At the speed of light, does time stop?

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Offline cheryl j

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At the speed of light, does time stop?
« on: 27/02/2013 15:53:56 »
I was listening to a lecture by astrophysicist Neil deGrasse Tyson and he made the remark that at the speed of light, time stops. He said from "the photons point of view," there is no period of time between when the photon is emitted from a star, and when it arrives millions of years of later, here on earth or somewhere else. No matter how hard I try to wrap my brain around that idea, it doesn't seem possible, because light does have a velocity, and velocity is distance/time. Hopefully I understood him correctly and am not misstating something.
« Last Edit: 27/02/2013 23:27:14 by chris »

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Offline Bill S

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Re: photons and time
« Reply #1 on: 27/02/2013 23:17:23 »
It seems that within the scientific community one can find experts who say quite definitely that photons do not "experience" time.  However, the most common response to this question seems to be that because one cannot assign a relativistic frame of reference to a photon, the question makes no sense, and therefore cannot have an answer.  It is argued that concepts mean something only if you can define how to measure them practically.

While one has to accept that all this is undoubtedly true, that photons cannot be considered as observers, and that no physical observer can attain the speed of light; the feeling that there is a cop-out lurking there somewhere is difficult to shake off.

I have recently tried using the CMBR as a way of clarifying the issue, but with little success.  I learned quite a bit about the CMB, but made no progress with the photon/time question.

I wish you luck sorting this one out.  If you succeed, PLEASE post your findings.

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Offline Bill S

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Re: At the speed of light, does time stop?
« Reply #2 on: 27/02/2013 23:52:39 »
Sascher Vongehr says:  http://www.science20.com/alpha_meme/fundamental_nature_light-75861

   Light has no time to see
       nor any space to be,
       nor even any energy.





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Offline JP

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Re: At the speed of light, does time stop?
« Reply #3 on: 28/02/2013 00:29:15 »
I hate to contradict Neil deGrasse Tyson, but if he claimed a photon has a "point of view," he's wrong.  This comes up a lot on the forum, so I'll just summarize the answer and link you to a previous thread.

The basic answer is that the idea of getting inside an object's "point of view" means we define a reference frame for that object.  Our theories tell us how to define reference frames for anything with rest mass (such as ourselves and clocks), but not for massless photons!  So we don't have a theory that tells us what a photon sees, and we don't know if it even makes sense to talk about time passing for a photon, since none of our measurement tools can ever move at the speed of light.

Here's a previous thread where it gets discussed in more detail: http://www.thenakedscientists.com/forum/index.php?topic=42037.0

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Offline yor_on

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Re: At the speed of light, does time stop?
« Reply #4 on: 28/02/2013 19:34:00 »
Using your own position in time and space Cheryl you would define a photon as having a speed, wouldn't you? And we have a speed defined for it called 'c'. So from our perspective it must take 'time'. But we also find it to be time less, as the only thing annihilating that light will be its interaction, although astronomical and other redshifts possibly can do the same? I'm not sure there, as we also have the definition of a light quanta of a unchanging energy intrinsically. Redshifts are you relating to a wave, but thought of as a photon its intrinsic energy shouldn't change ever.
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Offline Pmb

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Re: At the speed of light, does time stop?
« Reply #5 on: 01/03/2013 02:37:16 »
I agree with JP. It's not quite right to be speaking from a photons point of view. It's merely an extrapolation.

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Offline cheryl j

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Re: At the speed of light, does time stop?
« Reply #6 on: 01/03/2013 04:43:24 »
Well.That is why I stick to biology I guess. Except for the very difficult mathematics, does studying physics feel like smoking marijuana half the time? It's so conceptionally bizarre. I assume the mathematics makes it less so.

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Offline Pmb

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Re: At the speed of light, does time stop?
« Reply #7 on: 01/03/2013 05:00:18 »
Well.That is why I stick to biology I guess. Except for the very difficult mathematics, does studying physics feel like smoking marijuana half the time? It's so conceptionally bizarre. I assume the mathematics makes it less so.
He he! I guess it may feel like that to some people. I love it though. It gives me a better sense of the world in which I live. To me studying physics is a bit like looking up Mother Natures skirt. :)

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Offline JP

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Re: At the speed of light, does time stop?
« Reply #8 on: 01/03/2013 14:31:33 »
Well.That is why I stick to biology I guess. Except for the very difficult mathematics, does studying physics feel like smoking marijuana half the time? It's so conceptionally bizarre. I assume the mathematics makes it less so.
He he! I guess it may feel like that to some people. I love it though. It gives me a better sense of the world in which I live. To me studying physics is a bit like looking up Mother Natures skirt. :)

I gave up on biology in high school!  All the memorization made my head hurt.  I find physics easier because much of it can be derived mathematically from simple rules.  I guess it's just a matter of how you learn best!

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Offline yor_on

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Re: At the speed of light, does time stop?
« Reply #9 on: 02/03/2013 22:24:09 »
Hmm, first marijuana and now, memory loss?
Ah well, all disciplines have their ups and downs I guess :)
Even biology .
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Offline Bill S

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Re: At the speed of light, does time stop?
« Reply #10 on: 02/03/2013 23:35:18 »
If anyone has the time and patience to follow the link to Sascher Vongehr's articles, I would be fascinated to hear some comments from those who have more scientific know-how than I.

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Offline JP

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Re: At the speed of light, does time stop?
« Reply #11 on: 03/03/2013 00:13:01 »
He's making the same mistake mentioned above: that we can legitimately extrapolate to the "point of view of light" by just thinking about our own point of view if we could somehow reach the speed of light.  We can't, of course, and the theory that covers our sub-luminal speeds does not extrapolate to the reference frame of light.  This is a really common mistake, and it naturally comes from trying to extend the theory beyond it's limits--I used to post the same viewpoint on this site until I read up a bit more and realized why it was wrong. 

The problem is that in science we can only make theories about things that are testable, and "what does light experience"? is not testable by any means we currently have.  From a scientific viewpoint, it's unknowable for now.

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Offline Ethos_

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Re: At the speed of light, does time stop?
« Reply #12 on: 03/03/2013 02:12:44 »
He's making the same mistake mentioned above: that we can legitimately extrapolate to the "point of view of light" by just thinking about our own point of view if we could somehow reach the speed of light.

Precisely my friend................Just because experiment proves that as we accelerate closer to light speed, and the passage of time slows down for the traveler, it does not extrapolate into zero time at light speed. And this is because it can't be shown experimentally, and to assume this result is not scientific. Likewise, just because the universe is expanding and we assume that it all started from a singularity at the Big Bang, is also not achieved by the scientific method.

Sadly, many individuals that call themselves scientists only classify a particular view as scientific truth when those views fit their personal likes. And then turn right around and dismiss another view with similar evidence.

There is just as much evidence for zero time at light speed as there is for the Big Bang! What should that tell us about scientific bias?
« Last Edit: 03/03/2013 02:20:33 by Ethos_ »
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Offline JP

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Re: At the speed of light, does time stop?
« Reply #13 on: 03/03/2013 12:59:02 »
They're not analogous.  You can't extrapolate to light's point of view because our theories don't extrapolate to cover the case of moving at the speed of light and we can't possibly experiment there to develop a new theory that does cover that case.

In the case of the big bang, our theories do cover the period immediately after the big bang and we can access a lot of that period experimentally (through particle accelerators).  What we can't really talk about is what happened at the instant of the big bang, since that's where our theories break down and we can't do experiments. 

If you'd like to discuss the evidence for the big bang, let's start a new thread, since it's likely to derail this one.

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Offline yor_on

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Re: At the speed of light, does time stop?
« Reply #14 on: 04/03/2013 12:32:47 »
From a point of measuring you use a two way experiment for deciding a 'photon speed'. You and your detector together, and then a mirror placed some way away. The particle 'bounce' if you like in a so called 'elastic collision' with the mirror, or just think of it as a wave 'bouncing' from the mirror back to our detector. Don't really know any other way to do it myself although I've seen some very clever ideas elsewhere. Anyway, what you do when you measure that speed is that you use your local clock and your local ruler. The ruler defines your distance to the mirror, the clock defines 'segments' of time, ticking. Combining those you get a arbitrarily defined 'speed', arbitrarily as you can use some other 'segmentation' of time (or ruler), reaching another definition of 'speed'. But what seems to be true is that no matter what segmentation you use the light/photon still becomes a constant, no matter where you do that experiment, at a event horizon or on Earth.

And that is 'locality'. Giving you a same speed, no matter where you are. That 'speed' can then be seen as a real constant, defining the space and time you see. Why :) well, how do you measure, and see?
« Last Edit: 04/03/2013 12:40:17 by yor_on »
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Offline yor_on

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Re: At the speed of light, does time stop?
« Reply #15 on: 04/03/2013 12:54:07 »
There is one point more to it.
Gravity..

At the event horizon, assuming you being there measuring outwards into space, gravity should bend the 'path taken' as observed by you, making it take more 'time' for your measurement, all locally perceived. That situation is analogous to one in where you are inside a uniformly (constantly) accelerating spaceship (ignoring tidal forces) also finding the light to take more 'time'.

Now, why are those two analogous in my mind?

Gravity 'bends' SpaceTime, and inside that spaceship you must find a local 'gravity', making you 'weight something'. So strictly defined 'c' is 'c' as long as we only consider a 'flat SpaceTime', all as I get it. But using 'gravity' to define a 'shortest path' for that light, the two situations described becomes equivalent, and light will still be a local 'constant' relative the 'bending' of Space, measured locally. Well, as I get it.

It's not that complicated really, but the implications surely are.
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Offline yor_on

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Re: At the speed of light, does time stop?
« Reply #16 on: 04/03/2013 13:28:52 »
And a third.

Now consider a light clock  (<- - ->) traveling in space.

It uniformly moves away from you very fast, near the speed of light as you measure, and you see it as depicted above.
The light 'bounce' between those two mirrors ' ( <-and ->) becoming a 'clock', ticking.

From your perspective it must 'slow down'  as that light clock is moving away from you very fast, making the 'light bounces' traverse a lot of more space each time, than they would need to if the clock was perfectly still relative yourself (as being in the same room).

And it goes all out from light being a 'constant', same anywhere.

The question then becomes if this is true, does that light clock really slow down its 'time', and relative what If so?
Only your measurments, or inside that ship too?

That's what the 'twin experiment' discuss. And one twin are indeed expected to have aged less, the 'traveling one', as measured/compared when returning to Earth.

But it is also true that using the two way experiment to define a local 'time' inside the Spaceship, splitting that light into even 'chunks' relative your ruler, that speed never differs. We could instead have defined it as the light clock uniformly accelerating, and then found a 'bent space' between the mirror and detector inside that ship, but to point it out real good we can use a uniform motion instead, as all uniform motions becomes equivalent in matter of locally measuring a 'speed'. The mass may impose a 'bending', or if we talk accelerations, the 'relative mass' locally perceived to change your 'weight'. But in all uniform motions that relative mass is experimentally absent, locally measured, and, as I get it of course :)
=

What you can use is 'potential energy' to describe uniform motions. But all such 'energy' is relations between 'two frames of reference', as comparisons between points in space and time. And using suns to describe it they all move in different uniform motions relative you (Earth), also having different masses.

So depending on what Sun you compare your 'potential energy' to, in some thought up collision, you get different results. So what is your 'potential energy' alone? It got to be your mass, and it should be your motion. But all uniform motions are equivalent, locally measured. You need that other 'frame of reference' to find your 'potential energy'.


« Last Edit: 04/03/2013 13:48:14 by yor_on »
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Offline Bill S

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Re: At the speed of light, does time stop?
« Reply #17 on: 05/03/2013 19:13:46 »
Quote from: JP
The problem is that in science we can only make theories about things that are testable, and "what does light experience"? is not testable by any means we currently have.  From a scientific viewpoint, it's unknowable for now.

Is it me, or is there a dichotomy here?

Does a photon have a frame of reference?  We cannot say it does because relativity does not permit the designation of a frame of reference for a massless object.  Therefore the subject is anathema; it is simply speculation.  On the other hand a singularity, for which relativity provides no explanation, is quite acceptable on the basis that relativity breaks down at this point, so something else must provide the explanation, and undoubtedly will do so when we can find it, so it's OK to go on speculating about singularities.     




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Offline lightarrow

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Re: At the speed of light, does time stop?
« Reply #18 on: 05/03/2013 19:45:20 »
Quote from: JP
The problem is that in science we can only make theories about things that are testable, and "what does light experience"? is not testable by any means we currently have.  From a scientific viewpoint, it's unknowable for now.

Is it me, or is there a dichotomy here?

Does a photon have a frame of reference?  We cannot say it does because relativity does not permit the designation of a frame of reference for a massless object.  Therefore the subject is anathema; it is simply speculation.  On the other hand a singularity, for which relativity provides no explanation, is quite acceptable on the basis that relativity breaks down at this point, so something else must provide the explanation, and undoubtedly will do so when we can find it, so it's OK to go on speculating about singularities. 
But you don't put frames of reference in a singularity...

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Offline JP

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Re: At the speed of light, does time stop?
« Reply #19 on: 05/03/2013 20:25:01 »
Quote from: JP
The problem is that in science we can only make theories about things that are testable, and "what does light experience"? is not testable by any means we currently have.  From a scientific viewpoint, it's unknowable for now.

Is it me, or is there a dichotomy here?


As lightarrow pointed out, a reference frame and a singularity are not the same thing!   The issue is that one of the postulates of SR is that the speed of light is always measured as a constant in any inertial reference frame.  It is impossible for light to be in an inertial reference frame, since it would measure it's own speed at 0, rather than the speed of light, meaning that whatever it's "frame" means, it's not an inertial reference frame.

A singularity is where equations break down in a particular way, although it commonly gets used in physics to mean the region of space-time at the center of black holes where the equations of general relativity break down.  This is different than the light reference frame issue, since we don't formulate GR by excluding this break down region.  Rather, it's formulated from different considerations, and these breakdown points fall out of the equations, signifying the theory doesn't apply there and something more must be used. 

It's a subtle point, but while in GR, singularities exist over real regions of space-time and are approachable by experiment and current theory, reference frames for photons aren't even a valid thing in special relativity, so they can't be approached by theory or experiment--at least not in terms of current theories.

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Offline Pmb

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Re: At the speed of light, does time stop?
« Reply #20 on: 05/03/2013 22:09:25 »
Quote from: JP
I used to post the same viewpoint on this site until I read up a bit more and realized why it was wrong. 
Where did you read about it? I'm always looking for better ways to explain/describe things.

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Offline Pmb

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Re: At the speed of light, does time stop?
« Reply #21 on: 05/03/2013 22:17:00 »
Quote from: Ethos
\He's making the same mistake mentioned above: that we can legitimately extrapolate to the "point of view of light" by just thinking about our own point of view if we could somehow reach the speed of light.
It can be viewed as a limit though. We do this when we say that the proper time between two events seperated by a lightlike spacetime interval. This is the reason one cannot define a 3-momentum for a photon, i.e. because you'd be dividing by zero.

Regarding the so-called Big Bang singularity. In the Big Bang theory there is no event which can be called the big bang. As Peebles explains "If there were an instant, at a 'big bang,' when our universe started expanding it is not in the cosmology as now accepted, because no one has thought of a way to adduce objective physical evidence that such an event really happened."

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Offline JP

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Re: At the speed of light, does time stop?
« Reply #22 on: 06/03/2013 01:18:27 »
Quote from: JP
I used to post the same viewpoint on this site until I read up a bit more and realized why it was wrong. 
Where did you read about it? I'm always looking for better ways to explain/describe things.

One that I recall was this: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html

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Offline Pmb

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Re: At the speed of light, does time stop?
« Reply #23 on: 06/03/2013 02:06:58 »
Quote from: JP
I used to post the same viewpoint on this site until I read up a bit more and realized why it was wrong. 
Where did you read about it? I'm always looking for better ways to explain/describe things.

One that I recall was this: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html
Nice!!!

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Offline JP

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Re: At the speed of light, does time stop?
« Reply #24 on: 06/03/2013 17:10:22 »
The other neat thing about this question is that it shows that physics is about tying equations into nature, not just blindly applying math.  The reason I used to think that photons didn't "experience" time was that if you blindly apply the equations of SR to photons, you can get to that conclusion (technically, you'd have to take a limiting case). 

But if you look at nature, which says the speed of light is constant for all inertial reference frames, then you realized immediately that using the equations in that case is nonsense--since you're trying to define a frame where the speed of light has changed to 0 m/s.  And in fact, you can verify that the derivation of the equations of SR precludes them from covering the reference frame of photons.

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Offline 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 »

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Offline 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 »
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Offline 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 »

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Offline 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.
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Offline 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.
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Offline 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 »
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Offline 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 »

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Offline 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

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Offline 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?

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Offline 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 »

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Offline 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 :)
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Offline 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."
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Offline 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?
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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.
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Offline 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.

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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.
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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.
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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?

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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?
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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 »
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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.
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Offline 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.

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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 :)
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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. 

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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?
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