Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 16/03/2016 14:50:01
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Wilfred James asked the Naked Scientists:
A Question for the Naked Astronomers
I have discovered from many sources that anything that
travels at a speed close to that of light experiences a
slowing down of time. Some sources say that time stops
for anything travelling at the speed of light.
With these apparent facts in mind, how is time altered for photons
that travel at the speed of light?
Does the time dilation effect cause photons to appear to
be at longer wavelengths than when they were emitted from
distant astronomical objects? If this is so, wouldn't
this effect require a recalculation of distances based on
the apparent red shifts of distant objects?
It is hard to find any support for the idea that that a
photon retains its original launch energy when travelling
over inter-galactic distances. At the same time it seems
logical that a photon should lose some energy as it
travels immense distances. Could this loss of energy be
indicated by the lengthening of the wavelength of its
light?
Wilf James BSc.
What do you think?
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There are two forms of shift (photon energy change) that are fairly well understood.
Doppler occurs when light emitted from an object traveling at a different speed in relation to observer.
In cosmology this has been noted when comparing spectral lines from similar sources, on opposite sides of a spiral galaxy (where the left side is progressing toward and right is progressing away from observer). The lines have been shifted according to Doppler principle.
Gravitational shift occurs when the light originates in a different field of gravity than observed. This is believed to be due to time dilation outlined by principles of General Relativity. Time moves faster in stronger gravitational field. Light progressing into a stronger field is shifted toward the blue when progressing into stronger field and shifted toward red when progressing in weaker field of gravity.
There is a hypothesized cause for shift, know as "cosmological redshift" which is believed to be due to the Universe expanding. I don't buy it, and instead believe it's cause by the changing distribution of the field of gravity throughout the Universe. My opinion is in the minority. ;)
Edwin Hubble developed "hubble constant" based on luminosity and shift measured on Cepheid variables, stars that pulse on fairly regular intervals, thought to be of same relative luminosity. Comparing the different shifts to luminosity, and thus proved the universe is expanding.
Einstein and Hubble got together but it seems they didn't discuss gravitational shift. Well... The Lambda-CDM theory of the Universe didn't exist then. They obviously hadn't thought about the evolution of the universe much in that regard, or they would have likely discussed the thesis I've proposed.
Einstein's proclaimed his own "cosmological constant" was his greatest blunder. Curiously the general consensus is that his blunder wasn't wrong, although I side w/Einstein. We've seen evidence of galaxies gravitating toward one another, so there's no constant to keep that from happening. Tho Einstein's constant has to do with space being pulled in with gravity's persistent attraction.
The field of gravity permeates everywhere. We don't know if there is a limit to how far it reaches. It seems to be very much like light. It spreads out in all directions weakening according to the inverse square of the distance.
Unlike light, it doesn't require a chemical/nuclear reaction to be released. No, the gravity field persists as long as mass exists. The distribution of the field of gravity has evolved with the evolution of the Universe. Lambda-CDM model ignores that fact and instead insists the Universe is expanding, at an accelerating rate, based on cosmological redshift.
I get a little aggravated when ppl don't seen the logic... ;)
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Cosmic ray's witness the best evidence for near light speed time dilation effects. Photons are the smallest quanta we're aware of, AFAIK. They can be measured at differing energies based on frequency/shift. It's unlikely we'll be able to meter them in any other fashion (anytime soon).
Cosmic ray's are massive particles that travel near the speed of light. They seem to manage to travel great distances w/out decaying. Presumably it's due to time dilation, gravity/speed. I believe there's a correlation to frequency of decay based on altitude, which matches GR principles.
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Wilfred James asked the Naked Scientists:
A Question for the Naked Astronomers
I have discovered from many sources that anything that travels at a speed close to that of light experiences a
slowing down of time. Some sources say that time stops for anything travelling at the speed of light.
With these apparent facts in mind, how is time altered for photons that travel at the speed of light?
Does the time dilation effect cause photons to appear to be at longer wavelengths than when they were emitted from distant astronomical objects?
The effects of special relativity (relative velocity) that you hear about are only meant to be applied to objects that never travel at the speed of light. If you try to put the speed of light into the equations you get nonsense answers. The time dilation and length contraction of special relativity only make sense for things that have a rest frame.
If this is so, wouldn't this effect require a recalculation of distances based on the apparent red shifts of distant objects?
It is hard to find any support for the idea that that a photon retains its original launch energy when travelling over inter-galactic distances. At the same time it seems logical that a photon should lose some energy as it travels immense distances. Could this loss of energy be indicated by the lengthening of the wavelength of its
light?
Wilf James BSc.
What do you think?
Here is a link to a primer on distances in cosmology:
https://en.wikipedia.org/wiki/Distance_measures_(cosmology)
You'll notice that the astronomers take great care to account for basically everything. It isn't mentioned in the link but they also do their best to account for any dust that might be in the way when using luminosity. Generally speaking people who have dedicated their entire lives to studying the distances between us and celestial objects aren't likely to be missing any simple obvious thing that would drastically alter the calculations.
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how is time altered for photons that travel at the speed of light?
A photon does not have quantum properties that change irreversibly over time while it is "in flight", so it is not possible to state that light "experiences" time at all. The big changes occur when it is emitted and when it is detected; it is not possible to say very much about its state between these events, as an attempt to measure the properties require a detector.
Maxwell's equations provide another wave of viewing light in flight - they describe how the phase and position changes over time, but again it does so in a deterministic manner (but even more deterministic than quantum theory).
Does the time dilation effect cause photons to appear to be at longer wavelengths than when they were emitted from
distant astronomical objects?
In general, Yes. But sometimes we see a shorter wavelength, such as for the Andromeda galaxy which is on a collision course with the Milky Way galaxy.
If this is so, wouldn't this effect require a recalculation of distances based on the apparent red shifts of distant objects?
Yes - but it is often used in the reverse of what you suggest; the distance to really distant galaxies is mostly calculated from the red shift.
At the same time it seems logical that a photon should lose some energy as it travels immense distances.
The logic of entropy we develop on Earth - that a thrown ball slows down, or a cup of coffee/tea gets cold applies to macroscopic objects.
This logic does not apply in the quantum world (eg looking at photons).
At present it is thought that the range of the electromagnetic field is infinite (and, correspondingly, the rest-mass of the photon is zero).
This means that the photon does not lose energy when it travels between two points which are at rest relative to each other (and which are at the same gravitational potential, as JoeBrown mentions).
Could this loss of energy be indicated by the lengthening of the wavelength of its light?
Lengthening of the wavelength of light does indicate a loss of photon energy.
But given the theoretical result of "photons have infinite range", this red shift is interpreted as a different velocity and/or different gravitational strength and/or time dilation.