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I an having trouble trying to understand time as a physical entity. We can demonstrate the existence of air, the source of infection, electromagnetism, voltage, amperage, among many other physical entities. We can measure, weigh or see these items but when it gets to time my brain stops.

Does time have a physical manifestation, is it a particle or simply time is a mathematical expression. A constant pushed into formulas?

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    $\begingroup$ What kind of demonstration would convince you? I suspect you want a physical manifestation that "looks" like other things in the physics lab, such as a voltmeter moving or air in a container responding to pressure and volume changes? But if you accept the voltmeter, why not the clock? $\endgroup$ – Anders Sandberg May 11 '19 at 8:16
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/17056/2451 , physics.stackexchange.com/q/195290/2451 and links therein. $\endgroup$ – Qmechanic May 11 '19 at 8:18
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    $\begingroup$ Possible duplicate of What is time, does it flow, and if so what defines its direction? $\endgroup$ – John Rennie May 11 '19 at 9:20
  • $\begingroup$ I have read the 4 related questions and still it does not get to the point. Some say it is a mathematical entity, others say that time is what it is being measured by a clock. Other opinions relate time with space and connect it with relativity and it does not make any sense. Accept it like this is without an explanation, A demonstration is not an explanation. If time works in a formula to predict a position, a velocity is only a constant. Still time is not an object with properties, or is it? $\endgroup$ – caranax May 11 '19 at 15:07
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For fundamental questions on time you must refer to proper time, not to coordinate time. That means: Observers are measuring with their respective clocks time periods of worldlines. From such results in the form of coordinate time may always be retrieved the corresponding proper time of the corresponding worldline.

We can distinguish two kinds of worldlines: The spacetime interval of the worldlines of lightlike phenomena such as massless particles and fields is always zero, so they will not help us to understand what (proper) time is. In contrast, mass particles have worldlines with proper time. So when we want to understand what time is, it is sufficient to consider the proper time of mass particles.

The proper time of mass particles is a sort of "pulsebeat" of the particle. The particle has a certain frequency of aging. And this is what (proper) time is. In a second step, we may observe this pulsebeat of the particle, and the respective pulsebeat of all particles. However, the coordinate time we use is the proper time of each particle multiplied by the factors of time dilation - proper time is the time before time dilation, and coordinate time is the time after time dilation.

What we get is our observation, that is a complete coordinate manifold of time measurements of the universe. But each coordinate time measurement can be reduced to the proper time of the corresponding particle.

In one word: There is no common time axis, time is found separately within the different particles, it is their respective duration, their respective aging frequency and their respective proper time.

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In special relativity, i.e., in physics, spacetime is on a united footing. If you think that space is physical, there is good enough (and unavoidable) reason for you to say that time is physical. The experimental evidence for special relativity being true is tremendous. More strikingly, in general relativity (which makes the GPS work and has been proven incredibly accurate in multiple ways), spacetime is not only physical, it can change, i.e., it is dynamical--in particular, spacetime has $20$ degrees of freedom at each point. So, yes, time is very much physical.

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