Quantum Gravity’s Time Problem • Hacker News Board

Re: Quantum Gravity’s Time Problem

#21

Post by danbruc » Sat, Dec 03, 2016, 7:10 PM UTC

RobertoG wrote: post #15

danbruc wrote: post #13

Earlier quoted context omitted.

I would argue that solipsism and a universe simulation are actually very close. You observe x and it might be A or B but you have no ground truth on what A or B looks like or how to distinguish them. In case of solipsism you experience a universe around you but there seems to be nothing you could use to decide whether you are experiencing an independent outside universe through your senses or whether your mind is jus…

"I would argue that solipsism and a universe simulation are actually very close." I agree with you that from the point of view of testability both are very similar, but, in my view, from the point of view of logic reasoning are almost the opposite. Solipsism is just bad reasoning or, at most, a semantic trick: I define my mind as everything there is out there, so, obviously, everything is in my mind. But we have scie…

I define my mind as everything there is out there, so, obviously, everything is in my mind. But we have science, precisely, to try to understand what it is out there, and "out there" means outside of our minds.

But that is the very point of solipsism - because you are experiencing the outside world only through your senses in your mind, you are unable to decide whether the outside world even exists. The two options are 1) your body with your brain and your mind exists in the usual universe and you can learn about the universe through your senses and 2) the usual universe does not exist, only your brain or your body or something along that line actually exists and the entire universe you think exists outside of your body is just something your mind made up, your mind is simulating the universe for you.

So this is not about redefining mind to mean all the universe, it is about emptying the universe until only your body or your brain or whatever remains, dreaming or hallucinating or simulating the thing we just got rid of all day long so that you can not tell the difference.

Re: Quantum Gravity’s Time Problem

#22

Post by andrepd » Sat, Dec 03, 2016, 7:11 PM UTC

M_Grey wrote: post #19

abakker wrote: post #16

I have a question as a non physics/math person: We assume that light speed is a constant, but we also know that speed is measured as distance/time. We know that in the presence of high gravity (or proximity to very high mass), or very high speed, we experience time dilation, and that perspective and location matters WRT actual time. So, how do we assume that light speed is constant? Does the distance moved change rel…

It's constant within a given frame of reference .

No, the speed of light is the same in ALL inertial frames of reference. This is the crucial postulate from which all special relativity comes as a consequence.

Re: Quantum Gravity’s Time Problem

#23

Post by akeruu » Sat, Dec 03, 2016, 7:14 PM UTC

abakker wrote: post #20

M_Grey wrote: post #19

Earlier quoted context omitted.

It's constant within a given frame of reference .

So, really it isn't a universal constant, then? I guess the makes sense. Sub question - if photons are light particles, moving at the speed of light, then do they actually experience time dilation themselves? i.e. from the frame of reference of the photon, is the local time for the photon ticking away in one frame, while the outside world appears to move much much faster? Or, relative to us is time on the photon movi…

Actually for a photon time is so dilated, it does not exists anymore. From its perspective everything "happens" at the same "time". There are quite a few answers for this :

https://www.quora.com/Does-a-photon-experience-time

https://www.reddit.com/r/askscience/comments/2slx5n/how_is_i...

https://www.reddit.com/r/askscience/comments/2t8qr1/do_light...

Re: Quantum Gravity’s Time Problem

#24

Post by andrepd » Sat, Dec 03, 2016, 7:18 PM UTC

abakker wrote: post #20

M_Grey wrote: post #19

Earlier quoted context omitted.

It's constant within a given frame of reference .

So, really it isn't a universal constant, then? I guess the makes sense. Sub question - if photons are light particles, moving at the speed of light, then do they actually experience time dilation themselves? i.e. from the frame of reference of the photon, is the local time for the photon ticking away in one frame, while the outside world appears to move much much faster? Or, relative to us is time on the photon movi…

It is a universal constant. See above.

The photon travels at the speed of light c. If you look at the expression for the time dilation, you will see that it tends to infinity as v approaches c. Remember that this relates the time experienced between two frames moving relative to each other at a constant velocity v. However the fact is that the expression fails when v=c, since you have a division by zero. This reflects the fact that it is impossible to do a Lorentz boost that relates two frames moving relative to each other at c. This in turn means concept of time dilation in an object moving at the speed of light, such as a photon, is meaningless. There is no such thing as the time experienced from the point of view of the photon.

I realise I probably wasn't very clear, but it's difficult to lay it all out in a small paragraph.

Re: Quantum Gravity’s Time Problem

#25

Post by spuz » Sat, Dec 03, 2016, 7:19 PM UTC

abakker wrote: post #20

M_Grey wrote: post #19

Earlier quoted context omitted.

It's constant within a given frame of reference .

So, really it isn't a universal constant, then? I guess the makes sense. Sub question - if photons are light particles, moving at the speed of light, then do they actually experience time dilation themselves? i.e. from the frame of reference of the photon, is the local time for the photon ticking away in one frame, while the outside world appears to move much much faster? Or, relative to us is time on the photon movi…

The speed of light does not change for each given reference frame, i.e. it is constant. However, space and time are observed differently depending on a given reference frame.

With regards to your second question, photons experience no time whatsoever. From the photon's perspective, they travel the universe from the point of emission to the point of absorption instantly.

If you want to understand this, I highly suggest watching this video explaining reference frames: https://www.youtube.com/watch?v=aRDOqiqBUQY

And this playlist of the excellent PBS Space Time series: https://www.youtube.com/watch?v=YycAzdtUIko&list=PLsPUh22kYm...

Re: Quantum Gravity’s Time Problem

#26

Post by mark_edward » Sat, Dec 03, 2016, 7:21 PM UTC

M_Grey wrote: post #19

abakker wrote: post #16

I have a question as a non physics/math person: We assume that light speed is a constant, but we also know that speed is measured as distance/time. We know that in the presence of high gravity (or proximity to very high mass), or very high speed, we experience time dilation, and that perspective and location matters WRT actual time. So, how do we assume that light speed is constant? Does the distance moved change rel…

It's constant within a given frame of reference .

This is exactly wrong. Light speed is constant in all frames

Re: Quantum Gravity’s Time Problem

#27

Post by danbruc » Sat, Dec 03, 2016, 7:36 PM UTC

karmakaze wrote: post #17

RobertoG wrote: post #10

Earlier quoted context omitted.

I don't think universe as a simulation is epistemologically equivalent to solipsism or this "Last Thursdayism" thing (love the name, by the way). Solipsism is meaningless. What it means to say that there is nothing outside of your mind? Surely you can't predict or control what is going to happen to your experience, so how can you say that everything is "inside" your mind? what that even means? And if you are a solips…

"Experimental" is a key here which distinguishes the simulation hypothesis. There are two things we can do: 1. attempt to create a simulated universe 2. make predictions about our universe based on the hypotheis I would not rule either of these out as futile.

To some extend we are already simulating the universe, from the smallest scales like lattice QCD simulations because we can not analytically solve the problems to the largest scales to test or hypothesizes about the history of the universe and everything in between. But how would this help to shed any light on the problem of a [non-]simulated universe?

And as mentioned before, making predictions seems even more problematic to me. Where would you even start predicting things? Nobody has ever seen a universe and a universe simulator next to each other and could therefore say we should watch out for this or that to tell the two apart. You probably don't even have good reasons to assume that a simulation would be anything like the outside universe, our universe could be the equivalent of a Tetris level running on a phone somewhere in the real universe.

Re: Quantum Gravity’s Time Problem

#28

Post by jperras » Sat, Dec 03, 2016, 7:42 PM UTC

abakker wrote: post #16

I have a question as a non physics/math person: We assume that light speed is a constant, but we also know that speed is measured as distance/time. We know that in the presence of high gravity (or proximity to very high mass), or very high speed, we experience time dilation, and that perspective and location matters WRT actual time. So, how do we assume that light speed is constant? Does the distance moved change rel…

> We assume that light speed is a constant, but we also know that speed is measured as distance/time.

Correct. However, the crucial part that is missing from your thought process is that: 1) light can only travel in a "straight" line, and 2) "straight" lines are defined by the spacetime metric.

As a lower-dimensional analogy, think of what a "straight" line means on the surface of a sphere[1]. The shortest path between two points is, by definition, a "straight" line, but if the space itself that the path is embedded in is curved, then when viewed from a higher dimension (i.e. on the surface of the sphere it's straight for you, but if you were in space) it would appear curved.

The analogy fails when you attempt to think about it in higher dimensions, since humans have a very difficult time perceiving anything greater than the standard 3+1 spatial & time dimensions. But the math still holds.

Light will always travel in a straight line, it just so happens that gravity redefines what "straight" actually means[2].

1. http://mathworld.wolfram.com/GreatCircle.html 2. Explanation simplified for clarity.

Re: Quantum Gravity’s Time Problem

#29

Post by highd » Sat, Dec 03, 2016, 7:46 PM UTC

jperras wrote: post #28

abakker wrote: post #16

I have a question as a non physics/math person: We assume that light speed is a constant, but we also know that speed is measured as distance/time. We know that in the presence of high gravity (or proximity to very high mass), or very high speed, we experience time dilation, and that perspective and location matters WRT actual time. So, how do we assume that light speed is constant? Does the distance moved change rel…

> We assume that light speed is a constant, but we also know that speed is measured as distance/time. Correct. However, the crucial part that is missing from your thought process is that: 1) light can only travel in a "straight" line, and 2) "straight" lines are defined by the spacetime metric. As a lower-dimensional analogy, think of what a "straight" line means on the surface of a sphere[1]. The shortest path betwe…

Said another way - light follows the shortest path between two points. In flat spacetime, that is a line. In curved spacetime, it may be some other path - like a path traced by an ant walking on a sphere. These paths are called geodesics:

https://en.wikipedia.org/wiki/Geodesics_in_general_relativit...

Re: Quantum Gravity’s Time Problem

#30

Post by kurthr » Sat, Dec 03, 2016, 7:47 PM UTC

andrepd wrote: post #22

M_Grey wrote: post #19

Earlier quoted context omitted.

It's constant within a given frame of reference .

No, the speed of light is the same in ALL inertial frames of reference. This is the crucial postulate from which all special relativity comes as a consequence.

Although I think your comment is clearer... I think he's saying "within any given frame of reference", which is the same.

As you say:

The speed of light measured within ALL inertial (non-accelerated eg by GRAVITY!) frames of reference.