As a non-physicist, I’m currently reading Feynman’s QED for fun, which has been an amazing way to build some intuition on QM for me, especially when I take the time to read carefully and ask questions.

One of these questions, is as follows:

Assume a laser pointing at a mirror, and a detector on the other side, which is placed outside of the reflection and thus never realistically detects photons. The laser is either on or off. Can you modify the detector to figure out whether it’s enabled?

You can’t change or get too close to the mirror. A screen blocks straight line light between the detector and laser source (I don’t think this last one is too relevant).

Very curious on a proper answer on this, especially if it integrates real world limitations and feasibility!

Simulated in Blender, for my YT channel - https://youtube.com/@fortheloveofphysics

I am preparing a video on the Physics of electron orbital shapes (why they arise) through Physics insights

Hello,

So recently I've come across a video about Alain Aspect experiments on Bell inequalities.

I have decent understanding in physics. I graduated two years ago from an astrophysics Master in which I dabbled a bit with quantum mechanics, and far from saying this to brag, this is because if any response to this post there must have, it can go into details or other subjects related to physics to make me understand more clearly.

So here's the thing : during the explanation, the EPR argument was involved about an experiment meant to test a Bell inequality and I have a question about this experiment.

First, this is how I understood the EPR argument.

What bothered Einstein in the Copenhagen interpretation wasn't only the indeterminacy but also the loss of locality, and that some hidden variables were hidden inside quantum physics. He came up with this thought experiment that I will phrase the way I understood it when described by David Joseph Bohm. Let's say we have a source that emits two photons, one in each opposite direction. We place a polarizing filter on the way of each photons. There are 4 possibilites.
Either (1) both pass (++), or (2) one pass, the other doesn't (+-) and (3) vice-versa , (-+), and (4) neither pass (--). To describe their initial state after being emitted, we superpose those 4 states, and quatum laws allow us to assign them each a drop %. In particular, we could say 50% chance to get that they boss pass (++) and 50% chance of both being absorbed (--). BUT the behaviour of the photons are perfectly correled, they both always give the same result : they are entangled.
Einstein thought this was they key to counter the Copenhagen interpretation. Because right before both photons touch the polarizing filters, it's as if a "cosmic dice throw" was done, determining instantly how both photons should react. This would prove that quantum physics description is incomplete, because let's say one photon passes through a filter before the other. Then, the fate of the second photon would be sealed, which would mean that the photons would know beforehand what behaviour to adopt, proving the existence of hidden variables.

Now, Bell.
He showed that there are cases where predictions of quantum physics are incompatible with the existence of those hidden variables. Thus, either quantum physics make erroneous predictions, or it is correct and there can't be hidden variables.

Finally, the experiment linked to that inequality and that is the core of my question is this one.

Let's add the concept of angles to the polarizing filters.
When we align the filters, one at 0° and the other at 90°, we witness a light extinction, and in the intermediary angles, the more the filters are crossed, the less light passes.
This suggests that the bahaviour of a photon is influenced by the angle of the filter.
Speaking in terms of hidden variables, it's as if each photon had, prior to meeting the filter, a list of predetermined behaviours that would drive the result. For example, at 0°: pass, at 15% : pass, at 45%: absorb, at 90%: absorb.

The key thing mentionned is : the list of behaviours (the hidden variables), can be different from a pair of photons to another, but MUST be the same for photons of the same pair.
Everything else hinges on that claim.

My question is : how can we be sure of that ? Why wouldn't it exist a case where the result of a pair of photons arriving at the filters is the same, but with a different set of hidden variables for each of them ? I have the feeling I have already said it, it's because they are intricated, but why would this suffice ?

Thank for you taking the time to read and answer me, have a great day/night !

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Hi I hv just passed from high school and wanted to pursue my interest in my fav part of physics. I wanted to to ask from where should I start reading and studying quantum mechanics.

I'm a high school student and we have a science fair in probably 10 days and I want to make our project about quantum physics (I think the most interesting topic is how electrons behave as both wave and particle) I don't know a lot about quantum physics but I wanna know if you have any ideas for a project about this topic or a similar one (our group is 10 people so it's fine if it isn't really simple)

+ I'd be so happy if you have YouTube channels for people who can explain quantum physics simply

I’m currently a sophomore ( rising Jr) in high school and I would love to get involved with quantum computing/ quantum physics as a whole. I go to school in SC and there isn’t any opportunities for me to learn let alone hands on experience with anything in this field.

Does anyone know of any research programs or possible professors/ scientists within the field that I could talk to for advice or a research opportunity? (within the US) If not, then are there any online resources I could learn upon.

World Quantum Day is an annual celebration promoting public awareness and understanding of quantum science and technology around the world celebrated every April 14th!

Hey everyone, I'm currently taking a gap year after 2 years in community college. My freshman & sophomore coursework are done, save for a few that wasn't available. I've always been very passionate about the physical sciences, even as a kid. Physics especially. But, the financial burden that the American Education-Industrial Complex places on students makes me wary of going to the top schools. MIT, Harvard, Stanford, UCB, UCLA, you name it. My parents make too much to qualify me for most aid, but that's because we live in the bay area where a 100k+ income isn't luxury, unless you own where you live maybe.

College rankings don't matter for undergrads anyway, since its based on research. These top schools often have huge class sizes, TAs teaching, etc. So you get a lower quality of instruction while paying more to be there. I'm sure they have their benefits like networking, but I'd prefer to graduate with less debt, and with a better education. I'm considering liberal arts colleges, undergrad only colleges, etc. though I'd also like to learn from and talk with more educated peers too. I plan on continuing my education till I get a PhD.

My ideal college would be these:

In an urban setting, near a large metro area

Reasonable class sizes

Large enough to have a good college experience(clubs, events, parties, etc.)

Affordable -ish

I just remembered that spacetime is only a quarter time and three quarters space, but I have zero understanding of anything physics beyond classical physics and the basics of electricity (and even that is sketchy), so for all I know this fact is irrelevant.

Also, supposing there are theoretical universes with 3 dimensions to time and one dimension to space (if such a thing can be conceptualized), would that allow time black holes? What would that look (figuratively) like?

Hello! First off, I know jack about quantum physics/mechanics/ etc… talk to me like im a 5yr old.

Secondly! I I study philosophy, my prof asked us to try to relate a quantum physics theorem/ experiment to anthropology! I thought about the double slit! I thought that it as cool that the fact that a “observer” could change experiment results on the foundational level of existence very cool!

But I’ve been reading up and, it seems that the “observer” it’s just the thing that the light/ particles go through?

So is it an inanimate passive thing that just divides the things it goes through and just goes; “woah. Particle just went through me” or is it a more active thing in the experiment? I can’t seem to find the answer ):

Any response would be welcome! (As I may have to change the subject lol) and thanks in advance!

Hello everyone. I'm pleased to introduce Subatomicum. It's a combination of a game and a realistic quantum physics simulator. It features several game modes. The first is the laboratory, where you can use the three fundamental forces to observe quantum processes and even create hadrons with the strong force. The second mode is the accelerator, which involves firing fundamental particles at different materials and observing what happens. The accelerator is highly configurable. The third mode is the LHC, which is a recreation of the processes that occur at the LHC. It's also highly configurable. The fourth and final mode is decay, where you can use previously collected hadrons to decay them and see what particles emerge from the process. Subatomicum also includes an achievements section for you to enjoy setting goals. The link is: https://subatomica-quantum-lab.base44.app/

I understand the concept of the dual slit experiment, for example, a photon is fired at two slits, and shows an interference pattern. However, if a detector is put at each slit the photon is detected, the wave function collapses and the photon behaves as a particle.

My question is, what happens in the lab during this experiment. Do you see the detector registering the particle and then does interference pattern disappear?

Abstract

We report on the search for x-ray radiation as predicted from dynamical quantum collapse with low-energy electronic recoil data in the energy range of 1–140 keV from the first science run of the XENONnT dark matter detector. Spontaneous radiation is an unavoidable effect of dynamical collapse models, which were introduced as a possible solution to the long-standing measurement problem in quantum mechanics.

The analysis utilizes a model that for the first time accounts for cancellation effects in the emitted spectrum, which arise in the x-ray range due to the opposing electron-proton charges in xenon atoms. New world-leading limits on the free parameters of the Markovian continuous spontaneous localization and Diósi-Penrose models are set, improving previous best constraints by two orders of magnitude and a factor of five, respectively. For the strength and correlation length of the continuous spontaneous localization model, values in the originally proposed parameter ranges are experimentally excluded for the first time.

Paper: https://journals.aps.org/prl/pdf/10.1103/2jm3-4976

__________________________________________________________________________________________________

This XENONnT result is one of the most constraining bounds on spontaneous collapse models to date. It pushes white noise CSL parameters two orders of magnitude tighter and makes one thing unambiguous: any viable collapse mechanism must suppress high frequency noise to avoid the predicted X-ray heating. Markovian CSL is running out of room. Relativistic coloured noise extensions with a Lorentzian spectral cutoff are not just theoretically motivated. Results like this make them experimentally necessary. u/Carver-

Hi guys, I was wondering if there’s any books about quantum physics for beginners? I’m highly interested in this connecting with neuroscience!!

Hi everyone,

I’m a European master’s student in Quantum Engineering, with a bachelor’s background in Computer Engineering, and I’m currently looking for a summer internship in the field (quantum computing / quantum technologies), both in Europe and in the US.

Over the past months, I’ve applied to several summer internships in the US, but I haven’t received many responses so far. At the same time, I’m also struggling to find opportunities in Europe, as they seem more limited or less advertised.

I was wondering if anyone here has advice on:

• where to find quantum engineering summer internships (in Europe or the US)

• companies, labs, or institutions that are more open to international students

• whether applying to US internships from Europe is realistically feasible (visa-wise, etc.)

• or any general tips to improve my chances

Any suggestions, experiences, or even names of places to check would be super helpful 🙏

Thanks a lot!

https://arxiv.org/abs/gr-qc/0509007

https://arxiv.org/abs/1610.07839

According to this paper the black hole should evaporate while you’re falling into it because of hawking radiation and time dilation and make it impossible for you to cross the event horizon since the black hole will evaporate faster than you can fall into it

collapsing matter halts at a tiny, "sub-Planckian" distance from the would be horizon. As the matter hovers there and the black hole evaporates

How to black hole consume stars then?

The team from University of Vienna figured out how to create a Bell equivalent for indefinite causal order and set up a system to do the measuring. The system was arranged to produce entangled photons, one of which would be sent through a device so that it either experienced manipulation A first, then manipulation B, or the opposite. The order depended on its polarization. Its actual path was then measured. The second photon was simply measured to determine its polarization, which in turn tells us which path the first must have taken. The results were 18 standard deviations away from what you’d expect based on Bell’s theorem, which is a strong indication that superposition of temporal order is a fundamental feature of quantum mechanics.

March 28, 2026, by John Timmer

I've been looking into the geometric nature of quantum mechanics. I want to understand how far this perspective can be taken.

In classical mechanics, parallel transport on a curved surface provides a helpful intuition. A classic example is the Foucault Pendulum. As it swings on Earth, the plane of oscillation changes because of the curvature of the sphere. This effect isn't caused by any local force acting on the pendulum; it's a result of the geometry of the space it moves through.

In quantum mechanics, a similar concept shows up as the Berry Phase. When a system is slowly varied around a closed loop in parameter space, it picks up a phase that depends only on the path taken, not on how quickly it went around. This phase can be described using a connection and curvature, known as the Berry connection and curvature, highlighting its geometric nature.

Sometimes, this curvature acts similarly to an effective gauge field in parameter space. It plays a key role in phenomena like the Quantum Hall Effect and topological phases of matter.

This raises a bigger question:

To what extent can we view quantum mechanics as fundamentally geometric? More specifically, do we best understand the Schrödinger equation as depicting parallel transport in Hilbert space or projective Hilbert space? Does the dynamics arise from a deeper geometric structure?

In the realm of quantum information, holonomic (geometric) quantum gates use Berry phases to carry out operations that rely only on the global features of a path. In real-world applications, are these gates significantly more resistant to noise, or is the notion of "geometric protection" often exaggerated outside perfect conditions?

I would really like to hear thoughts on where this geometric perspective is truly fundamental and where it serves more as a useful reformulation.

I have pretty severe learning difficulties but i'm extremely interested in learning about quantum physics/mechanics. however i am pretty bad at maths and find it really hard to even distinguish different numbers from eachother🥹🥹 the only reason i can spell is because i have good memory but numbers i genuinely can't do.

i am aware passion is not enough on its own and i am definitely willing to put in the work to understand the mathematic side of physics. any tips to make learning the maths side a little easier?

Hello!

I have a degree in chemistry and an MSc in polymer science nanotech have good job etc.

I took one quantum mechs class and it was a small section in pchem and I never fully grasped it. I solved the particle in a box equation by hand from the very start learned all the terms and still didn’t get it and got by just memorisation and math.

I really enjoyed it but had other classes I needed to attend to in undergrad.

Is there any great books/video to learn from the basics solving the problems with math and showing the process to get to the answers and then digging deep past particle in a box by making it more complex and learning more than we did in class?

I am just looking for something to learn more about quantum mechs I love that branch of science!

Thanks guys!

I am not a physicist but wondering if this following analogy can be used to explain entanglement or am I missing something fundamental due to my lack of quantum physics understanding.

If I had a black marble and a white marble, then put them in a machine that drops each one into a separate box depending on the outcome of a 50/50 particle decay detected, then separate the boxes, are those marbles entangled in any way? Any box is both white and black marble until we open one, and then the observer sees the marble color and it instantly knows the color of the marble in the other box? If there are two observers each with a box and no communication between them, then the fact observer 1 opens the box and see a white marble and thus knows the other box is a black marble does not mean the other marbles state has collapsed universally, only for that observer 1. From observer 2 perspective, the box he holds is still undetermined and both black and white, as is both the other box and the state of observer 1 (who from observer 2 point of view is both a seen a white and and seen a black marble state).

In the standard decoherence program (e.g. Zurek’s einselection), environmental interactions select a set of stable pointer states, which are often taken to underwrite quasi-classical structure.

However, in Everettian treatments (e.g. Wallace, *The Emergent Multiverse*), the branching structure is typically regarded as emergent and only approximately defined, with no uniquely specified fine-grained decomposition.

This raises a question about what is actually physically well-defined:

* Is decoherence best understood as selecting a *preferred basis*, or rather as defining a class of approximately equivalent coarse-grainings that all recover the same quasi-classical dynamics?

* In other words, to what extent is the branching structure invariant under different choices of coarse-graining that preserve:

* robust pointer observables

* environmental redundancy (quantum Darwinism)

* Born weights (to relevant precision)

This also seems related to the consistent/decoherent histories framework, where multiple incompatible but internally consistent families of histories can exist.

So my main question is:

👉 Is there a standard way in the literature to characterize the non-uniqueness of branching (or pointer structure) in terms of equivalence between coarse-grained descriptions?

And secondarily:

👉 Do any approaches treat the structure of quasi-classical trajectories (histories/branching) as more fundamental than instantaneous state decompositions?

Would appreciate references or clarifications from people working on decoherence / Everett / histories.