# Questions on light

1. Aug 1, 2008

### epkid08

What is light if it has nothing to reflect off of? It's not observable. Can you even consider light something in the first place, as it has zero mass and no length. Is light equal to the progression of time? If you put a traveling photon through the time dilation equation, you get 0/0, what does this mean?

2. Aug 1, 2008

### CompuChip

At first sight this question looks analogous to "does a falling tree produce sound when nobody is there to hear it?". "Light" consists of photons, which you can view as packages of energy. Just like any other particle, basically. In fact it's quite easy to detect, as we can just build "photon detectors", unlike many other particles which we can only "see" indirectly (e.g. through their decay into other particles) but which we also believe to "exist".

It has zero rest mass, but it does carry energy from one place to another. It has "no length" but the same can be said of, for example, and electron.

No comment. Not because I don't want to help you, but because I don't want to get into the philosophical discussion "what is time?"

This means nothing, because it's not allowed. The time dilatation equation only holds for massive objects, which - as you said - the photon is not. (Technically the reason is that the equation is derived by transforming from, for example, your rest system to the rest system of the "moving" object, but since you are massive and must stay below the speed of light there is no transformation that will take you to the frame of something massless that must stay at the speed of light - or, for that matter - something travelling faster).

Last edited: Aug 1, 2008
3. Aug 1, 2008

### HallsofIvy

Why would you think light is not "observable if it has nothing to reflect off of" If you look directly at a light bulb you are seeing light that has not reflected off anything.

4. Aug 1, 2008

### epkid08

I'd say it reflects off of the bulb/filament/air/etc. If light was moving through space, you wouldn't be able to see it until it reflects off of something.

5. Aug 1, 2008

### epkid08

I thought electrons had microscopic length and mass.

This is my most important question. I thought time was a real dimension, a four dimensional distance between two volumes, or something like that. I wouldn't say it's a matter of philosophical discussion anyways, there is a right or wrong answer to it. I just need to know if light travels at a speed that equals the progression of time; I don't want a crazy, "Light is time" answer, just something that says whether or not there is a real distance, infinite distance, or zero distance from c to progression of time.

Last edited: Aug 1, 2008
6. Aug 1, 2008

### CompuChip

Indeed the have a mass, and if you'd have to call that "microscopic" depends on your definition of microscopic (I suppose if you have an electron microscope, it would :tongue:). You probably learned to see the electron as a small hard sphere. In modern physics, however, that view has been abandoned for quite some time (although we sometimes still use it for convenience) and we cannot assign a "length" to elementary particles. Of course they have some spatial extent, but, in fact "since quantum mechanics", this is not well-defined.

Time is a dimension in the same sense as the three dimensions we live in, with the difference that we seem to be stuck in one direction . For example, if you want to uniquely identify some event, you would not only have to say where it happened, but also when it happened. And special relativity shows us for example that spatial distances can shrink a little and time durations can stretch a little (we can "rotate" the time and space dimensions into each other partially, just as you can rotate the space dimensions themselves into each other).

I still cannot really answer your question because I don't know exactly what you mean by "progression of time" and what you mean by that last part of the sentence. "c" is a velocity (units, for example, m/s) or - sometimes - a number. What is the distance of 12 km/h to progression of time? Probably our problem is that I try to take everything you say strictly in the physical/mathematical sense. But - hey, what do you expect on a physics forum ;)

7. Aug 1, 2008

### HallsofIvy

Again why not? If I were in space also, looking directly at a star, I would "see" any light that went into my eye.

(And if you are going to complain that, in that case, it has "reflected" off the lens of my eye or retina, the I would have to conclude that you misunderstand the word "reflect". A photon originating in the filment of a lightbulb is not "reflecting" off it. A photon passing through glass, such as the bulb itself, is not "reflecting" off it.)

8. Aug 1, 2008

### schroder

Here is one way to look at it: Just as energy and mass are equivalent, so are space and time. Mass represents potential energy and time represents potential space. As mass releases energy, time releases space. If mass and time were to be depleted, we would have infinite space and infinite energy, the opposite of the singularity which was infinite time and infinite mass. At the present time, we have a mixture of energy and mass, space and time.

9. Aug 1, 2008

### epkid08

With all due respect, it does reflect off our retina, if anything. When I made that comment, I was mainly speaking about seeing a passing stream of light, as it travels; It wouldn't be traveling into our eyes, which was not my point, but it holds true. My point was that if light has nothing to reflect off of, i.e. an object, our retina(ultimatly our retina in all cases, humanly speaking), we wouldn't be able to see it.

As for the light bulb, a photon passing through glass leaves little reflection behind, but reflection none the less. (otherwise we couldn't tell where glass is, which we obviously can)

10. Aug 1, 2008

### epkid08

This is one take on it that I've struggled to come to. It basically says that time is not a physical dimension, but just a consequence of motion. How could one prove one or the other?

11. Aug 1, 2008

### Staff: Mentor

For you to see something, light must enter your retina and be absorbed, not reflected.

12. Aug 1, 2008

### epkid08

What I mean by that comment is that, if you see an object, light has reflected off of an object and made it back to your retina. Whether or not the image is 'reflected' or 'absorbed' really makes no difference to my point.

13. Aug 1, 2008

### CompuChip

Do you mind if that doesn't make any sense whatsoever to me?

That's usually the case, but it needn't have reflected off the object. It can also have originated at the object, as is the case with the sun and a light bulb, both producing photons. Most objects we see indeed don't generate photons, but they just reflect those coming from some (other) light source.

14. Aug 1, 2008

### octelcogopod

A photon is a particle, whether or not it can be observed is totally up to the perceiver.
Our eyes have adapted to absorbing photons and processing them..
It should be considered to be 'something' because it IS something..

Also, i consider 'time' to be nothing more than change.
I don't quite see the corrolation between light and time however.
You could say that light is the fastest the pace of time can go though, because no matter or energy can go faster than light, so /change/ will then happen no faster.

Your questions were a bit diluted to me, but I tried my best.

15. Aug 1, 2008

### schroder

I don’t mind at all. That reaction is to be expected when speaking about infinity of anything. However, most people are able to come to terms with the concept of a singularity which represents an infinite mass contained in no space and exists in infinite time, since there can be no change in such a situation. If you can manage to come to terms with that, you should be able to also come to terms with the opposite, which is an infinite energy spread out in an infinite space. The infinite mass has been translated into infinite energy, which is easy to understand since we know that mass and energy are equivalent. It is a bit more difficult to comprehend infinite time being translated into infinite space, but time and space are also equivocal in a similar way as mass and energy.

16. Aug 1, 2008

### CompuChip

As a side note, our eyes are only sensitive to photons very specific energies. We cannot see outside the visible spectrum, so we need machines to detect and visualise infrared, ultraviolet, x-ray, radio and all other EM radiation.

Schroder, I don't think it's the concept of infinity I'm having trouble with. It is the apparent equivalence of time and "space" that I don't get, and which you obviously find analagous to the equivalence of mass and energy. Indeed, one can convert between them using the same constant: $m \to E c^{-2}, d \to c t$. Usually when you do this you use light years as a unit. I still fail to see how "mass releases energy", "time releases space", whether you are trying to describe the Big Bang which supposedly started our universe and are asking whether there is a similar ending in which the universe is infinitely large and asymptotically non-expanding (IIRC that's just the same as: is our universe flat?) and finally, what all of this has to do with light.

17. Aug 1, 2008

### Staff: Mentor

That's quite different from having the light reflect off the retina. (Of course, the light from the object that reaches your eyes could be reflected light from some other source or emitted from the object itself.)
If you expect others to understand what you write, you'd be wise to pay attention to such differences in meaning.

18. Aug 1, 2008

### Kenny_L

In starwars, the laser bolts from tie fighters and x-wings can be seen from side on. Anyway, we still don't understand 'energy'. Right now, we just define things like 'particles' and 'waves' because we observe certain things acting as a discrete bundle or unit when moving around, while other things creates some other kind of propagating distortion effects.

And there's other things that we can't 'see'.....like the phenomena that provides gravity effects. And magnetic fields etc.

19. Aug 1, 2008

### schroder

“This is my most important question. I thought time was a real dimension, a four dimensional distance between two volumes, or something like that. I wouldn't say it's a matter of philosophical discussion anyways, there is a right or wrong answer to it. I just need to know if light travels at a speed that equals the progression of time; I don't want a crazy, "Light is time" answer, just something that says whether or not there is a real distance, infinite distance, or zero distance from c to progression of time. “

And you replied:

“Time is a dimension in the same sense as the three dimensions we live in, with the difference that we seem to be stuck in one direction. For example, if you want to uniquely identify some event, you would not only have to say where it happened, but also when it happened. And special relativity shows us for example that spatial distances can shrink a little and time durations can stretch a little (we can "rotate" the time and space dimensions into each other partially, just as you can rotate the space dimensions themselves into each other).”

So it seems to me that we are talking here about more than just “light”. But if you feel empowered to limit the range of this discussion, so be it.

20. Aug 2, 2008

### Kenny_L

You don't need to prove it. You just know it, because humans just defined 'time' by measuring something that's related to motion.....such as how 'long' you have to sit around for when something rotates a full cycle around something. Or how long it takes for something to rotate some fraction of that full cycle etc. And eventually, people started using smaller and smaller units of reference for this 'waiting' ....... such as how 'long' it takes for X amount of transitions of some gas atoms between two energy levels etc. Basically, we just use this convenient thing that we defined for referencing, and amazingly it has been an extremely important reference for our society.