Does Relative Motion make sense?

Question

If time slows down for a person in a spacecraft moving quickly away from Earth, does it also slow down for a person staying still while the Earth moves quickly away from them?

Answer 1

Yes it does, the effect is symmetric whilst the two observers do not accelerate relative to each other. It is true whether they are moving towards each other or away from each other. Also, the length of the spacecraft/Earth contracts relative to the other (by the same factor as the time dilation).

As Prof Zikzak rightly points out, these are real physical effects and not just a result of what one observer "sees" due to light having to travel an increasing distance from the other observer.

This can initially be confusing and can wrongly seem to lead to paradoxial results. The most famous example is the "Twin Paradox," in which one of two twins travels away from Earth at a high constant velocity, quickly accelerates back towards Earth, then travels at a constant velocity back to Earth. I'll go into this a bit, as it's apparent paradoxes like this that can make relative motion seem confusing.

For most of the journey, each twin is travelling with constant velocity relative to the other, and due to time dilation is ageing more slowly relative to each other. It may seem therefore that after the journey, each twin should find that the other one is younger than him.

However, there is no such paradox. The experiences of the two twins are not symmetric because of the acceleration of the travelling twin. However quickly it takes place, this period of acceleration is crucial. A rigorous analysis requires the use of "general relativity," ("special relativity" only deals with observers in constant motion relative to each other). But there are sometimes ad hoc ways to apply special relativity principles to these kinds of problems.

It turns out that the travelling twin is younger when they meet up after the journey. They both travel at the same speed relative to each other, but the journey is shorter to the travelling twin because the length between the Earth and the point to which he travels is contracted by his relative motion.

In the reference frame of the travelling twin, his twin on Earth ages quickly relative to himself during the period of acceleration. The effect of time contraction at this stage is greater than that due to the length dilation during the periods of constant motion, hence his twin is still older than him when he completes his journey.

Answer 2

This is the so-called twin paradox.

One twin flies off in a rocket and then returns to earth after say 20 years of flight time as measured by him on board the rocket.
At the same time, the other twin remains on earth.

For the twin left behind, it seems like the rocket has been gone 50 years (depending on the speed of the rocket).

But the twin in the rocket would have seen the other twin receeding at an equal speed in the opposite direction - so the 'paradox' is that the time effect should be symmetrical.

In fact, it has been show using atomic clocks on jets and satellites, that there is a real effect due to this type of experiment. The effect is asymmetrical and the clock on the jet has indeed run slower.

To resolve the paradox, the effect due to acceleration needs to be considered. The twin in the rocket has experienced different acceleration - he has to stop and turn around. The twin that remains at home has not experienced this acceleration.

General relativity is needed to correctly calculate the effects of the acceleration involved. It shows that the acceleration is the real contributory factor to produce the twin paradox.

Answer 3

This is NOT the twin paradox. Do not be confused.

Motion is indeed relative. What Special Relativity showed was that, because the speed of light is the same for everyone, then time must also be relative. There is no sense of absolute time.

So you will always experience time at the same rate, but you will measure the passage of time of someone moving with respect to you differently.

In the case you cite - which is symmetric - then the person on the spacecraft will view your clock as moving slower than his, and you will view his clock as moving slower than yours. You cannot choose one frame over another - that is the point of relativity.

Answer 4

Any inertial observer who sees a clock moving relative to their own rest frame observes it to run slow. So an observer on Earth watching a spacecraft go by observes the spacecraft's clock to run slow. Similarly, the astronaut on the spacecraft, so long as it drifts at constant speed, is entitled to consider herself at rest, and as she watches the earth speed by her, she observes its clocks running slow.

This is more than mere apperances. When a Relativist says "observes," apperances have been accounted for. We're not talking about light-travel times or anything like that. We're talking about two observers who really do measure time differently. When the astronaut in the spacecraft measures that he gets to Alpha Centauri in 37 seconds, it's because the light-years of distance *really did* contract to a distance easily covered in a few seconds.

Answer 5

Time does not slow down for a person in a spacecraft, nor does it slow down for a person staying still. For each person, time would APPEAR to slow down for the other (if it could be observed in the first place, which it can't). To further confuse the issue, note that direction is immaterial. That should give one a clue.

Answer 6

As I understand it, time slows time when you are at high energy, which is close to the earth or at high speeds. If the earth shot off, time would appear to slow down on earth from the perspective of a statioanry observer in space.

Answer 7

I don't think so... Isn't relative motion the return force caused by another original force? eg: pushing a boat away from a boat... cause and consequence?

Time only slows for the person in the spacecraft becasue they are the one causing force... I think

Answer 8

Time is an era between activities, and localized reference frames transferring relative to a minimum of one yet another will diploma that era in a diverse way while the relative velocity between them techniques that of sunshine. there's no prefered or absolute reference physique, it incredibly is why mild is a similar velocity for all inertial reference frames. while cosmologists estimate the age of the universe, thay are making use of education it incredibly is carried by ability of mild to make their estimates, and the cost of sunshine is a similar for all inertial frames. So it truly is not substantial the place you're.

Answer 9

just because you don't understand it, doesn't mean it doesn't make sense.