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.

 

[PeterDonis]

When things happen compared to your clock depends on the relative motion and the difference in gravitational potential.

No, it depends on your choice of coordinates/simultaneity convention.

Whatever your frame of reference, the speed of light relative to you is always c.

This is not correct either since we are talking about events distant from the observer's worldline. The "speed of light relative to you" at events not on your worldline is also a matter of your choice of coordinates. It does not always have to be $c$. What will always be $c$ is the speed you measure for light rays passing you--but that means you are measuring their speed at events on your worldline, not at distant locations.

 

[Nugatory]

What would occur in such situation?

Both astronauts will reach their respective stars at the same time using a frame in which the two stars are at rest. They will spend the same amount of time in flight, cover the same distance,and be moving at the same speed (but in different directions).

Using other frames, they will arrive at different times, travel at different speeds, and cover different distances. However: in no frame will their speeds reach or exceed $c$ (unless you do the experiment with flashes of light, in which case both will be moving at speed $c$ in opposite directions in all frames); and in all frames the distance traveled by each will be equal to the speed times the time in flight.

The best way to understand this problem is to write down the coordinates of the three events using coordinates in which both stars are at rest:
Event 1: the astronauts start their respective journeys at the event $x=0$, $t=0$
Event 2: left-moving astronaut reaches his star at the event $x=-D$, $t=D/v$
Event 3: right-moving astronaut reaches his star at the event $x=D$, $t=D/v$.
Here $v$ is the speed of the astronauts in this frame and $D$ is the distance from the starting point to each star.

Then use the Lorentz transformations to find when and where these events happen in some other frame, such as one in one or the astronauts is at rest (you can't do this with light flashes because there is no frame on which a light flash is at rest)... and that will tell you what happens.

It would be a good exercise to verify that the results you get this way are consistent with the formulas for time dilation, length contraction, and velocity addition: that is, speed times time in flight is equal to the distance traveled for both astronauts.

 

[vanhees71]

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.

In SR to the contrary of you believe time doesn't flow, but spacetime just is a four-dimensional pseudo-Euclidean affine 4D manifold with a fundamental form of signature (1,3) (or if you use the east-coast convention (3,1)). As soon as you forget to think of time and space in the Newtonian way, SR becomes much simpler!

 

[Janus]

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?

No, What is happening is that the way light behaves discloses something about the nature of time.
You can change the train car and platform experiment so that the train and platform observers are at the same point when they see the flashes.
In this scenario, the the light from the two flashes meet at the platform observer's position at the same moment that the train observer is passing him. Thus both observers detect both flashes simultaneously. They also both measure the speed of light for the two flashes to have the same speed relative to themselves.
The platform observer, knowing that he was exactly halfway between the points from which the flashes were emitted both when he sees the light from them and when the flashes were emitted concludes that the flashes were emitted simultaneously.
The train observer knows that he is halfway between the sources of the flashes when he detects the light from them, But not when they were emitted. Prior to seeing the flashes, he was nearer to one source than he was to the other. If he measures the speed of the light from both flashes as being the same relative to himself, the only way he could detect the two flashes reaching him a the same time is if they left the sources at different times. Thus he concludes that the flashes were not emitted simultaneously.
Thus we have two observers at the exact same point, seeing the exact same light flashes but coming to different conclusions as to whether those flashes originated simultaneously or not.

Another thing to avoid is to attach to much importance to light itself. What is really important is the speed at which light travels, 'c'. It is the fact that this invariant speed exists that leads to Relativistic effects. Light is just convenient to use in these examples because it is something that we can measure that moves at that speed, and it makes the problem easy to visualize. These Relativistic effects would still take place even if we didn't use light, But showing this to be the case would be more involved.

 

[Akriel]

No, What is happening is that the way light behaves discloses something about the nature of time.
You can change the train car and platform experiment so that the train and platform observers are at the same point when they see the flashes.
In this scenario, the the light from the two flashes meet at the platform observer's position at the same moment that the train observer is passing him. Thus both observers detect both flashes simultaneously. They also both measure the speed of light for the two flashes to have the same speed relative to themselves.
The platform observer, knowing that he was exactly halfway between the points from which the flashes were emitted both when he sees the light from them and when the flashes were emitted concludes that the flashes were emitted simultaneously.
The train observer knows that he is halfway between the sources of the flashes when he detects the light from them, But not when they were emitted. Prior to seeing the flashes, he was nearer to one source than he was to the other. If he measures the speed of the light from both flashes as being the same relative to himself, the only way he could detect the two flashes reaching him a the same time is if they left the sources at different times. Thus he concludes that the flashes were not emitted simultaneously.
Thus we have two observers at the exact same point, seeing the exact same light flashes but coming to different conclusions as to whether those flashes originated simultaneously or not.

Another thing to avoid is to attach to much importance to light itself. What is really important is the speed at which light travels, 'c'. It is the fact that this invariant speed exists that leads to Relativistic effects. Light is just convenient to use in these examples because it is something that we can measure that moves at that speed, and it makes the problem easy to visualize. These Relativistic effects would still take place even if we didn't use light, But showing this to be the case would be more involved.

I still see the issue in the nature of the light (or to be more exact, in the nature of the c), rather than in the nature of time. The moments when each observer detects the flashes are different, but it seems to occur due to the nature of the c, rather than that the dimension of time is somehow altered trough the relative speed of the observer.

It seems so that when you make the calculation you get a different values for t in each of the observers with each flash, but i don't see why i should conclude that it is due to the nature of time itself, rather than it just being a result of the constant of c. we already now that the c is a constant, so wouldn't it be more complex to assume that in besides of that, also the spacetime is somehow reacting to the experiement, rather than just being the scene where the experiement takes place?