• Rich
    3.2k
    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.
  • noAxioms
    555
    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.
  • Rich
    3.2k
    Thanks for the science lesson. Had nothing to do with GTR.
  • noAxioms
    555
    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.
  • Rich
    3.2k
    You're confusing it with velocitynoAxioms

    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.
  • noAxioms
    555
    Nothing more to say.T Clark
    I hear you.
  • Andrew M
    403
    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
  • Rich
    3.2k
    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.
  • Banno
    2.7k
    This thread is a sad indictment of the state of science education.
  • Rich
    3.2k
    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.
  • Banno
    2.7k
    Why did you assume I as talking about you?
  • Rich
    3.2k
    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.
  • Pierre-Normand
    1.1k
    Relativity is about RELATIVITYRich

    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).
  • Michael Ossipoff
    1.1k
    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
  • Rich
    3.2k
    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.
  • noAxioms
    555
    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.
  • Michael Ossipoff
    1.1k


    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
  • Rich
    3.2k
    Regardless of what's accelerating the spaceship, it's being accelerated to (and from) high speed with respect to the EarthMichael Ossipoff

    Or, a person on Earth could look at the spaceship and conclude that the Earth is accelerating away from the spaceship.
  • Michael Ossipoff
    1.1k
    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
  • Rich
    3.2k
    Fine. While you are waiting, why don't you do some reading also.
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