# Thread: Light from stars really from billions of years ago?

1. Originally Posted by bilby
Photons travel at c, and therefore experience no time, and no distance.
People keep saying that, but I don't understand why.

2. Originally Posted by Wiploc
Originally Posted by bilby
Photons travel at c, and therefore experience no time, and no distance.
People keep saying that, but I don't understand why.
Maybe they say it because understand relativity and time dilation. t = t0/(1-v2/c2)1/2.

Then again, if they don't understand relativistic time dilation, maybe they are just repeating what they heard someone who does understand it has said.

3. Originally Posted by skepticalbip
Originally Posted by Wiploc
Originally Posted by bilby
Photons travel at c, and therefore experience no time, and no distance.
People keep saying that, but I don't understand why.
Maybe they say it because understand relativity and time dilation. t = t0/(1-v2/c2)1/2.

Then again, if they don't understand relativistic time dilation, maybe they are just repeating what they heard someone who does understand it has said.
What if the photon was in water (0.75c or 225,000 km/s)? Would it experience any time?

4. Originally Posted by excreationist
Originally Posted by skepticalbip

Maybe they say it because understand relativity and time dilation. t = t0/(1-v2/c2)1/2.

Then again, if they don't understand relativistic time dilation, maybe they are just repeating what they heard someone who does understand it has said.
What if the photon was in water (0.75c or 225,000 km/s)? Would it experience any time?
No. c is not just the speed of light in free space. c is determined by the permittivity and permeability of the medium you are talking about so electromagnetic radiation travels at c (as established by the medium it is traveling through). People generally talk about c being the speed of light in free space because they are generally talking about light traveling through free space.

5. Originally Posted by excreationist
Originally Posted by skepticalbip
Originally Posted by excreationist
So are you saying that it isn't meaningful to say the earth seems to be 4.5 billion years old or that the universe seems to be about 13 billion years old?
It is meaningful to us humans because we share an inertial reference frame but would be meaningless to a photon. Also some critter traveling at close to c with respect to us would see the Earth and universe as having a different age than we do.
Well I'm talking about experiments done by humans not conscious photons...
Photons don't need to be conscious for us to understand that they exist for zero time in their own reference frame, and that as a result it is incoherent to describe any event for a photon as 'delayed'. A photon is emitted by a distant quasar, and absorbed by whatever detector you are using, simultaneously in the photon's reference frame. In your reference frame, these events are separated by many billions of light years of space, and many billions of years of time, but that's irrelevant - neither frame is preferred, both are correct and mathematically equivalent, according to the rules of general relativity as set out by Einstein, Lorentz, and others. You can do all your calculations in whichever frame makes them easiest, or in whatever frame takes your fancy, and then convert to any other frame to see how the same circumstances would appear to an observer in that other frame.

The question 'how does the photon "know" when it is emitted what measurement will be made to it when it's absorbed?' is most easily answered by understanding that in the photon's frame, these are simultaneous events, so there's no mystery, and no strange effect of the future on the past - a photon has neither future nor past. We do; But we're not moving at c, so we are able to experience time and distance. Photons are, so they can't.

6. Originally Posted by Wiploc
Originally Posted by bilby
Photons travel at c, and therefore experience no time, and no distance.
People keep saying that, but I don't understand why.
Photons don't have time or space, because they travel at c. They travel at c because they don't have mass. They don't have mass because they have been excommunicated by the Pope.

(sorry, it's been a long day, and my seriousity has evaporated).

7. Originally Posted by DBT
It is relevant to QM, ie, objective p wave collapse as a way that matter/energy may possibly work.
https://landing.newscientist.com/dep...ion-feature-3/
It doesn't seem to have many fans and is "still at an exploratory stage".

Anyway the objective collapse theory rejects that the cat is simultaneously alive and dead - that it is either alive or dead before the observation....

I wonder how it would work with the quasar... that it would involve particles or waves.... from what I've read of objective collapse theory it could collapse to particles itself sometimes.... but that might not match the observations of the experimenter....

8. Originally Posted by bilby
.....The question 'how does the photon "know" when it is emitted what measurement will be made to it when it's absorbed?' is most easily answered by understanding that in the photon's frame, these are simultaneous events, so there's no mystery, and no strange effect of the future on the past - a photon has neither future nor past. We do; But we're not moving at c, so we are able to experience time and distance. Photons are, so they can't.
What if the experiment was done using electrons instead of photons?

This seems to talk about the "delayed choice quantum eraser" and electrons:
https://www.preposterousuniverse.com...uantum-eraser/

Would using electrons still imply retrocausality? Also electrons apparently only move at about 1% of the speed of light...

9. Originally Posted by excreationist
Originally Posted by DBT
It is relevant to QM, ie, objective p wave collapse as a way that matter/energy may possibly work.
Anyway the objective collapse theory rejects that the cat is simultaneously alive and dead - that it is either alive or dead before the observation....
Exactly. The Copenhagen interpretation is not data, it is one of several attempts to understand the nature of the quantum world from the data. In the thought experiment, when the box is opened the cat will be found to be either alive or dead. What was the state of the cat before opening the box? Bohr, by the Copenhagen interpretation, says the cat would be in a state of superposition but Schrodinger (who posed the thought experiment to show the absurdity of that interpretation) says the cat would either be alive or dead. There were many physicists at the time and there still are many physicists who reject the Copenhagen interpretation.
I wonder how it would work with the quasar... that it would involve particles or waves.... from what I've read of objective collapse theory it could collapse to particles itself sometimes.... but that might not match the observations of the experimenter....
I think a major problem in trying to understand is that we don't really know the true nature of the QM world. Electrons and photons aren't really either particles or waves but using those descriptions is the only way we can currently make any sense of (or even describe) the results our measurements. The term wavicle has been coined as a term to replace particles and waves but it doesn't really help since we don't really know the true nature of 'wavicles'.

10. Originally Posted by skepticalbip
Exactly. The Copenhagen interpretation is not data, it is one of several attempts to understand the nature of the quantum world from the data. In the thought experiment, when the box is opened the cat will be found to be either alive or dead. What was the state of the cat before opening the box? Bohr, by the Copenhagen interpretation, says the cat would be in a state of superposition but Schrodinger (who posed the thought experiment to show the absurdity of that interpretation) says the cat would either be alive or dead. There were many physicists at the time and there still are many physicists who reject the Copenhagen interpretation.
I think that to connect Copenhagen interpretation with Shrodinger's thought experiment is incorrect. For example, the many-worlds interpretation also says that the cat is in a superposition of dead and alive.

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