[neilep (OP)]

Dear  Lightologists !


As a sheepy I of course can run really fast, I literally just completed the London marathon..............2004 !


That's fast !!!


Light was invented in 1845 by Al Luminatti when he went truffle foraging with his pet equine, Sauzzaje the 3rd of Hamlet House. This is true because I said so.


Light travels well fast that at it's top speed it does not perceive time.....weird eh ?   however, my kwescun is....




If you were to slow light down, would the photons then experience time ?






whajafink ?






hugs and shmishes




mwah mwah mwah


Neil xxxxx







The speed of light, so fast and bright,
It travels at an insane height,
Blink and you'll miss it, that's for sure,
Light's the universe's speedy tour.

[Halc]

Light travels well fast that at it's top speed it does not perceive time.....weird eh ?

Well, besides the point that subatomic things don't have 'experience', but that's probably not what's being asked.

If you were to slow light down, would the photons then experience time?

Only a time-like worldline defines a meaningful frame in which it has a temporal length. That means that a clock following that time-like line will log a certain amount of time. This is independent of any chosen frame.
This cannot happen for a space-like wordline nor a light-like worldline, which is what is followed by all things lacking proper mass.

All that jarjgon aside, if you slowed a photon down, it wouldn't be a photon, but the path taken by this slowed-down not-photon thing would indeed have a frame-independent temporal length, so in that sense, yes, it would experience time.

For instance, while light slows down in glass or some other medium with a refractive index, a photon does not. A photon at best can be said to be absorbed by the glass, briefly exciting some atom which in very short order emits a new photon in the same direction as the old one. It isn't the same photon, and it is a mistake to give a photon classic properties like that when it is a quantum thing, not a classical thing.
But bottom line is that you can have say a pipe in a square U shape with fast water running through it. You shine a light pulse from the side and it goes through the straight bottom part of the pipe to the other side. If the water flows fast enough, the light pulse will slow effectively to a stop relative to the pipe and will 'experience' as much time as does the pipe.

[Eternal Student]

Hi.

   That's quite a good answer from @Halc with very few words used.

    It's better to focus on making some sense of how light might be considered as being slowed down,  rather than to keep it at light speed and just ask if it experiences time.   But since Halc has already done one I might as well take a line or two to talk about the other.

    Suppose you don't consider time as something on a space-time diagram like that described by Halc or anything to do with an eigenvector of spacetime that has a negative eigenvalue for the metric in General relativity.  Instead just consider time as a record of change (possibly a cause or consequence of change).   Where there is change, time can be meaningfully identified and in our ordinary understanding of time we might say that change requires time to pass.
     Now in an expanding universe, light will be redshifted.  So its frequency is changing in any sensible co-ordinate system you care to use.   Just to be clear, the exact frequency it has is a frame dependant thing and we're not too interested in that.   However, the fact that the frequency is changing remains true in every co-ordinate system that you could (sensibly) choose.  So the changing of frequency is a frame independent characteristic that the photon has.   It's a short step to suggest that a photon experiences time because it does have a property that can change.   We can always use that to construct a notion of time (the photon was redder, so that will be defined as "later" in the lifetime of the photon etc.)

   I'm not going to fill in the details, it's just an alternative approach.  Time is complicated and modelled in various ways.   In an expanding universe you've got an easy option available for how you might get time to emerge for a photon. 

Best Wishes.

[alancalverd]

Time is what separates sequential events*. A photon cannot experience sequence as it has no memory.


*an original statement, but apparently my predecessor Prof A Einstein said "time is what prevents everything from happening at once", an arguably anthroponormative definition but having the same import.

[Janus]

The key here is that it is "c" which is the important thing, not the light itself.  As far as we know, light in a vacuum travels at c.  But even if this turned out not to be the case, it would not change the importance of the speed c as an invariant speed. It would just mean that the photon is not the truly "massless" particle we now consider it to be. 

[Zer0]

@OP

May i ask a follow-up Question?

[geordief]

The key here is that it is "c" which is the important thing, not the light itself.  As far as we know, light in a vacuum travels at c.  But even if this turned out not to be the case, it would not change the importance of the speed c as an invariant speed. It would just mean that the photon is not the truly "massless" particle we now consider it to be. 

Can I ask you  what are  some of the derivations that show  that c is an invariant speed and which have no connection to the speed of light?

I feel I should know this  but perhaps I don't  or perhaps you could jog my memory.

Is it perhaps based on the lorentz transformation between moving frames?Does the invariant speed c follow from that?

[hamdani yusuf]

Can I ask you  what are  some of the derivations that show  that c is an invariant speed and which have no connection to the speed of light?

In Special Theory of Relativity, invariance speed of light is taken as a postulate, hence not derived from some more fundamental axioms.

[evan_au]

in an expanding universe, light will be redshifted...a photon experiences time because it does have a property that can change

That is a fairly slow rate of change, not easily observable within a lab.

Another property that can change more rapidly with time is the amplitude of the electric and magnetic fields of a coherent light beam. In the absence of any measurable expansion of the local universe in the time that light takes to cross the lab, a light beam in a vacuum will keep the same amplitude, frequency and direction (ok - you need to ignore Earth's gravity too!). Since these are the main characteristics of light, the light itself is not changed by the passage of time.

Similarly, if you slow down light by putting it through glass or water, any light that managed to avoid absorption, scattering or reflection will be effectively identical to what went in, so they are unchanged by the "experience". Since there is no change, there is no experience of time (for the light itself).

[geordief]

Can I ask you  what are  some of the derivations that show  that c is an invariant speed and which have no connection to the speed of light?

In Special Theory of Relativity, invariance speed of light is taken as a postulate, hence not derived from some more fundamental axioms.

That was not what I was asking -unless your implication is that the invariant speed c  ,(contrary to what  @Janus   said) is completely linked to the speed of light in a vacuum .

[neilep (OP)]

@OP

May i ask a follow-up Question?

yes yes , of course 

[neilep (OP)]

phew !..thank ewe for all the well informed and articulate answers.

Ok, can I then ask a follow up ?

Hypothetically,  if ewe or I could travel at the speed of light armed with our sentience (yes it's impossible to travel at c).....and we travelled from the Sun to Earth....at the speed of c it would take 8 minutes yes ?, however, what would our perception of the time be ? instantaneous ?

[Halc]

Hypothetically,  if ewe or I could travel at the speed of light armed with our sentience (yes it's impossible to travel at c).....and we travelled from the Sun to Earth....at the speed of c it would take 8 minutes yes ?, however, what would our perception of the time be ? instantaneous ?

Let's say we wanted to visit Andromeda (2.5 million LY away). If we accelerated enough in that direction, the trip would take not 2.5 million years of our time, but as little time as you want, like say 47^3/4 seconds. It can be even less time if you accelerate even more.
Thing is, even in that frame, you will be stationary and light will be going by you at c still, and Andromeda will be coming at you (from only 47^3/4 light-seconds away) at pretty much light speed.

So since the trip can be made in as little subjective time as you like, our perception of time approaches zero, which is pretty much a 'yes' to your question.

PS and @chris
I don't see your sheepy insideout signature photo anymore. I see it, but it's blank.
Also, the special characters appear in my edit window but turn into question marks when I save my post. Things are not all working anymore.

[Eternal Student]

Hi.

@geordief   asked about  the speed c.
LaTex isn't working so we can't have mathematical symbols here in the post,  sorry.

    Basically, yes starting from the Lorentz transformation you would quickly obtain the relativistic velocity addition formula - see  https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity

   That will show you that a velocity with magnitude c (which is just what appears in the Lorentz transformation and not necessarily the speed of light) is mapped to another velocity with the same magnitude in any other inertial reference frame.

    The entire discussion was about assuming light may not travel at the speed c.  So, that's why @hamdani yusuf  said what they said (I would think).   In most textbook developments of special relativity it is common to start from an assumption about light and the invariance of its speed.  In this situation, the whole point is that you don't - we assume light has some other speed.

    You could still obtain the Lorentz transformation even if nature had been very unkind to physicists and never given them a massless particle that could be detected.   We were lucky, we had light, it did exhibit properties that strongly pointed us to the development of special relativity.   Assume light wasn't available or did not behave like that.  Provided special relativity is still a rule in nature, then there would have been signposts to it.
   See https://en.wikipedia.org/wiki/Experimental_testing_of_time_dilation   which describes how muons can be produced in earths upper atmosphere when cosmic rays come in.    We use this as a test of special relativity, specifically we suspect that muons moving fast relative to the lab frame would live long enough to reach the surface of the earth before decaying.   The main point is that the effects of relativity like this would still be there, in nature, and eventually we would notice:    Someone would have studied the half-life and decay of particles and they would have noticed that fast moving particles seem to live longer,  they would have said "that's weird, it's as if their clock is ticking slowly".   We now also have particle accelerators and with equipment like that to play with it's almost certain that we would have noticed strange effects when objects have high velocities relative to each other.    Basically, unless the important speed which we are calling c and appears in the Lorentz transformation was many orders of magnitude greater than it actually is, then there would have been signposts to relativity and eventually we would have noticed them.
        With some LaTex mathematical symbols (which, as I mentioned, we don't have the luxury of), we could demonstrate that the Poincare group of transformations is the only set of transformations between reference frames we should consider.   Non-linear transformations are also possible but the words "non-linear" should strike fear into the hearts of anyone who has ever studied some mathematics.   So you can rest assured that every linear transformation between reference frames would have been proposed and examined first.   The basic Poincare transformations like a rotation of the space frame won't explain the results you were getting from experiments so before too long the subset of the Poincare transformations which are just the Lorentz boosts would be all you have left before you move to non-linear transformations.   While mathematicians would be beginning to sweat and fear that  non-linear transformations would be needed, the last set of linear transformations would turn out to be the charm, they would work.  Obviously a Lorentz boost will work, we know that a Lorentz transformation was precisely what we needed.   There would be a constant which we can call c  in those transformations, it would have the dimensions of a speed etc.    The value of c would be chosen to match the experimental results we were obtaining.  Once you have the Lorentz transformations, the rest of the theory of special relativity follows.
   So, summarising all of that,  we could reasonably have obtained the Lorentz transformations just from empirical observations of strange effects when two objects have high velocities relative to each other.   Having a massless particle like light which did travel at c was a great help and it is a clear signpost to relativity.   It probably speeded up the recognition and development of relativity by many years.  Indeed taking, as an axiom, that the speed of light is an invariant will allow you (a lecturer) to develop the theory of relativity on the blackboard in front your students in 1 hour rather than over several lectures.   The key is that it wasn't essential, special relativity is just all the physics which you can obtain from the Lorentz transformation.   You can get to that (the Lorentz transformation) by other routes, you do not need to assume the speed of light is an invariant.   (However, in the world in which we do live, the speed of light is c, so you aren't doing any harm by taking that as an axiom and you will see it done in many textbooks and hear it suggested by many people.   You need this much space on a forum to explain why it isn't quite like that). 

I hope that helps.

Best Wishes.

[paul cotter]

I must say I was really impressed with ES and the "lateral" view of the question ie the red shift. Appealing to an engineering brain. Oh dear, I just crashed ES's latest post.

[Eternal Student]

Hi.

Thanks @paul cotter .   It's not really my idea, ideas like this have been discussed by people thinking about time.  Red-shift will work here instead of worrying about an emergence of time from any other geometry, so all I did was steal an idea and shorten it.   However, for me, keeping something short is an achievement.

Best Wishes.

[geordief]

Hi.

@geordief   asked about  the speed c.
LaTex isn't working so we can't have mathematical symbols here in the post,  sorry.

    Basically, yes starting from the Lorentz transformation you would quickly obtain the relativistic velocity addition formula - see  https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity

   That will show you that a velocity with magnitude c (which is just what appears in the Lorentz transformation and not necessarily the speed of light) is mapped to another velocity with the same magnitude in any other inertial reference frame.

    The entire discussion was about assuming light may not travel at the speed c.  So, that's why @hamdani yusuf  said what they said (I would think).   In most textbook developments of special relativity it is common to start from an assumption about light and the invariance of its speed.  In this situation, the whole point is that you don't - we assume light has some other speed.

    You could still obtain the Lorentz transformation even if nature had been very unkind to physicists and never given them a massless particle that could be detected.   We were lucky, we had light, it did exhibit properties that strongly pointed us to the development of special relativity.   Assume light wasn't available or did not behave like that.  Provided special relativity is still a rule in nature, then there would have been signposts to it.
   See https://en.wikipedia.org/wiki/Experimental_testing_of_time_dilation   which describes how muons can be produced in earths upper atmosphere when cosmic rays come in.    We use this as a test of special relativity, specifically we suspect that muons moving fast relative to the lab frame would live long enough to reach the surface of the earth before decaying.   The main point is that the effects of relativity like this would still be there, in nature, and eventually we would notice:    Someone would have studied the half-life and decay of particles and they would have noticed that fast moving particles seem to live longer,  they would have said "that's weird, it's as if their clock is ticking slowly".   We now also have particle accelerators and with equipment like that to play with it's almost certain that we would have noticed strange effects when objects have high velocities relative to each other.    Basically, unless the important speed which we are calling c and appears in the Lorentz transformation was many orders of magnitude greater than it actually is, then there would have been signposts to relativity and eventually we would have noticed them.
        With some LaTex mathematical symbols (which, as I mentioned, we don't have the luxury of), we could demonstrate that the Poincare group of transformations is the only set of transformations between reference frames we should consider.   Non-linear transformations are also possible but the words "non-linear" should strike fear into the hearts of anyone who has ever studied some mathematics.   So you can rest assured that every linear transformation between reference frames would have been proposed and examined first.   The basic Poincare transformations like a rotation of the space frame won't explain the results you were getting from experiments so before too long the subset of the Poincare transformations which are just the Lorentz boosts would be all you have left before you move to non-linear transformations.   While mathematicians would be beginning to sweat and fear that  non-linear transformations would be needed, the last set of linear transformations would turn out to be the charm, they would work.  Obviously a Lorentz boost will work, we know that a Lorentz transformation was precisely what we needed.   There would be a constant which we can call c  in those transformations, it would have the dimensions of a speed etc.    The value of c would be chosen to match the experimental results we were obtaining.  Once you have the Lorentz transformations, the rest of the theory of special relativity follows.
   So, summarising all of that,  we could reasonably have obtained the Lorentz transformations just from empirical observations of strange effects when two objects have high velocities relative to each other.   Having a massless particle like light which did travel at c was a great help and it is a clear signpost to relativity.   It probably speeded up the recognition and development of relativity by many years.  Indeed taking, as an axiom, that the speed of light is an invariant will allow you (a lecturer) to develop the theory of relativity on the blackboard in front your students in 1 hour rather than over several lectures.   The key is that it wasn't essential, special relativity is just all the physics which you can obtain from the Lorentz transformation.   You can get to that (the Lorentz transformation) by other routes, you do not need to assume the speed of light is an invariant.   (However, in the world in which we do live, the speed of light is c, so you aren't doing any harm by taking that as an axiom and you will see it done in many textbooks and hear it suggested by many people.   You need this much space on a forum to explain why it isn't quite like that). 

I hope that helps.

Best Wishes.

Ah yes,that has answered my question

When I first started asking questions about this subject some  14  years ago I thought relativity should require some particular way of going from one moving frame to another by virtue of the fact that it should be reversible.

I too assumed this would be a non linear equation of some kind  and  was a bit put out that is was just a plain linear  equation that seemed  to be using complex numbers.

Some people seem to prefer to consider c as a conversion factor between space and time (and other  measurements)

I see that with space and time c has the dimensions of speed.

What about mass and energy ?What would the corresponding   use for c in that situation?

Can there be other situations where c is the conversion factor between physical  properties?

What is mass divided by energy ,I wonder?

[Zer0]

@OP

May i ask a follow-up Question?

yes yes , of course 

Thanks!
I prefer to wait n watch, maybe Someone else will eventually ask what i want to.
(Interesting Responses so far)

[Eternal Student]

Hi.

What about mass and energy ?  What would the corresponding   use for c in that situation?

    You're presumably thinking of  E = mc²  or some similar relationship.

Can there be other situations where c is the conversion factor between physical  properties?

    A great many, I would think.

What is mass divided by energy ,I wonder?

   It can be interesting to take two quantities and just see what you get by taking a ratio.   Indeed there are quite a few times in physics where you could have made a good guess about the relationship between quantities just by examining the dimensions (or units of measurement) for those quantities.
     It's well worth experimenting with and quite educational.   However, eventually you may recognise that taking the ratio of two things doesn't always lead you to some new discovery or anything that we (human beings) can use.   
     For example, the ratio of a person's leg length  to the number of seconds it takes to find their mobile phone in the morning should be something with dimensions of speed.  It should mean something although it's not obvious what that is. It probably is related to every other thing which has dimensions of speed,  e.g.  the speed of a light.   However, the model you may need to represent that relationship could be so complicated it's beyond human understanding.   For all I know, all the particles in the universe share the same quantum wave function so that there can be quantum entanglement between all observables with the dimensions of speed - but a model that shows those sorts of relationships is way beyond my use or understanding.    For practical purposes, the ratio of person's leg length to the time it takes to find their phone, is some kind of speed but not usefully related to any other thing which has a speed.

Best Wishes.

[geordief]

Is it just a convention that c should be a very large number?

I think that sometimes it is given the value 1.

Would it be equally possible  to give it a very small number so that  in the expression e=mc^2 we might have the impression that it would take a numerically  huge  amount of mass to  render a numerically tiny amount of energy?

Or am I just confused?