Philosophy of Physics Philosophy of Physics Forum - Philosophical questions about our universe

 Jul 17th 2015, 11:35 AM #11 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 552 "Two astronauts in space compress a lightweight spring. After using it to fling themselves apart, they measure and agree their relative speed. This speed means each astronaut calculates that the other's mass has increased by a millionth. But the rest mass of one was twice that of the other, so he gained twice the energy. How do they agree how much energy they put into the spring?" I am not sure what I am failing to understand about your problem statement but the energy stored in the spring will be U(x) = k*x^2/2. Anyone who can agree with the compression distance x of the spring will agree about the stored energy. Anyone with a different view of x, because of their relative intertial frame speed, will also have a different view of the astronauts final speed as the spring uncoils. I am not sure if you are saying that there is a paradoxical violation of the conservation of energy here?
Jul 17th 2015, 12:59 PM   #12
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 Originally Posted by kiwiheretic I am not sure if you are saying that there is a paradoxical violation of the conservation of energy here?
Yes there is a violation. The two astronauts and the observer can all initially agree on an amount of energy - whether the source is a spring, or a piston and cylinder filled with gas. After the known amount of energy has been released we need to look at the inertial frames of the three people. There is a single relative speed between the two moving astronauts. When plugged into the Lorentz/gamma equation this gives a single proportional increase in mass.

In special relativity of course, each astronaut can consider themselves to be at rest. When the lighter astronaut works out the mass/energy gain of the moving heavier one, the answer is twice the figure that the heavier one calculates when he observes the lighter one. The observer at rest calculates a third different figure. (Your dimension x is initially the same for everyone. Afterwards, the relaxed length of the spring will also be the same to an accuracy of about a millionth.)

As I initially said, I think there is a radically different alternative which can answer this and all other problems with special relativity.

Last edited by AndrewS; Jul 17th 2015 at 01:10 PM.

 Jul 17th 2015, 01:34 PM #13 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 552 I agree with you that special relativity is leading you to the wrong answer here. However, special relativity only applies where there is no acceleration. Both astronauts were accelerated by the spring force, while it uncoiled, and at different rates due to their differing mass. So only general relativity (not special) can give you an answer here and, yeah, general relativity is going to lead to non agreement of all observers and there seems to be no resolution to this. No one seems to be able to say whose observation is more "correct". So yea, you seem to have stumbled across a limitation of special relativity. Try looking for the youtube video by DrPhysicsA on the twin paradox. He explains it better than me.
 Jul 18th 2015, 12:48 AM #14 Member   Join Date: Jul 2015 Posts: 84 'No one seems to be able to say whose observation is more "correct".' Yet common sense tells us which is correct. It is only the principle of relativity that is causing the problems. The things that make sense about special relativity are E=mc^2 and it is (partly) correct about time dilation. But these can be derived in a more understandable way from other principles. Have you had time to look at the strobe paradox? It is not just the theory's incompatibility with the conservation laws that causes problems. You are of course right that the theory does not apply to accelerations. However, if you read the other documents (that can be found by omitting the Strobe file name) I hope you will agree that it is illogical to excuse special relativity on these grounds.
 Jul 18th 2015, 10:09 AM #15 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 552 I believe this youtube video is what is basically equivalent to your strobe paradox. The difference is it uses one flash instead of several. So yes, it does result in disagreements between observers. (Haven't had a chance to look at your other materials yet, hopefully with the next couple of days or so.)
Jul 18th 2015, 10:54 AM   #16
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 Originally Posted by kiwiheretic Try looking for the youtube video by DrPhysicsA on the twin paradox. He explains it better than me.
I watched DrPhysicsA's first video and yes he was very clear. I disagreed from the outset because of course he said that if two bodies are in relative motion, no experiment can determine anything about their speeds other than their relative motion.

This is where I fundamentally differ from physicists. I contend that if one moves quickly through space then the light from all the visible stars and galaxies in front, and from those behind, would show frequency shifts in their light. These would be consistent with one having absolute motion with respect to the observable universe and, by implication, with respect to everything. I cannot think of a better definition of absolute motion. Absolute motion is much easier to measure in the case of rotation.

The refusal to accept the idea of absolute motion leads to problems such as the spring paradox. We are supposed to live in different worlds where we cannot necessarily agree on the properties of objects - their size, energy, momentum and time rate. You cannot even agree with yourself. If you take measurements of a planet, star or galaxy then these properties are changing depending on the time of day and year. Relativity is inconsistent with the all-important conservation laws.

I think the strobe paradox shows that, even when acceleration does not intervene, relativity predicts that the same experiment will have two different outcomes, in effect a red shift and a blue shift. I wish you could find time read my documents so you can consider an alternative theory whereby energy and momentum are conserved.

 Jul 18th 2015, 12:20 PM #17 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 552 The red shift would occur whether you are moving away from the galaxy or if the galaxy is moving away from you. The problem is no one knows where the centre of the universe is so its hard to know which point of reference is stationary. How do we even find the centre of mass of the universe? Did you watch the "earbot" video? It was quite short and gives a good explanation about why light flashes can lead to paradoxes (ie your strobe problem).
 Jul 18th 2015, 01:10 PM #18 Member   Join Date: Jul 2015 Posts: 84 Suppose two observers are moving relative to one another, if one measures equal red shifts in all directions but the other measures a blue shift in one direction, this would be an experiment in which we could say that the latter has absolute motion. This would disprove relativity's assumption that the relative motion between observers is all we can know. In my view, we are each entitled to consider ourselves at the centre of mass of our visible universe. If the universe is infinite, there is no other answer.
Jul 19th 2015, 03:38 PM   #19

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 Originally Posted by AndrewS Suppose two observers are moving relative to one another, if one measures equal red shifts in all directions but the other measures a blue shift in one direction, this would be an experiment in which we could say that the latter has absolute motion. This would disprove relativity's assumption that the relative motion between observers is all we can know. In my view, we are each entitled to consider ourselves at the centre of mass of our visible universe. If the universe is infinite, there is no other answer.
I've been spending some time pondering the original question and the conversations after. I have two comments to make.

1. For the spring problem we need to use GR if we are not going to use Classical Mechanics because the energy scale involved is very small...We need to account for the relatively small gravitational fields of the astronauts in order to talk about their resultant velocities. But the correction is very very small. If you are going to say this is a paradox of some kind then you are talking about correcting a very small deviation from the GR results.

2. As far as the red and blue shifts are concerned if you want to talk about stars or galaxies you have to include them as part of your system...You are no longer simply considering two astronauts and a spring. It looks to me that you are walking down the same road as Ernst Mach. Mach's principle deals with how the inertia of an object is defined in terms of the distribution of distant masses. Mach's principle was shown to be false by GR (Einstein backed Mach's principle until it was shown to be incompatible with GR.) I don't have the proof in hand but I could look it up if you like. I believe I saw a reference to it in Weinberg's text.

Before you have the chance to say that I'm missing the point and that your astronaut and strobe paradoxes show that GR is incorrect, please understand that GR has been tested rigorously for over a 100 years and has passed each test. If GR is wrong then it is wrong only to a small degree, just as Newtonian gravity was debunked by GR only to a small degree. The principles of GR have to be mainly correct or the experiments would have shown otherwise. GR may indeed be wrong on a very small scale, but I doubt we have the ability to measure a correction term that small.

-Dan
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