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Thread: Quantum gravity?

  1. #1
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    Quantum gravity?

    So, I am trying to understand the very basics of quantum gravity ideas.

    As usual I am confused.
    I thought we were trying to explain a simple force with no side effects of any type?

    If we think about the rocket or lift thought experiments.
    The force from the rocket motor or the tension in the lift cable produces an acceleration.
    Time dilations etc. etc. are results of the acceleration.

    If we then switch to a building on earth.
    The force stopping the building falling through the ground produces all the same time dilation scaling effects.
    I assumed the gravity is the balancing force only with no detectable side effects
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    Quote Originally Posted by PetTastic View Post
    So, I am trying to understand the very basics of quantum gravity ideas.

    As usual I am confused.
    I thought we were trying to explain a simple force with no side effects of any type?
    That description could be applied to Newtonian gravity. It doesn't work for GR where there is no force. And there are side effects.

    If we think about the rocket or lift thought experiments.
    The force from the rocket motor or the tension in the lift cable produces an acceleration.
    Time dilations etc. etc. are results of the acceleration.

    If we then switch to a building on earth.
    The force stopping the building falling through the ground produces all the same time dilation scaling effects.
    I assumed the gravity is the balancing force only with no detectable side effects
    Not quite. The reason you feel a force pushing on you when you are standing on the ground is because you are accelerating (when you are in free-fall and feel forces, that is because you are not accelerating).

    That is the equivalence principle. I'm not sure what any of this has to do with quantum gravity. But a theory of quantum gravity would have to preserve that equivalence principle.

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    So, if you think about the accelerating reference frame in a rocket under thrust.
    GR says the accelerating reference frame is indistinguishable from gravity.
    You can show it reproduces all of the effects of gravity. Gravitational time dilation gradient, gravitational redshift, blueshift and size effects.
    Time is running at a different speed a floor level to ceiling level while the rocket motor is running.

    Floor and ceiling co-moving & co-accelerating.
    There is a time delay between light travelling between floor and ceiling. The acceleration changes the velocity of the rocket in this time period.
    In a Newtonian model this would mean the light arrives a different speed to the surface it left, but in special relativity you get a red/blue shift.
    This also agrees with potential energy arguments (dropping and object inside an accelerating rocket).
    If there is red/blue shift there is time dilation etc. etc.
    Once you have time dilation you have length or in GR size effects.

    So the 'upward' force from a simple rocket motor reproduces all of the effects of gravity not just the forces.
    This is the equivalence principle.
    If the rocket lands the force from the rocket motor is replaced by the force on the landing legs.
    This force is equal and opposite to the force of gravity.

    Gravity force can have no side effects as it is the upward force opposing gravity that produces the time dilation etc.
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    Quote Originally Posted by PetTastic View Post
    Gravity force can have no side effects as it is the upward force opposing gravity that produces the time dilation etc.
    I would say that the time dilation etc. are side effects. After all, it doesn't happen if you consider gravity to be a simple force as in the Newtonian model. Then there are other side-effects like Lense-Thirring precession, event horizons, etc.

    In fact, the (pseudo) force of gravity you feel, as well as the time dilation, etc. are all "side effects" of the curvature of space-time.

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    Ok, I understand why I am confused.

    If you push or pull an object with a physical force you create a time gradient across it.
    If you drop and object in a gravitational field it is a uniform release of potential energy.
    Inside a lift falling under gravity there is no gravity.
    So quantum gravity does have side effects but they are uniform across the object.

    Thinking in terms of an accelerating rocket that is constantly gaining relativistic mass to an external observer.
    If you drop an object from the ceiling of the rocket.
    While it is falling it inside the rocket it is travelling with constant velocity to the external observer. So, it stops gaining relativistic mass until it hits the floor of the rocket.

    I think to an observer in the rocket this looks like the falling object loosing rest mass and gaining relativistic mass.
    No observer sees a time or redshift gradient across the falling object.

    So, to fix my confusion.
    Even though everything is relative and circular, I need to pick the mass centre of the total system when considering conservation of mass/energy.
    So, quantum gravity need to convert reset mass to relativistic mass uniformly across a falling object. (Also uniform time and scale effects)
    Does not sound a trivial quantum operation.
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    Quote Originally Posted by PetTastic View Post
    Ok, I understand why I am confused.

    If you push or pull an object with a physical force you create a time gradient across it.
    If you drop and object in a gravitational field it is a uniform release of potential energy.
    Inside a lift falling under gravity there is no gravity.
    So quantum gravity does have side effects but they are uniform across the object.

    Thinking in terms of an accelerating rocket that is constantly gaining relativistic mass to an external observer.
    If you drop an object from the ceiling of the rocket.
    While it is falling it inside the rocket it is travelling with constant velocity to the external observer. So, it stops gaining relativistic mass until it hits the floor of the rocket.

    I think to an observer in the rocket this looks like the falling object loosing rest mass and gaining relativistic mass.
    No observer sees a time or redshift gradient across the falling object.

    So, to fix my confusion.
    Even though everything is relative and circular, I need to pick the mass centre of the total system when considering conservation of mass/energy.
    So, quantum gravity need to convert reset mass to relativistic mass uniformly across a falling object. (Also uniform time and scale effects)
    Does not sound a trivial quantum operation.
    The discussion here of GR effects (time dilation, relativistic mass, etc.) is not addressing the issue of unifying gravitation and quantum mechanics. The challenge is to derive a quantum gravitational theory that looks like GR at macro scales where the quantization is imperceptible. If I am not mistaken that has not been achieved.

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    Quote Originally Posted by Hornblower View Post
    The discussion here of GR effects (time dilation, relativistic mass, etc.) is not addressing the issue of unifying gravitation and quantum mechanics. The challenge is to derive a quantum gravitational theory that looks like GR at macro scales where the quantization is imperceptible. If I am not mistaken that has not been achieved.
    Yes, I was just trying to understand what parts of GR effects are caused by gravitational forces and what is caused by acceleration.

    If a rocket motor can create all of the effects of gravitational time dilation, redshift and scaling, do we need a quantum theory of acceleration?
    Probably not, a physical force just equates to an electromagnetic force. As a total guess, the atoms are probably just going egg shaped as nucleus resists acceleration harder than the electron shells.

    Should we rename gravitational time dilation as acceleration time-dilation or even electromagnetic-acceleration time-dilation?
    Is it part of quantum gravity at all?
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    Quote Originally Posted by PetTastic View Post
    Is it part of quantum gravity at all?
    Unknown at this time.
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    Quote Originally Posted by PetTastic View Post
    Should we rename gravitational time dilation as acceleration time-dilation or even electromagnetic-acceleration time-dilation?
    Is it part of quantum gravity at all?
    A quantum theory of gravity would need to reproduce all the effects of GR and so would have to include gravitational time-dilation, etc.

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    Quote Originally Posted by Strange View Post
    A quantum theory of gravity would need to reproduce all the effects of GR and so would have to include gravitational time-dilation, etc.
    The issue I am wrestling with in my limited understanding of GR is does gravity cause gravitational time-dilation, etc?

    To me it looks like physical force producing the counter acceleration stopping the object falling with gravity that has all the side effects like time-dilation gradient.

    I can't find any difference between a rocket hovering in a gravitation field and a rocket accelerating through deep space using the same level of thrust.
    As far as I can understand things, both see the same level of time-dilation between the top and bottom of the rocket.
    So it is the rocket thrust that is causing it, not gravity.

    I need someone to point out what I have got wrong. To me it looks like gravity is just a simple but unblock-able force with no side effects.
    Just like releasing an object inside an accelerating rocket, it travels with constant velocity in the external reference frame until it hits the floor inside the rocket.
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    Quote Originally Posted by PetTastic View Post
    The issue I am wrestling with in my limited understanding of GR is does gravity cause gravitational time-dilation, etc?
    No. Both gravity and time-dilation are resulting effects of space-time curvature, in other words, the presence of energy.

    I can't find any difference between a rocket hovering in a gravitation field and a rocket accelerating through deep space using the same level of thrust.
    Yep. That is the equivalence principle.

    I need someone to point out what I have got wrong. To me it looks like gravity is just a simple but unblock-able force with no side effects.
    That is the Newtonian view.

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    Quote Originally Posted by Strange View Post
    No. Both gravity and time-dilation are resulting effects of space-time curvature, in other words, the presence of energy.
    If both do it then a hovering rocket would show double the effect, wouldn't it?
    It is experiencing both gravity and thrust.

    Quote Originally Posted by Strange View Post
    Yep. That is the equivalence principle.
    For the equivalence principle to work the effect either needs to be caused by gravity or the force opposing gravity. It can't be both.

    Quote Originally Posted by Strange View Post
    That is the Newtonian view.
    I can't avoid see things as far more complicated than that.
    In the Newtonian view there is no time-dilation gradient to explain.
    As I see it, the upward force opposing gravity generates all the desired effects required for equivalence principle apart from a cancelling downward acceleration.

    Gravity is only experienced by objects and observers who are being supported by an opposing physical force.

    Unsupported observers in free-fall, observing free-falling objects see no effects from gravity.

    Standing on the ground observing falling objects is the same as observing a rocket travelling with constant velocity from an accelerating rocket.

    I guess that the opposing 'physical' force is transmitted by virtual photons so electromagnetic in nature whereas gravity acts on mass.
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    Quote Originally Posted by PetTastic View Post
    If both do it then a hovering rocket would show double the effect, wouldn't it?
    It is experiencing both gravity and thrust.
    You seem to be implying that "thrust" causes gravitational effect. It doesn't. Acceleration does.

    A hovering rocket is in the same situation as one standing on the ground. But perhaps makes it clearer that remaining stationary in a gravitational field requires continual acceleration.


    For the equivalence principle to work the effect either needs to be caused by gravity or the force opposing gravity. It can't be both.
    I'm not quite sure what effect you mean. The equivalence principle says that it is impossible to distinguish acceleration from the effects of gravity. So, Galileo pointed out that if you were in a ship with no view of outside, you could not tell if you were moving or stationary. The equivalence principle goes one step further and says you can't tell the difference between uniforms acceleration and being in a uniform gravitational field.

    My understanding is that you can calculate things like time dilation using either. For example, if you have a centrifuge, then the gravitational time dilation of an object ebbing spun round can either be calculated from its relative velocity or by interpreting the force it experiences as if it were gravity.

    In the Newtonian view there is no time-dilation gradient to explain.
    Exactly, because Newtonian gravity is just a force with no side effects.

    Gravity is only experienced by objects and observers who are being supported by an opposing physical force.
    Only experienced as a force. Other effects of "gravity" (space-time curvature) are still present.

    Unsupported observers in free-fall, observing free-falling objects see no effects from gravity.
    No. Satellites, for example, are in free fall and appear to have no gravitational forces in operation (e.g. astronauts floating in the space station). But time dilation still applies.

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    What does quantum gravity have to do with any of this?

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    Quote Originally Posted by Hornblower View Post
    What does quantum gravity have to do with any of this?
    Quantum gravity needs to duplicate the effects of gravity.
    Otherwise the theory is untestable.
    The equivalence principle says the gravity in an accelerating rocket is indistinguishable from normal gravity.
    You can't have gravity without gravitational potential energy, that gives gravitational redshift, that requires gravitational time-dilation & then scaling effects.

    An object being accelerated upwards by a physical force that is producing acceleration shows all of the above effects.
    Adding equal gravity only produces an opposite acceleration that stops the object moving and has no other effects.

    What would a successful theory of quantum gravity have to produce in order to explain a link to relativity?
    Would it also need to work in accelerating rockets?
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    Quote Originally Posted by PetTastic View Post
    ...
    The equivalence principle says the gravity in an accelerating rocket is indistinguishable from normal gravity.
    That is not really the equivalence principle
    In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.
    ...
    What is now called the "Einstein equivalence principle" states that the weak equivalence principle holds, and that:[36]
    The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.
    Here "local" has a very special meaning: not only must the experiment not look outside the laboratory, but it must also be small compared to variations in the gravitational field, tidal forces, so that the entire laboratory is freely falling.
    So if you have a freely falling laboratory that is small, you cannot run an experiment that tells whether you are in a gravitational field or not.
    The rocket was mentioned in 1907 by Einstein (our 1 g of gravity is equivalent to a rocket in space accelerating at 1 g) and replaced by the freely falling laboratory.

    A quantum gravity theory will be a relativistic theory (both GR and QM are relativistic theories). It will give GR in the appropriate limits and QM in other appropriate limits.
    A quantum gravity theory will work everywhere including in an accelerating rocket.

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    Quote Originally Posted by Reality Check View Post
    A quantum gravity theory will work everywhere including in an accelerating rocket.
    This simplifies things a lot.
    If you think in terms of:
    (1) A free floating/ free falling observer
    (2) Conservation laws (No free energy machine)
    (3) Physical measurement & measurement using light & clocks give same results
    (4) Definition of the metre
    (5) Measurements relative to co-accelerating are constant


    (a)Velocity is constantly increasing. (1)
    (b)Relativistic effects are constantly increasing (1,5)

    Mass / energy is constantly increasing
    Blue-shift is constantly increasing
    Time dilation constantly increasing
    Size effects are constantly increasing (matter shrinking) (1,3,4).

    All of the above show a relative gradient in the direction of acceleration.
    Clocks / matter is running faster near the ceiling than the floor. (1)
    Matter is larger near the floor than the ceiling (3,4)
    Matter near the ceiling has higher mass the matter near the floor(1,2,5)

    Does this mean for quantum mechanics that even the Plank length and plank time have a gradient in the direction of acceleration?

    GR is very circular (chicken and egg). If your mind is twisted enough you could see gravity as a side-effect of matter shrinking with vacuum energy having about the right value to provide the energy required.
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    Sorry too late to edit.

    Nothing is happening to the photons is this version of events.
    The free-falling / free-floating observer sees them travel in straight lines and arrive with the same frequency wavelength they were emitted with.
    It is the matter that is changing size & speed.
    Smaller-faster atoms emitting shorter wavelength higher frequency photons & visa versa.
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    Quote Originally Posted by PetTastic View Post
    It is the matter that is changing size & speed.
    From an outside observer's viewpoint. The matter is not changing from its own POV.
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    Quote Originally Posted by Noclevername View Post
    From an outside observer's viewpoint. The matter is not changing from its own POV.
    Think of it in terms of your everyday experience.
    An atomic clock can tell you that time is travelling faster at head level than at floor level.
    The metre is defined in terms of time.
    So the metre is longer at floor level than head level.
    This implies that the atoms in your feet are larger and slower than the atoms in your head.
    Do you notice?
    You can change the size of matter in your body by just walking up a step.
    Mix this with the std model of particle physics or QED and there is an imbalance in forces being exchanged that itself looks like gravity.
    All fun but works much better if applied to cosmology at the same time as local experience.
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    Quote Originally Posted by PetTastic View Post
    Think of it in terms of your everyday experience.
    An atomic clock can tell you that time is travelling faster at head level than at floor level.
    The metre is defined in terms of time.
    So the metre is longer at floor level than head level.
    This implies that the atoms in your feet are larger and slower than the atoms in your head.
    Do you notice?
    You can change the size of matter in your body by just walking up a step.
    Mix this with the std model of particle physics or QED and there is an imbalance in forces being exchanged that itself looks like gravity.
    All fun but works much better if applied to cosmology at the same time as local experience.
    PetTastic

    Be careful. You seem to be crossing over from asking questions to advocating non-mainstream ideas in Q&A. There be infraction points.
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    Quote Originally Posted by PetTastic View Post
    This simplifies things a lot....
    No, it is a fact in the theory of quantum gravity. It applies to everything in the universe with mass and energy. This is because GR applies to everything in the universe with mass and energy and quantum gravity will do the same.
    Thus, this part of my post about the equivalence principle: A quantum gravity theory will work everywhere including in an accelerating rocket.

    The rest looks like a misunderstanding of the equivalence principle again. You ignore what local means in the equivalence principle: Here "local" has a very special meaning: not only must the experiment not look outside the laboratory, but it must also be small compared to variations in the gravitational field, tidal forces, so that the entire laboratory is freely falling.
    Special relativity says that other observers will measure increasing time dilation and length contraction due to the increasing speed of a free falling observer. Ditto for GR. For a freely falling observer who cannot look outside their laboratory and that laboratory is small enough that the gravitational field is uniform within what they can measure:
    • Mass is not increasing.
    • Energy not increasing.
    • There is no blue-shift or red-shift,
    • There is no "matter shrinking" (length contraction?, tidal effect?).
    • There is no "relative gradient in the direction of acceleration".
    • There is no "Clocks / matter is running faster" because there is no external observer.
    • There are no tidal effects as suggested by "Matter is larger near the floor than the ceiling".

    You end with "GR is very circular(chicken and egg). If your mind is twisted enough...". GR is not circular. For example, GR does not start with the equivalence principle and end up deriving the equivalence principle.
    GR is non-linear though. Simply put, the energy in a gravitational field contributes to the gravitational field. The Einstein field equations are non-linear similarly to many other non-linear systems of differential equations.
    Last edited by Reality Check; 2020-May-13 at 12:51 AM.

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    Quote Originally Posted by Swift View Post
    PetTastic

    Be careful. You seem to be crossing over from asking questions to advocating non-mainstream ideas in Q&A. There be infraction points.
    Oops sorry!
    I normally delete the alternative crap before posting, it just helps me think.

    Am ok with the length of the metre changing with time-dilation, as it is defined as distance light travels in 1/299,792,458 of a second?
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    I am ok with the definition of a metre which does not change with time dilation. If Alice has a ruler 1 meter long then it is always 1 meter long for Alice. It is Bob moving at a different speed that says Alice's clock is time dilated and her ruler is length contacted. Ditto for Bob in a different gravitational potential. Likewise for Charlotte moving at a different speed in a different gravitational potential and measuring different lengths and durations.

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    Quote Originally Posted by PetTastic View Post
    Am ok with the length of the metre changing with time-dilation, as it is defined as distance light travels in 1/299,792,458 of a second?
    What about when it was defined as the length of a standard bar? Or a proportion of the Earth's circumference? How about an inch?

    When dealing with GR/SR you need to be much more precise about what you mean. You seem to be mixing frames here as if there is some absolute frame that defines what is 'really' happening. If someone measures something as different it is not the same as saying the thing has changed in some fundamental manner.

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    Quote Originally Posted by Shaula View Post
    What about when it was defined as the length of a standard bar? Or a proportion of the Earth's circumference? How about an inch?

    When dealing with GR/SR you need to be much more precise about what you mean. You seem to be mixing frames here as if there is some absolute frame that defines what is 'really' happening. If someone measures something as different it is not the same as saying the thing has changed in some fundamental manner.
    Not sure about absolute frame.
    It is all a matter of how you normalise solutions to the field equations.
    By convention it is done so that the size of the observer is 1 but as everything is relative...

    All past definitions of the metre are pretty much equal for thinking about in context of GR.
    In physics it is only the measured value that matters. (OK?)
    The laws of physics don't care if your ruler is longer at the bottom of a diamond mine than it is at the top of a mountain, everything else is scaled equally.

    If we can go back to the OP and the equivalence principle.
    What is caused by gravity and what is caused by forces opposing gravity?
    You can't have an accelerating reference frame containing matter without a force accelerating it.

    Am I ok with the time-dilation gradient, changes in mass and size being caused by the force pushing against gravity or inertia?
    So not gravity?
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    Quote Originally Posted by PetTastic View Post
    Not sure about absolute frame....
    You should be sure that GR and SR have no absolute frame because that is basically what "relativity" means. Everything is relative to a reference frame.
    There is no "normalise solutions to the field equations" or "size of the observer is 1" in GR or SR.

    Back to the OP then:
    This is the equivalence principle. Not much discussion needed unless there are new aspects you are unclear about.

    Gravity causes acceleration. Forces opposing gravity cause less acceleration. Thus an apple on a table has an acceleration due to gravity and an opposing force from the table cancelling the acceleration.

    "time-dilation gradient, changes in mass and size" is still wrong. SR time dilation has relative velocities. GR time dilation has different gravitational potentials (there will be a gravitational field that varies, i.e. has a gradient).
    Mass and size of an object do not change in SR and GR. Measurements of length by another observer will be contracted. Showing length contraction is not a physical change in length is a reason for Charlotte in my Alice/Bob/Charlotte post. Bob, Charlotte and an infinite number of other possible observers can all measure different lengths of Alice's ruler at the same time. Her ruler cannot be different physical lengths at the same time.
    That observer can use the considered as "archaic and confusing" (in textbooks) concept of relativistic mass.

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    This whole thread is based on the equivalence principle.
    the equivalence principle is the equivalence of gravitational and inertial mass
    Hence all the stuff about rockets and lifts.

    Special relativity is a special case of GR where acceleration is zero.
    All of the scaling and dilation effects in GR are the general case of the SR.
    The velocities in SR are a result of acceleration that can only be modelled in GR.
    The time dilation and scaling in SR are the result of accumulated effects of acceleration that require GR to model.
    Acceleration time dilation is just as real as the simplified version in SR and therefore so are all the other effects unless you want the change the speed of light.

    The main point of Einstein's thought experiments with lifts and rockets is the show the equivalence of a uniform gravitational field to an accelerating reference frame.
    https://en.wikipedia.org/wiki/Genera...tonian_gravity

    Time dilation has no link to gravity gradients. There is increased time dilation at the bottom of a coal mine relative to the surface when the gradient is reversed, dropping towards zero at centre of planet.

    If you are in a rocket that is producing 1g of acceleration, inside the rocket you feel 1g of gravity with all the effects including red/blue shifts and time dilation and scaling.
    It does not matter if you are in deep space or about to hit the surface of neutron star head first it has no effect on the experienced gravity inside the rocket.
    Assuming the rocket is small in size compared to the size of the neutron star.

    In GR the free falling observer sees light travel in straight lines and not gain or lose energy.
    It sees you as being accelerated upwards at 1g by the upward force through you feet or chair.
    To the free falling observer you are constantly gaining speed, energy and therefore relativistic mass.
    The situation is identical to a free floating observer watching a rocket accelerate.

    The scaling effects change the size, speed and mass of matter. To me they look just as real as the SR simplified versions.

    So back to the question what is caused by gravity and what is caused by physical forces opposing gravity?

    The physical forces from a rocket, spring or cable are electromagnetic transferred by virtual photons.
    Gravity is probably not electromagnetic in nature.

    To me it looks like the physical forces are causing the time dilation gradient etc and the gravity has no side effects.

    Basically this comes down to: Do the virtual virtual photons and other bosons obey the same rules as 'real' photons in curved space-time?
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    Quote Originally Posted by PetTastic View Post
    ...The main point of Einstein's thought experiments with lifts and rockets is the show the equivalence of a uniform gravitational field to an accelerating reference frame.
    https://en.wikipedia.org/wiki/Genera...tonian_gravity.
    The equivalence principle is for GR not SR. You seem to have grasped the concepts of Einstein's elevator experiment.

    But then you write "The scaling effects change the size, speed and mass of matter" which is still not correct. There are no "scaling effects". There are relativistic effects from the gravitational field in GR. There are no changes in size or mass in SR or GR. There are changes in measurements of length and time duration. There is the rarely used, teaching aid of relativistic mass which is a change in measurements of mass. Similar real changes in measurement happen in SR, e.g. relative speed does not physically change the length of objects just what an observer measures.

    Gravity causes acceleration. Opposing forces reduce that acceleration. These forces can be electromagnetic.
    Gravity is certainly not electromagnetic in nature.
    Physical forces cannot cause whatever a "time dilation gradient" is. Gravity does have "side effects" such as gravitational time dilation (which is not a gradient). These are the relativistic effects that make GR not classical physics.

    Quote Originally Posted by PetTastic View Post
    Basically this comes down to: Do the virtual virtual photons and other bosons obey the same rules as 'real' photons in curved space-time?
    The answer is no because they are virtual particles
    Virtual particles do not necessarily carry the same mass as the corresponding real particle, although they always conserve energy and momentum ...
    A virtual particle does not precisely obey the energy–momentum relation m2c4 = E2 − p2c2. Its kinetic energy may not have the usual relationship to velocity. It can be negative.[5]:110 This is expressed by the phrase off mass shell.[4]:119

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