r/TheoreticalPhysics

Some Confusion about the Light Horizon and the Age of the Universe.

The universe is believed to be approximately 13.7 billion years old. We know this partly because the light horizon is about 13.7 billion light years away from us, meaning that it takes the light from the light horizon about 13.7 billion years to reach us here on Earth.

So in essence, when we see the light horizon we are not seeing it as it is NOW, but as it was 13.7 BILLION YEARS AGO. But, if the universe is 13.7 billion years old, then at the place where we are seeing the light horizon, wouldn't we be seeing the universe as it was in its infancy, basically the singularity before the Big Bang, or perhaps at a fairly short time afterwards?

Also, if we are measuring the distance to the light horizon, and thus (partly) determining the age of the universe, from Earth, does this mean that Earth is at the center of the universe, basically where the pre Big Bang singularity once was (or somewhere close)?

Am I the only one who has stumbled on these little dilemmas (if you can call them that), or is this something that physicists have resolved long ago, and I can go back to my layman's concerns?

Would appreciate some insights on this but I hope you can explain it in layman's terms, have some sympathy for us rubes! Thanks in advance.

In this video I go through the common 4D shapes, the tesseract and the pentatope, and show how they can perfectly represent what I propose as a better regard of the fourth dimension, which is the spatial capacity for motion.

Video Transcript:

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The tesseract is the 4D progression of a cube. Just as a cube is built from two squares with new lines connecting them in a new direction, the 4D tesseract is built from two cubes connected in what has been speculated as a new fourth direction orthogonal to 3D space.

What I’ve considered is not a new purely spatial direction that geometers still teach about, but instead a spacetime axis representing motion. Herman Minkowski and Albert Einstein popularized time as a fourth dimension, which paired with space gives us spacetime, which enables the spatial capacity for motion. Here the tesseract can perfectly represent a cube expanding and contracting.

This is another depiction of a tesseract, or hypercube, described as two cubes facing each other with all vertices connected by new 4D edges.

Instead of the expansion and contraction motion of the last example. This hypercube can represent simple positional motion from one space and time to another.

The pentatope is the dimensional progression from a triangle to a tetrahedron. The "4D edges" are added as lines from each vertex toward the center.

While the tesseract continues a cube's pattern of symmetry, expanding all part equally, the pentatope continues a tetrahedron's pattern of simplicity, moving only one vertex.

I am trying to understand a point that seems easy to state but hard to make precise.

In many discussions of emergent spacetime, locality is not assumed to be fundamental. Geometry is supposed to arise from something more primitive, such as entanglement structure, tensor networks, holography, causal structure, or an algebraic description.

My confusion is about the order of explanation.

If the underlying description does not already contain a background notion of nearby and far apart, what selects the particular subalgebras or degrees of freedom that later behave as local regions of spacetime?

Is there a known physical criterion that plays this role, such as factorization, error correction, modular structure, causality, symmetry, energy conditions, or some stability requirement?

I am not proposing a model. I am looking for the cleanest way this question is formulated in current theory, and for pointers to papers or frameworks where this selection problem is treated seriously.

I understand that the specific publication is normally reputable, but I am having trouble believing some of the conclusions the researchers have drawn. The article was published at the end of March of this year in a well-known journal.

I find it odd that there is only one reference to this article in any other publication discussing it, specifically given that the one in question is Popular Mechanics. I have read of others and experienced first-hand that Popular Mechanics isn't always the most reliable.

Especially as they often try to draw in readers/clicks to their articles as many sites do by giving somewhat flashy headlines as evidenced by "Scientists Say There’s a Place in Our Universe Where Time Moves Backwards." Which was published on May 7th also of this year. Now this might not necessarily warrant skepticism or a cynical response always.

However, I thought I would throw the subject to people with more experience in the field of Theoretical Physics than myself.

Hear me out, i think the point of life is death. It is known the big bang was a starting point. But what if anti matter is death. I mean that in a sense of less gravity, what if the reason why life was possible was because death wasn’t a concept until the earth started having bacteria to prove that. Look at every other plant in the system they’re just gases or rocks. Nothing to really define life as, but on earth even wood is considered life. These rocks still expand around a star so what if this thing we call life affects the gravity of a rock as it rotates around a star. If there’s a lot of life then it makes the rock heavier and thus more gravity. But once death happens we see from past extinctions that it liquifies so the rock is using matter to convert it to anti matter. That way it’ll be lighter and move around the sun faster. I mean the dinosaurs were life and after years we use them for oil sooooo

How is the potential determined to be Mexican hat potential when we look at spontaneous symmetry breaking in derivation foe mass term due to higgs field in lagrangian?

Hi everyone,

I’ve been reading up on the "It from Qubit" paradigm and the idea that spacetime geometry is fundamentally emergent from quantum entanglement, specifically looking at Mark Van Raamsdonk's work, Ryu Takayanagi, and the general ER=EPR conjecture.

I understand the basic premise: the connectivity and geometry of bulk spacetime correspond to the entanglement entropy of the boundary CFT. However, I’m struggling with a conceptual hurdle regarding the background independence of this setup, and I'd love to hear how the community views this.

If we say that space is built out of entanglement, we first have to define entanglement. In standard quantum mechanics, entanglement is defined relative to a specific factorization of the Hilbert space into subsystems: ℋ = ℋ_A ⊗ ℋ_B.

Usually, this factorization is inherently spatial, like having Alice's lab here and Bob's lab there.

My questions are:

1. If space does not fundamentally exist yet, what dictates the "correct" tensor product factorization of the underlying universal Hilbert space?
2. Are we forced to assume a preexisting algebraic structure that secretly smuggles a notion of locality back into the theory before spacetime even emerges?
3. In a fully background independent quantum gravity framework, how does the theory "know" which degrees of freedom to entangle to build the geometry we experience?

I feel like saying "spacetime comes from entanglement" is slightly circular if entanglement inherently relies on spatial separation to be meaningfully defined.

Am I misunderstanding how the Hilbert space is factored in these models? I would appreciate any insights, corrections to my premise, or recommendations for papers that directly address the ontological status of the Hilbert space in emergent gravity!

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Sister universe

Trying to understand how a stable universe like we see isn’t a wave function.

Hi !

Disclaimer : although the title seems quite philosophical, I wish to discuss what the equations of standard QM tell us about reality, and only that. Despite the aversion of this channel for crackpot philosophical digression (which I respect and support) it still seemed the more appropriate to me, due to the higher average level of posts and answers in this channel wrt to others. I hope this is fine.

1. Context :

A few days ago, I was chatting with a colleague from the philosophy department. She asked me "as a physicist, do you think QM challenges Aristotle's principle of non contradiction" ?

The Stanford encyclopedia for philosophy says that Aristotle's principle of non contradiction means that "It is impossible for the same thing to belong and not to belong at the same time to the same thing and in the same respect".

I believe what triggered my colleague's question is that she probably has read stuff like "in QM, an electron can be in several places at the same time", which is quite common in more or less pop science articles.

2. My attempt to construct a rigorous answer to her question

I believe QM does not contradict the principle of non contradiction, but it forces us to distinguish between what the system is (ie its state vector) vs what we can say about it (ie what we can measure, aka the spectrum of the operator).

Let's take the example of "can an electron be at different places at the same time" ? I believe the answer is no, because being in a superposition state does not mean being at different places at the same time.

Disclaimer : for simplification, I will assume the position basis is a 1D discrete basis. I will always assume that the state vector is normalized.

What the system is = the vector state. What I can say about the system = the outcome of a measurement (ie the eigenvalue that pops out if I apply the operator to the state vector).

a) Special case : the vector state is an eigenstate of the position operator.
This is the only case when I can fairly say "the system is at position x_i". In this special case, I recover the good old Newtonian perspective where 1) I can measure the position of the system without altering its state and 2) the position is both a fundamental aspect of what the system is (it's the fact that the ith coordinate of the state vector equals 1 in the position basis while all others equal 0) and it's what I can say about the system (it's the eigenvalue that pops out if I apply the position operator to the vector state).

b) General case : the vector state is in a superposition state. The question "what is the position of the system" is ill defined. The reason is that, unlike in the Newtonian perspective where the position is both a fundamental aspect of what the system is and what I can say about it, in QM position is only a fundamental aspect of what I can say about it.
It is true that there are several non-zero coordinates of the state vector in the position basis. But that does not mean that the system is at different positions at the same time. Special case a) has defined what it means to be at position x_i : it means the i_th coordinate of the state vector in the position basis has value 1. If the state is in a superposition state, there is no such coordinate : they all have values < 1.

Therefore QM is not in conflict with Aristotle's principle of non contradiction : either the system has a well defined single position, or it has no position at all.

What do you think of my attempt to answer this question ? Do I miss something ? Did I make any conceptual and/or physical mistakes ?

nb : I willingly let aside the delicate question of the collapse (or branching or whatever your favorite interpretation) of the state vector on one eigenstate during the measurement process. There, the question becomes really tricky of what really happens but I guess the honest answer is that current QM is floppy about it. I guess one could say that right at the "moment of the collapse" (whatever that means), the principle of non contradiction is somehow challenged because the system switches more or less instantaneously from a superposition state to an eigenstate for no obvious reason (in Copenhagen) or for an unknown hypothetical external reason (in objective collapse). Many world somehow manages to escape this contradiction, but the costs are quite high.

So im 17 and I like learning about physics not mathematical but more of theoretical and imaginative part of it.

I was able to imagine the curvature and how it might form bkackholes but why is it still incomplete. Also time dilation basics. And that an object free falling is actually its natural path and the surface applies force on an external body by resisting it from following to centre and why some accelerometers show 9.8m/s2 when kept on ground.

But I think that I am not having enough doubts that makes me wonder if I really understood it or I am just pretending to "understand" it. Im reading Relativity from fingerprint publications but again its more theoretical and imaginative rather than numerical.

I might not take theoretical physics as a career but I do like thinking on fundamental? Or not so quite fundamental things and just yeah. Thats it.

Any suggestions or advice for me :)

Today in quantum mechanics class, they introduced and analyzed the concept of the propagator to start discussing path integrals. Having also taken a course on complex systems a while ago, I noticed the similarity between the composition law of the propagator and the CK equation (Chapman-Kolmogorv equation) for Markov processes: I was wondering if anyone could give me some information regarding this connection, hoping it makes sense!

https://preview.redd.it/cxntoztvkk0h1.png?width=552&format=png&auto=webp&s=5a4d541790eee14eeaa5292b888eca342c6bef93

Ok, so I'll wait for all of the 'crackpot' comments I got last time, but if I'm wrong and the Earth doesn't expand with the Universe, than why is ^^ this true.

This is easily derived from a single mathematically equivalent modification to Einstein's relativity.

What happens inside a black hole when m²_eff = −ξR < 0 and w > 1/3?

Is there a surface inside every collapsing object where the effective mass of the scalar field goes negative?

Would that be an instability... or the collapse mechanism itself?

Three conditions. One consequence?

What would it imply if it were true?

When I open up the Book Modern Quantum Mechanics by Sakurai and Napolitano and skim through it, I find the defintion (1.97) which is:

" Probability for a' = |<a'|α>|^2 , provided that |α> is normalized. "

And then later on the statement:

"The probabilistic interpretation (1.97) for the squared inner product |<a'|α>|^2 is one of the fundamental postulates of quantum mechanics, so it cannot be proven."

Which means that the probabilistic interpretation of the inner product is one of the fundamental postulates of quantum mechanics.

On the other hand I have seen this episode of Star Trek: The Next Generation where Worf travels through the many worlds of quantum mechanics and in one of them he is married to Deanna Troi.

So my question is: Which one is the correct source to decide if quantum mechanics is probabilistic in its nature?

Hello all,

I’m in my final year of PhD and looking to graduate this fall. I looked extensively for a job related to my field and didn’t even get an interview. I work on cavity QED, phase transition and non-Gaussian light generation (broadly AMO physics/ Quantum Optics). There are many quantum computing related companies that have received significant funding and garnered a lot of attention, I was expecting that I would at least get one interview but that has not been the case.

I have a first-author publication in PRL and a second author in PRX Quantum and will have a couple more on arxiv soon. And the work is related to non-classical photonic states generated from light matter interactions which seems to the brief some companies like Infleqtion and Atom Computing are looking for. (I’m an international student, not sure if that really changes things or not ).

Do they prefer people with a postdoc or some extremely specific skill set? Are there any other industry roles that I can try for? I’d really appreciate any help as I’m close to graduating and would like to be able to make my rent. Thanks for reading and please share your thoughts on this if possible.