B Can there be time without mass?

[PeterDonis]

Can a universe of photons become more disordered?

Of course. You yourself described one mechanism for this: photon-photon interactions will produce fermion-antifermion pairs.

If not, then time cannot move forward.

Sure it can: the classical GR model of a radiation dominated universe still has a perfectly good definition of "time" that moves forward. (Note that in such a model, which idealizes "photons" as null radiation, there are no photon-photon interactions and no pair production.)

would the rate at which entropy increased be so low as to challenge the definition of time?

Any rate that isn't zero would still give you a second law "arrow" of time. But note that this is just a definition of which direction in time is the "forward" direction; it's not the same as a definition of time.

In a universe consisting only of photons, each photon might persist quadrillions of years without colliding with another photon - but during this life, its cumulative gravitational affect would be an important contributor to the geometry of the universe. And in a broad statistical sense, that contribution would betray its location.

Not necessarily; in the classical GR model of a radiation dominated universe, all you have is the energy density of radiation; there are no individual "photons" with locations.

 

[PeterDonis]

in this universe where all particles are massless, is there anything that prevents every interaction that ever did happen to will happen - from happening all at once?

Yes. The fact that arc length along null worldlines is zero does not mean the universe does not have 4 dimensions. Interactions still happen at distinct events and those distinct events are still distributed through 4 dimensions of spacetime.

 

[.Scott]

Of course. You yourself described one mechanism for this: photon-photon interactions will produce fermion-antifermion pairs.

OK. I didn't immediately recognize that as an increase in entropy. But it certainly would be. In due time, an equilibrium would be reached where the rate of fermion pair creation would equal the rate for pair destruction (not to mention other events). So the early pair productions would represent moves toward that equilibrium and thus an increase in entropy.

 

[.Scott]

Sure it can: the classical GR model of a radiation dominated universe still has a perfectly good definition of "time" that moves forward. (Note that in such a model, which idealizes "photons" as null radiation, there are no photon-photon interactions and no pair production.)

In such a case, how would entropy increase? Are Black Holes allowed to form? Does the distribution of matter get clumpy? If nothing statistically irreversible ever happens, you may have a minimal definition of time, but it will lack a direction.

 

[PeterDonis]

Are Black Holes allowed to form? Does the distribution of matter get clumpy?

There is no "matter" in such a universe, only radiation, which doesn't clump gravitationally the way matter (more precisely, non-relativistic matter) does, but that doesn't mean it doesn't clump at all; if it has density variations, there will be some clumping, yes.

If nothing statistically irreversible ever happens

The expansion of the universe itself is statistically irreversible if it is going to continue expanding forever.

 

[guptasuneet]

Actually, the "land dweller" would have to exit the spaceship first, then board it again some time later.

This is actually an instructive way to construct a "twin paradox" scenario. Suppose we pick a frame which we'll call the "land frame"; in this frame, a very, very long spaceship is moving to the right at some speed $v$ which is close to the speed of light. The clocks on the spaceship are all synchronized with each other as viewed in the spaceship's rest frame.

At time $t' = 0$ by the spaceship clocks, the "land dweller" jumps out of the spaceship at $x = 0$ in the "land frame" and immediately decelerates to a stop, so he is at rest in the "land frame". (We will say that the land frame clocks read $t = 0$ at this instant, and that the point on the spaceship where the land dweller jumped out is at $x' = 0$.) At this instant the land dweller's clock also reads zero, since it was sychronized with the spaceship clocks while he was on board the spaceship. Then the land dweller floats there and waits for a while, while the spaceship flies past him; then, after some time $T$ has passed on his clock, he immediately accelerates to speed $v$ and boards the spaceship. The time that he sees on the spaceship clock when he boards (this will of course be a different clock than the one he saw when he exited the spaceship, but all of the spaceship clocks are synchronized in the spaceship frame) will be greater than $T$.

Exactly, if the land dweller exits the ship, come to an immediate stop and jumps back in after some time (immediately going up to the ship's velocity), then the clocks on the spaceship will be ahead of the watch on his hand, because time moved faster for him, relative to the frame of reference of the spaceship.

 

[guptasuneet]

Yes. The fact that arc length along null worldlines is zero does not mean the universe does not have 4 dimensions. Interactions still happen at distinct events and those distinct events are still distributed through 4 dimensions of spacetime.

While the events will be distributed at distinct points in the 4-dimensional spacetime, for the massless photons, all interactions should still apparently happen instantaneously.

 

[Ibix]

While the events will be distributed at distinct points in the 4-dimensional spacetime, for the massless photons, all interactions should still apparently happen instantaneously.

No. Time isn't defined along null worldlines, so it's wrong to use time words like "instantaneously" for things that follow them.

 

[Nugatory]

While the events will be distributed at distinct points in the 4-dimensional spacetime, for the massless photons, all interactions should still apparently happen instantaneously.

That is a common misunderstanding, and one that you will have to give up at some point. The problem is that you are assuming the existence of an inertial frame in which the speed of the photon (properly speaking, "flash of light" - a photon is not what you think it is, but that's a different conversation) is zero, but the speed of light is $c$ in all inertial frames so there can be no such thing.

 

[jbriggs444]

Exactly, if the land dweller exits the ship, come to an immediate stop and jumps back in after some time (immediately going up to the ship's velocity), then the clocks on the spaceship will be ahead of the watch on his hand, because time moved faster for him, relative to the frame of reference of the spaceship.

Time dilation means that his watch ticks slow as reckoned in the rest frame of the [very long] spaceship.

The phrase "time moved faster for him" is inapt. Time moved at the same rate it always does for him. One second proper time per second of proper time. Just as it does for everyone.

 

[PeterDonis]

for the massless photons, all interactions should still apparently happen instantaneously.

For photons, there is no such thing as "apparently happen". The concept of "experienced time" simply does not apply to photons.

 

[Wes Tausend]

Strictly speaking, I would say No.

Time is merely a fundamental quality only assigned to the interaction of two or more masses in motion, or simply an event. Without involving mass nothing would happen, no event could occur and no space between particles would be present to form space-time. It makes no sense for time to exist in a universe devoid of particles and therefore devoid of space 'between' or motion 'of'.

Wes

 

[jbriggs444]

Strictly speaking, I would say No.

Time is merely a fundamental quality only assigned to the interaction of two or more masses in motion, or simply an event. Without involving mass nothing would happen, no event could occur and no space between particles would be present to form space-time. It makes no sense for time to exist in a universe devoid of particles and therefore devoid of space 'between' or motion 'of'.

Wes

An "event" is a four-dimensional coordinate in space-time. Nothing has to occur there for it to exist.

 

[Ibix]

Strictly speaking, I would say No.

Time is merely a fundamental quality only assigned to the interaction of two or more masses in motion, or simply an event. Without involving mass nothing would happen, no event could occur and no space between particles would be present to form space-time. It makes no sense for time to exist in a universe devoid of particles and therefore devoid of space 'between' or motion 'of'.

Wes

As Peter pointed out in #7, we're effectively talking about a radiation dominated FLRW spacetime here. The energy density depends on time, so is a clock in of itself.

 

[Tomas Vencl]

Strictly speaking, I would say No.

Time is merely a fundamental quality only assigned to the interaction of two or more masses in motion, or simply an event. Without involving mass nothing would happen, no event could occur and no space between particles would be present to form space-time. It makes no sense for time to exist in a universe devoid of particles and therefore devoid of space 'between' or motion 'of'.

Wes

As Peter pointed out in #7, we're effectively talking about a radiation dominated FLRW spacetime here. The energy density depends on time, so is a clock in of itself.

Yes, in this framework is effective to work with paradigma that interactions are happening in spacetime.
But I think that original question goes deeper, in fact, to unknown quantum gravity theory. And here, I agree with Wes, that I can imagine, that spacetime itself is gradually generated and formed by interactions. But this is far over current knowledge.

 

[PeterDonis]

I think that original question goes deeper, in fact, to unknown quantum gravity theory.

Discussion of quantum gravity theories is off topic in this forum; it belongs in the Beyond the Standard Model forum.

 

[guptasuneet]

Strictly speaking, I would say No.

Time is merely a fundamental quality only assigned to the interaction of two or more masses in motion, or simply an event. Without involving mass nothing would happen, no event could occur and no space between particles would be present to form space-time. It makes no sense for time to exist in a universe devoid of particles and therefore devoid of space 'between' or motion 'of'.

Wes

Does a photon traveling at c experience time?

A muon generated in the upper atmosphere (from cosmic rays) and traveling at 0.995c experiences time contraction of ~10 times, i.e. it travels the 15 km distance to Earth in 5 microseconds internal time, as compared to 50 microseconds external time. Similarly, the muon experiences the 15 km distance as 1.5 km from its frame of reference.

A photon traveling at c should experience instantaneous traveling in its internal frame of reference, with all external lengths contracted to zero.

Similarly, a massless particle traveling at c will not experience the passage of time. That's what I meant with time not existing without mass.

 

[Ibix]

Does a photon traveling at c experience time?

Time cannot be defined for things traveling at $c$.

A photon traveling at c should experience instantaneous traveling in its internal frame of reference, with all external lengths contracted to zero.

This is not correct, I am afraid. The formulae for length contraction and time dilation are not valid at $c$ because assumptions in their derivation are violated at that speed.

 

[phinds]

Does a photon traveling at c experience time?

No

A photon traveling at c should experience instantaneous traveling in its internal frame of reference, with all external lengths contracted to zero.

Similarly, a massless particle traveling at c will not experience the passage of time. That's what I meant with time not existing without mass.

All of that is wrong and is based on a common misconception which is that a massless object traveling at c can even HAVE a "frame of reference". It cannot.

https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/

 

[Ibix]

That's what I meant with time not existing without mass.

The problem here is one of writing precisely. "Time" is a word with several different meanings in relativity and you need to be careful with it.

If what you meant is that proper time cannot be defined along null worldlines then you are correct, although your reasoning from time dilation formulae isn't really valid as I noted above. But time can also mean coordinate time and it is perfectly possible to define coordinate time in terms of a grid of crossing light rays. In fact that's what the light clock does, as can be seen by extending virtual light rays through the mirrors on a Minkowski diagram. You get a grid of diamonds whose vertical diagonals are regular intervals of time.

 

[rc1]

I think Einstein thought about this "Gedanken experiment" as "sitting on a light beam" in his development of Special Relativity.
Try this site- https://sites.pitt.edu/~jdnorton/Goodies/Chasing_the_light/

 

[Herman Trivilino]

I understand the difference between traveling in a spaceship and staying on land, and why time contracts for the spaceship traveller and not for the land dweller.

Can you explain your understanding of that difference? The Principle of Relativity tells us that there is no difference between being at rest and moving in a straight line at a steady speed.

However, according to the Lorentz Transformation, if I am traveling at c with respect to another frame of reference outside, then the distance will tend towards zero and accordingly time taken will also tend towards zero.

The Lorentz transformation is derived using the assumption that you cannot travel at speed $c$ with respect to another frame of reference.

More specifically, the assumption is that a light beam will always travel at speed $c$ relative to you. So if you chase after it you can never catch it. It will recede from you at speed $c$ no matter how fast your attempt to catch it. Thus it is impossible for you to travel at speed $c$.

Note that this is not just some arcane bit of logic. It is a fact of life for engineers, scientists, and technicians working at places all around the world every day. They accelerate particles to high speeds understanding perfectly well that no matter how much energy they transfer to the particles, they can never get them to reach speed $c$.

 

[rc1]

The Interval is invariant under special relativity.
$s^2= x^2+y^2+z^2-t^2$

 

[Dylan007]

Massless particle will not experience the passage of time and cannot evolve or change. However, for a system where these massless particles interacts (and the system evolve and change) the passage of time, as well as mass, will emerge. So the system will experience the passage of time (and have mass) while the individual massless particles will not experience the passage of time.

P. S: As previously mentioned, although the massless particles does not experience time, spacetime with all its dimensions still exists. But it only makes sense to differentiate between time and space dimensions for objects / systems with mass (in other words it is not following a light path)

 

[PeterDonis]

Massless particle will not experience the passage of time

More correctly, the concept of "the passage of time" as it applies to particles with mass, is meaningless for a massless particle. However:

and cannot evolve or change.

This is wrong. The worldlines of massless particles still pass through distinct points of spacetime and the particles can evolve and change from one point of spacetime to the next. They just don't do it in a way that matches up with the concept of "the passage of time" that applies to particles with mass.

it only makes sense to differentiate between time and space dimensions for objects / systems with mass

This is wrong as well. You can perfectly well describe timelike and spacelike vectors in terms of appropriate combinations of lightlike vectors.

 

[exponent137]

Your best bet will be to find a good textbook (Taylor and Wheeler’s “Spacetime Physics” is my favorite and especially good for your particular question) and start over from a more solid foundation.

I found this book: https://ia801301.us.archive.org/22/...to Special Relativity [Taylor-Wheeler]PDF.pdf

On which page can I find about empty space-time, please?

 

[Ibix]

From the URL that's Taylor and Wheeler. Might be kinder to people's data costs not to link directly to a PDF of an entire book...

Almost the entire book is about special relativity. The last chapter mentions gravity, but the rest is about flat spacetime, which is empty of anything but test particles (and you don't have to put them there).

 

[member 728827]

For photons, there is no such thing as "apparently happen". The concept of "experienced time" simply does not apply to photons.

But, what happens when photons are slowed down? I'm not referring to the obvious refraction index, but to experiments like the one described in the article (I don't know if it's a reputable source...):

Researchers stop and store light for 60 seconds

 

[Ibix]

But, what happens when photons are slowed down?

Then they are travelling on timelike worldlines and proper can be defined along those worldlines.

 

[timmdeeg]

They don't seem to travel at all.

https://newatlas.com/stopping-light-inside-crystal/28610/
The photons are converted into atomic spin excitations (or "spin waves"), which can be stored in the crystal until the control beam is fired again and the spin waves are turned back into light, which finally escapes the crystal.