# Can you actually measure an object's velocity?

• Brainv2.1beta
In summary: There are also many people who have only a limited knowledge of relativity, and are not always able to distinguish between what is true and what is not. In summary, the OP has posted an incomplete and inaccurate summary of relativity.
Brainv2.1beta
Since everything in the universe is moving relative to the other, how can one measure an absolute velocity? Therefore, how can the speed of light be absolute?

Brainv2.1beta said:
Since everything in the universe is moving relative to the other, how can one measure an absolute velocity?

We can't and we don't

Therefore, how can the speed of light be absolute?

Light has the exceptional property that its speed (in vacuum) is independent of the speed of both source and receptor, this is why we always measure it to be c.

The universe has only one velocity that all observers will agree upon, and it happens to be c. If you ask "why is it that way?", we cannot give you an answer.

- Warren

What if there is a tunnel that lowers the speed of light, observers would observe 2 different "constants".

The speed of light cannot be lowered. All observers always agree on the speed of light.

- Warren

Ah, but it CAN.

No, it can't. Please provide evidence, or retract the claim.

- Warren

Brainv2.1beta said:

"Photons move at a speed less than c, unless they are traveling in vacuum. "
Did you notice the "vacuum" in my post?

When people refer to "the speed of light," they mean the speed of light in vacuum, which never changes. The decrease in propagation velocity in materials is due to frequent absorption and re-emission. In between those absorptions, the photons still travel at c.

- Warren

1effect said:

"Photons move at a speed less than c, unless they are traveling in vacuum. "
Did you notice the "vacuum" in my post?

Admittedly, I did not notice. However, there still is the problem that if over 50% of the universe can not be converted into a vacuum then the speed of light is not officially a constant.

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Brainv2.1beta said:

Now you do, try to remember it next time you post such stuff.

However, there still is the problem that if over 50% of the universe can not be converted into a vacuum then the speed of light is not officially a constant.

You are writing pure nonsense.

True, but it sounded good at the moment so I posted. :D

Brainv2.1beta said:
I would be suspicious of that article. There's a lot I don't understand, but the author made a big deal out of the variability of G (among other constants) without referencing any of the people who have so long worked on that (e.g. George Gillies). That would be like an article on QED that leaves out Schwinger - you have to ask why?

Brainv2.1beta said:
Admittedly, I did not notice. However, there still is the problem that if over 50% of the universe can not be converted into a vacuum then the speed of light is not officially a constant.

1effect said:
You are writing pure nonsense.

Agreed. The question has been asked, good answers have been given (and unfortunately mostly ignored by the OP). I'm locking the thread, it serves no further purpose.

If people are having trouble sorting the wheat from the chaff, I will point out that answers given by science advisors and staff are generally more reliable than answers given by non-science advisors. However, answers given by SA's and staff should not be taken as gospel, and we definitely encourage people to read standard textbooks and peer reviewed papers. We do not encourage "making it up as you go along" sorts of answers, though, that's one reason I'm locking the thread, we've seen too much of that here.

The internet presents some special problems with repsect to relativity. There are some fine resources on the topic in the WWW, there are also many incorrect and downwright wrong websites as well.

## 1. What is velocity and how is it measured?

Velocity is a measurement of an object's speed and direction. It is typically measured in meters per second (m/s) or kilometers per hour (km/h). Velocity can be measured using a variety of tools, such as a speedometer or a radar gun.

## 2. Is it possible to accurately measure an object's velocity?

Yes, it is possible to accurately measure an object's velocity using various scientific methods and tools. However, the accuracy of the measurement may depend on factors such as the precision of the measuring tool and the speed and direction of the object being measured.

## 3. Can the velocity of an object change over time?

Yes, an object's velocity can change over time if its speed and/or direction changes. This is known as acceleration, and it can be measured using tools such as accelerometers.

## 4. How does air resistance affect the measurement of an object's velocity?

Air resistance can affect the measurement of an object's velocity, especially for objects moving through air at high speeds. This is because air resistance can slow down the object's movement, resulting in a lower velocity measurement than the object's actual velocity. Scientists must take this into consideration when measuring an object's velocity in real-world situations.

## 5. Can velocity be measured for objects that are not in motion?

Velocity is a measurement of an object's speed and direction, so for an object to have a velocity, it must be in motion. Objects at rest do not have a velocity, but they may have other measurable properties, such as mass and volume, that can be used to calculate their potential velocity if they were to start moving.

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