A delightful quirk of relativity theory • Hacker News Board

Re: A delightful quirk of relativity theory

#191

Post by awb » Tue, Jan 18, 2022, 10:43 PM UTC

danbruc wrote: post #184

awb wrote: post #164

Earlier quoted context omitted.

From the article: > One of the key implications of the Heisenberg uncertainty principle is that it highlights the probabilistic nature of the universe

The article is wrong there. I can not provide a good source so I will just point you to this StackExchange question [1]. [1] https://physics.stackexchange.com/questions/63811/is-the-uni...

The article you linked to seems to say the universe is indeterministic:

> Indeterminism in Quantum Mechanics is given by another "evolution" that the wavefunction may experience: wavefunction collapse.

I’ll give you that it’s not a fully settled issue though:

> According to the Copenhagen interpretation of QM, it does mean the universe is non-deterministic. There are other interpretations of QM which allow for determinism though. Which is the correct interpretation (if any)? Currently, no one knows.

Here’s another article: https://www.theguardian.com/world/2021/apr/29/the-battle-for...

> Most physicists would not regard the events in our universe as deterministic, or clockwork if you insist; they are in fact considered probabilistic

> randomness (ie indeterminism) has been proven to be real by the National Institute for Science and Technology’s work on a randomness beacon.

Re: A delightful quirk of relativity theory

#192

Post by awb » Tue, Jan 18, 2022, 10:51 PM UTC

Diggsey wrote: post #187

awb wrote: post #61

Earlier quoted context omitted.

> Take the case of two people on Earth, one stationary and one travelling past in a car. They each have a different reference frame, and a different set events which make up their present. Now imagine that there is an alien government in Andromeda debating a possible invasion of Earth. In the reference frame of the person on Earth who is moving, the debate is still going on. But, in the reference frame of the station…

Actually no, the parent comment is correct: When you undergo an acceleration, it corresponds to a hyperbolic rotation of the spacetime around you (from your frame of reference). When the acceleration is small, the rotation is also very, very small; however at very large distances, even a small rotation can have a large effect. In the example, that "large effect" is the alien debate moving in time forward or backwards…

I don’t think the interpretation you describe is widely held. Check out the criticisms of the paradox:

https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argu...

Re: A delightful quirk of relativity theory

#193

Post by irthomasthomas » Tue, Jan 18, 2022, 10:57 PM UTC

awb wrote: post #61

irthomasthomas wrote: post #21

This would be more fun in a blog post. Twitter is a fucking horrible format for a complex analysis of special relativity. That aside, my favourite quirk from relativity is this: "The more you move in space, the less you move in time." Like relativity in general, it sounds simple enough, but it's consequences are profoundly shocking. (That is, if you take the time to understand it, and don't leap to the conclusion tha…

> Take the case of two people on Earth, one stationary and one travelling past in a car. They each have a different reference frame, and a different set events which make up their present. Now imagine that there is an alien government in Andromeda debating a possible invasion of Earth. In the reference frame of the person on Earth who is moving, the debate is still going on. But, in the reference frame of the station…

I once read a physicist on this, in a newspaper, who said "if I didn't believe in free-will, then I wouldn't bother getting up in the morning." I thought that was cute.

I think the term free-will has clouded the issue a bit. Forget about free-will. Scientists, instead, normally talk about Determinism vs indeterminism, or nondeterminism in CS. These have a more precise definition than that other thing. The universe is deterministic if the outcome of events are completely determined by previous events. Or the universe is nondeterministic if the outcome of events are not certain, but follow a random distribution.

As to the precise details of the speeds and distances etc, it is a thought experiment, swap the car for a rocket traveling past at 0.86c, if you like. Or instead of aliens deliberating all day, maybe they are more efficient than we are, and they make their decision instantly by flipping a digital coin powered by the perfectly nondeterministic quantum fluctuations of the vacuum.[0]

For earth rocket man, there is a moment before the quantum dice roll when the decision is indeterminable, it's completely random. But at the same moment in the stationary frame, the fleet is already on it's way. Unless you wish to argue that a stationary reference frame is somehow more valid than a moving one, you have to accept that in the moving frame, the event of the dice roll has still not occurred and should be random, but how can it be random, if it has already happened in the stationary frame. Hence the paradox.

I hope that cleared things up some?

  [0] The vacuum has an api: https://qrng.anu.edu.au/

Re: A delightful quirk of relativity theory

#194

Post by lofatdairy » Tue, Jan 18, 2022, 11:00 PM UTC

irthomasthomas wrote: post #21

This would be more fun in a blog post. Twitter is a fucking horrible format for a complex analysis of special relativity. That aside, my favourite quirk from relativity is this: "The more you move in space, the less you move in time." Like relativity in general, it sounds simple enough, but it's consequences are profoundly shocking. (That is, if you take the time to understand it, and don't leap to the conclusion tha…

I was thinking the same thing about this working better as a blog post. It's so frustrating for each sentence to be it's own line and to be interrupted every moment. The content itself is excellent, but I really wish there was a better platform for it than Twitter.

Re: A delightful quirk of relativity theory

#195

Post by Retric » Tue, Jan 18, 2022, 11:02 PM UTC

penteract wrote: post #156

Retric wrote: post #94

Earlier quoted context omitted.

That’s not how it works. But even beyond that you have the math wrong. Andromeda galaxy is only 2.5 million light-years from Earth. To get a 1 day difference in time dilation across 2.5 million years would take 1 in 913 million difference which someone traveling at even 100MPH (45m/s) isn’t going to see. (1 - (45^2/((3x10^8)^2))^0.5 simply isn’t enough.

I did the calculations from https://en.wikipedia.org/wiki/Special_relativity#Lorentz_tra... and it comes to ~0.25 years if the person in the car is moving towards Andromeda at 30m/s. γ is indeed very close to 1, so the humans' watches would still agree closely after 2.5 million years, but the humans would be a quarter of a light year apart at that point, so they need to disagree about something to get the same observ…

Direction of travel was undefined in the example, just a car on the earth. Suppose they where traveling 30m/s away from andromeda vs towards it suddenly the timing switches.

Chose a specific vector and both observers exactly agree with the events timing. Essentially the only thing they disagree about is what Andromeda’s location and velocity is. (Edited this several times, but it all simplified to choice of vectors.)

But even that’s assuming spaceships rather than “one stationary and one travelling past in a car” the guy in a car is never going to end up light years distant from the stationary person.

Re: A delightful quirk of relativity theory

#196

Post by fknorangesite » Tue, Jan 18, 2022, 11:02 PM UTC

deelly wrote: post #64

300bps wrote: post #22

Earlier quoted context omitted.

For what it’s worth, I clicked on the HN link on my iPhone and it opened in the Twitter app (that I never use) and it was extremely readable. Literally no problems reading it at all.

I can't say that buying IPhone with preinstalled Twitter app is very convenient way of reading twitter post.

I used my web browser and the powerful magic of...scrolling down.

I know shitting on twitter threads is a venerable HN tradition, but at this point I barely even know what people around here are complaining about anymore.

Re: A delightful quirk of relativity theory

#197

Post by fknorangesite » Tue, Jan 18, 2022, 11:04 PM UTC

d-d wrote: post #28

doctoboggan wrote: post #15

I like this, but it doesn't completely remove the "unintuitivenes" of relativity. For example, why do we measure in fractions of light speed? What happens when we are already rotated all the way to light speed and then fire another bullet? I guess there are other ways to reason about this, but it still doesn't feel intuitive to me.

Quoted post unavailable.

I look forward to your Nobel prize.

Re: A delightful quirk of relativity theory

#198

Post by afiori » Tue, Jan 18, 2022, 11:12 PM UTC

irthomasthomas wrote: post #193

awb wrote: post #61

Earlier quoted context omitted.

> Take the case of two people on Earth, one stationary and one travelling past in a car. They each have a different reference frame, and a different set events which make up their present. Now imagine that there is an alien government in Andromeda debating a possible invasion of Earth. In the reference frame of the person on Earth who is moving, the debate is still going on. But, in the reference frame of the station…

I once read a physicist on this, in a newspaper, who said "if I didn't believe in free-will, then I wouldn't bother getting up in the morning." I thought that was cute. I think the term free-will has clouded the issue a bit. Forget about free-will. Scientists, instead, normally talk about Determinism vs indeterminism, or nondeterminism in CS. These have a more precise definition than that other thing. The universe is…

> For earth rocket man, there is a moment before the quantum dice roll when the decision is indeterminable

Assuming earth rocket man's speed does not change, if he quickly alternates between 0c and 0.99c he could flip flop between "determinate" and "indeterminate".

Anyway for all system on earth the decision event would be in the relativistic present with no causality order

https://en.wikipedia.org/wiki/Minkowski_space#Causal_structu...

Part of the point of relativity is that t1<t2 is not a good test of "comes before"

Re: A delightful quirk of relativity theory

#199

Post by ncmncm » Tue, Jan 18, 2022, 11:20 PM UTC

vidarh wrote: post #104

awb wrote: post #75

Earlier quoted context omitted.

Exactly. Free will (or lack thereof) is an untestable theory, better categorized as a thought experiment or philosophical debate.

Free will isn't a theory before someone comes up with an actual definition. That is the problem. I keep asking people for a definition, and the outcomes are generally either "smoke and mirrors" (complexity hiding something not at all "free"), appeal to a "soul" which just leads to infinite regress, or failure to even try. Without someone willing to actually define what they mean, the discussion is moot - it makes no…

In any case, it has no place in any meaningful discussion of quantum mechanics.

Re: A delightful quirk of relativity theory

#200

Post by Diggsey » Tue, Jan 18, 2022, 11:49 PM UTC

awb wrote: post #192

Diggsey wrote: post #187

Earlier quoted context omitted.

Actually no, the parent comment is correct: When you undergo an acceleration, it corresponds to a hyperbolic rotation of the spacetime around you (from your frame of reference). When the acceleration is small, the rotation is also very, very small; however at very large distances, even a small rotation can have a large effect. In the example, that "large effect" is the alien debate moving in time forward or backwards…

I don’t think the interpretation you describe is widely held. Check out the criticisms of the paradox: https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argu...

That it has critics does not imply it's not widely held.

Regardless, that's just a dispute about terminology: sure we could say that we should only talk about events being "simultaneous" if they happen in exactly the same place - but in that case it's not a particularly useful term anymore. It's just an argument about how we apply words derived from newtonian intuitions to a relativistic context.

Mathematically, for a given reference frame we can draw a space-time diagram, and certain events will be above the horizontal line. We can draw the same space-time diagram for a different reference frame and have those same events be below the horizontal line.

Whether that horizontal line has any actual physical relevance doesn't really matter IMO, the maths says that events move in relation to the line when the frame of reference accelerates, and that's what the "paradox" is illustrating.

Having said that, if the "one way speed of light" exists (https://en.wikipedia.org/wiki/One-way_speed_of_light) and even if it's not the same in all directions, then this "plane of simultaneity" is definitely a physical thing, since you can send a beam of light to a target, measure how long it takes to return, and using the speed of light in that direction you can work out exactly when that beam hit the target in your local time - ie. you've found two events to be simultaneous from your frame of reference, that are not in the same location.