Time is an illusion, and these physicists say they know how it works [video]

What is this sentence referring to, if you don't mind me asking?

I know that some implementations of collision detection use volumes swept over time, but that's the only link I'm aware of.

Strong disagree on that part. High precision numerical analysis is always done with discrete time steps. It has been this way since the Finite Element method was invented. Anything non-linear (eg. collisions, large displacements, non-linear material behavior, non-linear boundary conditions, etc.) requires discrete steps to be calculated accurately.

The CCD research you linked below has very good uses cases (eg. robot control systems) but predicting the collision time and location of two simple solid shapes in a linear domain is absolutely not a "High precision simulation". There could be ways to use it to make a complex simulation more efficient (eg. dynamically adjust the steps to make them shorter right before a collision, densifying the mesh where you predict a collision, etc.) but at the end of the day your simulation is solved using discrete time steps.

Quantum mechanics describe the evolution of the state of system given the initial conditions and the hamiltonian describing it. That's exactly what the Schrodinger equation says and why it has a partial derivate of time. Usually in introductory treatments only the "time independent Schrödinger equation" is addressed, but that's a sub product on a common technique used to solve some family of differential equations.

In classical mechanics you can interpret time as another coordinate, but with so many privileges that it's a little far reached call it in the same way as space dimensions. Without the metric introduced in special relativity there is no way to justify put them in the same footing. For example, in classical mechanics you cannot rotate in a way that makes the time coordinate fall in a space coordinate, while you can do that in relativity, and it's called a boost.

The reason or physical principle from which this law of thermodynamics originats has always escaped me. Why can't there be a way to decrease entropy?

I think when a scientist says "free will does not exist", they mean that in a very particular sense - a theory of physics (or narrative) that includes free will has as much predictive power as one that doesn't. So from a scientific point of view it's parsimonious to exclude it.

As you rightly point out this has nothing to do with the subjective experience of free will, which is an aspect of consciousness that I expect most people are familiar with (scientists or no).

By the way, this is not directly related, but who are these scientists you speak of who claim to be unable to measure time? Time, as it is used in theories like classical mechanics or relativity, has been measurable with clocks for millenia. The measurements fulfil the role of time in those theories; the predictions they make have been tested to an extreme degree. This is the strongest evidence for time as an ontological primitive that science can produce!

(which is not inconsistent with the fact that it may behave differently outside those regimes)

I believe that human brains harness quantum uncertainty and chaos theory to combine stateful information (such as preference for chocolate icecream) and recursive processes to make 'free' decisions which aren't random and fundamentally unknowable to outsiders without destroying the human (similar to a quantum system which can't be measured in more than one dimension without destroying it).

Time is measured by clocks.

One idea is that the metric (geometry, gravity) could be a field just as matter is a field and people tried to apply the standard rules of (perturbative) QFT to gravity but failed. This is because the theory of gravity is unrenormalizable [0]. An interesting avenue in saving this line of thought is asymptotic safety where the idea is that gravity coupled to the standard model could actually be renormalizable in a certain sense [1].

In any case general relativity and quantum theory have so far been irreconcilable and there is now consensus on how to bridge the gap between those two theories. It is exactly because of this that most physicists will think of gravity and the other forces to be of a different nature.

When people say that gravity is not a force they mean that there is no known particle which acts as the intermediary of said force. For all the other forces we have a theory that explains the exertion of force via a particle.

Your first point states that "general relativity is just a model". Many physicists believe that it is more than a model but a true description of what the world really is like. I understand your urge to label theories as models, but ultimately the question is whether or not there is some level of ground truth that can be accessed in the form of mathematical theories.

[0] https://en.wikipedia.org/wiki/Renormalization#Renormalizabil... [1] https://en.wikipedia.org/wiki/Asymptotic_safety_in_quantum_g...

Ultimately physicists care not about the words but rather about the equations. Only equations allow them to predict the outcome of experiments (at least probabilistically). A physicists' ultimate dream is to find a single set of equations that predicts every experiment. Words are just used to describe what the equations do, to colleagues and laypeople alike.

So yes, our best model to describe gravitational phenomena is general relativity which describes these phenomena in terms of the bending of spacetime.

If you want to go back to a "force" description of general relativity you can do so as it's a free world. The description of all the other forces starts with a fixed background spacetime, so to do the same with gravity you'll have to contort the elegant covariant equations of general relativity into an ugly mess by expanding around such a background. And then your resulting "theory" will predict exactly the same experimental outcomes as general relativity. (Around flat space it is not so difficult, it produces Newton's law of gravity to the first approximation.)

All that just does not seem worthwhile. More importantly, the discovery of general relativity (and of the importance of general covariance) led to a paradigm shift in physics: all reasonable physicists now believe that any fundamental theory will not ultimately be a set of equations on a fixed background spacetime but rather a set of equations from which spacetime itself should somehow emerge. Very different, therefore, from quantum field theory which needs a fixed background and describes all the other forces in the standard model.

This was my attempt at addressing the "not like the other forces". For the quantum dynamics see the sibling comment https://news.ycombinator.com/item?id=41179419 . And thank you for teaching me about the "permanent" and "immanant" of a matrix, but I still have no idea what that comment is supposed to mean - sorry.

Yes, but it is the currently accepted model.

Whereas, for the other 3 fundamental interactions, the currently accepted model is in the form of a quantum field theory. For these 3 interactions, the models suggest force carrier particles, and these particles have been observed (electromagnetism: photon, weak interaction: bosons, strong interaction: gluons).

People have tried to construct a quantum field theory -like model for gravity, which would include gravitons as the force carrier particles. But all these attempts have failed to produce a consistent theory, and also nobody has ever measured or observed gravitons, so this is all hypothetical.

But if we get philosophical about this, gravity is not like other forces because the currently accepted model for gravity is not like the models for the other forces. Gravity is not a field with force carrier particles because we have not been successful at modelling it as such.

One can say that space time is also “field”, and mass creates distortions, so gravity is just like other forces interacting via field distortions, but iiuc, unlike nuclear forces, gravity does not have a quantized mathematical model.

Everything in physics a model. Quantum mechanics, electromagnetism, Newton's laws... Arguably everything in science is a model. But at some point, a model does so well at explaining observations that we take it to be a Truth. At one point, genes were 'just a model/theory,' but after the discovery of DNA, we took them to be real and physical. Seen in this way, there's nothing circular about either case.

It's uncontroversial to say that gravity is special because it is non-renormalizable. This means that one can't write down a quantum theory of gravity the same way that we write a quantum theory of electromagnetism, because it predicts infinite quantities that can't be removed from the theory. However string theory does unify quantum mechanics and gravity, and gravity is only "sort of special" in string theory (it does look in some ways like the other forces; the force carrier is the graviton). But exactly how spacetime emerges from string theory is still not fully understood.

1) Inertial mass and gravitational mass are exactly equal without an explanation why

or

2) The acceleration is just an effect of a curvature of space.

Imagine two bodies thrown exactly Northward on a sphere. Although their paths are parallel they would approach each other, as if there was an acceleration. This acceleration would be the same whatever their mass is.

Nevertheless I agree, that it is interesting to think about time as an series of changes or events instead of just as a dimension of 4D spacetime. I didn't watch the video yet (I prefer text), though, so I refrain from bothering you with may opinion on this topic.

Time is the expansion of the involved while simultaneously collapsing complexity permanently.

I think a universe with one particle is the same as one with none. As soon as you have two particles they can move toward each other, and spin can be observed.

A simple middle school formula really spells it out clearly.

If you’ve no distance or velocity, you don’t have time. If we pretend that velocity is heat (electron excitation, though technically a cloud), you can see why we can never get to absolute zero without discontinuity (black hole), so time increases or the universe gets larger. There is a reason being cold, slow, and dense are linked.

You can keep increasing the heat to “infinity”, making time slower and slower relatively.

I agree with your overall point though. All we really need to say, when we say something is an "illusion," is that the phenomenon in question is not as it intuitively appears. Our expectations were one thing, but because of x, y, and z, this can't be the case. We think of it one way, but it's actually another way.

It's tempting to draw this distinction upon metaphysical lines, e.g., we thought it was one way, but here's how it Really (TM) is. Again, this is totally unnecessary and adds nothing to our scientific understanding.

This article is one of my favorites on Wikipedia. The last bits are maybe the best for this question.

Somewhere in the very far future, time will lose all meaning. In that, the universe will be so big and so empty that if you ran time backwards it would not look meaningfully different than it running forwards. All matter that is left will have devolved to the basic particles and the universe will be in the maximally entropic state. So much so that the page correctly stops labeling time in any unit. It's not that if you could pop into this future and then look at your watch, that it would not tick. All laws of physics still apply, 'time' is passing still. It's that for the rest of the universe, 'time' has become something different. I'm not in the field anymore, but this maximal entropy and time's use are obviously linked.

A nanosecond or a gigayear are both essentially noise in the time spans that these events occur in. Time, very truly, is meaningless to our future dead universe.