Ask a physicist anything. (6)

[Naraoia]

Actually, it is. Or more precisely, it's defining yourself to be stationary, and working out the laws of physics from there. As there's no such thing as absolute velocity, to work out things like time dilation, you need a reference frame. Choosing one's own watch is an especially convenient way of doing this.

After all, there's no reason why we shouldn't define (0,0,0,0) in the most convenient way possible.

In the analogy, the motorcyclist was clocking himself, which is little use if he's not moving in his own version of time. If you're not going anywhere, your speed is zero even in fancy 4-dimensional hyperbolic spacetimes, right?

And... zero is not c.

I thought the whole point of that analogy was that every inertial observer could agree that everything has a spacetime speed of c? How does that work when any observer could find a reference frame in which they don't move at all?

Oops...

It seems I managed to re-confuse myself :o

I had to look in Wikipedia for this: it comes from the Latin celeritas, meaning 'swiftness', which is rather apt for the fastest thing in the universe.

Hey, that's pretty cool.

 

[AV1611VET]

Hey, that's pretty cool.

I'm the one that asked --

 

[sabercroft]

Ask a physicist anything: if there is an omniscient God who can observe the quantum state of all objects at any time, does the Copenhagen interpretation then mean that we have no free will?

 

[Wiccan_Child]

Ask a physicist anything: if there is an omniscient God who can observe the quantum state of all objects at any time, does the Copenhagen interpretation then mean that we have no free will?

No. An omniscient God can know in advance what decisions we will make, but they are still our decisions to make. That he knows what they are in advance doesn't determine what they're going to be; what determines which decision we make is still up to us, regardless of whether an omniscient being is aware of them.

 

[Wiccan_Child]

In the analogy, the motorcyclist was clocking himself, which is little use if he's not moving in his own version of time. If you're not going anywhere, your speed is zero even in fancy 4-dimensional hyperbolic spacetimes, right?

And... zero is not c.

You're not going anywhere in space, but you're travelling at c through time. By your own clock, your spatial velocity is zero, and your temporal velocity is c. By other people's clock, your spatial velocity is x and your temporal velocity is y, such that x[sup]2[/sup] + y[sup]2[/sup] = c[sup]2[/sup].

I thought the whole point of that analogy was that every inertial observer could agree that everything has a spacetime speed of c? How does that work when any observer could find a reference frame in which they don't move at all?

Oops...

It seems I managed to re-confuse myself :o

Any observer can find a reference frame in which she moves through space at zero, and that's the special reference frame where time is measured with respect to the observer herself. She also measures herself zipping through time at c. As she begins to accelerate and increase her spatial velocity, her temporal velocity decreases, such that her actual velocity across spacetime is still c.

So by her own clock, she starts off not moving in space, she defines herself as not moving in space, and so, by her watch, she is therefore moving at c through time.

Hey, that's pretty cool.

One day I will look like this man, and I will do science!

 

[sabercroft]

No. An omniscient God can know in advance what decisions we will make, but they are still our decisions to make. That he knows what they are in advance doesn't determine what they're going to be; what determines which decision we make is still up to us, regardless of whether an omniscient being is aware of them.

As far as my (ridiculously flimsy) understanding of the Copenhagen interpretation of quantum mechanics goes, the probability wave function of an object collapses once observed to a particular quantum state, and the probabilities of all other quantum states are all set to zero. How would an omniscient God avoid setting our quantum states to a fixed outcome by his act of observing us?

 

[Naraoia]

You're not going anywhere in space, but you're travelling at c through time. By your own clock, your spatial velocity is zero, and your temporal velocity is c. By other people's clock, your spatial velocity is x and your temporal velocity is y, such that x[sup]2[/sup] + y[sup]2[/sup] = c[sup]2[/sup].


Any observer can find a reference frame in which she moves through space at zero, and that's the special reference frame where time is measured with respect to the observer herself. She also measures herself zipping through time at c. As she begins to accelerate and increase her spatial velocity, her temporal velocity decreases, such that her actual velocity across spacetime is still c.

So by her own clock, she starts off not moving in space, she defines herself as not moving in space, and so, by her watch, she is therefore moving at c through time.

Oookay. All clear now, I think. Part of the problem was that when you said setting yourself as the origin was not nonsense, I thought that to mean that you can stay at the origin (i.e. stationary in your own space AND time). If you can't not move in time, then all my problems are solved

Thanks for bearing with me.

 

[Wiccan_Child]

Oookay. All clear now, I think. Part of the problem was that when you said setting yourself as the origin was not nonsense, I thought that to mean that you can stay at the origin (i.e. stationary in your own space AND time). If you can't not move in time, then all my problems are solved

Thanks for bearing with me.

No worries! It's hard to think in four dimensions, let alone describe it to someone else

 

[Jazer]

No. An omniscient God can know in advance what decisions we will make, but they are still our decisions to make.

He knows because He can look through time to see what decisions people make. He does not have to look to see if He does not want to.

 

[Wiccan_Child]

As far as my (ridiculously flimsy) understanding of the Copenhagen interpretation of quantum mechanics goes, the probability wave function of an object collapses once observed to a particular quantum state, and the probabilities of all other quantum states are all set to zero. How would an omniscient God avoid setting our quantum states to a fixed outcome by his act of observing us?

You're conflating the quantum mechanical and colloquial understandings of the word 'observer'. Humans ostensibly observe atoms all the time - by staring at my computer screen, I'm literally observing its atoms - but that doesn't make them quantum mechanical observers. Rather, an 'observer' is something that physically interacts with a system, forcing it to go from a superposition of states to a smaller superposition of states, or even to a single state.

So, the question is, is an omniscient being a quantum mechanical observer, interacting physically with every system? I would say no, it's not. Omniscience means you know everything, but that knowledge doesn't mean you have to be physically interacting with every particle in every moment in time. As an analogy, I can read about, and thus have knowledge of, a particular wavefunction collapse, but that knowledge doesn't mean I was the electron or photon that physically fiddle with that system to cause it to collapse. Similarly, I don't see why omniscience, which is simply knowledge, would cause every wavefunction in the universe to collapse. After all, which state would they collapse to?

 

[Wiccan_Child]

He knows because He can look through time to see what decisions people make. He does not have to look to see if He does not want to.

Surely his omniscience is an inherent trait, not a phenomenological experience?

 

[Naraoia]

No worries! It's hard to think in four dimensions, let alone describe it to someone else

It seems the problem wasn't the four dimensions per se, it was that one of them is Speshul

 

[hisgrace26]

According to conventional wisdom, Astronomers are able to identify the oldest star and then measuring the expansion rate back to the beginning in order to determine the age of the universe. Mr Physicist, what are some of the problems or assumptions that you think is wrong here. We want education from you, thanks.

 

[***Jamey***]

Why did you become a physicist?
What do you do with your degree?
Where did you study?

p.s what area of physics do you specialise in or what area to most enjoy?

 

[Wiccan_Child]

According to conventional wisdom, Astronomers are able to identify the oldest star and then measuring the expansion rate back to the beginning in order to determine the age of the universe. Mr Physicist, what are some of the problems or assumptions that you think is wrong here.

The biggest problem is that this isn't how astronomers determine the dynamics of the universe. There's no way of knowing whether the oldest known star is the oldest living star in the universe, nor how much older the dead stars are - after all, stars explode and die and their remnants gives rise to the next generation of stars. The oldest, most distant object we know of is the galaxy UDFj-39546284, 13.2 billion years old.

So, how do astronomers work out the age of the universe, and the rate of spacetime expansion? Well, the age of the universe is determined by looking at the redshift of stars to see their velocities relative to us. We see they are all accelerating away from us, and the further away they are, the faster they're moving. By reversing time, we see that all matter in the universe must have been condensed into a phenomenally dense singularity of matter and spacetime, approximately 13.5 billion years ago.

This is commonly called the beginning of the universe and the beginning of the Big Bang, but both of these are misconceptions: the Big Bang refers to the expansion of the universe from this singularity to its modern form. The 13.5 billion year long expansion is the Big Bang, not the singularity itself or the origin of the singularity (whatever that may have been). Moreover, the Big Bang theory tells us that the universe has been expanding for 13.5 billion years, so the universe is at least 13.5 billion years old - but for all we know, it could be much older. After all, we have no idea what that singularity was doing before it began to expand. Maybe it sat around for a few trillion years. Maybe it came about as the contraction of another universe.

We want education from you, thanks.

Alas, the UK government doesn't recognise me as an accredited university. I can give you my pearls of wisdom, but not a formal education.

 

[chris4243]

As far as my (ridiculously flimsy) understanding of the Copenhagen interpretation of quantum mechanics goes, the probability wave function of an object collapses once observed to a particular quantum state, and the probabilities of all other quantum states are all set to zero. How would an omniscient God avoid setting our quantum states to a fixed outcome by his act of observing us?

Even if someone were to measure every quantum event throughout spacetime, the accuracy of the measurement will be limited by the Uncertainty Principle. To be properly omniscient, the being would have to know the data without making a measurement as limited by physics and so would not collapse the wavefunction.

 

[sabercroft]

You're conflating the quantum mechanical and colloquial understandings of the word 'observer'. Humans ostensibly observe atoms all the time - by staring at my computer screen, I'm literally observing its atoms - but that doesn't make them quantum mechanical observers. Rather, an 'observer' is something that physically interacts with a system, forcing it to go from a superposition of states to a smaller superposition of states, or even to a single state.

From what I do understand, yes, you are interacting with your computer screen by observing it. Photons of light are being fired/bounced from the screen to your eyes, meaning that the Heisenberg principle applies, and hence so does the Copenhagen interpretation (assuming it's true).

Even if someone were to measure every quantum event throughout spacetime, the accuracy of the measurement will be limited by the Uncertainty Principle. To be properly omniscient, the being would have to know the data without making a measurement as limited by physics and so would not collapse the wavefunction.

If that were the case then an omniscient God would not ever observe the universe, and derive all of knowledge of it from his non-observing omniscience. Do you think that's what's happening?

 

[Jazer]

What is that stuff you see on PBS on TV about String Theory, Parallel Universe and so on. Is that really Physics? Do they really learn that stuff if you study Physics at the univeristy?

 

[sabercroft]

What is that stuff you see on PBS on TV about String Theory, Parallel Universe and so on. Is that really Physics? Do they really learn that stuff if you study Physics at the univeristy?

Short answer: yes.

The fact is that modern physics impinges on philosophy a lot more than I'm personally comfortable with, since it's not really intuitive to me at first glance what physics has to do with subjective perception and altered reality. but that's the way it goes, and part of the beauty as well.

 

[Wiccan_Child]

What is that stuff you see on PBS on TV about String Theory, Parallel Universe and so on. Is that really Physics? Do they really learn that stuff if you study Physics at the univeristy?

It's not mainstream stuff, so you don't get heavy lectures in it unless you're specialising in those fields. It's physics, but they're theories that don't have wide acceptance. String theory has not inconsiderable support, but it's woefully lacking in hard evidence. Parallel Universe is another tentative hypothesis and serves as a sort of explanation of weird quantum mechanical effects - but, again, there's no hard evidence behind it.