# The Naked Scientists Forum

### Author Topic: At the speed of light, does time stop?  (Read 17605 times)

#### simplified

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##### Re: At the speed of light, does time stop?
« Reply #25 on: 07/03/2013 15:55:43 »
Light speed is constant relatively of dominant gravitation and only in this gravitation. Another photon can't  be dominant gravitation for any photon.Any photon 'feels' gravitational field and energy of objects.Therefore it can have time.
« Last Edit: 07/03/2013 16:02:21 by simplified »

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #26 on: 07/03/2013 16:16:14 »
If gravity was a 'substance/force field' interacting, and we assumed that light too can interact without annihilating, as I think you need both to make it work, you would be correct. Do they? How?
« Last Edit: 07/03/2013 16:17:50 by yor_on »

#### simplified

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##### Re: At the speed of light, does time stop?
« Reply #27 on: 08/03/2013 15:27:18 »
If gravity was a 'substance/force field' interacting, and we assumed that light too can interact without annihilating, as I think you need both to make it work, you would be correct. Do they? How?
Photon interacts with gravitation.Their interacting defines energy of photon.Isn't it?Otherwise how does photon define  chose own energy and wave length relatively of a meeting
object?
« Last Edit: 08/03/2013 17:07:48 by simplified »

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #28 on: 08/03/2013 18:06:21 »
That's about definitions. You're thinking of blue (red) shifts right? And the way a gravitational acceleration can blue shift light to a inertial observer. Using a wave description a gravity well 'compress' the wave, but only as related to the inertial observer. Using a light quanta the definition is one of unchanging energy. so this one makes most sense from a wave picture where the blue shift you find is the relation between you, as constantly uniformly accelerating at one G on Earth, relative a wave unchanging. You need both frames to define and find it.

Gravity, as 'watching' a propagating light following a geodesic, does not change its energy by definition, The only thing that may change that light would be a 'expansion' of the space (geodesic) it propagates in and then you once more need to consider it from waves in a stretching 'space', as I find it very hard to see how a point particle, as a light quanta, can change energy without annihilating. Waves give us a very nice way of imagining the properties of light, but it does not take away the duality.

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #29 on: 08/03/2013 18:17:24 »
But you have a very good point there simplified, and it is one I'm wondering about too. If we assume that 'gravity' is what defines a space. Then where does the geodesic 'end' and a 'acceleration' (blue shift) start? If it so that the geodesic never ends, which should be the correct description, then what makes the light blue shift? Mass does it, but not space? I'm not sure on that one.

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #30 on: 08/03/2013 18:32:13 »
You could define it such as the only way to define a energy to light is to annihilate it, meaning, measuring it. You can do that close to a Neutron star for example, to then wander of to measure a 'same' laser light further and further away, each time keeping yourself 'at rest' relative the neutron star.

The light quanta's are per definition in a gravitational field made by that neutron star, and as we go further and further away, detecting that laser light, at rest at each detection. Will it lose energy? If it does we can define that as a ratio relative (each detection being at rest relative the neutron star) the distance you measure between the detector and origin (for simplicity's sake we assume that the laser comes from its surface), gravity, and what energy we once defined to it, being at rest with the laser.

So yes, energy must change, but only as defined relative the observer, his relative motion/acceleration, and the gravitational field he exist in, as he does his measurement.
=

forgot :)
We have to include his and the detectors mass too. As that should be a ever so small, truly minuscule, blueshift.

But the main point is that I don't expect anyone to be able to prove light quanta to change energy intrinsically, in a flat space, or any space for that matter. Which makes gravity a 'frame of reference', but only as defined from a observer being 'inertially' at rest, if I very loosely may use that expression. Maybe it's easier to consider it from the idea of a light quanta co-moving with that gravitational field, potential or geodesic?

If you imagine a gravitational field as having a 'speed', then the light quanta is at that 'speed' when it follows a geodesic. In that motto (and ever so loosely defined :)you might consider the light quanta as 'co-moving' in its geodesic. It's not until we introduce the observer and his detector, defined as being 'at rest' with the neutron star, (or 'accelerating' on Earth) we will find that light to change energy. In its 'natural state' (intrinsically) I don't expect it to be able to interact. Well, except in a annihilation.

The 'speed' of a gravitational field must then be defined from its mass, and if one use this expression we also must assume that if one found a way to negate Earths 'acceleration' (as well as other gravitational influences), you should be able to find that the energy of that light quanta indeed is the same as originally measured on its twin (in a ideal solution that is:).
==

I'm not happy with the idea of co-moving, thinking of it :) again. One could read that as if the particle must 'gain' something. What I mean by it is that the light quanta, to me, must be seen as being 'at rest' with the gravitational field, as if follows a geodesic. And in the end it becomes a philosophical question as we can't measure on the same particle twice. We can only define it such as all light quanta of a specific energy should be the same, and intrinsically unchanging. Because if they can change, you introduce a arrow intrinsically. Then you should find light able to aging too, and most all astronomical definitions we have should be wrong.

There should be some clearer definition of it, although I can't find it now. A geodesic is somewhat of a mystery to me too.

Thinking of it, any interaction assumed in a light quanta intrinsically presumes a arrow. You can't have a change without a arrow being involved, Which should mean that definitions of wave packets interacting with themselves, not annihilating, assume a arrow. To avoid that you will have to define a wave packet to some super position, as it seems to me?

(Then again, I have some weird ideas myself, that makes some sense, but I'm afraid only to me :) that is. But they don't build on a 'propagation' of light, and it is also from that point of view I defend them as 'intrinsically unchanging'. It's a hard thing to be objective when it comes to relativity:)
« Last Edit: 08/03/2013 22:33:53 by yor_on »

#### simplified

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##### Re: At the speed of light, does time stop?
« Reply #31 on: 09/03/2013 05:00:22 »
No problem with relative energy.Photon has real kinetic energy in Earth's gravitational field.Your escape (for example) creates only potential reducing of energy of the photon.And  only nearby approaching photon loses kinetic energy in your gravitational field.
« Last Edit: 09/03/2013 05:02:16 by simplified »

#### simplified

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##### Re: At the speed of light, does time stop?
« Reply #32 on: 09/03/2013 12:57:49 »
Kinetic energy of photon is obliged to correspond to the lost gravitation.It doesn't correspond in relativity.As the gravitation is the same to any observer. :P

#### Pmb

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##### Re: At the speed of light, does time stop?
« Reply #33 on: 09/03/2013 17:04:59 »
I gotta say that I'm puzzled by the language being used in this thread. It makes little sense to me. I think there are some wrong ideas about photons and gravity in this thread. For example
Quote
Light speed is constant relatively of dominant gravitation and only in this gravitation. Another photon can't  be dominant gravitation for any photon.Any photon 'feels' gravitational field and energy of objects.Therefore it can have time.
This makes no sense either. What does constant relatively of dominant gravitation and only in this gravitation mean? What does Another photon can't  be dominant gravitation for any photon mean?

The statement Therefore it can have time. makes no sense either. Obects can't be said to "have time". Only that they perhaps act according to the passing of time as all things do.
[/quote]
This is wrong. The speed of light changes as a photon moves through a gravitational field. I can't even understand what the following means
Quote
..constant relatively of dominant gravitation and only in this gravitation.
What is that supposed to mean? What does Another photon can't  be dominant gravitation for any photon. mean?

#### simplified

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##### Re: At the speed of light, does time stop?
« Reply #34 on: 09/03/2013 17:51:39 »
Time is a feeling of change.You can have no clock,but you should feel a change.Photon can react to changes,therefore it can have time.Delay of photon in gravitational field exists for observer in weaker gravitational field.And slowed time can show that photon travelled faster than "c",if photon's way was in weaker gravitational field.
Photon has no own gravitational field therefore another photon can travel at zero speed relatively of the photon.
« Last Edit: 09/03/2013 17:53:32 by simplified »

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #35 on: 09/03/2013 19:05:02 »
Simplified, you are introducing a intrinsic arrow there. Change always rely on a arrow, and so do 'sensing' something. They are forms of interaction in where you have two 'subjects' reacting to each other. That means time :)

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #36 on: 09/03/2013 19:16:07 »
Intrinsic = Belonging to a thing by its very nature.
Inherent = Existing as an essential constituent or characteristic

Intrinsic is the stronger word here, although one meets both discussing physics.

So, assuming you to be correct we now must find photons aging. If they do we must find a definition for at what rate, otherwise no astronomical observations will hold true, well, some will like 'Pluto is ... There..' but all revolving around decay of something will need to redefined in form of some relation relative that new 'photon decay' you propose. And all observations around a age of the universe and blue and red shifts must be redefined .

Take a look at this.

"Assume visible light traveling in vacuum and entering a transparent glass with refractive index 1.5. Because the glass is transparent, the light gets transmitted and comes out from the other side. Prior to entering the glass, the velocity of light was c. Just after the vacuum-glass interface, its velocity is c/1.5 (therefore there is deceleration!). After it comes out of the glass-vacuum interface, its velocity is again c (now there is acceleration from velocity c/1.5 to c). Where is the energy source for driving this deceleration and acceleration of the photons?
- B K Chandrasekhar (age 53)
Bangalore, India

A:

That's a really interesting question. It turns out that a blip of light has the same energy in the glass as it had outside. There is a universal quantum relation between energy and frequency, E=hf, where h is Planck's constant. The frequency doesn't change in the glass, ...... so the energy per quantum doesn't change,........ and neither does the number of quanta........ The velocity-dependent energy formula (mv2/2) familiar from classical physics only applies to particles with rest-mass traveling at much less than the speed of light.

In the vacuum, you can classically describe the light energy as entirely consisting of electric and magnetic field energies. In the glass, the field energy is reduced but the energy is still there in the kinetic energies of particles (mainly electrons) oscillating in response to the fields.

Mike W."

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #37 on: 09/03/2013 22:37:20 »
The point is that I don't think that photons 'react' Simplified. If they do they must have some intrinsic clock, and if we go from a propagating light universe the furthest away have traveled 13.7 billion years about. A geodesic, not that I fully get how that works practically, is defined as having no resistance. Space itself is defined this way to, and the geodesic is a expression of how the space is expected to distort relative mass, assuming that to be the major contributor to gravity. Then you have 'energy' that also is assumed to be able to create 'gravity', but neither of those attributes show a resistance.

Maybe you mean that, as it take a path there should be 'choice' involved in that path? Or 'forces' defining the 'walls' for the path taken? Would that be it?

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #38 on: 09/03/2013 22:57:54 »
Are you thinking of 'elastic collisions' here? Because if you do you might want to formulate it your way. If a elastic collision of a photon with a mirror allows it to 'bounce' away. Is that then a change? (but you know my take on those definitions already:) Treated as a system with conservation of energy all is as before the bounce, but treated as a 'bouncing photon' it sure is a change for it.

#### Pmb

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##### Re: At the speed of light, does time stop?
« Reply #39 on: 10/03/2013 03:52:30 »
Quote from: yor_on
If they do they must have some intrinsic clock, ..
Photons are funny things. Yes. That sort of have a clock. If you were to draw a diagram of a photon moving through space you'd be able to associate a clock reading time as the photon propagates. This is due to the wave function Psi = A exp(i (x - vt)/L). As the photon propagates the vector in the imaginary coordinate system rotates around and around until it stops and is absorbed some place. Feyman's book QED explains all this. Very nice read.

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #40 on: 10/03/2013 12:51:50 »
Do we have a experiment using a silvery surface, sending a 'photon' of a defined energy at it, measuring the energy change in that material, then another detector catching the 'bounce', measuring the energy there? Can one do that? from a interpretation of observers becoming part of what you measure your interaction will change the result. But if we imagine a photon caught on a film it should be absorbed, and according to how I think all photons only have two states. They 'exist', and then they annihilate. That is a photon definition although the same should be possible to state for a wave too. If it isn't we have two unique states, not to be mixed. But that experiment would be interesting to me.

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #41 on: 10/03/2013 12:58:19 »
The point I'm making here is that when you think of 'photons' as excitations of a wave you no longer define a duality. You define a excitation in a wave.

#### lean bean

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##### Re: At the speed of light, does time stop?
« Reply #42 on: 10/03/2013 16:28:15 »
Hope i'm not diverting subject with a screwy question.
Since the wave idea of light as been mentioned, how can something interact with a lightwave in a timespan shorter than the wave period of the lightwave?

In other words, lightwaves can only interact with anything as a 'whole' wavelength not half a wavelength. So, is this wave period(time) the same for  a photon when it interacts with something.
Or, can it be said both wave and photon interact with things instantly ? If so, why the need for a period?

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #43 on: 10/03/2013 16:52:40 »
A very sweet question Lean Bean, looking at it. Can you measure a time for the interaction? The 'speed' is 'c'. You can measure energy states changing over a distance, as the eye getting 'hit' by that photon, its 'energy' if one like interacting with 'stuff'' on the way to the to the brain, for that final interpretation. But the annihilation itself?

As you say, can we measure it?

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #44 on: 10/03/2013 17:23:57 »
I will now state that you are correct, I can't see how we could measure 'c' in its smallest constituents, as that annihilation must become. Think of it from Planck scale, define a smallest distance, define the annihilation as taking place at one smallest point, although the reactions from the annihilations energy will wander in diverse paths, as defined by interactions we measure. But any photon annihilation must become defined to one 'point' locally measured. It's not going to annihilate over a distance.
=

By smallest constituents when it comes to light I mean the time it takes for light to take one Planck length (being one Plank time). You can define other but those are the physically meaningful smallest definitions we use, as long as we're not going to discuss it from strings and branes :)
« Last Edit: 10/03/2013 17:41:07 by yor_on »

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #45 on: 10/03/2013 17:32:02 »
Better point out Lean Bean that we both become 'realists' here, especially if you agree. We expect things to make a real sense, in a experiment. We expect clear definitions of what light can, and can not do.

#### JP

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##### Re: At the speed of light, does time stop?
« Reply #46 on: 10/03/2013 18:17:04 »
Hope i'm not diverting subject with a screwy question.
Since the wave idea of light as been mentioned, how can something interact with a lightwave in a timespan shorter than the wave period of the lightwave?

In other words, lightwaves can only interact with anything as a 'whole' wavelength not half a wavelength. So, is this wave period(time) the same for  a photon when it interacts with something.
Or, can it be said both wave and photon interact with things instantly ? If so, why the need for a period?

That's one of the funny things about photons: they're not a light wave.  They can do some odd things because they aren't the same as a light wave.  Detection at a point is possible for a photon.

A light wave is one way of grouping many photons together so they behave collectively like a light wave over space and time.  You can't detect it at a point because when you try to measure it, you measure many photons striking your detector, and they don't all hit at the same point.  This means the light wave has a spread on the detector that's proportional to its wavelength.

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #47 on: 10/03/2013 20:16:58 »
So if we define it as taking a 'time', even though unmeasurable, then a photon is slightly 'faster' annihilating than a wave? Tells me what to bet on in a race then :)

#### JP

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##### Re: At the speed of light, does time stop?
« Reply #48 on: 10/03/2013 20:20:14 »
A classical wave is not the same as a photon.  You can't fairly compare the two, since they're different objects with vastly different properties.

#### yor_on

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##### Re: At the speed of light, does time stop?
« Reply #49 on: 10/03/2013 21:09:08 »
Would you like to expand on how they are the same, and from which point of view JP? You talk about a classical wave here, meaning a wave close to waves we see in nature I presume. But from where would one find them to be the same, a field?

#### The Naked Scientists Forum

##### Re: At the speed of light, does time stop?
« Reply #49 on: 10/03/2013 21:09:08 »

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