[xersanozgen (OP)]

When will the Sun's and Andromeda's photons meet?

Two photons (which have been emitted from the Sun and Andromeda on the same moment To) approach to each other.

The distance to the Andromeda Galaxy is 2.54 million light-years.

When will these photons meet?     Tı = ? 

[jeffreyH]

The midpoint would be 1.27 million light years. You cannot simply state that this is the point that they meet since a) the universe is expanding and b) there could be forces present along the photon paths that could deflect them. These are just two of the complications that could throw out any calculation.

[evan_au]

The Sun is thought to be about 4.5 billion years old.
A (younger) Andromeda galaxy would have existed well before that.
So photons from both would have passed each other about 4.5 billion years ago.

Andromeda is approaching our Milky Way galaxy; they are expected to collide in about 5 billion years.
So we could assume that they were about 5 million light-years apart at the time of the Sun's formation.
Photons from Andromeda would have been "in flight" for at least 5 million years prior to the Sun's formation, so the photons from the Andromeda galaxy would have reached the Sun as soon as the Sun burnt its way out of the dust cloud in which it formed.

[xersanozgen (OP)]

The midpoint would be 1.27 million light years. You cannot simply state that this is the point that they meet since a) the universe is expanding and b) there could be forces present along the photon paths that could deflect them. These are just two of the complications that could throw out any calculation.

Is 1.27 million  light years or merely 1.27 million years?

[xersanozgen (OP)]

The Sun is thought to be about 4.5 billion years old.
A (younger) Andromeda galaxy would have existed well before that.
So photons from both would have passed each other about 4.5 billion years ago.

Andromeda is approaching our Milky Way galaxy; they are expected to collide in about 5 billion years.
So we could assume that they were about 5 million light-years apart at the time of the Sun's formation.
Photons from Andromeda would have been "in flight" for at least 5 million years prior to the Sun's formation, so the photons from the Andromeda galaxy would have reached the Sun as soon as the Sun burnt its way out of the dust cloud in which it formed.

 To = the moment of starting to travel for  the Sun's and Andromeda's photons = now

Tı = the colliding or meeting moment = ?

[evan_au]

Is (it) 1.27 million  light years or merely 1.27 million years?

Both.

In a vacuum, light travels at "the speed of light".
- So in space, light travels 1 light-year in 1 year.
- And 1.27 million  light years in 1.27 million years

If you look at photons emitted now by the Sun and Andromeda, they will pass at the half-way point, which is 1.27 million  light years away, in 1.27 million years.

[Kryptid]

The midpoint would be 1.27 million light years. You cannot simply state that this is the point that they meet since a) the universe is expanding and b) there could be forces present along the photon paths that could deflect them. These are just two of the complications that could throw out any calculation.

The latest expansion rate measurement is about 71.9 kilometers per second per megaparsec. Since the Andromeda is 0.78 megaparsecs away, we'd expect the expansion rate to be about 56 kilometers per second between our two galaxies. Over a period of 1.27 million years, they'd only increase in distance from each other by about 237 light-years, an increase of less than 0.02 percent. So although it's true that the distance would change over that period of time, it's only a very small amount.

[evan_au]

You cannot simply state that this is the point that they meet since a) the universe is expanding

The cosmic expansion of the universe only applies over cosmic distances. The Andromeda galaxy is part of our "local cluster" of galaxies, which are gravitationally bound, and are not affected by today's cosmic expansion of the universe.

In fact, the Andromeda galaxy is moving towards the Milky Way galaxy at around 110 km/sec.
But as Kryptid calculates, this makes very little difference to the answer.

See: https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision

b) there could be forces present along the photon paths that could deflect them.

There is a low concentration of gas and dust between galaxies, and more within the galaxies, so some light could be absorbed.
- Most of the absorption would occur after the photon hit Earth's atmosphere (think cloudy days).
- There is a slight refraction if the photon hits Earth's atmosphere at an angle

Unlike charged particles (eg cosmic rays), photons are not deflected by the magnetic fields of galaxies.

So, barring a giant mirror between the galaxies, our view of the Andromeda galaxy is pretty clear.

[xersanozgen (OP)]

Is (it) 1.27 million  light years or merely 1.27 million years?

Both.

In a vacuum, light travels at "the speed of light".
- So in space, light travels 1 light-year in 1 year.
- And 1.27 million  light years in 1.27 million years

If you look at photons emitted now by the Sun and Andromeda, they will pass at the half-way point, which is 1.27 million  light years away, in 1.27 million years.

Yes, you are right certainly. Thanks.

In that case, the distance between two photons decreases by a value ' 2 c ' of velocity; or the collision speed is 2c.

In CERN, If the particules  accelerate up to big fractions of c; the collision speed can be bigger than ' c '.

[xersanozgen (OP)]

The latest expansion rate measurement is about 71.9 kilometers per second per megaparsec. Since the Andromeda is 0.78 megaparsecs away, we'd expect the expansion rate to be about 56 kilometers per second between our two galaxies. Over a period of 1.27 million years, they'd only increase in distance from each other by about 237 light-years, an increase of less than 0.02 percent. So although it's true that the distance would change over that period of time, it's only a very small amount.

Yes, neglectabl.

 In my opinion the sources can travel toward anywhere independently*, after emitting a photon packet; so, the distance or travelling way -at the moment of starting for the motion of photons- will be kept  by the same value.

* You can see this by using a photo-flash; after flashing you can carry the flash device to anywhere even opposite direction of light.

[paulggriffiths]

https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=71464.0;attach=23974;image

[Bill S]

In that case, the distance between two photons decreases by a value ' 2 c ' of velocity; or the collision speed is 2c.

Relative to an observer, or to each other?

[xersanozgen (OP)]

In that case, the distance between two photons decreases by a value ' 2 c ' of velocity; or the collision speed is 2c.

Relative to an observer *, or to each other?

According to an outmost / co-reference frame (space or Light Coordinate system : LCS)

This analysis is an simple application / example of LCS concept (alternative theory for special relativity). Please read that link:

https://www.thenakedscientists.com/forum/index.php?topic=71218.0

For detailed analysis of LCS concept and for calculation of universe's age: "Light Kinematics to analyze space-time"

http://adsabs.harvard.edu/abs/2013PhyEs..26...49E



( * ) 1-The light comes to an observer/receptor by always the speed ' c '; so he can perceive the events ( which are realized by bigger speed than c) in accordance with this condition.

2-  If an observer is an actor of analysis and experiment; his visual ability or visual restrictions can cause confusing or incorrect conclusions (our and Einstein's humanly mistake). Therefore we must prefer to use God's eyes for scientific integrity (This is not a new rule).

[evan_au]

In that case, the distance between two photons decreases by a value ' 2 c ' of velocity; or the collision speed is 2c.

There is not actually a collision - the photons pass each other.

Note that photons are massless, and always travel at c, in a vacuum.

In CERN, If the particles  accelerate up to big fractions of c; the collision speed can be bigger than ' c '.

Unlike photons, protons have a mass (even at rest), and so can never reach c.

If you work out the collision velocity as seen by one of the protons, the velocity of the oncoming proton is still less than c.

See: https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity

[xersanozgen (OP)]

If you work out the collision velocity as seen by one of the protons, the velocity of the oncoming proton is still less than c.

In CERN, If one of the protons has been accelerated up to 60 % c and the other's 70% c; the collision speed will be 1,30 c  . In other words, the distance between these photons decreases by the speed 1,30 c; so, if we assign one of them for reference role, the relative speed of other one must be 1,30 c.

V = (v + w) / (1 + v.w/c²)  This formula is valid for genuine relativity (If we throw a rocket from/over another rocket; the speed of relative rocket doesn't exceed the value ' c ').

If the objects get their speed by their own pattern independently ***(that Einstein had ignored this option; but, we can distinguish and internalize the types of relativity), the collision speed must find by additing directly (top limit 2c). In that case radial speeds can get a value up to 2.c

*** For example  If we consider two ambulances (which they travel toward each other by their maximum speed); the distance between them decrease by the speed (v + w). We cannot consider the value of maxsimum speed as top limit for the relative speed according to one of them (an ambulance cannot exceed this limit; but the distance can decrease by bigger value than a car's speed.

[xersanozgen (OP)]

If the objects get their speed by their own pattern independently ***(that Einstein had ignored this option; but, we can distinguish and internalize the types of relativity), the collision speed must find by additing directly (top limit 2c). In that case radial speeds can get a value up to 2.c

Radial speed is the escaping speed of sky object from Earth. In fact, it is the total of the vectoral components which the projections of universe expansion speed upon observational line for the Earth and sky object. So, the speeds of two objects are mentioned; therefore the top limit of radial speeds is  2.c .

The theory of SR declared a single c value for the top limit of radial speed. However, its formula (to transform the redshifts) is not correct.

[xersanozgen (OP)]

If the objects get their speed by their own pattern independently ***(that Einstein had ignored this option; but, we can distinguish and internalize the types of relativity), the collision speed must find by additing directly (top limit 2c). In that case radial speeds can get a value up to 2.c

Radial speed is the escaping speed of sky object from Earth. In fact, it is the total of the vectoral components which the projections of universe expansion speed upon observational line for the Earth and sky object. So, the speeds of two objects are mentioned; therefore the top limit of radial speeds is  2.c .

The theory of SR declared a single c value for the top limit of radial speed. However, its formula (to transform the redshifts) is not correct.

The burning phenomen had been wanted to define by Stahl (philogiston theory). This theory had an acceptance like reality for a long time.

Similarly, the theory SR is a first effort for light kinematics (actually Einstein's motivation was on the axle of Fitzgerald contraction; this aim caused to ignore the types of relativity; Einstein preferred to consider the genuine relativity for light's motion).

Whereas an alternative solution (LCS concept) is mentioned for analyzing light kinematics like the simple example which is presented at this topic.

[Janus]

Is (it) 1.27 million  light years or merely 1.27 million years?

Both.

In a vacuum, light travels at "the speed of light".
- So in space, light travels 1 light-year in 1 year.
- And 1.27 million  light years in 1.27 million years

If you look at photons emitted now by the Sun and Andromeda, they will pass at the half-way point, which is 1.27 million  light years away, in 1.27 million years.

Yes, you are right certainly. Thanks.

In that case, the distance between two photons decreases by a value ' 2 c ' of velocity; or the collision speed is 2c.

In CERN, If the particules  accelerate up to big fractions of c; the collision speed can be bigger than ' c '.

The "closing speed" between the particles as measured in the rest frame of the accelerator, can exceed c, however, the relative speed between the particles as measured by either particle cannot equal or exceed c.  For example, if both particle A and B have speeds of 0.999c relative to the accelerator, either particle will measure the  relative speed between the particles as being 0.999999499c

[evan_au]

V = (v + w) / (1 + v.w/c²)  This formula is valid for genuine relativity (If we throw a rocket from/over another rocket; the speed of relative rocket doesn't exceed the value ' c ').

This formula is also valid if you throw two rockets off a single rocket.

Consider that the Earth is one rocket, and the LHC throws two rockets (bunches of protons) in different directions.

Apply this formula, and you will see that the bunches of protons never exceed c compared to the Earth, and they never exceed c relative to each other.

[jeffreyH]

The velocity addition formula describes how time dilation affects perception in the proper time of a travelling object. It is all to do with relative velocities.