From 3.5 billion Reddit comments

1 point

·
29th Sep 2022

As was already mentioned eigenchris is an excellent physics channel, especially his relativity series. I also am enjoying a physics series on stars by physics almanac. Those are both youtube format. If you want a book, you can start with Einstein's book on relativity It's accessible to everyone. I first read it before taking a single physics class.

1 point

·
4th Aug 2020

>Ok, put a accelerometer on a solid surface and it will show 9.81 m/s2. What is it detecting if not gravity? There clearly is a reading of something and you are trying to convince me that it is registering nothing?

It is detecting the normal force. (The force of the Earth pushing against your feet, keeping you from falling through it.) That is why the detected acceleration points *away* from the Earth and not towards it.

Note that standing on the Earth is *not* free-fall (precisely because there is a proper force -- i.e. the normal force -- acting on you).

>In a free fall, the accelerometer is registering both gravity as a vector upwards at the value of 9.81 m/s2 and it's own acceleration as a vector downwards of the same value. Both vectors give 0 as a sum.

No. There is no circumstance where an accelerometer will *ever* measure the force of gravity.

Gravity is not a proper force, it is a fictitious force that arises due spacetime curvature. This was one of Einstein's key insights that led to the development of general relativity: that gravitationally-accelerated motion is actually inertial (non-accelerated) motion through a curved space.

This is also why you feel weightless (i.e. you feel no forces acting on you) when falling.

>>Special relativity features absolutely no warping of spacetime whatsoever.

>That is what I'm arguing about.

Well ... I don't know what to tell you. I'm afraid you are just outright mistaken. This stuff is textbook material for special and general relativity ...

>Maybe it is not correct to think about time dilatation without the space time concept itself.

Naturally, you do need the concept of spacetime in order to have any notion of time dilation, but you don't need *warping* of spacetime. This is, of course, the difference between special and general relativity. Both of them feature time dilation; only one of them features warping of spacetime.

>If the time is dilated and the time is a component of space time, how can the space time remain flat?

It's built into the structure of spacetime, which is not Euclidean (but that does not mean it is curved or warped in any way). You really should read up about Minokowski space, the Lorentz transformation, and the basics of special relativity.

Consider picking up a translated copy of Relativity: the Special and General Theory by Albert Einstein -- it was a book he wrote specifically to explain both special and general relativity to ordinary laymen with nothing more than a high school education. It uses nothing more than ordinary algebra, and meticulously walks you through most of the main thought experiments which lead Einstein through the theories' development. It's cheap too, only $10 USD.

>The idea is that lorentz formulas are just simplification that do not take the space time into account.

Come again? The Lorentz transformation is not just a simplification, they are a cornerstone of special relativity, and they are present in the spacetime featured in special relativity (so it clearly does take all the structure of Minkowski spacetime into account).

>They are just an approximation of the real phenomenon - a space time deformation that is equivalent to those described in GTR.

*Actual* spacetime -- that is to say, our universe -- is best described by general relativity*, yes.

*this technically remains to be seen: gravity is best described by general relativity, but all of the other forces as well as matter are best described by quantum field theory, which is the *specially*-relativistic version of quantum mechanics. Quantum field theory <em>can</em> be generalized to curved spacetime, but this does not give us a quantum theory of gravity, so it does not tell us exactly how particles gravitate and doesn't solve all of the problems that a true theory of quantum gravity would. It remains to be seen whether the correct theory of quantum gravity will be a quantum field theory in either flat or curved spacetime, or something else entirely such as string theory.

However, Lorentz transformations are not merely "an approximation," they are the exactly correct edge case of working in a spacetime that is perfectly flat. In reality, our spacetime is never perfectly flat, but we can approximate it as perfectly flat in many cases, and in those cases, yes, Lorentz transformations are recovered as an approximation. But that does not at all imply that all time dilation is gravitational time dilation (it's not). Even in general relativity, these are still two different effects with different origins: time dilation due to gravitation is due to the curvature/warping of spacetime, while time dilation due to relative velocity is due to the relative velocity between reference frames.

In general relativity, if you consider two reference frames which are at rest with respect to each other (no relative velocity), then *all* of any time dilation is going to be gravitational time dilation.

On the other hand, if you consider two reference frames which are at the same gravitational potential, *all* of any time dilation between them is going to be due to relative velocity (which is why the Lorentz transformations are exactly recovered in general relativity with the mentioned constraints).

< $25

< $25

< $50