<p>Your understanding is incorrect.</p>
<blockquote>
<p>General relativity states that all matter in the universe is moving forward through spacetime at the speed of light.</p>
</blockquote>
<p>You are talking about the four velocity here. The three velocity component being zero is consistent with a four velocity with magnitude <span class="math-container">$c$</span>. The Lorentz factor for a particle moving at three vector velocity <span class="math-container">$\vec v$</span> is <span class="math-container">$\gamma = \frac 1 {\sqrt{1-\frac{v^2}{c^2}}}$</span>, which is identically one when <span class="math-container">$v=0$</span>. The four velocity is <span class="math-container">$\mathbf U = \gamma\begin{bmatrix} c \\ \vec v\end{bmatrix}$</span>. This always has a magnitude equal to <span class="math-container">$c$</span> for all three vector velocities with magnitude less than <span class="math-container">$c$</span>, including <span class="math-container">$\vec v = 0$</span>.</p>

CC BY-SA 4.0
2021-11-24T02:51:19.373Z
Ah. Studying the four-velocity now, the word that stands out to me is "normalized". So the takeaway is that the four-velocity isn't actually representative of the objects actual velocity in four-dimensional spacetime, just it's direction, and it's actual velocity may have a magnitude greater or lesser than c?
Quasar
CC BY-SA 4.0
2021-11-24T03:21:35.690Z
These helped:

https://physics.stackexchange.com/questions/33840/why-are-objects-at-rest-in-motion-through-spacetime-at-the-speed-of-light

https://physics.stackexchange.com/questions/530868/is-the-magnitude-of-the-four-velocity-vector-in-spacetime-arbitrary

Of particular note, this answer from the first link: "Objects don't move through spacetime. Objects move through space. If you depict an object in spacetime, you have a world-line. The world-line doesn't move through spacetime, it simply extends across spacetime."

I think I understand better now.
Quasar
CC BY-SA 4.0
2021-11-24T01:41:58.927Z
2021-11-24T14:44:02.083Z
2021-11-24T14:44:02.083Z
David Hammen
<p>General relativity states that all matter in the universe is moving forward through spacetime at the speed of light. Objects that are stationary in space travel at c along the time axis, and objects moving in space experience time dilation such that, if you combine their speeds in time and space into a single vector, you will <em>always</em> get c. Object don't actually &quot;speed up&quot; or &quot;slow down&quot;, they just change their direction in spacetime.</p>
<p>Singularities exist at the bottom of a gravity well which causes extreme gravitational time dilation. Time is rotated fully into the spatial dimensions and matter that passes the event horizon completely stops moving forward in time. Ergo, matter within the event horizon must travel at the speed of light in space as it falls toward the center of mass.</p>
<p>And then the mass joins the singularity and just <em>stops</em>, in both time and space. Or in a ringulary, spins around the Centre of Mass at less than c. Either way, the universal constant is broken: matter in the singularity no longer travels at c in spacetime. So either general relativity completely breaks down at the singularity, or there is no singularity and something else entirely happens to the matter as it approaches the black holes center of mass.</p>
<p>Is my understanding accurate, or am I missing something?</p>

<black-hole><general-relativity><time-dilation><singularity>
Does general relativity imply that singularities cannot exist?
Quasar
# Does general relativity imply that singularities cannot exist?

(Asked by: username_0 on 2021-11-23)

General relativity states that all matter in the universe is moving forward through spacetime at the speed of light. Objects that are stationary in space travel at c along the time axis, and objects moving in space experience time dilation such that, if you combine their speeds in time and space into a single vector, you will *always* get c. Object don't actually "speed up" or "slow down", they just change their direction in spacetime.

Singularities exist at the bottom of a gravity well which causes extreme gravitational time dilation. Time is rotated fully into the spatial dimensions and matter that passes the event horizon completely stops moving forward in time. Ergo, matter within the event horizon must travel at the speed of light in space as it falls toward the center of mass.

And then the mass joins the singularity and just *stops*, in both time and space. Or in a ringulary, spins around the Centre of Mass at less than c. Either way, the universal constant is broken: matter in the singularity no longer travels at c in spacetime. So either general relativity completely breaks down at the singularity, or there is no singularity and something else entirely happens to the matter as it approaches the black holes center of mass.

Is my understanding accurate, or am I missing something?

**Tags:** black-hole, general-relativity, time-dilation, singularity


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## 1 answers

### Answer by username_1 on 2021-11-24. Score: 2

Your understanding is incorrect.

> General relativity states that all matter in the universe is moving forward through spacetime at the speed of light.

You are talking about the four velocity here. The three velocity component being zero is consistent with a four velocity with magnitude $c$. The Lorentz factor for a particle moving at three vector velocity $\vec v$ is $\gamma = \frac 1 {\sqrt{1-\frac{v^2}{c^2}}}$, which is identically one when $v=0$. The four velocity is $\mathbf U = \gamma\begin{bmatrix} c \\ \vec v\end{bmatrix}$. This always has a magnitude equal to $c$ for all three vector velocities with magnitude less than $c$, including $\vec v = 0$.

> **Comments:**
> **username_0**: Ah. Studying the four-velocity now, the word that stands out to me is "normalized". So the takeaway is that the four-velocity isn't actually representative of the objects actual velocity in four-dimensional spacetime, just it's direction, and it's actual velocity may have a magnitude greater or lesser than c?  
> **username_0**: These helped:  https://physics.stackexchange.com/questions/33840/why-are-objects-at-rest-in-motion-through-spacetime-at-the-speed-of-light  https://physics.stackexchange.com/questions/530868/is-the-magnitude-of-the-four-velocity-vector-in-spacetime-arbitrary  Of particular note, this answer from the first link: "Objects don't move through spacetime. Objects move through space. If you depict an object in spacetime, you have a world-line. The world-line doesn't move through spacetime, it simply extends across spacetime."  I think I understand better now.  

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