Who said all motion is relative

Press ESC to cancel. Skip to content Home Philosophy Who said all motion is relative? Ben Davis March 31, Who said all motion is relative? Are all motion relative? Is motion absolute or relative? How is velocity calculated? Is velocity a vector or scalar? What is constant speed? For all other obstacles either affect the body only, which is a dead thing; or, except for opinion and the yielding of reason itself, they do not crush nor do any harm of any kind; for if they did, he who felt it would immediately become bad.

Cummings American poet - Personal Records. Dated Notes. Maybe it's you who've moved away by standing still. Last update June 3, Jerome Lawrence 2 American playwright - To be angry is to move, to be brave is to stand still. Therefore, if you're angry, you'll run away. But true unification means to maintain the coordination of mind and body even when we are moving.

Forces acting up, down, or from side to side. You make theories to explain it all, but you might well remember that it was you that invented them all. For Mach there was no reason to believe the rest of the cosmos was doing what your little bit was doing, so science should only describe not try to explain.

Even description is relative. Special relativity tells you exactly how to calculate those disagreements. By my reckoning, if all speed is relative, then no mater how fast you go light should always race away from you at the same apparent speed. For further reading, I recommend this paper, Nothing but Relativity , in which it is shown that, assuming only the principle of relativity, the most general coordinate transformation involves an invariant speed.

We deduce the most general space-time transformation laws consistent with the principle of relativity. Thus, our result contains the results of both Galilean and Einsteinian relativity. The velocity addition law comes as a bi-product of this analysis. We also argue why Galilean and Einsteinian versions are the only possible embodiments of the principle of relativity.

It is an experimental fact that light moves at the same speed in every reference frame, no matter the underlying theory: see the experiment of Michelson and Morley. Every kinematic and dynamical quantity depends on the reference frame except the speed of light, which is the same for every observer. Besides the experimental result there is something deeper that comes from the principle of action at distance. All the interactions present in the universe do not propagate instantaneously, namely if something happens somewhere it takes a while for an observer located elsewhere to detect its consequences, and this is true as a matter of fact.

For example, if you take a small magnet generating a magnetic field and move it from its initial position it will take some time for an observer located far away to detect the change in the magnetic field generated. This said, as a consequence we must assume that there is somehow a velocity according to which the interactions propagate no matter what its nature is and also this velocity must be an upper limit for every other event in the universe, otherwise such events will happen before they can actually propagate and this would obstruct the initial assumption that they must propagate first in order to be detected.

In other words if there is a speed of "propagation" for action at distance then this speed must be an upper limit; also, it must not depend on the reference frame either, otherwise it would not be by definition the speed of propagation. This is the starting point of the special theory of relativity. That this velocity is accidentally the speed of light is proven experimentally. It does. Thats the clever bit! Of course, for this to be true, then what you get when you measure a meter or a second must change as your velocity changes.

This is what they mean when they say 'time slows down' as you get faster and it also explains why nothing that has mass can reach the speed of light. Mass means dimension which would have to become infinite for light to still be measured at 'c' relative to the mass and that can't happen.

To, to sum Gennaro's answer up to answer your title question, the velocity concerned is the speed of cause-effect propagation relative to the observer's rest frame. It measures how long it takes to cause-effect to propagate between experimental kit at rest relative to the observer in his or her rest frame laboratory. Try the VSauce video again - it does actually explain the matter. Try the bit from to about a few times, and think hard. One of the tricky parts about Relativity is that you need to hold a lot of concepts in your head at once - you really need to understand every part to get rid of the confusion.

There's nothing really special about light - it behaves the same way any other mass-less entity behaves. And that includes the fact that any observer, no matter his frame, perceives the speed of light the speed of information to be the same. It's really a property of the space-time itself, not something in the space-time.

You might want to ask why mass-less particles in particular behave this way in general, but that'd be a rather big new question :. To address your edit, this is exactly the point about all the observers agreeing on one speed of light.

As far as the ship-based observer is concerned, there actually isn't a finite maximum speed - they can accelerate ever faster provided they have enough fuel , and due to time-dilation which is not a simple trick, it's critical to understanding relativity , they will be moving ever faster - as far as they can tell.

From an observer on, say, Earth, they will be simply moving close to the speed of light, but to themselves, they can be going c or whatever.

The language is a bit confusing, especially when you only stay on the surface - ideas like "relative" and "absolute" have a slightly different meaning than you might think in the Relativity world we live in. So everyone can agree on one single speed of light - from any point of view, light always travels at the same speed.

However, that's pretty much the only thing they agree on - hence the misleading quote "everything is relative". So, why does that mean that we can't go around colonizing stars with a warp drive? After all, the observer on the ship can observe himself moving at c ignoring the fuel costs and other complications , so where's the "speed limit"?

If you only care about colonizing and exploring the galaxy, the only problem relativity brings is the cost of accelerating fast enough with respect to the source and target and slowing down again. Return trips are the tricky twin-paradox, time-dilating kind. But this also seems to be the way space-time works.

No matter what clever trick you use, you can't really get around this - if you used a "traversable wormhole", you'd again be travelling at much higher speeds than c as far as you're concerned, but as soon as you got back to your point of origin, you'd find that the trip took a lot longer than you thought - you never travelled faster than c with respect to your starting point.

This is a fundamental building block of the whole theory of relativity - the way space-time works. It's hard to see how you could "fix" relativity while keeping the confirmed observations - as the saying goes, "Special relativity, causality, FTL - pick any two".

I'd like to add a little to GreenBeans wonderful answer that not everything is relative. If we design our coordinate system to describe space isotropically and homogenously and describe time uniformly We sometime forget in relativity and in differential geometry that there is still an objective reality in co-ordinates, however bizarrely and human-centrically they may be defined and even though we think of them as human constructs.

At least in physics, for co-ordinates to be useful, there must be an objective, physical procedure for finding the physical point in spacetime labelled by given co-ordinates.

Let's look at this objective, nonrelative physics. Roughly, these are co-ordinates defined by the of rational multiples of displacements along linearly independent directions in space and time of uniform intervals marked out by unit measuring rods and clock ticks in each of the inertial frames.

Physics enters our geometry insofar that we make the physical postulate that the Euclidean geometrical notion of "straightedge" more generally, geodesic segment and the idealized constructions, defined by Euclid's postulates, of marking out a rational number times a unit length along a straight line are a good mathematical model of what we do when we take a ruler and do the same.

This is an experimental , objectively testable result. Likewise, the time co-ordinate enters an analogous description by marking out rational multiples of unit "ticks", where the ticks are defined either by Einstein's procedures with light, or, one can use the definition in Chapter 1 of [1] that "uniform" ticks are ones that the make the motion of a body uninfluenced by forces look uniform from an inertial frame.

There is another piece of objective, nonrelative physics essential to the invariant speed concept and that is Galileo's principle: the notion that an observer that there is no measurement that an observer in an inertial frame can do from within their own frame that can detect the observer's motion relative to any other frame.

This is most poetically described in Galileo's own Allegory of Salviati's Ship within his famous "Dialogue Concerning the Two Chief World Systems" the one that got him into heaps of trouble with Pope Urban II when the latter, having a bad hair day, got a bit bolshie at the implied slight on Papal Infallibility. Once you accept Galileo's relativity postulate as a piece of objective, reproducible, nonrelative physics, this means that co-ordinate transformations between inertial frames must form a group there's a little bit more to this assertion, as I show on my website [2] and also in a hopefully subject to review a forthcoming EJP article.

Then, once you accept that affine geometry models real systems of surveying procedures and time measurement, then the Copernican notion that Nature doesn't care where we put our origin translates to.

It then follows that the group action fulfils the equation:. So if we make the further physical postulate that co-ordinate transformations are continuous, then:. The physical postulate of continuous transformation encodes the everyday experimental result that, as we ride on a bus, we see trees and walkers in the street as we pass even though the bus is moving: we don't see their images shattered into disconnected chaotic sets!

Another active user on this site, Benjamin Crowell, has a wonderful description about how affine geometry with metric structure leads to the Lorentz transformation and the invariant speed concept in chapter 2 of his general relativity book[3]. The following is my own take on it. If we further postulate that there are collinear motions that are descibed by a matrix group parameterized by a real parameter such that group composition is a continuous function of this parameter, then the only transformation group in keeping with this physical postulate as well as Galileo's, Copernicus's and Transformation Continuity is of the form:.

Just think of it as a speedometer reading transformed in a nonlinear way that we'll discover below. Although this postulate sounds a bit technical, here is the physical idea: when we ride in a bus, as we accelerate from the busstop to cruising speed, and as we look out the window, we see the motions of trees and walkers relative to us change continuously and not jerkily. Four more objective, experimentally testable, non relative pieces of physics now enter:. We then use the orthogonal postulate together with isotropy to conclude that a co-ordinate transformation is unchanged if we rotate the co-ordinate system through any angle about the direction of motion.

We actually need the metric notion of orthogonal to define the rotation, and we assume that the Euclidean geometrical notion of rotation, expressed by a rotation matrix conserving the Euclidean inner product, corresponds to the physical notion of rotation.

Imposing this anitcommutation, we find:. Since we can absorb any real constant we like into the rapidity parameter and still get an additive rapidity parameter i. Thus we see that Special Relativity is simply Galileo's relativity with the assumption of absolute time relaxed.

To arrive at this conclusion, we have used the nonrelative, objective, experimentally reproducible physical postulates discussed in this answer. It should be stated that the first person to think along the lines of a relativity not predicated on light was Vladimir Ignatowski in [6]. Other references describing and building on his approach are given in the bibliography of my paper [2], of which a preprint can be seen on my website. You could also wait for a Kindle edition, or, as I did, buy a paperback and have it scanned.

Incidentally, he also has a special relativity book. He also has a fun read:. Although it's meant to be "Relativity Lite" for nonspecialists, nonetheless it does give some clear insights not present in more mathematical treatments, and so is a good read for physicists as well. Although there are several excellent answers, perhaps my answer will remove your confusion. In your statement "if all motion is relative Motion is just a space displacement.

How fast it is accomplished, is irrelevant. Speed is a rate of change of space displacement per unit of time. They are different "entities. An example might make it clearer: There are two ships in the sea, one is going east at mph, and the other is going west at mph.