# I Does light move, or does space and time move past light?

#### Troymteal

Could it be possible for light to not move at all but remain still while space and time moves past it? The light would just exists as the continuum of space time moves past light.

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#### PeroK

Homework Helper
Gold Member
2018 Award
The definition of motion is a change in position over time. By this definition, light moves.

Impossible.

By your definition we would have collided with the Sun or any Star which emits Light a very very long time ago.

#### Troymteal

Impossible.

By your definition we would have collided with the Sun or any Star which emits Light a very very long time ago.
Said sun and stars would be moving with space time as well

#### Troymteal

The definition of motion is a change in position over time. By this definition, light moves.
But could it be that position is what is really moving.

#### PeroK

Homework Helper
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2018 Award
Said sun and stars would be moving with space time as well
But could it be that position is what is really moving.
Spacetime does not move. Spacetime is the continuum within which motion can be defined.

Said sun and stars would be moving with space time as well
A star emits light in all directions.
Which way would your poor star move then? Every direction?

#### Dale

Mentor
Could it be possible for light to not move at all but remain still while space and time moves past it? The light would just exists as the continuum of space time moves past light.
There is no inertial reference frame where this is correct. You could construct a non-inertial coordinate system where this is correct, but the laws of physics would not be what we are used to at all.

#### Troymteal

A star emits light in all directions.
Which way would your poor star move then? Every direction?
You have heard that the universe is expanding

#### Nugatory

Mentor
Mentors' note: This post had been derailed by a more-heat-then-light argument involving personalities rather than physics. The offending posts have been removed and the primary rabble-rousers have been banned from the thread. Everyone is reminded to please focus on the physics of the matter, and when you see posts that you consider to be personal attacks, violations of the PF rules, or rude report them instead of replying.

#### Nugatory

Mentor
You have heard that the universe is expanding
We have, and in our cosmology section you will find many threads discussing what this means.

However, the effects of that expansion are utterly negligible (and you should not dispute that assertion unless and until you have tried calculating their approximate magnitude) at the scale of a solar system, so @Rada Demorn point is valid. Even if it weren't, you should also consider the point @Dale makes in post 8 above, in the context of two light bulbs one meter apart and shining in all directions.

Asking questions in a physics forum can be an effective way of learning, but you have to listen to the answers. Even more effective is to spend some time reading about what is already known, and for that we highly recommend the book "Spacetime Physics" by Taylor and Wheeler; that will get you caught up to what physicists knew about 100 years ago.

#### bhobba

Mentor
Could it be possible for light to not move at all but remain still while space and time moves past it? The light would just exists as the continuum of space time moves past light.
I do not know what you mean by space-time moving? For simplicity we will discuss non curved space-time - what is called inertial or flat space-time. That may be what is 'confusing' the situation for you. As conceived by physicists space-time also involves a coordinate system - do you mean that is moving? If so one of the strange properties of light is it does not matter how fast a space-time coordinate system moves light always travels away from it at the speed on light. In fact from relativity and coulombs law you can prove all of electromagnetism and that light moves at the speed it does relative to an inertial coordinate system:

As Nugatory correctly says cosmological 'effects' such as space-time expansion are negligible here on earth - for most practical purposes the earth can be considered an inertial frame. Strangely though although true for virtually all practical purposes the curvature of space time (general relativity) needs to be taken into account for the GPS to work - but such exceptions are rare.

Thanks
Bill

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Staff Emeritus
Imagine two light beams moving past each other. Are you arguing both are stationary and space is moving past them in two different directions?

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#### CWatters

Homework Helper
Gold Member
Imagine two light beams moving past each other. Are you arguing both are stationary and space is moving past them in two different directions?
A mirror or worse an infinity mirror would also appear to cause problems.

#### Dale

Mentor
You can make coordinate systems with four null vectors. So you can have each null ray “standing still” regardless of the direction.

Edit: actually, I am not sure if null vectors off axis are “standing still” in these coordinates

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"Does light move, or does space and time move past light?"

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