# Sense of Absolute in Relativity?

1. Apr 1, 2012

This could be very basic question, nevertheless its troubling me
In Feynman's lectures, he says that though motion is relative, there is an absolute motion between 2 frames. A high velocity mu-meson when moved linearly or circularly would live just as longer than stationary one. Or a twin in a spaceship would age more slowly than his counterpart on earth. Here, its the spaceship that is moving, as to compare the results, he would have to stop, and thus experience acceleration. Thus even if we argue that motion is relative, its the frame which is actually in motion that experiences time dilation.
But what if there is scenario that in a blank empty space two objects pass each other. No one would know who is in motion. No one has option to stop either. So when each one measures distance and speed of other, what will determine the difference in measurments. Even the nature does not know who is in motion, so ideally both of them should have same measurments when tallied. But according to SR, both measurments should be different..What am I missing?..I am assuming its possible to tally results without stopping

2. Apr 1, 2012

### ghwellsjr

When you are quoting someone, don't you think it important to provide the volume and page number so we can see what you are talking about?

As to your question, anyone can pick any frame to nail down the definition of what speed means and therefore who is experiencing time dilation. So if you want, you can say that the first object is stationary and the second one is experiencing all the motion and therefore all the time dilation, or you could do it the other way around, or you could pick a frame in which they are both in equal motion in opposite speeds in which case their two clocks would be ticking at the same rate. Each frame has its own definition for time and distances. As long as you stick to any one frame, you'll have no problem.

3. Apr 1, 2012

### yuiop

Stop right there! In SR there is no such thing as the frame that is "actually in motion". What you are missing is that in all cases where two clocks are brought together and tangible differences in ageing are observed, acceleration is involved and the observer that accelerates is always aware that they are the one that has accelerated. Acceleration is absolute while relative velocity is not. Where two systems have constant inertial motion relative to each other, they both measure the other system to be ageing more slowly in a reciprocal fashion and it is impossible to determine which system is "actually" moving or ageing more slowly. In SR the interpretation is that both systems are correct in their assumption that clocks tick more slowly. However this requires that you accept a logic system that allows A>B and B>A to both be true at the same time. In Lorentz Ether Theory (LET) the interpretation is that that the system with the greatest velocity relative to the Ether is the one that is "actually" time dilating the greatest, but effects on clocks and rulers due to motion relative to the ether causes all observers in constant motion to measure reciprocal values and be completely unable to determine which system is actually moving relative to the ether. This inability to be able to measure the motion relative to the ether causes most people to prefer the SR interpretation that dispenses with the notion of the ether all together and accept the rather crazy logic of SR.

4. Apr 2, 2012

### harrylin

Sorry I don't believe you. Perhaps he said something such as that some motion can be considered "absolute".
No, not at all. If two clocks start together in straight uniform motion, and next one is accelerated away and brought back, then that one's "time" will be behind on the first one's. A change of velocity has "absolute" effects.
According to SR, both measurements should be the same.

5. Apr 3, 2012

Thnx for the answer. @yuiop, u say say that when 2 inertial frames are in relative motion, there is no exact way of knowing which frame is ageing more or less slowly. As Harrylin has stated both the frames should be measured equally. Thus its contradicting.
Another question is what makes acceleration of a frame an absolute thing?.What makes acceleraing frame to move slower in time?..If you could show the maths behind it it could help..thnx

6. Apr 3, 2012

### Staff: Mentor

People in both frames are aging at exactly the same rate, since all their clocks are of identical designs.
Each individual observer in the S frame encounters a sequence of observers in the S' frame, and the times he sees displayed on their clocks are increasing more rapidly than the time displayed on his own clock. He is encountering later and later versions of the people in the S' frame.
Each individual observer in the S' frame encounters a sequence of observers in the S frame, and the times he sees displayed on their clocks are increasing more rapidly than the time displayed on his own clock. He is encountering later and later versions of the people in the S frame.
Before you begin studying the phenomena occurring in accelerating frames (which can be regarded as a sort of transition toward general relativity), you need to first get a better working understanding of special relativity. Keep up the hard work.

Chet

7. Apr 3, 2012

### harrylin

He and I meant quite the same; the problem with such words as "different" or "equally" is that they can be easily misunderstood, without the math. Perhaps you meant with "different" exactly the same as what I meant with "the same"!
Here's what I meant: If the one measures that their relative speed is exactly10'000 km/s, the other one will also measure that; and each will record exactly the same distance as function of time as the other if their clocks read the same at the instant that they pass each other, and each will measure the other as aging more slowly. Consequently the situation is perfectly symmetrical, so that there is absolutely no way of detecting "who is truly aging more or less slowly".

Now, what did you mean with "both measurements should be different"? The starting point of SR is that one does not know who is in motion, so that both of them should have same measurements.
I doubt that the math can explain why the math is so... The essential point is that the usual math is with respect to non-accelerated reference systems. How would you put that in the math?

The first explanation was that this is due to motion relative to the ether, a more widespread explanation is that this is due to the properties of Spacetime; and the most common "explanation" is perhaps that "this is just how nature works".

8. Apr 3, 2012