Black holes

[Farsight]

It's nice when you make definite statements, rather than vague ill-defined claims, because they can actually be evaluated. That one, of course, is false.

There is no reason in such an arrangement for the event A: "upper photon arrives at right side of lightclock" to be timelike separated from event B: "lower photon arrives at right side of lightclock". If it's spacelike separated, you can always choose a frame in which event B occurs first (for example, the rest frame of an observer moving at sufficiently high velocity in the correct direction).

We've been through this. Your motion through space alters your view of the world, but it doesn't alter the world. And gravitational time dilation is not symmetrical like the twins "paradox". When we replace the light beams with trains and bombs, you can't choose a frame where the upper train explodes. It's always the lower train. So my statement isn't false.

As for intervals, go back to the standard expression for a spacetime interval in flat Minkowski spacetime:

[latex]$ds^2 = -dt^2 + dx^2 + dy^2 + dz^2$[/latex]

We know it's related to Pythagoras' theorem, used in the Simple inference of time dilation due to relative velocity. One guy with a parallel-mirror light clock goes on an out-and-back trip whilst we stay at home with another one. There's no literal time flowing in the clocks, merely light moving at a uniform rate through space. The invariant interval is the light-path-length. It's the same in both cases, and we say it's time-like. A space-like interval is just the name we apply when two events are separated by a greater distance than light can travel in the intervening duration. We can kick that into touch by arranging our light beams like this: > .

 

[sol invictus]

We've been through this. Your motion through space alters your view of the world, but it doesn't alter the world.

Let me repeat your own statement for you: "It doesn't matter what coordinate system you use, the light in the lower parallel-mirror light clock never gets to the end before the light in the upper clock."

That's wrong, as my post demonstrates. Admit it, learn from it, and move on. Are you capable of that, Farsight?

And gravitational time dilation is not symmetrical like the twins "paradox". When we replace the light beams with trains and bombs, you can't choose a frame where the upper train explodes. It's always the lower train. So my statement isn't false.

Incoherent. Nothing my argument implies or relies on the situation being symmetrical, and I have no idea what your exploding trains are.

 

[Farsight]

...If someone takes the trouble to study what you've written, in an attempt to understand it; if they further think for themselves and do their own research; and if they write up, in some detail, an apparent contradiction in your idea (a pretty serious one, so it would seem), you do the cause of successfully communicating your idea to others great harm by persisting in ignoring them.

I've spoken at great length to posters here about Einstein's varying speed of light, which isn't my idea. However your physics knowledge is scant, you have no sincerity, and your sophistry is rubbish. I gave you plenty of my time, and it turned out that you were being deliberately dishonest, and a deliberate timewaster. Hence I now ignore you. Now go on, sling your hook kid, the big boys are talking physics.

So Farsight - you're saying he sees inbound light blueshifted?

With the artistic licence wherein our infalling observer doesn't get destroyed, yes. But note that's only what he sees. The inbound light doesn't change frequency. Conservation of energy tells you that.

So to you, a "wrong understanding" can make exactly the same set of predictions for every possible experiment that a "correct understanding" gives?

No. We reach very different predictions for black holes. The infalling observer bites the dust before the event horizon, Hawking radiation is out, they don't rotate, there's a parallel with the early universe re inflation, all sorts of interesting stuff opens up. We should move on from this discussion and talk about them. Or electromagnetism. It's amazing how people nowadays just don't understand it. It's like nobody has read the original Maxwell.

Time for tea, gotta go.

 

[edd]

Well I think it looks like Farsight thinks the answer to my question is that the infaller sees light coming from above blueshifted or not shifted at all. Would anyone like to buzz in from the other team? I think the answer has an impact on which clock appears to be running fastest.

 

[DeiRenDopa]

So Farsight - you're saying he sees inbound light blueshifted?

Also I was under the impression this situation had been under discussion - I think there is very little disagreement if any about observers that are stationary at the horizon.

(bold added)

Yes, it has been:

You can conduct the experiment when you and your two-parallel-mirror light clocks are in free fall. At all times the lower clock is below the upper clock, so you continue to see that the light beam in the lower clock goes slower than the light beam in the upper clock:

|----------------|
|----------------|

You can conduct the experiment when you and your two-parallel-mirror light clocks are in free fall. At all times the lower clock is below the upper clock, so you continue to see that the light beam in the lower clock goes slower than the light beam in the upper clock:

|----------------|
|----------------|

Let's be sure we're clear on precisely what you're saying. You're asserting that two light clocks in radial free fall near a black hole horizon, at relative rest, with one at larger r than the other, will not stay in sync (as judged by, say, an observer watching them from a freely falling position midway in between). Is that correct?

Because if so, you're wrong, and I can prove it easily. But first I want you to confirm that's what you're saying.

Let's be sure we're clear on precisely what you're saying. You're asserting that two light clocks in radial free fall near a black hole horizon, at relative rest, with one at larger r than the other, will not stay in sync (as judged by, say, an observer watching them from a freely falling position midway in between). Is that correct? Because if so, you're wrong, and I can prove it easily. But first I want you to confirm that's what you're saying.

No. Read what I said. I said You can conduct the experiment when you and your two-parallel-mirror light clocks are in free fall. At all times the lower clock is below the upper clock, so you continue to see that the light beam in the lower clock goes slower than the light beam in the upper clock. And you can't show me anything, because you and your two parallel-mirror light clocks are only falling because the speed of light at your feet is less than it is at your head. What are you going to do, presume it ain't so then reel off a pile of circular mathematics "proving" your own presumption?

No. Read what I said.

What you write is often incoherent and inconsistent, so "read what I said" doesn't help.

I said You can conduct the experiment when you and your two-parallel-mirror light clocks are in free fall. At all times the lower clock is below the upper clock, so you continue to see that the light beam in the lower clock goes slower than the light beam in the upper clock.

And that differs from my formulation.... how? Here it is again: "You're asserting that two light clocks in radial free fall near a black hole horizon, at relative rest, with one at larger r than the other, will not stay in sync (as judged by, say, an observer watching them from a freely falling position midway in between). Is that correct?"

If my formulation is accurate, you're wrong. If it isn't, you need to explain precisely why and how it differs from yours.

Farsight's response?

Silence.

 

[DeiRenDopa]

...If someone takes the trouble to study what you've written, in an attempt to understand it; if they further think for themselves and do their own research; and if they write up, in some detail, an apparent contradiction in your idea (a pretty serious one, so it would seem), you do the cause of successfully communicating your idea to others great harm by persisting in ignoring them.

I've spoken at great length to posters here about Einstein's varying speed of light, which isn't my idea. However your physics knowledge is scant, you have no sincerity, and your sophistry is rubbish. I gave you plenty of my time, and it turned out that you were being deliberately dishonest, and a deliberate timewaster. Hence I now ignore you. Now go on, sling your hook kid, the big boys are talking physics.

Noted*.

I wonder how many others - of the active participants here - will soon join me, in Ignored-by-Farsight-land?

* For the record (source):
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Farsight: Conclusion: what you don't do is try to hide the obvious with obviously facile assertions. People aren't that stupid.

Ziggurat: Who are these people of whom you speak?

Because so far, pretty much everyone BUT you agrees with DRD. And you agree with... nobody. Oh, sure, you claim Einstein agrees with you (by conveniently ignoring all the times he contradicts you), but don't you find it the least bit curious that nobody who's actually alive seems to be taking your side? Granted, the popularity of an idea doesn't prove its correctness or incorrectness, but why is it that you are unable to marshal any support from anyone else for your position? Even if you are the misunderstood genius you imagine yourself to be, what is it about your communications skills which so constantly fail to convince anyone?

Aren't you even the least bit curious?
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

 

[Mashuna]

It's true that the big boys are talking physics, and I'm enjoying reading it. And I guess that having an annoying little kid continually making the same mistakes is in some cases a useful catalyst for the discussion. It's just funny that Farsight considers himself one of the big boys.

 

[edd]

Mashuna: I've been finding some interesting things out on the way too.

Along the same lines I've found on the iTunes university Leonard Susskind's Cornell lectures on the subject can be recommended for those who have some solid grounding in college level physics but don't know anything about GR, if anyone reading is around there. I know some people who didn't get tensors for whom that was more helpful than the standard textbooks.

 

[edd]

I think reading back to DRD's summary just up there tht sol invictus has already pushed FS to the problems I was pointing at much later, albeit by using more of a special case.
It really is important to understand that the contention is about things falling through the event horizon and not those staying there. The event horizon is basically a locus of points on light cones for light rays heading directly out - and trajectories on a light cone are properly frozen (thanks to sol here in the past for clearing up one misunderstanding I'd somehow got there myself about such trajectories).
Now just because a point has such trajectories passing through it - or in the case of the event horizon along it - does not mean all trajectories through it are frozen.

My retina right now is on the surface of many light cones - it'd better be or it'd be kind of useless for seeing - but it certainly isn't following those trajectories and is not frozen in time (or again it'd be kind of useless). Equally an infalling observer passes through the light cones that define the event horizon but since it's going through and not along the physics does not need time to 'stop'.

 

[dlorde]

We normally think of the infalling observer moving faster and faster and faster as he approaches the black hole. If say he was "dropped from infinity", he'd be moving "at the speed of light" at the event horizon. However the speed of light at the event horizon is zero, and he can't be moving faster than that. So he doesn't fall faster and faster.

Forgive my uninformed interruption, but if this is true, and he can't be moving faster than c, which is zero at the EH, then his infalling speed must decrease to zero at the EH.

To my naive logic, this suggests that from some large distance outside the EH, he is accelerated inwards by the BH gravitational field to reach a maximum velocity at some distance from the EH, and then decelerates to zero at the EH (suggesting a negative gravitational gradient extending from the EH to the point of maximum velocity).

This doesn't sound right, but why is it wrong (assuming c == zero at the EH) ?

 

[edd]

dlorde: I take it it'd really bake your noodle to think of a large black hole passing at high speed through the space occupied by some lump of matter. Clearly it has to disappear from that volume and somehow be carried along by that black hole, and yet for Farsight it clearly has to remain at that fast moving event horizon.

It's not a problem for the rest of us as you simply see increasingly faint and heavily lensed light leaving the vicinity of the hole - but that's just outgoing light escaping - not the infalling matter.

At least I think that's the right way to think about it - someone else might well correct me.

 

[dlorde]

dlorde: I take it it'd really bake your noodle to think of a large black hole passing at high speed through the space occupied by some lump of matter. Clearly it has to disappear from that volume and somehow be carried along by that black hole, and yet for Farsight it clearly has to remain at that fast moving event horizon.

Consider my noodle baked

It's not a problem for the rest of us as you simply see increasingly faint and heavily lensed light leaving the vicinity of the hole - but that's just outgoing light escaping - not the infalling matter.

That's what I thought - until Farsight put me right

 

[Reality Check]

Hi dlorde, the in-falling observer moves faster and faster as they fall toward the black hole. Their velocity as they pass through the event horizon is not c. It is the same as any object falling in a gravitational field. The in-falling observer does not even notice passing through the EH!

Farsight is wrong when he states that the speed of light is zero at the event horizon. The local speed of light is always c. An external observer can measure a coordinate speed of light that depends on the coordinate system that they use. See the previous posts.

 

[Guybrush Threepwood]

Forgive my uninformed interruption, but if this is true, and he can't be moving faster than c, which is zero at the EH, then his infalling speed must decrease to zero at the EH.

To my naive logic, this suggests that from some large distance outside the EH, he is accelerated inwards by the BH gravitational field to reach a maximum velocity at some distance from the EH, and then decelerates to zero at the EH (suggesting a negative gravitational gradient extending from the EH to the point of maximum velocity).

This doesn't sound right, but why is it wrong (assuming c == zero at the EH) ?

I was wondering that too.

Hi dlorde, the in-falling observer moves faster and faster as they fall toward the black hole. Their velocity as they pass through the event horizon is not c. It is the same as any object falling in a gravitational field. The in-falling observer does not even notice passing through the EH!

According to MTW GR, sure, but not in FGR. What I'd like to see is farsight explain what happens to an infalling observer in his reality. I don't recall him doing that, he's explained what happens at the EH (time stops) but not how you get there.

 

[DeiRenDopa]

One thing which I think no JREF member, active in this thread, has so far done is to show that the Einstein quotes Farsight has posted, many times, are inconsistent with FGR, as presented by Farsight.

In this post I will attempt to do just that.

LOL. We've got sol misunderstanding the principle of equivalence and trying to say we really do accelerate upwards, still fighting shy of saying what his expression describes, and offering to provide a circular "proof". We've got Zig calling me a crank whilst clinging to his risible negative carpet, and wilfully ignoring the Schwarzschild blowup and getting all pompous about temperature. We got Dopa accusing me of "fundamental misunderstanding of the relationship between physics and reality" whilst proposing a tedious gedankenexperiment that tries to subvert what I said into multiple speeds for the object of your choice. We've got ben telling us the sun goes round the earth because all coordinate systems are equally valid. We've got Clinger still hiding behind maths and doubtless wittering on about homework. We've got ct wilfully ignoring the scientific evidence and Einstein and trying to play the "your theory" card. Oh, and dishonestly backing Dopa's and getting gravitational time dilation back to front. And last and least we've got RC asking me about the perihelion advance of Mercury when I've repeatedly pointed out the Gerber controversy.

And still nobody will address the hard scientific evidence of the Shapiro delay and light clocks losing synchronisation at different elevations:

|------------------|
|------------------|

Nobody will concede that this demonstrates with crystal clarity that the speed of light is not constant. And nobody will concede that that's what Einstein said:

1911: If we call the velocity of light at the origin of co-ordinates co, then the velocity of light c at a place with the gravitation potential Φ will be given by the relation c = co(1 + Φ/c²).

1912: On the other hand I am of the view that the principle of the constancy of the velocity of light can be maintained only insofar as one restricts oneself to spatio-temporal regions of constant gravitational potential.

1913: I arrived at the result that the velocity of light is not to be regarded as independent of the gravitational potential. Thus the principle of the constancy of the velocity of light is incompatible with the equivalence hypothesis.

1915: the writer of these lines is of the opinion that the theory of relativity is still in need of generalization, in the sense that the principle of the constancy of the velocity of light is to be abandoned.

1916: In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when die Ausbreitungs-geschwindigkeit des Lichtes mit dem Orte variiert [=the speed of light varies with the locality]. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust.

Clearly, Einstein expects - per these quotes - that the speed of light at a location will have just one value. And that that value will be a function of "the gravitation potential Φ".^^

Yes? No? Maybe? Don't know? Something else??

Also equally clearly, Farsight puts great stock in his parallel-mirror clocks, at different locations, or locations at which the gravitational potential differs, as "empirical evidence that the speed of light varies with gravitational potential"^.

To demonstrate an inconsistency, it is sufficient to show that Farsight's method can, logically, produce two distinct values for c, at one location (by definition there is only one permitted value for the gravitational potential at one location).

Here is just such a demonstration:
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
Here is a pair of parallel-mirror clocks (P-MLCs), per Farsight, but with a snapshot of Brain-M's bouncing light packets. The asterisks represents positions of the light packets. The P-MLCs are labeled. Per Farsight, P-MLC A is at a lower elevation than P-MLC Q. Here the light packets have just left the starting gate left-hand mirror:

|*-*-*-*-*-*-*-*-*-*-*-*| Q
|*-*-*-*-*-*-*-*-*-*-*-*| A

And here the light packet in P-MLC Q has reached the buffer right-hand mirror:

|*-*-*-*-*-*-*-*-*-*-*-*| Q
|*-*-*-*-*-*-*-*-*-*-*-*| A

The distance between the mirrors, in both P-MLCs, is 12 units; call these units btrs, abbreviated b. The P-MLCs 'tick' once per second; i.e. when the light packet reaches the right-hand mirror, one (local) second has elapsed (from the time the light packet left the left-hand mirror). The local speed of light - which is always the same (even Farsight agrees) - in these units is 12 btrs per second, b/s for short.

Per Farsight, the speed of light at location A is 3 b/s, when measured by the (distant) P-MLC Q.

Everything OK so far? As in, this is all exactly as Farsight prescribes?

Now consider another P-MLC, at a different location (elevation). Label this one F. As F is at a higher elevation, the Farsight diagram is much the same (I'm not going to repeat the 'light packets at the starting gate' diagram), except for the fact that F's elevation is not the same as Q's:

|*-*-*-*-*-*-*-*-*-*-*-*| F
|*-*-*-*-*-*-*-*-*-*-*-*| A

In this case the speed of light at location A is 6 b/s, when measured by the (distant) P-MLC F!

So, we have the speed of light at location A being both 3 b/s and 6 b/s!

Now if we do the experiment again, this time with all three P-MLCs, and add a ruler (to show the location of the light packets), Here's what we have:

|*-*-*-*-*-*-*-*-*-*-*-*| Q
|*-*-*-*-*-*-*-*-*-*-*-*| F
|*-*-*-*-*-*-*-*-*-*-*-*| A
|*-*-*-*-*-*-*-*-*-*-*-*| ruler

And:

|*-*-*-*-*-*-*-*-*-*-*-*| Q
|*-*-*-*-*-*-*-*-*-*-*-*| F
|*-*-*-*-*-*-*-*-*-*-*-*| A
|*-*-*-*-*-*-*-*-*-*-*-*| ruler

Conclusion: Using the Farsight method, the speed of light at a location is, per Einstein, a function of the gravitation potential. However, the gravitational potential that it is a function of is not the local gravitational potential, but the difference between the local gravitational potential and that of the location at which Farsight's other clock is located!

Not quite what Einstein meant, right. Yes? No? Maybe? Don't know? Something else??
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Now I posted this demonstration earlier, in this thread. So you may be wondering how Farsight responded. Here's how: "I've spoken at great length to posters here about Einstein's varying speed of light, which isn't my idea. However your physics knowledge is scant, you have no sincerity, and your sophistry is rubbish. I gave you plenty of my time, and it turned out that you were being deliberately dishonest, and a deliberate timewaster. Hence I now ignore you. Now go on, sling your hook kid, the big boys are talking physics."

What do you think?

^ in case you don't recall, here is one example (there are plenty of others):

Since you've admitted that you haven't been able to demonstrate that the math doesn't work, the only way you could possibly know that it's wrong is if you have empirical evidence. Do you have any empirical evidence?

Yes. I've given it repeatedly. The speed of light varies with gravitational potential just like Einstein said. But people who are convinced that it's absolutely constant absolutely refuse to accept it. If I arranged two parallel-mirror light clocks at different elevations, you know that they won't stay synchronised. You also know that there's no literal time flowing between the mirrors, just light moving. If we used say dust in space, you'd be able to see the light beams moving like this:

|-----------------|
|-----------------|

Think of the light beams as racehorses. When one gets ahead of the other you say it's moving faster than the other. If somebody tried to tell you they were going at the same speed, you'd laugh at them. When they then tried to tell you that space was falling down, you'd tell them to stop wasting your time with fairytales.

^^ Never mind that the first four clearly refer to SR, and that the fifth clearly distinguishes between SR and GR (and is fully consistent with his - Einstein's - own words on GR, in many other documents of which he is the author).

 

[W.D.Clinger]

how Farsight general relativity simplifies Einstein's GR

I've spoken at great length to posters here about Einstein's varying speed of light, which isn't my idea. However your physics knowledge is scant, you have no sincerity, and your sophistry is rubbish. I gave you plenty of my time, and it turned out that you were being deliberately dishonest, and a deliberate timewaster. Hence I now ignore you. Now go on, sling your hook kid, the big boys are talking physics.

What do you think?

I think Farsight's legendary modesty prevents him from taking full credit for the truly incredible simplification he has achieved with his Farsight Theory of General Relativity (FGR).

To appreciate the full extent of Farsight's simplifications, consider Einstein's 1916 paper on The Foundations of the General Theory of Relativity, which is generally regarded as the most important paper Einstein ever wrote on that subject.

Einstein's preface says the absolute differential calculus (aka differential geometry) and non-Euclidean manifolds are "necessary for general relativity". FGR does away with all that.

Einstein refers to research by Gauss, Riemann, Christoffel, Ricci, and Levi-Civita. FGR doesn't have anything to do with those guys.

Einstein goes on and on about those coordinate transformations that make standard GR so darn complicated:

...all imaginable systems of co-ordinates...with respect to any substitutions whatever...any chosen system of reference...any system of co-ordinates...any substitution of the co-ordinates...independent of the choice of co-ordinates...any other system of co-ordinates...any choice of the system of co-ordinates...

FGR forbids coordinate transformations.

Einstein talks about covariant this and covariant that, with some contravariant things thrown in for variety. That's all about coordinate transformations, so FGR tosses it.

That gets rid of everything in Einstein's sections 5, 6, and 7. Everything in Einstein's sections 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and 21 depends on sections 5, 6, and 7, so FGR does away with all the math that has made Einstein's general theory of relativity so notoriously difficult to understand.

That brings us to Farsight's greatest triumph.

All of standard GR's falsifiable predictions depend on purely theoretical assumptions that Farsight was able to avoid in FGR.

Although standard GR has stood up to experiment quite well so far, there may come a day when some experiment contradicts standard GR. When that happens, standard GR will be falsified...but FGR will survive!

Some small-minded folk may complain that FGR is unfalsifiable, but that's unfair. FGR does make one falsifiable assumption: that what Farsight says is true.

That assumption can't be controversial, else Farsight would have provided evidence for it. I mention it only to defend FGR against those who would otherwise accuse FGR of being unfalsifiable.

As it happens, sol invictus and DeiRenDopa have already succeeded in identifying falsifiable predictions made by FGR:
http://www.internationalskeptics.com/forums/showpost.php?p=8150884&postcount=817
http://www.internationalskeptics.com/forums/showpost.php?p=8151313&postcount=825

I am not a physicist. I don't know whether those predictions of FGR have yet been confirmed by experiment.

I do know we should not let Farsight's humility keep us from acknowledging the unprecedented simplicity of Farsight general relativity.

 

[Brian-M]

They're orthogonal to the direction of travel, so length contraction doesn't apply.

Who said anything about length contraction? I didn't. That's not what I was talking about.

Like Feynman said. The light is still moving at c, but now it looks slower to you, because your panning means you don't see travelling observer's lateral motion.

Yes. That's exactly what I'm talking about. If you follow the mirrors so you can't see the forward/lateral/upwards/whatever motion it looks slower, but really isn't.

When you look at the lower of your two clocks in the gif, it doesn't look like it's going slower. And it isn't really travelling a longer distance. Again length contraction doesn't apply. It is going slower.

Of course it's traveling a longer distance, the clock is moving at a higher velocity and so the light is bouncing off the mirrors at a more obtuse angle in order to bounce back and forth between these rapidly moving mirrors without missing them.

Since the light is now moving diagonally between parallel lines (the "lines" being the path of the mirrors) instead of at right angles, it must covering more distance, which means it must take longer to bounce back and forth between the mirrors.

If an observer is moving at a rate that keeps them a constant distance from the mirrors (or pans a telescope), this diagonal motion is not apparent. An illusion is created of the light bouncing back and forth off stationary mirrors at right angles at a slower rate.

But since the mirrors are moving the light isn't actually going any slower.

In the GIF, imagine the mirrors flying away from you into the screen. The bottom pair of mirrors are flying away from you faster, so the light has to travel a longer diagonal distance to get from one mirror to the other, creating the illusion that the light is slower.

But since you're also flying forward into the screen so that you're always a constant distance from the mirrors, it doesn't look like the mirrors are moving at different speeds, or even moving at all.

From the viewpoint of a stationary observer floating above, it's obvious that the bottom pair of mirrors is moving faster and that the light is zig-zagging at different angles between the different pairs of mirrors.

(If you're wondering how you can remain at a constant distance from the mirrors if the mirrors are moving at distant speeds, you're standing in a rotating space station. Since the top pair of mirrors are at a smaller radius from the hub, they travel at a slower speed than the lower mirrors.)

At this point I don't care if you get it or not, I'm just trying to make myself clear. If you want to know how this relates to what happens under gravity, re-read my previous point.

And gravitational time dilation is not symmetrical like the twins "paradox".

I thought the whole point of the twins paradox was that it wasn't symmetrical. You end up with one twin older than another.

 

[dlorde]

Hi dlorde, the in-falling observer moves faster and faster as they fall toward the black hole. Their velocity as they pass through the event horizon is not c. It is the same as any object falling in a gravitational field. The in-falling observer does not even notice passing through the EH!

Yes, this has been my understanding for some time.

Farsight is wrong when he states that the speed of light is zero at the event horizon. The local speed of light is always c. An external observer can measure a coordinate speed of light that depends on the coordinate system that they use. See the previous posts.

I realise that is the consensus position - I've been following the whole thread and have learned quite a bit from you guys. What I was clumsily trying to do was put myself into Farsight's shoes to see how he has come to his different interpretation where all infalling particles, including photons, end up timelessly squished on the event horizon forever.

I was hoping Farsight might be able to explain how, despite the rapidly increasing gravitational force closer to the BH, everything comes to a halt at the EH ? and how a BH forms & grows - is the singularity always the smallest possible, if, as soon as an EH forms, nothing can get past it? And what happens when the mass of matter accreted above the EH becomes greater than the mass of a BH of that size - does a new EH form around it, or does the EH expand somehow? How can it expand if time itself has stopped at the EH? How densely can the matter above the EH be packed? etc., etc.

Just curious to see how he explains it.

 

[DeiRenDopa]

As Farsight is completely ignoring me, he won't bother reading this post (or can't read it, if he has put me on Ignore). So the audience I'm writing it for is every other active participant in this thread*.

Earlier in this thread (this post), I took Farsight to task for being sloppy in his Gedankenexperiment setup, as it concerns two parallel-mirror light clocks, separated by ~0.3 m in elevation, in a room at rest on the surface of the Earth.

Not long afterwards, in two separate posts I showed - clearly - that his "exploding trains" analogy was also poorly specified (here and here).

(there's a separate sub-thread - starting with this post and this one - concerning a quite different physical scenario; Farsight applied the exploding trains analogy there too, but it's not directly relevant to the case I'm examining in this post).

What I want to talk about is these two posts:

The best way to appreciate this is to replace the light beams with trains. The first one to hit the buffers detonates a bomb on the other one. It's always the lower train that blows up. Your motion and your distance affects the way you see things, but you don't see the upper train blow up.

Really?

Let's take the two clocks trains separated by ~0.3 m in elevation (or vertically, if you prefer), in a room, in a lab. Not in freefall.

That's the optical clock-based experiment you have mentioned many times in this thread.

How does the bomb on the lower train know the upper train has hit its buffer?

For example, does the buffer send a flash of light (or radio, or gammas, or ...) out (either in a narrow beam or isotropically or any way in between), which the bomb on the other train receives, and then blows up?

If I am not mistaken - and I very well could be - the light-travel time between the two tracks/trains is far, far greater than the (remaining) time it would take the lower train to reach its buffer. And when it reaches its buffer, it sends a signal to the bomb on the other train, which means that other train will also explode (while sitting at its buffer).

If so, then we have two explosions, both of which - objectively - take place while the trains at their respective buffers.

If I am not mistaken - and I very well could be - the light-travel time between the two tracks/trains is far, far greater than the (remaining) time it would take the lower train to reach its buffer. And when it reaches its buffer, it sends a signal to the bomb on the other train, which means that other train will also explode (while sitting at its buffer).

Yep, it depends on the distances involved. You could arrange it so it's not clear which wins its race, or you could make it so it's unambiguous (a signal from one winning reaches the finish line of the other before the other does), but generally it's not just the race or clock we have to consider, but the observation of it as well (or the communication between the clocks).

As you might have expected, Farsight did not respond to either of these posts.^

Anyway, I decided to pursue this in some more detail, but not in this thread.

So I started a separate thread, Time for light to go 100m, vertically. Please feel welcome to join in the discussion in that thread (or merely lurk too, if you'd rather).

* lurkers: you are not my intended audience, but I'd love to hear from you, so you become active participants!

^ perhaps he also considers edd's "physics knowledge is scant", that edd "has no sincerity, and [edd's] sophistry is rubbish". I wonder if Farsight thinks he gave edd "plenty of my time, and it turned out that [edd was] being deliberately dishonest, and a deliberate timewaster"?

 

[Farsight]

...Along the same lines I've found on the iTunes university Leonard Susskind's Cornell lectures on the subject can be recommended for those who have some solid grounding in college level physics but don't know anything about GR, if anyone reading is around there. I know some people who didn't get tensors for whom that was more helpful than the standard textbooks.

Take care. Susskind will end up telling you about an elephant that's in two places at once. Take care with light cones too. Like reference frames and coordinate systems, they're abstract things. You can't point up to the sky and say "Look, there's a light cone. Your retina isn't actually "on the surface of many light cones". It's on the inside surface of your eye, your eye is in space, and light moves through space and terminates on your retina. It's important to stay very grounded with all this.