C, photons, infinity (and beyond!)

[Xamonas Chegwé]

Something has been puzzling me since Mistermack started his arguments about infinity not existing.

This is probably all wrong in some subtle way - or it can't be answered in any meaningful way - but I figured it makes an interesting thought exercise in any case. Here is a question for the relativity-heads.

Photons of different energies are continually being produced from all sorts of sources and flying off, at c, in all directions. Eventually, they collide with something and are absorbed or reflected - those that don't hit anything just keep going. They have no mass, hence no inertia, no friction, nothing to stop them or slow them down. There are photons 'out there' that have been travelling since immediately after the big bang and that will carry on travelling long after the sun has shone its last. With me so far? Hope so, that was the easy bit!

Now lets think about what happens to anything travelling at c. From our frame of reference (always assuming that we could 'observe' those photons without interracting with them - which I know is impossible) they are travelling at 3 x 108 ms-1 in all directions. However from the 'point of view' of a photon, the universe is infinitely compressed in the direction of its travel and has slowed to a stop. The photon 'experiences' no time at all and no distance travelled between its creation and its destruction - its birth and death are simultaneous.

So what of the photons that are still travelling, with nothing on a collision course? If they travel 'forever' but seemingly instantaneously, have they arrived yet?

OK, that last question was a bit facetious but it is the essence of my quandary. We have an asymptote - two even.
1. Infinite time (from any other frame of reference) passing in no time at all (from the photon's POV.)
2. Infinite distance (again, from any other reference frame) shrinking to zero length as far as the photon is concerned.

Did I get that wrong? Or is it really that boggling? Is that really what the maths says?

(PS. Forgive me repeatedly adopting the intentional stance when referring to the photon - I know it doesn't actually have a point of view - it just helps make things simpler.)

[The Curious Squid]

Tagging this for my ego search, don't have any answers but want to see the responses.

[Xamonas Chegwé]

Tagging this for my ego search, don't have any answers but want to see the responses.

You're in the same boat as me then.

[The Curious Squid]

Tagging this for my ego search, don't have any answers but want to see the responses.

You're in the same boat as me then.

It's one of those things in physics that makes me feel stupid because I can make it work in my head.

[Twiglet]

If you are a photon, time isn't meaninful for you. No second on your clock ever ticks, your path through space describes a frozen instant in time. For an object which possesses mass, the speed of light is not a finite thing, but the definition of a limit...if you could supply an infinite amount of energy to speed your motion, what is the final speed which you would attain?

The speed of light is.

There's no "why" for that in physics, it's just what seems to be true.

The question is transcendental. The theoretical answer borders on relgious. Counterexplanations simply lack any evidence to support them. Nature is in search of description. Only humans are after an explanation.

[Tigger]

What does c mean?
Take life seriously.

[Xamonas Chegwé]

There's no "why" for that in physics, it's just what seems to be true.

Actually, the why for me is the same as my description of the photon's POV. At c time outside has stopped and space has shrunk to a 2-dimensional plane perpendicular to your line of travel. To exceed c would do what to that arrangement? Time would go backwards? Space would become negative?

Subjectively, you can travel faster than c. Because as you approach the limit, time, for you, dilates and space contracts - you can travel a light year in a few days if you approach the speed of light close enough. But from any other frame of reference, you are constrained by c. To any observer not travelling the same course, at the same speed, your ship is moving close to, but less than, the speed of light, it is squashed into an almost flat disk and you, inside it, are moving so slowly that you are practically stopped.

When you think of it this way, it becomes obvious to me why the energy requirement to get 'a little closer' to c increases the closer you are - it is because from your point of view, you are not adding just a tiny fraction of c to your speed, you are increasing it a hundredfold, a thousandfold, a billionfold, heading closer and closer to infinite speed.

As far as time not existing for a photon, I quite take your point. It is equivalent to my description - any journey made by a photon is instantaneous. I can accept this, counter-intuitive as it is. But it is the photon that never collides with anything that is bugging me. It has no time but it makes an infinite journey. It is the physics equivalent of dividing by zero to this mathematician!

[newolder]

It has no time but it makes an infinite journey. It is the physics equivalent of dividing by zero to this mathematician!

A photon experiences no time but occupies an infinite space. Space is like inverse time, 1/t as t → 0. It is the physics equivalent of mathematical duality as far as I can tell.

[mistermack]

What we don't know is the nature of spacetime. Is it nothing, or something?
Can a photon travel through nothing, or is spacetime some sort of enabling 'medium'?
If it is, is it expanding faster than c, and can it expand for ever? If spacetime is limited in it's expansion, then at some point all those photons are going to be encountering a wall of 'nothing' through which they can't travel. As they cannot be destroyed, the most likely thing is that they will be somehow reflected back towards their origin.

That's my speculation over, but I have a question. Where do all the photons in the cosmic microwave background come from? If they have been travelling in straight lines since the big bang, you would think that there would be none heading our way any more.
I know there is a solid answer to this, as it was prediced long before it was discovered. Why do we still encounter it, and why in all directions?
.

[Twiglet]

When you think of it this way, it becomes obvious to me why the energy requirement to get 'a little closer' to c increases the closer you are - it is because from your point of view, you are not adding just a tiny fraction of c to your speed, you are increasing it a hundredfold, a thousandfold, a billionfold, heading closer and closer to infinite speed.

Exactly so, and as thought experiments go - there are plenty of galaxies which are moving away from us at very close to c. We can measure how fast from a stationary position, then move away from them at terrestrial speeds and retake the measurement (say from a passenger jet) and see if the result looks different.

From your own perspective, relativity puts no constraint on your ability to go 1mph faster. You dont need to go through the mental process of imagining yourself getting ever closer to "c" when you can just quite reasonably say that we are moving away from stuff at very close to c right now - and it doesn't seem to affect our ability to speed up or slow down in whatever direction.

The profound oddities (for me anyway) are - why 3x10m/s? Why does that speed measure times wavefront and not half that speed or a quarter or whatever. Why is the physical constant as it is? The second is just how weird relativity is.. you can walk away from someone throwing a ball toward you and the relative velocity of the ball slows. Walk towards them and it speed. If you were measuring the speed of light arriving from a torch they were shining at you, it would be identical regardless of whether you were walking towards, or away from - the torch. That's really bloody odd.

[ChildInAZoo]

What we don't know is the nature of spacetime. Is it nothing, or something?
Can a photon travel through nothing, or is spacetime some sort of enabling 'medium'?
If it is, is it expanding faster than c, and can it expand for ever? If spacetime is limited in it's expansion, then at some point all those photons are going to be encountering a wall of 'nothing' through which they can't travel. As they cannot be destroyed, the most likely thing is that they will be somehow reflected back towards their origin.

That's my speculation over, but I have a question. Where do all the photons in the cosmic microwave background come from? If they have been travelling in straight lines since the big bang, you would think that there would be none heading our way any more.
I know there is a solid answer to this, as it was prediced long before it was discovered. Why do we still encounter it, and why in all directions?
.

We encounter the background radiation because the light that we see is from distant parts of the universe. The expansion of the universe was, far back in the past, much faster than the speed of light, so we can trace a light path back in time "straight" back to a region of the universe that emitted radiation at what is called "the surface of last scattering".

[Xamonas Chegwé]

From your own perspective, relativity puts no constraint on your ability to go 1mph faster. You dont need to go through the mental process of imagining yourself getting ever closer to "c" when you can just quite reasonably say that we are moving away from stuff at very close to c right now - and it doesn't seem to affect our ability to speed up or slow down in whatever direction.

Yep - that's fucking weird all right. I kind of get it sometimes - I understand that from the POV of that galaxy, I am virtually stationary and also incredibly close - space having been compressed into something very close to a 2-d plane and time (for it) having been dilated to a point where any motion by myself in its own direction (or directly away from it) is imperceptible. But once I try to factor in the fact that there are millions (if not more) of galaxies travelling in pretty much every direction possible at comparable speeds, it starts to get a little fuzzy to think about!

What's really fucking mind-boggling is that, just as you start to think you have a handle on special relativity, some cunt throws general relativity in your face!

And if you're really lucky. Really, really lucky. You might get a few minutes to allow that to sink in before an even bigger cunt - a cunt that can swallow black holes for fun - just happens to mention that you might be a little bit interested in something called... well, it begins with a Q - I think that is enough!

[colubridae]

If you think that’s weird wait till you get to the single photon through a double slit experiment.

It’s why I am so fond of the mighty cadre of philosophers. Thousands of years their intellects have they been exercising.

Not only have they failed to produce anything even vaguely useful, but they didn’t even get close to the real nature of the universe. Aren’t they just exquisitely charming.


I have run an experiment shining laser light through an image of an everyday scene made up of tiny dots, such as this

dots.jpg (92.02 KiB) Viewed 1762 times

The scene can be rendered whole without dots, simply by placing a suitably sized circular mask at the focus of the light beam. Fourier analysis in the x-y plane.

[The Curious Squid]

The double slit experiment does confuse me but nowhere near as much as the concept of photons travelling for billions of years and being timeless. My brain just can't make those ideas gel.

[colubridae]

The double slit experiment does confuse me but nowhere near as much as the concept of photons travelling for billions of years and being timeless. My brain just can't make those ideas gel.

different strokes.