I Myths of Modern Popular Physics

[tzimie]

In our IT company sometimes we make presentations, sometimes even just for fun, not related to IT things. I suggested the presentation with the title above and many people are interested. They are quite smart, but there will be no formulas in my presentation.
Please review my list. Any suggestions/notes/advice?

1. Black holes
1.1 Spaghettification - human gets torn apart at event horizon (false, humans are torn apart approx 1s before singularity, for stellar size BH far away from horizon, for supermassive - deep inside)
1.2 Something spectacular happens when one crosses a horizon (apparent horizon retracts and never crosses your body).
1.3 There is a library inside a black hole )
1.4 In order to create a BH, matter must be compressed to immense density (again, true for small BH only).
1.5 When neutron star moves close to c, it becomes BH because a. It becomes "heavier" in GR (mention the obsolete concept of "relativistic mass") - false, b. it contracts (true but doesn't make it BH)
1.6 Time freezes near the horizon (true for hovering observers only, free falling observer can't be replaced with a sequence of hovering observers at same points)
1.7 So free falling observer would see the future of the universe (in fact, he accelerates so quickly that light behind him is redshifted, so he observes the universe slowed down)
1.8 As falling spaceship is "stuck" near the horizon for billion years, theoretically one can fly there and take him back (put Eddington–Finkelstein diagrams in Powerpoint)
1.9 As black holes evaporate, and it takes an infinite time to fall into it, it evaporated before you cross the horizon - (1.6-1.9 the same source of confusion)
1.10 Why spaceship can't avoid singularity - because it is in the future!

2. Rotating Black Holes
2.1 Ergospheres
2.2 Ring singularity (idealistic solution)
2.3 We really don't know what is inside because CTC is incompatible with QM so we need TOE.

3. GR in general
3.1 Again on "relativistic mass", are objects become "heavier" when move near c?
3.2 Are they heavier when heated (and this is true)
3.3 Mass of light. Does light gravitate? (talk about "sources" of gravity in GR)
3.4 Special cases, 2 parallel light beams in the same direction (don't attract) and opposite directions (do attract)
3.5 Energy is conserved (conservation is not well defined in curved spacetime).
3.6 GR is the only theory of gravity (mention Einstein–Cartan theory)

4. Cosmology
4.1 "When Universe was a size of an atom..."
4.2 "Big Bang" - not an explosion, where the name comes from
4.3 Why something infinite can expand.
4.3 "Universe at time t after Big Bang" is not a valid frame of reference and nothing is conversed there
4.4 Difference between Dark Energy and Dark Matter.

 

[nitsuj]

2.3 We really don't know what is inside because CTC is incompatible with QM so we need TOE.

isn't it a "myth" that there is such a volume inside...and that the surface of a black hole is a singularity

 

[robphy]

As suggested, I don't think "myth" is the appropriate term here.
Some of these are idealized thought experiments... akin to saying suppose I have a block on a frictionless incline...
...although these on your list are more speculative, often based on extrapolating our current understanding back to extreme situations. Often the goal is to highlight some effect, not make a prediction of what would really happen given all of the actual conditions of an experimental setup.

A myth is more like "the world is flat"...
or "the normal force is always equal to the weight" [i'm grading exams right now].
Ok... "GR is the only theory of gravity" is a myth...

It might certainly be the case that some of these speculations
have been "popularized" for the masses from their technical incarnation,
and may have gone through a game of "telephone".

 

[nitsuj]

oh and 3.2 regarding kinetic energy and "closed systems" in the context of trying to define mass of massive things beyond being defined as a bound system. perhaps your heated object lost electrical / nuclear potential in the process

I googled it and the wiki says mass is defined by a Center-of-momentum frame Hey that means technically heat does make an object more massive.

oh wow this is interesting

 

[Comeback City]

4.3 Why something infinite can expand.

I would recommend explaining this with the balloon analogy.

 

[A.T.]

I would recommend explaining this with the balloon analogy.

The balloon surface is not infinite. It is closed.

On the other hand, if you accept that something can be infinite in the first place, then accepting it can expand doesn't seem like a big step.

 

[Comeback City]

The balloon surface is not infinite. It is closed.

On the other hand, if you accept that something can be infinite in the first place, then accepting it can expand doesn't seem like a big step.

Balloon analogy explains how the distance between galaxies is increasing (aka expansion), regardless of (in)finiteness. All that NEEDS to be explained is the metric expansion.

 

[DrStupid]

Balloon analogy explains how the distance between galaxies is increasing (aka expansion), regardless of (in)finiteness.

And therefore it would miss the point. I would refer to Hilbert's Hotel instead: https://en.wikipedia.org/wiki/Hilbert's_paradox_of_the_Grand_Hotel

 

[Comeback City]

And therefore it would miss the point. I would refer to Hilbert's Hotel instead: https://en.wikipedia.org/wiki/Hilbert's_paradox_of_the_Grand_Hotel

The point is that metric expansion occurs whether or not it is infinite or finite (the surface of the balloon being finite is irrelevant), and therefore the balloon analogy can be used in order to explain it.

 

[tzimie]

I don't like balloon analogy because it suggests that Universe is finite, I rather think about stretching of infinite rubber sheet.

 

[tzimie]

2.3 We really don't know what is inside because CTC is incompatible with QM so we need TOE.

isn't it a "myth" that there is such a volume inside...and that the surface of a black hole is a singularity

More specifically, we don't know what is inside inner horizon (or do we?)
outer horizon is a singularity only in Schwarzschild metric, and this can be easily avoided.

 

[Comeback City]

I don't like balloon analogy because it suggests that Universe is finite

But it doesn't suggest that. That is just a result of overthinking something as simple as a balloon expanding. It only explains metric expansion (and also that there is no center to the universe).

I rather think about stretching of infinite rubber sheet

That works fine also, as long as you know that there is no center to this rubber sheet. That would suggest a center to the universe, which does not exist.

 

[A.T.]

It only explains metric expansion

Which is a different issue than 4.3.

 

[PeterDonis]

Thread closed for moderation.