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Author Topic:   Request for help re light speed, emission of light, and time
vimesey
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Posts: 919
From: Birmingham, England
Joined: 09-21-2011
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 (1)
 Message 1 of 9 (655067) 03-06-2012 11:04 AM

I was wondering if I could ask some of our resident scientists to talk me through something which is causing me some confusion.

I have seen it explained in the past, on this site (although finding this will take more time than I have, so sorry there's no link), that whilst from, say, our perspective, it takes a photon of light 8 minutes to travel from the sun to the earth, from the perspective of the photon, it takes no time at all. It is travelling spatially at the speed of light, and therefore from its perspective its movement through the dimension of time is zero (since spatial velocity plus velocity in time cannot exceed C). (In fact, it is always equal to C, for everything).

(I am no scientist, so apologies for any awful expressions here).

What I find confusing therefore, is that in certain circumstances, light can be seen to act as a wave. This indicates to my simple way of thinking that a photon of light travelling in direction x spatially, will during that journey alternate between travelling in a positive and then a negative and then a positive y direction, and so on.

If its y direction during its journey alternates between positive and negative, does this not mean that the photon of light is undergoing a number of changes in state during its journey ?

And if this is the case, how is that reconciled with (from the perspective of the photon) no time passing during its journey ? In order for its state to change, does this not necessarily mean that time is passing from its perspective ? A change in state requires time to pass, doesn't it ?

Can somebody please tell me where I am going wrong ?

Many thanks.

vimesey

Edited by vimesey, : Had another thought

 Replies to this message: Message 3 by Taq, posted 03-06-2012 7:32 PM vimesey has not yet responded Message 4 by kbertsche, posted 03-06-2012 8:09 PM vimesey has not yet responded

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 (1)
 Message 2 of 9 (655069) 03-06-2012 7:15 PM

Thread Copied from Proposed New Topics Forum
Thread copied here from the Request for help re light speed, emission of light, and time thread in the Proposed New Topics forum.
Taq
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 (1)
 Message 3 of 9 (655070) 03-06-2012 7:32 PM Reply to: Message 1 by vimesey03-06-2012 11:04 AM

 What I find confusing therefore, is that in certain circumstances, light can be seen to act as a wave. This indicates to my simple way of thinking that a photon of light travelling in direction x spatially, will during that journey alternate between travelling in a positive and then a negative and then a positive y direction, and so on.If its y direction during its journey alternates between positive and negative, does this not mean that the photon of light is undergoing a number of changes in state during its journey ?

If I understand it correctly, the wave function of light is a probability function (the chance of finding a photon within the area defined by the wave). Once a photon interacts with something then this wave function collapses to just one probability.

In Young's double slit experiment the photon actually "passes through" two slits at once and interferes with itself producing the interference patterns on the detector. I think it is incorrect to picture the photon physically being in the crest or trough at any one time. The photon is actually at the crest and the trough at the same time.

 This message is a reply to: Message 1 by vimesey, posted 03-06-2012 11:04 AM vimesey has not yet responded

kbertsche
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Posts: 1427
From: San Jose, CA, USA
Joined: 05-10-2007

 (1)
 Message 4 of 9 (655073) 03-06-2012 8:09 PM Reply to: Message 1 by vimesey03-06-2012 11:04 AM

quote:
I have seen it explained in the past, on this site (although finding this will take more time than I have, so sorry there's no link), that whilst from, say, our perspective, it takes a photon of light 8 minutes to travel from the sun to the earth, from the perspective of the photon, it takes no time at all. It is travelling spatially at the speed of light, and therefore from its perspective its movement through the dimension of time is zero (since spatial velocity plus velocity in time cannot exceed C). (In fact, it is always equal to C, for everything).

Not quite. From our perspective, the photon takes 8 minutes to travel from sun to earth. But also from our perspective, the photon does not age. If the photon carried a clock, the photon's clock would not move --from our perspective.

From the photon's perspective, the distance between sun and earth is contracted to zero, and the photon's trip happens instantly.

Edited by kbertsche, : Replaced second paragraph, which was wrong.

 This message is a reply to: Message 1 by vimesey, posted 03-06-2012 11:04 AM vimesey has not yet responded

 Replies to this message: Message 5 by PaulK, posted 03-07-2012 5:20 AM kbertsche has responded

PaulK
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 (1)
 Message 5 of 9 (655082) 03-07-2012 5:20 AM Reply to: Message 4 by kbertsche03-06-2012 8:09 PM

I don't think that's right. Shouldn't the distance between the Earth and the Sun be different in the photon's perspective (compared to ours), due to length contraction ?
 This message is a reply to: Message 4 by kbertsche, posted 03-06-2012 8:09 PM kbertsche has responded

 Replies to this message: Message 6 by NoNukes, posted 03-07-2012 6:17 AM PaulK has not yet responded Message 7 by kbertsche, posted 03-07-2012 10:07 AM PaulK has not yet responded

NoNukes
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 (1)
 Message 6 of 9 (655084) 03-07-2012 6:17 AM Reply to: Message 5 by PaulK03-07-2012 5:20 AM

 Shouldn't the distance between the Earth and the Sun be different in the photon's perspective (compared to ours), due to length contraction

Generally speaking, it is not all that helpful to consider things from the photons frame of reference, when the photo is propagating in a vacuum. But if we did so, the Lorentz-Fitzgerald equations suggest that length contraction would reduce the earth/sun distance to zero, which would mean that in the photon frame of reference, the photon would be in all places along its path at a single instant. However in such a frame, we could not verify that Einstein's postulate that the speed of light in all reference frames was c actually holds for the photon, making application of Einstein's special relativity rather problematic.

Besides that, I don't believe the question is about quantum mechanics. The wave function itself is complex, but the probability densities of a photon appearing at any one point in space are always positive. Perhaps this is what Taq is saying.

When light propagates, what is actually waving, and occupying positive and negative values are the coupled electromagnetic and magnetic fields associated with the radiation, and not the position of the photon. The concept of a photon traveling up and down some y or z axis as it propagates along the x axis is just wrong.

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 This message is a reply to: Message 5 by PaulK, posted 03-07-2012 5:20 AM PaulK has not yet responded

kbertsche
Member (Idle past 143 days)
Posts: 1427
From: San Jose, CA, USA
Joined: 05-10-2007

 Message 7 of 9 (655098) 03-07-2012 10:07 AM Reply to: Message 5 by PaulK03-07-2012 5:20 AM

Yes, I think PaulK and NoNukes are correct. I'll fix my original post. (Sorry, I've been under the weather and didn't think straight.)
 This message is a reply to: Message 5 by PaulK, posted 03-07-2012 5:20 AM PaulK has not yet responded

 Replies to this message: Message 8 by vimesey, posted 03-08-2012 7:29 AM kbertsche has not yet responded

vimesey
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Posts: 919
From: Birmingham, England
Joined: 09-21-2011
Member Rating: 7.2

 (1)
 Message 8 of 9 (655169) 03-08-2012 7:29 AM Reply to: Message 7 by kbertsche03-07-2012 10:07 AM

Thanks guys - I think you've shown me where I'm going wrong - I'm thinking in Newtonian/Euclidian terms, in a quantum environment.

Next job - to get my mind around the quantum analysis ! :-)

 This message is a reply to: Message 7 by kbertsche, posted 03-07-2012 10:07 AM kbertsche has not yet responded

Son Goku
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Posts: 1120
From: Ireland
Joined: 07-16-2005
Member Rating: 9.2

 (1)
 Message 9 of 9 (656920) 03-23-2012 6:12 AM

Others have already said all this, but I'll offer what I can.

Firstly the photon doesn't have a point of view, it's not possible to describe spacetime/the universe in terms of a coordinate system at rest with respect to the photon, because nothing can be at rest with respect to the photon.

If you attempt to, you'll see that the photon views all events along its path as simultaneous and coterminous, so its point of view can't be used to describe anything.

As other has mentioned the wave behaviour of the photon is its probability wave, known as the wave-function. The values of this wave are complex, you can think of them as a little 2D arrow at every point. The square of the length of this arrow is the probability to find the photon at that point. This wave of probabilities can evolve and change.

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