Why aren't standard rods and clocks affected by LC and TD?

[loislane]

Are measuring rods and clocks(the ones that are used as reference to ascertain LC and TD of the relatively moving rods and clocks) left out of the theory in principle?

 

[DrGreg]

What makes you think they are "left out of the theory"?

 

[loislane]

What makes you think they are "left out of the theory"?

That's what I am asking. Are they? Maybe answer first the thread's title question, and if that is explained by the theory then there is no need to answer the question in post #1.

 

[fresh_42]

That's what I am asking. Are they? Maybe answer first the thread's title question, and if that is explained by the theory then there is no need to answer the question in post #1.

The question was how did you come to assume this, and it's a justified question. Being rude is not the way we want to talk to each other.

 

[PeterDonis]

answer first the thread's title question

The answer to that question is "mu". Standard rods and clocks are affected by LC and TD.

 

[loislane]

The answer to that question is "mu".

"mu"?? what is that?

Standard rods and clocks are affected by LC and TD.

Ok, how exactly. You mean for instance that measuring standard rods are contracted?

 

[PeterDonis]

"mu"?? what is that?

It means that there is no valid answer to the question, because the question itself is not well-posed. For a quick reference, see here:

https://en.wikipedia.org/wiki/Mu_(negative)#.22Unasking.22_the_question

You mean for instance that measuring standard rods are contracted?

Yes; a standard measuring rod appears to be shorter relative to a frame in which it is moving, compared to a frame in which it is at rest.

 

[Ibix]

"mu"?? what is that?

Allegedly, it's a Chinese word used to mean "your question contains unexamined assumptions". For example - "have you stopped beating your wife?" If you answer yes, you imply that you used to beat her. If you answer no you imply that you continue to beat her. If you answer mu you point out that the question is structured on the assumptions that you have a wife and that you did beat your wife at some point in the past, and these assumptions should be examined before asking the question.

I don't speak a word of Chinese, so can neither confirm nor deny the accuracy of that, but Peter seems to be using it in that sense to address your title question.

Ok, how exactly. You mean for instance that measuring standard rods are contracted?

Anything with a length is length contracted when not observed in its rest frame..

 

[loislane]

Ok, thanks for the chinese class.

Yes; a standard measuring rod appears to be shorter relative to a frame in which it is moving, compared to a frame in which it is at rest.

Anything with a length is length contracted when not observed in its rest frame..

Here's the thing. I wonder how can one determine that some rod is contracted if the ruler one is using(even if only conceptually or ideally) is also contracted.
Maybe you are familiar with a thought experiment by Poincare about all the scales in the universe changing overnight, including the measuring rulers of course. And how nobody would notice since measurements would remain unaltered so the change would not really be physical.
Now in SR surely there are physical consequences but if I understood your reply you are saying that length contraction can be measured with a contracted ruler?
I don't think so, I guess you mean that the contraction of a moving rod is always compared with a Standard ruler that is not contracted, otherwise no contraction can be measured even in principle.
Now my question was if SR accounts for the fact that ideal clocks and rods cannot undergo dilation and contraction even in principle if they are to serve as standards against which to judge the LC and TD in the moving frames.

 

[strangerep]

Here's the thing. I wonder how can one determine that some rod is contracted if the ruler one is using(even if only conceptually or ideally) is also contracted.

Consider the "paradox" of the pole and the barn. Let's suppose that, when both are stationary, they have been measured (using standard rods) and the pole is found to be slightly longer than the barn. Upon boosting the pole, one encounters the well known paradox (also known as the ladder paradox). It is resolved by careful examination of simultaneity issues. I suspect the same thing applies for your question -- one must consider carefully how one would "compare" the rods...

 

[Ibix]

Here's the thing. I wonder how can one determine that some rod is contracted if the ruler one is using(even if only conceptually or ideally) is also contracted.
Maybe you are familiar with a thought experiment by Poincare about all the scales in the universe changing overnight, including the measuring rulers of course. And how nobody would notice since measurements would remain unaltered so the change would not really be physical.
Now in SR surely there are physical consequences but if I understood your reply you are saying that length contraction can be measured with a contracted ruler?
I don't think so, I guess you mean that the contraction of a moving rod is always compared with a Standard ruler that is not contracted, otherwise no contraction can be measured even in principle.
Now my question was if SR accounts for the fact that ideal clocks and rods cannot undergo dilation and contraction even in principle if they are to serve as standards against which to judge the LC and TD in the moving frames.

Length contraction and time dilation are always things that happen to other people. You will never be able to measure it happening to you because, as you say, your own rulers and clocks would rescale. This is kind of the point of relativity - you can always consider yourself to be at rest (as long as you aren't accelerating), so your rods are always their standard length. If they changed, this would imply that there was some absolute rest frame that you weren't in any more.

You can use your rods and clocks to observe length contraction and time dilation in moving rods and clocks. Simply deploy your rods, synchronise your clocks and, at some agreed time, record the positions of the ends of the moving rod. It'll come in short. Two photos of the same moving clock some time $\Delta t$ apart will show a smaller elapsed time. These effects are symmetric, of course. An observer at rest with respect to the moving rods would observe your rods to be length contracted and your clocks to be time dilated.

So there is no "standard" rod. There is only a rod that is at rest with respect to you. That one is not length contracted according to you; any rods in motion with respect to you are contracted.

 

[loislane]

Length contraction and time dilation are always things that happen to other people. You will never be able to measure it happening to you because, as you say, your own rulers and clocks would rescale. This is kind of the point of relativity - you can always consider yourself to be at rest (as long as you aren't accelerating), so your rods are always their standard length. If they changed, this would imply that there was some absolute rest frame that you weren't in any more.

You can use your rods and clocks to observe length contraction and time dilation in moving rods and clocks. Simply deploy your rods, synchronise your clocks and, at some agreed time, record the positions of the ends of the moving rod. It'll come in short. Two photos of the same moving clock some time $\Delta t$ apart will show a smaller elapsed time. These effects are symmetric, of course. An observer at rest with respect to the moving rods would observe your rods to be length contracted and your clocks to be time dilated.

So there is no "standard" rod. There is only a rod that is at rest with respect to you. That one is not length contracted according to you; any rods in motion with respect to you are contracted.

So you are confirming that measuring rods and clocks with which I am always referring to the ones that measure in the moving frame(and I already mentioned that it is understood this measurement is ideal but this doesn't detract from its physicality or we wouldn't have a physical theory to begin with) do not contract and dilate by definition(to avoid the rescaling issue), and that this point is not addressed or explained in the theory,i.e. there is not a relativistic theory of matter that justifies it(or LC and TD themselves, the theory doesn't enter into this which makes it more flexible too).

 

[Ibix]

So you are confirming that measuring rods and clocks with which I am always referring to the ones that measure in the moving frame(and I already mentioned that it is understood this measurement is ideal but this doesn't detract from its physicality or we wouldn't have a physical theory to begin with) do not contract and dilate by definition(to avoid the rescaling issue), and that this point is not addressed or explained in the theory,i.e. there is not a relativistic theory of matter that justifies it(or LC and TD themselves, the theory doesn't enter into this which makes it more flexible too).

I'm really not sure what you think I'm saying.

Measuring rods that are moving with respect to me will be contracted if I measure their length using rods at rest with respect to me and clocks synchronised according to the Einstein convention. However, someone at rest with respect to the moving rods using clocks synchronised with the same procedure will define their rods to be non-contracted and use them to measure my rods to be contracted.

The relativistic theory of matter would be quantum field theory - you'll have to ask (e.g.) @PeterDonis about that. However, one can see length contraction and time dilation as merely being the consequence of choosing different coordinate systems. I don't really need a theory of matter to explain a square turning into a diamond and back into a square as I rotate it, and I'm doing something quite similar when I change my choice of frame.

 

[PeterDonis]

this point is not addressed or explained in the theory,i.e. there is not a relativistic theory of matter that justifies it

LC and TD are not things that require a theory of matter; they only require a theory of spacetime and kinematics, which is what SR is. When you measure a ruler moving relative to you to be length contracted, using a ruler at rest relative to you, you are not measuring something that physically happened to the moving ruler; you are measuring something that arises from the way spacetime works. It's the spacetime analogue of looking at an object from a different angle in ordinary 3-dimensional geometry; it doesn't change anything about the object you're looking at.

 

[Dale]

you are saying that length contraction can be measured with a contracted ruler?

Yes. If Alice and Bob are moving relative to each other then in Alice's frame Bob's rulers are contracted and in Bob's frame Alice's rulers are contracted.

 

[loislane]

I don't really need a theory of matter to explain a square turning into a diamond and back into a square as I rotate it, and I'm doing something quite similar when I change my choice of frame.

LC and TD are not things that require a theory of matter; they only require a theory of spacetime and kinematics, which is what SR is. When you measure a ruler moving relative to you to be length contracted, using a ruler at rest relative to you, you are not measuring something that physically happened to the moving ruler; you are measuring something that arises from the way spacetime works. It's the spacetime analogue of looking at an object from a different angle in ordinary 3-dimensional geometry; it doesn't change anything about the object you're looking at.

Yes, all that is of course right and I'm aware of it and agree about SR not needing a theory of matter, that being probably a strong point as it makes the theory adaptable to possible different theories of matter. But I want to refer here to a point where the analogy with euclidean 3-geometry as presented by PeterDonis and Ibix(in the example of the square rotation) no longer works because we don't have the same tools and therefore cannot justify certain stipulations or tacit assumptions in the same way in euclidean geometry as in Minkowski geometry.

If I may elaborate it to achieve a better understanding this point can be summarized in the words of Max Born ("Einstein's theory of relativity" 1965 pg 211) by "The principle of the physical identity of the units of measure", or in other words any object that can act as a rigid ruler in the rest frame S retains that role in its new rest frame S' when boosted and the same tacit assumption hold for ideal clocks. Or as Einstein put it("The consequences of the principle of relativity in modern physics" 1910):"It should be noted that we will always implicitly assume that the fact of a measuring rod or clock being set in motion or brought back to rest doesn't change the length of the rod or the rate of the clock".
This stipulation was further stressed by Born in the above mentioned reference that I paraphrased but I quote here literally:"...it is a ssumed as self-evident that a measuring rod which is brought into one system of reference S and then into another S' under exactly the same physical conditions would represent the same length in each, and the same would be postulated for clocks...[...] This is the feature of Einstein theory by which it raises above the standpoint of a mere convention and asserts definite properties of physical bodies."(Born 1965 pp. 251-252)
The relevance of this tacit assumption that must be added in an ad hoc form to the theory since it is not justified nor derived from the mathematical formalism of the Minkowskian geometry and in this sense to answer Dr. Greg's question, one could say raher than being left out of the theory it is more appropriate to say that the theory separates artificially the measuring tools from the rest of the theory. Maybe it is best to use again Einstein's own words("Autobiographical notes" 1949):
"One is struck [by the fact] that the theory [of special relativity] [...] introduces two kinds of physical things, i.e., (1) measuring rods and clocks, (2) all other things, e.g., the electromagnetic field, the material point, etc. This, in a certain sense, is inconsistent; strictly speaking measuring rods and clocks would have to be represented as solutions of the basic equations (objects consisting of moving atomic configurations), not, as it were, as theoretically self-sufficient entities. However, the procedure justifies itself because it was clear from the very beginning that the postulates of the theory are not strong enough to deduce from them sufficiently complete equations. If one did not wish to forego a physical interpretation of the co-ordinates in general (something which, in itself would be possible), it was better to permit such inconsistency, with the obligation, however, of eliminating it at a later stage of the theory. But one must not legalize the mentioned sin so far as to imagine that intervals are physical entities of a special type, intrinsically different from other physical variables (‘reducing physics to geometry’, etc.)".

Now to come back to where the analogy used by PeterDonis breaks, it is trivial to see that euclidean geometry as a metric geometry that has a clear concept of rigid bodies and rigid rulers and thus has no difficulty justifying that even though we may observe changed lengths from our perspective the object measured in its position always retains its length as measured by any rigid ruler. Now in SR there are no rigid bodies since they contradict the theory(it would be possible to transmit info faster than light) so we don't have the luxury of deriving from their existence, like it is done in euclidean geometryand the 3D geometric example given by PD, the universality of standard lengths of rulers and rate of clocks mentioned by Born and Einstein and necessary to give the theory any physical meaning(that is to avoid the view that everythig amounts to "optical effects" and perspectives without physical consequences, this view is obviously false since we can observe the physical consequences in particle physics experiments, and all tests of SR to date (Hafele and Keating etc...). Minkowski geometry is not even a metrical(in the euclidean sense of distance function) geometry, it is an affine geometry without a notion of rigid ruler transportable to distant or rotated objects, it only allows parallel transport.

 

[loislane]

Yes. If Alice and Bob are moving relative to each other then in Alice's frame Bob's rulers are contracted and in Bob's frame Alice's rulers are contracted.

You must have missed my point, I mean what Einstein and Born say in the references mentioned above.

 

[Dale]

Now in SR there are no rigid bodies

There are no rigid bodies, but there are bodies which move rigidly (Born rigid motion).

Furthermore, if the laws of physics are the same in inertial frames then identically constructed clocks will keep time the same, so you can simply build the clock to be initially at rest in different frames.

 

[loislane]

There are no rigid bodies, but there are bodies which move rigidly (Born rigid motion).

True, but as you probably know Born rigid motions are limited to three degrees of freedom and six d.o.f. are needed to account for all possible rigid motions, so I don't think Born rigidity is relevant in this discussion.

Furthermore, if the laws of physics are the same in inertial frames then identically constructed clocks will keep time the same, so you can simply build the clock to be initially at rest in different frames.

Yes, this is the first postulate of SR, the implicit stipulation that Born and Einstein talk about above can be seen as part of it or as added third postulate making sure the equivalence of physical laws in all inertial frames is implemented. But again it remains artificial to have to make measuring rods and clocks special and having a special stipulation for them because unlike the case in classical mechanics the geometry doesn't allow rigid rulers so it is necesary to stipulate additionally a "principle of physical identity of the units of measurement".

 

[Dale]

six d.o.f. are needed to account for all possible rigid motions,

So what? You only need to be able to get enough motion to determine the Lorentz transform. As far as I can tell a single degree of freedom is more than sufficient.

so I don't think Born rigidity is relevant in this discussion.

Here I agree. I think that the whole topic of rigidity is irrelevant. However, insofar as rigidity is at all relevant it must be Born rigidity since that is the only rigidity compatible with SR.

Yes, this is the first postulate of SR, the implicit stipulation that Born and Einstein talk about above can be seen as part of it or as added third postulate making sure the equivalence of physical laws in all inertial frames is implemented. But again it remains artificial to have to make measuring rods and clocks special and having a special stipulation for them because unlike the case in classical mechanics the geometry doesn't allow rigid rulers so it is necesary to stipulate additionally a "principle of physical identity of the units of measurement".

Most modern approaches don't even bother. They come at it from a symmetry approach.

If you find yourself dissatisfied with the postulates, I would recommend looking into symmetry-based approaches rather than trying to patch up the postulates.

 

[Ibix]

Isn't Einstein making a distinction without a difference with his talk of two classes of objects? Any old atom is a clock - kick it into an excited state and watch what it emits when it decays. Any array of atoms is a ruler. Diffract some x-rays off it and see what you get.

What am I missing?

 

[PeterDonis]

six d.o.f. are needed to account for all possible rigid motions

In Newtonian physics, yes. Not in relativity. That is what the Herglotz-Noether theorem says (it's the theorem that says that all possible Born rigid motions can be described with only three degrees of freedom). Motions that do not meet the conditions of the H-N theorem are not rigid when relativistic effects are taken into account, even though they might seem to be in the Newtonian approximation.

 

[PeterDonis]

I think that the whole topic of rigidity is irrelevant.

Not if it's going to be used to justify the statement that it's impossible to have measuring rods and clocks in SR. Then it does need some discussion in order to refute that erroneous statement. See my next post.

 

[PeterDonis]

in SR there are no rigid bodies

This is not correct. What is correct is that a body in SR that is subjected to a force on only one part of it (such as a rocket being pushed by an engine at its rear) will not be rigid, because the force can only be propagated through the body at the speed of light. (And even this isn't completely right; see below.)

However, this is irrelevant to the theoretical construction of inertial frames in SR using measuring rods and clocks, because the rods and clocks are stipulated not to be subjected to any forces; they are moving inertially. The same is true for LC and TD; these "perspective effects" take place when measuring an object that is moving inertially, with respect to an inertial frame, constructed using measuring rods and clocks which are moving inertially, in which it has nonzero ordinary velocity. All of the theorems of Minkowski geometry can be realized using such constructions.

Also, even if we look at objects moving non-inertially, but with constant acceleration, we find that Born rigid motion is a natural state of equilibrium for such objects. For example, a rocket being pushed by an engine at its rear with constant proper acceleration will "settle" into a state of Born rigid motion described by a Rindler congruence of worldlines. A non-inertial frame--Rindler coordinates--can then easily be realized using measuring rods and clocks at rest relative to the rocket. Such a frame will obey different rules from an inertial frame (for example, the coordinate speed of light will not always be the same), but the rules are perfectly well-defined and are reflections of the same underlying Minkowski spacetime geometry.

Minkowski geometry is not even a metrical(in the euclidean sense of distance function) geometry

The lack of a positive definite metric does not mean Minkowski spacetime does not have a metric and is just an affine geometry. The metric of Minkowski spacetime does have a definite physical meaning. See above.

 

[stevendaryl]

Are measuring rods and clocks(the ones that are used as reference to ascertain LC and TD of the relatively moving rods and clocks) left out of the theory in principle?

Lorentz contraction and time dilation are all about idealized rods and clocks. If an ideal rod has length L in its rest frame, then it will have length $L \sqrt{1-\frac{v^2}{c^2}}$ in a frame in which the rod is moving at speed v (parallel to its length). If an ideal clock ticks once every $T$ seconds in its rest frame, then it will tick once every $T' = T/\sqrt{1-\frac{v^2}{c^2}}$ in a frame in which that clock is moving at speed v.

 

[loislane]

In Newtonian physics, yes. Not in relativity. That is what the Herglotz-Noether theorem says (it's the theorem that says that all possible Born rigid motions can be described with only three degrees of freedom). Motions that do not meet the conditions of the H-N theorem are not rigid when relativistic effects are taken into account, even though they might seem to be in the Newtonian approximation.

H-N theorem says what you write between parenthesis only.

Not if it's going to be used to justify the statement that it's impossible to have measuring rods and clocks in SR. Then it does need some discussion in order to refute that erroneous statement. See my next post.

Nobody stated that, only that one has to stipulate it tacitly without justification within the theory,and the claim is made by Born and Einstein.

This is not correct.

This was already settled by Laue in 1911 when it showed that rigid bodies cannot exist in SR so it is not worth arguing. You can accesss the paper here googling:"On the discussion concerning rigid bodies in the theory or relativity" by von Laue.

The lack of a positive definite metric does not mean Minkowski spacetime does not have a metric and is just an affine geometry.

Minkowski spacetime does have a metric tensor for sure, I used the word metric with the meaning it has for instance in "metric space". And of course Minkowski spacetime is defined as an affine space with a metric tensor.

 

[loislane]

Isn't Einstein making a distinction without a difference with his talk of two classes of objects? Any old atom is a clock - kick it into an excited state and watch what it emits when it decays. Any array of atoms is a ruler. Diffract some x-rays off it and see what you get.

What am I missing?

The problem comes when defining in a non-tautological way what an ideal clock is. And also in guaranteeing that it is not altered when transported. You have to stipulate it in an ad hoc way unrelated to the Minkowskian geometry and the Lorentz symmetry.

 

[stevendaryl]

Isn't Einstein making a distinction without a difference with his talk of two classes of objects? Any old atom is a clock - kick it into an excited state and watch what it emits when it decays. Any array of atoms is a ruler. Diffract some x-rays off it and see what you get.

What am I missing?

Not all objects maintain a constant shape, or a constant length. For instance, a blob of silly putty. And not all objects have behavior that is periodic.

An ideal measuring rod is one that keeps its shape while being (gently accelerated), and an ideal clock is some device that emits some signal at regular intervals.

 

[loislane]

So what? You only need to be able to get enough motion to determine the Lorentz transform. As far as I can tell a single degree of freedom is more than sufficient.

Well if you restrict to 2-dimensional Minkowski diagrams you are right. But not in the more realistic 4-dimensions.

Most modern approaches don't even bother. They come at it from a symmetry approach.

If you find yourself dissatisfied with the postulates, I would recommend looking into symmetry-based approaches rather than trying to patch up the postulates.

The problem is the symmetry in this case doesn't address the issue of the stability of measuring rods and clocks in the absence of rigid rulers, that is the sense of Einstein's lament.

 

[FieldTheorist]

Are measuring rods and clocks(the ones that are used as reference to ascertain LC and TD of the relatively moving rods and clocks) left out of the theory in principle?

Ah, but they are affected by relativistic effects (GR and SR). That's actually why they're useful. Standardized objects are objects that are forced to do something very precise by nature. Let's take the type-1A supernovas. They are forced to emit a certain spectrum of light. So if you know that when the light's wavelength was when it was emitted, e.g. $\lambda$, and you observe it with a wavelength of $\lambda '$, you can figure out how much the wavelength has been stretched whilst it traveled across spacetime, e.g.

$\frac{\lambda '}{\lambda} = a$

where a is a parameter that comes from GR. Thus, by knowing how much the light has been stretched due to GR, you can use GR to reverse engineer (reconstruct) the distance that the supernova is away from you.

 

[stevendaryl]

Well if you restrict to 2-dimensional Minkowski diagrams you are right. But not in the more realistic 4-dimensions.
The problem is the symmetry in this case doesn't address the issue of the stability of measuring rods and clocks in the absence of rigid rulers, that is the sense of Einstein's lament.

Could you relate this concern with nonideal clocks and measuring rods back to your original post? You were asking why ideal clocks and measurement rods are assumed to be unaffected by Lorentz contraction and time dilation. But that's not true, so isn't the original question answered?

I'm having trouble understanding your concern about nonideal clocks and rods. Is it that you are worried that the actual clocks and rods used in tests of SR might be affected by acceleration in such a way as to mimic the effects of SR? And so tests of SR might not be conclusive?

 

[loislane]

Ah, but they are affected by relativistic effects (GR and SR). That's actually why they're useful. Standardized objects are objects that are forced to do something very precise by nature. Let's take the type-1A supernovas. They are forced to emit a certain spectrum of light. So if you know that when the light's wavelength was when it was emitted, e.g. $\lambda$, and you observe it with a wavelength of $\lambda '$, you can figure out how much the wavelength has been stretched whilst it traveled across spacetime, e.g.

$\frac{\lambda '}{\lambda} = a$

where a is a parameter that comes from GR. Thus, by knowing how much the light has been stretched due to GR, you can use GR to reverse engineer (reconstruct) the distance that the supernova is away from you.

Not exactly. It is because the assumption about physical validity of ideal clocks and rods unaffected by LC/TD is undoubtedly confirmed empirically by the stability of atoms and their spectra that we can do that "reverse engineering".

 

[Dale]

Well if you restrict to 2-dimensional Minkowski diagrams you are right. But not in the more realistic 4-dimensions.

Even in 4D spacetime 3 degrees of freedom for Born rigid motion are more than enough to investigate the LT.

The problem is the symmetry in this case doesn't address the issue of the stability of measuring rods and clocks in the absence of rigid rulers

Again, so what? Why does that issue need to be addressed?

If we assume that the laws of physics have the appropriate symmetries then we get the Lorentz transform, regardless of whether or not those laws include rigid objects. This is the point of the symmetry approach, it does not depend on any specific laws, it comes directly from the symmetries.

I just don't see a problem here.

 

[loislane]

Could you relate this concern with nonideal clocks and measuring rods back to your original post? You were asking why ideal clocks and measurement rods are assumed to be unaffected by Lorentz contraction and time dilation. But that's not true, so isn't the original question answered?

The concern is Einstein's actually. I'm trying to come to terms with it.

I'm having trouble understanding your concern about nonideal clocks and rods. Is it that you are worried that the actual clocks and rods used in tests of SR might be affected by acceleration in such a way as to mimic the effects of SR? And so tests of SR might not be conclusive?

Not at all. Non inertial frames and accelerations hace not entered the discussion and they are anyway taken care by the clock postulate. SR tests are conclusive.

 

[loislane]

Even in 4D spacetime 3 degrees of freedom for Born rigid motion are more than enough to investigate the LT.

Not if you include rototranslations and those are motions found in physics.
Actually if you read Laue's paper you'll learn that infinite dof's are needed in Minkowski geometry for rigid bodies.

Again, so what? Why does that ah issue that needs to be addressed?

If we assume that the laws of physics have the appropriate symmetries then we get the Lorentz transform, regardless of whether or not those laws include rigid objects. This is the point of the symmetry approach, it does not depend on any specific laws, it comes directly from the symmetries.

I just don't see a problem here.

Ok, that's great. How dou you interpret Einstein's concern in that quote.