Humans travelling at light speed?

[mibaokula]

humans traveling at light speed?

just a little thought experiment. we know it is ludicrous to suggest that humans can travel at "c" because we have mass and it takes infinite energy to travel at c if you have mass. however, does this work for relative velocities: let us assume you are in a closed dark room standing completely still; suddenly a flash-light at the end of the room is turned on. we know that the flash light is expelling photons that are able to travel at c. my question is: with respect to each photon, what velocity do you (standing still) travel at?

 

[Pengwuino]

It makes no sense to ask what photons see. They do not have frames of reference where it makes sense to ask "what does a photon see". They don't experience time or space.

 

[mibaokula]

It makes no sense to ask what photons see. They do not have frames of reference where it makes sense to ask "what does a photon see". They don't experience time or space.

thanks i guess that would make sense. but that's no fun, light can experience time (we see galaxies as they were millions of years ago), light can experience space (black holes and other massive objects can "bend" light) but light has no reference frame in that particular thought experiment - in short, i don't get it

 

[ghwellsjr]

When we talk about a frame of reference associated with some object/observer, we define that object/observer to be at rest in that frame. We also define the speed of light to be c in any frame of reference that we are considering. A photon cannot both be at rest and traveling at c at the same time so it is meaningless to talk about a frame of reference associated with a photon.

 

[mibaokula]

When we talk about a frame of reference associated with some object/observer, we define that object/observer to be at rest in that frame. We also define the speed of light to be c in any frame of reference that we are considering. A photon cannot both be at rest and traveling at c at the same time so it is meaningless to talk about a frame of reference associated with a photon.

would the same principle apply if talking about two "photons" moving in opposite directions. can that be described using frames of reference?

 

[Doc Al]

would the same principle apply if talking about two "photons" moving in opposite directions. can that be described using frames of reference?

Sure it can be described. Just not from the frame of reference of a photon, which has no meaning.

 

[ghwellsjr]

would the same principle apply if talking about two "photons" moving in opposite directions. can that be described using frames of reference?

Yes, but remember, a photon, like any ray of light, is defined to travel at a speed of c. It makes sense in SR to talk about two photons being emitted in opposite directions simultaneously from a common source at rest in a reference frame so that they simultaneously hit a pair of targets equally spaced from the source. But when we use a different reference frame where the source and targets are in motion, the speeds of those two photons are redefined to be c in that new reference frame so that they do not hit the targets simultaneously.

 

[mibaokula]

Yes, but remember, a photon, like any ray of light, is defined to travel at a speed of c. It makes sense in SR to talk about two photons being emitted in opposite directions simultaneously from a common source at rest in a reference frame so that they simultaneously hit a pair of targets equally spaced from the source. But when we use a different reference frame where the source and targets are in motion, the speeds of those two photons are redefined to be c in that new reference frame so that they do not hit the targets simultaneously.

so in other words, looking at one frame of reference can have a completely different effect to another frame of reference. that makes me even more confused

 

[Pengwuino]

thanks i guess that would make sense. but that's no fun, light can experience time (we see galaxies as they were millions of years ago), light can experience space (black holes and other massive objects can "bend" light) but light has no reference frame in that particular thought experiment - in short, i don't get it

No, they do not experience either. The reason we see objects as they were millions of years ago is simply because it takes time for light to travel. However, that's in our frame of reference, not the photon's. Same idea with space.

 

[mibaokula]

No, they do not experience either. The reason we see objects as they were millions of years ago is simply because it takes time for light to travel. However, that's in our frame of reference, not the photon's. Same idea with space.

now I'm super confused. if light has a numerically defined speed - 300,000km/s in a vacuum. then surely a photon traveling at that speed experiences time. in other words, it would take 20 seconds for light to travel 6,000,000 km. And in general relativity, light does get effected by space ignoring frames of reference, right? the medium light propagates through is space, right? that's why it can travel in a vacuum.

so doesn't light get effected by space and time or am i completely misunderstanding your point?

 

[Ryan_m_b]

now I'm super confused. if light has a numerically defined speed - 300,000km/s in a vacuum. then surely a photon traveling at that speed experiences time. in other words, it would take 20 seconds for light to travel 6,000,000 km. And in general relativity, light does get effected by space ignoring frames of reference, right? the medium light propagates through is space, right? that's why it can travel in a vacuum.

so doesn't light get effected by space and time or am i completely misunderstanding your point?

Photons do not experience because they are non-conscious entities.

However to suggest an answer to your question (which may be an inappropriate answer because I may be wrong about this) if we imagine a hypothetical, conscious entity that is able to travel at light speed it would still not be able to "experience" anything. This is because time dilation at the speed of light becomes infinite.

 

[mibaokula]

Photons do not experience because they are non-conscious entities.

However to suggest an answer to your question (which may be an inappropriate answer because I may be wrong about this) if we imagine a hypothetical, conscious entity that is able to travel at light speed it would still not be able to "experience" anything. This is because time dilation at the speed of light becomes infinite.

sorry, what i meant by "experience" is the forces acting on it. i would say for example that a ball "experiences" gravity. I'm not crazy enough to say that the ball has a conscious or something. i (foolishly) mean that the ball is attracted to the Earth through the force of gravity. Likewise, i am saying that the force of gravity has an effect on the path of light on large scales and equally time has an effect on light because it doesn't take an instantaneous amount of time for light to travel a light year. Am i wrong?

 

[bcrowell]

FAQ: What does the world look like in a frame of reference moving at the speed of light?

This question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.

The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.

Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)

What if a system of interacting, massless particles was conscious, and could make observations? The argument given in the preceding paragraph proves that this isn't possible, but let's be more explicit. There are two possibilities. The velocity V of the system's center of mass either moves at c, or it doesn't. If V=c, then all the particles are moving along parallel lines, and therefore they aren't interacting, can't perform computations, and can't be conscious. (This is also consistent with the fact that the proper time s of a particle moving at c is constant, ds=0.) If V is less than c, then the observer's frame of reference isn't moving at c. Either way, we don't get an observer moving at c.

 

[Ryan_m_b]

sorry, what i meant by "experience" is the forces acting on it. i would say for example that a ball "experiences" gravity. I'm not crazy enough to say that the ball has a conscious or something. i (foolishly) mean that the ball is attracted to the Earth through the force of gravity. Likewise, i am saying that the force of gravity has an effect on the path of light on large scales and equally time has an effect on light because it doesn't take an instantaneous amount of time for light to travel a light year. Am i wrong?

No your not wrong, photons are affected by gravity, matter etc.

 

[mibaokula]

FAQ: What does the world look like in a frame of reference moving at the speed of light?

This question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.

The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.

Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)

What if a system of interacting, massless particles was conscious, and could make observations? The argument given in the preceding paragraph proves that this isn't possible, but let's be more explicit. There are two possibilities. The velocity V of the system's center of mass either moves at c, or it doesn't. If V=c, then all the particles are moving along parallel lines, and therefore they aren't interacting, can't perform computations, and can't be conscious. (This is also consistent with the fact that the proper time s of a particle moving at c is constant, ds=0.) If V is less than c, then the observer's frame of reference isn't moving at c. Either way, we don't get an observer moving at c.

so in short, until we are able to hypothesise a situation where we have infinite energy to travel at "c", it is impossible to even begin to describe what can happen at a photon's frame of reference

 

[mibaokula]

No your not wrong, photons are affected by gravity, matter etc.

so using my basic (probably naive) concept of velocity and distance, surely then there is no infinite time dilation if traveling at the speed of light (hypothetically). this is because it will still take you time to travel huge distances.

what confuses me is that if "c" is an actual numerical value, then how can time stop if traveling a distance? do you understand my confusion here?

 

[ghwellsjr]

so in other words, looking at one frame of reference can have a completely different effect to another frame of reference. that makes me even more confused

We don't "look" at a frame of reference, rather, we "define" an arbitrary reference frame from which we "look" at everything else. We can then use the Lorentz Transform to "see" what the same things "look" like according to a different frame of reference. Switching between frames of reference doesn't change the effect of anything that is happening, it only changes how we label what is happening.

 

[mibaokula]

We don't "look" at a frame of reference, rather, we "define" an arbitrary reference frame from which we "look" at everything else. We can then use the Lorentz Transform to "see" what the same things "look" like according to a different frame of reference. Switching between frames of reference doesn't change the effect of anything that is happening, it only changes how we label what is happening.

a-ah! but with the example of the light particles and the sources, two different effects are observed right?

but i don't understand why light (photons) are exempt from reference frames. photons, as i have tried to explain are affected both by space and time. what "criteria" must you meet in order to be "allowed" a frame of reference

 

[JDoolin]

The limit of acceleration.

Say you are looking at a planet in front of you, and you accelerate toward it.

The event that you are looking at moves into the past, and further away.

In the limit as you accelerate "to" the speed of light, the event moves infinitely far away, and infinitely into the past.

The image then approaches superluminally. In the limit, as you accelerate "to" the speed of light, the image approaches at infinite speed, and arrives in zero time.

Also, the frequency of the light coming to you, of course, goes to infinity; the wavelength goes to zero.

 

[mibaokula]

The limit of acceleration.

Say you are looking at a planet in front of you, and you accelerate toward it.

The event that you are looking at moves into the past, and further away.

In the limit as you accelerate "to" the speed of light, the event moves infinitely far away, and infinitely into the past.

The image then approaches superluminally. In the limit, as you accelerate "to" the speed of light, the image approaches at infinite speed, and arrives in zero time.

Also, the frequency of the light coming to you, of course, goes to infinity; the wavelength goes to zero.

thats if you are accelerating to the speed of light. However, if you are already moving at a constant speed of c, are you saying that there is an infinite time dilation. in other words, when we "see" the sun as it was 8 minutes ago, in the frame of reference of the light from the sun, what actually happened was that it arrived on Earth instantaneously but Earth went into the past to make it appear as though there was a delay of 8 minutes?

 

[JDoolin]

thats if you are accelerating to the speed of light. However, if you are already moving at a constant speed of c, are you saying that there is an infinite time dilation. in other words, when we "see" the sun as it was 8 minutes ago, in the frame of reference of the light from the sun, what actually happened was that it arrived on Earth instantaneously but Earth went into the past to make it appear as though there was a delay of 8 minutes?

Well, there is a big difference between looking in front of you and looking behind you. If you look behind you while traveling "at" the speed of light, then it would appear that the Earth was staying with you. The light from behind you is just keeping up.

If you look at the variety of good relativity books out there, you'll see a place where this type of discussion ought to be, but is notably lacking. It's the part of the twin paradox where they show the situation from the perspective of the stay-at-home twin, but they do not show the situation from the perspective of the traveling twin.

You can see it has been tried in Wikipedia here http://en.wikipedia.org/wiki/Talk:Twin_paradox/Archive_13#Specific_example
but, even though it's a simple calculation, because it is considered "original research" Wikipedia can't publish it.

I've read one version of the twin paradox even, where the traveling twin is put into a locked cell with no windows, to assure that you don't question what the traveling twin sees, because the traveling twin sees nothing at all!

When the traveling twin turns around, they will see the image of home shoot suddenly away during the acceleration phase, before rushing back superluminally during the return.

If you imagine zooming across a room, near the speed of light, the far wall will appear to be much further away, and the space across the room in front of you is likewise, far distant, but approaching superfast. The wall behind you would hardly appear to be receding at all.

 

[ghwellsjr]

a-ah! but with the example of the light particles and the sources, two different effects are observed right?

but i don't understand why light (photons) are exempt from reference frames. photons, as i have tried to explain are affected both by space and time. what "criteria" must you meet in order to be "allowed" a frame of reference

An observer can only be aware of the photons that hit his eyes or instruments located right next to him. For photons that he or his instruments emit, he cannot tell where they are at any particular time.

So light (photons) are certainly not exempt from reference frames, in fact, this ambiguity in knowledge of their whereabouts is one of the main reasons why Einstein came up with his Theory of Special Relativity. In any reference frame, light is defined to travel at c.

So, as I said before, there are not two different effects that are observed as a result of using two different frames. Things that are defined according to one frame because they could not have been observed, for example, the trajectory of a photon, also cannot be observed in any other frame, but are merely given a different definition.

I don't understand your question: 'what "criteria" must you meet in order to be "allowed" a frame of reference'. The only criterion for a frame of reference in Einstein's Special Relativity is that it is inertial, that is, not experiencing acceleration, either a change in speed or a change in direction.

 

[DrGreg]

I don't understand your question: 'what "criteria" must you meet in order to be "allowed" a frame of reference'. The only criterion for a frame of reference in Einstein's Special Relativity is that it is inertial, that is, not experiencing acceleration, either a change in speed or a change in direction.

There is also a second criterion, that the speed of light is the same (c) in all directions, and that clearly rules out a frame in which light (in one direction) can be stationary.

 

[mibaokula]

Thanks that really explains it well.
so in summary, light can be a frame of reference so long as it travels at c. time, sometimes, must be warped to hold this fact. light still travels at the same speed but time stops so light reaches a destination instantaneously. this must be true to obey Einstein's rules that c is the maximum speed. Correct?
what confuses me is that the "photon" doesn't experience time. so each photon of light is actually light from the very beginning of the Big Bang?

 

[ghwellsjr]

Thanks that really explains it well.
so in summary, light can be a frame of reference so long as it travels at c. time, sometimes, must be warped to hold this fact. light still travels at the same speed but time stops so light reaches a destination instantaneously. this must be true to obey Einstein's rules that c is the maximum speed. Correct?
what confuses me is that the "photon" doesn't experience time. so each photon of light is actually light from the very beginning of the Big Bang?

Please go back and read the posts at the beginning of your thread. I don't know why anything you have read in this thread would lead you to conclude that "light can be a frame of reference so long as it travels at c". All light travels at c in all frames by definition. You could say that any object with non-zero mass traveling at constant speed in a straight line in any direction "can be a frame of reference" but you cannot say that about a photon.

 

[IronHamster]

My interpretation of special relativity is this. If you were to race a beam of light, it would always beat you by 300,000,000 m/s in your frame of reference, since light goes at this speed relative to you no matter how fast you are going (relative to a "stationary" observer). This is a restatement of the second postulate.

If you were to travel at the speed of light, however, you would perceive the light traveling faster than you, and the other observer going light speed would perceive you traveling faster than them. What's more, the stationary observer would observe you both traveling at the same speed.

It's very strange.

 

[JDoolin]

Thanks that really explains it well.
so in summary, light can be a frame of reference so long as it travels at c. time, sometimes, must be warped to hold this fact. light still travels at the same speed but time stops so light reaches a destination instantaneously. this must be true to obey Einstein's rules that c is the maximum speed. Correct?
what confuses me is that the "photon" doesn't experience time. so each photon of light is actually light from the very beginning of the Big Bang?

Going the speed of light in Einstein Relativity is effectively the same as going at an infinite speed in Galilean Relativity. You can imagine relative speeds going faster and faster and faster, but you can't really imagine going at an infinite relative speed--except that it takes zero time to go from one place to another.

Effectively, from the photon's point of view (if it could be said to have a point-of-view, which really, it can't), it is going at that "infinite" speed. It is going an infinite distance in zero time. From any other point of view, it is gong a finite distance in a finite time, at the speed of light.

I'm not sure what you mean by "each photon of light." Are you referring to a certain light in particular

 

[ghwellsjr]

If you were to travel at the speed of light, however, you would perceive the light traveling faster than you, and the other observer going light speed would perceive you traveling faster than them. What's more, the stationary observer would observe you both traveling at the same speed.

It's very strange.

It's very strange because there is no basis in Special Relativity or anywhere else for attempting to discuss what would happen if you were to travel at the speed of light.

 

[mibaokula]

i meant that if light "experiences" zero time but infinite velocity, light in today's universe must actually be light existing at the dawn of time?

 

[nitsuj]

i meant that if light "experiences" zero time but infinite velocity, light in today's universe must actually be light existing at the dawn of time?

While there is some electro-magnetic energy that is still hanging around from the "dawn of time", there certainly is no visable light.

I don't know the mechanics, but understand a photon to be created when an electron in an atom experiences a change (increase) in energy levels. It spits out a photon to shed this unwelcomed increase in energy.

Photons come and go.

I see you are looking for the impact infinite time dilation has on a photon (electro-magnetism), maybe read about half-life of matter.

 

[mibaokula]

Doesn't what you say break a fundamental law: energy cannot be created or destroyed?
How can photons come and go?

 

[ghwellsjr]

Photons carry energy from one atom or molecule to another. The total energy is constant.

 

[nitsuj]

Doesn't what you say break a fundamental law: energy cannot be created or destroyed?
How can photons come and go?

Your blackest shirt on a sunny day will give you the feel of photons being "destroyed"

Your brightest light bulb can "create" the photons if it's too cloudy

 

[nitsuj]

sorry, what i meant by "experience" is the forces acting on it. i would say for example that a ball "experiences" gravity. I'm not crazy enough to say that the ball has a conscious or something.

how emotive descriptions of physics, despite transparency, can still be lost in translation to the point of requiring such an obvious reply it is humorous for most readers.

mibaokula, I would have replied the same, less the sorry.

 

[mibaokula]

Your blackest shirt on a sunny day will give you the feel of photons being "destroyed"

Your brightest light bulb can "create" the photons if it's too cloudy

surely the photons aren't destroyed but their wavelengths lengthened. i.e., visible light radiates on the black shirt and infra red is emitted? with the light-bulb, electricity in the circuit meets a very high resistance to the extent that visible light is radiates as the temperature reaches a critical level?

how emotive descriptions of physics, despite transparency, can still be lost in translation to the point of requiring such an obvious reply it is humorous for most readers.

mibaokula, I would have replied the same, less the sorry.

thanks - in my haste i often forget to describe my questions in fully scientific terms

 

[JDoolin]

what confuses me is that the "photon" doesn't experience time. so each photon of light is actually light from the very beginning of the Big Bang?

Well, if you want to make the case that the big bang produced atoms, which later became part of the sun, which released light. You could try to make the case that that light was "from" the big bang.

But for some period during that time, the energy from that photon was stored as potential energy, in the capacitive properties of a battery, or the distance between swirling particles in the formation of a star, as kinetic energy in the form of excited atoms (ions or electrons in upper valence shells), or as heat energy, in the form of extra velocity in the atoms, or as mass energy, in the form of binding energy in the nucleons.

The particles involved in that "energy storage" experienced time.

 

[mibaokula]

Well, if you want to make the case that the big bang produced atoms, which later became part of the sun, which released light. You could try to make the case that that light was "from" the big bang.

But for some period during that time, the energy from that photon was stored as potential energy in the form of excited atoms, or as heat energy, in the form of extra velocity in the atoms.

The particles involved in that "energy storage" experienced time.

i never thought about it like that. but still, energy in the form of photons does not experience time - though all surrounding the photon would appear to age at an infinite rate.

 

[mibaokula]

if you believe in string theory, and a photon is a string, how would the "string" experience the length compacting that objects traveling near to and at the speed of light experience.

 

[ghwellsjr]

surely the photons aren't destroyed but their wavelengths lengthened. i.e., visible light radiates on the black shirt and infra red is emitted? with the light-bulb, electricity in the circuit meets a very high resistance to the extent that visible light is radiates as the temperature reaches a critical level?

Photons always travel at c, by definition. The photons that come from a light bulb had no existence prior to their emmision when the light bulb gave up a packet of energy for each one and they end their existence when they hit something and that same exact amount of energy is absorbed by the target. It's possible for the target to re-emit a different photon some time later at a different wavelength and with a different amount of energy, but that is a new photon that may have gotten its energy from parts of many different photons.

 

[mibaokula]

Photons always travel at c, by definition. The photons that come from a light bulb had no existence prior to their emmision when the light bulb gave up a packet of energy for each one and they end their existence when they hit something and that same exact amount of energy is absorbed by the target. It's possible for the target to re-emit a different photon some time later at a different wavelength and with a different amount of energy, but that is a new photon that may have gotten its energy from parts of many different photons.

so energy cannot be created or destroyed but particles can?

 

[JDoolin]

if you believe in string theory, and a photon is a string, how would the "string" experience the length compacting that objects traveling near to and at the speed of light experience.

For me to believe in something, I have to either be able to experience it, visualize it, or understand it.

If I believed in string theory, I would be doing so without any comprehension of what I believed in. Let's calll me "string theory agnostic," since I've never heard any description of it that made any sense to me, or of any evidence to support it.

I'm much closer to believing in "deBroglie matter waves," but frankly, I don't really understand them, either.

https://www.physicsforums.com/showthread.php?t=458792 (Edit: The paper in post 10 really looks like it would be worth spending a few days working through.)

Using deBroglie waves you might (or might not) be able to make the case that all the mass-energy was somehow a superposition of waves moving at the speed of light.

But regardless of whether that's true or not, the end effect of all that are bodies that have mass, and experience time.

 

[JDoolin]

so energy cannot be created or destroyed but particles can?

Right.

 

[nitsuj]

surely the photons aren't destroyed but their wavelengths lengthened. i.e., visible light radiates on the black shirt and infra red is emitted? with the light-bulb, electricity in the circuit meets a very high resistance to the extent that visible light is radiates as the temperature reaches a critical level?

I understand it to be the energy of the photon is converted mostly to heat energy (infra-red?) I understood the photon to be a description of visible light in this context, which there would be essentially none reflected off the black shirt.

The atoms in the light bulb filament get hot. The electrons radiate the excess energy as brand new photons.

I think the difference in interpretation here is you are using a correct definition for a photon, a "packet" of pure electro-magnetic energy. I am using it to describe visible light.

 

[bernsten69]

I believe that the problem you are having 'imagining' what it would be like to be a photon is as follows:

There is a huge (literally infinite) difference between what an observer would observe as its speed approaches the speed of light versus what an 'observer' would observe while traveling AT the speed of light.

Time dilation, space contraction etc. are all effects of having a velocity less than C with respect to another frame of reference.

Rather than calculating the limit as V -> C, if you set V = C, you end up dividing by Zero, which, rather than giving us infinity (which is the limit as you approach C of 1/SQRT(1-v^2/c^2)), gives us a non-answer. Dividing by zero is a non-mathematical process that has no calculable result.

Therefore, if you ask the question, what does a person traveling at the speed of light observe, I believe the answer is non-calculable (probably because it is impossible) (and there is no reason to believe that it has anything to do with time-dilation or space contraction).

If you ask the question, what does a person accelerating towards the speed of light observe, the answer would be that, as you approached the speed of light, infinite time (and space contraction) with respect to the rest frame would pass(occur).

However, your question is neither of these two. Your question is what does light 'observe'. Light cannot 'observe' anything for a number of reasons, but here is a good one: there is no inertial frame of reference for 'light.' To observe something, you must interact with it in some manner. To interact with something, it must be able to be described in your frame of reference in some manner. From our frame of reference, we can see light interacting with matter and other electromagnetic waves. However, there is no known transformation from our frame of reference to 'light's' (non-existent) frame of reference. Attempting to perform this 'transformation' by imagining acceleration towards the speed of light, or even matter traveling AT the speed of light is improper. However, this is what you are attempting to do.

Here's another example why it doesn't make sense to try to imagine what light observes:

Among the often ignored solutions to Maxwell's equations are Advanced Waves which travel backwards in time. Attempting to imagine what a photon observes (travelling forward in time) makes just about as much sense as attempting to imagine what a photon observes traveling backwards in time. It would appear (from our reference frame), that the assumptions we typically make (ie, time progresses forward), may not apply to light.

Here's a better analogy: Attempting to determine what 'light' observes is akin to attempting to determine what the x-axis observes. Although it may have a value (ie, at position 5 for the x-axis) (or for light, a wavelength and direction of propagation), it does not 'observe' anything. There may be a formula that describes what value the X axis has at some point (just as those that describe the attributes of light), but that does not mean that it can 'observe' anything.

In some ways, space-time, matter and energy are all the same thing, but light has no mass and a constant velocity with respect to all reference frames. Therefore, attempting to ascribe a reference frame to it makes no sense.

 

[ghwellsjr]

Photons always travel at c, by definition. The photons that come from a light bulb had no existence prior to their emmision when the light bulb gave up a packet of energy for each one and they end their existence when they hit something and that same exact amount of energy is absorbed by the target. It's possible for the target to re-emit a different photon some time later at a different wavelength and with a different amount of energy, but that is a new photon that may have gotten its energy from parts of many different photons.

so energy cannot be created or destroyed but particles can?

If you have the right equipment, you can create particles with mass from energy or you can use some or all of the mass of particles to create energy. For example, it is possible to create an electron and a positron from pure energy. These two particles have the same mass the only difference being that the positron has a positive charge on it. However, the positron will soon be attracted to an electron (maybe the same one that was earlier created with it or a different one, it doesn't matter) and the two will annihilate each other giving up all the energy that was originally use to create them. This is an extreme example to show the conversion between mass and energy but there are many other examples where this is happening all the time. Whenever an atom or molecule emits a photon, it loses some mass. Whenever an atom or molecule absorbs a photon, its mass increases.

 

[mibaokula]

If you have the right equipment, you can create particles with mass from energy or you can use some or all of the mass of particles to create energy. For example, it is possible to create an electron and a positron from pure energy. These two particles have the same mass the only difference being that the positron has a positive charge on it. However, the positron will soon be attracted to an electron (maybe the same one that was earlier created with it or a different one, it doesn't matter) and the two will annihilate each other giving up all the energy that was originally use to create them. This is an extreme example to show the conversion between mass and energy but there are many other examples where this is happening all the time. Whenever an atom or molecule emits a photon, it loses some mass. Whenever an atom or molecule absorbs a photon, its mass increases.

in other words, energy is not created or destroyed etc. if there is net no energy, there must be a particle of positive energy and then a particle with negative energy to balance out (perhaps not the best example).
if you believe in the big bang (the universe came from nothing), why have we not been annihilated yet?

 

[ghwellsjr]

in other words, energy is not created or destroyed etc. if there is net no energy, there must be a particle of positive energy and then a particle with negative energy to balance out (perhaps not the best example).
if you believe in the big bang (the universe came from nothing), why have we not been annihilated yet?

I hope you're not thinking that my example of creating a pair of massive particles worked because one of them had positive energy and one of them had negative energy, no, they both have exactly the same positive energy.

You should ask your question about the big bang on another forum.

 

[mibaokula]

I hope you're not thinking that my example of creating a pair of massive particles worked because one of them had positive energy and one of them had negative energy, no, they both have exactly the same positive energy.

You should ask your question about the big bang on another forum.

yeah, forget about that. I'm digressing from the original question
if two bodies are moving at 99.9999999999999999999% the speed of light (hence they fit into Einstein's idea of reference frames and have inertia/mass) in the opposite direction, what happens

and someone said that light can only travel at the speed of light in any reference frame; what about refraction when light is slower in a denser medium

 

[bernsten69]

yeah, forget about that. I'm digressing from the original question
if two bodies are moving at 99.9999999999999999999% the speed of light (hence they fit into Einstein's idea of reference frames and have inertia/mass) in the opposite direction, what happens

and someone said that light can only travel at the speed of light in any reference frame; what about refraction when light is slower in a denser medium

There is a problem with your first statement that Special Relativity explains. Two bodies cannot be moving away from each other, each at 99.9999999999999999999% the speed of light. If this were possible, then light from one person shot at the other would never reach him. The speed of 'recess' between these two bodies is not the SUM of the speeds that they are traveling at with respect to a fixed reference frame (their origin/earth for example). If they both left from Earth going away in opposite directions at 99.9999999999999999999% the speed of light, if you were standing on earth, you would see each individual speeding away at exactly that speed (calculable from their doppler shifts). However, when you CHANGE your reference frame from that of Earth to that of one of the travelers, Special Relativity has a specific transformation that must be applied to what you see when you look at the other person's speed (via their doppler shift) (they do not add in the classical sense).

See http://www.desy.de/user/projects/Physics/Relativity/SR/velocity.html for a simple explanation and formula to answer your question.

If you ask WHY this happens, mathematically, it follows from the assumption that the speed of light is constant in Any reference frame. If you look up the derivation of Special Relativity, you will see how it follows from that base assumption.

The Speed of light is constant in any reference frame. In a dense medium, the speed of light is locally slowed, but the reference frame invarient remains the same. If you want to discuss what happens at the quantum level to 'slow' light, I am about 10 years behind in QFT (etc.), so someone else can better explain that effect. (I've since switched fields). However, slowing light with quantum effects does not change any of the effects of special relativity (as far as I recall).