Is there a universal "now"?

Question

When discussing relativity and spacetime, we learn that nothing, not even gravity, can travel faster than the speed of light. That gravity travels at exactly the speed of light makes the universe seem like a pond full of ripples. We only perceive the universe when those ripples reach the lens's of our telescopes.

When discussing time in the universe, therefore, we are forced to consider only one "now" - that's our "now" here on earth. The fact that nothing can interact with distant galaxies instantaneously suggests a universal "now" is a concept without meaning.

It's difficult to describe, even theoretically, how the entire universe might experience a "now", but the converse is not appealing. "Fooling" relativity doesn't seem possible.

(continued below)

I'd like to hear your comments on the following thought experiment. Consider what might happen if we mailed out letters to all the galaxies in the universe and, based on each galaxies distance from earth, gave everybody an exact date and time relative to earth-time at which to yell "Hooray!" When that moment came (millions or billions of years in the future, earth-time, if we sent the letters out today), would the effect not be a universal "now"?

Your thoughts are appreciated!

--- John

Does anyone plan on commenting directly on my thought experiment?

Answer 1

First off, awesome question.

I'm not an expert like some people here, but I love chatting physics and here's my two cents..

So if the only thing impeding a universal "now" is the speed of transmission of information, being limited by the speed of light; it is only a matter of removing or bypassing this barrier.

Everyone here has attempted to explain this matter via classical relatively, which is almost a hundred years old to explain this phenomena, but what of quantum mechanics?

I have read of experiments whereby oppositely charged particles, which must ALWAYS have reciprocal charges to each other, communicate at super-luminous speeds.

For example, let us start with a particle that’s positive, and its anti-particle negative. When physicists switched the positive charge to a negative charge, accordingly and superluminally, the other particle, no matter how far it was away from its partner, switched to negative, in order to correspond with its partner far away.

In theory at least, it is possible for information to travel instantaneously. With instant communication, a universal “now” is not only possible, but inevitable.

Even on Earth, even in close proximity to each other, different cultures produced different time-keeping techniques and calendars. All we did was agree to agree on one clock and one calendar.

Answer 2

something that is categorically true, regardless of who believes it. for example, it is true that i have asked questions on this website, whether or not i can provide evidence or whether anyone believes it is true. this is a universal truth, ie. whatever conditions you decide upon and whatever you believe, it is still true. a non-universal truth would be something like "chocolate is good". that is something true, but not in every context and situation. for example, some people don't think it is good, as they don't like it (!), and it is not good for your health in too high a quantity. thus, it is not universally true. i think you are talking about something more fundamental. you seem to describe the reaction some people feel when they learn some major new concept about life, the universe and/or everything. it is what drives philosophy, and what you call universal truth is probably philosophy.

Answer 3

Well, yes, there can be a universal "now", provided only that all recipents properly account for the speed of light and the expansion of space. To do so, they only need know of the physics of each, which I think we do well enough, and the locations of all other communicatees. But it is only through computation, and not through any true universal "now". And since there are portions of the universe that are receeding from use at close to the speed of light, perhaps even beyond, we'd be limited to at most to the observable universe.

As an example, Earth transmits a message outward that includes our location and says the universal "now" will be exactly in 50 billion years. We wait. The recipients receive the message, some of them billions of years from now, and compute the time relative to them that the message was originally sent via the location of Earth information. Then they also wait. No problem.

Answer 4

As you say, a universal 'now' would have very little meaning. If you photograph a distant star (1,000 light years away) now, you are photographing photons that have traveled for about 1,000 years and the star may have disappeared in a supernova long ago. What we see in the distant sky is history. Columbus discovered the new world in 1492 but it is meaningless to ask what he had for lunch 'now.' A universal 'now' is a major communications problem where events in curved space would have to be calculated for an infinite number of members and sub-members. Perhaps the only universal 'now' proceeded the Big Bang (by a groan?).

Answer 5

A universal now? According to Newton, yes. According to Einstein, no. Experimentation over the last century indicates that Einstein is probably right.

Einstein's theory of special relativity states that simultaneity is also relative to one's frame of reference; two events that appear to happen at the same time to one observer don't necessarily appear so to another observer in another inertial frame. So regarding your thought experiment, supposing you timed it so all the recipients shouted "hooray" at the same time (all technical difficulties aside) it would only appear simultaneous from your point of view on Earth. Observers in other galaxies would hear the shouting completely out of sync.

Answer 6

We humans are very tied down to our material reality.
We have not yet transcended to a place of connectiveness to the rest of existance, of which we are comprised.
To experience a universal now would only require a deep connection between all of the universe.
This is already occuring, and it is known by the name GOD.
There are other aspects of God, but this one in particular which you speak of has been the factor that draws so many to religion. Connectiveness.
Within your mind you have to power to be aware of all that is, because it is all the same simple element which can best be described as pure energy. Even solid mass like earth and rocks are at their deepest level, moving energy.
We too are pure energy and can process all sorts of energy surrounding us. Through evolution we gained ability to "receive" and "interpret" energy within a limited spectrum.

Through technology we have bathed ourselves in harmful energy outside of that spectrum, to further expand our ability.

We have seen evidence through history of how one person can be affected by another who is a great distance away.
Where there could not have been a transfer of information through a known channel, somehow a message was sent.
By what means could such a thing occur?

Answer 7

This is the problem Einstein tackled in the Theory of Special Relativity, and it certainly is a mind-bender. You will like the animation and explanation on the page link below.

Answer 8

There is no universal "now." Contrary to what some other responders have said, this is not simply a result of the _distance_ between reference frames, but is because of their _speed_ relative to each other. In such a case, each observer thinks his own clock is running faster than everybody else's. "A" thinks that "B"'s clock is slow; and "B" thinks that "A"'s clock is slow. That makes it impossible for them to agree on a "now."

Here's a thought experiment to illustrate the point:

Say two spaceships pass each other at a relative speed of 1/2 the speed of light. At the moment they cross paths, they both synchronize their clocks to "12:00 noon."

By a prior agreement, the captain of ship "A" waits until his clock says "1:00 pm", and then he fires a radio signal (at the speed of light) toward ship "B".

When the captain of ship "B" receives the signal, he looks at his clock, and records the time in his log. What he writes is this: "Received signal at 1:44 pm".

Later, when they're sharing a drink, both captains agree that this is a reasonable result; but the two of them have completely different accounts of the sequence of events.

Here is Captain "A"'s version:
"I was standing still, while "B" was moving at speed c/2. That made his clock run slow--only 87% as fast as mine. At 1:00, I poured a cup of coffee, and fired my signal. At that moment, my clock said "1:00" but his slow clock said "12:52". He was 1/2 light-hour away at the time. Over the next hour, he receded an additional 1/2 light-hour, at which point the radio signal finally overtook him. At that time, its was 2:00 by my clock, but only 1:44 by his clock."

But Captain "B"'s version is completely different:
"I was standing still, while "A" was moving at speed c/2. That made HIS clock run slow--only 87% as fast as mine. At 1:00, I had a cup of tea. At that time, "A" had receded a distance of 1/2 light-hour, and his slow clock said "12:52". Several minutes later (about 1:09 by my clock), Captain "A"'s slow clock finally reached "1:00", and he fired his signal. By that time, he had receded a little further, and was now 0.58 light-hours (34.6 light minutes) away. It took 34.6 minutes for the signal to reach me: by that time, it was 1:44."

So you see the discrepancies here. Neither captain can agree about "when" things were happening on the other's ship. Who had their 1:00 beverage "first"? Captain "A" says the coffee happened before the tea. Captain "B" says the tea happened before the coffee. There is no way (as long as they're in relative motion) for them to arrange to have their beverages at the "same time." There is no universal "now" -- It's all relative.

Answer 9

There is no universal now, that can be defined by people. Our optical sensors (eyes) catch the electromagentic radiation late at further distances. There is much of the universe we are not designed to see as well. Perhaps now can be defined by other beings who have a way to see larger scales, but not even everything we know is computable.

Answer 10

Relativistic events need not be strictly ordered. Time and space mix in the metric. No two inertial frames can be proven to be synchronous unles they are local ("touching"). Time is wholly maleable depending on point of view.

So is space. A relativistic universe has four distinct distances: luminosity (inverse square), angular diameter, parallax, and proper motion. No two of them need agree to maintain consistency.