# Speed of light: making non-Newtonian physics tangible

1. Jun 17, 2011

### dotancohen

I am trying to understand the implications of a fixed speed of light, mostly to explain it to my nieces and daughters.

The distance to Aldebaran is 65 light years. I understand that means that it's distance is equal to the distance that light would travel in a vacuum in 65 years. But how much time passes for the photon? Did it experience 65 years of travel? Assuming that a hypothetical massless clock were traveling with the photon, would that clock have registered 65 years?

Also, people like to say that the light that we are seeing left Aldebaran 65 years ago. I understand that this is incorrect, rather, that the light left now (in our frame of reference). Does that imply that the photon felt the time pass between Aldebaran and here instantly? Id est, a massless clock on that photon would have measured no time at all?

And if that photon were to glint in my daughter's eye and bounce back towards Aldebaran, how much time would have passed on Aldebaran between the photon's departure and it's return? Due to the motion of celestial objects, would the photon have to travel a different path (i.e., be bounce back at an angle!=0 degrees) or would reflecting it back in the direction that we see Aldebaran get it back home?

Thanks.

2. Jun 17, 2011

### tiny-tim

hi dotancohen!
if the photon uses our kind of clock, it would register no time at all … our kind of clock goes slower as the clock moves faster, and although no clock could move at the speed of light, it could get as close as we like to the speed of light, and so the rate of the clock could get as close as we like to zero

(in practice, the photon would choose to measure time by counting wave lengths )
no, it's correct, the light did leave Aldebaran 65 years ago in our frame of reference.
well, we're moving at 18 miles per second round the Sun, and the Sun is moving …

so it would be a very slightly different direction, same as throwing a ball from a train and throwing it back again …

but the difference in angle would be far smaller than the spread of even a laser beam!

(and it's 65 years each way, in either our frame or Aldebaran's frame)

3. Jun 17, 2011

### dotancohen

Hi!

I see, so the photon would have felt that it passed instantly from Aldebaran to Earth? Does that imply that the photon is on every point of a ray at the same instant?

So that follows that the photon would have passed 0 wavelengths? Somehow this does not seem right to me.

Oh. Are we seeing then Aldebaran as it was 65 years ago, or as it is now (in our time of reference)?

Erik Idle would like to have a word with you!

I was referring to the motion of Aldebaran, not the Earth's motion.

But how much time passed on Aldebaran in the meantime? 130 years?

Thanks!

4. Jun 17, 2011

### Janus

Staff Emeritus
From the Photon's point of view the distance between Earth and Aldebaran would have contracted to no distance at all. This is if the photion could be said to have a point of view. In reality it can't. Photons do not have a valid frame of reference from which we van make these types of determinations.
As it was 65 yrs ago.
It doesn't really matter, all that mattewrs is the relative difference in speed between the Earth and Aldebaron, regardless of which one you consider as moving.

Yesw, 130 yrs.

5. Jun 17, 2011

### tiny-tim

hi dotancohen!
if the photon is silly enough to use our type of clock, then it must use a clock which has stopped, so yes according to a stopped clock the photon passes instantly from Aldebaran to Earth, and is on every point at the same instant

how would it feel?

it would feel that using a stopped clock is stupid, so it would use a clock that made sense, ie it would count wavelengths!
no, it would take 0 time on a stopped clock, and a very long time if it counts wavelengths

6. Jun 17, 2011

### dotancohen

I take this to mean that the clock would have stopped, not that it is a stopped clock (i.e., there is nothing intrinsically preventing the clock from ticking). Since the photon could count it's wavelengths could it in fact measure the distance from Aldebaran to Earth, even though it could not measure the time?

7. Jun 17, 2011

### PAllen

This discussion of photonic experisnce is all a bit silly, but I'll bite: Ok, how does a photon count wavelengths? If we're going to mix classical and quantum terminology, my understanding is that a given photon contributes to the amplitude of one wavecrest. Therefore, it is in no position to count any wavelengths. You can count wave arrivals at a given detector (intersection of null cones (for each wave crest) with a timelike world line); or you can count wavelengths along a path of simultaneity.

8. Jun 17, 2011

### tiny-tim

if it can measure the distance, then it can measure the time … all it has to do is to multiply the distance by one, and even the dimmest photon (and believe me, individual photons are really dim ) can do that!
ah, i never said it would count its own wave lengths!

it would use a standard frequency … blue would give it "imperial", and red would give it "metric"

9. Jun 17, 2011

### PAllen

OK, here is my perception of the photon view of existence. The photon does not live in spacetime at all, or a manifold, or any set with any type of metric structure (no measure of distance or time exists). It lives in an ordered set of cardinality aleph-c, consisting of the events from its emission to its absorption. The order is causality order.

Last edited: Jun 17, 2011
10. Jun 18, 2011

### dotancohen

So we could say that today Aldebaran is different than how we see it? I ask because I seem to remember it being mentioned in Physics 3 (a course that I took years ago) that in our frame of reference things are as we see them today (for us). I am referring to light cones and simultaneity and such.

I see. I just read the Twin Paradox article on wikipedia and I now understand that a theoretical massless clock on the photon would have experienced 0 time, and an identical clock on Aldebaran would have experienced 130 years.

11. Jun 18, 2011

### tiny-tim

no, that's definitely wrong

in our frame of reference things are never as we see them today, we always see things in our own past
for a photon going from Aldebaran to Earth and back, yes