3 ms·

> Specifically, Einstein was puzzled by a difference that didn’t make a difference, a symmetry that didn’t make sense. It’s still astonishing to drop a wad of c

• hollerith 7y ago
  No you are not misunderstanding. The author of the article doesn't understand elementary physics as well as you do, which means he or she should NOT be trying to write an article about relativity or the future of physics. Welcome to modern journalism. In the paragraph after your quote is another point the author almost certainly gets wrong: >When Einstein realized that a person falling freely would feel weightless, he described the discovery as the happiest thought of his life. I have read elsewhere that this "happiest thought" was that there is no experiment that someone falling freely could do that would reliably distinguish between being stationary and falling freely, with an emphasis on measurements of the speed of light. I am pretty sure that it was common knowledge among physics PhDs back then that people falling freely feel weightless. It is not like physics PhD are incapable of jumping while noticing the sensations in their bodies when the sensations are a straightforward consequence of Newtonian mechanics.
  • ChrisLomont 7y ago
    >The author of the article doesn't understand elementary physics as well as you do Don't be so quick to make such judgments: There is ample scientific interest in the differences between inertial and gravitational mass [1]. That they seem the same is a profound question known to Einstein. He explicitly assumed they were the same in his equivalence principle [2]. So their equivalence is a postulate, not derived from deeper theories. But one day we may find it is not consistent over time. It most certainly is a deep question in physics. There are other theories of gravity, taken seriously, as possible future modifications to relativity, such as Brans-Dicke theory [3], that remove this equivalence principle, yet still agree with all observational evidence to date. Current experimental evidence puts them equivalent to under 1 part in 10^-15 [2], with more experiments still being done. [1] https://en.wikipedia.org/wiki/Mass#Inertial_vs._gravitational_mass https://en.wikipedia.org/wiki/Mass#Inertial_vs._gravitationa... [2] https://en.wikipedia.org/wiki/Equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle [3] https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory
• DiogenesKynikos 7y ago
  In Newtonian physics, you can think of every object as having an inertial mass (m_i), which controls how it responds to forces, and a gravitational mass (m_g), which describes how strongly it pulls on other masses through gravity. The gravitational mass can be thought of as the "gravitational charge," in analogy to electric charge. In Newtonian mechanics, there is no reason for m_i to equal m_g. It is simply a coincidence. If they were not always equal, then some objects would fall faster than others in a vacuum. The interesting thing is that there exists a class of forces which are proportional to mass - so-called "fictitious forces." These are forces like centrifugal force, which only appear to exist if you are in an accelerating frame of reference. You only feel centrifugal forces if you're spinning, for example, and the "force" you feel is just your body trying to keep on going in a straight line. Fictitious forces always act on an object with a force proportional to its inertial mass. Einstein's insight was essentially to view gravity as a fictitious force. The hint is that m_g = m_i. In General Relativity, gravity is not a "real" force. The effects of gravity are caused by geometry being non-Euclidean. If you want to understand this better, you can start by reading about the "Equivalence Principle": https://en.wikipedia.org/wiki/Equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle