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#54553 10/21/15 06:23 PM
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This is a thought experiment, so we can accept that there is a celestial body (let’s call it Astra) exactly 10 ly from Earth; in Earth’s RF. We can also accept a space trip to/from Astra at 0.8c, without complicating things with acceleration and turning.

Alice and Bob are on Earth. Bob remains there while Alice makes the return trip to Astra at 0.8c. (No, they are not twins).

In Bob’s RF the trip takes 25 years, but in Alice’s RF it takes only 15 years. Bob and Alice agree that her speed, throughout, was 0.8c. Alice cannot, therefore, have measured the distance as 20 ly.

15*0.8=12, so in her RF the journey was only 12 light years. Earth and Astra must be only 6 light years apart. Both RFs must be considered as right. We don’t actually know anything about distances in space, because there is no absolute distance. (?)


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I have been sitting here thinking about that and doodling a bit. You are correct. There are no absolute distances. However, we can make useful distance measurements by assuming that the local FR is absolute. This trick of course only works if we use it for large enough distances and small enough differential velocity so that the relativistic dilation is negligible. So we can measure the 10 LY distance to Astra with enough confidence to use it in making all our measurements from Earth. The distance will vary in response to a whole lot of different variables, but the variation produced by those disturbances is still negligible.

Of course now I wonder if the relativistic dilation would affect our trajectory. Could we go straight there and then reverse the trajectory and come straight back? Or would our trajectory need to be adjusted?

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
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Originally Posted By: Bill
Of course now I wonder if the relativistic dilation would affect our trajectory. Could we go straight there and then reverse the trajectory and come straight back? Or would our trajectory need to be adjusted?


The adjustment idea is interesting. If the craft were unmanned and pre-programmed would it have to be programmed to go, at 0.8c, to a body 10ly away, or 6ly away? OK,I know that over simplifies the necessary programming, but you probably see what I mean.


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I assume that, since the distance is different and the clock is running differently it would have to be taken into account. But then I don't even know how to program an orbit to take a satellite into Earth orbit. And mostly when I run into that sort of thing it is in science fiction, where all they have to say is "program the transit", or in speculative articles about interstellar travel, where the details are pretty much ignored.

Now they have mentioned that you have to allow for the proper motions of the stars, but that would be included in the variations due to relativity.

Bill Gill


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I really don't get the problem?

Set up two identical atomic clocks sit them side by side and check they hold accuracy for a year. Now send one of the clocks up to the ISS and when it comes back in a couple of months check it .. THE CLOCKS ARE OUT IT'S BEEN DONE SEVERAL TIMES.

It's a repeat of the Hafele–Keating experiment done back in the 70's where they flew an atomic clock around in a plane.
(https://en.wikipedia.org/wiki/Hafele-Keating_experiment)

You don't need to go to 0.8c to see the effect.

That experiment has been done several times in different version on the ISS ... search it.

There are actually two dilations on the ISS which is why the experiments have been done so many times it extends the original Hafele–Keating experiment.

First time goes faster on the ISS because they are in lower gravity of earth (89%). Second time slows down for them because they are moving so fast (27,600 km/h).

The two effects are opposite but for them the slower time dilation based on speed dominates but scientifically they wanted to check the two effects are truly additive.

As for true measure of distance Bill S find us a patch of space that isn't moving and we will do the test for you smile

Incase you don't get the problem our patch of space is hurtling around the milky way galactic centre at 828,000 kilometers per hour. When you are in transit to you other celestial body I have no idea how you are going to work out what you actual motion is, at the end of the day you are heading to a target that is all you have told me. I have no way to know the motions of the space between those two bodies without more detail. If the two bodies are both in the milky way your motion can never be zero as you have the galactic spiraling to worry about. All I can say is earth is probably not a good area because we have an inbuilt velocity just to stay in position within the galaxy.

Now if you could actually stop motion totally you can move at 828,000 kilometers per hour in an arc in our own galaxy smile

Last edited by Orac; 10/23/15 06:19 AM.

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Now if you want to go beyond the silly layman stuff and look at direct evidence of length contraction done almost everyday.

Do some reading on the bunching functions in a particle accelerators. Now those charged particles for a given field strength will sit a distance apart normally.

Now as you accelerate the particles, the particles and their electric fields contract and they bunch up in the direction of motion ... QFT at work smile

Same effect would happen with a solid, it is just there is too much neutral particle mass in a solid to turn it and keep it off the wall in particle accelerator. If you can build us a big enough magnet to keep a solid off the wall of the accelerator we could try it for you.

The RHIC was the first accelerator able to accelerate Ions (so normal atoms with extra or deficit electrons) to help provide enough force to keep the atoms off the walls. That was a huge step up from just circulating charged particle like protons and electrons in that there is plenty of neutral mass in an atom. Those atoms are highly deformed in the direction of motion as you could well guess. The LHC does lead Ions for a month or so each run as it's part in a joint program.

What you really want is the ultimate extension of this have enough charge in a solid object to be able to circulate it in an accelerator. We will get there one day and it is the definitive test and more likely than the one you are asking with space.

The problem is simple you need a larger collider radius, stronger magnetic field or more charge on the object to provide the turning force in the accelerator. So there are 3 different ways to tackle the problem.

BTW from the RHIC website has some nice images of how the bunches end up looking like pancakes because they are so shortened up the motion direction they almost look like a single plane. The collision looks something like two pizzas hitting each other
https://www.bnl.gov/rhic/physics.asp

Last edited by Orac; 10/23/15 07:58 AM.

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Originally Posted By: Orac
It's a repeat of the Hafele–Keating experiment done back in the 70's where they flew an atomic clock around in a plane.
(https://en.wikipedia.org/wiki/Hafele-Keating_experiment)

Is that the one sponsored by Hewlett Packard? I recall hearing about that, although I thought they did it in the 60s. HP did it when they introduced their off-the-shelf cesium clock. For them it was a promotional stunt as much as a science experiment.

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I don't know the background sorry Bill well before my time but the first was 1972 as given in the link. I laughed at the buying seat tickets for Mr clock. It's been repeated and refined numerous times with ever increasing accuracy.

To be honest most big universities have the ability to do this experiment for there undergrads as part of a relativity coarse. They mostly use two atomic clocks and lift one up a few meters or up a building in a lift which is how it was done for me.

Mostly however this stuff is historic and of little interest as SR fits so heavily into QM it's not something seriously under any consideration of being wrong. All the "new" theories of everything like string theory etc all embody SR because of the mountain of support for it.

Last edited by Orac; 10/25/15 12:40 PM.

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I was interested because at the time I was an electronics technician and the production of an off-the-shelf atomic clock was big news. At that time Hewlett Packard was the big test equipment supplier. They were at the leading edge of test equipment development. I remember when they announced their test of relativity I was really impressed with how simple it was if you had a set of good clocks.

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Boy that took some finding in the HP archives
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1965-04.pdf

Hank Taylor shares some interesting background on it on page 21 of his memoirs
http://www.kennethkuhn.com/hpmuseum/hpmemory/an/pdf/Taylor_Memoir_120503.pdf

LOL try this today and make sure you name isn't Mohamad
Quote:
The clocks usually traveled !/2-fare as "children" (230 Ibs. each) and occasionally as excess baggage or cabin freight.

It appears to that it was checking the relativity adjustments made to certified time clocks in each country. So what they did was take these two clocks around to make sure each countries National Standard agreed with each other.

So it would probably be classified as an indirect test of relativity but historically I found the whole thing fascinating. Things are so much easier these days you forget how hard this stuff would have been to do. The efforts just to set the national standards was just the beginning of that process that embedded relativity in the middle of science and development.

Compare that to the sort of modern atomic clock on a chip which comes in at 35 grams and around $1500.
http://www.microsemi.com/products/timing...le-atomic-clock

The irony for those that don't accept relativity is that it is embedded in most of there new gadgets that they use smile

Last edited by Orac; 10/27/15 07:04 AM.

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I remember how amazing it was when HP brought out their commercial atomic clock. Before that all atomic clocks had been custom made and were expensive and delicate. Back in 1968 and 69 I worked at a satellite communications station in Hawaii. When our custom made clock had to be reset they took it to the University of Hawaii and set it to their maser. Keeping time back then was a chore. Heck I remember when the best we had at work, back in the late 50s and early 60s, was a really good crystal oscillator in a full sized single bay rack. Every morning one of the technicians had to synchronize it with the time broadcast from WWV, the national time standard. It had a display that showed the phase of the local frequency with respect to the phase of the WWV broadcast. The technician carefully looked at that and made small adjustments to the frequency of our standard to match. And that was what we used to calibrate all of our time sensitive equipment, primarily frequency counters.

And I agree that too many people who make strange claims about relativity being all wrong would be left in the lurch if it was wrong, because so many things wouldn't work. And of course the HP test was just one of a huge number that have proved relativity is right at finer and finer levels ever since.

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Of course there is a lot of complicated stuff about who is moving and how fast, but at the end of it all, it seems we cannot say that there is an absolute distance between any two objects any more than we can attribute absolution motion to anything.

If that is correct, the next question must be: Is there any scenario in which two objects that are seen as being separated in space in one RF can be seen as contiguous in another?


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Originally Posted By: Bill S.
Of course there is a lot of complicated stuff about who is moving and how fast, but at the end of it all, it seems we cannot say that there is an absolute distance between any two objects any more than we can attribute absolution motion to anything.

You are correct it is the underpinnings of SR/QM all you can ever do is give the relative motion and distance to your reference frame. Later you may find your reference frame is actually moving or subject to gravity (or worse, we will get to that) and so the measurements are only relative to you and at that particular moment in time. The measurement may be useful for a period of time and a volume of space but there is no way to generalize how long or how far.

Originally Posted By: Bill S.
If that is correct, the next question must be: Is there any scenario in which two objects that are seen as being separated in space in one RF can be seen as contiguous in another?

Nope there isn't any, look at the formulation of spacetime, you have a different time at every point in space that is what it says and you write it's coordinates [x, y, z, t]. All you can ever do is at some point in time have areas of space that are approximately the same enough to do some measurements that might make sense to you locally. Quantum mechanics goes further and says each of those points is evolving over time in a state of superposition.

So lets push you a bit more Bill S, and deal with superposition which extensions often gets described as weird and spooky but is actually neither. You need to understand why superposition exists and to do that you need to start with it's general description

https://en.wikipedia.org/wiki/Superposition_principle

Newton and classic physics know this thing as cause and effect. The problem with classical physics is it gives no idea why such a thing should be connected you just have to accept it.

Superposition is the mechanism that QM says is responsible for why cause and effect are connected. It is telling you the universe as far as we can tell is a linear system and we can write the relationship as Schrödinger's wave equation and connect everything thru descriptions of energy or information.

The superposition which connects cause and effect, because it is a wave can have some strange properties like entanglement. A lot of science media tend to suggest that entanglement is defying cause and effect when it's actually the reverse it is showing just how connected they. The problem is the connection is not in the normal space dimensions of classical physics.

So the lesson here is no you can not rely on any measurement as that measurement is in superposition with the universe. Measurements are useful locally in so much as they allow you to interact with the universe and effect outcomes. Those measurements however are distinctly local and distinctly transient to a point in time.

That realization lead to an experimental test called the Quantum Eraser experiment (https://en.wikipedia.org/wiki/Quantum_eraser_experiment).

The experiment throws up how heavily cause and effect are connected that a change in the superposition ignores time ordering of measurement events. Yes some event in the future can make your measurement wrong it's just rare for us to see it.

Now if you got the full connection you should be able to realize you can play with the time evolution by continually measuring which is called the Quantum zeno effect. Topically this had a writeup last week (http://phys.org/news/2015-10-zeno-effect-verifiedatoms-wont.html)

There is nothing new in any of this it has been known by anyone who cared to look at the science for 85 years. Special relativity and QM are different frameworks to describe the same thing a universe in which there are no absolutes and space and time are connected via cause and effect.

So there you have your answer that no measurement can ever be absolute, they can only ever be relative and only guaranteed to be correct at the moment you took it smile

Perhaps you need to consider what you are doing when you measure something .. try thinking in QFT terms.

Last edited by Orac; 10/29/15 09:00 AM.

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if the star is 10 ly from earth , and the speed at which alice travels to the star is 0.8c then the trip will take 12.5 earth years.

why would bob think it would take 25 years?

doesn't he own a calculator?

maybe he is thinking about how long it would take her to go
the 12.5 years to the star and the 12.5 years to return from
the star ... 12.5 + 12.5 = 25 earth years.


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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Originally Posted By: Orac
Nope there isn't any, look at the formulation of spacetime, you have a different time at every point in space that is what it says and you write it's coordinates [x, y, z, t]. All you can ever do is at some point in time have areas of space that are approximately the same enough to do some measurements that might make sense to you locally. Quantum mechanics goes further and says each of those points is evolving over time in a state of superposition.


If your spacetime coordinates for one point are x,y,z,t and for another point are x1 ,y1, z1 ,t1; if one could, in principle, travel between the two at c, wouldn’t they appear, in the RF of the traveller to be superimposed?


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Originally Posted By: Bill S.
If your spacetime coordinates for one point are x,y,z,t and for another point are x1 ,y1, z1 ,t1; if one could, in principle, travel between the two at c, wouldn’t they appear, in the RF of the traveller to be superimposed?

Correct .. at c all points in the universe overlap as [0, 0, 0, t] to that observer and there is no problem with that.

Go back to what we discussed above about cause and effect, nothing means anything until you connect two events, that is what the wave function is doing. Again stop thinking in classical physics its wrong has been for 85 years, the overlapping events in space means nothing each individual event is still unique to QM. That is the message QM has been screaming at us for 85 years. QM really doesn't give a rats if those events are separated in time or space they are unique and identifiable.

Now here is the kicker to get in your head .... ready

The only way to measure an event in the QM field, is to use the QM field in a measurement device.

That is the irony for those who won't let go of classic physics that everytime they measure something they are using QM. Why? because they need an energy transfer which is the only thing they can measure. You can call it information or energy both are the same thing. So stop worrying about events in the same point of space you can only measure with QM and it still sees them as individual events and that is why it isn't an issue.

The only people who have issues with this are those who want to make space more important than time. Why because apparently space is so important to them because they live in it and time is just this other thing. QM says that is rubbish and happily encodes in either smile

QM tells you that you can put a very large amount of information in the same point in space and still measure each individual one ... there are hundreds of experiments to show this and one was done last week

http://phys.org/news/2015-10-physicists-experimentally-quantum-hilbert-hotel.html

IBM holds the current record for overlapping encoding putting 35 bits into a single electron. (http://news.stanford.edu/news/2009/january28/small-012809.html)
Our poor control of QM and the uncertainty principle are the things that stops them going further.

Technically there are an infinite number of individually addressable states (Coulomb potentials) where quantum bits can be stored in an electron the problem for us is the encode and measurement device aren't that good smile

Perhaps take the question in reverse? What is the maximum amount of information the universe can hold and Lubos gives you the answer here
http://physics.stackexchange.com/questions/4118/how-many-bytes-can-the-observable-universe-store

The answer is roughly 1x10E120 bits. A few other comments show the other ways of doing the calculation and end at the same number.

As he notes in that response the encoding and possible future encoding is in all sorts of things, some spacelike and some timelike.

So now you should see the physics of the universe as how it really is with space and time both equally important.

You live in a Quantum universe which sometimes can be simplified to classical physics but can never be understood by it.

Last edited by Orac; 10/31/15 04:24 PM.

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