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Speed of Light

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Jefferson :
Hi everyone, I'm not very familiar with this topic, especially the concept below:
"The speed of light is constant from all forms of reference."
https://www.youtube.com/watch?v=-Irlq3TFr8Q

However, can someone use this to explain the following scenario to me?
What if 2 objects travel in opposite directions towards a centre point (i.e) Earth at 225 000 000 m/s (i.e. 0.75c)

[ 225 000 000 m/s ] ⚽1 ----------------->>>> 🌎 <<<<----------------- ⚽2  [ – 225 000 000 m/s ]

Then what would ⚽1 measure ⚽2's speed to be?
By Galilean's Relativity Theory, it would be – 450 000 000 m/s (i.e. 1.5c), which exceeds c. But what about Einstein's theory?
If the measured speed is still 300 000 000 m/s (i.e. c), then what happens to the excess?

DrDusk:

--- Quote from: Jefferson  on August 27, 2019, 10:16:48 pm ---Hi everyone, I'm not very familiar with this topic, especially the concept below:
"The speed of light is constant from all forms of reference."
https://www.youtube.com/watch?v=-Irlq3TFr8Q

However, can someone use this to explain the following scenario to me?
What if 2 objects travel in opposite directions towards a centre point (i.e) Earth at 225 000 000 m/s (i.e. 0.75c)

[ 225 000 000 m/s ] ⚽1 ----------------->>>> 🌎 <<<<----------------- ⚽2  [ – 225 000 000 m/s ]

Then what would ⚽1 measure ⚽2's speed to be?
By Galilean's Relativity Theory, it would be – 450 000 000 m/s (i.e. 1.5c), which exceeds c. But what about Einstein's theory?
If the measured speed is still 300 000 000 m/s (i.e. c), then what happens to the excess?

--- End quote ---
Ah it's good that you are thinking about this. Fyi this is not actually necessary to know in terms of HSC Physics because it requires the use of formulas known as the 'Lorentz Transformations'. They are used to describe displacement and time of another moving 'object' when you are in a moving reference frame and are given by





In the HSC you only need to consider time & mass dilation and length contraction. You do not need to know concepts like what you've mentioned above, like what velocity you would measure in that scenario.

Theory wise Galilean Relativity cannot be applied here as it is incorrect with such large velocities. The speed you measure will still be less than the speed of light. There is no 'excess'.

He would thus measure it as -0.96c using the above formula for v..

DrDusk:
One thing you can note that I forgot to add in is that these equations by making one assumption bring out the Galilean relativity formulas.

If you take really small velocities, small in comparison to the value of 'c', then you can make the assumption that:

If we assume u and v are 100 meters/second, they will approximately be 0.00000033c. We can clearly see that 1-0.00000033^2 is literally almost 1.

So using that assumption we get

which if you look at it, it is just the same as adding/subtracting the velocities using Galilean Relativity!

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