Special relativity and the flow of time?

[Akriel]

Pardon me if this has been asked before, but I'm confused with the implication that SR makes, that there is no objective moment of present. Do the science still believe that the time flows (i don't mean the arrow of time, but the phenomena of flowing/passing, eq. flowing of a river)? I mean, how can it flow if the spacetime itself is not bound to any present moment? In comparison, a river only flows if you observe it from a present moment.

 

[jbriggs444]

Pardon me if this has been asked before, but I'm confused with the implication that SR makes, that there is no objective moment of present. Do the science still believe that the time flows (i don't mean the arrow of time, but the phenomena of flowing/passing, eq. flowing of a river)? I mean, how can it flow if the spacetime itself is not bound to any present moment? In comparison, a river only flows if you observe it from a present moment.

The notion of time "flowing" has never been scientific. It is, at best, a metaphor.

 

[PeterDonis]

i'm confused with the implication that SR makes, that there is no objective moment of present

That's not quite what SR says. For you, traveling along your worldline in spacetime, there is an "objective moment of present"--it's the one you're experiencing right now. What SR says is that there is no unique way for you to say what is happening "now" at places distant from you--for example, what is happening on the Sun "now", at the same "moment" you are experiencing now. This is called "relativity of simultaneity".

 

[Dale]

Do the science still believe that the time flows (i don't mean the arrow of time, but the phenomena of flowing/passing, eq. flowing of a river)?

I don't know of anything in classical physics that corresponds to this. As far as I know there is no aspect of the math of classical physics that relates time and a river flow.

A river flow is usually modeled as an incompressible fluid flow, sometimes including viscous shear forces at the surface of the river bed. Time is usually modeled as a coordinate or a parameter.

 

[Akriel]

That's not quite what SR says. For you, traveling along your worldline in spacetime, there is an "objective moment of present"--it's the one you're experiencing right now. What SR says is that there is no unique way for you to say what is happening "now" at places distant from you--for example, what is happening on the Sun "now", at the same "moment" you are experiencing now. This is called "relativity of simultaneity".

Is this lack of unique way of saying what happens in distant places related to the distance or to the difference in the volume of mass/gravity between me and the distant location?

about the train car and platform-thought experiement mentioned on wikipedia-article about the RoS: if the action of light actually changes by changing the point of observation, then isn't this actually an issue in nature of light rather than in nature of time?

 

[Akriel]

I don't know of anything in classical physics that corresponds to this. As far as I know there is no aspect of the math of classical physics that relates time and a river flow.

A river flow is usually modeled as an incompressible fluid flow, sometimes including viscous shear forces at the surface of the river bed. Time is usually modeled as a coordinate or a parameter.

The phenomena that changed yesterday to this present moment, we both know what i ment.

 

[David Lewis]

When things happen compared to your clock depends on the relative motion and the difference in gravitational potential. Distance away isn't relevant.

 

[Akriel]

When things happen compared to your clock depends on the relative motion and the difference in gravitational potential. Distance away isn't relevant.

Is the relative speed factor even when talking about photons etc. that has no mass?

 

[David Lewis]

No. Whatever your frame of reference, the speed of light relative to you is always c.

 

[Akriel]

No. Whatever your frame of reference, the speed of light relative to you is always c.

Isn't this about the nature of the light/nature of the cosmos bound to the restriction set by the speed of light rather than the nature of the time?

 

[Dale]

The phenomena that changed yesterday to this present moment

What phenomenon is that?

There is nothing in the math for that either. Again, in the math of classical physics time is either a coordinate or a parameter. No single value is given preferential status either way.

 

[David Lewis]

It has to do with the nature of time, and also the nature of space. They work together.

 

[Akriel]

No. Whatever your frame of reference, the speed of light relative to you is always c.

Also what woke my curiosity is a thought experiement: If we pick two stars that are in relatively fixed positions in relation to each other, take two astronauts right in the middle in between them, and then send one astronaut towards the star 1, and other towards the star 2, both at 99% the speed of light. Both astronauts are supposed to reach their star at the same time x, but yet their relative speed cannot exceed the speed of light. What would occur in such situation?

 

[Akriel]

What phenomenon is that?

There is nothing in the math for that either. Again, in the math of classical physics time is either a coordinate or a parameter. No single value is given preferential status either way.

that something we both call time in our every day experience. that was my original question: does our personal experience of time passing have any position in physics? is it still considered as physical phenomenon, or is it thought just as some aberration of human brain? it obivously exist on some level, cause i remember what i did 5 minutes ago and am pretty sure that i still exist still after 5 minutes.

 

[phinds]

Also what woke my curiosity is a thought experiement: If we pick two stars that are in relatively fixed positions in relation to each other, take two astronauts right in the middle in between them, and then send one astronaut towards the star 1, and other towards the star 2, both at 99% the speed of light. Both astronauts are supposed to reach their star at the same time x, but yet their relative speed cannot exceed the speed of light. What would occur in such situation?

You need to Google "relativistic speed addition" since you see to thing that 99%c + 99%c > c which it is not (at least not in the way you think)

 

[phinds]

that something we both call time in our every day experience. that was my original question: does our personal experience of time passing have any position in physics? is it still considered as physical phenomenon, or is it thought just as some aberration of human brain? it obivously exist on some level, cause i remember what i did 5 minutes ago and am pretty sure that i still exist still after 5 minutes.

Google "proper time" and "coordinate time"

 

[Akriel]

You need to Google "relativistic speed addition" since you see to thing that 99%c + 99%c > c which it is not (at least not in the way you think)

Then replace the astronauts with photons moving at the speed of light.

 

[phinds]

Then replace the astronauts with photons moving at the speed of light.

You can't. Photons have no inertial frame of reference and thus can't be treated in the way you want.

 

[Akriel]

You can't. Photons have no inertial frame of reference and thus can't be treated in the way you want.

that's funny, i think i could just place a light bulb, or a star between the star 1 and 2, and make the thought experiement. how exactly will this thought experiement fall apart?

 

[Dale]

that something we both call time in our every day experience. that was my original question: does our personal experience of time passing have any position in physics?

Our actual subjective experience of time is far too complicated to be described by physics. There is no "fun" which we can input into a physics equation in order to make time "fly", nor any of our other subjective experiences regarding time.

In particular, our persistent visceral sensation of the present moment being something more real than any other moment is not part of physics.

 

[Akriel]

Our actual subjective experience of time is far too complicated to be described by physics. There is no "fun" which we can input into a physics equation in order to make time "fly", nor any of our other subjective experiences regarding time.

In particular, our persistent visceral sensation of the present moment being something more real than any other moment is not part of physics.

I understand the problem, though i wouldn't forget that no physicist ever has experienced this universe beyond the present-centric image created by our brain. Anyways that answers my question and I'm happy with it.

 

[phinds]

that's funny, i think i could just place a light bulb, or a star between the star 1 and 2, and make the thought experiement. how exactly will this thought experiement fall apart?

You have not proposed any specific "thought experiment" with this scenario but what's clear is that you do not understand special relativity and you seem to be unwilling to pursue the reading suggestions that I have given you but instead want to continue to propose scenarios based on your lack of knowledge.

 

[Akriel]

You have not proposed any specific "thought experiment" with this scenario but what's clear is that you do not understand special relativity and you seem to be unwilling to pursue the reading suggestions that I have given you but instead want to continue to propose scenarios based on your lack of knowledge.

for sure i don't understand it, and that's why I'm here. that's why i proposed this thought experiement, or idea, or however you want to call it, of sending two photons departing from each other towards the stars at distance x from the source. simple experiement, totally doable, i asked what would happen. you see the problem I'm seeing there? relative speed of two photons cannot exceed the speed of light, but yet they are supposed to reach their destination with the speed of light. the best i can come up with is that when measured from the photon 1 the photon 2 seems not to move from the source. i took a glance at the wikipedia-articles of the topics what you suggested but they didn't seem to answer to my question.

 

[phinds]

relative speed of two photons cannot exceed the speed of light

And I gave you an answer for that, which is that you can't DO that calculation because it requires an inertial frame of reference for photons and such does not exist. That is, it is not meaningful to say "what is the speed of this thing, relative to a photon (even if the thing is another photon)". There IS NO "relative to a photon" because a photon does not have an inertial frame of reference.

 

[Dale]

when measured from the photon 1

This is undefined. As phinds mentioned there is no inertial reference frame of a photon

 

[Akriel]

copy that.

 

[David Lewis]

Both astronauts are supposed to reach their star at the same time x, but yet their relative speed cannot exceed the speed of light. What would occur in such situation?

The distance to the stars is less for the astronauts than it is for those at rest with respect to the stars.

 

[David Lewis]

Then replace the astronauts with photons moving at the speed of light.

The relative speed between photons is 2c when they are moving in opposite directions.

 

[phinds]

The relative speed between photons is 2c when they are moving in opposite directions.

Yes but that's a recession velocity, not proper motion of one relative to the other which I think is what he was asking about.

 

[PeterDonis]

Is this lack of unique way of saying what happens in distant places related to the distance or to the difference in the volume of mass/gravity between me and the distant location?

No. It's related to your choice of coordinates--there are an infinite number of ways to choose coordinates, and they give different answers to the question of what is happening "now" at places distant from you.

if the action of light actually changes by changing the point of observation

It doesn't. Wikipedia is not a good source for things like this. You need a good SR textbook like Taylor & Wheeler.