How fundamental is light to the physics of the universe?

[geordief]

Practically and theoretically.

 

Please correct me if I am wrong * but all "distances" (except in the special case of frames of reference that do not move with respect to each other or are subject to important gravitational fields **) can only be calculated (in a way that all observers can agree upon) when we take into account the nature of light and its propagation.

 

So is this a bedrock of our physics or is it incidental *** ?

 

Do distances between things actually exist at all unless we perform an experiment (which always requires light ) to measure them ?

 

(ridiculously) Would they "collapse" upon themselves without the presence of EM radiation to "justify their existence" ?

 

If EM radiation did not exist would some other phenomenon present itself to fulfill the role "vacated" by it?

 

 

 

* I may of course be making /compounding many errors(please take my embarrassment as a given)

 

** any other exceptional circumstances?

 

*** It is hard to find antonyms for fundamental

[swansont]

 

If EM radiation did not exist would some other phenomenon present itself to fulfill the role "vacated" by it?

 

Basically you are asking if the laws of physics were different, what they would be. It's impossible to say.

[Sensei]

Photons in core of star prevents total collapse of it.

 

[math]p^+ + p^+ \rightarrow D^+ + e^+ + V_e + 0.42 MeV[/math]

 

[math]e^+ + e^- \rightarrow \gamma + \gamma + 1.022 MeV[/math]

Annihilation of previously created positron with electron, creates 2.433 times more energy than the first reaction.

Absorbed, scattered photons accelerate particles.

 

[math]D^+ + p^+ \rightarrow ^3_2He + \gamma + 5.49 MeV[/math]

Deuterium and proton fuse together, producing photon with plentiful of energy.

5 times more than in the above annihilation.

Edited November 1, 2015 by Sensei

[geordief]

Basically you are asking if the laws of physics were different, what they would be. It's impossible to say.

Is it as stark as that?

After all have I not heard of scenarios close to what is termed the Big Bang where light had not formed ?

( or have I misheard /misunderstood and are they saying that light was simply trapped in the initial conditions that they think existed around then and so there was no time in the past yet theorised when light did not exist?)

[Strange]

You seem to be saying that because we can use light to measure distance, that distance must be defined by light. This seems just as illogical as the claim that because we can use change to measure time, time is defined by change. It is refreshing to see someone apply the same logic to space as to time, even if it is equally wrong.

 

It is, for example, possible to use the Einstein Field Equations to model a universe with no energy; there fore no light and no change. Time and space still exist in such a universe and, without looking it up, I think there is even expansion as in our universe.

[geordief]

Photons in core of star prevents total collapse of it.

 

[math]p^+ + p^+ \rightarrow D^+ + e^+ + V_e + 0.42 MeV[/math]

 

[math]e^+ + e^- \rightarrow \gamma + \gamma + 1.022 MeV[/math]

Annihilation of previously created positron with electron, created 2.433 times more energy than the first reaction.

Absorbed, scattered photons accelerate particles.

Does that loosely fit in with my idea that , without light the space between "things" would no longer exist ?(so light would be entirely fundamental to our understanding of "separateness" (of course I do not follow your maths or even your follow up)

[Sensei]

Does that loosely fit in with my idea that , without light the space between "things" would no longer exist ?

Not really.

 

(so light would be entirely fundamental to our understanding of "separateness" (of course I do not follow your maths or even your follow up)

I was referring to your what if "EM radiation did not exist".

 

1 MeV = 1000000 eV

1 eV = 1.602176565 * 10^-19 Joules.

So for example proton-Deuterium fusion reaction is releasing

5.49*10^6 * 1.602176565 * 10^-19 = 8.79594934185*10^-13 J

Edited November 1, 2015 by Sensei

[geordief]

You seem to be saying that because we can use light to measure distance, that distance must be defined by light. This seems just as illogical as the claim that because we can use change to measure time, time is defined by change. It is refreshing to see someone apply the same logic to space as to time, even if it is equally wrong.

 

It is, for example, possible to use the Einstein Field Equations to model a universe with no energy; there fore no light and no change. Time and space still exist in such a universe and, without looking it up, I think there is even expansion as in our universe.

Well(hopefully to clarify) my inclination is to "believe" that light is not fundamental but everything I have read leads me to feel that a universe without light (em radiation) is is an absurdity.

 

I would like this not to be the case (for the absurdity to hold water) but I know that what I like does not count.

[Strange]

Does that loosely fit in with my idea that , without light the space between "things" would no longer exist ?(so light would be entirely fundamental to our understanding of "separateness" (of course I do not follow your maths or even your follow up)

 

As another example, dark matter does not take part in electromagnetic interactions. But it is still spread out through space.

[geordief]

 

As another example, dark matter does not take part in electromagnetic interactions. But it is still spread out through space.

Thanks . Yes that is very easy for me to understand.

 

I look forward to learning about dark matter in my lifetime

[studiot]

 

Is it as stark as that?

 

Yes it is exactly as stark as that.

 

Even in non relativistic physics light plays a fundamental role because it is a result of a form wave motion, as predicted from the physical laws that govern it.

 

If these equations did not result in some form of wave motion would that mean that wave motion was excluded?

 

So water waves, sound waves and all the mechanics associated with wave motion would be lost to our universe.

Dislocation theory in solid mechanics would not work,

Quantum effects would fails so electronics, chemical reactions, ................

This list of physics that would not hold is mind bogglingly large if the wave equation were not true.

Edited November 1, 2015 by studiot

[swansont]

Is it as stark as that?

 

After all have I not heard of scenarios close to what is termed the Big Bang where light had not formed ?

 

( or have I misheard /misunderstood and are they saying that light was simply trapped in the initial conditions that they think existed around then and so there was no time in the past yet theorised when light did not exist?)

There was a period where the universe was opaque, because the mean free path for light was very short. But light existed.

[MigL]

Furthermore, light is not the only way to measure distances ( even so that all observers agree ).

I can easily use, say sonar, to calculate a distance, and any observer ( knowing some physics, would be able to make the appropriate transformations to get exactly the same results.

[geordief]

Furthermore, light is not the only way to measure distances ( even so that all observers agree ).

I can easily use, say sonar, to calculate a distance, and any observer ( knowing some physics, would be able to make the appropriate transformations to get exactly the same results.

Is light(em radiation) not the only method of measuring distance in a vacuum?

 

Do distances (in the space-time sense) have any meaning if not measured in a vacuum?.

Edited November 2, 2015 by geordief

[studiot]

 

Is light(em radiation) not the only method of measuring distance in a vacuum?

 

 

No.

[Strange]

Do distances (in the space-time sense) have any meaning if not measured in a vacuum?.

 

Yes.

 

Distance does not depend on us measuring them - otherwise the universe could not have evolved in our absence. And if you are using "measure" in a more abstract sense of something like it requires light to be able to travel for a distance to exist, then there was a time (the first 380,000 years) when the universe was much, much larger than the mean free path of light.

Edited November 2, 2015 by Strange

[Sorcerer]

You seem to be saying that because we can use light to measure distance, that distance must be defined by light. This seems just as illogical as the claim that because we can use change to measure time, time is defined by change. It is refreshing to see someone apply the same logic to space as to time, even if it is equally wrong.

 

It is, for example, possible to use the Einstein Field Equations to model a universe with no energy; there fore no light and no change. Time and space still exist in such a universe and, without looking it up, I think there is even expansion as in our universe.

If you can't measure something it doesn't exist.

 

I can't measure the flying spaghetti monster.

 

Logical positivists aren't illogical

[swansont]

!

Moderator Note

 

FYI - discussion of clocks has been split

http://www.scienceforums.net/topic/91969-distance-and-clocks-split-from-how-fundamental-is-light/

 

[robinpike]

Practically and theoretically.

 

Please correct me if I am wrong * but all "distances" (except in the special case of frames of reference that do not move with respect to each other or are subject to important gravitational fields **) can only be calculated (in a way that all observers can agree upon) when we take into account the nature of light and its propagation.

 

So is this a bedrock of our physics or is it incidental *** ?

 

Do distances between things actually exist at all unless we perform an experiment (which always requires light) to measure them?

 

(ridiculously) Would they "collapse" upon themselves without the presence of EM radiation to "justify their existence" ?

 

If EM radiation did not exist would some other phenomenon present itself to fulfill the role "vacated" by it?

 

 

You seem to be saying that because we can use light to measure distance, that distance must be defined by light. This seems just as illogical as the claim that because we can use change to measure time, time is defined by change. It is refreshing to see someone apply the same logic to space as to time, even if it is equally wrong.

 

It is, for example, possible to use the Einstein Field Equations to model a universe with no energy; there fore no light and no change. Time and space still exist in such a universe and, without looking it up, I think there is even expansion as in our universe.

 

Hi geordief, you appear to be asking two questions (I’ve marked the question in bold above)…

1) Do distances exist if it is not possible to perform an experiment to measure the distance?

2) Do distances always require light to measure them?

 

I will have a go at helping with the second part…

 

The following example may help to expand on Strange’s point as well (I’ve marked that in bold too).

 

Light takes just over a second to go from the earth to the moon. But this time is dependent on whether the observer is stationary in the earth and moon's frame of reference, or if the observer is moving relative to the earth and moon's frame of reference.

 

However, light itself always measures that distance to be the same value. Indeed, it appears to be the only ‘measuring rod’ we have that behaves the same in all reference frames.

 

For example, a pulse of light is sent from the earth to the moon...

 

At that moment an astronaut, near to that point on the earth, is flying in a direction away from the moon and also sends a pulse of light to the moon (from the back of his spaceship).

 

And another astronaut, again near to that point on the earth, is flying towards the moon and he too sends a pulse of light to the moon (from the front of his spaceship).

 

The three pulses of light are moving alongside each other on their way to the moon, albeit at slightly different frequencies but nonetheless at the same speed as each other.

 

All three observers agree that the three pulses were sent from the same point in space and at the same moment (for this discussion we will assume that this has been achieved). And all three observers agree that the three pulses of light reach the moon together. This means that the distance travelled by each pulse of light is the same distance. (Hopefully further discussion is not needed to agree to that conclusion.)

 

So is the light measuring that distance?

 

Well yes, that must be the case because it is a repeatable experiment that, regardless as to who creates the light, the light always agrees with the other pieces of light as to that distance. In this respect, there is no cunning conspiracy by the light to fool us as to what distance the light is measuring.

 

But that does not mean that the observers are able to agree as to what that distance is.

 

To see why, let’s look at how each observer sees the event.

 

The earth observer sees the moon neither moving closer nor moving further away from him during the experiment. So the distance between the earth and the moon remains constant during the experiment.

 

The observer on the spaceship that is moving away from the moon, says he is stationary, and it is the earth and the moon that are moving away from him during the experiment. So he considers the point in space that he released his pulse of light (and the other two pulses of light) is still right next to where he is in space. So the three pulses of light have a longer distance to travel to the moon, since the moon is moving away from that point in space during the time it takes the light to reach the moon.

 

On the other hand, the observer on the spaceship that is moving towards the moon says he is stationary, and it is the earth and the moon that are moving towards him during the experiment. So he considers the point in space that he released his pulse of light (and the other two pulses of light) is still right next to where he is in space. So the three pulses of light have a shorter distance to travel to the moon, since the moon is moving towards that point in space during the time it takes the light to reach the moon.

 

The three observers can calculate the distance each other measures as the distance of the earth to the moon, but it is not possible to say whose, if any, is the 'real distance'. The distance that the light has measured is a measurement that all particles of light agree on and therefore could be said to be the 'real distance', but we do not know what that value is, but as a value it does exist.

Edited November 4, 2015 by robinpike

[studiot]

 

2) Do distances always require light to measure them?

 

I will have a go at helping with the second part…

 

The following example may help to expand on Strange’s point as well (I’ve marked that in bold too).

 

Light takes just over a second to go from the earth to the moon. But this time is dependent on whether the observer is stationary in the earth and moon's frame of reference, or if the observer is moving relative to the earth and moon's frame of reference.

 

However, light itself always measures that distance to be the same value. Indeed, it appears to be the only ‘measuring rod’ we have that behaves the same in all reference frames.

 

For example, a pulse of light is sent from the earth to the moon...

 

At that moment an astronaut, near to that point on the earth, is flying in a direction away from the moon and also sends a pulse of light to the moon (from the back of his spaceship).

 

And another astronaut, again near to that point on the earth, is flying towards the moon and he too sends a pulse of light to the moon (from the front of his spaceship).

 

The three pulses of light are moving alongside each other on their way to the moon, albeit at slightly different frequencies but nonetheless at the same speed as each other.

 

All three observers agree that the three pulses were sent from the same point in space and at the same moment (for this discussion we will assume that this has been achieved). And all three observers agree that the three pulses of light reach the moon together. This means that the distance travelled by each pulse of light is the same distance. (Hopefully further discussion is not needed to agree to that conclusion.)

 

So is the light measuring that distance?

 

Well yes, that must be the case because it is a repeatable experiment that, regardless as to who creates the light, the light always agrees with the other pieces of light as to that distance. In this respect, there is no cunning conspiracy by the light to fool us as to what distance the light is measuring.

 

But that does not mean that the observers are able to agree as to what that distance is.

 

To see why, let’s look at how each observer sees the event.

 

The earth observer sees the moon neither moving closer nor moving further away from him during the experiment. So the distance between the earth and the moon remains constant during the experiment.

 

The observer on the spaceship that is moving away from the moon, says he is stationary, and it is the earth and the moon that are moving away from him during the experiment. So he considers the point in space that he released his pulse of light (and the other two pulses of light) is still right next to where he is in space. So the three pulses of light have a longer distance to travel to the moon, since the moon is moving away from that point in space during the time it takes the light to reach the moon.

 

On the other hand, the observer on the spaceship that is moving towards the moon says he is stationary, and it is the earth and the moon that are moving towards him during the experiment. So he considers the point in space that he released his pulse of light (and the other two pulses of light) is still right next to where he is in space. So the three pulses of light have a shorter distance to travel to the moon, since the moon is moving towards that point in space during the time it takes the light to reach the moon.

 

The three observers can calculate the distance each other measures as the distance of the earth to the moon, but it is not possible to say whose, if any, is the 'real distance'. The distance that the light has measured is a measurement that all particles of light agree on and therefore could be said to be the 'real distance', but we do not know what that value is, but as a value it does exist.

 

 

I don't see how this answers the question, which by the way was originally confined to measurement in a vacuum

 

 

post 14

Is light(em radiation) not the only method of measuring distance in a vacuum?

 

This was already answered in my post 15.

 

The floating micrometer i used to measure distance in my first post after leaving school could operate in a vacuum if required.

[robinpike]

Please correct me if I am wrong * but all "distances" (except in the special case of frames of reference that do not move with respect to each other or are subject to important gravitational fields **) can only be calculated (in a way that all observers can agree upon) when we take into account the nature of light and its propagation.

 

 

 

I don't see how this answers the question, which by the way was originally confined to measurement in a vacuum

 

 

This was already answered in my post 15.

 

The floating micrometer i used to measure distance in my first post after leaving school could operate in a vacuum if required.

 

 

Hi studiot, I don't understand - why mention a vacuum? Erm... but anyway a vacuum is what is between the earth and the moon!?.

 

I understood the opening question as being concerned with measuring distances when different frames of reference are used by the measurers. How would a floating micrometer be used to measure a distance between objects if the micrometer is in a different frame of reference to the objects?

[geordief]

@robinpike

thanks.I do need and did benefit from that resume but I kind of take it as a given that in your earth/moon /spacecraft scenario that the distance as agreed by the 3 observers in the 3 different frames of reference is a function of the speed of light (even if I still struggle with the actual mathematics involved).

 

My question (if I can still summarize it) was whether this is universally the case.

 

Strange has convinced me otherwise as he has pointed out that dark matter seems to exist in isolation from the effects of em radiation and so ,presumably we might need some other mechanism to define or measure distances in that context..

 

Hope that clarifies my question (which I think Strange did resolve for me in a way that was quite clear) and my understanding of the answers.

Edited November 4, 2015 by geordief

[studiot]

 

Hi studiot, I don't understand - why mention a vacuum? Erm... but anyway a vacuum is what is between the earth and the moon!?.

 

 

 

I didn't introduce the question.

I did, however, quote the post where the questiion was first posed

Did you not read it?

 

 

Is light(em radiation) not the only method of measuring distance in a vacuum?

 

I'm not sure about whether the vacuum is important to the poster or not.

It could also be that vacuum was mentioned because that is the only place light has velocity c.

 

As to measurement of distance.

 

Two basic methods are recognised.

They are respectively called line standards and end standards.

 

My micrometer is an end standard and requires simultaneous access to both ends of the mesureand.

No relativity corrections or reference frames are therefore required.

 

Note that large distances are not the only ones we measure, smaller ones are just as important.

 

So I am simply objecting to a blanket statement/question that there is only one means of measurement of length or distance.

Edited November 4, 2015 by studiot

[robinpike]

Thanks studiot, I did read all the posts but didn't pick up where some of the threads were going! Anyway, geordief understood them which is the main point - and for me, posting about the consequences of the constant speed of light is always good practice for clarity of thought!