If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: Physics library>Unit 16

Lesson 2: Minkowski spacetime

# Galilean transformation and contradictions with light

Here we'll see how classical physics predicts scenarios that disagree with what we observe in nature. Something's got to give….

## Want to join the conversation?

• What if I had a race with light? My friend shined a torch. Light traveled with velocity c & I ran with velocity 0.9c (both with respect to my friend). Then wouldn't the speed of light with respect to me be, (c - 0.9c) = 0.1c?
• No, light would move at c for you. That's the whole point of the postulate "Light speed is constant for any frame of reference".
• At around , Sal says that the speed is independent of the inertial frame of reference. But if Sal and Sally are traveling at different speeds, how can they observe the speed of light as being the same? Isn't Sally closer to the speed of light, so she has greater relative velocity compared to it?
• What you are saying makes common sense but that is not how velocities that are a significant fraction of the speed of light work. Any observer that is not accelerating will always measure light in a vacuum traveling at the same speed.
• Bear with me... I just had a dumb idea. What if we imagine this: Sally's frame of reference is perpendicular on mine (like a z axis, from a 3rd dimension). Can we imagine a situation where Sally's photon's path is a slice of an expanding sphere, such that the part of the sphere that i see, is equal to the part of the sphere in Sally's frame?
• can you elaborate , the thought seems interesting
• When both Sally and I shoot a photon each from our respective flashlights at exactly the same position as me at time t=0, in my understanding both the photons should move together regardless of my or Sally's relative speeds. Sally will observe the photons moving slower than I will observe them since she's moving faster relative to me in the same direction as the photons, but because she is aware of her speed, she should also be able to recognize that the speed of light has not slowed. If she were to start moving at the speed of light, she would find from that point onwards that the distance between her spaceship and the photons she's been chasing has become locked into a constant and they are both moving at the same speed. Am I missing something here?
• No all observers will always measure the speed of light in a vacuum as the same value. All constant velocity motion is relative, you can't tell if you are moving or other things are.
• So can i understand the video in one sentence like : Scientists notice that the Newtonian system doesn't work by observing our universe and find out that the velocity of light always stay the same in any frame of reference ?
Is that right ?
And if so, what is the experiment that can prove Galileo or Newton wrong ?
• For one thing, time dilation can be proved with radar. I studied that on some web pages a few years ago, and I thought I understood it for about a day, then lost it. But maybe Sal can "bring me back".
(1 vote)
• what if i am in a black hole just before the event horizon and time would be moving at very high speeds for me according to some sources. if everything's speed is increasing what about light would it still be the same (3*10^8) in my frame of reference.
• Speed of light is always the same in all frames of reference. That is the primary boundary condition for special and general relativity.
• At , here speed of light seems to be half of the real value, and you said that it makes sense, but then you told that speed of light regardless of the frame of reference should be 3*10^8 .so i did not get it. Please help me.
• He's telling you what the Galilean reasoning would suggest and then he's showing that it's wrong when you try to apply it to light, which is why the video has the title it has.
• At , i'm not exactly sure what he means by "faster than the speed of light" because when you think about it, wouldn't the light travel that 1.5x10^8m with the ship and then travel the other 1.5x10^8m to be a total of 3.0x10^8m away from Sal in one second?
• When you measure the speed of light when traveling through a vacuum it will always travel at 3 * 10^8 m/s regardless of the velocity of the source of the light when it was emitted. So if you have a rocket going 1.5 * 10^8 m/s and they fire a laser at you they see the light as traveling at 3 * 10^8 m/s and you see it traveling at 3 * 10^8 m/s as well not 4.5 * 10^8 m/s.