Does objectivism accept Einstein's relativity? Einstein's relativity seems to imply that the nature of the object depends on the observer (not in the idealist sense, but in the frame of reference sense). Is that simply wrong, does an entity have a nature which exists independently of any observer and properties which all observers will agree upon?
For the benefit of those who may not be as well versed in Einstein's theories as the question may assume, it would be helpful for the questioner to clarify what is meant by "idealist sense" versus "frame of reference sense," and what aspects of the "nature of the object" are alleged (by whom?) to change when the object is moving at nearly the speed of light relative to a distant observer, or vice versa -- and by what means the distant observer would be able to observe the nature of the fast moving object. Or is the object "stationary" and the distant observer is the one moving? Does relativity say that it doesn't matter which is "stationary" and which is "moving" in any absolute sense, and all that matters is movement between the object and the distant observer? One must also consider how long it will take light to travel from the object to the distant observer, if the distant observer is relying on light to know the "nature" and condition of the object.
The main Objectivist criticisms of modern physics that I know of pertain to fundamental epistemological issues of the cognitive methodology of the physicists, rather than primarily to the conclusions which the physicists reach by means of their cognitive methodology. Leonard Peikoff's book, The DIM Hypothesis, for example, includes an entire chapter on modern-era physics (Chapter 6), with subsections on Newton, Positivism, Einstein, Quantum Mechanics, and String Theory (Theory of Everything). The section on Einstein spans nearly six pages (pp. 115-120). It shows that Einstein followed a "mixed mode" of cognitive integration, a combination of otherworldliness (deduction from a priori axioms) and secularism (seeking to relate theory to observed facts). Page 116 observes:
[In special relativity, Einstein] introduces into physics a new, axiomatic fundamental: the constancy for all observers of the speed of light. Again, he seeks no physical explanation of this fact, and offers for it no empirical basis. He does not even invoke the groundbreaking 1887 experiments on light performed by Michelson and Morley. His method of validating the axiom is to relate it, not to fact, but to other ideas, and he does this in essence by consulting aesthetic criteria. For example, Maxwell's electromagnetic equations, he says, would be simpler and more beautiful if combined with the light postulate; so on that basis alone the postulate deserves to be part of the foundation of physics.
Earlier, on p. 115, the DIM book describes special relativity as follows (deriving from the "light postulate"):
Special relativity begins with the observed fact that lengths, times, and masses, as measured by an observer, vary according to his motion relative to the body being observed.
A key point that is confusing to me here is whether the actual lengths, times, and masses vary, or merely the measurements of them by an observer using light or similar phenomena as the basic means of knowing where the measured object is and what its characteristics are, and also whether the object is considered to be moving relative to a "stationary" observer or vice versa. Presumably an observer accompanying the object would detect no changes in the object's attributes at all as a result of another traveler moving by at nearly the speed of light, or if the object and its accompanying observer are the ones who are moving relative to the other observer. If there is any actual change in an object's attributes as a result of relative motion, why wouldn't it matter who is moving and who is stationary? (And, for that matter, what would be the "frame of reference" for determining what is stationary and what is moving? My understanding, for example, is that there is such a thing as an "intertial frame of reference" and non-inertial frames of reference resulting from accelerations or rotations of the frame of reference relative to an inertial frame. Refer to the Wikipedia article on "Inertial frame of reference.") The DIM excerpt above says "observed fact," suggesting that the attributes of a moving object really do change with speed. But the same excerpt also says "as measured by an observer," which suggests that the changes in attributes are only illusions resulting from the speeds involved and the use of light to make the observations. I'm really not sure which interpretation the DIM excerpt intended to express.
In any case, the DIM discussion of Einstein concludes with the identification of his cognitive methodology as M1, meaning Type 1 misintegration -- reliance on deduction from a priori axioms, along with a preference toward tying the deductions to reality whenever possible. This is also known as the "Many from One" mode of cognitive integration, as contrasted with true integration (I), which consists in finding the "One from within the Many" (integrating concretes to form abstractions).
The question asks: "does an entity have a nature which exists independently of any observer and properties which all observers will agree upon?" Even if an object's attributes do actually change as a result of an observer zipping by, that isn't necessarily a contradiction of the object's identity. The law of identity doesn't say what an entity's identity is; the law says only that an entity has an identity and it is what it is. If it is a fact that attributes can change as an observer zips past, then the capacity of an object to respond that way to such causal factors is simply part of the identity of the object. Relative to ordinary human experience, however, it certainly seems fantastic to suggest that an object can change (especially as seen by a local observer close to the object) merely because some other distant, fast-moving observer happens to be looking at it. (Later quantum physicists do say such things.*)
(*) Update: For those who may be interested in an Objectivist appraisal of quantum mechanics, the section of DIM Chapter 6 on Quantum Mechanics (mentioned above) may prove very helpful. Page 123 discusses Heisenberg's ideas of probability as metaphysical rather than epistemological, "unactualized probabilities," and Nature as limbo, followed on p. 124 by "Schroedinger's cat" and the condition of the cat in the box as indeterminate (neither dead nor alive) until someone opens the box to look inside. The DIM discussion goes on to explain: "Astonished by their discovery that the measurement of nothing can make it something, quantum physicists came to call such an occurrence the 'measurement miracle.'" For corroboration, an independent (non-Objectivist) book titled, Quantum: A Guide for the Perplexed, by Dr. Jim Al-Khalili (Sept. 2004) may also prove very helpful.
Update: QM and DIM
The comments have continued to question the accuracy of the DIM book in its assessment of quantum mechanics (QM). There is a great deal more that can be said about that, but it belongs in a separate Objectivist Answers topic, since the present topic is Einstein and relativity. Here are at least two recent Objectivist Answers discussions relating to QM and DIM:
Those who seriously question the veracity of DIM and want to resolve their doubts really will need to read the book first-hand in some detail, understand what it actually says and does not say, and check the extensive set of references which the book provides in the Endnotes. The QM section alone is acompanied by Endnotes 74 through 83. DIM is one of the most heavily footnoted and documented works I have ever seen from a leading Objectivist intellectual.