Thoughts On GW150914

Jim1348
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RE: Here is a very key

Quote:
Here is a very key point in what follows :
Quote:
the rate that time passes is always slower in a stronger gravitational field than a weaker one

... where the weaker field may include the case where there is ( almost ) no field at all ie. our astronaut distantly viewing the black hole merger.


Doesn't that provide a method of time travel, into the future at least? As you move away from the stronger gravitational field into the weaker one, time speeds up, and you see events sooner. Though time always seems to pass at the same rate for you. And I don't know how you go back in time. But that is the case in any time travel scheme. How do you know that it is the "future"?

Mike Hewson
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RE: RE: Here is a very

Quote:
Quote:
Here is a very key point in what follows :
Quote:
the rate that time passes is always slower in a stronger gravitational field than a weaker one

... where the weaker field may include the case where there is ( almost ) no field at all ie. our astronaut distantly viewing the black hole merger.

Doesn't that provide a method of time travel, into the future at least? As you move away from the stronger gravitational field into the weaker one, time speeds up, and you see events sooner. Though time always seems to pass at the same rate for you. And I don't know how you go back in time. But that is the case in any time travel scheme. How do you know that it is the "future"?


Yep, you will travel into the future at a slower personal rate with respect to those in lower gravity fields. This was correctly illustrated in the movie Interstellar when the crew that went down to the first planet they visited ( quite nearby the black hole ) spent a few hours by their reckoning but on return to the mother ship the guy who'd stayed was some ~ 25 years older. I think the quoted rate was about seven years per hour .... however unlike Special Relativity this is always asymmetric and all parties will agree to the relative rates and who has the faster/slower clocks. If the astrophysicist chap ( mother ship ) was watching them stuff about on that planet's surface - getting swamped by tidal waves etc - then it would be an intensely boring slow-mo for him to view over that quarter century.

You'd know it was into the future by biological cues if you like. Indeed at the end of the movie the main adult character ( Matthew McConaughey ) finally catches up with his daughter who is then in late age while he is barely older since their last meeting when she was a ten something year old child. Again both would realise whose clock had what rate.

I'll preempt something I will explain in detail later : in Special Relativity ( no acceleration and no gravity ) we get mutual/symmetric clock slowing by relative ( high ) velocity. That occurs as the clocks are always changing their separation. But in General Relativity you can get one of the clocks slowing down even though there is no distance change b/w the two. That is acceleration can occur without relative spatial movement : this is what we call gravity. This is a most bizarre conclusion on everyday grounds and the secret to understanding that is time. It can be a real mind bender ! :-)))

Cheer, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

AgentB
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RE: Here is a very key

Quote:

Here is a very key point in what follows :
Quote:
the rate that time passes is always slower in a stronger gravitational field than a weaker one

... where the weaker field may include the case where there is ( almost ) no field at all ie. our astronaut distantly viewing the black hole merger. This has been tested over a separation of a few hundred feet in altitude on Earth ( the lower clock ran slower with respect to the higher clock ). If the Eddington expedition had performed spectroscopy on the starlight of those stars studied, they would have noticed a lowering of the frequency of those spectral lines in addition to spatial path deflection.

OK I had wondered whether the simulations included any frequency shift of the stars behind. Then i thought, if it were a stationery, not spinning common garden variety black hole then any photons would gain going in and lose going out and so be untouched. Is that not the case?

Mike Hewson
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RE: RE: Here is a very

Quote:
Quote:

Here is a very key point in what follows :
Quote:
the rate that time passes is always slower in a stronger gravitational field than a weaker one

... where the weaker field may include the case where there is ( almost ) no field at all ie. our astronaut distantly viewing the black hole merger. This has been tested over a separation of a few hundred feet in altitude on Earth ( the lower clock ran slower with respect to the higher clock ). If the Eddington expedition had performed spectroscopy on the starlight of those stars studied, they would have noticed a lowering of the frequency of those spectral lines in addition to spatial path deflection.

OK I had wondered whether the simulations included any frequency shift of the stars behind. Then i thought, if it were a stationery, not spinning common garden variety black hole then any photons would gain going in and lose going out and so be untouched. Is that not the case?


Yup. If that stationary configuration is constant then it would be so.

{ Bear in mind the relative distance of source and observer. A very distant star might still look frequency shifted if the observer is deeper into the well than the source. So we have to be quite careful about specifying the situations we speak of .... }

Roger Penrose came up - some time ago - with the idea that you could snatch energy from a rotating black hole by a clever type of gravitational boosting somewhat like what we do to punt space probes to the outer solar system. I see no reason why this could not apply to photons .....

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

astro-marwil
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Hallo! Is the gravitational

Hallo!
Is the gravitational wave by prinzip free of dispersion or was it not observed at GW150914?
Maybe, there was no material concentration within a radius of several hundred lightyears around GW150914. In no scientific papers about GW150914 the words dispersion or absorption is included. That mean to me, that the GW doesn´t interact with real materia, dispersing energy from the GW. This seems to me somewhat peculiar.

Kind regards and happy crunching
Martin

Mike Hewson
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RE: Hallo! Is the

Quote:

Hallo!
Is the gravitational wave by prinzip free of dispersion or was it not observed at GW150914?
Maybe, there was no material concentration within a radius of several hundred lightyears around GW150914. In no scientific papers about GW150914 the words dispersion or absorption is included. That mean to me, that the GW doesn´t interact with real materia, dispersing energy from the GW. This seems to me somewhat peculiar.

Kind regards and happy crunching
Martin


I think that is still an open question. Absorption/dispersion etc issues require rather higher order analysis and more data to get there. Presently we have only measured either strength at emission ( Taylor/Hulse ) or at reception ( GW150914 ) but these are different objects. To work out changes during propagation for a given wave you need ( direct/indirect/proxy ) information from more than one locale.

Interestingly in this adjacent thread the article referred to mentions one FermiLAT event which is consistent with GW150914 - that having an awfully broad margin of fit though. It is intriguing that the energy was in about the right range : 'hard xrays'.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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Proper Time ... or ... Get

Proper Time ... or ... Get Your Own Clock !

A good definition of proper time is :

Quote:
The proper time that passes between two events is the time ticked off by a clock that passes through said two events.


Notes :

- it is a directly measurable quantity or may be deduced from measurement.

- there are no 'absolute time values' so this means only a measured interval ie. one later clock reading minus an earlier clock reading.

- no clock goes at the speed of light. As a matter of consistency we say that the proper time for a photon that goes through two events is zero. This agrees, for example, with limiting values based upon sub-light speed objects.

Example : at first they were called 'mesotrons'. This name didn't last for long, only a few weeks. Then it was called the 'mu mesotron', followed by 'mu meson' and then 'muon'. Later 'meson' was used for something else entirely. Muon stuck. Think of it as a fat electron. It doesn't last long at all in human terms. It decays to something elses in a typical timeframe which is quoted as a half-life. This means that any single muon has a constant probability of decay in any given time interval. One can deduce this probability by observing very many muons ( in identical circumstances ), do the math and this probability is evident.

What 'identical' circumstances ? Now if you study these muons at low speed - with respect to the speed of light - then the number for the half life is about 2 microseconds. So per one hundred muons at some point in time, about 50 will remain as such approximately two microseconds later.

So here is the curious thing : some muons are produced in cosmic ray showers eg. amazingly energetic protons firing into Earth's atmosphere from outer space. These muons are rather more stable as they go close to light speed. Their half life is much longer and so their probability of decay per time interval must be less. Right ? Well, yes and no.

From the point of view of human observers, yes. For the muon, no. The muon has a slower clock at higher speed as we will measure it. It's proper time b/w creation ( from a cosmic ray collision with an atmospheric atom's nucleus ) until decay ( into other things ) is the interval in question. When averaged over many instances etc the half life is still that same value as at low speed, but only when we use the clock that runs with the muon.

So the language/logic here is that proper time is the 'right' one to use for such things. That's the usage at least. When something is assessed from the point of view of something co-moving - a clock that goes through two events - that is the 'natural' rate to consider.

Be careful. In reality, and all too often implicitly, we can only truly merit the comparisons b/w two proper time intervals. So sitting here at my study desk I have a clock that for at least a while will serve as my proper time. Someone passing by in a car going on an up-country trip will have, say, their dashboard clock sufficing as their proper time measuring device. By a measuring system that I may have set up, this traveller's time from going past my house to some distant destination will have some value. However that is my business and my viewpoint. I haven't got a particular single clock going through the start and endpoints of the other person's journey. There is one of those : it is the in-car clock. Not mine. My clocks either measure my own personal time or another time interval altogether ( which is not labelled as 'proper' ).

Let's expand our thinking. I'll get a wrist watch and wear it all the time. Sleep with it on. Get up in the morning. Go to the toilet, brush teeth, snatch breakfast, pat my wife, kiss the dog, drive to work, do stuff there, knock off, drive to hardware store, buy light globes, drive rest of way home, kiss my wife and pat the dog this time, have dinner ....

The wrist watch that I wear if very carefully compared with my wife's wrist watch - we are each present at the kiss/pat events - both measure proper times. A proper time interval for me. A proper time interval for her. If those timepieces are sufficiently precise we will detect a disagreement about those intervals. We have each undergone different velocities, different accelerations, slightly different gravity field strengths etc .... as the day progresses. In human lifetimes the effect is ridiculously small. Indeed you could argue that you could never measure it. But if you get out of the human range of experiences - go to The Suburb of The Muons for instance - then the effect is definitely seen.

So what is the comparison b/w the proper times of a distant observer of a black hole and another that descends into the black hole ? Next up :

You Take The High Road And I'll Take The Low Road !

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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You Take The High Road And

You Take The High Road And I'll Take The Low Road !

OK. The reason for discussing the comparison b/w distant and descending viewpoints around black holes is that what happens to material bodies tells us much of what happens to light also. If you please we could do this as a Q & A ... :-0

Setup : two observers sitting in space rather far away from a black hole, but nearby one another so that each with an identically constructed clock may calibrate one against the other's. Let them do so and hence mark their starting times ( some mutual time value that we may as well call 'zero' ) and rate of value transition as equal ie. one second passing on one clock will equal one second passing on the other clock ( initially at least ). Let us label the observer that will be staying put with her clock HighGal and the other with his clock LowGuy. LowGuy is going down and even worse won't be coming back later on. HighGal gets to live out her expected lifetime otherwise unperturbed by approaches to black holes*. We will make the reasonable a priori assumption that from now on any difference in clock readings will be due to different circumstance/situations in the relativistic sense and not, say, intrinsic fault in the technical operation of either device. For this discussion let us allow arbitrary precision of either without regard to technical aspects including quantum mechanics. Either clock will function as long as we like too. Also ignore the rest of the Universe for the moment.

Questions 1 : Suppose LowGuy chickens out of his legally binding contract and doesn't go down.

(a) What will we see at their common location one thousand years later and why ?

(b) What will be the clock readings ? Same or different ? What could we deduce either way ?

{ Answers to be submitted with a one hundred dollar note please. Points awarded for humor, but only if not also subject to forum moderation. :-)) }

Cheers, Mike.

* Sadly she did sign up for an interview series with Sixty Minutes though.

{ Ouch ! Mike you are a naughty commentator .... }

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Kavanagh
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I am a sucker for these sorts

I am a sucker for these sorts of questions. There will be two sets of dry bones with magically ticking clocks telling the same time. Oh! Who set that thousand years?

Richard

AgentB
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RE: (a) What will we see

Quote:

(a) What will we see at their common location one thousand years later and why ?


i guess this depends where we are, whether LowGuy and HighGal both have been equally bone idle before turning into bones, and what we have been doing for our thousand years?

LowGuy and HighGal being equally idle is a reasonable assumption, based on the fact the rest of the Universe is not watching. If the Universe is observing then strange quantum-like time effects will occur if a bathroom is involved.

If i don't have a clock, do i know what a thousand years is? (*)

Quote:

(b) What will be the clock readings ? Same or different ? What could we deduce either way ?

Same, if different then you know who has been to the bathroom.

Gravity wave meets 100 dollar note

(*) I tick therefore i am?

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