Same thing. See the example above with the sun. Let's say it flashes 10 times a minute (every 6 seconds).
Without relativity, the trip takes 18 minutes, so the Earth observer sees 180 flashes in 9 minutes, twice the actual rate. Relativity says the fast-moving clock is dilated and only flashes 156 times (my guess was off) in those 18 Earth minutes. Still appears faster to the Earth observer. At 0.5c, the Doppler effect doubles the pace, and relativity removes only about a sixth of the pace. — noAxioms

Pretty sure this is wrong. Doppler changes wavelength/frequency of light. Relativity changes rate of time. Unrelated.

Pretty sure this is wrong. Doppler changes wavelength/frequency of light. Relativity changes rate of time. Unrelated. — T Clark

Go through the example, ignoring relativity or not.

Does the half-lightspeed pulse generator not take 18 minutes (in Earth frame) to travel 9 light minutes? Does the first pulse not take 9 minutes to be seen, and the last not get seen immediately? What part isn't clear?

I browsed this thread rather quickly. It seems not to have occurred to many participants that the Lorentz transformations from which time dilation, space contraction, and the relativity of simultaneity all derive are transformation formulas that apply to *inertial* frames of reference only. An inertial frame of reference is a frame of reference within which test masses that aren't subjected to any external force move in straight lines at constant speed. The effects from gravity are neglected in the framework of Special Relativity.

The twin paradox that was alluded to earlier arises when one neglects that the traveler twin can't be considered at rest in one single inertial frame of reference if she is going to eventually turn around and come back to Earth. The situation must therefore be analysed with respect to at least three frames of reference: one in which the Earth (and the twin remaining at home) remains at rest at all times, a second one in which the traveling twin is at rest during the outward journey, and a third one in which she is at rest during the returning journey. (We can imagine, for simplicity, that the acceleration period when the travelling twin reverses course is very short).

Considering the mutual Lorentz transformations of time and space coordinates between those three frames of reference solves the paradox through highlighting the asymmetry between the situations of the two twins. The traveling twin must effectively "jump" from one frame of reference to another one between the two legs of her trip, and at that time (as measured by her local clock) the definition of simultaneity with the corresponding time on Earth shifts. This shift accounts for the fact that her local clock marks a shorter duration than the clock on Earth over the duration of the whole trip in spite of the fact that, as referred to her own reference frames (on both legs of the trip) less time elapse on Earth that does on the ship on each separate leg of the trip.

Well, yes, the Earth is always accelerating - it is revolving around the sun, which involves acceleration. — T Clark

Which one is slowing down? The sun or the Earth. Which is accelerating relative to the other.

I understand this time dilation stuff had been around for a long time and tickles the fancy of people who enjoy the Sci Fi aspect, but it is so full of holes in so many respects. The reason there is isn't more analysis of the problem is because it had no practical relevance so the fantasy story continues.

What is important is the t time in STR is not the t in GTR. There is no t in GTR.

Effects on acceleration on clocks and measurements had nothing to do with the biological sense of time and duration passing. This is a huge confusion.

As for acceleration itself, the equations do not provide and means for establishing what is accelerating. I am accelerating as I write this but I don't feel it. Feeling acceleration is not part of the equations.

Anyone who is interested in this question is going to have to do a ton of research but I am convinced it is a Sci Fi story, no more.

Anyone who is interested in this question is going to have to do a ton of research but I am convinced it is a Sci Fi story, no more. — Rich

Nothing more to say.

The Earth is accelerating — Rich

we will make the spacecraft with propulsion. it is needed in order for the spacecraft to leave earth. when you create something with the intention of acceleration then it will accelerate. yes i understand that from each viewpoint both seem to be moving away from each other but the spacecraft has been built in order to accelerate. the only acceleration that earth is experiencing is its orbit around the sun and that wont change when the spacecraft leaves.

There is no variable in the equations about "experiencing" acceleration.

Two measurements, and from each point of view, something is accelerating.

How we ever got from a theory about gravitation to two vilification entities aging at different rates, I have no idea. It would be a great research project to find out who started this all. I know why it it to perpetuated. It sells books just like Black Holes.

well isn't saying that anything could be accelerating depending on the viewpoint just as silly? it is when you put it against actual propulsion systems in a spacecraft that allow it to move forward. i understand that the time stuff is weird and it may very well amount to nothing so lets ignore it (i was merely exploring the topic in my original post after doing some research on time dilation).

your acceleration point still doesn't make sense.

Two measurements, and from each point of view, something is accelerating. — Rich

yes that something is the machine that has been built to accelerate. if you dismiss the idea of having your own perspective of time depending on gravity and velocity, then why aren't you so quick to dismiss having your own perceptive of acceleration.

well isn't saying that anything could be accelerating depending on the viewpoint just as silly? — David Solman

Look at the equation.

Do you feel propulsion when sleeping? You are still accelerating.

what you're suggesting is that the spacecraft doesn't move and that for your whole trip on the spacecraft, the whole universe moves around you? come on. that sounds as ridiculous as time dilation.

I'm suggesting that STR and GTR are a complete mess. But that doesn't v keep people from concocting stories about time travel and the sorts. I wish people spent more time truly digging into stuff rather than spin tall tales. GTR is a to theory about gravity. There is no t in GTR.

Look at the equation. — Rich

Yes, you should do this. It is covered in 7th grade physics. F=MA or A=F/M which still works even under relativity.
The force on Earth is primarily the weak gravitational fields of the moon and sun, which actually cancel out over a month/year respectively. Net vector of almost zero. The space craft has much greater acceleration due to orders-of-magnitude greater force to mass ratio, and it is applied in a consistent vector (at least until mid-trip where it turns around), unlike the gravity.

You describe the ship/earth as symmetrical cases, each moving away from the other's point of view, but that isn't acceleration. If it was a symmetrical situation, one twin would not be older when they are reunited. The difference is acceleration, or more precisely, the moment of acceleration. Both twins can accelerate equally (for control purposes if you like), but the one with the greater moment will be younger when the reunite.

Thanks for the science lesson. Had nothing to do with GTR.

Had nothing to do with GTR. — Rich

That's right. It has to do with knowing what acceleration is.
You're confusing it with velocity. Yes, in the frame of the fast moving ship, Earth has the high velocity, but from any point of view, it is the ship, or the frame of the ship, that does the significant accelerating.
Just throwing GTR term around or labeling as science fiction the description of others isn't helping.

Gravity plays a trivial role in the scenario being discussed. We're not computing it to 7+ digits here. You only need to do that if you're actually writing the navigation software that needs to get the ship to its target.

You're confusing it with velocity — noAxioms

GTR has zero relevance to biological systems. It's about the problems of measurement. Every body in the Universe is accelerating. There is no privileged frame of reference. There is no t in GTR. It is some manufactured space-time thing. End of story.

Nothing more to say. — T Clark

I hear you.

GTR has zero relevance to biological systems. It's about the problems of measurement. — Rich

There isn't a problem with measurement. Precisely-measured differences in clock tick rates have been observed that are consistent with the predictions of special and general relativity.

The Hafele–Keating experiment was a test of the theory of relativity. In October 1971, Joseph C. Hafele, a physicist, and Richard E. Keating, an astronomer, took four cesium-beam atomic clocks aboard commercial airliners. They flew twice around the world, first eastward, then westward, and compared the clocks against others that remained at the United States Naval Observatory. When reunited, the three sets of clocks were found to disagree with one another, and their differences were consistent with the predictions of special and general relativity. — The Hafele–Keating experiment

There isn't a problem with measurement. Precisely-measured differences in clock tick rates have been observed that are consistent with the predictions of special and general relativity. — Andrew M

The problem was and continues to be determining simultaneity. This was the root issue. The answer that was present by STR is bizarre. There is a privilege reference frame. It is the one where the train is hitting the car. But I don't want to get into all of the messiness of Relativity. Einstein didn't receive a Nobel prize for it and for good reason. He actually tried to elevate it to ontology for heaven sakes. Clocks do not measure human biological aging (real time), they measure space traversal with the purpose to establish order tsimultaneity - a completely different story. The sci fi book writers who wrote about relatively are way ahead of themselves.

This thread is a sad indictment of the state of science education.

The reason no one understands Relativity is because it is a mess and the paradoxes it creates is evidence of this. But to elevate to some equations into some ontology is absolutely insane, which pretty much describes the hubris of science and its priests.

Why did you assume I as talking about you?

Relativity is about RELATIVITY. It's not about time travel or how clocks control the biological aging process. It's amazing how people get caught up in their own stories. The making of myths.

Relativity is about RELATIVITY — Rich

Actually, I remember reading that Einstein had come to have some misgivings about his theory being named "Theory of Relativity". He thought this was unfortunate and had a tendency to give rise to misleading interpretations. That's because there is much more of an emphasis, in the so-called theory of "relativity", about *invariants*. The speed of light, for instance, in an invariant. Spatial distances between events, or the speeds of point particles, already had been regarded to be relative to a frame of reference in the framework of classical mechanics, but the frame relativity of (instantaneous) distance, duration, and simultaneity all are direct consequences of the invariant geometry of Lorentzian space-time (in special relativity) and of the invariant intrinsic curvature of space-time represented by the metric tensor (in general relativity).

When Rich says that there it no 't' in GR, I think he means to say that there is no time dependence of the invariant structure of the metric tensor (that is, of the geometry of space-time). That is true in a sense. It's just because space-time incorporates the temporal dimension and hence does not, as a whole, have a variable geometry as a function of time. This would just not make sense since there isn't a temporal dimension external to space-time. However, from an empirical standpoint, for finite creatures such a us who live in a particular moment in time and are interested in making predictions about the future (and explaining the past) the equations of General Relativity can be put to use operationally to factor out the time dimension relative to a specific local frame of reference relevant to us and predict how strong the gravitational field will be in each point of space relative to this specific frame of reference, and how it will vary as a function of time (as measured by local stationary clocks).

Not necessarily. Because of Special Theory of Relativity's Receprocity one can say that Earth is accelerating away from the spaceship, so it is the clocks in the Earth that are slowing down. — Rich

No. There's no such principle. The spaceship's engines are accelerating the spaceship, not the Earth.

Time dilation has been observed in various ways. It has been observed that less time transpired in the system that was accelerated to high speed, in comparison to an unaccelerated system. ...when their clocks were compared later.

Shouldn't we leave physics to the physicists?

We live in a culture in which science is important and highly valued. Therefore, there's a strong tendency to worship science. That's our culture's main religion now.

As for interstellar travel, if our civilization ever achieves fast interstellar travel, we'll probably have good enough robotic technology by then, that there won't be a need for humans to be on the starship.

The robotic probes will, by that time, be able to find out all that we want to know from visits to other solar-systems.

Of course it will take a long time, and so, as you said, it would have to be done as a longterm investment, (if at all).

I don't believe that just because something becomes technically possible it should be done.

Michael Ossipoff

spaceship's engines are accelerating the spaceship, — Michael Ossipoff

You won't find "Spaceship engines" as a variable in any Relativity equations.

The rest is literally Sci Fi.

You won't find "Spaceship engines" as a variable in any Relativity equations. — Rich

What about the F? Sounds pretty much like a variable to me.

I'd said;

"The spaceship's engines are accelerating the spaceship [not the Earth]",

Rich says:

You won't find "Spaceship engines" as a variable in any Relativity equations. — Rich

Regardless of what's accelerating the spaceship, it's being accelerated to (and then back from) high speed with respect to the Earth. It's the spaceship, and not the Earth that's being accelerated.

When an equation in physics includes a force or an acceleration, the equation doesn't typically specify or label the origin of the force, or the name of the thing that caused the force.

Rich says:

The rest is literally Sci Fi.

I would remind Rich that interstellar travel was the topic of the thread's original post.

Maybe a better example of science fiction is Rich's incredible mis-statements about physics.

Rich is referring to this:

As for interstellar travel, if our civilization ever achieves fast interstellar travel, we'll probably have good enough robotic technology by then, that there won't be a need for humans to be on the starship. — Michael Ossipoff

Prominent computer scientists predict advances in robotics, on a timescale shorter than the timescale predicted by phyicists for feasible interstellar travel.

A long time ago, the radio astronomer, Bracewell, discussed the obvious likely eventual feasibility of robotic interstellar probes.

Physicists say that interstellar vehicles are still very far off. That statement, too, isn't science-fiction.

I continued:

The robotic probes will, by that time, be able to find out all that we want to know from visits to other solar-systems.

Hardly a doubtful sci-fi suggestion, given the planetary probes that have recently been in use.

Of course it will take a long time, and so, as you said, it would have to be done as a longterm investment, (if at all).

Even at near-relativistic speeds, it would take a long time to get results from interstellar probes.

Yes I was talking about something that couldn't be feasible and worthwhile to do for a long time. (Arthur Clarke has pointed out that any interstellar probe that could be sent any time soon would be passed-up by faster ones sent out later.)

I don't consider it at all certain that there will be any reason, or perceived reason, or motivation, to do interstellar travel, or probes, even when it becomes more feasible.

Michael Ossipoff

Regardless of what's accelerating the spaceship, it's being accelerated to (and from) high speed with respect to the Earth — Michael Ossipoff

Or, a person on Earth could look at the spaceship and conclude that the Earth is accelerating away from the spaceship.

Or, a person on Earth could look at the spaceship and conclude that the Earth is accelerating away from the spaceship. — Rich

The distinction between uniform and accelerated motion, relating to why only the spaceship experiences time-dilation, and why its clocks read slow when compared to ours, is commonly discussed in typical articles on this subject.

Michael Ossipoff

Fine. While you are waiting, why don't you do some reading also.