[mxplxxx (OP)]

Given we can't seem to go forwards or backwards in Spacetime, how likely is it that Spacetime is real?

It can be strongly argued that the past is just memories and the future is determined by the current state of the present. It seems likely that reality is a type of finite-state machine (as is a computer). A finite-state machine does not need the concept of time to perform. Things happen one after another. So why not reality?

It seems that the present changes state at the speed of light. But for anything that achieves light speed, the concept of time vanishes. So what does this mean for Spacetime?

Finally, if time is relative as Einstein suggests, it would seem that Spacetime is different for each object in the universe. This seems to indicate it is a mathematical concept only.

[geordief]

As I understand  this , spacetime is a mathematical model that allows us to make predictions concerning motion in space.

It is also the only theory to date  (apart perhaps from quantum mechanics) that allows us to make to same measurements of events anywhere at all  from any place ,time or circumstance  (barring singularities apparently which is where the theory falls off a cliff)

Whether "space time  is real " seems  to be a philosophical point of view from what I have gathered rather than a subject for physics at present.

[mxplxxx (OP)]

Thx for that interesting reply.

[PmbPhy]

Given we can't seem to go forwards or backwards in Spacetime, how likely is it that Spacetime is real?

Where did you ever get the idea that we can't go forward in time? That's most certainly not true. And the notion of real has no place in science. As Einstein wrote

"The physical world is real." This is supposed to be the basic hypothesis. What does "hypothesis" mean here? For me, a hypothesis is a statement whose truth is temporarily assumed, whose meaning, however, is beyond all doubt. The above statement seems intrinsically senseless though, like someone saying "The physical world is a cock-a-doodle-do." It appears to me that "real" is an empty, meaningless category (draw) whose immense importance lies only in that I place certain things inside it and not certain others. It is true that this classification is not a random one ....... now I see you grinning and expecting me to fall into pragmatism so that you can bury me alive. However, I prefer to do as Mark Twain, by suggesting that you end the horror story yourself.
     Real and unreal seem to me like right and left. I admit that science deals with the "real" and am nonetheless a "realist." - Letter from Albert Einstein to Eduard Study (Sept. 25, 1918)

Spacetime is quite real. I'm very busy right now so I'll get back to this later as to why that is my contention. I'll address the rest of your post at that time.

[mxplxxx (OP)]

Great Scott, as the Doc in Back to the Future movies is wont to say, time-travel may disrupt the Spacetime continuum. We seem to be stuck in the present and can go neither forwards nor backwards in time.

There are lots of hints in our current knowledge of physics that says Spacetime isn't real. The major hint  is the speed of light which is a constant in Spacetime irrespective of the frame of reference it is measured in. Bosons, which move at the speed of light in a vacuum, do so apparently without any acceleration being involved. Impossible? Seemingly but physics has always ignored this fact which makes our current theories of reality pretty shaky.

It may be that the only thing that moves at the speed of light is the present. A boson in this scheme of things is just a fixed point in a space that is full of events approaching it at the speed of light. Spacetime may be much better understood as the state history of fixed points in space. 

[PmbPhy]

There are lots of hints in our current knowledge of physics that says Spacetime isn't real.

Where on Earth did you ever get the idea that any current knowledge says that spacetime isn't "real"? I know as a fact that no relativist would ever make such an assertion. I know that I wouldn't, that's for certain.

The major hint  is the speed of light which is a constant in Spacetime irrespective of the frame of reference it is measured in.

Huh?? What led you to believe such a thing, i.e. that the invariance of the speed of light implies that spacetime isn't "real"?

Bosons, which move at the speed of light in a vacuum, ...

Not necessarily. That some bosons, such as photons, move at the speed of light, it's not a defining property of bosons. For example: The W and Z bosons have mass. See: https://en.wikipedia.org/wiki/W_and_Z_bosons

The mass of the W boson is 80.385±0.015 GeV/c². The mass of the Z boson is 91.1876±0.0021 GeV/c².

Bosons, which move at the speed of light in a vacuum,so apparently without any acceleration being involved. Impossible? Seemingly but physics has always ignored this fact which makes our current theories of reality pretty shaky.

Physics has never ignored such a problem because such problems don't exist. If you apply your argument to photons, which do move at the speed of light, there still isn't a problem because no acceleration is involved because photons are created already moving at the speed of light.

I'm sorry my friend but your understanding of physics is off and that's the reason for what you see as unsolved problems in physics.

[mxplxxx (OP)]

Has the speed of W and Z Bosons ever been measured?

[evan_au]

Has the speed of W and Z Bosons ever been measured?

Their mass is known to be around W: 80 GeV/c², and Z: 91 GeV/c², so their speed is < c.

Their actual speed depends on how much momentum they were created with.

Their lifetime is about 3x10^-25s, so even if they were created traveling at relativistic speeds, they would only travel an average of t x v < t x c = 3x10^-25s x 3x10⁸m/s = 9x10^-17 meters, or about 1% of the diameter of a proton. (They are unlikely to travel more than the width of a hydrogen atom, even if the time dilation were extreme.)

In other words, they are so unstable that they decay before they can exit the proton beam to reach a detector that would measure their speed.

So in practice, the energy of the W or Z boson (and their speed) is estimated by adding up the energy of the decay products that do make it to a detector (and accounting for the loss of energy due to neutrinos, which are almost undetectable).

See: https://en.wikipedia.org/wiki/W_and_Z_bosons

[mxplxxx (OP)]

So, the answer is "no their speed has never been measured". So it could be c. Many physicist, it would seem, believe that photons also have mass. Physics is far less a cut and dried science than you would have us believe. Many of the old theories are being questioned in light of their inability to incorporate newly discovered phenomena (think dark matter) - not to mention their inability to describe gravity. Spacetime may just be one of many mathematical theories that can predict the future. Fixed, stateful points in space may just be another way of looking at Statetime. But it does have the advantage of treating space as a fluid which allows for propagation of particles in waves and allows quantum theory to be reformulated as a theory of fluid dynamics (https://www.wired.com/2014/06/the-new-quantum-reality/) . 

Actually, the idea of Spacetime as a continuum appeals. This introduces the possibility that Spacetime is eternal in nature with digital simulations of it being possible at smaller and smaller and larger and larger scales. Is quantum theory just a digital representation of Spacetime? Is a galaxy from the universes point of view a particle. Doe a photon have a TOE that includes particles smaller than itself.

.... but I am rambling!!!

[puppypower]

Space-time integrates the concepts of time and space, as though together they act as one thing. Propagation forward in time, is reference dependent, which implies time depends on position in space. At a given position in space, there is a certain configuration of matter and energy, which defines how time will propagate.

Theoretically, space and time can also act as independent things. For example, acceleration has the units of d/t/t, which is one part distance and two parts time. Acceleration has the units of space-time plus time. Space-time is more concerned with velocity; d/t. In Special relativity, the operating variable, for defining time and distance, is velocity; d/t. Say something quantum steps in distance, in zero time; electron transition, in this case, this can be modeled as extra distance potential in space-time.

The idea of separate time and distance is my own theory. It is based on an observation that has been around since before the birth of Albert Einstein and the other fathers of modern physics. This observation is from photography and is called motion blur.

Motion blur, occurs in photography, when the shutter speed of the camera is slower than the action speed. Since a still photo will stop time, the difference in speed; delta (d/(t=0)) is expressed as uncertainty in distance; motion blur. Motion blur creates the impression of motion, even with time stopped, because  time (delta speed) is conserved, and expressed as uncertainty in distance. Time and space are interchangeable at some level, allowing extra time or distance potential to be expressed as special cases in space-time.

[Atkhenaken]

Time is just an intellectual device which we have created to measure spin rotations. You can assign numbers to represent time in a mathematical sense though. The problems begin when you forget that the numbers which represent time are not spacial in nature. Its when you mistake time numbers for distance numbers that you really get into a fuddle. This is what a lot of physicists have done in the past. Thus, they come up with intellectual ideas such as space/time which is a logical impossibility.

[geordief]

Time is just an intellectual device which we have created to measure spin rotations. You can assign numbers to represent time in a mathematical sense though. The problems begin when you forget that the numbers which represent time are not spacial in nature. Its when you mistake time numbers for distance numbers that you really get into a fuddle. This is what a lot of physicists have done in the past. Thus, they come up with intellectual ideas such as space/time which is a logical impossibility.

what is a spin rotation?

[Atkhenaken]

what is a spin rotation?

A rotation of a planet or a clock. The clock being a device which mimics the rotation of a planet.

[geordief]

I understand that time  will also measure processes that do not involve such movements  . The rate of decay of subatomic particles is an example. Nothing moves  and yet the process is regular and that is how we time the movements of other things.

[evan_au]

The clock being a device which mimics the rotation of a planet.

The reference clock used for scientific purposes is an atomic clock, which does not involve rotation, and is not directly related to the rotation of a particular planet.

[PmbPhy]

Has the speed of W and Z Bosons ever been measured?

I'm sure that it has, even if indirectly.

[PmbPhy]

So, the answer is "no their speed has never been measured". So it could be c.

That's hardly a logical conclusion. One doesn't assume something can be true merely because it hasn't been observed. E.g. I'm 100% certain that my cat cannot travel at the speed of light even though I've never tried an experiment to deduce it.

Many physicist, it would seem, believe that photons also have mass.

That is quite incorrect. If a physicist said that a photon has mass then they're not talking about its proper mass (aka rest mass), they're talking about its relativistic mass.

Physics is far less a cut and dried science than you would have us believe.

That's quite incorrect. What are you basing the claim on?

[geordief]

All sub-atomic particles spin at the speed of light. How can something decay if it is not spinning? Impossible!

Have you a reference for that.? I have heard of the term "spin" when applied to sub atomic particles but I think it is said to be different from what we normally think of as "spin".

I am not sure it means these particles are in motion  but I cannot say as I have not studied this area.

[geordief]

They must be spinning. Otherwise how can they decay if they are not spinning? Its just a matter of logical certainty.

I prefer to rely on interpretations based on observations rather than anyone's  conclusions based on "logical certainty".

You haven't provided me with any reference to particles "spinning at the speed of light" as I requested  . Did you make that up ,then?

[geordief]

Apologies . I thought spin  was one of those made up names like "charm"  and "strange".

I had been told elsewhere  that no motion was involved in radioactive decay. I have no idea if spin is involved in that  process and what properties  related to motion  there is in spin -or indeed what "spin" is when it is at home .