B Can there be time without mass?

[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.

 

[vanhees71]

Photons are no classical point particles. It doesn't make much sense to say they'd travel along worldlines whatsoever. They are not localizable. See also the very interesting discussion going on in the quantum theory forum:

https://www.physicsforums.com/threads/single-photon-states.1048604/

 

[PeterDonis]

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

"Photons being slowed down" is not a good description of what is happening in these experiments.

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

This is not a good description of what is happening in the experiments referred to.

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.

While this ordinary language description still leaves a lot out, at least it acknowledges that the photons have to be "converted" into something else--photons themselves are not being "slowed down" or "stopped".

Further discussion of these experiments belongs in a separate thread in the quantum physics forum. It is off topic here.

 

[guptasuneet]

Thanks everyone for participating in the discussion and for some mind opening replies. What I have understood from the discussion till now is:

1. Spacetime is definable independent of the presence of any particles in it
2. Concept of time cannot be defined for a particle with zero rest mass
3. Interactions between the particles with zero rest mass can be used to define / measure time, even in a spacetime consisting only of such particles with zero rest mass

Not sure if the above summary is proper and request that I may please be corrected.

 

[PeroK]

Thanks everyone for participating in the discussion and for some mind opening replies. What I have understood from the discussion till now is:

1. Spacetime is definable independent of the presence of any particles in it
2. Concept of proper time cannot be defined for a particle with zero rest mass
3. Interactions between the particles with zero rest mass can be used to define / measure time, even in a spacetime consisting only of such particles with zero rest mass

Not sure if the above summary is proper and request that I may please be corrected.

The only thing I'd add is to emphasise that proper time cannot be defined for a particle with zero rest mass.

 

[PeterDonis]

Moderator's note: An off topic subthread has been deleted.