# Speed of light hypothetical question

• anti666
In summary: If the distance between A and B was 0.9ly, than under normal circumstances light would travel from A to B in one year. However in my given scenario the light the source inside the room would reach B in less than one year?It is possible that the light would reach B in less than one year under normal circumstances. However, in your scenario where the room is moving, the light would reach B in one year.
anti666
Slight theoretical question that i have difficulty wrapping my head around.

Lets imagine a confined room traveling trough space at .9c on a line defined by points A and B. Let's say that in this room is a light source. If this light source is turned on than does the light inside the room still travel at c in every direction?

If so then let's assume that the direction that the room travels in is from A to B, length of the room is the same as distance between A and B and finally that the light source is at the center of the room.
Now if the light source is turned on when it is in point A, than the light would reach point B faster than c?

I'm not aiming at FTL travel, just wondering if light itself could in some cases be made travel faster than c or observed to be traveling faster than c?

anti666 said:
Slight theoretical question that i have difficulty wrapping my head around.

Lets imagine a confined room traveling trough space at .9c on a line defined by points A and B. Let's say that in this room is a light source. If this light source is turned on than does the light inside the room still travel at c in every direction?

Yes it does still travel at c in every direction.

anti666 said:
If so then let's assume that the direction that the room travels in is from A to B, length of the room is the same as distance between A and B and finally that the light source is at the center of the room.
Now if the light source is turned on when it is in point A, than the light would reach point B faster than c?

No. A person sitting in the room would still observe the light to travel at speed c. That is an amazing mystery, isn't it? Google "Space Time Diagram" and see if you can get more insight into this. There are other posts here that shed "light" on the mystery. And others may jump in here to provide you with more insight as well.

anti666 said:
I'm not aiming at FTL travel, just wondering if light itself could in some cases be made travel faster than c or observed to be traveling faster than c?

Again, the answer is no. There are no known cases of light itself being made to travel faster than c.

bobc2 said:
No. A person sitting in the room would still observe the light to travel at speed c.

Yes i can understand how the observer in the room would 'see' the light traveling at c.

However i think that the light itself would reach point B faster than c in the given scenario. If not, than why?

If the distance between A and B was 0.9ly, than under normal circumstances light would travel from A to B in one year. However in my given scenario the light the source inside the room would reach B in less than one year?

bobc2 said:
There are no known cases of light itself being made to travel faster than c.

I am aware of that, but could it be theoretically possible based on what we know today?

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anti666 said:
If the distance between A and B was 0.9ly, than under normal circumstances light would travel from A to B in one year. However in my given scenario the light the source inside the room would reach B in less than one year?
You are assuming classical vector addition laws apply. The velocity of the train doesn't add linearly with the velocity of the light with respect to point B. The vector addition law in SR is $s = \frac{u + v}{1 + (vu / c^{2})}$.

anti666 said:
Yes i can understand how the observer in the room would 'see' the light traveling at c.

However i think that the light itself would reach point B faster than c in the given scenario. If not, than why?

It would not, because:

X = ct

t = X/c

where X is the distance traveled to point B and t is the time taken to travel the distance. If you are sitting in the room watching, then that equation is the same whether the room is moving with respect to some outside reference or not. And the value of c does not change. Thus you get the same answer for t, either way (that is when you are just considering only what is going on from the point of view of the observer inside the room).

anti666 said:
If the distance between A and B was 0.9ly, than under normal circumstances light would travel from A to B in one year. However in my given scenario the light the source inside the room would reach B in less than one year?

Now, this is a different question. You are now comparing the observations of a rest observer compared to the observer in the moving room. Clock readings and distances traveled will certainly be different for two observers in relative motion with respect to each other. The observer at rest could report a reading on his clock at the instant the light reaches point B. But the observer in the moving room will report a different time on his clock. Also, the two observers will report different distances between points A and B. But, again, both will measure the speed of light to be c.

anti666 said:
I am aware of that, but could it be theoretically possible based on what we know today?

No. There is no theory based on anything new we know today that would change this.

This space-time diagram attempts to indicate a 4-dimensional geometric explantion for the strange situation you're dealing with. Apologies if it just makes it more confusing. In this case we show just one of our normal 3 dimensions (X1) along with the 4th dimension (X4) of Minkowski space-time.

Here we have a red rocket and a blue rocket moving in opposite directions with respect to some black reference system. Each is moving along its own 4th dimension (world line), i.e., X4, at speed c. The strange thing that seems to happen with special relativity is that a given observer's X1 axis (one of the normal spatial directions) rotates in the 4-D universe so as to always result in photon world lines bisecting the angle between X4 and X1. Note that this is the case for the black rest system as well as the blue and red coordinate systems.

So, you see in the diagram that the blue and red guys live in two different 3-D cross-sections of the 4-D universe. Quite mysterious for sure! This of course results in different views of clocks and distances. The blue guy sees the red rocket as being shorter than his own and the red guy sees the blue rockete as being shorter than his. Red and Blue read different clock times (positions along the 4th dimension).

WannabeNewton said:
The vector addition law in SR is $s = \frac{u + v}{1 + (vu / c^{2})}$.

It seems to me that using c as a constant would not make sense, if one would try to prove that c could be greater (in some hypothetical scenario) than what we currently believe it to be.

bobc2 said:
Now, this is a different question. You are now comparing the observations of a rest observer compared to the observer in the moving room. Clock readings and distances traveled will certainly be different for two observers in relative motion with respect to each other. The observer at rest could report a reading on his clock at the instant the light reaches point B. But the observer in the moving room will report a different time on his clock. Also, the two observers will report different distances between points A and B. But, again, both will measure the speed of light to be c.

But light from the source inside the room would reach B before the source itself would reach B (even from rest observer standpoint), thus exceeding the speed that the room itself is traveling?From searching a bit i have found actually something that somewhat resembles what I'm trying to get at here. Its http://www.nasa.gov/centers/glenn/technology/warp/ideachev.html#alcub" (found it by accident actually)

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anti666 said:
It seems to me that using c as a constant would not make sense, if one would try to prove that c could be greater (in some hypothetical scenario) than what we currently believe it to be.

What?

But light from the source inside the room would reach B before the source itself would reach B (even from rest observer standpoint), thus exceeding the speed that the room itself is traveling?

Why would the source itself reach B before the light?

From searching a bit i have found actually something that somewhat resembles what I'm trying to get at here. Its http://www.nasa.gov/centers/glenn/technology/warp/ideachev.html#alcub" (found it by accident actually)

The key here is that in that scenario you are not moving through LOCAL SPACE at greater than c, so there is no problem. (If it does work at least, it doesn't at the moment and probably won't anytime in the forseeable future.)

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Drakkith said:
What?

Lets say you are trying to calculate the wattage of a lightbulb. On it it says 60W. Now are you really going to use that figure of 60W as a constant for your calculations to find the real and accurate wattage of the lightbulb?

Drakkith said:
Why would the source itself reach B before the light?
anti666 said:
But light from the source inside the room would reach B before the source itself would reach B (even from rest observer standpoint), thus exceeding the speed that the room itself is traveling?

anti666 said:
It seems to me that using c as a constant would not make sense, if one would try to prove that c could be greater (in some hypothetical scenario) than what we currently believe it to be.

C is a conversion factor and nothing more. I could set c = 1 and you wouldn't even see it in the equation in the natural units.

anti666 said:
Lets say you are trying to calculate the wattage of a lightbulb. On it it says 60W. Now are you really going to use that figure of 60W as a constant for your calculations to find the real and accurate wattage of the lightbulb?

That's not really a good example. The 60W is what the bulb is supposed to put out. I would measure the power usage and light given off to determine how close to 60W it actually is.

In any case, the speed of light has been measured as c, and so many things work as a result of using c in calculations that if it were NOT correct we would have noticed by now. Energy conversions, power outputs, how different EM effects work, ETC.

anti666 said:
Slight theoretical question that i have difficulty wrapping my head around.

Lets imagine a confined room traveling trough space at .9c on a line defined by points A and B. Let's say that in this room is a light source. If this light source is turned on than does the light inside the room still travel at c in every direction?

If so then let's assume that the direction that the room travels in is from A to B, length of the room is the same as distance between A and B and finally that the light source is at the center of the room.
Now if the light source is turned on when it is in point A, than the light would reach point B faster than c?

Though the above is a badly formed question , I will try to answer it. Suppose that the room moves in the direction of the +x axis at speed v with respect to a certain frame of reference, with B at the rear and C at the front. The light emitted from the midpoint (A) has the following equations:

1. In the frame of the room:

$$ct_B=d/2$$
$$ct_C=d/2$$

where d is the distance BC.

So, in this case :

$$t_B=t_C=d/(2c)$$

2. In the frame wrt which the room moves at speed v:

$$ct'_B+vt'_B=d/2$$
$$ct'_C=d/2+vt'_C$$

In this case:

$$t'_B=d/(2(c+v))$$
$$t'_C=d/(2(c-v))$$

In this case it would 'appear" that the light moves at $c+v$ and $c-v$ respectively but this is only due for the fact that you are observing the COMBINED movement of the room and of the light front. This is called "separation speed" or "closing speed" and it is the speed of two DIFFERENT entities that cover ONE common distance.

I'm not aiming at FTL travel, just wondering if light itself could in some cases be made travel faster than c or observed to be traveling faster than c?

Not really, see above.

anti666 said:
It seems to me that using c as a constant would not make sense, if one would try to prove that c could be greater (in some hypothetical scenario) than what we currently believe it to be.
The invariance of c is well established by a wide variety of experiments that did not assume it's invariance.

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

Drakkith said:
That's not really a good example. The 60W is what the bulb is supposed to put out. I would measure the power usage and light given off to determine how close to 60W it actually is.

In any case, the speed of light has been measured as c, and so many things work as a result of using c in calculations that if it were NOT correct we would have noticed by now. Energy conversions, power outputs, how different EM effects work, ETC.

This was not meant to be a 'good example', it was meant just to illustrate what i meant with my original comment.
I have no difficulty accepting that c (as in speed of light in vacuum) is the constant we believe it to be. But i have difficulty seeing how c is the upper limit of the speed that light can (seem to) travel at in every (theoretically) possible scenario. And it becomes even more difficult when i take into consideration that the light source and the observer are also moving independently from each other.
However the following quote seems to be the answer i was looking for.

ctxyz said:
In this case it would 'appear" that the light moves at $c+v$ and $c-v$ respectively but this is only due for the fact that you are observing the COMBINED movement of the room and of the light front. This is called "separation speed" or "closing speed" and it is the speed of two DIFFERENT entities that cover ONE common distance.

anti666 said:
I have no difficulty accepting that c (as in speed of light in vacuum) is the constant we believe it to be. But i have difficulty seeing how c is the upper limit of the speed that light can (seem to) travel at in every (theoretically) possible scenario.
The first, the invariance of c, logically implies the second, the universal speed limit. You cannot have one without the other.

Do you understand the traditional derivation of the Lorentz transform from Einstein's two postulates?

## 1. What is the speed of light?

The speed of light, denoted by the symbol 'c', is a physical constant that represents the speed at which light travels in a vacuum. It is approximately 299,792,458 meters per second or about 670,616,629 miles per hour.

## 2. Is the speed of light constant?

Yes, according to Einstein's theory of relativity, the speed of light in a vacuum is constant and independent of the observer's reference frame.

## 3. Can anything travel faster than the speed of light?

According to our current understanding of physics, nothing can travel faster than the speed of light. The speed of light is considered to be the maximum speed at which all matter and information in the universe can travel.

## 4. What would happen if the speed of light was infinite?

If the speed of light was infinite, then time would essentially stop and the fundamental laws of physics would break down. This hypothetical scenario is not possible according to our current understanding of the universe.

## 5. How is the speed of light measured?

The speed of light is measured using various methods, including the time it takes for light to travel a known distance or the frequency of electromagnetic waves. One of the most accurate methods is through the use of interferometry, which measures the interference patterns of light waves.

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