# Tag Info

12

The rubber-sheet analogy is often used to "explain" the basics of GR to beginners, but actually it has nothing to do with real gravity. It acts much more like a scalar field (the up/down freedom degree) - and there were several attempts to build a scalar gravity. But the correct description turned out to be tensorial and purely geometrical. GR has 10 ...

5

To add to Hindsight's great answer: one of the reasons that the analogy fails is the same reason why Nordström's Scalar Theory of Gravitation fails: Waves on rubber sheets are described by linear wave equations; at least in the small amplitude limit. However, by analogy with Maxwell's equations, waves in gravitation should bear energy. But we are also ...

5

Have a read through Did the Big Bang happen at a point? and the answers to it. The singularity at the Big Bang is the zero time limit of the equation (the FLRW metric) that describes the expansion of the universe. Most physicists believe that this is a mathematical artefact and does not describe what actually happened. It seems likely that some quantum ...

5

Square the proper time $d \tau = dt / \gamma$, multiply by $c^2$, rearrange, and take the square root: $$\left(\frac{d\tau}{dt}\right)^2 = \gamma^{-2} = 1 - \left(\frac{\bf{v}}{c}\right)^2 \Rightarrow \sqrt{\left(c \frac{d\tau}{dt}\right)^2 + {\bf{v}}^2} = c$$ The proper time $d\tau$ is the time elapsed in the frame moving with respect to the lab frame, in ...

4

Yes it is - well, sort of. The coordinate invariant form of velocity is the four velocity, and the magnitude of any four velocity is always $c$ (or $1$). So even if you are stationary in space in your chosen coordinate system the magnitude of your four velocity is still $c$. Whether moving at the speed of light on the time axis is a good way to state this ...

3

I realize that in essence there is no object which can be considered as "not moving in space". No object at all is moving in space if you are taking the point of view of its reference frame! The law of conservation of energy is requiring that the energy of its mass (e = mc2) is "transported through time", or in other words, that time is passing for ...

3

First of all, physics does not ever talk about the question of existence, but about useful descriptions and predictions of observations. No physicist will ever prove to you he is not just a figment of your imagination but he can prove to you that Newton's law works pretty well for what you see. In the scientific method, a theory is indeed used until it ...

2

Your questions are in no way challenging. The answer to both questions is the same: it could be, we do not know. Actually, you could also ask the opposite: how do we know that physical space is equivalent to the continuum (the real line) instead of being a larger infinite (by this a mean an ordered field of larger cardinality, such as the surreal line)

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It's just a drawing convention. Rather than "vertical", time is orthogonal to the x-axis. The reason it is not shown vertical is because the paper surface is 2D, and the author uses the vertical axis for drawing the altitude with respect to the ground. Just recall the way you draw the 3D axis. Here, the author uses X (horizontal),Y (vertical) and time ...

1

The aim of special relativity and of spacetime (in particular: the Minkowski space time) is not to know about what time is. Spacetime is showing a relation between space and time from an observer's view only - and this whatever time is in reality (including the question if time exists or not). The result is that time (i.e. the value measured by clocks) may ...

1

This sounds a bit in the spirit of John Wheeler's geometrodynamics. He hoped to find in the dynamic geometry of GR a way for "mass without mass, charge without charge, field without field" to somehow emerge just from vacuum gravitational fields interacting. He contemplated "geons" which would be packets of gravitational waves held together on the short ...

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