# Is the speed of light the limit of our universe?

1. Apr 20, 2012

### bedsheet010

I have a question about physics theory pertaining to the singularity beginning our particular universe.

The laws physics may be thought of as “A logical explanation of the workings of our particular universe”. If you are going to accept that our universe is one of many others, how can you be sure that these laws will be applicable in a different universe?

In the attempt to describe the origin of multiple “parallel universe's”, perhaps the origin of ours may be described.

Under our accepted laws of physics, time is continuous and irreversible.

Let me stray from my thinking to provide a necessary example to convey my thinking. Consider a truly irreversible chemical reaction. Consider the beginning of this reaction to be t=a. As the reaction proceeds, it will eventually stop at t=b. If it is possible to travel beyond the speed of light then theoretically it is possible to take an irreversible reaction from t=b to t=a, hence reversing the irreversible. In contrast, take the “big bang” to be t=a, and present time to be t=b. If it were possible to travel beyond the speed of light, then based on accepted theories it would be possible to travel from t=b to t=a. Furthermore, it would be possible to travel back further than the “big bang”. Since the “big bang” is accepted as the beginning of matter in our universe, you would have essentially brought matter to where matter doesn't exist.

Logic dictates that this is not possible. How can something exist when it doesn't?

Perhaps e=mc^2 does not describe the limits and principles of light, but rather the limits of our particular universe?

Our particular existence can be described through three visible dimensions of length, width and height. Time, can be considered a dimension. What if time was in fact not a 4th dimension, but a “governing dimension” of our universe? What if the “natural speed limit” was best was to describe our universe?

If it is accepting that there are multiple oscillating universes, then why can time not be a description of understandable, perceivable, tangible dimension but rather the oscillating limit of our universe? If there are parallel universes, could the limit of the speed of light be the line between our universe and another, parallel universe? Could the speed limit of light simply be line between our universe and the other universes?

2. Apr 20, 2012

### Steely Dan

That is the thinking of some who study cosmology, to attempt to explain away why some of the physical parameters are so finely tuned to make life possible. In a general sense you don't need to explain how the universe ended up with the parameters it did if there are an infinite number of universes with that take all values of the physical parameters.

This is not a meaningful argument. You are starting with the assumption that the reaction is irreversible and then proving that either the assumption is wrong or that time travel is problematic. But that's fine -- there's no issue with the assumption being wrong, because you haven't defined what it even means for a process to be truly irreversible. I don't think you can without biasing this thought experiment.

Hypotheses for how you could actually execute time travel in our universe invariably use time dilations between various frames within our universe. They are not licenses to arbitrarily jump back X years of time, so there isn't a meaningful way to return to earlier than the beginning of the universe in this context.

This more or less already exists in special relativity, though I don't know what you mean by "governing dimension." One way of stating the basic premise of special relativity is that the speed limit of travel through the combined concept spacetime is the speed of light; you can maximize your travel through time by standing still, or maximize your travel through space by going the speed of light through space (and therefore taking zero time).

This is getting a bit too esoteric for me. Sorry, not really sure how to interpret this :)

3. Apr 20, 2012

### Staff: Mentor

You cannot be sure that the same laws will apply in a separate universe.

4. May 15, 2012

### Jacksterlark

There ar particles that can travel faster than the speed of light. So the answer is no, but for an object such as a human being it is impossible because you would need an infinite amount of energy.

5. May 15, 2012

### twofish-quant

We aren't. In fact, some of the latest thinking in physics says that the laws of physics could be random.

Here is an interesting paper

http://space.mit.edu/home/tegmark/dimensions.pdf

The trouble is that if you do that you quickly end up with universes that are incomprehensible. One of the ideas that Tegmark brings up is that the universe has a 3+1 structure with a speed of light, because if you had any other sort of universe, then intelligent life couldn't exist.

Having a speed of light separates things into a "past" and a "future". If you have any other sort of universe, then there is no past and there is no future, and without a "past" and a "future" it's hard to imagine how intelligence could exist.

6. May 15, 2012

### twofish-quant

That's not the big problem.

The big no-no is that you can't send a signal faster than light. The problem with sending signals faster than light is that sending signals faster than light ends up being the same thing as sending signals back in time, and that causes all sorts of problems. The "light barrier" is what keeps the past and the future separate.

Also, I'm not prepared to say that it can't be done... All I can say is

1) no one has ever observed it - (we thought we did a few months ago, but it looks like it wasn't there)
2) it would cause, big, big problems if it happened.

Right after it was announced that CERN may have developed FTL, I was half-expecting to get a mysterious e-mail from my future self, saying that yes, they did find FTL.

Now it might be possible that there is another universe in which FTL happens all the time, but in that universe, it's hard to see how life would have evolved.

7. May 15, 2012

### GeorgeDishman

OK, mix AB and CD at t=a to irreversibly produce AD and BC at t=b.

You could take it even farther back before t=a. Having done so, you have AD and BC while the original you has AB and CD at the same time. Your reaction products have not been changed and his are yet to react (but now he can see what he is going to produce).

Current theory doesn't go back to t=0, but it does say that space since t~0 has been expanding. Trying to go back to that time would be like dropping a ball into an inverted cone, it bounces around as it gets lower but eventually stops at the point.

Matter was produced later, the big bang as an event can be considered the beginning of time and space.

It doesn't describe light, it tells you that mass is a form of energy and that c2 is the conversion factor from units of kg to Joules (as "25.4" is the conversion factor from inches to millimetres).

8. May 15, 2012

### GeorgeDishman

What evidence do you have for that?

9. May 15, 2012

### Mark M

There have already been excellent answers, but I'd just like to add something. Remember that FTL speeds violate causality - they can allow situations such as the Twin 'Paradox' to becomes actual paradoxes. Any transmittance of information must travel slower than light.

Lorentz transformations are what keep the rule - time dilation, and length contraction. When adding velocities, you must use special relativity's formula: $$s = \frac{vu}{1+\frac{vu}{c^{2}}}$$ No matter what two slower than light velocities you substitute in, you can never achieve a FTL speed.

10. May 27, 2012

### rorix_bw

I don't accept the premise within the field of science. The laws try to explain our observable universe. If there are other universes that we cannot observe, then we cannot prove they exist or do not exist, and we can say nothing about them that can be verified.

How do you prove that a thing exists? You measure it, or you measure an effect that can *only* be caused by it (which is actually quite hard...). How do you prove that multiple universes exist when you can't measure them?

Therefore they are indistingishable from the clockwork universe of the Magical Space Goblin who turns a giant handle to move the stars and planets. This can also be said to exist, and you also can neither prove nor disprove its existence.
That's not true. At light speed time stops. This is part of the Law of Relativity. Plus there is some debate about whether or not the arrow of time is reversible or not.

11. May 27, 2012

### twofish-quant

How hard have you tried?

Seriously. One consequence of a lot of physics theories is are multiple universes. The thing that makes science different from other methods of inquiry is rather than "giving up" you ask the question "just how *would* we prove/disprove the existence of other universes?"

People have come up with clever ideas like Boltzmann's brain. Suppose that there were an large number of universes, then which one you find yourself in should be random, therefore if you have a theory that generates a large number of universes, the vast majority of which look different than what you see, then there's something wrong.

There's also the quantum suicide experiment, which I plan to do to celebrate my 150th birthday.

You aren't trying hard enough.

12. May 27, 2012

### shreyakmath

This isnt really the case. You cant neglect quantum effects even at large scale.
In the relativistic framework, speed of light is the limit,but in the quantum mechanical framework velocities greater than c(called tachyonic velocities) are possible. Many researchers have claimed to have produced tachyons

13. May 28, 2012

### twofish-quant

What is the situation is that it's possible to create pairs of particles in coherent states, so that if you measure something in one particle, you get a fixed measurement in another particle. From that you can show that either the information in the particle isn't localized to one particle, or else it and the particles are exchanging internal information *instantaneously*.

The weird thing is that even if the particles are exchanging internal information, there is apparently no way of using this to transmit information. Which is odd and probably says something deep about the universe.

No they haven't. On curious thing is that you can have "stuff" travel faster than light. The big limit is that you can't send *information* faster than light. If you could then you could send a message into your past. Which is weird.

The limit on the speed of light is what separates the future from the past. Without that barrier, the future and the past get mixed, which would results in a very different universe, and perhaps one in which intelligent life is impossible.

14. May 28, 2012

### Mark M

This is false. A theory that contains tachyons has an unstable vacuum, such as the early bosonic string theories. This is a defect in a theory, and the prediction of tachyons is taken to mean a particular theory is fundamentally flawed.

As twofish-quant pointed out, quantum mechanics never allows for faster-than-light transfer of information. Entanglement is a correlation, but it is dependent on the information available to a particular observer. Alice may take a measurement on particle A, and hence know the state of particle B, but Bob is still unaware of the state of B or A. For Alice to send this information to Bob, she would need to do it slower-than-light.

15. May 28, 2012

### GeorgeDishman

This is something I see sometimes stated which puzzles me, "instantaneously" in what frame?

16. May 28, 2012

### Staff: Mentor

@GeorgeDishman: In some frame, depending on the setup. But space-like separated (read: light cannot travel between these points in spacetime) in all frames.

(text in "[ ]" from me)
I doubt that a lot of scientists thought that. At least in my environment, the usual opinion was that OPERA did some measurement error.

17. May 28, 2012

### rbj

twofish, i don't think your reply cuts it. the scientific method does require that eventually theories submit themselves to the test of falsifiability. i think a theory can stick around for a while, while scientists think about ways of testing an operational difference that can be observed that is predicted by the theory. i.e. something would be observationally, measurably different that the theory predicts as opposed to what would be predicted without that theory.

general relativity made a falsifiable prediction about the perihelion precession of Mercury that wasn't tested until Eddington. what would happen to GR if that measurement came out differently than what GR predicted?

what happened to the Big Bang theory when Penzias and Wilson discovered cosmic microwave background radiation? what, then, happened to the Steady State theory?

the aether theory of EM made a prediction that could be tested with the Michaelson-Morley experiment. what happened to this theory when the experiment turned out differently than what it predicted?

string theory or M theory cannot live on indefinitely as a "scientific theory" if the physicists that are working on it cannot come up with a falsifiable prediction of an experimental outcome that would turn out differently if the theory was valid compared to if it was not.

if no falsifiable prediction of an experimental outcome is ever formulated for a theory of multiple universes, then that theory's status as "scientific" weakens to the point where it is no longer scientific. it becomes philosophy, wishful thinking, an "interesting notion", even a religious notion, but if it never makes a falsifiable prediction, if it can never be measured in any way, shape, or form, then it ceases to be science.

i would say that the most fundamental big limit is that no interaction moves from cause to effect faster than c. and i would not call c merely the "speed of light", i would call it the speed of propagation of all "instantaneous" interactions (strong, EM, gravity) and it is a fundamental property of space and time. and, as long as c is real, positive, and finite, it doesn't really matter what the speed is, we cannot tell the difference.

it doesn't matter if we are communicating by means of waving electric charges back-and-forth at each other, or if we're as big as gods and i'm waving a good-sized planet around to send out gravity waves, if there is a third party who is equal distance between you and me, and they see my charge or planet move and then see your charge or planet get perturbed as a consequence, that is the fundamental manifestation of the "speed limit" of c. and it doesn't need to be any particular finite value, life would be the same for us in any case, so it is operationally meaningless what the speed limit is, only that there is a speed limit. that is, for this third party who is equal distance from both you and me, in our universe there is necessarily a delay between cause and effect, cause precedes effect, and that delay is proportional to the distance between the locations in 3-space of where cause and effect are. that is the reason we cannot send information faster than c.

18. May 28, 2012

### twofish-quant

What I'm disputing is that the idea of multiple universes is inherently non-falsifiable. You can argue that it's *hard* to falsify, but unless you try and fail, I wouldn't mark it as impossible to falsify.

Also, "real science" is a lot more complicate they what the philosophers seem to think, and I'm skeptical to appeals to the "scientific method." I'm not sure that such a thing really exists.

People would look for experimental error, and if they could find some excuse to make the results fit, they'd use it. Over time, people would come up with more and more elaborate excuses for why the theory fails, until at some point people realize that some other simpler explanation fits the results better.

This is what happens in reality. If I come up with a single experimental result that doesn't fit theory, I assume the experimental result is wrong. This is because usually the experiment result *is* wrong (i.e. superliminal neutrinos).

Well, the initial results in the 1960's in fact ruled out big bang. When the did atmospheric tests of the CMB, it turned out that it looked nothing like a black body, and the results seems to confirm steady state. It took a few years for new data to come in.

Also the steady state theory is making something of a comeback now in the form of eternal inflation.

People came up with ways of patching the theory. It turns out that there was a simpler explanation. Again it took about 15 years or so.

They haven't. That doesn't mean that they can't. Also various forms of string theory are easily falsibile since they predict things that are obviously wrong.

In the words of the Spartans..... If.....

You give up too easily. There are some very interesting ideas on *how* to falsify multiple universes. Also some of the things in multiple universes are easily falsifiable. You can use the CMB background to put limits on how close an alternative universe could be.

Conversely, if there is a subtle asymmetry in CMB, then that could be a sign of an colliding universe.

Medieval theologians spent several hundred years trying to mathematically prove the existence of God before giving up.

19. Jun 10, 2012

### Eric333

It's quite possible that alternate universes, possibly with differeing levels of "dark matter" mean that light may still be the limiting factor, but it may move much faster than light in our galaxy

20. Jun 11, 2012

### galacticfan

In fact, we have some evidence to show that particles are able to travel faster than the speed of light. Based on an old theory, scientists at NIST have exploited this rule. It relies on forcing one pulse to propagate through a second one. If the second pulse is moving at a speed close to the speed of light, it should in theory be possible to make the first one travel faster than the speed of light.

The concept is called four wave mixing, and it involves combing four different frequencies in such a way as to produce a new signal containing four separate frequencies.

Even the theory proved successful, it was short lived. However, it shows that these rules have loopholes and can be exploited.