# Time dilation and the reference frame of the vacuum

• B
• danb
In summary, the twin paradox in special relativity states that the traveling twin experiences a real, frame-invariant effect in which the time evolution of all moving matter is slowed down. However, there is no consensus on the physical mechanism that causes this time dilation, and the principle of relativity rules out absolute motion through the vacuum as part of the mechanism. The only aspect of the traveler's motion that could initiate a physical mechanism is its absolute velocity through the vacuum, leading to the conclusion that the vacuum must have its own reference frame. This raises questions about physical causation in the universe and the validity of the principle of relativity.
danb
TL;DR Summary
The vacuum MUST have its own reference frame in order for the frame-invariant time dilation in the twin paradox to have a physical cause.
The point of the twin paradox in special relativity is that the traveling twin experiences a real, frame-invariant effect in which the time evolution of all moving matter is slowed down. If you read a hundred articles and textbooks on SR, you'll see a hundred variations on the message that the symmetry of the paradox is broken by the fact that the traveler passes through two or more reference frames, either by accelerating or by having two moving observers who synchronize clocks when they pass each other.

However, the number of articles that explain what physical mechanism causes the time dilation is exactly zero. Even worse, the relativistic principle that there's no preferred reference frame explicitly rules out absolute motion through the vacuum as part of the mechanism. My question is this: How can that principle possibly be true? It's literally inconsistent with physical determinism.

The causal ordering of events in the traveler's voyage can be summarized as follows:
1. The traveler moves.
2. Some unknown quantum effect occurs.
3. The traveler's time evolution is slowed down.
But what aspect of the traveler's motion is involved in step 1? Acceleration can be ruled out by the three-observer experiment (time dilation without acceleration) and a quick trip to the Sun and back (acceleration with not more than about 16 minutes of total time lag).

It's inconceivable that motion relative to Earth could be part of any causal mechanism. That would be like saying you can make your car accelerate by moving your foot through the air relative to the gas pedal. It would imply action at a distance with one random object out of all the objects in the spaceship's vicinity, which which it has an equal number of other relative velocities.

So what's left? There's only one aspect of the traveler's motion that can initiate a physical mechanism to slow down the traveler's time evolution: its absolute velocity through the vacuum. The vacuum MUST have its own reference frame. If it doesn't, there's no information available to determine what the rate of time dilation should be. What other explanation can there be?

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danb said:
Summary:: The vacuum MUST have its own reference frame in order for the frame-invariant time dilation in the twin paradox to have a physical cause.

The point of the twin paradox in special relativity is that the traveling twin experiences a real, frame-invariant effect in which the time evolution of all moving matter is slowed down.

No. the point of the twin paradox is that the "traveling" twin experiences less total accumulated time than the "stay at home" twin does. It never claims that the traveling twin's time slows down in an absolute sense.
According the stay at home twin, this difference is due to the traveler aging slowly during phases of trip.
According to the traveler, it is distance between his twin and the turn around point that undergoes length contraction, thus shortening the time spent between the two. He will also say that his twin aged slower than he did during both legs of the trip, but aged very quickly during the turn around. It is this period of rapid aging that accounts for his stay at home twin to be older when they meet up again.
The point is that neither twin's view of how the age difference came to be is any more valid than the others.

Special Relativity is not about how movement physically effects clocks and other processes. It is how the measurement of time and space differs between reference frame which have a relative motion with respect to each other.

the point of the twin paradox is that the "traveling" twin experiences less total accumulated time than the "stay at home" twin does. It never claims that the traveling twin's time slows down in an absolute sense.
Something must have happened in an absolute sense. The twins begin and end the experiment in the same place at the same time, and in the same reference frame. That's a real, objective result, and therefore it must have had a real, objective cause.
The point is that neither twin's view of how the age difference came to be is any more valid than the others.
I'm not interested in whose view is valid. I'm interested in what made one of the twins physically younger than the other.
Special Relativity is not about how movement physically effects clocks and other processes. It is how the measurement of time and space differs between reference frame which have a relative motion with respect to each other.
I'm not asking a question about relativity. I'm asking a fundamental question about physical causation in the universe. That's why I posted the question in the Quantum Interpretations and Foundations forum. I can accept the probabilistic nature of quantum mechanics, because it's at least consistent with some kind of deterministic underlying reality, in analogy with Brownian motion. It's more complicated, of course, but it still allows for the possibility of explaining the apparent low-dimensional nondeterminism of a "particle" with the high-dimensional determinism of some distributed, wavelike phenomenon.

But relativity is worse than that. Something physically happened to the traveling twin (OR the earthbound twin, but not both), and something must have physically caused that effect. There's only one aspect of the twins' motions that could have activated any physical mechanism to implement that effect, and it's inconsistent with the principle of relativity. I think the idea of frame-invariant time dilation without a preferred frame is magical thinking. I don't think it's physically possible.

Moderator's note: Moved thread to relativity forum.

danb said:
I think the idea of frame-invariant time dilation without a preferred frame is magical thinking. I don't think it's physically possible.

This forum is not for personal opinions or personal speculation. Relativity has no preferred frame, and its prediction of differential aging for objects that follow different paths through spacetime is well confirmed by experiment. So you're just going to have to learn to deal.

I am tempted to close the thread at this point since the attitude you are expressing in the above quote is not going to lead to any productive discussion. However, there are some other points in your post the response to which might give you food for thought, so I'll follow up on those with a separate post. Bear in mind, though, that assertions that relativity, a theory with no preferred frame whose predictions, including those of differential aging, are very well confirmed by experiment, is not "physically possible" are not going to get you anywhere.

danb said:
Something must have happened in an absolute sense.

Something did: two twins followed different worldlines through spacetime, and those different worldlines had different path lengths. It's no different than two people taking different routes from New York to Los Angeles finding different readings on their odometers when they meet up at the end of the trip.

danb said:
The twins begin and end the experiment in the same place

Yes.

danb said:
at the same time

No. Their clocks show different elapsed times. There is no "same time" in any absolute sense.

danb said:
in the same reference frame

There is no such thing. All objects are always "in" all reference frames, and no reference frame is preferred over any other.

danb said:
I'm interested in what made one of the twins physically younger than the other.

The fact that they followed different paths through spacetime that have different lengths. That's an objective physical fact that entirely explains the differential aging of the twins.

danb said:

Yes, you are, since relativity is the physical theory we have that explains the phenomenon you are asking about.

danb said:

Sure, but if you want the physics explanation of "physical causation" in the particular case you are asking about, you have to go to the physical theory that provides that explanation.

danb said:
That's why I posted the question in the Quantum Interpretations and Foundations forum.

And the above is why I moved it to the relativity forum. Nothing you are asking about has anything to do with QM or QFT at all, let alone quantum foundations or interpretations. It's straightforward special relativity.

Relativity has no preferred frame
To repeat what I said in my previous post:
I'm not asking a question about relativity. I'm asking a fundamental question about physical causation in the universe. That's why I posted the question in the Quantum Interpretations and Foundations forum.

assertions that relativity, a theory with no preferred frame whose predictions, including those of differential aging, are very well confirmed by experiment, is not "physically possible" are not going to get you anywhere.
Assertions of relativity are very well confirmed by all experiments to date. I agree with that, I never contradicted it, and I have no intention of doing so. However, the principle of physical determinism requires that there be some causal pathway from the motion of the two twins to the difference in their ages.

To rephrase the question I asked in my first post: How can the physical age difference between the two twins have a physical cause without a mechanism that responds to their absolute velocities in some universal reference frame? Again, it's not a question about relativity. I know what relativity says about the phenomenon.

danb said:
What other explanation can there be?
The geometrical explanation.

danb said:
How can the physical age difference between the two twins have a physical cause without a mechanism that responds to their absolute velocities in some universal reference frame?
The geometry is that mechanism and does not require a universal frame

danb said:
the principle of physical determinism requires that there be some causal pathway from the motion of the two twins to the difference in their ages.

There is: the different path lengths of their paths through spacetime.

danb said:
know what relativity says about the phenomenon.

Then why don't you accept it? Relativity gives you a perfectly good explanation that does not require a preferred frame.

danb said:
How can the physical age difference between the two twins have a physical cause without a mechanism that responds to their absolute velocities in some universal reference frame?
The same way that when two cars drive different routes of different lengths between the same two cities, their odometers wil lread differently at the destination. A clock measures distance through spacetime the same way that an odometer measures distance through space, and in the twin paradox the two travellers have taken different paths through spacetime.

two twins followed different worldlines through spacetime, and those different worldlines had different path lengths.
In classical physics, the shortest path between two points is a straight line. Are you saying the twin who went to Alpha Centauri and back took a shorter route than the twin who stayed on Earth?
There is no "same time" in any absolute sense.
All objects are always "in" all reference frames
The reunion of the twins is a single event. My point was that the measurement of their age difference isn't an artifact of measurements made when they're at different points in spacetime, or by observers in different reference frames.
The fact that they followed different paths through spacetime that have different lengths. That's an objective physical fact
HA Lorentz disagreed. His theory of the vacuum made the same predictions without reference to spacetime.
relativity is the physical theory we have that explains the phenomenon you are asking about.
if you want the physics explanation of "physical causation" in the particular case you are asking about, you have to go to the physical theory that provides that explanation.
Time evolution of matter is a quantum-mechanical process. It occurs in the absence of gravity. It occurs in the absence of motion. Its ultimate mechanism has zero inherent connection with relativity, except that the formulas of relativity provide a way to calculate the degree to which the phenomenon slows down under certain circumstances.
Nothing you are asking about has anything to do with QM or QFT at all, let alone quantum foundations or interpretations. It's straightforward special relativity.
Gauge theory gravity makes the same predictions as GR without curved spacetime, and Lorentz's ether theory makes the same predictions as SR without the notion of spacetime. There's no proof that curvature of space is a physical reality. There's no proof that the vacuum doesn't have a reference frame, only that it hasn't been detected yet.

danb said:
In classical physics, the shortest path between two points is a straight line. Are you saying the twin who went to Alpha Centauri and back took a shorter route than the twin who stayed on Earth?
Yes, exactly. The difference is the metric. The Euclidean metric is ##ds^2=dx^2+dy^2+dz^2## so the shortest path is a straight line. In contrast the metric in spacetime is ##d\tau^2=dt^2-dx^2-dy^2-dz^2## so a straight line is the longest path.

danb said:
There's no proof that curvature of space is a physical reality
There certainly is. Spacetime curvature is tidal gravity and tidal gravity is experimentally well established.

danb said:
Summary:: The vacuum MUST have its own reference frame in order for the frame-invariant time dilation in the twin paradox to have a physical cause.
I am curious about why you don’t consider the two postulates to constitute a physical cause.

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petecc and PeroK
danb said:
The reunion of the twins is a single event.

A single point in spacetime, yes, but that doesn't mean it has any absolute "time" attached to it.

danb said:
My point was that the measurement of their age difference isn't an artifact of measurements made when they're at different points in spacetime

Yes, that's correct.

danb said:
Are you saying the twin who went to Alpha Centauri and back took a shorter route than the twin who stayed on Earth?
A shorter path through spacetime, yes.

danb said:
The reunion of the twins is a single event. My point was that the measurement of their age difference isn't an artifact of measurements made when they're at different points in spacetime, or by observers in different reference frames.
You are confusing the phenomenon with the phenomenology. The phenomenon is that when the clocks are compared at the reunion, less time has elapsed on the clock of the traveling twin.

It's the explanation of this phenomenon that can involve measurements made when they're at different locations.

You are claiming that the explanation necessarily involves a preferred frame of reference, and that is not the case. You have even gone on to point out that such explanations exist, and indeed they do, but the point is that those explanations are not necessary. There are a number of explanations that do not involve a preferred reference frame.

It seems you lament the lack of a physical explanation of time dilation? It's a consequence of the existence of an invariant speed. But then you could go a layer deeper and lament the lack of a physical explanation of that. And so on, ad infinitum.

danb said:
In classical physics, the shortest path between two points is a straight line. Are you saying the twin who went to Alpha Centauri and back took a shorter route than the twin who stayed on Earth?

As others have pointed out, yes, absolutely they did. If you get that the 'interval' or 'proper time' (the time as measured by the observer taking the route) between two events in space-time is like distance between two points (via different routes) in Euclidean space, then you're half way there.

The twin 'paradox' is like a triangle in space-time, with the traveling twin going the 'long' way between two of the corners (via the third). If space-time was Euclidean, then the traveling twin would arrive back older than the stay at home twin. The difference is just that the geometry of space-time isn't Euclidean, so the traveling twin actually took the shorter route.

Just like distances between points in space don't depend on the particular choice of coordinates, the interval or proper time doesn't depend on the frame of reference used. A triangle with given 'distances' between its corners, is still a triangle with those distances in any frame of reference.

Nugatory, vanhees71, Dale and 1 other person
Sometimes physicists take their equations too seriously. The metric is just a function that helps in quantifying various consequences of motion and gravity. When you say the metric makes the path through spacetime "shorter", that's just a fancy way of saying that less time elapses for the traveler. So, essentially, the relativistic "explanation" of why less time elapses for the traveler is ... less time elapses for the traveler.

Saying that relativity "explains" time dilation is like saying you can "explain" how a clock works by pointing out that the short hand goes around in a circle twice a day and the long hand goes around twelve times every time the short hand goes around once. That doesn't explain why the hands move as they do, it only describes how much they move. An explanation would involve describing the gears and motor and timing device behind the clock face that keep the hands moving.

As for lamenting a lack of infinite explanations, that's a classic perfectionist's argument. The perfect is the enemy of the good. An explanation of a phenomenon at any given level of abstraction is a description of whatever mechanism at a lower level of abstraction controls the phenomenon in question. If the relativistic "explanation" of time dilation is that zooming around the galaxy at high speeds is a "shorter" path between two events than just staying put, I'm sorry, but I just can't take that seriously. Gauge theory gravity shows that spacetime is NOT a proven fact.

And again, I want to repeat that I don't really think a relativity forum is the right place for my question. I know vaguely what relativity has to say about my question, and I'm not really interested in that. I think there's a question of fundamental physics that has more to do with the quantum-mechanical nature of the vacuum. Not trying to keep the argument going, but a couple people made new posts this morning.

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weirdoguy
danb said:
If the relativistic "explanation" of time dilation is that zooming around the galaxy at high speeds is a "shorter" path between two events than just staying put, I'm sorry, but I just can't take that seriously.
That is your choice, but it does not imply any deficiency in the theory. The geometric explanation satisfies all of your listed objections even if you refuse to take it seriously.

Motore, vanhees71, weirdoguy and 2 others

## 1. What is time dilation?

Time dilation is a phenomenon in which time appears to pass at different rates for objects that are moving at different speeds relative to each other. This is a consequence of Einstein's theory of relativity and is caused by differences in the reference frames of the objects.

## 2. How does time dilation occur?

Time dilation occurs because the speed of light is constant in all reference frames. This means that as an object moves faster, time appears to slow down for that object relative to a stationary observer. This effect becomes more significant as the speed of the moving object approaches the speed of light.

## 3. What is the reference frame of the vacuum?

The reference frame of the vacuum is a theoretical concept used in physics to describe the properties of empty space. It is a reference frame in which there is no matter or energy present, and all objects are considered to be at rest. This reference frame is used to study the effects of time dilation and other phenomena in the absence of external forces.

## 4. How does the reference frame of the vacuum affect time dilation?

The reference frame of the vacuum is important in understanding time dilation because it is the frame in which the speed of light is constant. This means that any differences in the perceived passage of time between two objects in different frames can be attributed to their relative speeds, rather than any external factors. This allows scientists to accurately measure and predict the effects of time dilation.

## 5. Can time dilation be observed in everyday life?

Yes, time dilation can be observed in everyday life, although the effects are usually very small. For example, the Global Positioning System (GPS) satellites in orbit experience time dilation due to their high speeds, which must be accounted for in order for the system to accurately calculate positions on Earth. However, for most everyday situations, the effects of time dilation are too small to be noticeable.

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