', they will attract each other.

They can be falling and not on the floor.
Due to the mass of each test mass they will attract each other at a known calculable rate/force, this can be tested for, but there will be an extra part to that ‘attraction’ due to both heading towards the same point, centre of mass/earth, the extra ’tells’ you there’s more to it than just rocket acceleration and so you discern you are not in a  ‘gravitationally 'empty space'. :)
« Last Edit: 23/10/2012 16:34:18 by lean bean »

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #13 on: 23/10/2012 16:41:23 »
I'm slow Lb :)
Where do you get the 'extra' from? My way of looking at it it won't matter if two free falling masses (in geodesics) are inside or outside that ship. Assuming the ship to be roomy, 10 ly for example, the masses will have ample time to follow their geodesics. The only 'extra' I can imagine there would be the way the mass of the ship itself contribute?

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #14 on: 23/10/2012 16:54:44 »
Are you thinking of it as the masses having one preferred point (gravitational potential) that they deviate too following their geodesics on Earth, whereas on the ship their geodesics will express itself differently as the mass is 'surrounding' them? Maybe you are correct there, assuming that we have this predefined notion of what a planet will do to geodesics? what that goes back to is if those situations (black box scenarios) truly are equivalent. What we can do there is to compare Einsteins equations regarding the equivalence principle with experimental data. And then gravity becomes equal to a constant uniform acceleration as compared with the way light 'bends'. He used a (constantly accelerating) elevator, fitted with a small hole from where light falls in, as compared to how light deviate from a 'straight path', as defined by the earthbound observer, propagating past the sun. But yes, I think you might be correct there, as the way the mass is distributed must play a role for the geodesics. So now we have two, Earths spin and the gravitational potentials 'endpoint', both defining slightly different geodesics.
« Last Edit: 23/10/2012 17:01:11 by yor_on »

#### lean bean

• Guest
##### Re: Can I prove that I'm moving/accelerating?
« Reply #15 on: 23/10/2012 17:54:42 »
I'm slow Lb
No your not slow, it could be my depiction is misleading. But, if you insist… : )  : )

Try again.
Forget the ship and the test masses mutual attraction for each other for a minute.
Two test masses are falling to earth, each of their paths will converge at earth’s centre, that is, whilst their falling they will seem to be ‘attracting’ each other ONLY because they are heading to the same point, earth’s centre of mass and not because of the test masses  attraction for each other.

Now, on top of this, you have the test masses actual mutual  attraction for each other, this part is  a known calculable, anything beyond the calculated rate of attraction to each other is because of something else, here it is because their paths are also converging on earth’s centre.
So, because of this extra bit of what seems like attraction, you discern you are not in a ‘gravitationally empty space'.
In empty space away from a planet but in your ship, the test masses will only attract each other at the calculable rate, ignoring here the ship around them, because they are not now also heading to the centre of a planet. All this talk of mutual attraction, I feel as if I writing a love story. :)

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #16 on: 23/10/2012 20:02:10 »
Yes, that's how I got you :) That you were thinking of how the geodesics might differ between infalling to a planet as compared to the same geodesics inside a constantly uniformly accelerating ship in a space. But then you have the spin aka Coriolis force too that will differ between a spinning planet and a rocket. None of those though are a obstacle to the way gravity is defined in relativity as I understands it.

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #17 on: 23/10/2012 20:18:53 »
In a black box scenario I expect you to find it impossible to differ those geodesics, although you might find their paths confusing. Remember that the same could be created by the box being asymmetrical, massing more on one side than the other. That would also influence those geodesics as all 'mass attract', but assuming you enclosed inside a perfect box as you observe it, you would only be able to guess to the reason why the geodesics act as they do. And also, as i understands it any other experiment you could do should, if the equivalence principle is correct, be indistinguishable from a same experiment done on Earth.
==

The way to test for a external gravity, is to assume yourself inside that box, free floating (weightless), and then watch your stones geodesics relative that perfect box, as described form the inside, the way they 'attract' each other, or not.
« Last Edit: 23/10/2012 20:25:25 by yor_on »

#### lean bean

• Guest
##### Re: Can I prove that I'm moving/accelerating?
« Reply #18 on: 28/10/2012 15:58:56 »
Hi yor_on  :)
The way to test for a external gravity, is to assume yourself inside that box, free floating (weightless), and then watch your stones geodesics relative that perfect box, as described form the inside, the way they 'attract' each other, or not.
Is that suppose to be a practical way or is it theoretical ?
Where do you place the centre of mass of your ‘stone system’ so you can observe their motion relative to each other or the box ?
Do you place it at the centre of mass of your perfect box?
How do you set this up without the box influencing the stones or the stones influencing the box at the start? if one stone is nearer to a wall the wall will influence that stone more.  :)

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #19 on: 29/10/2012 05:23:38 »
You can find it in Lecture Notes on General Relativity: by Matthias Blau. Albert Einstein center for fundamental physics (Bern). And no, it's not theoretical. All mass attract all mass, always as I understands it.

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Can I prove that I'm moving/accelerating?
« Reply #20 on: 29/10/2012 05:31:39 »
As for how to set up such a system :)
That will mean that you set a lot of limits everywhere, assuming idealized boxes etc, so in that motto it sure becomes theoretical. How can one ever prove that one have isolated the 'gravity' as coming from just one 'isolated system', when gravity is the metric of Space? Maybe it's possible practically, but I suspect there always will be some doubt.

#### The Naked Scientists Forum

##### Re: Can I prove that I'm moving/accelerating?
« Reply #20 on: 29/10/2012 05:31:39 »