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Jun 20th 2019, 11:54 AM   #11
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 Originally Posted by topsquark One of the main principles of SR is that all observers measure the speed of light to be the same: c. So the relative speed between a photon's frame and any other (slower than c) will be c. -Dan
Yes, but what about relative velocity?

Jun 20th 2019, 12:17 PM   #12

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 Originally Posted by benit13 Yes, but what about relative velocity?
The relative velocity between a particle moving at the speed of light and any other particle moving less than the speed of light will be c. This is what sparked the Relativity thing.

-Dan
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Jun 20th 2019, 12:19 PM   #13
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 Originally Posted by topsquark The relative velocity between a particle moving at the speed of light and any other particle moving less than the speed of light will be c. This is what sparked the Relativity thing. -Dan
The direction is what I'm trying to get at. Sure, every photon that literally collides with you will be coming directly towards you at a speed of c, but what information does that provide you about the emitter? If the emitter is also travelling at speed c, it's relative velocity differs depending on its direction of travel and you can perform Doppler imaging to find out whether the direction is towards you or away from you.

 Jun 20th 2019, 05:11 PM #14 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 398 four men vs three natures The questions about photon will be chaotic forever with thought in only 3D space. @wavesearcher , Woody: From the viewpoint of photon any 3D space point is the same ( 3D space ineffective in another guy ****9876's word), the direct result is constant velocity movement in 3D space ineffective, in turn velocity of light is a constant in any inertial frame in SR. But when you observe cosmos, you are from your viewpoint (your angle) and you are not photon in nature. @Dan: Why you try to escape C vs C? It's true in nature, for example, one photon vs another photon. @benit: C vs C is also C, direction ineffective... but it's not this guy who can tell more...haha Last edited by neila9876; Jun 20th 2019 at 05:14 PM.
 Jun 20th 2019, 06:19 PM #15 Junior Member   Join Date: Jun 2019 Posts: 8 I am thinking of something like this. When a photon leaves the surface of the Sun, emitted by say a helium atom there, it takes almost 8 minutes by our time for it to reach the Earth and be absorbed by an atom of a leaf perhaps. But to the photon it took no time and traveled no distance so its being emitted from an atom on the sun and absorbed here by a leaf is one spacetime event. In fact I wonder from the point of view of light has any time passed since the big bang and has the universe expanded?
 Jun 20th 2019, 07:51 PM #16 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 398 space vs time @wavesearcher: Time is more complicated and secret than space, so I often put it aside temporarily. I care more about how much time left for me this life because of the absolute suppression and sealing off and disruption and disturbing by the local dark force (江门地方黑恶势力钟永康)...I am in bad health... In respect of cosmos, when I see threads elsewhere talking "how big is universe?", "what's outside universe?", etc...I reply: with only xyz 3D space thought, someone might think to die and not able to understand it.
Jun 21st 2019, 03:04 AM   #17
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 Originally Posted by neila9876 @benit: C vs C is also C, direction ineffective... but it's not this guy who can tell more...haha
No one is arguing that the relative velocity will have a magnitude less than c.

The Lorentz transformations as most people know them (e.g. on wikipedia: https://en.wikipedia.org/wiki/Lorentz_transformation) assume that the moving frame travels along the x-axis relative to the other one, so the only term that changes is the x-axis term. If an observer is looking at several objects which are all travelling at speed c but in different directions, then one cannot just blindly apply the same equation for all frames of reference because the frames are no longer just moving in the x-axis; it's a 3D problem, not a 1D problem. To solve the situation, you have to translate the axes each time so that for each object the "x-axis" corresponds to the line following the object's velocity.

Time and space doesn't just become meaningless because $\displaystyle \gamma$ has the same value.

Jun 21st 2019, 03:54 AM   #18
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 Originally Posted by wavesearcher I am thinking of something like this. When a photon leaves the surface of the Sun, emitted by say a helium atom there, it takes almost 8 minutes by our time for it to reach the Earth and be absorbed by an atom of a leaf perhaps. But to the photon it took no time and traveled no distance so its being emitted from an atom on the sun and absorbed here by a leaf is one spacetime event. In fact I wonder from the point of view of light has any time passed since the big bang and has the universe expanded?
Why would a photon "travel no distance"? It travelled 1 AU to get to Earth from the Sun. From the photon's perspective it travelled 1 AU and did so in 8 minutes.

I think the misconception is the idea of "time stopping" which is often used in descriptions of special relativity. When an object moves at the speed of light, it will still have a clock and the clock will tick. From your perspective, it will tick just the same as any normal clock; nothing special happens just because you're travelling at the speed of light. However, consider also some external stationary observer with their own clock. They will observe that your clock is not ticking compared to theirs.

There are modified Lorentz transformations for photons, but I don't have them to hand.

Last edited by benit13; Jun 21st 2019 at 04:20 AM.

Jun 21st 2019, 04:00 AM   #19

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 Originally Posted by benit13 Why would a photon "travel no distance"? It travelled 1 AU to get to Earth from the Sun. I think you're under the impression that travelling at the speed of light is somehow going to make everything irrelevant. That's not true.
That has to do with something I mentioned. The photon doesn't "see" that it has moved any distance. The Fitzgeral contraction shrinks the distance to 0. However according to any outside observer traveling at less than the speed of light (relative to any frame you want) the photon does travel the full distance.

-Dan
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 Jun 21st 2019, 04:48 AM #20 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 398 @no one: How big is universe? What's outside universe?

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