# Universe frame of reference

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1. May 24, 2015

### Stephanus

Dear PF Forum,
Sorry if I ask again, because I want a detailed information.
Does the universe has a frame of reference?

Two twins. A stays, B "travels". (Travel here is confusing, B will say I stay and A travels) and B "turns around" and come back.
B ages more slowly then A.

Q1: Why?
A1:
Because B turns around.
Q2: But in B point of view, isn't A who travels to him?
A2: No, because B accelerates.
Q3: So, the difference between A and B is that B feels the acceleration?
A3: Yes
Q4: Supposed, B sling shot a black hole and come back, doesn't he feel the acceleration?
A4: No, he doesn't feel.
Q5: Both doesn't feel acceleration, does B ages slowlier than A?
A5: Yes, because B changes its inertia frame of reference.
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My question here.
1. How can every atoms, energies and time in B know that they are changing their inertia frame of reference?
Because in Qestion number 2, it might be that A who travels to B in B point of view, because B doesn't feel acceleration.
2. Does the universe know that B is changing its inertial frame of reference? Does the universe "slow down" B clock?
3. Does atoms and energies and time in B know such as you travel in a train with a closed window?
So you have no idea that you're moving or not. But by looking at your GPS you know that you are moving

2. May 24, 2015

### Simon Bridge

No.

They are all being acted on by an unbalanced force.

The Universe knows about time dilation in the same way the Universe knows about perspective and parallax. It's an effect of geometry.
However, it is unclear what you mean by "does the Universe know" something.

This question is incomplete - do they know what?

Someone could be spoofing the GPS signal: how would you know?
The idea is that there is no experiment you can conduct to discover an absolute state of uniform motion - but you can conduct experiments to discover changes in your motion. This is just Galilean relativity. There is no such thing as absolute motion.

Note: a gravitational system (where you experience a gravitational force) is not an inertial system.

Last edited: May 24, 2015
3. May 24, 2015

### Stephanus

Hi Simon, thanks again for your answer.
I mean how do the clock (and time and atom and energy, all the property of the universe) knows that they are changing inertial frame of reference, although they sling shot so, the wouldn't feel acceleration. And time in B slows down.

What do you mean by unbalanced force here? Acceleration?

4. May 24, 2015

### PWiz

[Simon beat me to it this time, hehe]
Firstly, there is no universal / preferred frame of reference. This is what Gallileo said and this is what is universally (pardon the pun) accepted.

Secondly, the twin paradox no longer remains a symmetric problem when one twin turns around. This is because it is impossible to "turn" without accelerating. Think of it as "switching" inertial frames instantaneously ( for more, read about MCRFs). So when one of the twins has turned and finally settled into a new inertial frame, this frame's worldline does not coincidence with twin A's worldline at the same "spot" ( event really) as the frame in which twin B left, and this results in an aging discrepancy. This is best understood by drawing a spacetime diagram. (Sorry for the loose language here)

Secondly, (proper) acceleration can be felt by the body that undergoes it - acceleration is absolute. If a spacecraft approaches you with an increasing velocity, you will be able to tell what is actually accelerating, depending upon whether you feel some "force" or not (ignoring gravitational forces for simplicity here).
Again this is not an SR issue, but something Newton taught us hundreds of years before Einstein.

GR comes in when you want to distinguish whether a force that you feel is arising due to being in a gravitational potential well or due to being subjected to proper acceleration, and it can get a little a messy as you try to find a coordinate transformation which can reduce your spacetime metric tensor to the trivial Minkowski one. But the point is, you will be able to tell if you're accelerating or not using some calculations.

Finally, try not to think of the universe as a living thing - for scientific purposes, explanations become difficult (although it's perfectly fine from a philosophical point of view, which is not discussed here).

5. May 24, 2015

### Simon Bridge

... when you look at a distant building, it appears smaller (the effect is called "perspective"), how does the building know how small to look? The sum of the internal angles of (euclidean) triangles is 180deg ... how do they know to do that?
This is the same way that the clocks know to run slow etc.
You don't think that objects looking smaller with distance is remarkable because you are used to it.

Note: where did you get the idea that a slingshot does not involve acceleration (or the feeling of acceleration)?

I mean if you get hit by a car, you know you were hit by a car right? You understand why you do don't you? The car exerted a force on your body and it probably hurt. There is no question that you experienced a force that someone standing off to the side didn't.

The accelerated twin knows they are the accelerated twin because they felt the force that caused the acceleration.
In the usual description the turn-around is very fast compared with the overall journey so the force would have been quite big. When the twins meet up, one of them would say, "Gosh I felt quite a kick back there!" ...

[By "feel a force" and "experience an acceleration" I do not mean that you have to be conscious of the force ... it could be too small for you to register it but it still happens.]

6. May 24, 2015

### Stephanus

Thanks PWiz for the answer.

No, of course not. It's just that how I phrase my question.

7. May 24, 2015

### Staff: Mentor

The basic question of the twin paradox is one of symmetry. How can we tell which one gets the time dilation given the apparent symmetry between the two. In the usual SR case the symmetry is broken by the proper acceleration. In the gravitational turn around there is no proper acceleration, but the symmetry is broken by the curvature.

Is both cases the thing that breaks the symmetry doesn't by itself tell you quantitatively the amount of time measured by each twin. It just tells you that they may not be equal. In all cases the time is given by evaluating the metric, g, along the worldline, P, in any coordinates you choose, dx.
$$\tau=\int_P \sqrt{g_{\mu \nu}dx^{\mu}dx^{\nu}}$$