I have a question about physics, specially photons and the speed of light. I've heard it said, put roughly, that when traveling at the speed of light, as a photon does, there is no time. So from the point a photon is emitted to when it is absorbed by an object, no matter how much time appears to pass from our frame of reference, from the standpoint of the photon, no time at all has passed. The emission to absorption by another object is for a photon instantaneous. Is that roughly correct? My next question, then, is how do we know the age of the Universe? I thought it was partly because the farthest light we can see is 13.8 billion light years away. But then, is that only from "our frame of reference"? How does that work from the standpoint of a photon?

Yes, on a frame of reference that's moving at the speed of light, time does not advance.
But time advances on any frame of reference that doesn't move at the speed of light.
And, since this speed is finite, we can measure an age, or the time that photons take to go from point A to point B.

Ok thanks poc, that makes a lot of sense.

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So about past and future then... a photon that is emitted today, say from a star 30 light years away from us, it instantaneously reaches the observers on Earth 30 years from now (from our standpoint), in some sense?

(April 25, 2014 at 1:49 pm)Pickup_shonuff Wrote: Ok thanks poc, that makes a lot of sense.

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So about past and future then... a photon that is emitted today, say from a star 30 light years away from us, it instantaneously reaches the observers on Earth 30 years from now (from our standpoint), in some sense?

From the photons perspective the journey was an instant. From our perspective, the journey took 30 years. The concept of "now" is not universal. From the photon perspective the time it left is now. From you perspective the time it left was 30 years ago.

Yeah my brain is trying very hard to "make sense" of that. My brain isn't very good at it though.

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"The photon's perspective" is a weird idea in of itself.

(April 25, 2014 at 2:13 pm)Pickup_shonuff Wrote: Yeah my brain is trying very hard to "make sense" of that. My brain isn't very good at it though.

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"The photon's perspective" is a weird idea in of itself.

It helps to think of the universe...past, present, and future as a single object. "Now" can be defined as all the events on a line that passes through that object. Perspective is just looking at a particular line.

(April 25, 2014 at 2:13 pm)Pickup_shonuff Wrote: Yeah my brain is trying very hard to "make sense" of that. My brain isn't very good at it though.

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"The photon's perspective" is a weird idea in of itself.

What part does not make sense to you?

(April 25, 2014 at 2:42 pm)Chuck Wrote:

(April 25, 2014 at 2:13 pm)Pickup_shonuff Wrote: Yeah my brain is trying very hard to "make sense" of that. My brain isn't very good at it though.

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"The photon's perspective" is a weird idea in of itself.

What part does not make sense to you?

Basically that light travels at a definite and finite speed and at that speed time ceases to be.

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Isn't it also said that space is expanding faster than the speed of light? So what does THAT mean in relation to time?

(April 25, 2014 at 4:24 pm)Pickup_shonuff Wrote: Isn't it also said that space is expanding faster than the speed of light? So what does THAT mean in relation to time?

It means that from our perspective, photons emitted from regions of space expanding faster than the speed of light will never reach us.

From the perspective of the photons, they already did.

Put that in your pipe and smoke it.

I've been wondering about constraints on the emission of photons. Like if a particular photon is destined to never be absorbed, it may not be possible for it to be emitted in the first place.

Is that just theoretical, or is it 'true' ??