Here's something that bugs me about dimensionality and time - would be grateful for enlightenment...?

I think you're asking a difficult question to answer, but, according to Einstein, time is another dimension of space-time like up and down, left and right, and forwards and backwards. The thing is that you are always travelling at the speed of light, but it is normally through the dimension of time. As soon as you move in any other direction, you are travelling slower though time, so that your total velocity remains the same. It is only the direction of travel which has changed. If you could move at the speed of light through any other of the dimensions you would not move through time at all.

You asked about a fifth dimension. Well we don't know but there are plenty of scientists who speculate on the possibility. String theory suggests that there are many other dimensions, but they are wrapped up tightly so that we are not aware of them. Another scientist has recently suggested that there are actually two dimensions of time, not one. That may seem strange but it does simplify some of the equations in cosmology.

Why is time uni-directional? Nobody really knows, but time is considered to move in the direction of increasing entropy. That means in the direction that things fall apart but do not spontaneously reform. Time travelling in the other direction would be possible but statistically unlikely.

I suspect that your question is much more a philosophical one that one of physics.

In a very fundamental sense Physics and Physicists do not actually know anything! The scientific method being based upon falsifiability. Physicists can be reasonably certain of what doesn't work and has theories that have yet to be shown false for all the rest. Communication using the negative all the time would be so cumbersome that its just not practical so it isn't used, which can lead to a false belief that physicists and physics know the truth. We have theories, often spectacularly good theories, that all remain to be proved false.

Mathematics is the best tool that physics has for expressing its theories. The mathematics used knows absolutely nothing about the physics.

So even if some of the mathematics used is treating time and distance as the same that does not make the two things the same. Rather that for the particular problem being considered treating the two ideas as similar works.

There are lots of open philosophical questions in Physics: such as why does the universe seem to like 'beautiful' mathematics.? Just why has a search for beauty in physics been so fruitful in generating good theories? The question of why is time like distance is one of these questions of beauty and simplicity. But just because the math is beautiful when the two are considered the same does not make them the same. The universe has many secrets.

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Personally I experience three directional dimensions and then something completely different called time. In my own personal experience time and distance are completely different.

I have to be very cautious when I extrapolate from my personal experience to what theoretical physics is suggesting takes place at very small scales and just as carefull when I extrapolate from the very small back up to my human experiences.

Science has looked hard for ways to treat time like a spatial dimension. It would make the math so much more, well, beautiful. No luck so far. Most physical processes are symmetric with respect to time. The reference lists some of each.

The various superstring theories require at least 11 dimensions of spacetime. I still haven't heard of one with more than one temporal dimension.

If you want to think of it that way, the main ratchet preventing time from flowing in reverse is quantum mechanical. The field most directly affected is thermodynamics.

Actually time is a scalar quantity like mass or temperature.

Its rate is a function of an object's position in a gravitational field and also changes with velocity just like mass.

However, just like mass and temperature, time cannot go negative.

I think that "time directionality" has to be with the 2nd principle of Thermodynamics. We need to create entropy for fixing memories, then we have to remember past, and can't have memories from future. The "directionality" is in the structure of our perception, not in the Physics itself. Just an opinion...

Does time really fit the definition of a "dimension"?

Dimensions don't contain things by definition, we just use them to maintain a sense of direction.

Dimensions aren't tangible. Whether they existed or not, wouldnt everything be the same? We just use them to describe shape and direction.

I guess.

You live in three dimensional space, time is distance demonstrated or measured in terms of speed. time does not exist.