The h Bar

3:53 AM

>
.How should we best figure out how small research findings are connected to the larger picture and their relationship to other areas of the research field. That is, how can we best connect the dots. For example, what softwares or neural networks are available presently that can propose a network of research topics to visualise the strength of their connections and topology given an input research result (something more general than a citation map that connects topics based on possibly implications and historical interactions)?

The cool thing about computers is one can instantly narrow a broad question by turning it into a software request question

4:30 AM

@ACuriousMind Woah, I just had a thought

Jan 18 at 23:16, by ACuriousMind

@obe Hallucinogens are not known for respecting the laws of physics :P

The only reason things seem to obey the laws of physics is because we interpret them to.

So if humans weren't around to witness things, would the laws of physics apply?

@SirCumference that seems excessively anthropocentric

@JohnRennie Well yeah, but it's food for thought

The laws of physics as perceived by mankind could be drastically different from the laws perceived by another animal

@SirCumference that seems a rather meaningless statement to me

@JohnRennie Well, think about it. ACM half-jokingly said that hallucinogens are not known for respecting the laws of physics. Does that imply that the laws of physics are determined by what we perceive them to be?

You could as well argue that the laws of physics look different for an accelerating observer and an inertial observer. We get round this by formulating those laws in a covariant way.

Other animals might use different coordinate systems to us i.e. different perceptions of space and time, but when the laws have a covariant formulation that doesn't matter.

If your laws are dependent on your perception then they are very poor laws.

4:43 AM

I suppose. But can we match up what a person taking drugs sees with what a sober person sees?

That's cognitive neuroscience not physics

@JohnRennie Eh, it ties back to my original question. Whether the laws of physics are determined by how our brain perceives them

No. As I said we try to formulate laws in a covariant way, which means they are the same for all observers. Those observers just need to understand what coordinates they are using.

Working out what coordinates a person uses under the influence of hallucinogenic drugs is cognitive neuroscience. I'm sure those experiments would be fun, especially if you're the one taking the drugs, but they aren't relevant to the physical laws.

Yes, but say the person observes something radically different. If someone with severe psychosis or a similar ailment observes the laws of physics differently from us, can we match them up? If not, wouldn't that imply that physical laws are only determined by those unaffected by such ailments?

Pretty sure if you account for all the possible mechanisms that the visual and sensory imagery on how the laws are perceived by that mentally altered person, the laws of physics should have a covariant formulation that relates back to normal mental states. However I suspect at such level, the covarient formulation will be full of complicated parameters to be practical

If not, then we have a problem about the objectivity of reality itself...

4:51 AM

@Secret That's what I'm wondering

We scientists assumes reality is sufficiently objective in order to formulate our models to explain phenomenon. If there exists "perspective dependent laws of physics", then it is almost the same as saying magic is real, which is very weird and very very hard to to be sure

@SirCumference So you have a hallucination that cars are moving very slowly. Does that make it safe to step out onto the highway in busy traffic?

@JohnRennie Huh, that's a good argument. Gimme a minute to think...

Having said that, I am still not sure how I should view energy nonconservation of the FLRW metric. Initially I thought I can explain away the apparent energy nonconservation by using something similar to covariance (that different reference frames and the curvature of spacetime will account for the missing energy and thus is a coordinate effect like how kinetic energy has different values in different inertial frames),
but Slereah, Johnrennie and Jim clarified to me that this nonconservation is real because it all depends on whether there exists a global time killing vector

@JohnRennie I guess you have a solid point. I can't deny that those cars could kill us, regardless of how we perceive it

Wow, redundancy. I need sleep.

Q: How are beliefs restricted by an objective reality?

4:59 AM

3

Consider the following belief A human can survive if they don't drink water for 1000 days We knew from biology that this is practically improbable as there are very few people who can survive for 18 days without water (Andreas Mihavecz in 1979). However, there are also beliefs that are unr...

for more discussion about beliefs and objective reality

I dunno if that falls under philosophy. Physics is indeed the study of the Universe.

The Universe's objectivity seems relevant

To, me philosophy and physics canhave many overlaps, and I and my friends do treat philosophy seriously.

With new tools in today's century, it seems philosophy are more capable to answer questions than in the past

But yeah, our universe seemed sufficiently objective enough

@Secret I think philosophy is more about the questions than the answers themselves. Very often something "philosophical" will make you ponder for a while, and you'll reach no answer.

Though the questions themselves can be important in their own right

even though quantum mechanics is probabilistic, the underlying evolution of the wavefunction is deterministic (epistemic or ontic interpretations), or that no matter how many times we replicate an experiment, the results are more or less returning the same distribution of observables (instrumentalist), or that how our knowledge on what the outcomes will be is also more or less conistent (Qbism)

In short, everything in physics seemed quite consistent and apparently different outcomes can be related consistently by simple rules as far we know

@SirCumference yeah, it can be quite a while before the question lead to actual outcomes. I recal one of my friends in a phlosophy club said that philosophy seemed to raise the questions, and other faculties solve the questions for them

However, when it comes to informing social policies, philosophical discussion seemed to have effect almost on par with art

IMO, philsophy usually becomes the most practical when it comes to ethical decisions

bbc.com/news/technology-36517340

such as this harmful AI topic recently

I hate it when I can't just use my university to get a paper

Q: Wormhole paradox: portal inside portal

5:14 AM

(To be expanded later when I get back home) Inspired from the phrase "negative activation entropy" in one of my literature reviews: Can we have a deep entropic well in phase space that is quite localised?

0

I found this image on Facebook referring to the Portal videogame. But I wonder... What would really happen if this was in real life, supposing the portal was a wormhole?

wormholes

I have once done a simple calculation on this scenario with basic portals (I.e. Ignoring GR curvatures). The result is that somewhere where the two portals intersect at the centre, they will get stuck because they bumped into each other's fixed point

5:40 AM

@DavidZ: Homework???

Non-mainstream possibly, or unclear, but homework?

Well ER=EPR is pretty mainstream, just still theoretical

I am not sure if I classify that as homework, given it's more conceptual

@JohnRennie actually, I don't remember whether wormhole solutions will mean the two mouths have to be located in two different universes. But it seems based on your comment on that question, it seemed to suggest even if you have a wormhole where both mouths are in the same universe, it is still impossible to stuff one mouth into another

@Secret well consider a railway tunnel. How would you place one end of a railway tunnel inside the other? Even allowing the ground to be elastic.

I suppose it will work like placing one end of a tubing into another, thus part of the tubing will end up being encased by another. But I think for wormholes it might be more complicated since there's literally an identification between two points in spacetime with no notion of tubing of finite length

(Will draw a picture later, but the result is kinda similar to when you try to force one end of a garden hose into another for the railway tunnel case)

David Z

6:03 AM

@JohnRennie Yeah, it reads like a homework-like question to me. It's just not explicitly identified as such.

6:16 AM

It's looking like a nice day in Chester:

A good day to stay inside and do some physics :-)

Hritik Narayan

7:14 AM

If we point all of the lasers that we have on Earth at one point, would we have remotely enough energy to make a micro black hole?

@JohnRennie Every day is a good day to stay inside and do physics :P

@HritikNarayan Suppose the Schwarzschild radius of your proposed black hole is $r$ then the time taken for light to cross the black hole is $t = 2r/c$. If the total power of the lasers is $P$ then the energy inside your region $2r$ is $E = Pt = 2Pr/c$.

The Schwarzschild radius is given by $r = 2GM/c^2$, where in this case $M$ is the mass equivalent to the energy $E = Mc^2$ or $M = E/c^2$.

Comparing the two equations I get the power required as $P = c^6/4G$ which is about $10^{60}$ watts.

So that would be a no then :-)

Hritik Narayan

Ouch. 10^60. That's a big no.

7:32 AM

Interestingly the power required is independent of the black hole size. That's because the mass of a black hole is proportional to the black hole radius not the black hole volume.

Hritik Narayan

That was what I was wondering about after seeing your work there, that's pretty cool

No micro black holes for a very very long time, then

@ACuriousMind @BalarkaSen Is there a difference between the exterior algebra and the Grassman algebra?

Some sources say it's the same, some seem to imply that there's also a contraction operator defined.

7:54 AM

user228700

8:13 AM

@JohnR: Morning! :-)

Hi :-)

You're later than usual today ...

user228700

Yes, I, erm...gchat? :-/

On my way ...

8:38 AM

@Slereah I wouldn't make a difference there.

@SirCumference If a tree falls in a forest and nobody is around to hear, does it make a sound? :P

Dawood ibn Kareem

@KyleKanos The speaker is clearly British. I want to know why the querent thought he might be an Australian or a New Zealander.

Dawood ibn Kareem

8:57 AM

@JohnRennie So if the answer had been yes, we could have made a really big black hole?

@DawoodibnKareem the power required is independent of the size, but total energy isn't. Most ultra high power lasers can only supply that power for a very short time - sometimes on the femtosecond scale.

So in practice we could make only a very small black hole because we couldn't sustain the laser power for long enough to make a big one.

Dawood ibn Kareem

@JohnRennie Or until they get sucked into a nearby black hole.

I was going to suggest that trying this experiment, to verify that your mathematics is correct, probably isn't the greatest idea in the world.

Q: Link between the Grassman algebra and spinors

1

What is the exact link between spinors and the Grassman algebra? I'm pretty sure there's one, based on the following : The Berezin integral in path integrals is done over the Grassman algebra of $\Bbb C$ There's a mapping from the tangent bundle of a supermanifold (which is locally $\Bbb R^n \...

supersymmetry group-representations spinors clifford-algebra

plz halp

9:34 AM

@Slereah You misunderstand what that theorem is saying. It's saying that given a spin manifold $M_0$, then we may choose a supermanifold $M_1$ whose bosonic part is $M_0$ and whose fermionic part is the exterior algebra bundle of the spinor bundle (as a vector bundle) such that $T_p M_0 + S_p = T_p M_1$. But that's not a deep result, it has nothing to do with the spinor quality of the bundle, you'd have the same relation for any vector bundle on $M_0$.

The interesting part is that you can choose this isomorphism canonically, but actually I think that also works for any other vector space

Ah, alright

Is there a particular reason we use "Grassmann algebra" rather than exterior algebra for spinors?

They are the same

You'll also find mathematician call the exterior algebra Graßmann algebra as such, it's a matter of preference

I've seen a thing where the grassmann algebra was slightly different

basically an extra operator on the exterior algebra