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## Physics library

### Course: Physics library>Unit 16

Lesson 2: Minkowski spacetime

# Starting to set up a Newtonian path–time diagram

We introduce a graph that's a lot like your good old position-vs-time graph—but with a twist!

## Want to join the conversation?

• At , Sal says that "as we get more into physics, we'll see that maybe we shouldn't necessarily think of time as driving; maybe position, in some ways, is driving time." What is Sal referring to? Is there a relevant Khan Academy video or Wikipedia page?
• Is it that if at , the photon at "c" velocity travels the same distance in less time than the spaceship, making it feel as if less time has passed? Or not? I get a little confused with relativity.
• For an earth-based observer, the photon in the spaceship travels a longer distance at the same speed, so that means more time must pass between ticks of the clock.

This video might help
• hey sal, what is the meaning of word space-time?
• It is the three known spatial dimensions plus the dimension of time. Einstein viewed these as not separate dimensions, but all parts of the same structure known as space-time.
• at when sal draws the graph of position of light he doesn't take into consideration the fact that the person is moving with constant velocity .

In the future videos you will learn how that the speed of light (3 x 10^8 m/s) is absolute - it is always constant no matter from what frame of reference we look at it (even if we are moving).

The Newtonium way of looking at things thought of space and time like this;
1) The passage of time is the same for all frame of references, meaning time is absolute.
2) Measured space is the same for all frames of references.

But as you will learn, this can't be.

Instead, Einstein's Theory of Special Relativity talks about how light is absolute, but space and time are not.

Now we know space and time are not absolute, they are really just different directions in this continuum spacetime. That is why people say space/the universe has 4 dimensions, the last one being time. Space = 3D and time = 1D, when you put these together you get Spacetime which is is representing that time isn't a separate absolute, and space a separate absolute, they are just different directions of spacetime.

This switch from Newtonian/Galilean thinking may take a while to sink in - just keep learning!

Hope this helps,
- Convenient Colleague
• what is the equation for time dilation due to speed
• There is no time dilation in Newtonian mechanics it is part of Special Relativity.
• Given that, "Inertial frames obey Newton's first law, while non-inertial frames don't. That happens because non-inertial frames are accelerating or spinning somehow, which creates pseudo forces within them, denying the first law." Why do accelerating frames create non-inertial forces actin on the accelerating masses (but they do not come from the environment they are intrinsic to the mass that is accelerating. When a body orbits the earth the earth gravity supplies the force responsible for the acceleration in the orbit,but if this is taken to be a bucket full of water the water is pushed against the bottom of the pail so it doesn't fall out becayus their is a centripetal force on the water outward and this is the so called non-inertial forse (we can feel this force when we accelerate around9are carried around relative to the ground) on a merry-go-round.
• There is no force pushing the water against the bottom of the bucket but there is a force from the bucket pushing on the water to make it accelerate in a circle. There only seems to be a force pushing the water against the bottom of the bucket from a non-inertial frame rotating with the bucket.

When you look at the bucket from a non-spinning frame of reference all of the forces make sense but from the spinning frame of reference there is this mysterious force pulling on the bucket and water.
• What does sal mean by “inertial frame of reference” because it would only mean with respect to some other frame right? So I am confused at that part ,because we can’t define an absolute inertial frame of reference.
• An inertial frame of reference is one that follows the law of inertia: a body maintains its state of motion unless an outside force acts on it. A physicist inside an inertial frame can test for whether it is inertial by watching what happens to a pendulum, or a dropped ball, etc.
• What are inertial and non inertial frames of reference? Thank you.
• An inertial frame of reference is simply a frame of reference where the observer/object is moving at a constant velocity (i.e. undergoing no acceleration). A non-inertial frame of reference is one that is undergoing an acceleration, either speeding up, slowing down or spinning.
• Hi,

1) So yes, the speed of light is absolute. It will always move 3x10^8 m/s relative to us. Based upon that, we can measure time an position differently (they may not agree for different frame of references).
2) Based upon this, I have question;

Basically, I'll think about it 2 ways.
1) I am floating backwards. I shoot light forward. That light WILL move 3x10^8 m/s with respect to me.
2) It will also move that fast if I am sitting still on a chair and I shoot light photons.

So what changes in those 2 situations? Time of when we shoot light? Position of the light?
• All motion is relative. So if you are "stationary" or floating with some velocity you are always stationary with respect to your self. So any motion you have is with respect to another reference frame. Because of this there is nothing to explain why the observed speed of light in vacuum is the same for you.

The perceived discrepancy comes in when you are comparing the measured speed of light between reference frames that are moving with relation to each other.

To compare reference frames you need to use the Lorentz Transformation to translate the space-time coordinates.

Here is a shortened like to a play list on the Fermilab YouTube channel on Relativity that are fairly good: shorturl.at/nyCOX