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Comment by ulam2 | Hacker News Reader

It wouldn't surprise me if they find that time too is discrete.

One experiment suggests that time is an emergent property of quantum entanglement. [1]

"A static, entangled state between a clock system and the rest of the universe is perceived as evolving by internal observers that test the correlations between the two subsystems. We implement this mechanism using an entangled state of the polarization of two photons, one of which is used as a clock to gauge the evolution of the second: an "internal" observer that becomes correlated with the clock photon sees the other system evolve, while an "external" observer that only observes global properties of the two photons can prove it is static."

[1] https://arxiv.org/abs/1310.4691

I'm working in a field where m=1 and hbar as a semiclassical scale is given as order of N (number of bosons or fermions), i.e. I don't do actual real physics...

But.... a thought experiment:

If a particle experiences time differently dependent to its speed in relative to a frame of reference, i.e. that time experienced is scaled according to its speed. Since speed is definitely not discrete, scaling it with time, even though time in itself is discrete as you claimed, still makes the time experienced for said particle to be continuous, no? Is this not a contradiction?

> Since speed is definitely not discrete,

Why is it definite that speed isn't discrete? Distance is discrete in plank length, and time is discrete in plank time.

Speed being distance over time, is one discrete unit over another. Isn't it by definition discrete?

At a macro level we may abstract over it's discreteness, but isn't it necessarily discrete if it's made up of discrete units?

Isn't that one of the things that fall out of relativity? that i could never be able to say if i'm the one at rest or if you are at rest?

I would be surprised if it is not, given the success of spacetime treating it as just another dimension.

What does that have to do with it being discrete?

"A Planck time unit is the time required for light to travel a distance of 1 Planck length in a vacuum"

https://en.wikipedia.org/wiki/Planck_units#Planck_time_and_l...

I don't know about any experimental proofs that either time or space is definitely discrete, or definitely continuous.

Both Planck length and Planck time are way too short for current experimental techniques.

Slightly unrelated but your comment led me to actually learning what the Planck length is, and the Wikipedia article for it has a great section on how to visualize it [1].

Figured others might enjoy this!

1. https://en.wikipedia.org/wiki/Planck_length#Visualisation

I am unconvinced that time is a real dimension, sure we can model movement/interactions of particles as a 4th dimension, but nothing in our understanding of physics requires it being a physical reality. IMO it's less space-time and more just space, it's just the rate of change slows with more mass.

There's at least one prominent physicist [1] who is working on showing that time is an emergent property of quantum physics rather than a fundamental one.

[1] https://www.preposterousuniverse.com/blog/2013/10/18/is-time...

If time isn't real, then when you say "rate of change", I ask "rate of change with respect to what?"

He's not claiming time isn't real, which the definition of "real" is difficult and troubling to define in itself.

I think he doesn't consider time to be 4th (or part of 3+1 or \R^3 \times \R ) dimension... or in heat equation language: that the domain is a not parabolic cylinder.

Check out "The Order of Time" by Carlo Rovelli! You might find it interesting. He explains (WAY more eloquently than I could) that time is most likely an interpretation of underlying physical law, rather than a fundamental part of it!

An illusion, if just a fancy one.

I have a hard time buying that, given the Lorentz Transform. Time is just as much a real dimension as space is. (Or are the dimensions of space also "an interpretation of underlying physical law"?)

I believe the Lorentz Transformation more than I believe eloquent arguments.

Correct https://simple.wikipedia.org/wiki/Planck_time

https://en.wikipedia.org/wiki/Planck_units#Planck_time_and_l...

Very much not correct. The Plank time is a rough scale at which we know that our theories do not work. That might mean that a new discrete theory appears there (which is frankly quite probable), but we most certainly do not know whether that is the case and it is just as possible that the underlying theory is continuous.

That doesn’t say it’s discrete. Only that there is a limit to precise measurement.

> That doesn’t say it’s discrete.

Possibly not discrete, as we envisage it - the small scale does not usually follow human intuition at all.

Though I would say that "there is a limit to measurement" of length and time entirely. Not just to accurate measurement.

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