[Bored chemist]

I can see what you were trying to do and it is absolutely brilliant.

I'm not sure he was trying to make that point.

[alancalverd]

However, you (Alancalverd) must also know that there is no way you could convert that photon into a particle anti-particle pair.

It happens when photons with energy exceeding 1.02 MeV approach heavy nuclei. The p-e pair annilhilates and emits two 510 keV photons, which are easy to detect. It's such a simple experiment that we use it to calibrate the 1 MeV point for small accelerators.

But to return to the root topic, it is quite clear that we have two models of gravitation. One relies on the common observation that massive bodies attract, and the other, the hypothesis that massive bodies warp spacetime. The beauty of the Einsteinian model is that it predicts gravitational lensing without having to ascribe mass to a photon, and degenerates to the Newtonian approximation for two nonzero masses.

[Eternal Student (OP)]

Hi McQueen

What a terrific return on a two billion dollar plus investment AND it has increased our knowledge of the Universe we live in beyond belief.

    You write in a proficient and poignant manner but I'm a less proficient reader:  I'm just not sure how much of this was meant in sarcasm.  That money could eliminate quite a lot of famine, for example.
   

The talk about gravity waves is very interesting

   I haven't checked all the facts you quote about LIGO but the spirit of what you've said seems reasonable.

Since work done is equal to force x distance it is possible to calculate the work in Joules that an 80 Kg man  taking a single step would exert at a distance of  a 1000 km. It turns out this force would be equal to 850/1000,000 = 0.000085 J at the LIGO site.

    That calculation seems to contain some errors.   Work done is equal to   Force  x   The distance that force is moved in the direction of the force.    So one concern is that the force exerted by the man was almost perpendicular (not in the same direction) to his displacement from the LIGO apparatus.
    At times during your calculation you seem to have compared things with different units.  It is difficult to determine that an energy of  5.43 x 10-14 eV    is more or less than  a length of  8.418 x 10-20 m.
    None the less, I'm sure that tremors are a problem at LIGO as you stated.  It is something of a miracle that LIGO works at all and it was never expected to start detecting results of any significance as quickly or as easily as it seems to have done.

[Colin2B]

Would you care to put a number to the mass of a photon? 

Think you can put a value on the equivalence substituting hf for E in the famous equation

In principle you can derive a relativistic mass for the photon, but this is not often used nowadays.
The m in E=mc² is rest mass, more properly m₀, which for a photon is 0, so E=0 for m₀. The energy of the photon is in the second part of the famous equation E=pc and it is possible to have momentum without mass.
Because all forms of energy (of which mass contains a lot concentrated in a small volume) affect spacetime it is easier to use energy as the common currency rather than mass.

[gem]

Hi all,
Yes thanks for that Colin, I totally agree, your actually, stating something that was also said in a previous post:
(Energy is the currency of the physical world)
😎

In regards to ES having a problem with the use of a ‘thought experiment/ analogy’ of the Sun disappearance, I believe there are lots of examples of these used through out mans development of theories, and Einstein/Newton are documented to having resorted to using them, to great benefit.

[Eternal Student (OP)]

Hi.

    Firstly, don't rush to call it a "thought experiment",  you're giving an urban myth some level of recognition it does not deserve.  It's not as if anyone did the experiment (i.e. thought about this example) while developing the theory -there's good reason to believe they didn't.

    How do you define "thought experiment"?  Is it just something that you can imagine but can't do in reality?  "How would the earth's orbit be affected if the sun disappeared?"  does fit that description, I have to admit that.    "How would the earth's orbit be affected if all hats suddenly turned green?"  That's another thought experiment by this definition.

    My idea of a "thought experiment" is that it does provide information and insight about the theory being developed    OR    that it can be resolved with the theory you are working on.   The thought experiment with all hats turning green is actually more useful in developing a theory of gravity.  At least there are theories which don't preclude the possibility of all hats turning green.  GR is also a fully competent theory of gravity and quite sufficient to resolve the thought experiment where the hats suddenly turn green:  The colour of hats isn't a factor in determining gravitation so there would be no change in earth's orbit.  All things considered the hats turning green is a more useful thought experiment for GR.

   I think it's been established that GR cannot resolve the situation with a sudden disappearance of the sun. So, it's more likely that you (gem) are suggesting it provides some insight and information about the theory of GR.  What is this insight that you feel is gained from it?

   I don't think this situation was originally intended to be a "thought experiment".  I think it has evolved from what was just an example.  Go back fifty years and imagine people were told "gravity isn't instantaneous action at a distance like we thought".  Now that isn't easily understood or imagined - what practical difference should it make?  So someone came up with an easily visualised example - "well, it's a bit like this.... if the sun suddenly disappeared then...".   The sudden disappearance of the sun is much more like an example situation to help visualise something about GR.   It simply doesn't fit the criteria to be considered a "thought experiment"  (IMHO).

   I am suggesting (to everyone) that this example has now become an extremely harmfull example.  It misleads  people more than any insight it brings.  In no small part this is because people have come to think of it as a "thought experiment" which should have a resolution under GR  and/or  provide insight,  while it was probably never meant to be more than a bad example of how changes in the metric field can propagate and evolve with time.

   So, I don't have a problem with "thought experiments" in general.  It's just that this isn't one (or it shouldn't be one).

[Halc]

How do you define "thought experiment"?

Typically it is an exercise in following through on a proposed set of rules. It's a mathematical exercise. So Einstein's train thought experiment was designed to illustrate the implications of light speed being a frame independent constant. Relativity of simultaneity is trivially demonstrated. That doesn't prove RoS, but it does prove it given the validity of the constant light speed premise.
Supposing something impossible like violation of conservation laws (suppose Earth suddenly orbited the other way) cannot yield any useful information about a universe that forbids the premise.  But a train going 0.8c is not forbidden, even if it is pretty impractical to do.

At least there are theories which don't preclude the possibility of all hats turning green.

It means somebody turned off the machine that puts artificial images on the hats, leaving only the actual green hats. Queen of England kind of found this out the hard way:
https://www.boredpanda.com/queen-elizabeth-green-screen-outfit-funny-photoshop-battle/?utm_source=google&utm_medium=organic&utm_campaign=organic

I think it's been established that GR cannot resolve the situation with a sudden disappearance of the sun.

Neither can it resolve how I could go back in time and kill my mom, and still exist to do the time travel. There are resolutions to that, but not from GR, which does not formally support that sort of thing. QM does allow cause after effect, but not in a way that information can be sent, and the time travel thing seems definitely to be information sent outside one's future light cone.

What is this insight that you feel is gained from it?

So try something a little more valid. Put a strong string on Earth and yank it away.  How long before a distant orbiting thing deviates from its path?  That's a valid scenario. Plenty of gravitational waves.

Go back fifty years and imagine people were told "gravity isn't instantaneous action at a distance like we thought".

I think it is a mistake to characterize gravity as 'action' at all. It's a field, not something that radiates towards us. If I step in the Atlantic, I get wet. Sure, if the Pacific were to be drained somehow (not impossible), the beach where I got wet would eventually (not instantly) drop in water level and I'd not get wet. But my getting immediately wet isn't due to instantaneous action at a distance from the Pacific ocean holding sea level up. Sea level is like a field. It is already there.

[jeffreyH]

I think it is a mistake to characterize gravity as 'action' at all. It's a field, not something that radiates towards us

Do you understand what action means?

[Colin2B]

(despite Colin2B's attempts at platitude) I'm still ashamed to say that I have propagated the myth by using the example with one other person in the past.

I’m sorry you think my remarks trite, meaningless, prosaic, dull or insipid. That was not my intention, I merely wanted to show a different way of thinking about this 

We (that'll be at least Halc and I) don't think Einstein was motivated by that thought experiment, it is more like an urban myth. 

I agree. As I said in my reply, I think the Wiki reference may have come from Brian Green. Thinking about it, I would suggest his book ‘preposterous universe’. So we can classify it as an urban myth like Galileo dropping weights off a tower and Newton being hit by a falling apple.

  I see you have also heard of this example, it is widespread. 

Strange that it is so widespread, I wonder why. Also interesting that so many respected and prominent physicists working in GR use it.
Thinking about John Wheeler using a less dramatic version. He became a close friend of Einstein helping develop some of the mathematical extensions to GR, so he would have understood Einstein’s thinking on many issues and how that thinking may have developed.
There may be a clue here https://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/general_relativity_pathway/index.html
You will be familiar with John Norton’s work on the history of relativity, and if you look down at the section ‘Adjusting Newton’s theory of gravitation’ we see that the key issue is:
“Newton's celebrated theory of gravitation presumed instantaneous action at a distance. The sun now exerts a gravitational force on the earth now with a magnitude set by Newton's inverse square law. The key part was the "now." If the sun were to move slightly, the resulting alteration in the force it exerts on the earth would be felt by us instantaneously according to Newtonian theory.”
Interesting to note the more reasonable suggestion used by Wheeler ‘If the sun were to move slightly’ rather than the exaggerated ‘disappear’. John goes on:
“The change needed was, apparently, straightforward. In the revised theory, a change in the sun should not be felt here on earth instantly, but only after a time lag of around 8 1/3 minutes, the approximate time light takes to propagate from the sun to the earth. Then absolute simultaneity would no longer be needed in the theory.”
If we follow the article on we can see a chain of thinking from the widely discussed instantaneous action, to trajectories of cannon balls to the famous "the happiest thought of my life." and the consideration of falling bodies in general.
So, although he didn’t use Brian Green’s cosmic catastrophe, he was clearly influenced by discussion at the time which was considering the problem of the time between a change in the sun’s position and that change’s effect.

  You frequently mention that it is the job of a university to correct and fix these misconceptions. 

Do I? Frequently?
I wasn’t aware I had said that once, let alone frequently. Could you point out where please.
I did say:
“I think one concern you expressed was that students would need to relearn when moving to higher study. They will have to do that anyway as some pregraduate teaching contains approximations and simplifications that look quite different later on - not to mention teachers who are just plain wrong!”
But that’s not saying it is the job of Universities to fix it, just that they are forced to do it by circumstance.

[Eternal Student (OP)]

Hi.
Lot's of replies, thanks and I'll get around to checking all the links and information from everyone.  Thank you.
I'm just making a few quick responses to start with:

@Colin2B
   "Preposterous Universe".   I think this is Sean Carroll's book / web-page / social media thing and not Brian Greene's thing.   Probably doesn't change much of what you said.
   "platitude" it does imply an attempt to ease social or emotional discomfort was made (even if it is trite, meaningless and ineffective).  Thank you for that.
    I may have misunderstood some of what you said about universities but the spirit of it is there.  I think some of the problem is that different parts of the world describe things differently.   What is a pre-graduate?  Is it what we would call an under-graduate?  Anyway, it shouldn't be left to Universities to fix misconceptions.  There is some obligation not to put these misconceptions into public circulation in the first place, especially if you are (for example) a school teacher or a forum moderator.  You don't need to be lecturing in a university to be teaching people in some context and shaping the development of future scientists.   I mean how do you (moderators and not just Colin) see your role?  Is it just entertainment - because you're selling yourselves short if that's all you think you should do.    Well, that's my view and the main thrust of this discussion - should we stop using this example?

@Halc
  Something may have gone wrong with the quote at the end of your post.
Thanks for the humorous link about hats turning green.  I don't see much disagreement with what a thought experiment should be or should achieve, or anything else you've said.  I've got to read Colin2B's history of development before I do much more, he/she is suggesting it has been seriously corrupted rather than never having been intended as a thought experiment.

    I'm going to read all the links and info from various people before I write anything else and I've got real-life stuff to do first.
Thank you to everyone for your time.

[Eternal Student (OP)]

Hi again.

There may be a clue here https://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/general_relativity_pathway/index.html

    This is a quality resource and it has kept me occupied for hours.  Thank you, thank you.  I think you've already pulled the text that is most relevant to this forum thread and put it directly into your earlier post.
    I don't seem to have much disagreement with or from you, Colin.

Neither can it resolve how I could go back in time

   ? I can't see the relevance or follow the sub-text here.  Was it that I used the phrase "go back fifty years"?

So try something a little more valid. Put a strong string on Earth and yank it away.

    Actually that does sound like a better version of the example.  I may use something like that.  Thank you.

It's a field, not something that radiates towards us.

   Yes, I think I see what you're saying.  There is a metric field that is defined on all of spacetime.  The metric field is a (0,2) tensor valued field but otherwise it's much like familiar fields such as the electric or magnetic field.  The fields don't propagate, they just "are there" having a value at each point in space and time.  It's just that sometimes, at a fixed spatial position we would observe a pattern of change in the values of the field over a time interval    and that pattern of change will be repeated at a point further along in space at  over a time interval    .  There may be some attenuation in the amplitude (or magnitudes of each entry for the tensor) and possibly some spread in the time interval for this pattern of change  - but overall, it "looks like" a wave propagating through the field and the quantity    is identified as a speed of propagation.
   The name "gravity wave" gravitational wave is perhaps an unfortunate one but it is one that is used and it falls outside the scope of this thread to criticise the physicist's who thought of that.  Only under certain motions of gravitating masses  (in what is essentially a vacuum outside of those masses) can we actually see these repeating patterns of change appearing to propagate through the metric field like a wave.  Anyway, I'm not taking the blame for naming something a "gravity wave" gravitational wave,  or trying to identify an amount of energy that is often said to be "radiated away" in a such a wave.  Somebody else did that.

    The Einstein Field Equations differ from Maxwell's equations in a way that could also make a wave-like propagation of changes in the field values hard (impossible?) to identify:  They are non-linear differential equations so solutions do not simply add together like some superposition principle for electric and magnetic fields.  If a wave in the electric field propagated through space to approach a region with a static source, we would still see the wave propagate but all values of the field are just appropriately translated upwards (or downwards) according to the field from that source.  If a gravity wave reached a region of space where some existing gravitating source became significant, then the overall appearance of the metric in that region becomes ..?complicated?.. ?unrecognisable as any wave? (I don't know, the only "gravitational wave" solutions to the E.F.E. that I've seen are vacuum solutions).

   Late editing:  I've deleted the rest, I'm not really sure what the problem was that you were trying to point out.    Also, note the correction gravity wave ---> gravitational wave  (thanks, evan-au for pointing that out).
   

 - - - - - - - - -

Where next?
    It seems that proposing an alternative example to illustrate the (delayed) action of gravity might be sensible.  I guess we would have to continue using the sudden disappearance of the sun if there is nothing else that serves as a visual example.  Halc has suggested one alternative already.  I'm thinking that we have recorded actual gravitational waves and maybe we should just use that directly.
   Halc's idea needs some consideration.  As I understand it GR determines how matter must move unless it is acted on by an external force.  So one of the keys to make his/her idea work is that the gravitating source being pulled by the string MUST actively be pulled for a time, it must be experiencing acceleration in its local inertial co-ordinates.  Simply giving the string a quick jerk to start the mass traveling off on some other straight line (technically geodesic path through spacetime) won't do.
    I'm now getting interested in other things that PopSci articles report and may drift off to another topic but I'd like to say thank you again to everyone who spent some time here.

Best wishes.

[gem]

Hi all
So ES all is good in your world ?
Yanking mass and accelerating it, is a more satisfactory starting point of a thought experiment/analogy?

How does that work exactly ?
What is the string/mechanism  going to be attached to, that allows changes in motion/velocity?

Although given Halc stated gravity is not action maybe you can yank this imaginary piece of string without considering the force or energy input required, and any consequences as to where the action / reaction of yanks occur.
But if you’re happy, that’s nice,  but I am not to sure how many others you have convinced as to the premise of your irritation and lack of a credible solution 🤔

[Eternal Student (OP)]

But if you’re happy, that’s nice

     In my limited experience, most people who spend some hours engaged with forums aren't "happy".  They're just trying to keep themselves occupied.  But, on the whole I'm OK and wouldn't want to discuss anything other than science.
    What's more important is that I seem to have upset you.  I'm sorry for that, it wasn't my intention.  I've actually been very grateful for your input.
   You sound like someone who could go on to be involved in science and maybe end up teaching some people yourself.  I'm sure you'll do well and I wish you all the best.  Please don't disregard the possibility that some examples in GR do seem to have become corrupted.  You can use them, you can do whatever you like.  Just make the best decisions you can.

I am not to sure how many others you have convinced

   Don't worry, I'm not moving on because I think I've "won" something.  This is a discussion not a set of commandments to follow.  The seeds of information are already there in the thread if people want to read it and my position on the matter is shifting according to what other people have said.  Halc's idea needs some consideration and Colin2B has presented a good source that indicates a weaker form of this example may have actually influenced Einstein's development of GR.

[evan_au]

The name "gravity wave" is perhaps an unfortunate one... motions of gravitating masses

The name "Gravity Wave" is not actually the right name.
- Gravity Waves may be easily generated and observed in a container with a layer of oil floating on a layer of water.
- The Gravity Waves propagate on the oil/water interface
See: https://en.wikipedia.org/wiki/Gravity_wave

What you are discussing is a "Gravitational Wave".
- I'm not sure that it is any more descriptive, but it is the standard term to describe the propagating distortions in space time caused by accelerating masses.
- It is impossible to generate detectable Gravitational Waves with any masses we have in the Solar System
See: https://en.wikipedia.org/wiki/Gravitational_wave

[Eternal Student (OP)]

Thanks evan-au.    Strike through corrections have been made in the original post.

[Halc]

It seems that proposing an alternative example to illustrate the (delayed) action of gravity might be sensible.

The trouble with the yanking thing is trying to describe how the force is applied from a distance. It can all be solved with simply a transfer of momentum, which physics allows.

We have some mass somewhere which we need to remove ASAP, but it cannot just disappear. So what if it suddenly just accelerates to any arbitrary speed in some direction?  Physics allows that.

Earth is just sitting there, and there's an incoming ping-pong ball coming in at some absurd momentum.  It has all the momentum we want to give to Earth.  It hits something (a small gas cloud?) off at some appropriate distance and explodes.  So now we have this expanding ball of momentum that just happens to be Earth size when it hits Earth.  The planet absorbs perfectly all the momentum and suddenly has whatever velocity you want to give it. OK, so nobody survives this. We don't care. We care about the object orbiting away at say 10 light seconds to see if it alters its path in those 10 light seconds.

The biggest reaction would be if the Earth was suddenly moving towards the orbiting thing. Its gravity should be much greater as it approaches, so the orbiting thing would have the greatest deviation from its normal path.  But it won't. It will track its original path for at least 10 seconds before it begins to deviate from that.  And no, it will not go in a straight line after that because Earth is not gone, it's just not where it usually was. Oh yea, and it's a ball of plasma. Hope nobody was inconvenienced. All in the name of science, eh?

[Eternal Student (OP)]

OK Halc.  Although you may be over-thinking this.

    I don't see a problem with an idealised long stiff string and something pulling it from far away.  Far enough that the spatial curvature around the earth isn't determined by it (so that'll be at infinity - but for practical purposes, just far away and negligible in effect).
   It's not the reality of the situation that is the biggest problem.  It's just ensuring that GR really can resolve the situation and the example is therefore useful as a learning aid even under some scrutiny by the diligent student.

    We can leave out the string all together.  It could be said that if the sun was rapidly accelerated toward the earth, then the earth's trajectory can't be affected for 8 (and a bit) minutes under GR.  While under Newtonian gravity a change in trajectory would be evident before this. 
    More than that under Newtonian gravity we human beings would soon start to see and feel strange gravitational effects (like things falling upward on one side of the planet).  While under GR, the paths of objects in free fall won't change for 8+ minutes.  There's lots of easily understood visualisation that could be done.

    Any student wishing to follow this example through more carefully with some Mathematics can find a way to provide that acceleration to the sun.  They can add stiff strings, photon rockets or high momentum incoming ping-pong balls if they need to.  The main thing is that they don't hit a wall that stops them proceeding with the Mathematics as soon as they look at the E.F.E. and realise that the proposed stress-energy tensor isn't even continuous let alone differentiable.

[gem]

Hi all,

So we do seem to be in a pickle, coming up with an alternative thought experiment.
 Halc
You’re not ignoring the laws of conservation of momentum are you, what’s your coefficients  of restitution for these collisions, gasses and solids ?

ES I don’t understand this:
  “More than that under Newtonian gravity we human beings would soon start to see and feel strange gravitational effects (like things falling upward on one side of the planet).  While under GR, the paths of objects in free fall won't change for 8+ minutes. “

At what point in proceedings are you anticipating these physical realities
For Newton’s postulate ?

Given Earth is currently, approximately accelerated in its orbit at 6.0 x 10^-3 m/s^2
 

[Colin2B]

The sudden disappearance of the sun is much more like an example situation to help visualise something about GR.   It simply doesn't fit the criteria to be considered a "thought experiment" (IMHO).

I agree, but I am pretty ambivalent about it's use.

I've never used it directly myself. In the thread mentioned I was interested in pointing to the latter half of the article in the link and hadn't even noticed that this example had been used - notice he also uses the breaking string example. Therin lies a problem, because even if we don't use it directly, it is so widespread that suggesting someone read such and such an article, website or paper might well direct them to this example, and not all those will be Popsci it will include well respected experts in GR.
Anyway, 'nuff said on this before I get on to misrepresentations of QM!

This is a quality resource and it has kept me occupied for hours. Thank you, thank you.  I think you've already pulled the text that is most relevant to this forum thread and put it directly into your earlier post.
I don't seem to have much disagreement with or from you, Colin.

John is one of those really nice guys who is also an excellent researcher and thinker. You should find something on his site about thought experiments in general. Lots of good material and he’s happy to share.

  Any student wishing to follow this example through more carefully with some Mathematics can find a way to provide that acceleration to the sun.  They can add stiff strings, photon rockets or high momentum incoming ping-pong balls if they need to.  The main thing is that they don't hit a wall that stops them proceeding with the Mathematics as soon as they look at the E.F.E. and realise that the proposed stress-energy tensor isn't even continuous let alone differentiable.

Haven’t had chance to read through all the latest posts, but this point caught my eye.
The objective of this site is outreach to a general public and interested amateurs to try and stimulate an interest in science. As such it is very unlikely that the average questioner will understand tensors, let alone stress-energy tensors. For that reason the level of answer we give is very different from that on say physicsstackexchange.
Take this question which is somewhat typical "How long could humanity survive if the sun went out?" https://www.thenakedscientists.com/forum/index.php?topic=82152.0
(Note: We have also had " What would happen to the earth is the sun suddenly dissapeared")
We don't answer this by starting to discuss nuclear fusion and the technicalities of why the sun cannot suddenly blink out, and the audience expects little maths, just a straight forward answer they can understand.  Obviously some of the questions go a little deeper, but in general we don’t expect to go much beyond school physics in terms of technical understanding.
I don’t say this to put you off your quest, but just to put some perspective for this site.

Can I make one plea on terminology. A gravity wave is one where the returning force on the displacement is gravity eg a water wave. In GR we refer to gravitational waves to differentiate.

I think it is a mistake to characterize gravity as 'action' at all. It's a field, not something that radiates towards us.

I think you, like me, enjoy looking at physics from different perspectives and use the most appropriate at the time.  I remember as an undergrad we were set the exercise of deriving Newton’s laws from GR in the weak field limit, a useful reminder that in the approximations they are still valid.
I assume you are familiar with the work of Fokker, Tetrode & Schwarzschild and the Wheeler, Feynman analysis of this in terms of electrodynamics and particle action. It’s an interesting look at the relationship between field theory and stationary action, bit old but still interesting, particularly coming from action man himself.

[alancalverd]

Never mind disappearing suns. You can consider something like a very sensitive gravimeter and a moving mass like a simple pendulum. An observer sees the pendulum making the usual sinusoidal oscillations.  With his other eye, he sees the gravimeter deflect.

Did the deflection correlate with the apparent position of the cannonball (v_g = c) or was there a phase lag?

If there is a phase lag, what are the implications for cosmology? 

If v_g > c, does that imply that the gravimeter knows that the pendulum is moving before the pendulum actually moves? Time travel?

If v_g < c does that explain the accelerating expansion of the universe?