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

  • Thread starter Troymteal
  • Start date
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.
 

PeroK

Science Advisor
Homework Helper
Insights Author
Gold Member
2018 Award
9,112
3,284
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.
 
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
 
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.
 
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?
 
27,532
4,032
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.
 
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
11,980
4,481
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
11,980
4,481
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.
 
9,076
1,995
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:
http://www.cse.secs.oakland.edu/haskell/Special Relativity and Maxwells Equations.pdf

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
 
Last edited:

Vanadium 50

Staff Emeritus
Science Advisor
Education Advisor
22,605
4,884
Imagine two light beams moving past each other. Are you arguing both are stationary and space is moving past them in two different directions?
 
Last edited:

CWatters

Science Advisor
Homework Helper
Gold Member
10,505
2,285
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.
 
27,532
4,032
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
 
Last edited:

Want to reply to this thread?

"Does light move, or does space and time move past light?" You must log in or register to reply here.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top