## Main Question or Discussion Point

I am a laymen. I just thing that if the speed of light is constant then all acceleration should be measured against it. It doesn't move. Everything moves through it. Is that how it works? This has been making me anxious.

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Ibix
if the speed of light is constant then all acceleration should be measured against it.
I'm not sure what this is supposed to mean. Measuring an acceleration against a speed? It's like comparing the height of your house to the speed limit on the road outside.
It doesn't move. Everything moves through it. Is that how it works?
Again, I don't think this makes any sense. Light definitely moves.

jbriggs444
Homework Helper
2019 Award
I just thing that if the speed of light is constant then all acceleration should be measured against it.
The constancy of the speed of light makes it useless as a tool for measuring the speed of anything else. No matter how fast the "anything else" is moving or how it accelerates, light will always be moving at light speed relative to it.

If the result of a measurement is certain beforehand then taking the measurement yields zero information.

FactChecker
Gold Member
Since the speed of light is, itself, measured with respect to an inertial reference frame, position, velocity, and acceleration are easier to measure with respect to that reference frame.

How do we know that light is moving?

Mister T
Gold Member
I am a laymen. I just thing that if the speed of light is constant then all acceleration should be measured against it.
Start by understanding why people say that the speed of light is constant. What they mean is that if you see two objects race past you, and only one of them is a beam of light in a vacuum, then that beam of light will always be the faster of the two. This is true no matter how fast that other object moves! Moreover, to someone co-moving with that object, the beam of light will have the same speed as it does for you. That's what they mean by "constant" in this context.

There are people do indeed measure all speeds relative to the speed of light in a vacuum. They assign it a value of "1" and all other speeds are less than one but greater than or equal to zero.

Start by understanding why people say that the speed of light is constant. What they mean is that if you see two objects race past you, and only one of them is a beam of light in a vacuum, then that beam of light will always be the faster of the two. This is true no matter how fast that other object moves! Moreover, to someone co-moving with that object, the beam of light will have the same speed as it does for you. That's what they mean by "constant" in this context.

There are people do indeed measure all speeds relative to the speed of light in a vacuum. They assign it a value of "1" and all other speeds are less than one but greater than or equal to zero.
That makes a lot of sense to me. As though things are slowing down and becoming denser from 1?

Mister T
Gold Member
That makes a lot of sense to me. As though things are slowing down and becoming denser from 1?
Nothing is becoming denser. A car moving at a speed of 60 mi/h is not any less dense than it would be if it were moving at a speed of 30 mi/h.

Nothing is becoming denser. A car moving at a speed of 60 mi/h is not any less dense than it would be if it were moving at a speed of 30 mi/h.
Sorry, that was dumb.
And it's speed is measured in relation to an observer fixed to the earth.
I guess I just think it should be measured against the speed of light. That should be the standard. As though everything is moving through light. It would make things simpler.

PeterDonis
Mentor
2019 Award
I guess I just think it should be measured against the speed of light.
That is how we measure speeds. The speed of light is $1$. All other speeds are smaller.

However, you can't measure things that aren't speeds against a speed. So your suggestion in the OP to measure accelerations, for example, against the speed of light, makes no sense. Nor would it make any sense to try to measure positions or times against the speed of light.

I think you need to take a step back and consider all of this more carefully.

FactChecker
Gold Member
Sorry, that was dumb.
And it's speed is measured in relation to an observer fixed to the earth.
I guess I just think it should be measured against the speed of light. That should be the standard. As though everything is moving through light. It would make things simpler.
There is a big difference between using the speed of light as a unit of measure of velocity versus using any light (going in ?? direction) as a reference frame. You need to be clear regarding what you are talking about. I doubt very much that it would make things simpler.

PeterDonis
Mentor
2019 Award
it's speed is measured in relation to an observer fixed to the earth
That's because all speeds are relative. You can't just have a speed; it has to be a speed relative to something.

Also, the something that speeds are relative to has to be something that can be at rest. Light can't be at rest, so it can't be a thing that is taken as "fixed" as a frame of reference.

You need to consider this carefully as well.

Forgive my ignorance. I'm simply troubled by this mystery. As I understand it, it's not known why the speed of light is constant. I will think deeply about the questions you've posed, do some more reading, and reply.
I think that space moves and light doesn't. Entertain it as a shift in perspective.

PeterDonis
Mentor
2019 Award
As I understand it, it's not known why the speed of light is constant.
While there is a sense in which this is true, it does not mean you can just speculate however you wish.

I think that space moves and light doesn't.
This doesn't make sense. And you should not be speculating about any of this at your current state of knowledge. You should be working to achieve a better understanding of what we do know first. And we know a lot.

Dale
Mentor
I think that space moves and light doesn't. Entertain it as a shift in perspective.
Good luck trying to make that into a self consistent framework that can be used to make experimental predictions. Once you have done that, you should easily be able to publish it in a peer reviewed journal. Then we would be glad to discuss it here.

Dale
Mentor
I actually can speculate however I wish.
Yes, but not here.

Doubt and asking questions actually makes a mind stronger (stranger as well).
I doubt that there is any scientific study which concludes that allowing students to pursue their own personal speculations is an effective way to teach relativity.

PeterDonis
Mentor
2019 Award
Perhaps it will protect you from the poison of arrogance which seems to have prevented you from considering my point.
I have considered your points and responded to them. The fact that the responses are apparently not what you wanted to hear does not make them invalid. But it does indicate that further discussion in this thread is pointless.