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#56049 - 06/14/16 08:56 PM Vectors and time reversal.
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
This thought experiment involves a friction-free snooker table, in a vacuum. The cue-ball is alone on the table, placed centrally on the balk cushion.

At t=0 the cue strikes it, sending it straight up the table at constant velocity, v, with constant momentum, p.

At t=1 it crosses the green spot.
At t=2 it crosses the blue spot.
At t=3 it arrives at the pink spot; at which point, time is reversed.

It crosses the blue and green spots at t=2 and t=1, respectively.

What happens to v and p, both are vectors, so at t=2 and t=1, in reversed time, the ball is travelling towards the balk cushion, but the vectors point away from it?
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#56052 - 06/15/16 02:03 AM Re: Vectors and time reversal. [Re: Bill S.]
paul Offline
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Registered: 03/21/06
Posts: 4136
constant velocity?
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#56053 - 06/15/16 08:17 AM Re: Vectors and time reversal. [Re: paul]
paul Offline
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Registered: 03/21/06
Posts: 4136
the green , blue and pink spots are not lined up with each other.

the 3 spots are not in a straight line so the ball would
experience a change in velocity if it did cross the green
spot and the blue spot and then it vectored to the pink spot.

however you can buy more pink spots and place another pink spot
on the table lined up with the other 2 spots and then they would
all be in a straight line.

or you could simply exchange the green spot with the brown spot
in the OP if thats the way you do things.




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3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.

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#56055 - 06/15/16 02:34 PM Re: Vectors and time reversal. [Re: Bill S.]
Orac Offline
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Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
Originally Posted By: Bill S.
What happens to v and p, both are vectors, so at t=2 and t=1, in reversed time, the ball is travelling towards the balk cushion, but the vectors point away from it?

Now place yourself at the other far end bounce cushion and time is always running backwards according to you even when you are waiting to strike the ball ... you may need to think about it for a second smile

Time only goes the right way when the ball has bounced and is on it's way back.

Bill S new word for the day to learn about => Hamiltonian
https://en.wikipedia.org/wiki/Hamiltonian_mechanics

Quote:
The time evolution of the system is uniquely defined by Hamilton's equations

You have created a Hamiltonian description and it's just a mathematical description. The article even tells you how to deal with your vector descriptions and the physical meaning.

Want me to prove to you that your description is completely meaningless ... pick up the ball somewhere in it's travels.

So which way is your time running when I pick the ball up? laugh


Edited by Orac (06/15/16 02:37 PM)
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#56057 - 06/15/16 05:57 PM Re: Vectors and time reversal. [Re: Orac]
paul Offline
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Registered: 03/21/06
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oh , Im on orachnids ignore list ... LOL




you may need to think about it for a second smile
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#56067 - 06/16/16 06:31 PM Re: Vectors and time reversal. [Re: Bill S.]
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
Quote:
At t=1 it crosses the green spot.


Of course, that should have been the brown spot!

Peter Ebdon plays snooker (very well) in spite of being colour blind; but even he would not put the green on that spot. smile
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#56068 - 06/16/16 07:42 PM Re: Vectors and time reversal. [Re: Orac]
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
Quote:
Now place yourself at the other far end bounce cushion and time is always running backwards according to you even when you are waiting to strike the ball ... you may need to think about it for a second


I thought about it for a few seconds, but wherever I stand, time runs the same way.


Thanks for the link. I understand that Roger Penrose's father tought him calculus the evening before he started it at school. It would take me a lot longer, and I don't have the time; so if there is something there that would enlighten me, it will need to be simplified. Lots! smile

Quote:
So which way is your time running when I pick the ball up?


Same way it was before you picked it up.
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#56069 - 06/16/16 11:55 PM Re: Vectors and time reversal. [Re: Bill S.]
paul Offline
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Registered: 03/21/06
Posts: 4136
is this your thought experiment or someone elses?

the reason Im asking is because if it were possible to
reverse time then both v and p would be in the reversed direction.

my reasoning on this is that when the ball strikes the cue
at the balk cushion its momentum would be transferred into
the cue due to its v and mass.

if the experiment is only referencing time as in a frame by frame
event such as a video that is stopped as the ball reaches the pink spot and then reversed frame by frame then in that case both v and p would still point the same way.

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#56071 - 06/17/16 03:23 AM Re: Vectors and time reversal. [Re: Bill S.]
Orac Offline
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Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
Originally Posted By: Bill S.
Thanks for the link. I understand that Roger Penrose's father tought him calculus the evening before he started it at school. It would take me a lot longer, and I don't have the time; so if there is something there that would enlighten me, it will need to be simplified. Lots! smile

Instead of using the colors on the table make it a distance from the point you choose (you selected the cushion) and write the equation for speed and time.

The formula is straight forward:
distance = speed ball hit at * time from ball strike

That is the hamiltonian you are describing in your thoughts. So select a table a length (say 2m), select a suitable speed say (0.25 m/sec) now write the distance for each of the 10 seconds following the ball strike.

Do you see the problem with your hamiltonian ... => time from ball strike only conceptually only goes forward then your distance can only ever get bigger. When the ball hits the time reversal pink spot the distance actually starts decreasing the weird behaviour you think is strange.

So what is required is to negate one of the terms in the hamiltonian at the point of reversal.

That can be done in one of TWO WAYS.
1.) Turn speed into a velocity one way being +ve the other -ve
2.) Invert time at the reversal point, notionally its zero at the reversal point so time has a +ve and -ve component

Surprised me but Paul correctly worked that out but he went for both, they don't both reverse you get to choose one.

The problem is there exists no selective process you can choose that tells you which is right or wrong or even if it has physically correct meaning.

What I was trying to get you to realize in your thought experiment was that your statement time reverses at the pink spot is totally SUBJECTIVE.

I can solve your problem in two other ways than your choice

1.) By simply putting time = 0 at the pink spot and it doesn't matter which way time runs the calculation will work. So time is zero at the pink spot and radiates out from that point.
2.) I can simply have an invisible bumper at the pink spot and the ball actually bounced inverting it's velocity.

So even if I could physically do your experiment and view it I would not in any way deduce time had reversed and it introduces no anomaly.

Originally Posted By: Bill S
Same way it was before you picked it up.

So now we take the other choice you didn't consider that when the ball reached the pink spot you the observer started going backward in time. If the pink ball is a point with a time reversal why can't the point you are observing from be one.

Picking up the ball is a metaphor for some event that is nothing to do with what is being observed but what the observer does or has done to them.

Your experiment assumes you the observer are GOD and know exactly what is actually happening .. an impossibility for us mere mortals.


Edited by Orac (06/17/16 07:15 AM)
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#56072 - 06/17/16 12:10 PM Re: Vectors and time reversal. [Re: paul]
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
Quote:
the reason Im asking is because if it were possible to
reverse time then both v and p would be in the reversed direction.


For the sake of discussion, I'm accepting relativity. This means that t=2 (for example) is an immutable spacetime event. It cannot occur with v and p running R to L, then L to R.
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#56073 - 06/17/16 12:19 PM Re: Vectors and time reversal. [Re: Orac]
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
Quote:
Your experiment assumes you the observer are GOD and know exactly what is actually happening .. an impossibility for us mere mortals.


This is more-or-less my starting point. There are too many impossibilities to allow physical time reversal to work. Or are there? Do we impose the "impossibilities" through our own maths and physics.

BTW; I am "god"; so are you; but that's a whole different line of logic. smile
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#56074 - 06/17/16 12:26 PM Re: Vectors and time reversal. [Re: paul]
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
Quote:
is this your thought experiment or someone elses?


It's my thought experiment, but I got the idea from somewhere. I'll let you know if/when I remember where, because I'm fairly sure there were other time related issues there as well.

Yes - you are "god", too. Feels good, doesn't it? smile
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#56075 - 06/17/16 01:40 PM Re: Vectors and time reversal. [Re: Bill S.]
paul Offline
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Registered: 03/21/06
Posts: 4136
well if its about "spacetime" then I personally dont care.

I just cant think of a valid reason to care.

right , thats it.
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#56076 - 06/17/16 02:37 PM Re: Vectors and time reversal. [Re: Bill S.]
paul Offline
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Registered: 03/21/06
Posts: 4136
Quote:
Yes - you are "god", too. Feels good, doesn't it?


at first when I read that I pictured a fish swimming in a
stream minding his own business carrying out his daily
routine who was startled by a plopping
sound as if something had been thrown into the stream ...

and I can see through the eyes of the fish as he swims over
to the location where he heard the sound coming from ...

hes looking at a shinny bright metallic object with a
big fat juicy worm dangling from it ...

the shinny metallic object and the big fat juicy worm
then begin to jump up and down as if it ha been startled by
the fish , and then the fish sees a tiny clear plastic line
that is attached to the bright shinny object that leads
up and out of the stream.

at first the fish was hungry but now hes curious so he quickly
darts through the water following the plastic line then suddenly
he is out of the water flying through the air and out of one
of his eyes he sees a man standing on the bank holding a
fishing rod and quickly tugging it up and down with a really
big grin on his face ...

and standing just behind the man is a really huge grizzly bear
who towers high above the mans head and to the right of the mans
head is a brown blur that is quickly approaching the mans head
the fish glances at the bears head , the bear has a really big grin on its face.

about that time the fish plops back into the stream and as
he does he hears a extremely large splash and feels a jolt
as he is swimming back to where the bright shinny metallic
object was he notices that the plastic line is now laying flat
on the bottom of the stream and the worm has crawled off of
the bright shinny metallic object ...

the fish swims over to the worm and asks the worm if he
would like to see the inside of his stomach and the worm said
that was exactly what the man asked you when he saw you just
before he tossed me into the stream.

as the fish is looking at the worm crawling away , the water
is quickly changing to a bright shinny red color the next thing the fish feels is something hes never experienced before ... there is no water ... he cant breath .. then . he sees the bears teeth he both hears and feels a loud crunch and then darkness.

the moral of this story is that theres always something bigger
than you are.

and of course the bait got away.
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3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.

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#56077 - 06/17/16 08:29 PM Re: Vectors and time reversal. [Re: paul]
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
OK, forget about spacetime, if you wish.

At t=2 the ball crosses the blue spot, R to L.

At t=3 it arrives at the pink spot; at which point, time is reversed.

At what time does the ball cross the blue spot, L to R?
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#56079 - 06/17/16 08:50 PM Re: Vectors and time reversal. [Re: Bill S.]
paul Offline
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Registered: 03/21/06
Posts: 4136
if time could be reversed then the measurements
would also be reversed.

t=2

time is a measurement and nothing else.

so its like using a measurement tape.

measured out to t=3 then back to t=2

why?

because if time were forwarded again at the blue spot

then history would repeat in the same order.

and the ball would once again approach the pink spot
or t=3

else

if you advanced the time measurement to t=4
as the ball crosses the blue spot then
time could not repeat if the time were forwarded
at the blue spot because the next available time
measurement would be t=5

and only 3 units of time would have occured.

t=1 t=2 and t=3





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3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.

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#56081 - 06/19/16 01:39 PM Re: Vectors and time reversal. [Re: Bill S.]
Orac Offline
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Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
Originally Posted By: Bill S.
For the sake of discussion, I'm accepting relativity. This means that t=2 (for example) is an immutable spacetime event. It cannot occur with v and p running R to L, then L to R.

immutable spacetime event???? ... in relativity? Really?

Refresher ... Newtonian physics has immutable space and time events. Einstein wanted that feature in Relativity but he lost the argument and eventually worked it out that observers may see things differently.

To be blunt unless you are god you can't even identify that the ball didn't just go past t2 and keep sailing on and you started observing it go backward ... in a nutshell you have no way to know your observation is "absolute" or "real". This goes back to the time problem at the event horizon and you wanting to make it real.

You need to stop making things "real" and "immutable" just because you observe them. Go and read Einsteins train thought experiment.

I can tell you from your setup I can't determine a thing, I don't have enough information.


Edited by Orac (06/19/16 02:17 PM)
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#56082 - 06/19/16 01:55 PM Re: Vectors and time reversal. [Re: Bill S.]
Orac Offline
Megastar

Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
Originally Posted By: Bill S.
This is more-or-less my starting point. There are too many impossibilities to allow physical time reversal to work. Or are there? Do we impose the "impossibilities" through our own maths and physics.

In your experiment there is no way to sort forward from backward time in your experiment, you haven't given me enough to sort that out.

The biggest problem with your experiment is the result, which starts from an out there suggestion of a point at which time reverses.

So to make the "result fit" I am thinking what motions could happen to "observe that", and more direct out there results would be things like enter a worm hole and come out at the same entry point but going backwards. You are constructing the far out idea of a point at which time could reverse so I assume I am allowed to put a wormhole there as time is going backwards. So I believe in worm holes no but nor do I believe in points at which time reverses.

You need to have it clear in you mind when you talk of reversing time are you talking of really doing that or just reversing event order because the two things are not the same. Reversing event order is reasonably easy to do, reversing actual time not so much.

Paul correctly worked the LOGIC fail, that you can't be talking about real actual time as you won't go backwards because through the same section of space you initially were going forward. So why didn't you go backwards when coming thru the same section of space originally.

If this was your start point I think you need a new one smile


Edited by Orac (06/19/16 02:26 PM)
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#56084 - 06/20/16 09:16 AM Re: Vectors and time reversal. [Re: Orac]
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
Quote:
To be blunt unless you are god you can't even identify that the ball didn't just go past t2 and keep sailing on and you started observing it go backward ... in a nutshell you have no way to know your observation is "absolute" or "real".


While this is all, undoubtedly, true; games of snooker are played, and they are subject to the laws of physics. If you apply your reasoning to the events of the macro-world, how would a player know what any ball was going to do?
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#56085 - 06/20/16 09:26 AM Re: Vectors and time reversal. [Re: Orac]
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
Quote:
If this was your start point I think you need a new one


A tourist in Ireland stopped his car and asked a local man: “Could you direct me to Ballynanty?” The man thought for a moment, then said: “Ah, to be sure, if I was going to Ballynanty, I’d not start from here”. smile


Edited by Bill S. (06/20/16 09:27 AM)
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